- Email: [email protected]
Effects of compaction method and rubber content on the properties of concrete paving blocks
Construction and Building Materials 28 (2012) 164–175
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of compaction method and rubber content on the properties of concrete paving blocks Tung-Chai Ling ⇑ Faculty of Construction and Land Use, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
a r t i c l e
i n f o
Article history: Received 28 May 2011 Received in revised form 19 August 2011 Accepted 21 August 2011 Available online 13 October 2011 Keywords: Compaction method Crumb rubber Concrete paving block Properties
a b s t r a c t A wide variety of recycled waste has been successfully used in the production of concrete paving blocks. It is known that the mechanical properties of these concrete products tend to be inconsistent, which is understandable in view of the range of mix designs as well as the variety of materials and compaction methods that were adopted in the production. In this study, recycled waste tyre (crumb rubber) was used to replace sand by volume at the level of 0%, 10%, 20% and 30% in order to investigate how the soft rubber particles behave under plant-machine compaction method during the production of rubberized concrete paving blocks (RCPB). In the hardened stage, the physical properties as well as mechanical properties of RCPB including density, compressive strength, bending strength and skid resistance were studied. The results showed that as a small proportion (10%) of soft rubber particles was included in the mixture, the particles easily distorted and filled the voids between the solid particles. This filling mechanism reduced the porosity of concrete mixtures and effectively developed an adequate adhesion between the particles, resulting in higher gain in strengths. On the contrary, as the rubber ratio increased more than 10%, which the deformability is more predominant than the filling mechanism, this results in higher total stress concentrations and rebound stress of rubber particles, thus, increasing the porosity and micro-cracks, resulting in loss in strengths. Nevertheless, the presence of rubber in concrete did not demonstrate brittle failure, but rather a ductile which had an ability to withstand post-failure loads. In comparison, the mechanical properties of plant-made RCPB performed better than that of corresponding manually-made RCPB. Therefore, plant-compaction method is recommended for future RCPB production and crumb rubber content used to replace sand by volume should be kept at or less than 10%. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Research background
Semi-dry concrete block has become increasingly important material in the construction industry. The fabricated concrete blocks can vary in size, strength and durability, depending upon their usage and the need in construction. To achieve better durability, high utility, consistent quality, and good appearance of SCB, the design concept of the production is mainly based on the combination of low water–cement (w/c) ratio and high compaction method. Presently, there are three general methods for the production of semi-dry concrete blocks at high and uniform standards: (1) hand ramming compaction; (2) manual or machine tamping; (3) high frequency of vibration and compaction methods. Current practice at most of the commercial plants is the (3) method, high frequency of vibration and compaction method which is fully automated, fast and efficient. The final product also has higher density, better strength as well as lower permeability and lesser pore structure.
One of the major consequences of the rapid growth of population, economic and industrialisation is the massive generation of solid wastes and by-product materials. Most of these waste materials are currently landfilled worldwide. As a result, an innovative solution to meet these challenges is necessary. Owing to the advantages and the successful development of the production methods of semi-dry concrete block, it is expected that introduction of solid waste and by-product materials in concrete blocks could considerably reduce the waste management problem. Previous studies have shown that it is possible to utilise most of the solid wastes or by-products in the production of concrete block products. Extensive works have been conducted by Poon et al. [1–6] on the use of construction and demolition wastes such as recycled concrete aggregates and contaminants (tiles, brick, glass and wood) for the production of concrete block products. In their works, a combination of compaction method (manual and high compression pressure) was used for the production of concrete paving blocks (CPB) at laboratory scale. The CPB were prepared with little amount of water in such a way the mix was cohesive
⇑ Tel.: +852 66975982. E-mail addresses: [email protected], [email protected] 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.08.069
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
but with zero slump. The semi-dry cohesive concrete mix was poured into a 200 100 60 mm mould in three layers at equal depth. For the first two layers, manual compaction was applied using a hammer and a wooden stem. After the third layer was poured, a compression force at a rate of 600 kN/min was applied until the force reached 500 kN. The test results showed that it was feasible to produce CPB with 50% recycled concrete aggregate and 50% of crushed clay brick that satisfied the minimum requirement of ETWB of Hong Kong. In order to test the feasibility of the production technique, the same mix proportion of CPB were produced at a commercial plant setting. An automatic block making machine with a combined vibration and compaction force of 80 psi for 12 s was used. They concluded that the properties of plant-produced CPB were comparative with those laboratory-produced CPB except a slightly higher compressive strength was found in the plant-produced CPB. Several authors [7–11] have reported that incorporating recycled glass in CPB is one of the promising solutions for waste glass recycling. According to the reported data [7], one conventional block making manufacturer could use as much as 1000 tonnes recycled glass per day (as a replacement of natural aggregates) in the CPB production. Previous research studies data have shown considerable scatter on the maximum possible content of recycled glass that could be used as natural aggregates replacement in CPB. Meyer et al. [7] found that it was possible to produce plant-CPB containing up to 100% glass as aggregate. However, studies conducted by Byars et al. [8] indicated that if the glass aggregate exceeded 40%, the amount of moisture could not be absorbed by glass aggregate, therefore, that may result in slump and the produced plant-CPB may not be able to retain its original shape. Turgut [9] demonstrated that no damage was observed for all the concrete blocks prepared with recycled glass at laboratory setting under a controlled w/c and moderate compaction technique (160 MPa). Other studies have been carried out [12–14] which aimed to create useful by-products such as fly ash and bottom ash in the production of CPB and pressed blocks. Naik et al. [12] investigated the effect of using 15% and 25% of fly ash as cement replacement in the production of paving stone at a plant setting. Results showed that none of the produced paving stone satisfied the minimum strength requirement according to ASTM C 936. They concluded that the presence of a frog (groove) on the bottom part of the brick mould had affected some of the physical properties of the blocks during the production. Holt and Raivio [13] conducted a preliminary laboratory to look on the effect of different residues as cement replacement in CPB. In the laboratory, an Intensive Compaction Tester (ICT-1000R) with a pressure of 100 kPa, simulated to the plant manufacture setting was used. Based on the laboratory test results, one residue was selected to replace 10% and 15% of cement in CPB to assess its feasibility during the production at full-scale plant setting. In terms of fresh and hardened properties, the CBP containing residue at 10% and 15% showed comparable results as compared to reference CBP. Karasawa et al. [14] worked on the use of large amount of fly ash as fine aggregate replacement in making compacted CPB. The results revealed that as the amount of fly ash increased to 25% in the mix, a large plastic deformation and deterioration of dimension quality was observed during demoulding. They further noted that it was possible to control the plastic deformation by increasing the fineness modulus in the fine aggregate compositions. Freidin [15] used manual block-making machine (Cinva–Ram) with compaction pressure of 4 MPa to prepare building blocks containing fly ash and bottom ash. Results showed that there was no complication for the produced blocks to meet the specified requirement for shape, surface appearance as well as strength. Among the waste, waste tyre is one of the major environmental problems faced by most municipalities in the world due to its not
165
readily biodegradable nature. Majority of such tyres were disposed at landfill and eventually will become mosquito breeding places and the worst case when it is burnt. In recent years, concerning the possible use of recycled waste tyre (crumb rubber) in the production of CPB has been investigated [16–21]. Preliminary results [16] demonstrated that the production process of making CPB mixed with crumb rubber by using manually-operated hand press machine (Cinva–Ram) was quick and simple. Test results have shown that partial replacement of natural aggregates by an equal volume of crumb rubber varying from 10% to 20% resulted in a better flexibility and energy absorption. This improvement may be due to the ability of the crumb rubber to undergo large elastic deformation before the failure of the CPB took place. Moreover, some of the unique properties of CPB produced using rubber aggregate were light weight, higher impact resistance, higher toughness and plastic deformation, which might offer offering a great potential for it to be used in sound barriers and pavement structures. An attempt was made by Ling et al. [17] to evaluate how the crumb rubber contents and w/c ratios affected the properties of manualcompacted CPB. The laboratory test results of CPB incorporating crumb rubber ranging from 0% to 50% showed that the higher the rubber content in the concrete mix, the lower was the compressive strength. Sukontasukkul and Chaikawe [16] also noticed that as the rubber content increased to 20%, there was a great reduction in strength at approximately 82.5%. Thus, the demand for more practical and effective use of crumb rubber to its maximum possibility in the production of rubberized concrete paving blocks (RCPB) to improve some of the engineering properties while maintaining strength has become intense. The influence of soft rubber particles in the production of RCPB under different compaction methods are investigated in this present work. The quality assurance from raw materials to manufacturing process and finishing products is critically evaluated. The hardened properties of RCPB including density, compressive strength, bending strength and skid resistance specimens made by plant-machine and hand-ramming method are studied and compared. 3. Details of research study Fig. 1 shows the flowchart of the present study. This study was divided into three major parts. In the first part, the physical properties of raw materials used for the production of rubberized concrete paving blocks (RCPB) were assessed. In the second part, the influence of soft rubber particles and high compaction force on the fresh and physical properties of RCPB produced was monitored and assessed. Immediately, a same concrete mix was used to produce manually-made RCPB using hand-ramming compaction method. In the final part, the density, skid resistance as well as compressive and bending strengths of both plant-made and manually-made RCPB were determined and compared. A total of approximately 4300 RCPB including control specimens were produced in this study. 3.1. Raw materials The RCPB comprised ordinary Portland cement (OPC), granite aggregate, coarse sand, fine sand and crumb rubber. OPC used throughout the study conformed to BS 12 [22]. The superplasticizer (SP) used was Rheobuild 1000 (an aqueous solution of a Ca–Napthalene Sulphonated). Due to the high viscosity, the SP was dissolved with mixing water before being added into concrete mix. Granite aggregate which had a fineness modulus of 5.29 and flakiness index of 17.07% was used as coarse aggregate. Coarse and fine sand had a maximum particle size less than 9.50 mm and 4.75 mm and fineness modulus of 3.02 and 1.78 were used as fine aggregate
166
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
Sand & granite
Raw materials -Physical attributes check
Material hopper Material dosing Cement, rubber & additive
Water
Cement & rubber
Water
Base concrete mixer
Face concrete mixer
Plant-made compaction machine Plant-made RCPB on wood pallet
Collection of extra base mix at the end of plant-production
Fresh concrete -Shape, dimension & weight checking Finished Products -Visual properties & physical attribute checking
Elevator Production of manual-made RCPB
Curing chamber
Curing in air
Lowerator
Comparison of mechanical properties Fig. 1. The flowchart of research study.
