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Mechanical and durability properties of concretes containing recycled lime powder and recycled aggregates
Construction and Building Materials 53 (2014) 253–259
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Mechanical and durability properties of concretes containing recycled lime powder and recycled aggregates Antonios Kanellopoulos a,⇑, Demetrios Nicolaides b, Michael F. Petrou c a
University of Cambridge, Department of Engineering, Trumpington Street, CB2 1PZ Cambridge, UK Frederick University Cyprus, Department of Civil Engineering, 7 Y. Frederickou Street, 1036 Nicosia, Cyprus c University of Cyprus, Department of Civil and Environmental Engineering, PO BOX 20537, 1678 Nicosia, Cyprus b
h i g h l i g h t s Recycled lime powder (RLP) does not seem to have any pozzolanic properties. RLP shows very good potential as a partial cement replacement material. Replacement of natural aggregates affects marginally the mechanical properties. We found good correlation exists between sorptivity and open porosity.
a r t i c l e
i n f o
Article history: Received 3 October 2013 Received in revised form 25 November 2013 Accepted 26 November 2013 Available online 22 December 2013 Keywords: Concrete Recycled waste Recycled fillers Recycled aggregates mechanical properties Durability
a b s t r a c t According to a recent report by the European Commission, within the European Union, the construction and demolition wastes come to at least 450 million tons per year. Roughly 75% of the waste is disposed to landfill, despite its major recycling potential. The bulk constituents of demolition debris are concrete (50–55%) and masonry (30–40%) with only small percentages of other materials such as metals, glass and timber. In Cyprus, at present, recycling of waste materials is practically inexistent and almost the entire demolition waste products are disposed in landfill sites, with all possible economic, technical and environmental impacts. This research paper presents the evaluation and the effective reuse of waste construction materials, such as recycled lime powder (RLP) and recycled concrete aggregates (RCA), disposed to landfill sites in Cyprus, due to the lack of a lucid recycling policy and knowledge. Results show that both RLP and RCA have the potential to produce good quality and robust concrete mixtures both in terms of mechanical and durability performance. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction The conservation of earth’s resources and sustaining the natural habitat has become the most crucial and serious problem for the continued existence and wellbeing of humankind. The natural sources of energy and materials have been consumed in great quantities by the rapid industrialisation and population growth over a span of more than two hundred years since the industrial revolution, resulting in global environment changes that humankind have never experienced. The problem we are facing today is that approximately 20% of the world’s population, mostly living in Western Europe, North America and Japan, in order to maintain its standard of living consumes almost 80% of the world’s resources and energy [1]. Meanwhile, the remaining 80% of global population has rapidly improved the rate of industrialisation in pursuit of a better life. One can easily understand that the result of this ⇑ Corresponding author. Tel.: +44 1223 766683. E-mail address: [email protected] (A. Kanellopoulos).
perpetual race for urbanisation encourages the continuation of consumption of natural resources at high rates. There are simply neither enough natural resources nor energy to continue our technological evolution as usual. It is imperative to find solutions that will assist to sustain the environment, without being an obstacle in the developmental race that meets the needs of not only the present but also future generations. The need for new infrastructure to support a minimum standard of life will increase as human population will continue growing, putting enormous demand on natural resources and the supply of construction materials. It is widely accepted that concrete is the most versatile construction material in the world. It can be fabricated in all sorts of conceivable geometries, it has excellent mechanical and durability properties, its availability is widespread since its constituent materials are most readily available anywhere in the world; and more importantly its acquisition is relatively affordable especially compared to steel. This popularity of concrete is being translated into an annual worldwide production in the range of 12 billion tons, which accounts for about 1.7 billion tons of concrete per year per
0950-0618/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.11.102
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person living on this planet [2,3]. However, this popularity comes at price: concrete production has an enormous impact on the environment, a fact which at the past has been significantly overlooked. First of all the production of concrete requires vast amounts of natural resources each year. Then, the production of each ton of Portland cement releases almost one ton of carbon dioxide into the atmosphere. It is estimated that the cement industry alone is responsible for about 7% of worldwide CO2 emissions, not to mention that the cement production is also very energy intensive. Third, the production of concrete requires large amounts of water, that fulfils certain standards, and water availability is a matter of concern these days. Lastly, the demolition and disposal of concrete structures creates another major environmental burden. The building sector has a huge environmental impact and the EU job market depends heavily on the construction sector. Buildings and construction consume a lot of natural resources and generate a lot of waste. The sector uses more than 50% of all materials extracted from the earth and generates more than 450 million tonnes/year of waste in the EU according to a recent announcement by the European Commission [4]. Managing and disposing of old buildings is also problematic [5]. Within the European Union, the construction and demolition wastes come to at least 450 million tons per year. Roughly 75% of the waste is disposed to landfill, despite its major recycling potential. Some Member States (in particular Denmark, The Netherlands and Belgium) investigated thoroughly the technical and economic feasibility of recycling, achieving recycling rates of more than 80%. On the other hand, the South European countries recycle very little of their construction wastes [6]. The bulk constituents of demolition debris are concrete (50–55%) and masonry (30–40%) with only small percentages of other materials such as metals, glass and timber [7]. In the United States of America of the approximately 2.7 billion metric tons of aggregates used, about 70% are used in structural concrete, whereas about 30% is being utilised in pavements and road works [8,9]. Although, incentives for use of processed aggregates are given to promote use of RCA, though a large part of the production is suitable only as fill or construction base [8,9]. Several researchers studied the use of RCA to partially or globally replace natural aggregates in the production of concrete [10,11]. Density and water absorption ratio are the properties having the greatest differences in comparison with natural aggregates. These differences are mainly attributed to the lower density of the adhered mortar in the recycled aggregate, as reported by many authors [12,13] and have a negative impact on the concrete mixes. However, there are a few studies that prove that concrete made with coarse aggregates deriving from concrete recycling have mechanical properties similar to those of conventional concretes [14]. On the other hand, there is scepticism in the use of the fine fraction of these recycled aggregates. Not many studies have been conducted using fine fractions due to the belief that their greater water absorption can jeopardize the final results, particularly for replacement ratios exceeding 30% [15,16]. Moreover, recycled fine aggregates contain a larger amount of adhered mortar, which results in difficulties in procuring the required slumps, as well as a substantial increase in deformation, and sharp drops in the modulus of elasticity and strengths. Some researchers examined the effect of the addition of silica fume on the basic properties of recycled concrete (RC) [14]. However, very few projects have been able to provide conclusive evidence regarding the effect of mineral admixtures on the properties of recycled concrete. Finally, although a significant amount of work has been conducted regarding the mechanical behaviour of concretes made with RCA, very limited work can be found in the literature about their durability aspects [17–19]. The replacement of natural aggregates with recycled coarse aggregates increases dramatically the water demand.
Therefore, dispersing agents have been used by several researchers. Locally the recycling of construction wastes is a critical issue, considering that at the present time recycling of waste materials is practically inexistent and the fact that landfill sites are becoming increasingly difficult to come by, due to the small size of the island. This deficiency is getting sharper considering the confinement of the island due to the existing political issue. At the same time, resource supply or feed material can be guaranteed in Cyprus, where replacement of infrastructure is occurring, natural aggregate resources are very limited, and environmental regulations encourage recycling. This research work was considered imperative at national level, as it was aiming to enhance the stateof-the-art knowledge concerning the reuse of waste materials in construction industry and also boost the recycling process in Cyprus with all possible social, economic, technical and environmental benefits. The specific area lacks extensive international experimental data and is of primary interest in the case of Cyprus (arid climate, large CO2 emissions in cities, and existence of reactive siliceous aggregates). Furthermore, the effect of the addition of a waste lime filler, deriving from the aggregates used in the production of asphalt concrete, on the properties of concrete was investigated. 2. Materials and mixtures The experimental work was carried out in two stages. The first stage consisted of the evaluation as cement replacement material of a lime powder that comes as a by-product from the production of asphalt concrete. In the second stage of the work the production of robust concrete mixtures using two gradings of recycled aggregates, sourced locally, was investigated. A large number of mixtures were produced and tests were performed to determine their fresh, mechanical and durability properties. 2.1. Materials 2.1.1. Cement To investigate the potential of recycled lime powder as cement replacement material, ordinary Portland cement CEM-I 32.5 was used. For the second part of the work, standard Portland pozzolan cement with a strength category of 42.5 N/ mm2 with sulphate resistance properties and low heat of hydration was used. All cements used in this study conform to the requirements of EN 197-1:2000 [20]. 2.1.2. Natural aggregates Graded crushed calcareous coarse aggregates in two different particle size gradings (8–20 mm and 4–10 mm) and two different gradings of calcareous sand (0– 4 mm and 0–2 mm) were used. All natural aggregates were supplied by a local quarry. 2.1.3. Recycled concrete aggregates The recycled aggregates were obtained from concrete rubbles available in Cyprus, using mobile impact crushers. It has to be mentioned that these rubbles represent a best-case scenario since they produced from leftover reinforced concrete laboratory specimens. Where necessary, material was refined in laboratory from a dry mill in order to allow the maximum grain sizes required for concrete mixes. Two fractions of recycled coarse aggregates were used in this work, namely 4– 10 mm and 8–20 mm. The remaining fractions (<4 mm and > 20 mm) were rejected, by applying sieving operation. Note that for the production of all concrete mixtures the same particle size ranges for both natural and recycled aggregates were used. Both types of aggregates were tested regarding their grading, flakiness index, abrasion resistance, density and water absorption [21–24]. Table 1 summarises the properties of both natural and recycled aggregates used in this study. It is noticeable that the abrasion resistance of recycled aggregates, as this derived from the Los Angeles test, is comparable to the corresponding value for natural aggregates. This was attributed mainly to the poor quality of natural aggregates available on the island, which have water absorptions in the range of 2.7–4%. In addition, the water absorption of recycled materials was well higher than the corresponding values for natural aggregates, due to the existence of the hardened cement paste covering the surfaces of recycled materials. 2.1.4. Recycled lime powder The recycled lime powder (RLP) used in this study is a waste material of the asphalt concrete production process. Calcareous aggregates used in this process are being heated at 800 °C in special kilns, then the surface dust is blown away
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A. Kanellopoulos et al. / Construction and Building Materials 53 (2014) 253–259 Table 1 Properties of Recycled Concrete Aggregates (RCA) and Natural Aggregates (NA). Coarse aggregates
Moisture content (%) Flakiness index (%) Los Angeles (LA) (%) Apparent particle density (Mg/m3) Particle density Particle density in SSD WA 24 (%)
RCA 4/10
RCA 8/20
NA 4/10
NA 8/20
2.00–3.00 9.00 – 2.59 2.09 2.28 9.37
1.95–3.00 5.00 28.60 2.60 2.21 2.37 7.02
0.12–0.27 – – 2.71 2.42 2.53 4.50
0.06–0.28 3.00 29.00 2.71 2.44 2.54 4.00
and finally the aggregates are mixed with the bituminous material. This dust is nothing but limestone powder. In Cyprus alone, it is estimated, that 230,000 tonnes (for 2010) from this dust are being disposed in landfills. Pure, standardised, limestone powder (SLP) has been used widely in the past mainly for the production of self-compacting concrete mixtures [25]. Performing X-ray Diffraction Crystallography and particle size analysis, it was found the RLP has very similar characteristics to the standardised lime powders used locally [26]. Table 2 shows how RLP compares to SLP while Fig. 1 illustrates the crystallographic profiles of both powders. 2.2. Mixtures During the first stage of the experimental work four different cement replacement percentages were investigated, ranging from 5% to 20%. To study the potential of the RLP as cement replacement material, a control mortar mixture was developed incorporating cement, water, superplasticiser (SP) and fine sand with maximum grain size 2 mm. Keeping the water-to-cement ratio (w/c) constant, cement was replaced gradually by 5%, 8%, 10% and 20% by volume. To investigate the use of RCA, a control concrete mixture was designed containing only natural aggregates (coarse and fine). Subsequently eight mixes were prepared, replacing portions, by weight, of natural coarse aggregates, with the corresponding coarse fractions of RCA. Specifically, four different mixes replacing 10%, 30%, 50%, and 100% of natural coarse aggregates 8–20 mm and three different mixes replacing 30%, 50% and 100% of natural coarse aggregates 4–10 mm were prepared. Furthermore, one more mix replacing totally both fractions of natural coarse aggregates was also prepared. Water-to-cement ratio was kept constant in all mixtures and moreover the fine aggregates used were the same for both the control mixture and the mixtures containing the replacement of coarse aggregates. Table 3 shows the constituents for control mortar and concrete mixtures.
Workability of concrete mixtures was assessed by common slump test as described in EN12350-2 [28]. Water and SP contents remained unaltered, since the effect of RCA on the workability of mixtures was to be investigated.
3.3. Specimen preparation Six cubes (50 50 50 mm) were cast for each of the mortar mixtures as described in BS3892-1 [29]. All mortar cubes were carefully compacted using an electric motor vibrating table. The specimens were demoulded after 24 h from casting and placed at water tank (20 °C ± 2 °C) until the day of testing. For concrete specimens, mould filling was conducted according to the requirements specified in European Standards EN 12390-2:2009 and EN 13670:2009 [30,31]. Moulds were filled on an electric motor vibrating table in two layers, each being followed by adequate mechanical vibration (about 12 s each). In total, fifteen standard cube moulds (100 100 100 mm), three standard prism (cross section 100 100 mm, length L = 500 mm) and 9 standard cylindrical moulds (diameter D = 100, height H = 200 mm) have been cast and treated for each different mixture produced. Ten of the cube specimens were used for the determination of the compressive strength (fcu) at seven and twenty-eight days, while the remaining five used for the determination of open porosity and sorptivity at 28 days. The prism and cylindrical specimens were used to obtain the flexural strength (fcf) and chloride penetrability respectively at 28 days. The test specimens were left to set for 24 h after casting and then they were demoulded and received the same treatment. All concrete specimens were cured in a water tank (tap water), with a water temperature of 20 °C ± 2 °C, as described by EN 12390-2:2009, until the age of testing (7 and 28 days).
3.4. Hardened mortar testing 3. Experimental procedures 3.1. Mixture production For the production of the mixtures a 20 L capacity dough mixer has been used for the production of mortars and a 100 L capacity fixed-pan planetary type cylindrical mixer with rotating blades has been used for the production of concrete mixtures. The same mixing procedure has been carefully followed in all mixtures. At first, the aggregates have been dry-mixed followed by the cement to produce a homogeneous dry mixture. Then, 80% of the water has been added, followed by the rest of the water content in addition with the superplasticiser. 3.2. Fresh properties Mortars were examined for their flowability according to European standard EN1015-3, which determines the consistency of fresh mortars by flow table [27]. Since w/c was kept constant the SP quantity was adjusted, where needed, to achieve the same flow as the control mortar with an acceptable deviation of ±5 mm, as the standard dictates. Note that the flow table experimental procedure was followed to make sure that all mixtures have the same workability level as the control mortar mix. The effect of RLP on the workability of the mixtures is beyond of the scope of the present study.
Table 2 Physical properties of SLP and RLP. Property
SLP
RLP
CaCO3 Fineness <18 lm <2 lm Specific surface area Specific gravity
96%
95%
96% 7% 3659 cm2/g 2.8 g/cm3
97% 9% 3509 cm2/g 2.7 g/cm3
The purpose of the compressive strength tests in mortar cubes was to obtain the so called Strength Activity Index (SAI) as described in BS3892-1. SAI is the ratio of the average compressive strength of the mortar mixture under investigation to the average compressive strength of the control mortar mixture. Normally, SAI is being reported as a percentage. All mortar cubes were tested in a 250 kN compression test frame.
3.5. Hardened concrete testing 3.5.1. Mechanical properties From each concrete mixture, standard cubes (100 100 100 mm) were used to determine conventional compressive strength, according to EN 12390-3:2009. All cubic specimens were tested on a 5000 kN servo hydraulic compression frame that conforms to EN 12390-4:2009. In addition, the prismatic specimens (cross section 100 100 mm, length L = 500 mm) used for the determination of flexural strength, as described by EN 12390-5:2009, were tested in three-point bend configuration using an 100 kN servo hydraulic flexural frame.
3.5.2. Durability properties 3.5.2.1. Open porosity. The open porosity of each mixture has been tested on three standard cube specimens (100 100 100 mm) in accordance with ASTM C64297 [32]. After the treatment period, the cubes have been weighted both below water and in air before being oven dried at a temperature of 105 °C until constant mass, which usually lasted a period between 7 and 10 days. By combining the three measured masses, the open porosity has been evaluated.