in base and face concrete mixes, respectively. Crumb rubber was a fine material produced by mechanical shredding with the gradation close to that of sand. Two particle sizes of crumb rubber were used: 1–3 mm and 1–5 mm as a partial substitute for sand in the production of face and base layers of RCPB, respectively. The density of 1–3 mm and 1–5 mm dense crumb rubber were 596 kg/m3 and 606 kg/m3, whereas their fineness moduli were 4.52 and 4.74, respectively. 3.2. Mix design A total of four mixes were prepared in a commercial plant setting. Each of the RCPB produced consists of two layers, base (bottom) and face (surface) layer. The raw materials used for base concrete mix were cement, granite aggregate, coarse sand and 1– 5 mm crumb rubber. The mix proportion for control base concrete mix was 1:1.9:3.8 (cement:granite aggregate:coarse sand), with an SP/cement ratio of 0.06. The weight ratio of granite aggregate to coarse sand of all the mixtures was kept at 1:2. The w/c was in
the range 0.39–0.45 for 290–330 kg/m3 cement content, as appropriate to the plant production setting. To produce a good surface appearance (approximately 5 mm thick), the raw materials used for face concrete mix consisted of high volume cement, fine sand and 1–3 mm crumb rubber. The mix proportion for control face mix was 1:2.3 (cement:fine sand). The w/c was in the range of 0.23–0.29 for 570–620 kg/m3 cement content, as appropriate to the smooth facing layer production. The volume fraction of crumb rubber was varied at approximately 0%, 10%, 20% and 30% for sand replacement in the mix for RCPB-0, RCPB-10, RCPB-20 and RCPB-30, respectively. The details of all the RCPB mixes design are shown in Table 1. 3.3. Manufacturing process of plant-made RCPB 3.3.1. Concrete mixing Semi-dry concrete mixing was used at plant production setting for the production of rubberized concrete paving blocks in this study. During the concrete mixing, two independent mixers were
Table 1 Mix composition and ratio of RCPB mixtures. Mix notation
RCPB-0 RCPB-10 RCPB-20 RCPB-30
Mix ratio
Cement content (kg/m3)
w/c ratio
Rubber content (%)
Face (C:FS)
Base (C:A:CS)
Face
Base
Face
Base
Face
Base
1:2.3 1:2.1 1:1.9 1:1.7
1:1.8:3.8 1:1.8:3.4 1:1.8:3.0 1:1.8:2.6
617 585 604 574
328 317 274 286
0.23 0.23 0.29 0.26
0.45 0.43 0.48 0.39
0 8.8 21.6 30.4
0 9.7 19.4 29.0
C-cement, FS-fine sand, CS-coarse sand.
Demoulded fresh density (kg/m3)
2170 2140 2100 2030
167
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
used with appropriate capacity and worked in parallel, to ensure face layer was added on the base layer for a better appearance. Initially, granite aggregate, coarse sand, cement and 1–5 mm crumb rubber were mixed in a base concrete mixer for approximately 1 min. After mixing for 1 min, the required amount of water (SP thoroughly pre-mixed) was added to the dry materials and mixed for another 1 min until the desired moisture content was obtained. Similar mixing process was also employed for face concrete mix. 3.3.2. Fabrication of plant-made RCPB After concrete mixing process was completed, both base and face concrete mixes were transferred from pan mixers to their feed hoppers, respectively. The amount in the feed hoppers was controlled by an automatic weighting system. The hopper discharged a correct amount of concrete into steel mould with individual internal dimensions of 210 mm length, 105 mm width and 60 mm depth. When the mould was filled by the base concrete mix, preliminary vibrations together with high compaction pressing were applied (see Table 2). After that, the base concrete mix was poured into the mould again for the second layer, and then intermediate compaction and vibration were applied for 2.2 second. Finally, face concrete mix was filled up the mould for another (final) compaction of 1.2 second was applied. Hydraulic ram was released after the compaction and the head was lifted to allow early stripping of RCPB from the steel moulds. Previously, because the mechanics of RCPB mixed with rubber particles had not been fully researched, considerable reliance was placed on experience (trial and error) to select an optimum pressing time and w/c for the production of RCPB products at plant setting. The optimum time of vibration at 5.2 second was determined in the plant. But good compaction was more difficult to achieve in RCPB when the concrete mix contained 20% or 30% of crumb rubber. Therefore, trail and error approach was taken to adjust the w/c ratio that can optimise the use of 20% and 30% of soft rubber particles in the production of RCPB. After various tries, an ‘optimum parameter’ was adopted (see Tables 1 and 2). 3.3.3. Monitoring of fresh plant-made RCPB After demoulding, pallets were removed from the lowerator and all the RCPB were stood individually and were separated from one another by thickness of the mould walls. Fig. 2 shows 54 fresh RCPB specimens on a pallet that was produced by one compactions pressing. All the RCPB produced were visually checked and their height was monitored by height control device. Any significant change in the physical appearance indicates something awry in the parameters setting during the production. Fig. 3 indicates some of the ‘‘expanded’’ RCPB that were being rejected as the pallet passed through the height control device. Besides, the appearance and colour variations of RCPB were also checked by manual sampling of five samples from the pallets (see Fig. 4). Weight and dimensions of the sampling samples were determined as shown in Fig. 5. The thickness of the face layer was also measured in accordance to the requirement of MA 20 [23]. Table 3 shows the results of physical properties of fresh plantmade RCPB. Because of low specific gravity of rubber particles,
Table 2 Vibration time and speed was adopted during the production of RCPB. Sequence of vibration applied
Time (s)
Speed (Hz)
Preliminary Intermediate Final (for face) Total Vibration Time (s)
1.8 2.2 1.2 5.2
60 58 60 –
Fig. 2. Height checking.
Fig. 3. Some of the rejected RCPB.
1
2
3 4
5
Fig. 4. RCPB sampling.
weight of fresh plant-made RCPB decreases with the increase in the percentage of rubber content. However, the decrease in weight of rubber was found to be less important when rubber content was at 10% of the total sand volume. Based on the visual observations of the fresh plant-made specimens, no honeycombs, cracks, and outstanding deformation were found on RCPB containing 0% and 10% of crumb rubber. For RCPB containing 20% or 30% of crumb rubber, some cracks and delamination between the face and base layers were clearly observed. This may be attributed to the increase in degree of compressibility by
168
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
increasing the rubber content (during compaction). Therefore, once the RCPB were released from the compaction force (after demoulding), which in turn changed the volume and dimensions (expansion takes place due to the stress released from rubber particles) of the RCPB specimens. The deformability mechanism of the presence of high elastically deformable crumb rubber during the compaction and moulding is illustrated by Fig. 6. Fig. 7 shows a typical pattern of cracks of varying orientations and dimension change of fresh RCPB-30 specimens. Another possible reason for the cracks development may be due to the increase in shear plane between the mould wall and all sides of fresh RCPB specimens. This is because the sand–rubber mixture could created higher shear resistance compared to sand alone at the same compaction [24]. Aslantas [25] also reported similar problem for concrete block produced by a combined vibration and pressure method. He stated that high water content in the mix had caused the surface problems and instability of concrete at the demoulding stage. Also, care should be taken with the use of high content of rubber in the concrete mix because it can gradually affected the concrete slump [26].
Fig. 5. Dimensions measurement.
Table 3 Physical properties of fresh plant-made RCPB. Mix notation
Rubber content (%)
Total depth (mm)
Thickness of facing layer (mm)
Weight (kg)
Visual observations Very good, no cracking Good
RCPB-0
0
59.6
5.5
2.82
RCPB10 RCPB20 RCPB30
10
59.8
5.0
2.82
20
59.2
5.0
2.74
30
59.8
3.3
2.68
3.3.4. Curing All the plant-made RCPB samples were cured under an elevated curing temperature at temperature of 25 ± 3 °C with humidity of 95 ± 3% for the first day. After 1 day, the 1-day hardened plantmade RCPB samples on pallets were then removed from roller-conveyors mounted on the outlet side of the press, and placed on to an elevator. The samples were then collected and further cured at room temperature of 32 ± 3 °C with 65 ± 5% relative humidity until the testing on the 28th day. The effects of different curing conditions on the properties of plant-made RCPB can be found in [20].
Some cracking Some cracking, delamination
When compaction force is applied
When compaction force is released
Mould wall
H1 < H2
RCBP
RCBP Steel plate
B1
<
B2
Fig. 6. RCPB during (a) compaction and (b) demoulding.
Fig. 7. Cracks and dimensions change of RCPB-30 (a) side view (b) plan view.
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
169
3.3.5. Investigation of 1-day hardened plant-made RCPB 3.3.5.1. Physical appearance. The visual properties and physical attribute of 1-day plant-made RCPB samples were checked in accordance to the requirement of the quality specified by MA 20 [23]. Figs. 8 and 9 show some of the finished products that rejected due to some faults present. For each mix design, more than 1000 RCPB specimens were produced. The total number of samples produced and rejected is given in Table 4. The rejection rate increased with an increase in the rubber content which mainly due to the decrease of its feasibility during the production. The rejection samples of RCPB-20 and RCPB-30 shown in Table 4 were included in the redundant samples sourced from those failed trial and error mixes.
Fig. 8. Checking and selecting of rejected RCPB.
3.3.5.2. Surface colour. Fig. 10 shows surface colours of plant-made RCPB containing different content of crumb rubber. The RCPB mixed with 20% and 30% of crumb rubber were slightly darker than those controlled RCPB (0%) and RCPB mixed with 10% of crumb rubber. This slight colouration would not cause significant problem when it was applied in pedestrian area or low traffic volume pavement.
3.4. Manufacturing process of manually-made RCPB
Fig. 9. Rejected RCPB.