3.5.2.2. Sorptivity. The sorptivity (liquid capillary absorption coefficient) is obtained from a simple short-term one-dimensional experiment [33]. During this test, liquid uptake is governed by capillary and viscous forces. In this case two cubic specimens (100 100 100 mm) were used. After oven drying at 105 °C, the specimens were allowed to absorb ethanol from a free liquid surface by capillary rise. During the capillary absorption of ethanol, the increase in weight of the specimens over time
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CaO
14000 13000 12000 11000 10000
Lin (Counts)
9000 8000 7000
CaO
6000 5000 4000
CaO
3000
CaO
CaO
2000
C2S
C2S
1000 0 5
10
20
30
40
50
60
2-Theta - Scale Fig. 1. Crystallographic profiles for SLP (black curve) and RLP (red curve). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 3 Constituents for control mortar and concrete mixtures (quantities are in kg/m3). Constituents
Control – mortar mix
Control – concrete mix
CEM-I 32.5 CEM-II 42.5R Water Coarse aggregates 8–20 mm 4–10 mm Sand 1 (crashed limestone) Sand 2 (fine limestone sand) Superplasticiser
400 – 200
– 400 200
– – – 1200 5.5
655 264 560 108 6.4
Table 4 Chloride ion penetrability based on charge passed (according to [35]). Charge passed Q (C)
Chloride ion penetrability
>4000 2000–4000 1000–2000 100–1000 <100
High Moderate Low Very low Negligible
(t) was noted. The sorptivity was determined by the slope of the cumulative absorbed volume of liquid per unit inflow surface versus t½ plot, as described in the literature [33–35].
3.5.2.3. Chloride penetrability. To assess the chloride penetrability of concrete mixtures presented in this study a very popular and well-known experimental procedure was adopted. The rapid chloride permeability test (RCP) is a standardised ASTM test that exists for 24 years [36]. The experiment assesses concrete’s ability to resist chloride intrusion. In fact it measures the electrical conductivity or resistance of a 50 mm thick concrete disc specimen, cut from the middle section of a standardised cylindrical specimen, for a period of 6 h. It is assumed that the resistivity is directly related to the pore network or concrete permeability, although some researchers claim that the relation is not perfect [37]. The resistivity is measured in terms of total electric charge (measured in Coulombs, C) passed through the specimens, and these measurements are used for the classification of mixtures as shown in Table 4. During the test, the specimens are positioned in a measuring cell with a fluid reservoir at each face of the specimen. For the ASTM/AASHTO test, one reservoir is filled with sodium chloride (3.0% NaCl) solution and the other with sodium hydroxide (0.3 M NaOH) solution. The reservoir containing the NaCl is
connected to the negative terminal and the NaOH reservoir to the positive terminal of the microprocessor power supply unit. The specimens used in the tests described in this paper were cured under the same conditions. The experiments took place at 28 days and the average of three specimens for each time interval was recorded.
4. Results and discussion 4.1. Mortars’ strength activity index Replacing cement with RLP resulted in robust mortar mixtures with good mechanical performance at both 7 and 28 days. At low replacement percentages the SAIs are as high as 90% at both testing days. Increasing cement replacement results in gradual reduction of the strength activity index. Table 5 summarises the SAIs and compressive strength results for mortars. As it can be seen from the above table at low cement replacements with RLP (5% and 8%) the reduction in strength does not exceed 18%. Replacing cement up to 20% results in significant reduction of compressive strength. These results indicate that at low replacement percentages RLP has the potential to substitute cement in concretes designated for structural applications. Although at larger substitution percentages the strength is being reduced significantly, RLP can be used as cement replacement in concretes for non-structural applications. In the light of the above discussion, in order to explore the potential of RLP in structural concrete it has been decided to produce two concrete mixtures, based on the mix design shown in Table 3, replacing cement with RLP by 5% and 8% by volume. 4.2. RLP concretes – mechanical properties Table 6 shows the values obtained for compressive and flexural strengths at two different time intervals for the control mixture and the mixtures containing cement replacement with RLP. Results justify what has been revealed by previous experiments on mortars and SAIs. Both concretes at 5% and 8% cement replacement attain relatively large values of both compressive and flexural strengths. It can be noted that the reduction in compressive strength for both percentage replacement levels at both testing dates is somewhat smaller than the corresponding reductions reported in
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A. Kanellopoulos et al. / Construction and Building Materials 53 (2014) 253–259 Table 5 Compressive strengths and SAIs for mortar mixtures. 7 days
Control Substitution 5% 8% 10% 20%
28 days
fcu (MPa)
SAI (%)
Reduction (%)
fcu (MPa)
SAI (%)
Reduction (%)
30.1
–
–
32.1
–
–
28.1 24.7 18.9 15.7
93.3 82.1 62.8 52.2
6.7 17.9 37.2 47.8
30.1 27.3 20.7 17.4
93.4 85.0 64.5 54.2
6.6 15.0 35.5 45.8
Table 6 Compressive and flexural strengths for control and RLP concrete mixtures. 7 days
Control RLP 5% RLP 8%
28 days
fcu
fcf
fcu reduction (%)
fcu
fcf
fcu reduction (%)
52.8 50.1 44.7
– – –
– 5.1 15.3
63.8 60.2 54.6
7.1 6.7 5.9
– 5.6 14.4
Table 7 Sorptivity, porosity and RCP values for control and RLP concrete mixtures. 28 days
Control RLP 5% RLP 8%
p Sorptivity (mm/ min)
Porosity (%)
RCP (Coulombs)
0.087 0.101 0.119
17.8 18.9 19.8
5181 5294 5400
Table 5, concerning the mortar mixtures. A possible explanation for this observation is that the presence of coarse aggregates in the concrete mixtures have increased the stiffness of the composite material resulting in specimens that sustain marginally larger compressive stresses than the ones made with mortars. Even the mixture containing 8% replacement of cement with RLP produces an almost 45 MPa concrete that conforms the criteria for a C32/ 40 concrete as specified in EN206-1 [38]. 4.3. RLP concretes – durability properties Cement replacement with RLP seems to reduce to some extent the durability performance of the produced concretes in a similar manner as it reduced their mechanical performance. Table 7 summarises the results, at 28 days, on sorptivity, open porosity and rapid chloride permeability for the control and the cement replaced mixtures. Substituting 5% of the cement with RLP results in: 14% increase in the sorptivity, about 6% increase of the open porosity and a 2% increase of the value of rapid chloride permeability. Further replacement of cement with RLP (8%) results in: an additional 13% increase of the sorptivity values, a total 10% increase of the open porosity and a total 4% increase in the rapid chloride permeability. It seems that the most affected durability indicator is the sorptivity which exhibits the larger variations with increasing cement replacement percentage. The above durability properties’ results show that the RLP to some extent fails to refine the pore network of the matrix and as result the produced mixtures are to a certain extent more susceptible to liquid absorption and chloride penetration. The explanation for this phenomenon is that RLP is not a standardised material and therefore contains impurities that affect its performance as filler. Albeit being a waste material that results in mixtures with somewhat reduced mechanical and durability performance, RLP shows a very good potential as a cement replacement material. Experimental results suggest that RLP can
be used as filler in concrete mixtures, reducing cement quantity without significantly affecting the structural performance of the produced material. 4.4. RCA concretes – fresh properties It is clearly observed a drop of slump, as the percentage of replacement is increased (Table 8). This is becoming more evident in the case of the coarser fraction (8–20 mm), whereas the drop of slump in the case of replacement of part of the 4–10 mm fraction is not that pronounced. The existence of high amount of hardened cement paste on the surface of the recycled aggregates is the main reason for the lower slump values, as the percentage of replacement is increased. Moreover, the lower slump values for the mixtures containing RCA can be also justified by the relatively large flakiness index numbers of the RCA compared to natural aggregates. As it is known, flaky and elongated particles can cause workability issues in concrete [39]. Nevertheless, it is important to notice that even for the total replacement of both fractions of natural aggregates (4–10 mm and 8–20 mm), the slump value is not detrimentally low. 4.5. RCA concretes – mechanical properties It is important to highlight the small impact of the natural aggregates replacement, on the compressive strength of concrete mixes. As it is indicated on Table 9, the replacement percentage does not have a serious effect on the average compressive strength of each mix. The highest difference is in the case of the total replacement of both fractions of natural aggregates with recycled, where the strength drop percentage is approximately 10%. However, even in this case the strength reduction is regarded small, especially considering the fact that natural aggregates have not been used. This could be attributed to several reasons. Most importantly, is regarded the additional cement content which is introduced into the mix through the cement paste which covers the recycled aggregates. In combination with the poor quality of the natural aggregates available in Cyprus, used in the commercial concrete mixes, are the profound reasons for the compressive strengths of mixtures containing recycled concrete aggregates to be very similar to the values obtained when natural aggregates are used. The flexural strength of beams containing recycled concrete aggregates reduces, as the replacement percentage increases (Table 9). The highest drop of flexural strength was observed when both fractions of natural aggregates were totally replaced with recycled. It is strongly believed that the main reason of this flexural strength drop is the poor quality of the interfacial bond developed between the old cement paste covering the recycled aggregates and the new cement matrix. This has been verified recently both by nanoindentation experiments and numerical modelling of the interfacial transition zone in mortars containing recycled aggregates [40].
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Table 8 Slump variation with aggregate replacement. Fraction of natural aggregates which is replaced by RCA
Percentage of natural aggregates replacement
Slump (mm)
Control mix 8/20
0% Replacement 10% Replacement 30% Replacement 50% Replacement 100% Replacement 30% Replacement 50% Replacement 100% Replacement 100% Replacement
200 195 168 168 134 198 193 170 188
4/10
8/20–4/10
Table 9 Average compressive and flexural strength of concrete mixes containing recycled concrete aggregates. Fraction of natural aggregates which is replaced by RCA
Control mix 8/20
4/10
8/20–4/10
Percentage of natural aggregates replacement
0% Replacement 10% Replacement 30% Replacement 50% Replacement 100% Replacement 30% Replacement 50% Replacement 100% Replacement 100% Replacement
7 days
28 days
fcu (MPa)
fcu (MPa)
fcf (MPa)
52.2 49.5 49.5 49.9 52.6 50.6 50.8 50.0 47.0
63.8 61.7 60.7 60.2 62.7 58.8 60.9 60.9 58.1
7.1 6.7 6.7 6.1 6.0 5.9 5.9 6.0 5.4
Table 10 Durability tests results of concrete mixes containing recycled concrete aggregates. Fraction of natural aggregates which is replaced by RCA
Percentage of natural aggregates replacement
p Sorptivity (mm/ min)
Porosity (%)
RCP (Coulombs)
Control mix 8/20
0% replacement 10% replacement 30% replacement 50% replacement 100% replacement 30% replacement 50% replacement 100% replacement 100% replacement
0.087 0.085 0.111 0.108 0.117 0.105 0.100 0.112 0.144
17.8 17.7 19.0 19.1 21.2 18.8 19.4 19.8 22.9
5181 5474 5331 5504 5432 6131 5863 6349 6505
4/10
8/20-4/10
4.6. RCA concretes – durability properties
0.1400 R² = 0.98
Sorptivity (mm/ √min )
The experimental work also emphasised on the durability aspect of concrete mixes containing recycled concrete aggregates. Properties such as liquid capillary absorption, porosity and rapid chloride permeability were investigated, and the results are summarized in Table 10. The increase in porosity is clearly attributed to the porous cement paste covering the surface of each recycled aggregate. The values of open porosity are increased, as the percentage of natural aggregates replacement is increased. This is more evident when higher percentages of recycled aggregates of larger sizes (i.e. 8–20 mm) are used in the mixtures. This is due to the increased amount of cement paste found on the surfaces of aggregates of larger sizes, leading to higher values of open porosity. Sorptivity has been affected in a similar manner. The excess cement paste surrounding RCA, results in increasing sorptivity values with increasing RCA replacement percentage. All mixes manufactured for the purposes of the presented study were classified as highly permeable, when the resistance of the specimens to chloride ion penetration was determined. The permeability values of all mixes, independently of the percentage of natural aggregates replacement, were very close. However, a small increase was noticed in mixes using only natural aggregates 8–20 mm and portions of both natural and recycled aggregates
0.1500
0.1300 0.1200 0.1100 0.1000 0.0900 0.0800 R² = 0.77
0.0700 16
17
18
19
20
21
Open porosity (%)
22
23
24
8/20 RCA 4/10 RCA
Fig. 2. Sorptivity variation with open porosity for various replacement levels of two different RCA fractions.
4–10 mm. The increase was owing to the presence of larger natural aggregates in higher proportions. The poor quality of the natural aggregates becomes more intense in aggregates of larger sizes, whereas this effect becomes less prominent in lower fractions of aggregates.
A. Kanellopoulos et al. / Construction and Building Materials 53 (2014) 253–259
Fig. 2 shows how sorptivity is correlated with open porosity for the two different RCA gradings at various replacement levels. As it can be observed increase in the sorptivity is particularly well correlated to the increase of the open porosity, especially for the 4/10 RCA fraction. This result agrees with similar observations made by the authors on self-compacting concrete [26]. 5. Conclusions Recycled lime powder (RLP) shows good potential as cement replacement filler, especially at low replacement percentages. The presented study showed that replacements up to 8% reduce somewhat the compressive strength of the mixture, resulting in a robust material both in terms of fresh and mechanical properties. Durability performance of the mixtures containing RLP seems to be more affected by the cement replacement. Being a waste material, RLP contains impurities that result to the increase of all durability indicators (sorptivity, porosity and chloride permeability). Nonetheless, RLP shows very good potential as cement replacement filler. The abrasion resistance results of recycled aggregates were comparable to the corresponding value for natural aggregates due to the poor quality of natural aggregates available on the island. The water absorption of recycled materials was well higher than the equivalent value for natural materials, due to the hardened cement paste covering the surfaces of recycled materials. The drop of the slump value with the increase of the percentage of recycled aggregates in the mixtures was attributed to the high amount of hardened cement paste covering the surface of the recycled aggregates. However, even for the total replacement of both fractions of natural aggregates, the slump value was not detrimentally low. The replacement of natural aggregates with recycled had a small impact on the compressive strength of concrete mixes. This was attributable to the additional cement content which was introduced into the mix through the old cement paste and to the poor quality of the natural aggregates available in Cyprus. The flexural strength of beams containing recycled concrete aggregates reduced, as the replacement percentage increases due to the poor quality of the interfacial bond developed between the old cement paste covering the recycled aggregates and the new cement paste. The increase in porosity was clearly attributed to the porous cement paste covering the surface of each recycled aggregate and the values of open porosity are increased, as the percentage of natural aggregates replacement is increased. This was more evident when higher percentages of recycled aggregates of larger sizes were used in the mixtures, due to the increased amount of cement paste found on the surfaces of aggregates of larger sizes. All mixes were classified as highly permeable and the permeability values of all mixes, independently of the percentage of natural aggregates replacement, were very close. For the determination of liquid capillary absorption, sorptivity was measured, and was recorded an increase of the sorptivity values, as a result of the presence of recycled aggregates. The results of this study are strongly encouraging in the possible use of ‘‘green’’ concrete in several, high value applications. References [1] Sakai K, Noguchi T. The sustainable use of concrete. CRC Press – Taylor & Francis Group; 2012. ISBN 978-0415667203. [2] Mehta PK. Concrete technology for sustainable development. Concrete Int 1999;21:47–53. [3] Mehta PK. Greening of the concrete industry for sustainable development. Concr Int 2002;24:23–8.