Table 4 Number of total samples produced and rejected for each mix design RCPB. Mix notation
Total production sample
Rejected sample
% of rejection rate
RCPB-0 RCPB-10 RCPB-20 RCPB-30
1116 1012 1126 1046
60 100 446 648
5.4 9.9 39.6 62.0
RCPB-0
RCPB-10
The production of manually-made RCPB has been studied in a side experiment. Only base concrete mix collected from wood pallet at the end of each batch plant production was used to prepare manually-made RCPB. The compaction method used to produce manually-made RCPB was a modified method from ASTM D 698 [27] (see Fig. 11). This manually-made method was proven to achieve minimum target strength of 30 MPa [17], which stratified for the application in trafficked area less than 3 tonnes gross weight. The procedure of the modified manual-compaction method is described below. Manual compactions were applied by using a hammer at each layer of concrete mixture after the steel moulds were filled in two layers of about equal depth with internal dimensions of 200 100 60 mm. To ensure the mixture was well compacted at each layer; 50 drops (five drops in horizontal direction and ten drops in vertical direction) of 1.86 kg square hammer (25 mm 25 mm) were uniformly applied from a height of up to 15 cm onto the mixture directly. After the second layer was compacted, a final mix will fill up the empty space within the mould. Excessive materials were removed from the blocks surface and the surface was then flattened with a trowel. The manually-made RCPB were removed from the steel moulds 1 day after casting
RCPB-20
RCPB-30
Fig. 10. Surface colours of four different plant-made RCPB specimens.
170
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
H1 H2
H3 25mm
25mm
Hammer weight (N) Hammer size Drop height (cm) Total number of drops Number of drop per layer Compaction rate (drops/min)
ASTM D 698 [19] 24 50 mm in diameter 30
Modified method 18 25×25 mm in rectangular 15
75
100
25
50
25
50
V1 – V10
V7 V6 V5
H1 - H5
Test method
Rammer pattern for compaction in 200×100×60 mm steel mould Fig. 11. Summary of modified hand-ramming compaction method for manually-made RCPB.
and cured in air at an average room temperature of 30 ± 3 °C with 65 ± 5% relative humidity until the 28 days of testing. 3.5. Mechanical properties test methods The compressive strength was determined in accordance with MA 20 [23]. Prior to the testing, RCPB were soft capped with two pieces of 4 mm thick plywood to ensure a flat surface during loading. A modified British Standard (BS 6073-1) [28] method was used for the three-point bending strength test. Load was applied to a central line of RCPB while being simply supported over a span of 150 mm until rupture occurred. Deflection and energy absorption were automatically recorded in the data acquisition system and the modulus of rupture (MOR) was then calculated. The MOR is giLF ven according to equation: r ¼ 32 BD 2 and expressed in MPa. Where, L is the span length (mm), F the maximum applied load (N), B the average width of the sample (mm), and D its average thickness (mm). The dry density of RCPB was determined according to BS 6073-2 [29]. The density of the RCPB is simply the mass of air dried sample divided by its volume, expressed in kg/m3. The skid resistance was determined in accordance with ASTM E 303 [30]. Four swings were made and an average result was calculated for each specimen. All the testing at hardened stage was conducted at 28 days after casting. An average result of three samples for manually-made RCPB and five samples for plant-made RCPB were reported. 4. Results and discussion on the 28-day hardened properties 4.1. Compressive strength Fig. 12 shows the effects of percentage of crumb rubber replacing fine aggregate on the compressive strength of RCPB. In general, the compressive strength results obtained in this study for both plant-made and manually-made methods are in agreement with previous studies, in which the compressive strength of RCPB decreased as the percentage of crumb rubber increased [16–19]. A study by Sukonrasukkul and Chaikaew [16] reported that the inclusion of 20% of crumb rubber with fineness modulus of 4.98, 3.77 and 2.62 into concrete paving blocks decreased the compressive
strength as much as 84.4%, 78.1%, 85.0%, respectively. In this study, when crumb rubber with a fineness modulus of 4.74 was used as 20% of sand replacement decreased the compressive strength of 50.0% and 52.6% for plant-made and manually-made RCPB, respectively. As for a study by Ling et al. [17], 20% of crumb rubber incorporated in concrete paving blocks with water-to-cement ratio of 0.45, 0.50 and 0.55 caused a decrease of 33.6%, 18.3% and 38.0% in compressive strength, respectively. It is worth to note that the compressive strength of plant-made RCPB increased, as the proportion of crumb rubber in concrete was 10%. This point of inflexion can be explained by the mechanism behaviour shown in Fig. 13. As a small proportion (10%) of crumb rubber was included in the mixture, the soft rubber particles easily distorted and filled the voids between the solid particles (natural aggregates) under a compression force of plant-made machine. This filling mechanism was found to reduce the porosity by filling up the free pore volume in the concrete mixtures (see Fig. 13b). Furthermore, under this circumstance, the rubber particles had bonded well with cement matrix (see Fig. 14a) which in turn resulted in a better compressive strength of RCPB. A perfect adherence between rubber and cement matrix has also been observed in other work [31]. However, it is impossible to achieve ‘‘zero’’ porosity by increasing the rubber particles in the mixtures. A larger porosity was observed with an increasing rubber ratio higher than 10%, because at this point the deformability is more predominant than the filling mechanism. This indicates that the deformability of RCPB increased significantly after an optimum close packing is reached. Also, as the crumb rubber content increases beyond this limit, it increased the total stress concentrations in the concrete mixture and therefore rebound stress of rubber particles occurred. This, in turn, results in micro-cracks between the interfaces in concrete matrix and considered to loss the strength significantly. Fig. 14b shows the loss of adhesion between the rubber particles and the surrounding cement paste. As can be seen in Fig. 12, the compressive strength of RCPB greatly depends on the compaction methods used. Regardless of crumb rubber content, the compressive strength of plant-made RCPB was higher than the corresponding manually-made RCPB. This finding can be explicated through the packing behaviour of the particles in the RCPB whereby the higher level of compaction
171
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
60
Compressive strength (MPa)
plant-made manually-made Ling et al. (w/c=0.45)
50
Ling et al. (w/c=0.50) Ling et al. (w/c=0.55) Sukontasukkul and Chaikaew (FM=4.98)
40
Sukontasukkul and Chaikaew (FM=2.62) Sukontasukkul and Chaikaew (FM=3.77)
30
20
10
0 0
10
20
30
Rubber content by total sand volume (%) Fig. 12. Effects of rubber content on the compressive strength.
Fig. 13. Mechanism behaviour between soft rubber and solid particles (a) under compaction force and (b) once released from the force.
Fig. 14. Observation of undisturbed facture surface resulting from compression test (a) RCPB-10 and (b) RCPB-30.
was; the higher the packing and better strength was achieved. For instance, in the case of RCPB-0 without the influence of crumb rubber particles, when the concrete mixture only consists of rigid/solid particles, applying higher compaction force (by plant-made machine) is able to enhance the particles distribution and ensure a better packing density of the system per unit volume. Note that the decrease of the porosity in the mixtures, which composed of only rigid particles, was mainly attributed to the particles movement and rearrangement with the applied stress because no changes to the shapes and size are expected in solid particles.
The relative strength, Cp/Cm (Cp and Cm being the compressive strengths of plant-made and manually-made RCPB, respectively) was used to compare the relative differences between the strength obtained from two compaction methods. It was noted in Table 5 that the relative compressive strength ranged from 1.92 to 3.48 depending on the percentage use of crumb rubber. Increased in rubber content reduced the value of relative compressive strength. For RCPB-0 without crumb rubber, the use of plant-made method enhanced the compressive strength by 2.55 times as compared to corresponding mix manufactured through manually-made
172
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
Table 5 Relative properties of plant-made and manually-made RCPB mixes. Mix notation
RCPB-0 RCPB-10 RCPB-20 RCPB-30
Compressive strength (MPa) Plantmade
Manuallymade
31.1 42.5 15.6 11.7
12.2 12.2 5.8 6.1
Hardened density (kg/m3)
Relative compressive strength
2.55 3.48 2.69 1.92
Plantmade
Manuallymade
2063 2138 1994 1918
1917 1931 1836 1740
Relative density
Flexural strength (MPa)
1.08 1.11 1.09 1.10
Plantmade
Manuallymade
4.59 5.24 3.30 2.54
2.75 2.67 0.84 0.90
Relative flexural strength
1.67 1.96 3.93 2.82
Fig. 15. SEM image of (a) plant-made and (b) manually-made RCPB-0.
method. From observations under microstructures images, it can be seen that the microstructure of concrete mix matrix was improved. In fact, the concrete matrix shown in Fig. 15a, enriched by the finest sand particles, might firmly embed the pore structure in the matrix, although there was no additional filler in the admixtures. For manually-made method, Fig. 15b shows a lose interface between the particles in the admixture. 4.2. Relationship between compressive strength and hardened density As can be seen in Table 5, the density of manually-made RCPB ranged from 1740 to 1930 kg/m3, whereas the density of plantmade RCPB ranged from 1917 to 2138 kg/m3. This indicated that there was great enhancement in density of RCPB made through
Plant-made
plant-made method. The increase in density was about 9.3% in average. But the incorporation of the crumb rubber decreased the density of both plant-made and manually-made RCPB. As expected, the decrease in density resulted in decreasing compressive strength of RCPB. Fig. 16 shows a statistical relation between 28-day compressive strength and density of both plantmade and manually-made RCPB. In this case, compressive strength was treated as the dependent parameter, whereas density was considered as the independent variable. Higher R-squared values indicated that the proposed method explains the relationship significantly. Therefore, it is suggested that if a higher density can be achieved for a RCPB, it will in turn increase in compressive strength, regardless of rubber content and compaction methods.
Manually-made
Compressive strength (MPa)
60
50
y = 0.0001e6.119x 40
R2 = 0.98
30
y = 0.007e3.9147x 20
R2 = 0.86
10
0 1700
1800
1900
2000
2100
Density (kg/m3) Fig. 16. Relationship between compressive strength and density.