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[4] European Commission – Science for Environment:, accessed on: 03/10/2013. [5] European Quality Association for Recycling: , accessed on: 03/10/2013. [6] Corinaldesi V, Moriconi G. Behavior of cementitious mortars containing different kinds of recycled aggregate. Constr Build Mater 2009;23:289–94. [7] Tam VWY. Economic comparison of concrete recycling: a case study approach. Resour Conserv Recy 2008;52:821–8. [8] Gilpin R, Menzie DW, Hyun H. Recycling of the construction debris as aggregate in the Mid-Atlantic region. USA. Resour Conserv Recycl 2004;42:275–94. [9] ACI committee 555. ACI 555-01: removal and reuse of hardened concrete. ACI; 2001. [10] Evangelista L, de Brito J. Mechanical behaviour of concrete made with fine recycled concrete aggregates. Cem Concr Compos 2007;29:397–401. [11] Khatib JM. Properties of concrete incorporating fine recycled aggregate. Cem Concr Res 2005;35:763–9. [12] Dhir RK, Limbachiya MC, Leelawat T. Suitability of recycled concrete aggregate for use in BS 5328 designated mixes. Struct Build 1999;134:257–74. [13] Dhir RK, Henderson NA, Limbachiya MC. Use of recycled concrete aggregate. London: Thomas Telford Publishing; 1998. [14] Gonzalez-Fonteboa B, Martine-Abella F. Concretes with aggregates from demolition waste and silica fume: materials and mechanical properties. Build Environ 2008;43:429–37. [15] Wainwrigth A, Trevorrow YYU, Wang Y. Modifying the performance of concrete made with coarse and fine recycled concrete aggregates, demolition and reuse of concrete and masonry. In: Erik KL, editor, Proceedings of the third international RILEM symposium. 1993. p. 319–30, ISBN 0-412-32110-6. [16] Portland Cement Association. Recycled aggregate for reinforced concrete? Concr Technol Today 2002: p. 5–6. [17] Levy S, Helene P. Durability of recycled aggregates concrete: a safe way to sustainable development. Cem Concr Res 2004;34:1975–80. [18] Gómez-Soberón J. Porosity of recycled concrete with substitution of recycled concrete aggregate: an experimental study. Cem Concr Res 2002;32:1301–11. [19] Evangelista L, de Brito J. Durability performance of concrete made with fine recycled concrete aggregates. Cem Concr Compos 2010;32:9–14. [20] EN 197. Cement – Part 1: composition, specifications and conformity criteria for common cements, 2000. [21] EN 933. Tests for geometrical properties of aggregates Part 1: Determination of Particle Size Distribution, 2012. [22] EN 933. Tests for geometrical properties of aggregates Part 3. Determination of particle shape. Flakiness index, 2012. [23] EN 1097-2. Tests for mechanical and physical properties of aggregates. Methods for the determination of resistance to fragmentation, 2010. [24] EN 1097-6. Tests for mechanical and physical properties of aggregates. Determination of particle density and water absorption, 2000. [25] Domone PL. Self-compacting concrete: 11 years of case studies. Cem Concr Comp 2006;28:197–208. [26] Kanellopoulos A, Petrou MF, Ioannou I. Durability performance of selfcompacting concrete. Constr Build Mater 2012;37:320–5. [27] EN 1015-3. Methods of test for mortar for masonry. Determination of consistence of fresh mortar (by flow table), 1999. [28] EN 12350-2. Testing fresh concrete – slump test, 2009. [29] BS3892-1. Pulverized-fuel ash. Specification for pulverized-fuel ash for use with Portland cement, 1997. [30] EN 12390-2. Testing hardened concrete. Making and curing specimens for strength tests, 2009. [31] EN 13670. Execution of concrete structures, 2009. [32] ASTM C642-97. Standard test method for density, absorption, and voids in hardened concrete, 1997. [33] Rilem TC. 116-PCD. RILEM technical recommendation: determination of the capillary absorption of water of hardened concrete. Mater Struct 1999;32:178–9. [34] Gummerson RJ, Hall C, Hoff WD. Water movement in porous building materials – II. Hydraulic suction and sorptivity of brick and other masonry materials. Build Environ 1980;15:101–8. [35] Hall C. The water sorptivity of mortars and concretes: a review. Mag Concr Res 1989;41:51–61. [36] ASTM C1202. Standard test method for electrical indication of concrete’s ability to resist chloride ion penetration, 2009. [37] Mindess S, Young JF, Darwin D. Concrete. In: 2nd ed. Upper Saddle River: Pearson Education Inc., 2003. [38] EN 206-1. Concrete. Specification, performance, production and conformity, 2000. [39] Bouquety MN, Descantes Y, Barcelo L, de Larrard F, Clavaud B. Experimental study of crushed aggregate shape. Constr Build Mater 2007;21:865–72. [40] Xia J, Li W, Corr DJ, Shah SP. Effects of interfacial transition zones on the stress– strain behavior of modeled recycled aggregate concrete. Cem Concr Res 2013;52:82–99.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Mechanical and durability properties of concretes containing recycled lime powder and recycled aggregates Antonios Kanellopoulos a,⇑, Demetrios Nicolaides b, Michael F. Petrou c a
University of Cambridge, Department of Engineering, Trumpington Street, CB2 1PZ Cambridge, UK Frederick University Cyprus, Department of Civil Engineering, 7 Y. Frederickou Street, 1036 Nicosia, Cyprus c University of Cyprus, Department of Civil and Environmental Engineering, PO BOX 20537, 1678 Nicosia, Cyprus b
h i g h l i g h t s Recycled lime powder (RLP) does not seem to have any pozzolanic properties. RLP shows very good potential as a partial cement replacement material. Replacement of natural aggregates affects marginally the mechanical properties. We found good correlation exists between sorptivity and open porosity.
a r t i c l e
i n f o
Article history: Received 3 October 2013 Received in revised form 25 November 2013 Accepted 26 November 2013 Available online 22 December 2013 Keywords: Concrete Recycled waste Recycled fillers Recycled aggregates mechanical properties Durability
a b s t r a c t According to a recent report by the European Commission, within the European Union, the construction and demolition wastes come to at least 450 million tons per year. Roughly 75% of the waste is disposed to landfill, despite its major recycling potential. The bulk constituents of demolition debris are concrete (50–55%) and masonry (30–40%) with only small percentages of other materials such as metals, glass and timber. In Cyprus, at present, recycling of waste materials is practically inexistent and almost the entire demolition waste products are disposed in landfill sites, with all possible economic, technical and environmental impacts. This research paper presents the evaluation and the effective reuse of waste construction materials, such as recycled lime powder (RLP) and recycled concrete aggregates (RCA), disposed to landfill sites in Cyprus, due to the lack of a lucid recycling policy and knowledge. Results show that both RLP and RCA have the potential to produce good quality and robust concrete mixtures both in terms of mechanical and durability performance. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction The conservation of earth’s resources and sustaining the natural habitat has become the most crucial and serious problem for the continued existence and wellbeing of humankind. The natural sources of energy and materials have been consumed in great quantities by the rapid industrialisation and population growth over a span of more than two hundred years since the industrial revolution, resulting in global environment changes that humankind have never experienced. The problem we are facing today is that approximately 20% of the world’s population, mostly living in Western Europe, North America and Japan, in order to maintain its standard of living consumes almost 80% of the world’s resources and energy [1]. Meanwhile, the remaining 80% of global population has rapidly improved the rate of industrialisation in pursuit of a better life. One can easily understand that the result of this ⇑ Corresponding author. Tel.: +44 1223 766683. E-mail address: [email protected] (A. Kanellopoulos).
perpetual race for urbanisation encourages the continuation of consumption of natural resources at high rates. There are simply neither enough natural resources nor energy to continue our technological evolution as usual. It is imperative to find solutions that will assist to sustain the environment, without being an obstacle in the developmental race that meets the needs of not only the present but also future generations. The need for new infrastructure to support a minimum standard of life will increase as human population will continue growing, putting enormous demand on natural resources and the supply of construction materials. It is widely accepted that concrete is the most versatile construction material in the world. It can be fabricated in all sorts of conceivable geometries, it has excellent mechanical and durability properties, its availability is widespread since its constituent materials are most readily available anywhere in the world; and more importantly its acquisition is relatively affordable especially compared to steel. This popularity of concrete is being translated into an annual worldwide production in the range of 12 billion tons, which accounts for about 1.7 billion tons of concrete per year per
0950-0618/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.11.102
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person living on this planet [2,3]. However, this popularity comes at price: concrete production has an enormous impact on the environment, a fact which at the past has been significantly overlooked. First of all the production of concrete requires vast amounts of natural resources each year. Then, the production of each ton of Portland cement releases almost one ton of carbon dioxide into the atmosphere. It is estimated that the cement industry alone is responsible for about 7% of worldwide CO2 emissions, not to mention that the cement production is also very energy intensive. Third, the production of concrete requires large amounts of water, that fulfils certain standards, and water availability is a matter of concern these days. Lastly, the demolition and disposal of concrete structures creates another major environmental burden. The building sector has a huge environmental impact and the EU job market depends heavily on the construction sector. Buildings and construction consume a lot of natural resources and generate a lot of waste. The sector uses more than 50% of all materials extracted from the earth and generates more than 450 million tonnes/year of waste in the EU according to a recent announcement by the European Commission [4]. Managing and disposing of old buildings is also problematic [5]. Within the European Union, the construction and demolition wastes come to at least 450 million tons per year. Roughly 75% of the waste is disposed to landfill, despite its major recycling potential. Some Member States (in particular Denmark, The Netherlands and Belgium) investigated thoroughly the technical and economic feasibility of recycling, achieving recycling rates of more than 80%. On the other hand, the South European countries recycle very little of their construction wastes [6]. The bulk constituents of demolition debris are concrete (50–55%) and masonry (30–40%) with only small percentages of other materials such as metals, glass and timber [7]. In the United States of America of the approximately 2.7 billion metric tons of aggregates used, about 70% are used in structural concrete, whereas about 30% is being utilised in pavements and road works [8,9]. Although, incentives for use of processed aggregates are given to promote use of RCA, though a large part of the production is suitable only as fill or construction base [8,9]. Several researchers studied the use of RCA to partially or globally replace natural aggregates in the production of concrete [10,11]. Density and water absorption ratio are the properties having the greatest differences in comparison with natural aggregates. These differences are mainly attributed to the lower density of the adhered mortar in the recycled aggregate, as reported by many authors [12,13] and have a negative impact on the concrete mixes. However, there are a few studies that prove that concrete made with coarse aggregates deriving from concrete recycling have mechanical properties similar to those of conventional concretes [14]. On the other hand, there is scepticism in the use of the fine fraction of these recycled aggregates. Not many studies have been conducted using fine fractions due to the belief that their greater water absorption can jeopardize the final results, particularly for replacement ratios exceeding 30% [15,16]. Moreover, recycled fine aggregates contain a larger amount of adhered mortar, which results in difficulties in procuring the required slumps, as well as a substantial increase in deformation, and sharp drops in the modulus of elasticity and strengths. Some researchers examined the effect of the addition of silica fume on the basic properties of recycled concrete (RC) [14]. However, very few projects have been able to provide conclusive evidence regarding the effect of mineral admixtures on the properties of recycled concrete. Finally, although a significant amount of work has been conducted regarding the mechanical behaviour of concretes made with RCA, very limited work can be found in the literature about their durability aspects [17–19]. The replacement of natural aggregates with recycled coarse aggregates increases dramatically the water demand.
Therefore, dispersing agents have been used by several researchers. Locally the recycling of construction wastes is a critical issue, considering that at the present time recycling of waste materials is practically inexistent and the fact that landfill sites are becoming increasingly difficult to come by, due to the small size of the island. This deficiency is getting sharper considering the confinement of the island due to the existing political issue. At the same time, resource supply or feed material can be guaranteed in Cyprus, where replacement of infrastructure is occurring, natural aggregate resources are very limited, and environmental regulations encourage recycling. This research work was considered imperative at national level, as it was aiming to enhance the stateof-the-art knowledge concerning the reuse of waste materials in construction industry and also boost the recycling process in Cyprus with all possible social, economic, technical and environmental benefits. The specific area lacks extensive international experimental data and is of primary interest in the case of Cyprus (arid climate, large CO2 emissions in cities, and existence of reactive siliceous aggregates). Furthermore, the effect of the addition of a waste lime filler, deriving from the aggregates used in the production of asphalt concrete, on the properties of concrete was investigated. 2. Materials and mixtures The experimental work was carried out in two stages. The first stage consisted of the evaluation as cement replacement material of a lime powder that comes as a by-product from the production of asphalt concrete. In the second stage of the work the production of robust concrete mixtures using two gradings of recycled aggregates, sourced locally, was investigated. A large number of mixtures were produced and tests were performed to determine their fresh, mechanical and durability properties. 2.1. Materials 2.1.1. Cement To investigate the potential of recycled lime powder as cement replacement material, ordinary Portland cement CEM-I 32.5 was used. For the second part of the work, standard Portland pozzolan cement with a strength category of 42.5 N/ mm2 with sulphate resistance properties and low heat of hydration was used. All cements used in this study conform to the requirements of EN 197-1:2000 [20]. 2.1.2. Natural aggregates Graded crushed calcareous coarse aggregates in two different particle size gradings (8–20 mm and 4–10 mm) and two different gradings of calcareous sand (0– 4 mm and 0–2 mm) were used. All natural aggregates were supplied by a local quarry. 2.1.3. Recycled concrete aggregates The recycled aggregates were obtained from concrete rubbles available in Cyprus, using mobile impact crushers. It has to be mentioned that these rubbles represent a best-case scenario since they produced from leftover reinforced concrete laboratory specimens. Where necessary, material was refined in laboratory from a dry mill in order to allow the maximum grain sizes required for concrete mixes. Two fractions of recycled coarse aggregates were used in this work, namely 4– 10 mm and 8–20 mm. The remaining fractions (<4 mm and > 20 mm) were rejected, by applying sieving operation. Note that for the production of all concrete mixtures the same particle size ranges for both natural and recycled aggregates were used. Both types of aggregates were tested regarding their grading, flakiness index, abrasion resistance, density and water absorption [21–24]. Table 1 summarises the properties of both natural and recycled aggregates used in this study. It is noticeable that the abrasion resistance of recycled aggregates, as this derived from the Los Angeles test, is comparable to the corresponding value for natural aggregates. This was attributed mainly to the poor quality of natural aggregates available on the island, which have water absorptions in the range of 2.7–4%. In addition, the water absorption of recycled materials was well higher than the corresponding values for natural aggregates, due to the existence of the hardened cement paste covering the surfaces of recycled materials. 2.1.4. Recycled lime powder The recycled lime powder (RLP) used in this study is a waste material of the asphalt concrete production process. Calcareous aggregates used in this process are being heated at 800 °C in special kilns, then the surface dust is blown away
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A. Kanellopoulos et al. / Construction and Building Materials 53 (2014) 253–259 Table 1 Properties of Recycled Concrete Aggregates (RCA) and Natural Aggregates (NA). Coarse aggregates
Moisture content (%) Flakiness index (%) Los Angeles (LA) (%) Apparent particle density (Mg/m3) Particle density Particle density in SSD WA 24 (%)
RCA 4/10
RCA 8/20
NA 4/10
NA 8/20
2.00–3.00 9.00 – 2.59 2.09 2.28 9.37
1.95–3.00 5.00 28.60 2.60 2.21 2.37 7.02
0.12–0.27 – – 2.71 2.42 2.53 4.50
0.06–0.28 3.00 29.00 2.71 2.44 2.54 4.00
and finally the aggregates are mixed with the bituminous material. This dust is nothing but limestone powder. In Cyprus alone, it is estimated, that 230,000 tonnes (for 2010) from this dust are being disposed in landfills. Pure, standardised, limestone powder (SLP) has been used widely in the past mainly for the production of self-compacting concrete mixtures [25]. Performing X-ray Diffraction Crystallography and particle size analysis, it was found the RLP has very similar characteristics to the standardised lime powders used locally [26]. Table 2 shows how RLP compares to SLP while Fig. 1 illustrates the crystallographic profiles of both powders. 2.2. Mixtures During the first stage of the experimental work four different cement replacement percentages were investigated, ranging from 5% to 20%. To study the potential of the RLP as cement replacement material, a control mortar mixture was developed incorporating cement, water, superplasticiser (SP) and fine sand with maximum grain size 2 mm. Keeping the water-to-cement ratio (w/c) constant, cement was replaced gradually by 5%, 8%, 10% and 20% by volume. To investigate the use of RCA, a control concrete mixture was designed containing only natural aggregates (coarse and fine). Subsequently eight mixes were prepared, replacing portions, by weight, of natural coarse aggregates, with the corresponding coarse fractions of RCA. Specifically, four different mixes replacing 10%, 30%, 50%, and 100% of natural coarse aggregates 8–20 mm and three different mixes replacing 30%, 50% and 100% of natural coarse aggregates 4–10 mm were prepared. Furthermore, one more mix replacing totally both fractions of natural coarse aggregates was also prepared. Water-to-cement ratio was kept constant in all mixtures and moreover the fine aggregates used were the same for both the control mixture and the mixtures containing the replacement of coarse aggregates. Table 3 shows the constituents for control mortar and concrete mixtures.
Workability of concrete mixtures was assessed by common slump test as described in EN12350-2 [28]. Water and SP contents remained unaltered, since the effect of RCA on the workability of mixtures was to be investigated.
3.3. Specimen preparation Six cubes (50 50 50 mm) were cast for each of the mortar mixtures as described in BS3892-1 [29]. All mortar cubes were carefully compacted using an electric motor vibrating table. The specimens were demoulded after 24 h from casting and placed at water tank (20 °C ± 2 °C) until the day of testing. For concrete specimens, mould filling was conducted according to the requirements specified in European Standards EN 12390-2:2009 and EN 13670:2009 [30,31]. Moulds were filled on an electric motor vibrating table in two layers, each being followed by adequate mechanical vibration (about 12 s each). In total, fifteen standard cube moulds (100 100 100 mm), three standard prism (cross section 100 100 mm, length L = 500 mm) and 9 standard cylindrical moulds (diameter D = 100, height H = 200 mm) have been cast and treated for each different mixture produced. Ten of the cube specimens were used for the determination of the compressive strength (fcu) at seven and twenty-eight days, while the remaining five used for the determination of open porosity and sorptivity at 28 days. The prism and cylindrical specimens were used to obtain the flexural strength (fcf) and chloride penetrability respectively at 28 days. The test specimens were left to set for 24 h after casting and then they were demoulded and received the same treatment. All concrete specimens were cured in a water tank (tap water), with a water temperature of 20 °C ± 2 °C, as described by EN 12390-2:2009, until the age of testing (7 and 28 days).
3.4. Hardened mortar testing 3. Experimental procedures 3.1. Mixture production For the production of the mixtures a 20 L capacity dough mixer has been used for the production of mortars and a 100 L capacity fixed-pan planetary type cylindrical mixer with rotating blades has been used for the production of concrete mixtures. The same mixing procedure has been carefully followed in all mixtures. At first, the aggregates have been dry-mixed followed by the cement to produce a homogeneous dry mixture. Then, 80% of the water has been added, followed by the rest of the water content in addition with the superplasticiser. 3.2. Fresh properties Mortars were examined for their flowability according to European standard EN1015-3, which determines the consistency of fresh mortars by flow table [27]. Since w/c was kept constant the SP quantity was adjusted, where needed, to achieve the same flow as the control mortar with an acceptable deviation of ±5 mm, as the standard dictates. Note that the flow table experimental procedure was followed to make sure that all mixtures have the same workability level as the control mortar mix. The effect of RLP on the workability of the mixtures is beyond of the scope of the present study.