2200
173
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
4.3. Three-point bending strength There is no doubt that if RCPB are used for pavement application, the RCPB are more liable to break under traffic (fail in bending) than being crushed (fail under compression). Hence, it is necessary to evaluate the bending conditions of the plant-made and manually-made RCPB produced. Fig. 17 illustrates the effect of compaction methods on modulus of rupture (MOR) in varying crumb rubber content. A similar trend of MOR as in compressive strength was observed. It was, however, the MOR results of manually-made RCPB samples that were very discouraging, particularly at higher percentage of rubber content. For example, the MOR of manually-made RCPB-20 was only a quarter (25%) of the corresponding plant-made RCPB-20. This could be due to the combined reasons of low water-to-cement ratio, low compaction force of manually-made method as well as the presence of high content of soft rubber particles in the concrete mixtures. Therefore, to prevent the low MOR, avoid insufficient water, insufficient compaction, and too much soft rubber particles from occurring simultaneously during the production of RCPB. Fig. 18 shows the typical load–deflection curve of (a) plantmade and (b) manually-made RCPB with varying percentage of crumb rubber. Maximum deflections were observed for RCPB incorporating 30% of crumb rubber for both plant-made and manually-made method. This showed that the rubber particles
Plant-made
increased the deformability property of RCPB. It was observed that when RCPB-20 and RCPB-30 achieved its maximum load, they were not completely fractured, but withstand post-failure loads and then fail gradually (ductile behaviour). The toughness is known as energy absorption capacity and is generally calculated from the area under load–deflection curve up to where the point failure is plotted. For a given rubber content, even though the deflection value of plant-made RCPB was lower, but the toughness and fracture energy was found to be larger than the manually-made RCPB. This is due to the higher strength response. Hence, the plant-made RCPB was able to absorb larger quantities of energy after the peak load and prior to the final failure than manually-made RCPB prepared with the same percentage of crumb rubber. Ling et al. [21] reported that concrete paving blocks containing crumb rubber performed better impact resistance than a plain concrete paving block. They found that the plain concrete block was broken completely after the 3rd drops of falling weight; whereas the block with 30% crumb rubber only suffered small cracking after the 9th drops and maintained the integrity of the block structure. 4.4. Skid resistance Skid resistance was measured in accordance to ASTM E 303 [30]; four swings of the pendulum were made for each individual surface of RCPB. Prior to the test, the concrete block surface was
Manually-made
Modulus of rupture (MPa)
6
5
4
3
2
1
0 RCPB-0
RCPB-10
RCPB-20
RCPB-30
Mix notation Fig. 17. Modulus of rupture of plant-made and manually-made RCPB.
RCPB-0
RCPB-10
RCPB-0
RCPB-10
RCPB-20 RCPB-30
RCPB-20
Fig. 18. Three-point bending strength responds of (a) plant-made and (b) manually-made RCPB.
RCPB-30
174
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
Plant-made
Manually-made
Brithish pendulum number (BPN)
90 80 70 60 50 40 30 20 10 0 RCPB-0
RCPB-10
RCPB-20
RCPB-30
Mix notation Fig. 19. Skid resistance of plant-made and manually-made RCPB.
thoroughly cleaned and dried. The effect of crumb rubber content of both plant-made and manually-made RCPB is shown in Fig. 19. In general, all RCPB produced in this study satisfied ASTM requirement. As seen in the figure, it showed that increasing percentages of crumb rubber in RCPB decreases the skid resistance. It was found that the highest skid resistance of 82.6 BPN and 78.0 BPN were achieved for plant-made and manually-made RCPB without crumb rubber. The RCPB without rubber particles on surface layer might create more friction as the pendulum passed across. Ling et al. [17] also showed that the inclusion of crumb rubber in concrete paving blocks lowered the skid resistance. However, results reported by [16] indicate the opposite trend. For a given rubber content, plant-made RCPB showed slightly higher values than those corresponding manually-made RCPB with an average increment of 10% in BPN value. The improvement was more pronounced at a higher level of rubber content (30%) over the lower level of rubber content (10%), which accounted for a 15% and 5.6% gain in BPN, respectively. 5. Conclusions and recommendations Based on the investigation in this study, the following conclusions can be drawn: It is not feasible to produce RCPB at commercial plant setting when the concrete incorporated more than 10% of soft rubber particle. Also, when the RCPB contain 20% or 30% of crumb rubber, delamination or cracks may occur on the fresh RCPB specimens due to the increase in degree of compressibility, which in turn causes a change in volume and dimensions when demoulding. The rejection rate of RCPB produced increase with an increase in the rubber content due to the decrease of its feasibility during the production. Besides, the surface appearance of RCPB mixed with 20% and 30% of crumb rubber were slightly darker than that of RCPB mixed with lower substitution ratios of crumb rubber. Inclusion of small proportion (10%) of crumb rubber in plantmade RCPB slightly improved the strengths, probably due to the filling mechanism of soft rubber particles. On the contrary, larger porosity was observed with an increasing rubber ratio more than 10% due to the deformability which is more predominant than filling mechanism and therefore resulted in a significant strength reduction.
There was a great enhancement in density when RCPB was produced by plant-made method. The increase in density was about 9.3% in average as compared to manually-made method. As expected, the incorporation of crumb rubber decreased the density of both plant-made and manually-made RCPB. It is suggested that if a higher density can be achieved would produce a higher compressive strength. In the practical viewpoint, plantmade method is recommended for future RCPB production. The use of rubber particles in RCPB increased the deformability and toughness property. It was observed that when rubber particles were incorporated, the RCPB were able to absorb larger quantities of energy after the peak load and prior to the final failure. Also, the presence of rubber in concrete did not demonstrate brittle failure, but rather a ductility which had the ability to withstand post-failure loads. Increasing percentages of crumb rubber in RCPB decreased the skid resistance. For a given rubber content, plant-made RCPB showed slightly higher values than those corresponding manually-made RCPB with an average increment of 10% in BPN value. However, there is no significant impact to apply both plantmade and manually-made RCPB for pavement application as both satisfied ASTM requirement.
Acknowledgements The authors express their sincere thanks to the Ministry of Science, Technology and Innovation (MOSTI), Malaysia under IRPA research Grant No. 03-02-06-0129-EA0001. Technical help provided by Sunway Paving Solution (M) Sdn. Bhd. is gratefully acknowledged. References [1] Poon CS, Chan D. Paving blocks made with recycled concrete aggregate and crushed clay brick. Constr Build Mater 2006;20(8):569–77. [2] Poon CS, Chan D. Effects of contaminants on the properties of concrete paving blocks prepared with recycled concrete aggregates. Constr Build Mater 2007;21(1):164–75. [3] Poon CS, Lam CS. The effect of aggregate-to-cement ratio and types of aggregates on the properties of pre-cast concrete blocks. Cem Concr Compos 2008;30(4):283–9. [4] Poon CS, Kou SC, Lam L. Use of recycled aggregates in molded concrete bricks and blocks. Constr Build Mater 2002;16(5):281–9. [5] Poon CS, Kou SC, Wan HW, Etxeberria M. Properties of concrete blocks prepared with low grade recycled aggregates. Waste Manage 2009;29(8):2369–77.
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175 [6] Zhao X, Ling TC, Kou SC, Wang QY, Poon CS. Use of wastes derived from earthquakes for the production of concrete masonry partition wall blocks. Waste Manage 2011;31(8):1859–66. [7] Meyer C, Egosi N, Andela C. Concrete with waste glass as aggregate. In: Recycling and re-use of glass cullet, proceeding of the international symposium concrete technology unit of ASCE, University of Dundee; 2001. [8] Byars EA, Zhu HY, Morales B. CONGLASSCRETE I. Final report. The waste & resources action programme, UK; 2004. [9] Turgut P. Properties of masonry blocks produced with waste limestone sawdust and glass powder. Constr Build Mater 2008;22(7):1422–7. [10] Turgut P, Yahlizade ES. Research into concrete blocks with waste glass. Int J Environ Sci Eng 2009;1(4):202–8. [11] Lam CS, Poon CS, Chan D. Enhancing the performance of pre-cast concrete blocks by incorporating waste glass – ASR consideration. Cem Concr Compos 2007;29(8):616–25. [12] Naik TR, Kraus RN, Chun YM, Ramme BW, Singh SS. Properties of field manufactured cast-concrete products utilizing recycled materials. ASCE J Mater Civil Eng 2003;15(4):400–7. [13] Holt E, Raivio P. Use of gasification residues in compacted concrete paving blocks. Cem Concr Res 2005;36(3):441–8. [14] Karasawa A, Suda S, Naito H, Fujiwaran N. Application of fly ash to concrete paving block. In: Proceedings of the 7th international conference on concrete block paving, Sun City, South Africa; 2003. p. 343–52. [15] Freidin C. Cementless pressed bocks from waste products of coal-firing power station. Constr Build Mater 2007;21(1):12–8. [16] Sukonrasukkul P, Chaikaew C. Properties of concrete pedestrian block mixed with crumb rubber. Constr Build Mater 2006;20(7):450–7. [17] Ling TC, Nor HM, Hainin MR. Properties of concrete paving blocks incorporating crumb rubber and SBR latex. Road Mater Pavement 2009;10(1):213–22.
175
[18] Ling TC. Prediction of density and compressive strength for rubberized concrete blocks. Constr Build Mater 2011;25(11):4303–6. [19] Ling TC, Nor HM, Lim SK. Using recycled tyre in concrete paving blocks. ICEWaste Resour Manage 2010;163(1):37–45. [20] Ling TC, Nor HM, Hainin MR, Lim SK. Long term strength of rubberized concrete paving blocks. ICE-Constr Mater 2010;163(1):19–26. [21] Ling TC, Nor HM, Hainin MR, Chik AA. Laboratory performance of crumb rubber concrete block pavement. Int J Pavement Eng 2009;10(5):361–74. [22] BS 12. Specification for Portland cement. British Standards Institution; 2001. [23] MA20. Specification for concrete segmental paving units. Concrete Masonry Association of Australia, 1996. [24] Ghazavi M. Shear strength characteristics of sand-mixed with granular rubber. Geotech Geol Eng 2004;22(3):401–16. [25] Aslantas O. A study on abrasion resistance of concrete paving blocks. Master thesis. Middle East Technical University; 2004. [26] Khatib ZK, Bayomy FM. Rubberized Portland cement concrete. ASCE J Mater Civil Eng 1999;11(3):206–13. [27] ASTM D 698. Standard test methods for laboratory compaction characteristics of soil using standard effort (12 400 ft-lbf/ft3 (600 kN-m/m3)). American society of testing and materials; 1997. [28] BS 6073-1. Precast concrete masonry units – Part 1: Specification for precast concrete masonry units. British Standards Institution; 1891. [29] BS 6073-2. Precast concrete masonry units – Part 2: Guide for specifying precast concrete masonry units. British Standards Institution; 2008. [30] ASTM E 303. Standard test method for measured surface frictional properties using British pendulum tester. American society of testing and materials; 1993. [31] Herna´ndez-Olivaresa F, Barluengaa G, Bollatib M, Witoszek B. Static and dynamic behaviour of recycled tyre rubber-filled concrete. Cem Concr Res 2002;32(10):1587–96.
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of compaction method and rubber content on the properties of concrete paving blocks Tung-Chai Ling ⇑ Faculty of Construction and Land Use, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
a r t i c l e
i n f o
Article history: Received 28 May 2011 Received in revised form 19 August 2011 Accepted 21 August 2011 Available online 13 October 2011 Keywords: Compaction method Crumb rubber Concrete paving block Properties
a b s t r a c t A wide variety of recycled waste has been successfully used in the production of concrete paving blocks. It is known that the mechanical properties of these concrete products tend to be inconsistent, which is understandable in view of the range of mix designs as well as the variety of materials and compaction methods that were adopted in the production. In this study, recycled waste tyre (crumb rubber) was used to replace sand by volume at the level of 0%, 10%, 20% and 30% in order to investigate how the soft rubber particles behave under plant-machine compaction method during the production of rubberized concrete paving blocks (RCPB). In the hardened stage, the physical properties as well as mechanical properties of RCPB including density, compressive strength, bending strength and skid resistance were studied. The results showed that as a small proportion (10%) of soft rubber particles was included in the mixture, the particles easily distorted and filled the voids between the solid particles. This filling mechanism reduced the porosity of concrete mixtures and effectively developed an adequate adhesion between the particles, resulting in higher gain in strengths. On the contrary, as the rubber ratio increased more than 10%, which the deformability is more predominant than the filling mechanism, this results in higher total stress concentrations and rebound stress of rubber particles, thus, increasing the porosity and micro-cracks, resulting in loss in strengths. Nevertheless, the presence of rubber in concrete did not demonstrate brittle failure, but rather a ductile which had an ability to withstand post-failure loads. In comparison, the mechanical properties of plant-made RCPB performed better than that of corresponding manually-made RCPB. Therefore, plant-compaction method is recommended for future RCPB production and crumb rubber content used to replace sand by volume should be kept at or less than 10%. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Research background
Semi-dry concrete block has become increasingly important material in the construction industry. The fabricated concrete blocks can vary in size, strength and durability, depending upon their usage and the need in construction. To achieve better durability, high utility, consistent quality, and good appearance of SCB, the design concept of the production is mainly based on the combination of low water–cement (w/c) ratio and high compaction method. Presently, there are three general methods for the production of semi-dry concrete blocks at high and uniform standards: (1) hand ramming compaction; (2) manual or machine tamping; (3) high frequency of vibration and compaction methods. Current practice at most of the commercial plants is the (3) method, high frequency of vibration and compaction method which is fully automated, fast and efficient. The final product also has higher density, better strength as well as lower permeability and lesser pore structure.
One of the major consequences of the rapid growth of population, economic and industrialisation is the massive generation of solid wastes and by-product materials. Most of these waste materials are currently landfilled worldwide. As a result, an innovative solution to meet these challenges is necessary. Owing to the advantages and the successful development of the production methods of semi-dry concrete block, it is expected that introduction of solid waste and by-product materials in concrete blocks could considerably reduce the waste management problem. Previous studies have shown that it is possible to utilise most of the solid wastes or by-products in the production of concrete block products. Extensive works have been conducted by Poon et al. [1–6] on the use of construction and demolition wastes such as recycled concrete aggregates and contaminants (tiles, brick, glass and wood) for the production of concrete block products. In their works, a combination of compaction method (manual and high compression pressure) was used for the production of concrete paving blocks (CPB) at laboratory scale. The CPB were prepared with little amount of water in such a way the mix was cohesive
⇑ Tel.: +852 66975982. E-mail addresses: [email protected], [email protected] 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.08.069
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
but with zero slump. The semi-dry cohesive concrete mix was poured into a 200 100 60 mm mould in three layers at equal depth. For the first two layers, manual compaction was applied using a hammer and a wooden stem. After the third layer was poured, a compression force at a rate of 600 kN/min was applied until the force reached 500 kN. The test results showed that it was feasible to produce CPB with 50% recycled concrete aggregate and 50% of crushed clay brick that satisfied the minimum requirement of ETWB of Hong Kong. In order to test the feasibility of the production technique, the same mix proportion of CPB were produced at a commercial plant setting. An automatic block making machine with a combined vibration and compaction force of 80 psi for 12 s was used. They concluded that the properties of plant-produced CPB were comparative with those laboratory-produced CPB except a slightly higher compressive strength was found in the plant-produced CPB. Several authors [7–11] have reported that incorporating recycled glass in CPB is one of the promising solutions for waste glass recycling. According to the reported data [7], one conventional block making manufacturer could use as much as 1000 tonnes recycled glass per day (as a replacement of natural aggregates) in the CPB production. Previous research studies data have shown considerable scatter on the maximum possible content of recycled glass that could be used as natural aggregates replacement in CPB. Meyer et al. [7] found that it was possible to produce plant-CPB containing up to 100% glass as aggregate. However, studies conducted by Byars et al. [8] indicated that if the glass aggregate exceeded 40%, the amount of moisture could not be absorbed by glass aggregate, therefore, that may result in slump and the produced plant-CPB may not be able to retain its original shape. Turgut [9] demonstrated that no damage was observed for all the concrete blocks prepared with recycled glass at laboratory setting under a controlled w/c and moderate compaction technique (160 MPa). Other studies have been carried out [12–14] which aimed to create useful by-products such as fly ash and bottom ash in the production of CPB and pressed blocks. Naik et al. [12] investigated the effect of using 15% and 25% of fly ash as cement replacement in the production of paving stone at a plant setting. Results showed that none of the produced paving stone satisfied the minimum strength requirement according to ASTM C 936. They concluded that the presence of a frog (groove) on the bottom part of the brick mould had affected some of the physical properties of the blocks during the production. Holt and Raivio [13] conducted a preliminary laboratory to look on the effect of different residues as cement replacement in CPB. In the laboratory, an Intensive Compaction Tester (ICT-1000R) with a pressure of 100 kPa, simulated to the plant manufacture setting was used. Based on the laboratory test results, one residue was selected to replace 10% and 15% of cement in CPB to assess its feasibility during the production at full-scale plant setting. In terms of fresh and hardened properties, the CBP containing residue at 10% and 15% showed comparable results as compared to reference CBP. Karasawa et al. [14] worked on the use of large amount of fly ash as fine aggregate replacement in making compacted CPB. The results revealed that as the amount of fly ash increased to 25% in the mix, a large plastic deformation and deterioration of dimension quality was observed during demoulding. They further noted that it was possible to control the plastic deformation by increasing the fineness modulus in the fine aggregate compositions. Freidin [15] used manual block-making machine (Cinva–Ram) with compaction pressure of 4 MPa to prepare building blocks containing fly ash and bottom ash. Results showed that there was no complication for the produced blocks to meet the specified requirement for shape, surface appearance as well as strength. Among the waste, waste tyre is one of the major environmental problems faced by most municipalities in the world due to its not
165
readily biodegradable nature. Majority of such tyres were disposed at landfill and eventually will become mosquito breeding places and the worst case when it is burnt. In recent years, concerning the possible use of recycled waste tyre (crumb rubber) in the production of CPB has been investigated [16–21]. Preliminary results [16] demonstrated that the production process of making CPB mixed with crumb rubber by using manually-operated hand press machine (Cinva–Ram) was quick and simple. Test results have shown that partial replacement of natural aggregates by an equal volume of crumb rubber varying from 10% to 20% resulted in a better flexibility and energy absorption. This improvement may be due to the ability of the crumb rubber to undergo large elastic deformation before the failure of the CPB took place. Moreover, some of the unique properties of CPB produced using rubber aggregate were light weight, higher impact resistance, higher toughness and plastic deformation, which might offer offering a great potential for it to be used in sound barriers and pavement structures. An attempt was made by Ling et al. [17] to evaluate how the crumb rubber contents and w/c ratios affected the properties of manualcompacted CPB. The laboratory test results of CPB incorporating crumb rubber ranging from 0% to 50% showed that the higher the rubber content in the concrete mix, the lower was the compressive strength. Sukontasukkul and Chaikawe [16] also noticed that as the rubber content increased to 20%, there was a great reduction in strength at approximately 82.5%. Thus, the demand for more practical and effective use of crumb rubber to its maximum possibility in the production of rubberized concrete paving blocks (RCPB) to improve some of the engineering properties while maintaining strength has become intense. The influence of soft rubber particles in the production of RCPB under different compaction methods are investigated in this present work. The quality assurance from raw materials to manufacturing process and finishing products is critically evaluated. The hardened properties of RCPB including density, compressive strength, bending strength and skid resistance specimens made by plant-machine and hand-ramming method are studied and compared. 3. Details of research study Fig. 1 shows the flowchart of the present study. This study was divided into three major parts. In the first part, the physical properties of raw materials used for the production of rubberized concrete paving blocks (RCPB) were assessed. In the second part, the influence of soft rubber particles and high compaction force on the fresh and physical properties of RCPB produced was monitored and assessed. Immediately, a same concrete mix was used to produce manually-made RCPB using hand-ramming compaction method. In the final part, the density, skid resistance as well as compressive and bending strengths of both plant-made and manually-made RCPB were determined and compared. A total of approximately 4300 RCPB including control specimens were produced in this study. 3.1. Raw materials The RCPB comprised ordinary Portland cement (OPC), granite aggregate, coarse sand, fine sand and crumb rubber. OPC used throughout the study conformed to BS 12 [22]. The superplasticizer (SP) used was Rheobuild 1000 (an aqueous solution of a Ca–Napthalene Sulphonated). Due to the high viscosity, the SP was dissolved with mixing water before being added into concrete mix. Granite aggregate which had a fineness modulus of 5.29 and flakiness index of 17.07% was used as coarse aggregate. Coarse and fine sand had a maximum particle size less than 9.50 mm and 4.75 mm and fineness modulus of 3.02 and 1.78 were used as fine aggregate
166
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
Sand & granite
Raw materials -Physical attributes check
Material hopper Material dosing Cement, rubber & additive
Water
Cement & rubber
Water
Base concrete mixer
Face concrete mixer
Plant-made compaction machine Plant-made RCPB on wood pallet
Collection of extra base mix at the end of plant-production
Fresh concrete -Shape, dimension & weight checking Finished Products -Visual properties & physical attribute checking
Elevator Production of manual-made RCPB
Curing chamber
Curing in air
Lowerator
Comparison of mechanical properties Fig. 1. The flowchart of research study.
in base and face concrete mixes, respectively. Crumb rubber was a fine material produced by mechanical shredding with the gradation close to that of sand. Two particle sizes of crumb rubber were used: 1–3 mm and 1–5 mm as a partial substitute for sand in the production of face and base layers of RCPB, respectively. The density of 1–3 mm and 1–5 mm dense crumb rubber were 596 kg/m3 and 606 kg/m3, whereas their fineness moduli were 4.52 and 4.74, respectively. 3.2. Mix design A total of four mixes were prepared in a commercial plant setting. Each of the RCPB produced consists of two layers, base (bottom) and face (surface) layer. The raw materials used for base concrete mix were cement, granite aggregate, coarse sand and 1– 5 mm crumb rubber. The mix proportion for control base concrete mix was 1:1.9:3.8 (cement:granite aggregate:coarse sand), with an SP/cement ratio of 0.06. The weight ratio of granite aggregate to coarse sand of all the mixtures was kept at 1:2. The w/c was in
the range 0.39–0.45 for 290–330 kg/m3 cement content, as appropriate to the plant production setting. To produce a good surface appearance (approximately 5 mm thick), the raw materials used for face concrete mix consisted of high volume cement, fine sand and 1–3 mm crumb rubber. The mix proportion for control face mix was 1:2.3 (cement:fine sand). The w/c was in the range of 0.23–0.29 for 570–620 kg/m3 cement content, as appropriate to the smooth facing layer production. The volume fraction of crumb rubber was varied at approximately 0%, 10%, 20% and 30% for sand replacement in the mix for RCPB-0, RCPB-10, RCPB-20 and RCPB-30, respectively. The details of all the RCPB mixes design are shown in Table 1. 3.3. Manufacturing process of plant-made RCPB 3.3.1. Concrete mixing Semi-dry concrete mixing was used at plant production setting for the production of rubberized concrete paving blocks in this study. During the concrete mixing, two independent mixers were
Table 1 Mix composition and ratio of RCPB mixtures. Mix notation
RCPB-0 RCPB-10 RCPB-20 RCPB-30
Mix ratio
Cement content (kg/m3)
w/c ratio
Rubber content (%)
Face (C:FS)
Base (C:A:CS)
Face
Base
Face
Base
Face
Base
1:2.3 1:2.1 1:1.9 1:1.7
1:1.8:3.8 1:1.8:3.4 1:1.8:3.0 1:1.8:2.6
617 585 604 574
328 317 274 286
0.23 0.23 0.29 0.26
0.45 0.43 0.48 0.39
0 8.8 21.6 30.4
0 9.7 19.4 29.0
C-cement, FS-fine sand, CS-coarse sand.
Demoulded fresh density (kg/m3)
2170 2140 2100 2030
167
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
used with appropriate capacity and worked in parallel, to ensure face layer was added on the base layer for a better appearance. Initially, granite aggregate, coarse sand, cement and 1–5 mm crumb rubber were mixed in a base concrete mixer for approximately 1 min. After mixing for 1 min, the required amount of water (SP thoroughly pre-mixed) was added to the dry materials and mixed for another 1 min until the desired moisture content was obtained. Similar mixing process was also employed for face concrete mix. 3.3.2. Fabrication of plant-made RCPB After concrete mixing process was completed, both base and face concrete mixes were transferred from pan mixers to their feed hoppers, respectively. The amount in the feed hoppers was controlled by an automatic weighting system. The hopper discharged a correct amount of concrete into steel mould with individual internal dimensions of 210 mm length, 105 mm width and 60 mm depth. When the mould was filled by the base concrete mix, preliminary vibrations together with high compaction pressing were applied (see Table 2). After that, the base concrete mix was poured into the mould again for the second layer, and then intermediate compaction and vibration were applied for 2.2 second. Finally, face concrete mix was filled up the mould for another (final) compaction of 1.2 second was applied. Hydraulic ram was released after the compaction and the head was lifted to allow early stripping of RCPB from the steel moulds. Previously, because the mechanics of RCPB mixed with rubber particles had not been fully researched, considerable reliance was placed on experience (trial and error) to select an optimum pressing time and w/c for the production of RCPB products at plant setting. The optimum time of vibration at 5.2 second was determined in the plant. But good compaction was more difficult to achieve in RCPB when the concrete mix contained 20% or 30% of crumb rubber. Therefore, trail and error approach was taken to adjust the w/c ratio that can optimise the use of 20% and 30% of soft rubber particles in the production of RCPB. After various tries, an ‘optimum parameter’ was adopted (see Tables 1 and 2). 3.3.3. Monitoring of fresh plant-made RCPB After demoulding, pallets were removed from the lowerator and all the RCPB were stood individually and were separated from one another by thickness of the mould walls. Fig. 2 shows 54 fresh RCPB specimens on a pallet that was produced by one compactions pressing. All the RCPB produced were visually checked and their height was monitored by height control device. Any significant change in the physical appearance indicates something awry in the parameters setting during the production. Fig. 3 indicates some of the ‘‘expanded’’ RCPB that were being rejected as the pallet passed through the height control device. Besides, the appearance and colour variations of RCPB were also checked by manual sampling of five samples from the pallets (see Fig. 4). Weight and dimensions of the sampling samples were determined as shown in Fig. 5. The thickness of the face layer was also measured in accordance to the requirement of MA 20 [23]. Table 3 shows the results of physical properties of fresh plantmade RCPB. Because of low specific gravity of rubber particles,
Table 2 Vibration time and speed was adopted during the production of RCPB. Sequence of vibration applied
Time (s)
Speed (Hz)
Preliminary Intermediate Final (for face) Total Vibration Time (s)
1.8 2.2 1.2 5.2
60 58 60 –
Fig. 2. Height checking.
Fig. 3. Some of the rejected RCPB.
1
2
3 4
5
Fig. 4. RCPB sampling.
weight of fresh plant-made RCPB decreases with the increase in the percentage of rubber content. However, the decrease in weight of rubber was found to be less important when rubber content was at 10% of the total sand volume. Based on the visual observations of the fresh plant-made specimens, no honeycombs, cracks, and outstanding deformation were found on RCPB containing 0% and 10% of crumb rubber. For RCPB containing 20% or 30% of crumb rubber, some cracks and delamination between the face and base layers were clearly observed. This may be attributed to the increase in degree of compressibility by
168
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
increasing the rubber content (during compaction). Therefore, once the RCPB were released from the compaction force (after demoulding), which in turn changed the volume and dimensions (expansion takes place due to the stress released from rubber particles) of the RCPB specimens. The deformability mechanism of the presence of high elastically deformable crumb rubber during the compaction and moulding is illustrated by Fig. 6. Fig. 7 shows a typical pattern of cracks of varying orientations and dimension change of fresh RCPB-30 specimens. Another possible reason for the cracks development may be due to the increase in shear plane between the mould wall and all sides of fresh RCPB specimens. This is because the sand–rubber mixture could created higher shear resistance compared to sand alone at the same compaction [24]. Aslantas [25] also reported similar problem for concrete block produced by a combined vibration and pressure method. He stated that high water content in the mix had caused the surface problems and instability of concrete at the demoulding stage. Also, care should be taken with the use of high content of rubber in the concrete mix because it can gradually affected the concrete slump [26].
Fig. 5. Dimensions measurement.
Table 3 Physical properties of fresh plant-made RCPB. Mix notation
Rubber content (%)
Total depth (mm)
Thickness of facing layer (mm)
Weight (kg)
Visual observations Very good, no cracking Good
RCPB-0
0
59.6
5.5
2.82
RCPB10 RCPB20 RCPB30
10
59.8
5.0
2.82
20
59.2
5.0
2.74
30
59.8
3.3
2.68
3.3.4. Curing All the plant-made RCPB samples were cured under an elevated curing temperature at temperature of 25 ± 3 °C with humidity of 95 ± 3% for the first day. After 1 day, the 1-day hardened plantmade RCPB samples on pallets were then removed from roller-conveyors mounted on the outlet side of the press, and placed on to an elevator. The samples were then collected and further cured at room temperature of 32 ± 3 °C with 65 ± 5% relative humidity until the testing on the 28th day. The effects of different curing conditions on the properties of plant-made RCPB can be found in [20].
Some cracking Some cracking, delamination
When compaction force is applied
When compaction force is released
Mould wall
H1 < H2
RCBP
RCBP Steel plate
B1
<
B2
Fig. 6. RCPB during (a) compaction and (b) demoulding.
Fig. 7. Cracks and dimensions change of RCPB-30 (a) side view (b) plan view.
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
169
3.3.5. Investigation of 1-day hardened plant-made RCPB 3.3.5.1. Physical appearance. The visual properties and physical attribute of 1-day plant-made RCPB samples were checked in accordance to the requirement of the quality specified by MA 20 [23]. Figs. 8 and 9 show some of the finished products that rejected due to some faults present. For each mix design, more than 1000 RCPB specimens were produced. The total number of samples produced and rejected is given in Table 4. The rejection rate increased with an increase in the rubber content which mainly due to the decrease of its feasibility during the production. The rejection samples of RCPB-20 and RCPB-30 shown in Table 4 were included in the redundant samples sourced from those failed trial and error mixes.
Fig. 8. Checking and selecting of rejected RCPB.
3.3.5.2. Surface colour. Fig. 10 shows surface colours of plant-made RCPB containing different content of crumb rubber. The RCPB mixed with 20% and 30% of crumb rubber were slightly darker than those controlled RCPB (0%) and RCPB mixed with 10% of crumb rubber. This slight colouration would not cause significant problem when it was applied in pedestrian area or low traffic volume pavement.
3.4. Manufacturing process of manually-made RCPB
Fig. 9. Rejected RCPB.
Table 4 Number of total samples produced and rejected for each mix design RCPB. Mix notation
Total production sample
Rejected sample
% of rejection rate
RCPB-0 RCPB-10 RCPB-20 RCPB-30
1116 1012 1126 1046
60 100 446 648
5.4 9.9 39.6 62.0
RCPB-0
RCPB-10
The production of manually-made RCPB has been studied in a side experiment. Only base concrete mix collected from wood pallet at the end of each batch plant production was used to prepare manually-made RCPB. The compaction method used to produce manually-made RCPB was a modified method from ASTM D 698 [27] (see Fig. 11). This manually-made method was proven to achieve minimum target strength of 30 MPa [17], which stratified for the application in trafficked area less than 3 tonnes gross weight. The procedure of the modified manual-compaction method is described below. Manual compactions were applied by using a hammer at each layer of concrete mixture after the steel moulds were filled in two layers of about equal depth with internal dimensions of 200 100 60 mm. To ensure the mixture was well compacted at each layer; 50 drops (five drops in horizontal direction and ten drops in vertical direction) of 1.86 kg square hammer (25 mm 25 mm) were uniformly applied from a height of up to 15 cm onto the mixture directly. After the second layer was compacted, a final mix will fill up the empty space within the mould. Excessive materials were removed from the blocks surface and the surface was then flattened with a trowel. The manually-made RCPB were removed from the steel moulds 1 day after casting
RCPB-20
RCPB-30
Fig. 10. Surface colours of four different plant-made RCPB specimens.
170
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
H1 H2
H3 25mm
25mm
Hammer weight (N) Hammer size Drop height (cm) Total number of drops Number of drop per layer Compaction rate (drops/min)
ASTM D 698 [19] 24 50 mm in diameter 30
Modified method 18 25×25 mm in rectangular 15
75
100
25
50
25
50
V1 – V10
V7 V6 V5
H1 - H5
Test method
Rammer pattern for compaction in 200×100×60 mm steel mould Fig. 11. Summary of modified hand-ramming compaction method for manually-made RCPB.
and cured in air at an average room temperature of 30 ± 3 °C with 65 ± 5% relative humidity until the 28 days of testing. 3.5. Mechanical properties test methods The compressive strength was determined in accordance with MA 20 [23]. Prior to the testing, RCPB were soft capped with two pieces of 4 mm thick plywood to ensure a flat surface during loading. A modified British Standard (BS 6073-1) [28] method was used for the three-point bending strength test. Load was applied to a central line of RCPB while being simply supported over a span of 150 mm until rupture occurred. Deflection and energy absorption were automatically recorded in the data acquisition system and the modulus of rupture (MOR) was then calculated. The MOR is giLF ven according to equation: r ¼ 32 BD 2 and expressed in MPa. Where, L is the span length (mm), F the maximum applied load (N), B the average width of the sample (mm), and D its average thickness (mm). The dry density of RCPB was determined according to BS 6073-2 [29]. The density of the RCPB is simply the mass of air dried sample divided by its volume, expressed in kg/m3. The skid resistance was determined in accordance with ASTM E 303 [30]. Four swings were made and an average result was calculated for each specimen. All the testing at hardened stage was conducted at 28 days after casting. An average result of three samples for manually-made RCPB and five samples for plant-made RCPB were reported. 4. Results and discussion on the 28-day hardened properties 4.1. Compressive strength Fig. 12 shows the effects of percentage of crumb rubber replacing fine aggregate on the compressive strength of RCPB. In general, the compressive strength results obtained in this study for both plant-made and manually-made methods are in agreement with previous studies, in which the compressive strength of RCPB decreased as the percentage of crumb rubber increased [16–19]. A study by Sukonrasukkul and Chaikaew [16] reported that the inclusion of 20% of crumb rubber with fineness modulus of 4.98, 3.77 and 2.62 into concrete paving blocks decreased the compressive
strength as much as 84.4%, 78.1%, 85.0%, respectively. In this study, when crumb rubber with a fineness modulus of 4.74 was used as 20% of sand replacement decreased the compressive strength of 50.0% and 52.6% for plant-made and manually-made RCPB, respectively. As for a study by Ling et al. [17], 20% of crumb rubber incorporated in concrete paving blocks with water-to-cement ratio of 0.45, 0.50 and 0.55 caused a decrease of 33.6%, 18.3% and 38.0% in compressive strength, respectively. It is worth to note that the compressive strength of plant-made RCPB increased, as the proportion of crumb rubber in concrete was 10%. This point of inflexion can be explained by the mechanism behaviour shown in Fig. 13. As a small proportion (10%) of crumb rubber was included in the mixture, the soft rubber particles easily distorted and filled the voids between the solid particles (natural aggregates) under a compression force of plant-made machine. This filling mechanism was found to reduce the porosity by filling up the free pore volume in the concrete mixtures (see Fig. 13b). Furthermore, under this circumstance, the rubber particles had bonded well with cement matrix (see Fig. 14a) which in turn resulted in a better compressive strength of RCPB. A perfect adherence between rubber and cement matrix has also been observed in other work [31]. However, it is impossible to achieve ‘‘zero’’ porosity by increasing the rubber particles in the mixtures. A larger porosity was observed with an increasing rubber ratio higher than 10%, because at this point the deformability is more predominant than the filling mechanism. This indicates that the deformability of RCPB increased significantly after an optimum close packing is reached. Also, as the crumb rubber content increases beyond this limit, it increased the total stress concentrations in the concrete mixture and therefore rebound stress of rubber particles occurred. This, in turn, results in micro-cracks between the interfaces in concrete matrix and considered to loss the strength significantly. Fig. 14b shows the loss of adhesion between the rubber particles and the surrounding cement paste. As can be seen in Fig. 12, the compressive strength of RCPB greatly depends on the compaction methods used. Regardless of crumb rubber content, the compressive strength of plant-made RCPB was higher than the corresponding manually-made RCPB. This finding can be explicated through the packing behaviour of the particles in the RCPB whereby the higher level of compaction
171
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
60
Compressive strength (MPa)
plant-made manually-made Ling et al. (w/c=0.45)
50
Ling et al. (w/c=0.50) Ling et al. (w/c=0.55) Sukontasukkul and Chaikaew (FM=4.98)
40
Sukontasukkul and Chaikaew (FM=2.62) Sukontasukkul and Chaikaew (FM=3.77)
30
20
10
0 0
10
20
30
Rubber content by total sand volume (%) Fig. 12. Effects of rubber content on the compressive strength.
Fig. 13. Mechanism behaviour between soft rubber and solid particles (a) under compaction force and (b) once released from the force.
Fig. 14. Observation of undisturbed facture surface resulting from compression test (a) RCPB-10 and (b) RCPB-30.
was; the higher the packing and better strength was achieved. For instance, in the case of RCPB-0 without the influence of crumb rubber particles, when the concrete mixture only consists of rigid/solid particles, applying higher compaction force (by plant-made machine) is able to enhance the particles distribution and ensure a better packing density of the system per unit volume. Note that the decrease of the porosity in the mixtures, which composed of only rigid particles, was mainly attributed to the particles movement and rearrangement with the applied stress because no changes to the shapes and size are expected in solid particles.
The relative strength, Cp/Cm (Cp and Cm being the compressive strengths of plant-made and manually-made RCPB, respectively) was used to compare the relative differences between the strength obtained from two compaction methods. It was noted in Table 5 that the relative compressive strength ranged from 1.92 to 3.48 depending on the percentage use of crumb rubber. Increased in rubber content reduced the value of relative compressive strength. For RCPB-0 without crumb rubber, the use of plant-made method enhanced the compressive strength by 2.55 times as compared to corresponding mix manufactured through manually-made
172
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
Table 5 Relative properties of plant-made and manually-made RCPB mixes. Mix notation
RCPB-0 RCPB-10 RCPB-20 RCPB-30
Compressive strength (MPa) Plantmade
Manuallymade
31.1 42.5 15.6 11.7
12.2 12.2 5.8 6.1
Hardened density (kg/m3)
Relative compressive strength
2.55 3.48 2.69 1.92
Plantmade
Manuallymade
2063 2138 1994 1918
1917 1931 1836 1740
Relative density
Flexural strength (MPa)
1.08 1.11 1.09 1.10
Plantmade
Manuallymade
4.59 5.24 3.30 2.54
2.75 2.67 0.84 0.90
Relative flexural strength
1.67 1.96 3.93 2.82
Fig. 15. SEM image of (a) plant-made and (b) manually-made RCPB-0.
method. From observations under microstructures images, it can be seen that the microstructure of concrete mix matrix was improved. In fact, the concrete matrix shown in Fig. 15a, enriched by the finest sand particles, might firmly embed the pore structure in the matrix, although there was no additional filler in the admixtures. For manually-made method, Fig. 15b shows a lose interface between the particles in the admixture. 4.2. Relationship between compressive strength and hardened density As can be seen in Table 5, the density of manually-made RCPB ranged from 1740 to 1930 kg/m3, whereas the density of plantmade RCPB ranged from 1917 to 2138 kg/m3. This indicated that there was great enhancement in density of RCPB made through
Plant-made
plant-made method. The increase in density was about 9.3% in average. But the incorporation of the crumb rubber decreased the density of both plant-made and manually-made RCPB. As expected, the decrease in density resulted in decreasing compressive strength of RCPB. Fig. 16 shows a statistical relation between 28-day compressive strength and density of both plantmade and manually-made RCPB. In this case, compressive strength was treated as the dependent parameter, whereas density was considered as the independent variable. Higher R-squared values indicated that the proposed method explains the relationship significantly. Therefore, it is suggested that if a higher density can be achieved for a RCPB, it will in turn increase in compressive strength, regardless of rubber content and compaction methods.
Manually-made
Compressive strength (MPa)
60
50
y = 0.0001e6.119x 40
R2 = 0.98
30
y = 0.007e3.9147x 20
R2 = 0.86
10
0 1700
1800
1900
2000
2100
Density (kg/m3) Fig. 16. Relationship between compressive strength and density.
2200
173
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
4.3. Three-point bending strength There is no doubt that if RCPB are used for pavement application, the RCPB are more liable to break under traffic (fail in bending) than being crushed (fail under compression). Hence, it is necessary to evaluate the bending conditions of the plant-made and manually-made RCPB produced. Fig. 17 illustrates the effect of compaction methods on modulus of rupture (MOR) in varying crumb rubber content. A similar trend of MOR as in compressive strength was observed. It was, however, the MOR results of manually-made RCPB samples that were very discouraging, particularly at higher percentage of rubber content. For example, the MOR of manually-made RCPB-20 was only a quarter (25%) of the corresponding plant-made RCPB-20. This could be due to the combined reasons of low water-to-cement ratio, low compaction force of manually-made method as well as the presence of high content of soft rubber particles in the concrete mixtures. Therefore, to prevent the low MOR, avoid insufficient water, insufficient compaction, and too much soft rubber particles from occurring simultaneously during the production of RCPB. Fig. 18 shows the typical load–deflection curve of (a) plantmade and (b) manually-made RCPB with varying percentage of crumb rubber. Maximum deflections were observed for RCPB incorporating 30% of crumb rubber for both plant-made and manually-made method. This showed that the rubber particles
Plant-made
increased the deformability property of RCPB. It was observed that when RCPB-20 and RCPB-30 achieved its maximum load, they were not completely fractured, but withstand post-failure loads and then fail gradually (ductile behaviour). The toughness is known as energy absorption capacity and is generally calculated from the area under load–deflection curve up to where the point failure is plotted. For a given rubber content, even though the deflection value of plant-made RCPB was lower, but the toughness and fracture energy was found to be larger than the manually-made RCPB. This is due to the higher strength response. Hence, the plant-made RCPB was able to absorb larger quantities of energy after the peak load and prior to the final failure than manually-made RCPB prepared with the same percentage of crumb rubber. Ling et al. [21] reported that concrete paving blocks containing crumb rubber performed better impact resistance than a plain concrete paving block. They found that the plain concrete block was broken completely after the 3rd drops of falling weight; whereas the block with 30% crumb rubber only suffered small cracking after the 9th drops and maintained the integrity of the block structure. 4.4. Skid resistance Skid resistance was measured in accordance to ASTM E 303 [30]; four swings of the pendulum were made for each individual surface of RCPB. Prior to the test, the concrete block surface was
Manually-made
Modulus of rupture (MPa)
6
5
4
3
2
1
0 RCPB-0
RCPB-10
RCPB-20
RCPB-30
Mix notation Fig. 17. Modulus of rupture of plant-made and manually-made RCPB.
RCPB-0
RCPB-10
RCPB-0
RCPB-10
RCPB-20 RCPB-30
RCPB-20
Fig. 18. Three-point bending strength responds of (a) plant-made and (b) manually-made RCPB.
RCPB-30
174
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175
Plant-made
Manually-made
Brithish pendulum number (BPN)
90 80 70 60 50 40 30 20 10 0 RCPB-0
RCPB-10
RCPB-20
RCPB-30
Mix notation Fig. 19. Skid resistance of plant-made and manually-made RCPB.
thoroughly cleaned and dried. The effect of crumb rubber content of both plant-made and manually-made RCPB is shown in Fig. 19. In general, all RCPB produced in this study satisfied ASTM requirement. As seen in the figure, it showed that increasing percentages of crumb rubber in RCPB decreases the skid resistance. It was found that the highest skid resistance of 82.6 BPN and 78.0 BPN were achieved for plant-made and manually-made RCPB without crumb rubber. The RCPB without rubber particles on surface layer might create more friction as the pendulum passed across. Ling et al. [17] also showed that the inclusion of crumb rubber in concrete paving blocks lowered the skid resistance. However, results reported by [16] indicate the opposite trend. For a given rubber content, plant-made RCPB showed slightly higher values than those corresponding manually-made RCPB with an average increment of 10% in BPN value. The improvement was more pronounced at a higher level of rubber content (30%) over the lower level of rubber content (10%), which accounted for a 15% and 5.6% gain in BPN, respectively. 5. Conclusions and recommendations Based on the investigation in this study, the following conclusions can be drawn: It is not feasible to produce RCPB at commercial plant setting when the concrete incorporated more than 10% of soft rubber particle. Also, when the RCPB contain 20% or 30% of crumb rubber, delamination or cracks may occur on the fresh RCPB specimens due to the increase in degree of compressibility, which in turn causes a change in volume and dimensions when demoulding. The rejection rate of RCPB produced increase with an increase in the rubber content due to the decrease of its feasibility during the production. Besides, the surface appearance of RCPB mixed with 20% and 30% of crumb rubber were slightly darker than that of RCPB mixed with lower substitution ratios of crumb rubber. Inclusion of small proportion (10%) of crumb rubber in plantmade RCPB slightly improved the strengths, probably due to the filling mechanism of soft rubber particles. On the contrary, larger porosity was observed with an increasing rubber ratio more than 10% due to the deformability which is more predominant than filling mechanism and therefore resulted in a significant strength reduction.
There was a great enhancement in density when RCPB was produced by plant-made method. The increase in density was about 9.3% in average as compared to manually-made method. As expected, the incorporation of crumb rubber decreased the density of both plant-made and manually-made RCPB. It is suggested that if a higher density can be achieved would produce a higher compressive strength. In the practical viewpoint, plantmade method is recommended for future RCPB production. The use of rubber particles in RCPB increased the deformability and toughness property. It was observed that when rubber particles were incorporated, the RCPB were able to absorb larger quantities of energy after the peak load and prior to the final failure. Also, the presence of rubber in concrete did not demonstrate brittle failure, but rather a ductility which had the ability to withstand post-failure loads. Increasing percentages of crumb rubber in RCPB decreased the skid resistance. For a given rubber content, plant-made RCPB showed slightly higher values than those corresponding manually-made RCPB with an average increment of 10% in BPN value. However, there is no significant impact to apply both plantmade and manually-made RCPB for pavement application as both satisfied ASTM requirement.
Acknowledgements The authors express their sincere thanks to the Ministry of Science, Technology and Innovation (MOSTI), Malaysia under IRPA research Grant No. 03-02-06-0129-EA0001. Technical help provided by Sunway Paving Solution (M) Sdn. Bhd. is gratefully acknowledged. References [1] Poon CS, Chan D. Paving blocks made with recycled concrete aggregate and crushed clay brick. Constr Build Mater 2006;20(8):569–77. [2] Poon CS, Chan D. Effects of contaminants on the properties of concrete paving blocks prepared with recycled concrete aggregates. Constr Build Mater 2007;21(1):164–75. [3] Poon CS, Lam CS. The effect of aggregate-to-cement ratio and types of aggregates on the properties of pre-cast concrete blocks. Cem Concr Compos 2008;30(4):283–9. [4] Poon CS, Kou SC, Lam L. Use of recycled aggregates in molded concrete bricks and blocks. Constr Build Mater 2002;16(5):281–9. [5] Poon CS, Kou SC, Wan HW, Etxeberria M. Properties of concrete blocks prepared with low grade recycled aggregates. Waste Manage 2009;29(8):2369–77.
T.-C. Ling / Construction and Building Materials 28 (2012) 164–175 [6] Zhao X, Ling TC, Kou SC, Wang QY, Poon CS. Use of wastes derived from earthquakes for the production of concrete masonry partition wall blocks. Waste Manage 2011;31(8):1859–66. [7] Meyer C, Egosi N, Andela C. Concrete with waste glass as aggregate. In: Recycling and re-use of glass cullet, proceeding of the international symposium concrete technology unit of ASCE, University of Dundee; 2001. [8] Byars EA, Zhu HY, Morales B. CONGLASSCRETE I. Final report. The waste & resources action programme, UK; 2004. [9] Turgut P. Properties of masonry blocks produced with waste limestone sawdust and glass powder. Constr Build Mater 2008;22(7):1422–7. [10] Turgut P, Yahlizade ES. Research into concrete blocks with waste glass. Int J Environ Sci Eng 2009;1(4):202–8. [11] Lam CS, Poon CS, Chan D. Enhancing the performance of pre-cast concrete blocks by incorporating waste glass – ASR consideration. Cem Concr Compos 2007;29(8):616–25. [12] Naik TR, Kraus RN, Chun YM, Ramme BW, Singh SS. Properties of field manufactured cast-concrete products utilizing recycled materials. ASCE J Mater Civil Eng 2003;15(4):400–7. [13] Holt E, Raivio P. Use of gasification residues in compacted concrete paving blocks. Cem Concr Res 2005;36(3):441–8. [14] Karasawa A, Suda S, Naito H, Fujiwaran N. Application of fly ash to concrete paving block. In: Proceedings of the 7th international conference on concrete block paving, Sun City, South Africa; 2003. p. 343–52. [15] Freidin C. Cementless pressed bocks from waste products of coal-firing power station. Constr Build Mater 2007;21(1):12–8. [16] Sukonrasukkul P, Chaikaew C. Properties of concrete pedestrian block mixed with crumb rubber. Constr Build Mater 2006;20(7):450–7. [17] Ling TC, Nor HM, Hainin MR. Properties of concrete paving blocks incorporating crumb rubber and SBR latex. Road Mater Pavement 2009;10(1):213–22.
175
[18] Ling TC. Prediction of density and compressive strength for rubberized concrete blocks. Constr Build Mater 2011;25(11):4303–6. [19] Ling TC, Nor HM, Lim SK. Using recycled tyre in concrete paving blocks. ICEWaste Resour Manage 2010;163(1):37–45. [20] Ling TC, Nor HM, Hainin MR, Lim SK. Long term strength of rubberized concrete paving blocks. ICE-Constr Mater 2010;163(1):19–26. [21] Ling TC, Nor HM, Hainin MR, Chik AA. Laboratory performance of crumb rubber concrete block pavement. Int J Pavement Eng 2009;10(5):361–74. [22] BS 12. Specification for Portland cement. British Standards Institution; 2001. [23] MA20. Specification for concrete segmental paving units. Concrete Masonry Association of Australia, 1996. [24] Ghazavi M. Shear strength characteristics of sand-mixed with granular rubber. Geotech Geol Eng 2004;22(3):401–16. [25] Aslantas O. A study on abrasion resistance of concrete paving blocks. Master thesis. Middle East Technical University; 2004. [26] Khatib ZK, Bayomy FM. Rubberized Portland cement concrete. ASCE J Mater Civil Eng 1999;11(3):206–13. [27] ASTM D 698. Standard test methods for laboratory compaction characteristics of soil using standard effort (12 400 ft-lbf/ft3 (600 kN-m/m3)). American society of testing and materials; 1997. [28] BS 6073-1. Precast concrete masonry units – Part 1: Specification for precast concrete masonry units. British Standards Institution; 1891. [29] BS 6073-2. Precast concrete masonry units – Part 2: Guide for specifying precast concrete masonry units. British Standards Institution; 2008. [30] ASTM E 303. Standard test method for measured surface frictional properties using British pendulum tester. American society of testing and materials; 1993. [31] Herna´ndez-Olivaresa F, Barluengaa G, Bollatib M, Witoszek B. Static and dynamic behaviour of recycled tyre rubber-filled concrete. Cem Concr Res 2002;32(10):1587–96.