Table 2 Physical properties of SLP and RLP. Property
SLP
RLP
CaCO3 Fineness <18 lm <2 lm Specific surface area Specific gravity
96%
95%
96% 7% 3659 cm2/g 2.8 g/cm3
97% 9% 3509 cm2/g 2.7 g/cm3
The purpose of the compressive strength tests in mortar cubes was to obtain the so called Strength Activity Index (SAI) as described in BS3892-1. SAI is the ratio of the average compressive strength of the mortar mixture under investigation to the average compressive strength of the control mortar mixture. Normally, SAI is being reported as a percentage. All mortar cubes were tested in a 250 kN compression test frame.
3.5. Hardened concrete testing 3.5.1. Mechanical properties From each concrete mixture, standard cubes (100 100 100 mm) were used to determine conventional compressive strength, according to EN 12390-3:2009. All cubic specimens were tested on a 5000 kN servo hydraulic compression frame that conforms to EN 12390-4:2009. In addition, the prismatic specimens (cross section 100 100 mm, length L = 500 mm) used for the determination of flexural strength, as described by EN 12390-5:2009, were tested in three-point bend configuration using an 100 kN servo hydraulic flexural frame.
3.5.2. Durability properties 3.5.2.1. Open porosity. The open porosity of each mixture has been tested on three standard cube specimens (100 100 100 mm) in accordance with ASTM C64297 [32]. After the treatment period, the cubes have been weighted both below water and in air before being oven dried at a temperature of 105 °C until constant mass, which usually lasted a period between 7 and 10 days. By combining the three measured masses, the open porosity has been evaluated.
3.5.2.2. Sorptivity. The sorptivity (liquid capillary absorption coefficient) is obtained from a simple short-term one-dimensional experiment [33]. During this test, liquid uptake is governed by capillary and viscous forces. In this case two cubic specimens (100 100 100 mm) were used. After oven drying at 105 °C, the specimens were allowed to absorb ethanol from a free liquid surface by capillary rise. During the capillary absorption of ethanol, the increase in weight of the specimens over time
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CaO
14000 13000 12000 11000 10000
Lin (Counts)
9000 8000 7000
CaO
6000 5000 4000
CaO
3000
CaO
CaO
2000
C2S
C2S
1000 0 5
10
20
30
40
50
60
2-Theta - Scale Fig. 1. Crystallographic profiles for SLP (black curve) and RLP (red curve). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 3 Constituents for control mortar and concrete mixtures (quantities are in kg/m3). Constituents
Control – mortar mix
Control – concrete mix
CEM-I 32.5 CEM-II 42.5R Water Coarse aggregates 8–20 mm 4–10 mm Sand 1 (crashed limestone) Sand 2 (fine limestone sand) Superplasticiser
400 – 200
– 400 200
– – – 1200 5.5
655 264 560 108 6.4
Table 4 Chloride ion penetrability based on charge passed (according to [35]). Charge passed Q (C)
Chloride ion penetrability
>4000 2000–4000 1000–2000 100–1000 <100
High Moderate Low Very low Negligible
(t) was noted. The sorptivity was determined by the slope of the cumulative absorbed volume of liquid per unit inflow surface versus t½ plot, as described in the literature [33–35].
3.5.2.3. Chloride penetrability. To assess the chloride penetrability of concrete mixtures presented in this study a very popular and well-known experimental procedure was adopted. The rapid chloride permeability test (RCP) is a standardised ASTM test that exists for 24 years [36]. The experiment assesses concrete’s ability to resist chloride intrusion. In fact it measures the electrical conductivity or resistance of a 50 mm thick concrete disc specimen, cut from the middle section of a standardised cylindrical specimen, for a period of 6 h. It is assumed that the resistivity is directly related to the pore network or concrete permeability, although some researchers claim that the relation is not perfect [37]. The resistivity is measured in terms of total electric charge (measured in Coulombs, C) passed through the specimens, and these measurements are used for the classification of mixtures as shown in Table 4. During the test, the specimens are positioned in a measuring cell with a fluid reservoir at each face of the specimen. For the ASTM/AASHTO test, one reservoir is filled with sodium chloride (3.0% NaCl) solution and the other with sodium hydroxide (0.3 M NaOH) solution. The reservoir containing the NaCl is
connected to the negative terminal and the NaOH reservoir to the positive terminal of the microprocessor power supply unit. The specimens used in the tests described in this paper were cured under the same conditions. The experiments took place at 28 days and the average of three specimens for each time interval was recorded.
4. Results and discussion 4.1. Mortars’ strength activity index Replacing cement with RLP resulted in robust mortar mixtures with good mechanical performance at both 7 and 28 days. At low replacement percentages the SAIs are as high as 90% at both testing days. Increasing cement replacement results in gradual reduction of the strength activity index. Table 5 summarises the SAIs and compressive strength results for mortars. As it can be seen from the above table at low cement replacements with RLP (5% and 8%) the reduction in strength does not exceed 18%. Replacing cement up to 20% results in significant reduction of compressive strength. These results indicate that at low replacement percentages RLP has the potential to substitute cement in concretes designated for structural applications. Although at larger substitution percentages the strength is being reduced significantly, RLP can be used as cement replacement in concretes for non-structural applications. In the light of the above discussion, in order to explore the potential of RLP in structural concrete it has been decided to produce two concrete mixtures, based on the mix design shown in Table 3, replacing cement with RLP by 5% and 8% by volume. 4.2. RLP concretes – mechanical properties Table 6 shows the values obtained for compressive and flexural strengths at two different time intervals for the control mixture and the mixtures containing cement replacement with RLP. Results justify what has been revealed by previous experiments on mortars and SAIs. Both concretes at 5% and 8% cement replacement attain relatively large values of both compressive and flexural strengths. It can be noted that the reduction in compressive strength for both percentage replacement levels at both testing dates is somewhat smaller than the corresponding reductions reported in
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A. Kanellopoulos et al. / Construction and Building Materials 53 (2014) 253–259 Table 5 Compressive strengths and SAIs for mortar mixtures. 7 days
Control Substitution 5% 8% 10% 20%
28 days
fcu (MPa)
SAI (%)
Reduction (%)
fcu (MPa)
SAI (%)
Reduction (%)
30.1
–
–
32.1
–
–
28.1 24.7 18.9 15.7
93.3 82.1 62.8 52.2
6.7 17.9 37.2 47.8
30.1 27.3 20.7 17.4
93.4 85.0 64.5 54.2
6.6 15.0 35.5 45.8
Table 6 Compressive and flexural strengths for control and RLP concrete mixtures. 7 days
Control RLP 5% RLP 8%
28 days
fcu
fcf
fcu reduction (%)
fcu
fcf
fcu reduction (%)
52.8 50.1 44.7
– – –
– 5.1 15.3
63.8 60.2 54.6
7.1 6.7 5.9
– 5.6 14.4
Table 7 Sorptivity, porosity and RCP values for control and RLP concrete mixtures. 28 days
Control RLP 5% RLP 8%
p Sorptivity (mm/ min)
Porosity (%)
RCP (Coulombs)
0.087 0.101 0.119
17.8 18.9 19.8
5181 5294 5400
Table 5, concerning the mortar mixtures. A possible explanation for this observation is that the presence of coarse aggregates in the concrete mixtures have increased the stiffness of the composite material resulting in specimens that sustain marginally larger compressive stresses than the ones made with mortars. Even the mixture containing 8% replacement of cement with RLP produces an almost 45 MPa concrete that conforms the criteria for a C32/ 40 concrete as specified in EN206-1 [38]. 4.3. RLP concretes – durability properties Cement replacement with RLP seems to reduce to some extent the durability performance of the produced concretes in a similar manner as it reduced their mechanical performance. Table 7 summarises the results, at 28 days, on sorptivity, open porosity and rapid chloride permeability for the control and the cement replaced mixtures. Substituting 5% of the cement with RLP results in: 14% increase in the sorptivity, about 6% increase of the open porosity and a 2% increase of the value of rapid chloride permeability. Further replacement of cement with RLP (8%) results in: an additional 13% increase of the sorptivity values, a total 10% increase of the open porosity and a total 4% increase in the rapid chloride permeability. It seems that the most affected durability indicator is the sorptivity which exhibits the larger variations with increasing cement replacement percentage. The above durability properties’ results show that the RLP to some extent fails to refine the pore network of the matrix and as result the produced mixtures are to a certain extent more susceptible to liquid absorption and chloride penetration. The explanation for this phenomenon is that RLP is not a standardised material and therefore contains impurities that affect its performance as filler. Albeit being a waste material that results in mixtures with somewhat reduced mechanical and durability performance, RLP shows a very good potential as a cement replacement material. Experimental results suggest that RLP can
be used as filler in concrete mixtures, reducing cement quantity without significantly affecting the structural performance of the produced material. 4.4. RCA concretes – fresh properties It is clearly observed a drop of slump, as the percentage of replacement is increased (Table 8). This is becoming more evident in the case of the coarser fraction (8–20 mm), whereas the drop of slump in the case of replacement of part of the 4–10 mm fraction is not that pronounced. The existence of high amount of hardened cement paste on the surface of the recycled aggregates is the main reason for the lower slump values, as the percentage of replacement is increased. Moreover, the lower slump values for the mixtures containing RCA can be also justified by the relatively large flakiness index numbers of the RCA compared to natural aggregates. As it is known, flaky and elongated particles can cause workability issues in concrete [39]. Nevertheless, it is important to notice that even for the total replacement of both fractions of natural aggregates (4–10 mm and 8–20 mm), the slump value is not detrimentally low. 4.5. RCA concretes – mechanical properties It is important to highlight the small impact of the natural aggregates replacement, on the compressive strength of concrete mixes. As it is indicated on Table 9, the replacement percentage does not have a serious effect on the average compressive strength of each mix. The highest difference is in the case of the total replacement of both fractions of natural aggregates with recycled, where the strength drop percentage is approximately 10%. However, even in this case the strength reduction is regarded small, especially considering the fact that natural aggregates have not been used. This could be attributed to several reasons. Most importantly, is regarded the additional cement content which is introduced into the mix through the cement paste which covers the recycled aggregates. In combination with the poor quality of the natural aggregates available in Cyprus, used in the commercial concrete mixes, are the profound reasons for the compressive strengths of mixtures containing recycled concrete aggregates to be very similar to the values obtained when natural aggregates are used. The flexural strength of beams containing recycled concrete aggregates reduces, as the replacement percentage increases (Table 9). The highest drop of flexural strength was observed when both fractions of natural aggregates were totally replaced with recycled. It is strongly believed that the main reason of this flexural strength drop is the poor quality of the interfacial bond developed between the old cement paste covering the recycled aggregates and the new cement matrix. This has been verified recently both by nanoindentation experiments and numerical modelling of the interfacial transition zone in mortars containing recycled aggregates [40].
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Table 8 Slump variation with aggregate replacement. Fraction of natural aggregates which is replaced by RCA
Percentage of natural aggregates replacement
Slump (mm)
Control mix 8/20
0% Replacement 10% Replacement 30% Replacement 50% Replacement 100% Replacement 30% Replacement 50% Replacement 100% Replacement 100% Replacement
200 195 168 168 134 198 193 170 188
4/10
8/20–4/10
Table 9 Average compressive and flexural strength of concrete mixes containing recycled concrete aggregates. Fraction of natural aggregates which is replaced by RCA
Control mix 8/20
4/10
8/20–4/10
Percentage of natural aggregates replacement
0% Replacement 10% Replacement 30% Replacement 50% Replacement 100% Replacement 30% Replacement 50% Replacement 100% Replacement 100% Replacement
7 days
28 days
fcu (MPa)
fcu (MPa)
fcf (MPa)
52.2 49.5 49.5 49.9 52.6 50.6 50.8 50.0 47.0
63.8 61.7 60.7 60.2 62.7 58.8 60.9 60.9 58.1
7.1 6.7 6.7 6.1 6.0 5.9 5.9 6.0 5.4
Table 10 Durability tests results of concrete mixes containing recycled concrete aggregates. Fraction of natural aggregates which is replaced by RCA
Percentage of natural aggregates replacement
p Sorptivity (mm/ min)
Porosity (%)
RCP (Coulombs)
Control mix 8/20
0% replacement 10% replacement 30% replacement 50% replacement 100% replacement 30% replacement 50% replacement 100% replacement 100% replacement
0.087 0.085 0.111 0.108 0.117 0.105 0.100 0.112 0.144
17.8 17.7 19.0 19.1 21.2 18.8 19.4 19.8 22.9
5181 5474 5331 5504 5432 6131 5863 6349 6505
4/10
8/20-4/10
4.6. RCA concretes – durability properties
0.1400 R² = 0.98
Sorptivity (mm/ √min )
The experimental work also emphasised on the durability aspect of concrete mixes containing recycled concrete aggregates. Properties such as liquid capillary absorption, porosity and rapid chloride permeability were investigated, and the results are summarized in Table 10. The increase in porosity is clearly attributed to the porous cement paste covering the surface of each recycled aggregate. The values of open porosity are increased, as the percentage of natural aggregates replacement is increased. This is more evident when higher percentages of recycled aggregates of larger sizes (i.e. 8–20 mm) are used in the mixtures. This is due to the increased amount of cement paste found on the surfaces of aggregates of larger sizes, leading to higher values of open porosity. Sorptivity has been affected in a similar manner. The excess cement paste surrounding RCA, results in increasing sorptivity values with increasing RCA replacement percentage. All mixes manufactured for the purposes of the presented study were classified as highly permeable, when the resistance of the specimens to chloride ion penetration was determined. The permeability values of all mixes, independently of the percentage of natural aggregates replacement, were very close. However, a small increase was noticed in mixes using only natural aggregates 8–20 mm and portions of both natural and recycled aggregates
0.1500
0.1300 0.1200 0.1100 0.1000 0.0900 0.0800 R² = 0.77
0.0700 16
17
18
19
20
21
Open porosity (%)
22
23
24
8/20 RCA 4/10 RCA
Fig. 2. Sorptivity variation with open porosity for various replacement levels of two different RCA fractions.
4–10 mm. The increase was owing to the presence of larger natural aggregates in higher proportions. The poor quality of the natural aggregates becomes more intense in aggregates of larger sizes, whereas this effect becomes less prominent in lower fractions of aggregates.
A. Kanellopoulos et al. / Construction and Building Materials 53 (2014) 253–259
Fig. 2 shows how sorptivity is correlated with open porosity for the two different RCA gradings at various replacement levels. As it can be observed increase in the sorptivity is particularly well correlated to the increase of the open porosity, especially for the 4/10 RCA fraction. This result agrees with similar observations made by the authors on self-compacting concrete [26]. 5. Conclusions Recycled lime powder (RLP) shows good potential as cement replacement filler, especially at low replacement percentages. The presented study showed that replacements up to 8% reduce somewhat the compressive strength of the mixture, resulting in a robust material both in terms of fresh and mechanical properties. Durability performance of the mixtures containing RLP seems to be more affected by the cement replacement. Being a waste material, RLP contains impurities that result to the increase of all durability indicators (sorptivity, porosity and chloride permeability). Nonetheless, RLP shows very good potential as cement replacement filler. The abrasion resistance results of recycled aggregates were comparable to the corresponding value for natural aggregates due to the poor quality of natural aggregates available on the island. The water absorption of recycled materials was well higher than the equivalent value for natural materials, due to the hardened cement paste covering the surfaces of recycled materials. The drop of the slump value with the increase of the percentage of recycled aggregates in the mixtures was attributed to the high amount of hardened cement paste covering the surface of the recycled aggregates. However, even for the total replacement of both fractions of natural aggregates, the slump value was not detrimentally low. The replacement of natural aggregates with recycled had a small impact on the compressive strength of concrete mixes. This was attributable to the additional cement content which was introduced into the mix through the old cement paste and to the poor quality of the natural aggregates available in Cyprus. The flexural strength of beams containing recycled concrete aggregates reduced, as the replacement percentage increases due to the poor quality of the interfacial bond developed between the old cement paste covering the recycled aggregates and the new cement paste. The increase in porosity was clearly attributed to the porous cement paste covering the surface of each recycled aggregate and the values of open porosity are increased, as the percentage of natural aggregates replacement is increased. This was more evident when higher percentages of recycled aggregates of larger sizes were used in the mixtures, due to the increased amount of cement paste found on the surfaces of aggregates of larger sizes. All mixes were classified as highly permeable and the permeability values of all mixes, independently of the percentage of natural aggregates replacement, were very close. For the determination of liquid capillary absorption, sorptivity was measured, and was recorded an increase of the sorptivity values, as a result of the presence of recycled aggregates. The results of this study are strongly encouraging in the possible use of ‘‘green’’ concrete in several, high value applications. References [1] Sakai K, Noguchi T. The sustainable use of concrete. CRC Press – Taylor & Francis Group; 2012. ISBN 978-0415667203. [2] Mehta PK. Concrete technology for sustainable development. Concrete Int 1999;21:47–53. [3] Mehta PK. Greening of the concrete industry for sustainable development. Concr Int 2002;24:23–8.
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[4] European Commission – Science for Environment: