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Mechanical strength, flexural behavior and fracture energy of Recycled Concrete Aggregate self-compacting concrete
Structures 23 (2020) 34–43
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Mechanical strength, flexural behavior and fracture energy of Recycled Concrete Aggregate self-compacting concrete Saif I. Mohammed, Khalid B. Najim
T
⁎
Civil Engineering Department, University Of Anbar, University Campus, Ramadi, Iraq
A R T I C LE I N FO
A B S T R A C T
Keywords: Recycled Concrete Aggregate (RCA) Self-compacting concrete (SCC) Taguchi method Structural behavior Mechanical properties Fracture energy
The possibility of using Recycled Concrete Aggregates (RCA) to produce self-compacting concrete (RASCC) was investigated. Three parameters that are recycled coarse aggregate, recycled fine aggregate and Superplasticizer were studied with four different percentages of replacement. Compressive, flexural strength and modulus of elasticity tests were carried out in order to investigate the mechanical properties of the studied mixes. Based on the result of Taguchi analyses for the compressive strength, four mixes were selected to be in-depth studied in terms of flexural behavior. Eight 100 × 150 × 1200 mm reinforced concrete beams were tested under two-point loading system. Flexural stiffness (k) and flexural toughness (I) were determined. The cracks pattern, propagation and their tortuosity were determined by utilizing of image processing technique using the fractal theory. Based on the experimental results, it was found that hardened properties and the flexural stiffness and toughness generally decreased with RCA incorporation. However, SCC has 39 MPa compressive strength still achievable even with 100% RCA replacement. Surface cracks fracture energy parameters were also determined. It was found that the classical definition of fracture energy was in agreement with the deterioration in strength, stiffness, toughness, for the tested beams with incorporating RCA.
1. Introduction: Concrete is the most widely used construction material around the world. The demand for concrete was rapidly increased in the last fifty years as a result of the population growth that, of course, has been leading to increase the need to implement residential and infrastructures projects. This leads to increase the demand for the construction raw materials, especially the aggregate that counts about 70–80% of the total concrete volume [1,2]. It was recently found that the consumption of aggregate in construction industry reached 48.3 billion metric tons in 2015 worldwide. This consumption annually increases by about 5% while the highest consumption was found to be in Asia and Pacific [3], as shown in Fig. 1. By considering this clear increase in aggregate consumption, natural aggregate (NA) could no longer meet the needs of construction industry in some counties in terms of both quantity and quality [4]. One the other hand, the attention towards the negative environmental impact of construction industry that consumes massive quantities of virgin materials such as aggregate has recently increased [5]. Therefore, alternative aggregate resources should be found. Recycled Concrete Aggregate (RCA) could be a part of the solution because it provides an alternative aggregate to natural aggregates (NA). ⁎
Additionally, it helps to dispose of construction and demolition waste (C&DW), reduces landfill space, diminishes environmental pollution, decreases transport costs and protects environmental balance [6]. RCA is defined, according to the British standard [7], as “the aggregates derived from concrete waste only”. In Germany after World War II, for example, RCA was used in reconstructing the destroyed buildings [8]. In Iraq, there are massive quantities of the concrete rubble generated from the repeated wars that happened during the last three decades, as shown in Fig. 2-A. Also, the concrete security barriers (precast concrete walls) that being used for security purposes (Fig. 2-B) need to be disposed after enhancing the security situation recently. The waste concrete that produced from testing the concrete specimens in the concrete laboratories at the universities and the research and quality control centers over the world needs to be considered by recycling these materials in a beneficial matter. The reuse of these waste materials would definitely contribute in achieving the sustainable development goals by processing them to aggregate (RCA) to be used in producing concrete meanwhile reducing the use of the raw materials. Self-compacting concrete (SCC) is defined as sophisticated version of the high performance concrete. It was discovered in Japan, at the University of Tokyo in early of 1980s. The purpose was to produce
Corresponding author. E-mail address: [email protected] (K.B. Najim).
https://doi.org/10.1016/j.istruc.2019.09.010 Received 13 August 2019; Received in revised form 20 September 2019; Accepted 20 September 2019 2352-0124/ © 2019 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
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S.I. Mohammed and K.B. Najim
2. The used materials and the studied mixes Ordinary Portland Cement (OPC) confirms the Iraqi specification IQ No. 5/1984 [13] with specific gravity of 3.14 and specific surface area 2600 cm2/gm was used in this investigation. The natural coarse aggregate (NCA) was crushed angular shape with nominal maximum size of (5–14) mm. The recycled concrete coarse aggregate (RCA) was produced by crushing the tested concrete specimens at the laboratories of university of Anbar-Iraq. Thousands of specimens are annually tested at this laboratory whether for undergraduate/postgraduate students or for the infrastructure projects for quality control goal. Around 700 (100 × 100 mm) concrete cubes having compressive strength ranged between 25 and 30 MPa were crushed after storing them at the laboratory conditions for a year. These specimens are crushed using a crusher to the maximum of 14 mm and minimum 4.75 mm particles size. The particles that passing through 4.75 mm sieve were used as recycled fine aggregate (RFA) in this investigation in addition to the natural fine aggregate. The sieve analyses of the natural and recycled used aggregate were presented in Fig. 3. Table 1 shows the physical properties of used aggregate. The aggregate tests were carried out based on IQ No.45 [14]. Silica fume (SF) powder having 120% strength activity index at 7 days was utilized as a mineral admixture. The SF was blended with tested mixes by 12% wet of cement content based on the manufacturing data sheet and the literature. Sika ViscoCrete-5930 was used as a chemical admixture to satisfy SCC requirements. It is free of chlorides and compatible with ASTM C494 [15] types G and F. The percentage replacements of RCA are presented in Table 2.
Fig. 1. Demand on construction aggregates worldwide [3].
durable concrete that is used in heavily reinforced and complicated concrete structures. The durability problems in concrete structures were noted in Japan in the 1970s due to lack of skilled workers [9]. It is considered as a latest innovation in concrete technology due to its numerous advantages over conventional concrete. It is a special type of concrete that spreads through the congested reinforcement, consolidates under its own weight without any external vibration. It provides excellent filling ability, passing ability and exhibits good segregation resistance [10]. These properties could be achieved by using high powder contents (cement and mineral additives and admixtures such as fly ash, silica fume, and limestone, etc.) in addition to Superplasticizer. The use of RCA to produce recycled aggregate SCC (RASCC) could combine the advantages of using RCA and SCC. However, this type of concrete should be deeply investigated. Many previous studies were carried out to investigate the use of recycled aggregate that produced from demolishing old building in producing SCC. Banda [11] and Bal investigated the effect of using different quantities of RCA that produced from a demolished building about 25 years old on the properties of SCC. The results were compared with conventional concrete (100% natural aggregate (NA) concrete) in terms of mechanical properties (compressive strength, flexural strength, and splitting tensile Strength). The percentages of replacement of NA with RCA were 10%, 20%, 30% and 40% by weight. It was found that the mechanical properties decreased with increasing the percentage of replacement of RCA. However, it was recommended that RCA could be efficiently used with up to 30% replacement of RCA. Manzi et al. [12] found that structural SCC with up to 40% of fine and coarse recycled aggregate could be produced with no significant difference than conventional concrete. Structural concrete in terms of compressive, flexural, and tensile strengths in addition to elastic modulus were produced even with 100% replacement due to the improvement in the microstructure. The aim of this research is to investigate the use of RCA that produced from recycling the tested/crushed concrete specimens in producing RASCC as a first step before studying the other waste concrete as previously mentioned. Structural behavior under flexural loading of this concrete with different RCA contents in addition to flexural stiffness and toughness of reinforced concrete beams were studied. The cracks pattern, propagation and their tortuosity were also studied. This was carried out by employing the image processing technique using the fractal theory while the previous investigations used traditional methods. Surface fracture energy parameters were also determined for the studied mixes via the reinforced concrete beams.
3. Design of experiments and mix proportions All prepared mixes of SCC were designed based on the European guidelines of EFNARC 2005 [16]. Trial and error approach was used to find out the proportions that meet the SCC requirements. The mix proportions of the reference mix with a target compressive strength (fcu) of 55 MPa are presented in Table 3. After achieving the reference mix, Taguchi method of experiment design was set considering four level of replacement on three parameters. These parameters are coarse and fine aggregate in addition to super plasticizer as demonstrated in Table 4. This creates standard Taguchi orthogonal array which are special standard experimental design requires a small number of experimental trials to find the main factor that affect the output. To consider the aforementioned parameters, 64 experiments are required. However; Taguchi approach suggests L16 orthogonal array based on Minitab 17 software that was used in this study. In other words, only 16 instead of 64experiments are required to sufficiently consider and optimize these parameters. Tables 4 and 5 respectively show the standard Taguchi orthogonal array L16 of the tested mixes and their proportions. It was found that that the recycled coarse aggregate has more effect than other two parameters followed by the recycled fine aggregate. SP has a marginal effect on the studied properties within the range of the tested SP content. Based on Taguchi analyses of the strength at 28 days four mixes were selected to study the structural behavior and fracture energy parameters of the selected mixes. These four mixes are; reference mix (S0), the mix provided the highest strength (Sop), the mix provided the mean strength (Sm) and the mix provided the lowest strength (SW). The mix proportions for the selected mixes are shown in Table 6. 4. Taguchi technique It is a powerful tool developed by Genichi Taguchi (1950) to design high quality systems [17]. It is used to design the experiment by creating a standard orthogonal array in order to taking into account the effect of a number of factors on the targeted value in addition presenting the experiments plan. This allows collecting the necessary data 35
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Fig. 2. The concrete waste.
Table 4 The Standard Taguchi orthogonal array L16.
Fig. 3. Particle size distribution of natural and recycled aggregates.
Experiment number
Recycled coarse aggregates
Recycled fine aggregates
Superplasticizer
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16
25% 25% 25% 25% 50% 50% 50% 50% 75% 75% 75% 75% 100% 100% 100% 100%
20% 30% 40% 50% 20% 30% 40% 50% 20% 30% 40% 50% 20% 30% 40% 50%
2.25% 2.5% 2.75% 3% 2.5% 2.25% 3% 2.75% 2.75% 3% 2.25% 2.5% 3% 2.75% 2.5% 2.25%
Table 1 The physical properties of aggregate. Physical properties
SO3 % Specific gravity Absorption % Materials finer than 75 µm% Attached mortar %
Coarse aggregate
Fine aggregate
Natural
Recycled
Natural
Recycled
0.03 2.67 0.68% – –
– 2.51 2.01% – 43%
– 2.7 0.8% 1.5% –
– 2.48 6.53% – –
Table 5 The proportions of RCA mixes (kg/m3). Mix
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16
Table 2 The parameter and their levels of replacement. Parameter and Unit
Recycled Coarse Aggregate Recycled Fine Aggregate Superplasticizer
Symbol
RCA RFA SP
Parameter levels Level 1
Level 2
Level 3
Level 4
25% 20% 2.25%
50% 30% 2.5%
75% 40% 2.75%
100% 50% 3.0%
Table 3 Mix proportions of reference mix. Materials
(kg/m3)
Typical range by mass (kg/m3) European Guidelines 2005[40]
Cement (kg/m3) Fine aggregate (kg/m3) Coarse Aggregate (kg/m3) Water (kg/m3) Silica fume (kg/m3) Superplasticizer (kg/m3)
450 950 770 200 54 9
380–600 48–55% of total aggregate weight 750–1000 150–210 —— ——
CA
FA
NCA
CRA
NFA
RFA
577.5 577.5 577.5 577.5 385 385 385 385 192.5 192.5 192.5 192.5 0 0 0 0
192.5 192.5 192.5 192.5 385 385 385 385 577.5 577.5 577.5 577.5 770 770 770 770
760 665 570 475 760 665 570 475 760 665 570 475 760 665 570 475
190 285 380 475 190 285 380 475 190 285 380 475 190 285 380 475
Water
Cement
S.F
SP (l/m3)
200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200
450 450 450 450 450 450 450 450 450 450 450 450 450 450 450 450
54 54 54 54 54 54 54 54 54 54 54 54 54 54 54 54
10.125 11.25 12.375 13.5 11.25 10.125 13.5 12.375 12.375 13.5 10.125 11.25 13.5 12.375 11.25 10.125
to determine which factor affects the experiment more than others with a minimum amount of experiments to reduce the required time and resources. The first concept of Taguchi that should be discussed is the signal to noise ratio “S/N”. The term of “Signal” represents the desired value for the output characteristic while the term of “Noise” represents the undesired value for the output characteristic. S/N offers a measurable product or process characteristic to deviate from its target value [18]. The S/N ratio depends on the characteristics quality of the product/process to be optimized. Usually, there are two categories of the performance characteristics in the analysis of the S/N ratio: 36
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Table 6 Mix proportions for the selected mixes. Mix
S0 Sop Sm Sw
Cement (kg/m3)
450 450 450 450
Fine aggregate (kg/m3)
Coarse Aggregate (kg/m3)
natural
RCA
Natural
RCA
950 760 608 475
0 190 342 475
770 577.5 288.75 0
0 192.5 481.25 770
• Smaller is better: choose when goal is to minimize the response. The n
∑i =1 Yi2⎞
(1)
⎠
• Larger is better: choose when goal is to maximize the response. The S/N ratio is calculated as given in Eq.[19]:
1 S / N = −10 log10 ⎜⎛ n ⎝
n
∑i =1
1 ⎞ ⎟ Yi2 ⎠
SF (kg/m3)
SP (L/m3)
fcu MPa
200 200 200 200
54 54 54 54
9 10.125 10.8 13.5
57 55 51 39
strength as RCA (coarse and fine) increases as illustrated in Fig. 4. This could be attributed to the weakness of RCA strength in comparison with the natural aggregate in addition to the bonding between the RCA surface and the new mortar i.e. weaker and double ITZ in comparison to the standard aggregate as previously concluded [21]. This could be flaws and cracks initiation points that speeds up the cracks distribution within the new ITZ and then the concrete body. Regarding RFA, same behavior was noted which can be justified by the increase in the absorbed water by the fine aggregate in addition to its strength in comparison to the natural fine aggregate.
S/N ratio can be calculated as given in Eq.[19]:
1 S / N = −10 log10 ⎛ ⎝n
Water (kg/m3)
(2)
5.2. Flexural strength (fr)
where, S/N is the signal to noise ratio, N is the number of observations while yi is the observed data. The classical experimental design methods are sometimes too complicated and not easy to be used. When the numbers of the studied parameters are large, huge number of experiments should be carried out. Taguchi technique is employed to solve this problem by using a special design of orthogonal arrays to study the entire parameter with minimal number of experiments. Orthogonal arrays are special standard experimental design that requires only a small number of experimental trials to find the main factors that affect the results.
The modulus of rupture (fr) was determined using concrete prisms having dimensions of 100 × 100 × 400 mm. This test was carried out using 140 KN capacity testing machine with two-point loading system according to BS EN 12390-5 [22]. The average test results of three specimens were recorded for each mix to determine the modulus of rupture (fr). Fig. 5 shows the modulus of rupture results at 28 days. It can be noticed that the flexural strength (modulus of rupture (fr)) significantly decreased with increasing RCA content. The reduction in flexural strength could be attributed to the same reasons that affected the compressive strength.
5. Experimental tests and results of mechanical properties 5.1. Compressive strength
5.3. Modulus of elasticity
150 × 150 × 15 mm cubic specimens were used to determine the compressive strength of the studied mixes according to BS 1881: part 116 [20]. The average of test results of three specimens was recorded. The results showed that there is a significant reduction in compressive
Modulus of elasticity test was carried out using standard cylinders of 150 mm diameter and 300 mm height according to ASTM C469-02 [23]. The average test result of three specimens was recorded for each mix. The modulus of elasticity of concrete (Ec) depends on the modulus
Fig. 4. Effect of RCA content on the compressive strength. 37
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Fig. 5. Effect of recycled aggregate content on the flexural strength.
Fig. 6. Effect of RCA content on the modulus of elasticity.
reference beams (S0). The load-deflection curves of the tested beams were used to determine the flexural toughness (I) and flexural stiffness (k). Fractal dimension (D) was used to describe the cracks path and length by employing the image processing technique that would be later explained as suggested elsewhere [24,25]. The density, mechanical properties and water absorption of the selected mixes based on Taguchi approach is presented in Table 7. It is clear that there is a reduction in concrete quality/studied properties in parallel with recycled aggregate incorporation.
of elasticity of concrete constituents especially the modulus of elasticity of aggregate. Thus, Ec decreases which increasing of RCA replacement, as shown in Fig. 6. The reduction in modulus of elasticity can be attributed to the low modulus of elasticity of RCA compared with NA in addition to the other factors that negatively affected the strength as previously mentioned. 6. Results of tested beam specimens Eight steel- reinforced concrete beams were prepared, cast and tested to study their structural behavior and fracture energy parameters under bending load. The test setup and the details of steel reinforcement are illustrated in Fig. 7. The reference beam was cast using SCC with 0% RAC replacement. The other six beams were cast using RASCC with 25%, 56%, and 100% of RCA replacement based on Taguchi analysis as earlier explained representing S, Sop, Sm, and Sw, respectively, see Table 6. This means that the Taguchi technique analysis can be counted as stage one in this study while these selected mixes is stage two that studied in terms of their structural behavior in addition to mechanical properties. These beams were tested until the failure took place. The behavior of the modified beams (Sop, Sm and Sw) was compared with the
6.1. Load-deflection results Table 8 shows the results for all tested reinforced concrete beams. It shows the load at first crack (Pcr), deflection at first crack (δcr), ultimate load (Pu) and the deflection at the ultimate load (δu). The results show that first-crack and ultimate load capacity clearly decreased as RCA content increased. The deflection of the tested beams that incorporated RCA was greater than that for the reference SCC beams. The first crack was noted to be occurred earlier in parallel with the increase of RCA percentage replacement. This could be attributed to the reduction in RCA strength itself in addition coupled with reduction in the bond strength between aggregate interface and mortar in comparison with 38
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Fig. 7. Details of the tested beams.
The flexural stiffness k represents the linear part of the load=deflection curve. It was calculated as the slope (ΔF/Δδ) between 10% and 50% of the ultimate load representing the range before first crack initiation and at the beginning of the elastic-plastic stage based on Turatsinze and Garros [26] as previously used by other researchers [24,27]. The obtained results show that the flexural stiffness decreased with increase the incorporation of RCA, as shown in Table 8. This highly attributed to the same reasons that caused the reduction in concrete strength as previously mentioned.
fiber on SCC [24]. Table 8 shows the results of the flexural toughness indices. Flexural toughness indices were found that the decreased with increasing RCA content for all examined mixes as compared to the reference mix (S0). Where, first crack load, ultimate load (first crack and ultimate moments) decreased with increased the RCA percentage replacement. This could be attributed to the lower strength of RASCC as earlier explained. However, it can be noted that the reduction also increased with increasing the applied load i.e. the reduction percentage in I0 was less than that for I5 which less than I10. This is justifiable if the cracks propagation and the bonding characteristics between the RCA and mortar in addition to other mentioned reasons are considered. It can be concluded that the ultimate deflections of RASCC beams were larger than that of beams made using NA and this increase was found to be in parallel with the increase of RCA replacement. This is logical if the reduction in the flexural toughness indices considered where all these indices decreased with RCA incorporation. Such behavior could be attributed to the two ITZ existent that could speeds up the cracks growth. Contradictory results were noted for the first crack deflection in comparison with the ultimate load deflection where it increased with RCA incorporation as seen in Table 8. Such behavior could be due to the stress relaxation. The ratio of the ultimate deflection to the first crack deflection is defined as ductility displacement (µ) which is noted to be continually decreased with RCA increased. This means that the absorbed energy decreases with RCA increase which will be later discussed.
6.3. Flexural toughness (I)
6.4. Fractal dimension (D) test and results
The area under load-deflection curve is commonly known as flexural strength. Whereas, the flexural toughness indices (I5, I10 and I20) are calculated based on ASTM C 1018 [28] by dividing the area up to a specified deflection (3δ, 5.5δ and 10.5δ) to the area up to first crack (δ). Fig. 9 shows that I5, I10 and I20 corresponding to 3δ, 5.5δ and 10.5δ, respectively. These indices were previously calculated and used to study the effect of incorporating rubber aggregate on the flexural toughness of rubberized self-compacting concrete [29], and also the effect of steel
The fractal dimensions for all tested beams were calculated by employing image processing technique that was recently utilized to determine the tortuosity of the cracks of steel fibers reinforced concrete beams [24]. These beams were painted by white color to easily recognize the cracks especially at the mid-third that was photographed as no significant cracks have been noted elsewhere, Digital camera of 14 Mega pixels was used in this investigation. The beams were photographed during the test with increasing the applied load until reaching the ultimate load. ImageJ v1.44p software was used to analysis the
Table 7 Mechanical and hardened properties of selected mixes. Mix.
Density (kg/m3)
fcu (MPa)
fr (MPa)
Ec (GPa)
Water absorption (%)
S0 Sop Sm Sw
2412 2389 2271 2186
57 55 51 39
5.50 5.25 5.00 4.50
32.0 31.5 28.5 26.0
2 3 5 7
the NA. The roughness of the RCA that could increase the mechanical interaction could compensate for the reduction in strength due to the use of RCA as earlier explained. Fig. 8 shows the load-deflection curve for the examined beams that is going to be used in calculating the flexural stiffness and toughness. 6.2. Flexural stiffness k
Table 8 Load-deflection results of the tested beams. Mix
Pcr (kN)
δcr (mm)
Pu (kN)
δu (mm)
µ
k (KN/mm)
I5
I10
I20
S0 Sop Sm Sw
27 24 23 19
1.97 2.23 2.42 2.74
78 75 72 70
21.2 19.09 17.24 15.07
10.76 8.56 7.12 5.50
13 12.15 11.2 9.89
9.96 8.53 8.1 7.72
26.3 22.83 21.4 19.6
62.1 55.9 51.3 48.3
39
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Fig. 8. Load-deflection curve of the investigated mixes.
Table 9 Fractal dimension of the tested beams. Mix
S0 Sop Sm Sw
D 10 kN
20 kN
30 kN
40 kN
50 kN
60 kN
70 kN
– – – –
– – – 0.542
0.74 0.78 0.82 0.95
0.88 0.92 0.98 1.03
0.95 0.98 1.05 1.10
1.07 1.12 1.13 1.17
1.17 1.15 1.14 1.12
Fig. 9. Flexural toughness index calculation [28].
images after converting them from colored to binary images in order to obtain the fractal dimension. The photographed part of the tested beams were cropped into 900 × 300 pixels i.e. 300 mm × 120 mm followed by applying the brightness/contrast feature to manually adjust the bracket of upper/lower limits of the grey scale histogram. Then, 1px median filter was applied followed by manually removing the noise using the ‘eraser’ tool. D was calculated for all tested beams for two faces (front and back) at specified loads (10, 20, 30, 40, 50, 60 and 70 kN) for each beam as shown in Table 9 employing Image Processing Technique. It represents the linear regression coefficient of Log count vs. Log box size. The fractal box count tool installed in ImageJ software tool v1.44p was used with the following box sizes: 2, 3, 4, 6, 8, 12, 16, 32 and 64. Fig. 10 shows a binary image of the cracks of the reference (S0) beam, as an example. This technique was deeply explained elsewhere [24,29]. Table 9 shows that with 10 and 20 kN applied load, there was no visible cracking except for Sw mix (highest RCA replacement) while at 30 kN
Fig. 10. An example of binary image (A) and the fractal dimension calculation (B) for S0 beam frnt face.
the cracks clearly appeared. Indeed, D value increased with incorporating RCA when the applied load increased from up to 60 kN, the rate of this increment almost decreased with increasing of applied load. Additionally, this rate increased with increasing of RCA percentage replacement. This behavior contradicts the flexural toughness indices and flexural stiffness as Table 8 demonstrates. This could be attributed to the fewer number of resulted cracks with applying the load for the mixes incorporating RCA coupled with elongating their paths. Namely, increase their tortuosity up to a certain applying load with increase the mouth opining (width). Such behavior could be explained by the hypothesis of this concrete has 40
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Gd = Gf a
two ITZ and the new ITZ is weaker than that for concrete made using NA. ITZs could be working as cracks attraction points leading to decrease the number of cracks and increase the tortuosity and crack width. This finding confirms the results of flexural toughness as less energy was absorbed i.e. I values decreased as the absorbed energy related to cracks number and tortuosity. When the applying load exceeds 60 kN, the D value starts to marginally decrease which could be attributed to the stress relaxation. Generally, this contradictory in the results of flexural toughness indices and D value could also understandable if the methodology of calculating them is considered. The D value is calculated based on image processing technique that considered the number and the tortuosity of the cracks while toughness indices are calculated using load-deflection curve as previously mentioned. To deeply understand this point, more research needs to be carried out
(3)
where Gf is the concrete fracture energy at observation scale and a the “Euclidean length” (the height of the cross section) that moderated based on fractal theory as explained elsewhere [31]. The following equations that proposed by [24] is used as follow, where by substituting: 1−d
δ a∗ = a ⎛ ⎞ ⎝a⎠
,
(4)
Eq. (1) becomes:
δ 1−D Gd = Gf a∗ = Gf a ⎛ ⎞ ⎝a⎠
(5)
Or
Gd δ 1−D = a⎛ ⎞ Gf ⎝a⎠
6.5. The surface cracks fracture energy parameters
(6)
δ is the grid size that suggested to be equal to 5 mm according to Guo et al. [34]. This was attributed to the size of used fine aggregate in the studied concrete that is almost having same size (4.75 mm). The calculated value of Gd was considered as an indicator to the absorbed/
It was previously stated that fracture energy governs the behavior of reinforced concrete structural members particularly in tension when the concrete as a matrix is resisting the load alone [30]. In another words, fracture energy should be studied in addition to strength when cracks initiation/propagation is important i.e. structural behavior of concrete. Therefore, in this study, fracture energy parameters were studied as the concrete matrix would be affected by replacing NA with RCA due to the differences in their characteristics as previously mentioned in results of mechanical strength section. Darwin et al. [30] defined fracture energy Gf as the required energy to form cracks/micro cracks per unit area. It is known that cracks/ micro-cracks initiate due to strain localization at weak points/flaws in a concrete matrix. After crack formation, crack tip would be created leading to the creation of fracture process zone that represent the region ahead of crack tip which is important in concrete nonlinear fracture mechanics. Once the cracks propagate/widen (open) due to increase tensile strength (ultimate tensile stress), the energy is dissipated leading to stress release. It is important to understand that the direction plane of stress is perpendicular to the fracture area. It was stated that the actual crack length plus the fracture process zone is known as the effective crack length [31–33]. In concrete, it is highly that the single crack that caused fracture is affected by other cracks/micro-cracks that are distributed within the concrete matrix by absorbing/dissipating energy as a result of their tortuosity i.e. propagation and elongation. Image processing technique was used in this investigation to quantify the surface fracture energy in addition to calculate the fracture energy parameters. The fracture energy was previously calculated following the guidelines established by RILEM (1985) that used closedloop flexural testing machine under crack mouth opening displacement (CMOD) control, see David et al. [30] for more explanation and details. In this study, not only the main crack that caused concrete fracture was considered but also the other formed cracks in the middle third of the tested beams (Mode I) was taken into account in the calculations. This approach simulates the reality more precisely than the traditional one as it takes the effect of the most visible cracks/micro cracks on the fracture energy. The fracture energy parameters were calculated based on previous studies [31,34,35]. Namely, they are a combination of different approaches in order to consider the previous literature as suggested elsewhere [24].
Gf
dissipated energy due to RCA incorporation that affected the cracks propagation, width, and elongation. 6.5.2. Fracture energy based on fracture profile Fracture energy was classically/mathematically defined as follow in order to be calculated:
Gf =
W A
(7)
W represents the total dissipated work while represent the Euclidian fracture surface (b × d), where d = a in this case. The work that done by the applied external load on a structural member was suggested to represent the area under the load-deflection curve while Af represents the area of rupture [34]. The total dissipated work due to the applied load by the surface of rupture is calculated as follow:
δ 1−D W = 2γsAf = 2γsba ∗ = 2γsb ⎡a ⎛ ⎞ ⎤ ⎢ ⎝a⎠ ⎥ ⎣ ⎦
(8)
where γs is the density of the surface energy of concrete, while the fracture energy is as follow:
Gf =
W δ 1−D = 2γs ⎛ ⎞ A a ⎝ ⎠
(9)
Or
Gf γs
1−D
δ = 2⎛ ⎞ a ⎝ ⎠
(10)
6.5.3. Fracture energy based on classical definitions Hillerborg [36] correlated the fracture area with a loaded concrete member (beam) Failure. It was concluded that the required work to cause failure to the tested concrete beam is equal to the required energy to create the rupture area. This relation can be mathematically expressed as follow:
Gf = 6.5.1. Fracture energy based on surface cracks dimension (mm) The surface fracture energy is highly depending on surface micro/ macro-cracks tortuosity that responsible for dissipating the fracture energy. This, in turn, depends on the tensile stress [34] in addition to concrete ingredients, strength, external conditions, etc. Carpinteri and Spagnoli [35] suggested a formula to calculate the total dissipated energy Gd at the crack surface per unit thickness as follow:
W0 −Ws b (d − a0 )
(11)
The failure work is the net of the work carried out by the external applied load (W0) minus the done work by a concrete member selfweight (Ws). The sectional dimensions of a concrete member represent by b and d i.e. width and depth, respectively. Petersson [37] stated that the resultant work due to applying an external load mathematically represents the area under the stress-strain curve whereas the done work 41
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S.I. Mohammed and K.B. Najim
by the weight of the concrete beam itself is approximated by 2P0u0. P0 represents the external load that applying to a concrete member that causes the bending moment which equivalent to the moment results by the self-weight of this member. It is calculated as P0 = mg (2S − L)/2S while u0 is displacement at the ultimate load. The tested beam mass and the acceleration of gravity respectively represented by m and g whereas S is the distance between supports, L is the clear span, and the dimension of the initial notch a0 was considered zero in this study. Eq. (10) was used by Darwin et al. [30] to determine the fracture energy Gf.
Gf = (W0 + mgδu )/ A
Table 11 Comparison of classical fracture energy values. Mix
S0 Sop Sm Sw
Gf, J/m2 (Eq. (9))
Gf, J/m2 (Eq. (10))
Gf, J/m2 (Eq. (11))
@ first crack
Failure
@ first crack
Failure
@ first crack
Failure
3507 3524 3666 3421
109,820 94,627 81,283 69,731
3593 3620 3765 3529
110,741 95,445 81,979 70,322
10,638 10,704 11,132 10,412
330,720 285,000 244,800 210,000
(12) under certain loading. Again, this behavior is in agreement with fractal dimension results as presented in Table 9. The presented results of fracture energy (Gf) that calculated based on the classical equations (Section 5.5.3) shows that Gf calculated from Eqs. (9) and (10) were very closed for both first crack and ultimate loads. There was a gradual decrease in Gf with increasing RCA content, as seen in Table 11, which resulted from the ultimate load. This is understandable if the strength of the mixes and the RCA itself taken into account in addition to the ITZ that important in initiating the micro cracks/cracks, as mentioned previously. For the applied load causing first crack, Sm mix provided marginal increase in Gf in comparison with the other tested mixes that could be attributed to the stress relaxation. Eq. (11) provided highly overestimated values of Gf in comparison with Eqs. (9) and (10) for both first crack and ultimate loads. This could be attributed to the traditional approach in calculating Gf i.e. does not considering the fractal dimension that modified to be used in calculating Gf based on fractal theory. Nevertheless, the trend of reduction in Gf with RCA increase is clear. This means that the classical definition of fracture energy was in agreement with the deterioration in strength, stiffness, toughness, for the tested beams with RCA incorporation. However, only the D values were increased with RCA incorporation which can be attributed to the existence of two ITZ that could increase the cracks tortuosity within a local concrete matrix but for less cracks number. Finally, based on the results of the tested concrete mixes and beams, RCA could be successfully used in producing concrete to be used in structural applications. More experimental investigations are required to confirm these results especially the correlation between flexural behavior, fractal dimension with the absorbed energy. Also, studying the durability of the produced concrete should be considered which is ongoing.
where W0, m, g, are respectively represented the area under the loaddeflection curve, mass of the tested concrete beam, the gravity acceleration. δu is the failure mid-span deflection while A is the cross-sectional area above the notch of the tested beam. Kazemi et al. [38] employed Eq. (11) that taken from Bazant and Planas 1997 [39] to calculate the fracture energy of a concrete beam. Where, δu is the maximum deflection at mid-span of the tested beams and P0 is the external applied load.
Gf = (W0 + 2Pδu )/ A
(13)
6.6. Results and discussion of fracture energy parameters In concrete, the dissipation of energy depends on many factors like concrete quality, applied load, incorporation of fibers and other reinforcement, member geometry, etc. However, cracks that resulted from different causes are the main factor that controls the concrete ductility/energy dissipation. Fractal dimension of the surface cracks has recently used [24] in quantifying/determining the crack tortuosity that could be an indicator of energy absorption. Table 10 demonstrates fracture energy dimensions (Gd/Gf) of the surface cracks continually increased with applied load increasing (stress increased). Same behavior was noted with Recycled Aggregate (RA) increase for all applied loads except when the loading reached 70 kN. Such behavior could be justified as with load increasing the cracks propagating and elongating in addition to increase the cracks width which leads to absorb the applied energy. This confirms the D results as seen in Table 9, as explained in Section 6.4. It is stated that when D increases the cracks are elongating and propagating [24]; however, the crack opening mouth increase also increase the absorb energy before the failure. This means that the applied load (energy) should be increased in order to cause failure to the tested concrete member in addition to the gained stress relaxation. Same results was noted for the dimensionless fracture energy parameter i.e. fracture profile (Gf /ɤs), as demonstrated in Table 10. It shows that a linear relationship between (Gd /Gf) and (Gf /ɤs). This previously attributed to their dependency on the modified beam height (a*) based on the fractal theory that presented in Eq. (2), Section 5.5, for steel fiber self-compacting concrete [24]. However, steel-fibers works to elongate/propagate the cracks path while the RCA could also doing same job but in different mechanisms as previously explained. Due to the questionable weakness between the RCA and the matrix as a result of the double ITZ and other previously mentioned reasons, the cracks could propagate around the RCA rather that concrete matrix
7. Conclusions It was concluded that the incorporation RCA caused a decrease in the mechanical strength including compressive, splitting and flexural strengths in addition to modulus of elasticity that slightly decreased with adding RCA. However, this reduction did not affect the potential use of the tested concrete as a structural concrete in terms of the required mechanical strength. Where, even with 100% RAC replacement the compressive strength is 39 MPa which is adequate to be structurally used. The structural behavior of the tested beams that containing RCA was noted to be deteriorated in comparison with the SCC beams. The first-crack load and ultimate load capacity decreased with RCA
Table 10 Surface cracks fracture energy dimension (Gd /Gf) mm, and fracture energy based on fracture profile (Gf /ɤs) values with loading stages. Mix.
S0 Sop Sm Sw
30 kN
40 kN
50 kN
60 kN
70 kN
Gd/ Gf
Gf / γs
Gd/ Gf
Gf / γs
Gd/ Gf
Gf / γs
Gd/ Gf
Gf / γs
Gd/ Gf
Gf / γs
61.94 70.97 81.32 126.54
0.82 0.94 1.08 1.68
99.73 114.26 140.13 166.11
1.32 1.52 1.86 2.21
126.54 140.13 177.8 210.76
1.68 1.86 2.37 2.81
190.32 225.6 233.4 267.42
2.53 3.00 3.11 3.56
267.42 249.83 241.48 219.54
3.56 3.33 3.21 3.00
42
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increasing. This coupled with the increasing of the deflection that corresponding to the first-crack and ultimate loads. However, these decreases could be qualitatively counted within the permissible range from the structural engineering point of view. The results of this study show that both the tested concrete mechanical properties and the tested behavior of the studied beams were deteriorated in parallel with RCA incorporation. Nevertheless, the examined RCA can be used in producing structural Self-compacting concrete as the strength/structural behavior satisfied the requirements to be structurally used. However, more research is needed to be carried out in order to examine the use of RCA that produced from the demolished building and security concrete precast panels to generalize this conclusion in addition to test the durability of the produced concrete which is ongoing.
2005;21(6):845–50. [18] Fowlkes WY, Creveling CM, Derimiggio J. Engineering methods for robust product design: using Taguchi methods in technology and product development. MA: Addison-Wesley Reading; 1995. [19] Yokoyama, Y., Taguchi methods: design of experiments. Vol. 4. 1993: Amer Supplier Inst. [20] B.S. 1881:Part 116 / (1983), “Methods for Determination of Compressive Strength of Concrete Cubes”. 1983. [21] Nassar R, Soroushian P. Strength and durability of recycled aggregate concrete containing milled glass as partial replacement for cement. Constr Build Mater 2012;29(2):368–77. [22] BSEN12390-5, Testing hardened concrete. Flexural strength of test specimens. British Standard Institution, 2009. [23] Astm C. 469. Standard test method for static modulus of elasticity and Poisson’s ratio of concrete in compression. West Conshohochen: Am Soc Test Mater 2001. [24] Najim KB, Saeb A, Al-Azzawi Z. Structural behaviour and fracture energy of recycled steel fibre selfcompacting reinforced concrete beams. J Build Eng 2018;17:174–82. [25] Najim KB, Hall MR. Crumb rubber aggregate coatings/pre-treatments and their effects on interfacial bonding, air entrapment and fracture toughness in self-compacting rubberised concrete (SCRC).. Mater Struct 2013;46(12):2029–43. [26] Turatsinze A, Garros M. On the modulus of elasticity and strain capacity of selfcompacting concrete incorporating rubber aggregates. Resour Conserv Recycl 2008;52(10):1209–15. [27] Najim KB, Hall M. Mechanical and dynamic properties of self-compacting crumb rubber modified concrete.. Constr Build Mater 2012;27:521–30. [28] Standard, A., C1018. Standard Test Method for Flexural Toughness and First-Crack Strength of Reinforced Concrete (Using Beam with Third-Point Loading. ASTM International, West Conshohocken, PA, 1997. [29] Hall MR, Najim, KB. Structural behaviour and durability of steel-reinforced structural Plain/Self-Compacting Rubberised Concrete (PRC/SCRC). Constr Build Mater 2014;73:490–97. [30] Darwin David, et al. Fracture energy of high-strength concrete. ACI Mater J 2001;98(5). [31] Gdoutos EE, Konsta-Gdoutos MS, Danoglidis PA. Portland cement mortar nanocomposites at low carbon nanotube and carbon nanofiber content: a fracture mechanics experimental study. Cem Concr Compos 2016;70:110–8. [32] Jenq Y, Shah S. Two parameter fracture model for concrete. J Eng Mech 1985;111(10):1227–41. [33] Park K, et al. Prediction of interfacial fracture between concrete and fiber reinforced polymer (FRP) by using cohesive zone modelling. Cem Concr Compos 2015;63:122–31. [34] Guo L, et al. Study on the flexural fatigue performance and fractal mechanism of concrete with high proportions of ground granulated blast-furnace slag. Cem Concr Res 2007;37(2):242–50. [35] Carpinteri A, Spagnoli A. A fractal analysis of size effect on fatigue crack growth. Int J Fatigue 2004;126(2):125–33. [36] Hillerborg A. The theoretical basis of a method to determine the fracture energy GF of concrete. Mater Struct 1985;18(4):291–6. [37] Petersson, PE. Crack growth and development of fracture zones in plain concrete and similar materials. Technical Report, Lund Institute of Technology, 1981. LUTVDG/TVBM-1006. [38] Kazemi MT, Fazileh F, Ebrahiminezhad MA. Cohesive crack model and fracture energy of steel-fiber-reinforced-concrete notched cylindrical specimens. Mater Civ Eng 2007;19(10):884–90. [39] Bazant PZ, Planas J. Fracture and size effect in concrete and other quasibrittle materials 1997, USA, Florida: CRC Press. [40] BIBM C, ERMCO E, EFNARC (2005) The European guidelines for self-compacting concrete. Specification, Production and Use.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] Neville AM. Properties of concrete Vol. 4. London: Longman; 1995. [2] Alexander M, Mindess S. Aggregates in concrete. London: Taylor & Francis; 2005. [3] Yehia S, et al. Strength and durability evaluation of recycled aggregate concrete. Int J Concr Struct Mater 2015;9(2):219–39. [4] Gomes P, et al. Comminution and sizing processes of concrete block waste as recycled aggregates. Waste Manage 2015;45:171–9. [5] World Commission on Environment Development, Bruntland Commission), Our common future. 1987, Oxford University Press Oxford. [6] Tam VW, Tam CM. Re-use of construction and demolition waste in housing developments. 2008. [7] BS EN 206-1-Part: 2, Concrete complementary. 2006, London/UK: British Standards, Institution. [8] Thomas C, et al. Evaluation of the fatigue behavior of recycled aggregate concrete. JCLP J Cleaner Prod 2014;65:397–405. [9] Okamura H, Maekawa K, Ozawa K. Development of self-compacting high performance concrete. Doboku Gakkai ronbunshu. 1. J Struct Mech Earthquake Eng 1995;522:23. [10] Assaad J, Khayat KH, Daczko J. Evaluation of static stability of self-consolidating concrete. ACI Mater J 2004;101:207–15. [11] Pandaa KC, Balb, PK. Properties of self compacting concrete using recycled coarse aggregate in Chemical, Civil and Mechanical Engineering Tracks of 3rd Nirma University International Conference on Engineering (NUiCONE 2012) 2012, ELSEVIER: Procedia Engineering (2013) Ahmedabad, India. p. 159–164. [12] Manzi S, Mazzotti C, Bignozzi MC. Self-compacting concrete with recycled concrete aggregate: study of the long-term properties. Constr Build Mater 2017;157:582–90. [13] IQstandardNo.45, Portland Cement I.s. Indtitution, Editor. 1984: Baghdad, Iraq. [14] IQNo.45, Natural aggregate that used in construction and concrete Iraqi Standard Institution 1984. [15] ASTM C494, Standard Specification for Chemical Admixtures for Concrete. 2004, ASTM International West Conshohocken, PA. [16] European Guidelines for Self-compacting Concrete: specification, production and use. 2005. [17] Basavarajappat S. Wear studies on metal matrix composites: a Taguchi approach
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Structures journal homepage: www.elsevier.com/locate/structures
Mechanical strength, flexural behavior and fracture energy of Recycled Concrete Aggregate self-compacting concrete Saif I. Mohammed, Khalid B. Najim
T
⁎
Civil Engineering Department, University Of Anbar, University Campus, Ramadi, Iraq
A R T I C LE I N FO
A B S T R A C T
Keywords: Recycled Concrete Aggregate (RCA) Self-compacting concrete (SCC) Taguchi method Structural behavior Mechanical properties Fracture energy
The possibility of using Recycled Concrete Aggregates (RCA) to produce self-compacting concrete (RASCC) was investigated. Three parameters that are recycled coarse aggregate, recycled fine aggregate and Superplasticizer were studied with four different percentages of replacement. Compressive, flexural strength and modulus of elasticity tests were carried out in order to investigate the mechanical properties of the studied mixes. Based on the result of Taguchi analyses for the compressive strength, four mixes were selected to be in-depth studied in terms of flexural behavior. Eight 100 × 150 × 1200 mm reinforced concrete beams were tested under two-point loading system. Flexural stiffness (k) and flexural toughness (I) were determined. The cracks pattern, propagation and their tortuosity were determined by utilizing of image processing technique using the fractal theory. Based on the experimental results, it was found that hardened properties and the flexural stiffness and toughness generally decreased with RCA incorporation. However, SCC has 39 MPa compressive strength still achievable even with 100% RCA replacement. Surface cracks fracture energy parameters were also determined. It was found that the classical definition of fracture energy was in agreement with the deterioration in strength, stiffness, toughness, for the tested beams with incorporating RCA.
1. Introduction: Concrete is the most widely used construction material around the world. The demand for concrete was rapidly increased in the last fifty years as a result of the population growth that, of course, has been leading to increase the need to implement residential and infrastructures projects. This leads to increase the demand for the construction raw materials, especially the aggregate that counts about 70–80% of the total concrete volume [1,2]. It was recently found that the consumption of aggregate in construction industry reached 48.3 billion metric tons in 2015 worldwide. This consumption annually increases by about 5% while the highest consumption was found to be in Asia and Pacific [3], as shown in Fig. 1. By considering this clear increase in aggregate consumption, natural aggregate (NA) could no longer meet the needs of construction industry in some counties in terms of both quantity and quality [4]. One the other hand, the attention towards the negative environmental impact of construction industry that consumes massive quantities of virgin materials such as aggregate has recently increased [5]. Therefore, alternative aggregate resources should be found. Recycled Concrete Aggregate (RCA) could be a part of the solution because it provides an alternative aggregate to natural aggregates (NA). ⁎
Additionally, it helps to dispose of construction and demolition waste (C&DW), reduces landfill space, diminishes environmental pollution, decreases transport costs and protects environmental balance [6]. RCA is defined, according to the British standard [7], as “the aggregates derived from concrete waste only”. In Germany after World War II, for example, RCA was used in reconstructing the destroyed buildings [8]. In Iraq, there are massive quantities of the concrete rubble generated from the repeated wars that happened during the last three decades, as shown in Fig. 2-A. Also, the concrete security barriers (precast concrete walls) that being used for security purposes (Fig. 2-B) need to be disposed after enhancing the security situation recently. The waste concrete that produced from testing the concrete specimens in the concrete laboratories at the universities and the research and quality control centers over the world needs to be considered by recycling these materials in a beneficial matter. The reuse of these waste materials would definitely contribute in achieving the sustainable development goals by processing them to aggregate (RCA) to be used in producing concrete meanwhile reducing the use of the raw materials. Self-compacting concrete (SCC) is defined as sophisticated version of the high performance concrete. It was discovered in Japan, at the University of Tokyo in early of 1980s. The purpose was to produce
Corresponding author. E-mail address: [email protected] (K.B. Najim).
https://doi.org/10.1016/j.istruc.2019.09.010 Received 13 August 2019; Received in revised form 20 September 2019; Accepted 20 September 2019 2352-0124/ © 2019 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
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S.I. Mohammed and K.B. Najim
2. The used materials and the studied mixes Ordinary Portland Cement (OPC) confirms the Iraqi specification IQ No. 5/1984 [13] with specific gravity of 3.14 and specific surface area 2600 cm2/gm was used in this investigation. The natural coarse aggregate (NCA) was crushed angular shape with nominal maximum size of (5–14) mm. The recycled concrete coarse aggregate (RCA) was produced by crushing the tested concrete specimens at the laboratories of university of Anbar-Iraq. Thousands of specimens are annually tested at this laboratory whether for undergraduate/postgraduate students or for the infrastructure projects for quality control goal. Around 700 (100 × 100 mm) concrete cubes having compressive strength ranged between 25 and 30 MPa were crushed after storing them at the laboratory conditions for a year. These specimens are crushed using a crusher to the maximum of 14 mm and minimum 4.75 mm particles size. The particles that passing through 4.75 mm sieve were used as recycled fine aggregate (RFA) in this investigation in addition to the natural fine aggregate. The sieve analyses of the natural and recycled used aggregate were presented in Fig. 3. Table 1 shows the physical properties of used aggregate. The aggregate tests were carried out based on IQ No.45 [14]. Silica fume (SF) powder having 120% strength activity index at 7 days was utilized as a mineral admixture. The SF was blended with tested mixes by 12% wet of cement content based on the manufacturing data sheet and the literature. Sika ViscoCrete-5930 was used as a chemical admixture to satisfy SCC requirements. It is free of chlorides and compatible with ASTM C494 [15] types G and F. The percentage replacements of RCA are presented in Table 2.
Fig. 1. Demand on construction aggregates worldwide [3].
durable concrete that is used in heavily reinforced and complicated concrete structures. The durability problems in concrete structures were noted in Japan in the 1970s due to lack of skilled workers [9]. It is considered as a latest innovation in concrete technology due to its numerous advantages over conventional concrete. It is a special type of concrete that spreads through the congested reinforcement, consolidates under its own weight without any external vibration. It provides excellent filling ability, passing ability and exhibits good segregation resistance [10]. These properties could be achieved by using high powder contents (cement and mineral additives and admixtures such as fly ash, silica fume, and limestone, etc.) in addition to Superplasticizer. The use of RCA to produce recycled aggregate SCC (RASCC) could combine the advantages of using RCA and SCC. However, this type of concrete should be deeply investigated. Many previous studies were carried out to investigate the use of recycled aggregate that produced from demolishing old building in producing SCC. Banda [11] and Bal investigated the effect of using different quantities of RCA that produced from a demolished building about 25 years old on the properties of SCC. The results were compared with conventional concrete (100% natural aggregate (NA) concrete) in terms of mechanical properties (compressive strength, flexural strength, and splitting tensile Strength). The percentages of replacement of NA with RCA were 10%, 20%, 30% and 40% by weight. It was found that the mechanical properties decreased with increasing the percentage of replacement of RCA. However, it was recommended that RCA could be efficiently used with up to 30% replacement of RCA. Manzi et al. [12] found that structural SCC with up to 40% of fine and coarse recycled aggregate could be produced with no significant difference than conventional concrete. Structural concrete in terms of compressive, flexural, and tensile strengths in addition to elastic modulus were produced even with 100% replacement due to the improvement in the microstructure. The aim of this research is to investigate the use of RCA that produced from recycling the tested/crushed concrete specimens in producing RASCC as a first step before studying the other waste concrete as previously mentioned. Structural behavior under flexural loading of this concrete with different RCA contents in addition to flexural stiffness and toughness of reinforced concrete beams were studied. The cracks pattern, propagation and their tortuosity were also studied. This was carried out by employing the image processing technique using the fractal theory while the previous investigations used traditional methods. Surface fracture energy parameters were also determined for the studied mixes via the reinforced concrete beams.
3. Design of experiments and mix proportions All prepared mixes of SCC were designed based on the European guidelines of EFNARC 2005 [16]. Trial and error approach was used to find out the proportions that meet the SCC requirements. The mix proportions of the reference mix with a target compressive strength (fcu) of 55 MPa are presented in Table 3. After achieving the reference mix, Taguchi method of experiment design was set considering four level of replacement on three parameters. These parameters are coarse and fine aggregate in addition to super plasticizer as demonstrated in Table 4. This creates standard Taguchi orthogonal array which are special standard experimental design requires a small number of experimental trials to find the main factor that affect the output. To consider the aforementioned parameters, 64 experiments are required. However; Taguchi approach suggests L16 orthogonal array based on Minitab 17 software that was used in this study. In other words, only 16 instead of 64experiments are required to sufficiently consider and optimize these parameters. Tables 4 and 5 respectively show the standard Taguchi orthogonal array L16 of the tested mixes and their proportions. It was found that that the recycled coarse aggregate has more effect than other two parameters followed by the recycled fine aggregate. SP has a marginal effect on the studied properties within the range of the tested SP content. Based on Taguchi analyses of the strength at 28 days four mixes were selected to study the structural behavior and fracture energy parameters of the selected mixes. These four mixes are; reference mix (S0), the mix provided the highest strength (Sop), the mix provided the mean strength (Sm) and the mix provided the lowest strength (SW). The mix proportions for the selected mixes are shown in Table 6. 4. Taguchi technique It is a powerful tool developed by Genichi Taguchi (1950) to design high quality systems [17]. It is used to design the experiment by creating a standard orthogonal array in order to taking into account the effect of a number of factors on the targeted value in addition presenting the experiments plan. This allows collecting the necessary data 35
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Fig. 2. The concrete waste.
Table 4 The Standard Taguchi orthogonal array L16.
Fig. 3. Particle size distribution of natural and recycled aggregates.
Experiment number
Recycled coarse aggregates
Recycled fine aggregates
Superplasticizer
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16
25% 25% 25% 25% 50% 50% 50% 50% 75% 75% 75% 75% 100% 100% 100% 100%
20% 30% 40% 50% 20% 30% 40% 50% 20% 30% 40% 50% 20% 30% 40% 50%
2.25% 2.5% 2.75% 3% 2.5% 2.25% 3% 2.75% 2.75% 3% 2.25% 2.5% 3% 2.75% 2.5% 2.25%
Table 1 The physical properties of aggregate. Physical properties
SO3 % Specific gravity Absorption % Materials finer than 75 µm% Attached mortar %
Coarse aggregate
Fine aggregate
Natural
Recycled
Natural
Recycled
0.03 2.67 0.68% – –
– 2.51 2.01% – 43%
– 2.7 0.8% 1.5% –
– 2.48 6.53% – –
Table 5 The proportions of RCA mixes (kg/m3). Mix
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16
Table 2 The parameter and their levels of replacement. Parameter and Unit
Recycled Coarse Aggregate Recycled Fine Aggregate Superplasticizer
Symbol
RCA RFA SP
Parameter levels Level 1
Level 2
Level 3
Level 4
25% 20% 2.25%
50% 30% 2.5%
75% 40% 2.75%
100% 50% 3.0%
Table 3 Mix proportions of reference mix. Materials
(kg/m3)
Typical range by mass (kg/m3) European Guidelines 2005[40]
Cement (kg/m3) Fine aggregate (kg/m3) Coarse Aggregate (kg/m3) Water (kg/m3) Silica fume (kg/m3) Superplasticizer (kg/m3)
450 950 770 200 54 9
380–600 48–55% of total aggregate weight 750–1000 150–210 —— ——
CA
FA
NCA
CRA
NFA
RFA
577.5 577.5 577.5 577.5 385 385 385 385 192.5 192.5 192.5 192.5 0 0 0 0
192.5 192.5 192.5 192.5 385 385 385 385 577.5 577.5 577.5 577.5 770 770 770 770
760 665 570 475 760 665 570 475 760 665 570 475 760 665 570 475
190 285 380 475 190 285 380 475 190 285 380 475 190 285 380 475
Water
Cement
S.F
SP (l/m3)
200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200
450 450 450 450 450 450 450 450 450 450 450 450 450 450 450 450
54 54 54 54 54 54 54 54 54 54 54 54 54 54 54 54
10.125 11.25 12.375 13.5 11.25 10.125 13.5 12.375 12.375 13.5 10.125 11.25 13.5 12.375 11.25 10.125
to determine which factor affects the experiment more than others with a minimum amount of experiments to reduce the required time and resources. The first concept of Taguchi that should be discussed is the signal to noise ratio “S/N”. The term of “Signal” represents the desired value for the output characteristic while the term of “Noise” represents the undesired value for the output characteristic. S/N offers a measurable product or process characteristic to deviate from its target value [18]. The S/N ratio depends on the characteristics quality of the product/process to be optimized. Usually, there are two categories of the performance characteristics in the analysis of the S/N ratio: 36
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Table 6 Mix proportions for the selected mixes. Mix
S0 Sop Sm Sw
Cement (kg/m3)
450 450 450 450
Fine aggregate (kg/m3)
Coarse Aggregate (kg/m3)
natural
RCA
Natural
RCA
950 760 608 475
0 190 342 475
770 577.5 288.75 0
0 192.5 481.25 770
• Smaller is better: choose when goal is to minimize the response. The n
∑i =1 Yi2⎞
(1)
⎠
• Larger is better: choose when goal is to maximize the response. The S/N ratio is calculated as given in Eq.[19]:
1 S / N = −10 log10 ⎜⎛ n ⎝
n
∑i =1
1 ⎞ ⎟ Yi2 ⎠
SF (kg/m3)
SP (L/m3)
fcu MPa
200 200 200 200
54 54 54 54
9 10.125 10.8 13.5
57 55 51 39
strength as RCA (coarse and fine) increases as illustrated in Fig. 4. This could be attributed to the weakness of RCA strength in comparison with the natural aggregate in addition to the bonding between the RCA surface and the new mortar i.e. weaker and double ITZ in comparison to the standard aggregate as previously concluded [21]. This could be flaws and cracks initiation points that speeds up the cracks distribution within the new ITZ and then the concrete body. Regarding RFA, same behavior was noted which can be justified by the increase in the absorbed water by the fine aggregate in addition to its strength in comparison to the natural fine aggregate.
S/N ratio can be calculated as given in Eq.[19]:
1 S / N = −10 log10 ⎛ ⎝n
Water (kg/m3)
(2)
5.2. Flexural strength (fr)
where, S/N is the signal to noise ratio, N is the number of observations while yi is the observed data. The classical experimental design methods are sometimes too complicated and not easy to be used. When the numbers of the studied parameters are large, huge number of experiments should be carried out. Taguchi technique is employed to solve this problem by using a special design of orthogonal arrays to study the entire parameter with minimal number of experiments. Orthogonal arrays are special standard experimental design that requires only a small number of experimental trials to find the main factors that affect the results.
The modulus of rupture (fr) was determined using concrete prisms having dimensions of 100 × 100 × 400 mm. This test was carried out using 140 KN capacity testing machine with two-point loading system according to BS EN 12390-5 [22]. The average test results of three specimens were recorded for each mix to determine the modulus of rupture (fr). Fig. 5 shows the modulus of rupture results at 28 days. It can be noticed that the flexural strength (modulus of rupture (fr)) significantly decreased with increasing RCA content. The reduction in flexural strength could be attributed to the same reasons that affected the compressive strength.
5. Experimental tests and results of mechanical properties 5.1. Compressive strength
5.3. Modulus of elasticity
150 × 150 × 15 mm cubic specimens were used to determine the compressive strength of the studied mixes according to BS 1881: part 116 [20]. The average of test results of three specimens was recorded. The results showed that there is a significant reduction in compressive
Modulus of elasticity test was carried out using standard cylinders of 150 mm diameter and 300 mm height according to ASTM C469-02 [23]. The average test result of three specimens was recorded for each mix. The modulus of elasticity of concrete (Ec) depends on the modulus
Fig. 4. Effect of RCA content on the compressive strength. 37
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Fig. 5. Effect of recycled aggregate content on the flexural strength.
Fig. 6. Effect of RCA content on the modulus of elasticity.
reference beams (S0). The load-deflection curves of the tested beams were used to determine the flexural toughness (I) and flexural stiffness (k). Fractal dimension (D) was used to describe the cracks path and length by employing the image processing technique that would be later explained as suggested elsewhere [24,25]. The density, mechanical properties and water absorption of the selected mixes based on Taguchi approach is presented in Table 7. It is clear that there is a reduction in concrete quality/studied properties in parallel with recycled aggregate incorporation.
of elasticity of concrete constituents especially the modulus of elasticity of aggregate. Thus, Ec decreases which increasing of RCA replacement, as shown in Fig. 6. The reduction in modulus of elasticity can be attributed to the low modulus of elasticity of RCA compared with NA in addition to the other factors that negatively affected the strength as previously mentioned. 6. Results of tested beam specimens Eight steel- reinforced concrete beams were prepared, cast and tested to study their structural behavior and fracture energy parameters under bending load. The test setup and the details of steel reinforcement are illustrated in Fig. 7. The reference beam was cast using SCC with 0% RAC replacement. The other six beams were cast using RASCC with 25%, 56%, and 100% of RCA replacement based on Taguchi analysis as earlier explained representing S, Sop, Sm, and Sw, respectively, see Table 6. This means that the Taguchi technique analysis can be counted as stage one in this study while these selected mixes is stage two that studied in terms of their structural behavior in addition to mechanical properties. These beams were tested until the failure took place. The behavior of the modified beams (Sop, Sm and Sw) was compared with the
6.1. Load-deflection results Table 8 shows the results for all tested reinforced concrete beams. It shows the load at first crack (Pcr), deflection at first crack (δcr), ultimate load (Pu) and the deflection at the ultimate load (δu). The results show that first-crack and ultimate load capacity clearly decreased as RCA content increased. The deflection of the tested beams that incorporated RCA was greater than that for the reference SCC beams. The first crack was noted to be occurred earlier in parallel with the increase of RCA percentage replacement. This could be attributed to the reduction in RCA strength itself in addition coupled with reduction in the bond strength between aggregate interface and mortar in comparison with 38
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S.I. Mohammed and K.B. Najim
Fig. 7. Details of the tested beams.
The flexural stiffness k represents the linear part of the load=deflection curve. It was calculated as the slope (ΔF/Δδ) between 10% and 50% of the ultimate load representing the range before first crack initiation and at the beginning of the elastic-plastic stage based on Turatsinze and Garros [26] as previously used by other researchers [24,27]. The obtained results show that the flexural stiffness decreased with increase the incorporation of RCA, as shown in Table 8. This highly attributed to the same reasons that caused the reduction in concrete strength as previously mentioned.
fiber on SCC [24]. Table 8 shows the results of the flexural toughness indices. Flexural toughness indices were found that the decreased with increasing RCA content for all examined mixes as compared to the reference mix (S0). Where, first crack load, ultimate load (first crack and ultimate moments) decreased with increased the RCA percentage replacement. This could be attributed to the lower strength of RASCC as earlier explained. However, it can be noted that the reduction also increased with increasing the applied load i.e. the reduction percentage in I0 was less than that for I5 which less than I10. This is justifiable if the cracks propagation and the bonding characteristics between the RCA and mortar in addition to other mentioned reasons are considered. It can be concluded that the ultimate deflections of RASCC beams were larger than that of beams made using NA and this increase was found to be in parallel with the increase of RCA replacement. This is logical if the reduction in the flexural toughness indices considered where all these indices decreased with RCA incorporation. Such behavior could be attributed to the two ITZ existent that could speeds up the cracks growth. Contradictory results were noted for the first crack deflection in comparison with the ultimate load deflection where it increased with RCA incorporation as seen in Table 8. Such behavior could be due to the stress relaxation. The ratio of the ultimate deflection to the first crack deflection is defined as ductility displacement (µ) which is noted to be continually decreased with RCA increased. This means that the absorbed energy decreases with RCA increase which will be later discussed.
6.3. Flexural toughness (I)
6.4. Fractal dimension (D) test and results
The area under load-deflection curve is commonly known as flexural strength. Whereas, the flexural toughness indices (I5, I10 and I20) are calculated based on ASTM C 1018 [28] by dividing the area up to a specified deflection (3δ, 5.5δ and 10.5δ) to the area up to first crack (δ). Fig. 9 shows that I5, I10 and I20 corresponding to 3δ, 5.5δ and 10.5δ, respectively. These indices were previously calculated and used to study the effect of incorporating rubber aggregate on the flexural toughness of rubberized self-compacting concrete [29], and also the effect of steel
The fractal dimensions for all tested beams were calculated by employing image processing technique that was recently utilized to determine the tortuosity of the cracks of steel fibers reinforced concrete beams [24]. These beams were painted by white color to easily recognize the cracks especially at the mid-third that was photographed as no significant cracks have been noted elsewhere, Digital camera of 14 Mega pixels was used in this investigation. The beams were photographed during the test with increasing the applied load until reaching the ultimate load. ImageJ v1.44p software was used to analysis the
Table 7 Mechanical and hardened properties of selected mixes. Mix.
Density (kg/m3)
fcu (MPa)
fr (MPa)
Ec (GPa)
Water absorption (%)
S0 Sop Sm Sw
2412 2389 2271 2186
57 55 51 39
5.50 5.25 5.00 4.50
32.0 31.5 28.5 26.0
2 3 5 7
the NA. The roughness of the RCA that could increase the mechanical interaction could compensate for the reduction in strength due to the use of RCA as earlier explained. Fig. 8 shows the load-deflection curve for the examined beams that is going to be used in calculating the flexural stiffness and toughness. 6.2. Flexural stiffness k
Table 8 Load-deflection results of the tested beams. Mix
Pcr (kN)
δcr (mm)
Pu (kN)
δu (mm)
µ
k (KN/mm)
I5
I10
I20
S0 Sop Sm Sw
27 24 23 19
1.97 2.23 2.42 2.74
78 75 72 70
21.2 19.09 17.24 15.07
10.76 8.56 7.12 5.50
13 12.15 11.2 9.89
9.96 8.53 8.1 7.72
26.3 22.83 21.4 19.6
62.1 55.9 51.3 48.3
39
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Fig. 8. Load-deflection curve of the investigated mixes.
Table 9 Fractal dimension of the tested beams. Mix
S0 Sop Sm Sw
D 10 kN
20 kN
30 kN
40 kN
50 kN
60 kN
70 kN
– – – –
– – – 0.542
0.74 0.78 0.82 0.95
0.88 0.92 0.98 1.03
0.95 0.98 1.05 1.10
1.07 1.12 1.13 1.17
1.17 1.15 1.14 1.12
Fig. 9. Flexural toughness index calculation [28].
images after converting them from colored to binary images in order to obtain the fractal dimension. The photographed part of the tested beams were cropped into 900 × 300 pixels i.e. 300 mm × 120 mm followed by applying the brightness/contrast feature to manually adjust the bracket of upper/lower limits of the grey scale histogram. Then, 1px median filter was applied followed by manually removing the noise using the ‘eraser’ tool. D was calculated for all tested beams for two faces (front and back) at specified loads (10, 20, 30, 40, 50, 60 and 70 kN) for each beam as shown in Table 9 employing Image Processing Technique. It represents the linear regression coefficient of Log count vs. Log box size. The fractal box count tool installed in ImageJ software tool v1.44p was used with the following box sizes: 2, 3, 4, 6, 8, 12, 16, 32 and 64. Fig. 10 shows a binary image of the cracks of the reference (S0) beam, as an example. This technique was deeply explained elsewhere [24,29]. Table 9 shows that with 10 and 20 kN applied load, there was no visible cracking except for Sw mix (highest RCA replacement) while at 30 kN
Fig. 10. An example of binary image (A) and the fractal dimension calculation (B) for S0 beam frnt face.
the cracks clearly appeared. Indeed, D value increased with incorporating RCA when the applied load increased from up to 60 kN, the rate of this increment almost decreased with increasing of applied load. Additionally, this rate increased with increasing of RCA percentage replacement. This behavior contradicts the flexural toughness indices and flexural stiffness as Table 8 demonstrates. This could be attributed to the fewer number of resulted cracks with applying the load for the mixes incorporating RCA coupled with elongating their paths. Namely, increase their tortuosity up to a certain applying load with increase the mouth opining (width). Such behavior could be explained by the hypothesis of this concrete has 40
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S.I. Mohammed and K.B. Najim
Gd = Gf a
two ITZ and the new ITZ is weaker than that for concrete made using NA. ITZs could be working as cracks attraction points leading to decrease the number of cracks and increase the tortuosity and crack width. This finding confirms the results of flexural toughness as less energy was absorbed i.e. I values decreased as the absorbed energy related to cracks number and tortuosity. When the applying load exceeds 60 kN, the D value starts to marginally decrease which could be attributed to the stress relaxation. Generally, this contradictory in the results of flexural toughness indices and D value could also understandable if the methodology of calculating them is considered. The D value is calculated based on image processing technique that considered the number and the tortuosity of the cracks while toughness indices are calculated using load-deflection curve as previously mentioned. To deeply understand this point, more research needs to be carried out
(3)
where Gf is the concrete fracture energy at observation scale and a the “Euclidean length” (the height of the cross section) that moderated based on fractal theory as explained elsewhere [31]. The following equations that proposed by [24] is used as follow, where by substituting: 1−d
δ a∗ = a ⎛ ⎞ ⎝a⎠
,
(4)
Eq. (1) becomes:
δ 1−D Gd = Gf a∗ = Gf a ⎛ ⎞ ⎝a⎠
(5)
Or
Gd δ 1−D = a⎛ ⎞ Gf ⎝a⎠
6.5. The surface cracks fracture energy parameters
(6)
δ is the grid size that suggested to be equal to 5 mm according to Guo et al. [34]. This was attributed to the size of used fine aggregate in the studied concrete that is almost having same size (4.75 mm). The calculated value of Gd was considered as an indicator to the absorbed/
It was previously stated that fracture energy governs the behavior of reinforced concrete structural members particularly in tension when the concrete as a matrix is resisting the load alone [30]. In another words, fracture energy should be studied in addition to strength when cracks initiation/propagation is important i.e. structural behavior of concrete. Therefore, in this study, fracture energy parameters were studied as the concrete matrix would be affected by replacing NA with RCA due to the differences in their characteristics as previously mentioned in results of mechanical strength section. Darwin et al. [30] defined fracture energy Gf as the required energy to form cracks/micro cracks per unit area. It is known that cracks/ micro-cracks initiate due to strain localization at weak points/flaws in a concrete matrix. After crack formation, crack tip would be created leading to the creation of fracture process zone that represent the region ahead of crack tip which is important in concrete nonlinear fracture mechanics. Once the cracks propagate/widen (open) due to increase tensile strength (ultimate tensile stress), the energy is dissipated leading to stress release. It is important to understand that the direction plane of stress is perpendicular to the fracture area. It was stated that the actual crack length plus the fracture process zone is known as the effective crack length [31–33]. In concrete, it is highly that the single crack that caused fracture is affected by other cracks/micro-cracks that are distributed within the concrete matrix by absorbing/dissipating energy as a result of their tortuosity i.e. propagation and elongation. Image processing technique was used in this investigation to quantify the surface fracture energy in addition to calculate the fracture energy parameters. The fracture energy was previously calculated following the guidelines established by RILEM (1985) that used closedloop flexural testing machine under crack mouth opening displacement (CMOD) control, see David et al. [30] for more explanation and details. In this study, not only the main crack that caused concrete fracture was considered but also the other formed cracks in the middle third of the tested beams (Mode I) was taken into account in the calculations. This approach simulates the reality more precisely than the traditional one as it takes the effect of the most visible cracks/micro cracks on the fracture energy. The fracture energy parameters were calculated based on previous studies [31,34,35]. Namely, they are a combination of different approaches in order to consider the previous literature as suggested elsewhere [24].
Gf
dissipated energy due to RCA incorporation that affected the cracks propagation, width, and elongation. 6.5.2. Fracture energy based on fracture profile Fracture energy was classically/mathematically defined as follow in order to be calculated:
Gf =
W A
(7)
W represents the total dissipated work while represent the Euclidian fracture surface (b × d), where d = a in this case. The work that done by the applied external load on a structural member was suggested to represent the area under the load-deflection curve while Af represents the area of rupture [34]. The total dissipated work due to the applied load by the surface of rupture is calculated as follow:
δ 1−D W = 2γsAf = 2γsba ∗ = 2γsb ⎡a ⎛ ⎞ ⎤ ⎢ ⎝a⎠ ⎥ ⎣ ⎦
(8)
where γs is the density of the surface energy of concrete, while the fracture energy is as follow:
Gf =
W δ 1−D = 2γs ⎛ ⎞ A a ⎝ ⎠
(9)
Or
Gf γs
1−D
δ = 2⎛ ⎞ a ⎝ ⎠
(10)
6.5.3. Fracture energy based on classical definitions Hillerborg [36] correlated the fracture area with a loaded concrete member (beam) Failure. It was concluded that the required work to cause failure to the tested concrete beam is equal to the required energy to create the rupture area. This relation can be mathematically expressed as follow:
Gf = 6.5.1. Fracture energy based on surface cracks dimension (mm) The surface fracture energy is highly depending on surface micro/ macro-cracks tortuosity that responsible for dissipating the fracture energy. This, in turn, depends on the tensile stress [34] in addition to concrete ingredients, strength, external conditions, etc. Carpinteri and Spagnoli [35] suggested a formula to calculate the total dissipated energy Gd at the crack surface per unit thickness as follow:
W0 −Ws b (d − a0 )
(11)
The failure work is the net of the work carried out by the external applied load (W0) minus the done work by a concrete member selfweight (Ws). The sectional dimensions of a concrete member represent by b and d i.e. width and depth, respectively. Petersson [37] stated that the resultant work due to applying an external load mathematically represents the area under the stress-strain curve whereas the done work 41
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by the weight of the concrete beam itself is approximated by 2P0u0. P0 represents the external load that applying to a concrete member that causes the bending moment which equivalent to the moment results by the self-weight of this member. It is calculated as P0 = mg (2S − L)/2S while u0 is displacement at the ultimate load. The tested beam mass and the acceleration of gravity respectively represented by m and g whereas S is the distance between supports, L is the clear span, and the dimension of the initial notch a0 was considered zero in this study. Eq. (10) was used by Darwin et al. [30] to determine the fracture energy Gf.
Gf = (W0 + mgδu )/ A
Table 11 Comparison of classical fracture energy values. Mix
S0 Sop Sm Sw
Gf, J/m2 (Eq. (9))
Gf, J/m2 (Eq. (10))
Gf, J/m2 (Eq. (11))
@ first crack
Failure
@ first crack
Failure
@ first crack
Failure
3507 3524 3666 3421
109,820 94,627 81,283 69,731
3593 3620 3765 3529
110,741 95,445 81,979 70,322
10,638 10,704 11,132 10,412
330,720 285,000 244,800 210,000
(12) under certain loading. Again, this behavior is in agreement with fractal dimension results as presented in Table 9. The presented results of fracture energy (Gf) that calculated based on the classical equations (Section 5.5.3) shows that Gf calculated from Eqs. (9) and (10) were very closed for both first crack and ultimate loads. There was a gradual decrease in Gf with increasing RCA content, as seen in Table 11, which resulted from the ultimate load. This is understandable if the strength of the mixes and the RCA itself taken into account in addition to the ITZ that important in initiating the micro cracks/cracks, as mentioned previously. For the applied load causing first crack, Sm mix provided marginal increase in Gf in comparison with the other tested mixes that could be attributed to the stress relaxation. Eq. (11) provided highly overestimated values of Gf in comparison with Eqs. (9) and (10) for both first crack and ultimate loads. This could be attributed to the traditional approach in calculating Gf i.e. does not considering the fractal dimension that modified to be used in calculating Gf based on fractal theory. Nevertheless, the trend of reduction in Gf with RCA increase is clear. This means that the classical definition of fracture energy was in agreement with the deterioration in strength, stiffness, toughness, for the tested beams with RCA incorporation. However, only the D values were increased with RCA incorporation which can be attributed to the existence of two ITZ that could increase the cracks tortuosity within a local concrete matrix but for less cracks number. Finally, based on the results of the tested concrete mixes and beams, RCA could be successfully used in producing concrete to be used in structural applications. More experimental investigations are required to confirm these results especially the correlation between flexural behavior, fractal dimension with the absorbed energy. Also, studying the durability of the produced concrete should be considered which is ongoing.
where W0, m, g, are respectively represented the area under the loaddeflection curve, mass of the tested concrete beam, the gravity acceleration. δu is the failure mid-span deflection while A is the cross-sectional area above the notch of the tested beam. Kazemi et al. [38] employed Eq. (11) that taken from Bazant and Planas 1997 [39] to calculate the fracture energy of a concrete beam. Where, δu is the maximum deflection at mid-span of the tested beams and P0 is the external applied load.
Gf = (W0 + 2Pδu )/ A
(13)
6.6. Results and discussion of fracture energy parameters In concrete, the dissipation of energy depends on many factors like concrete quality, applied load, incorporation of fibers and other reinforcement, member geometry, etc. However, cracks that resulted from different causes are the main factor that controls the concrete ductility/energy dissipation. Fractal dimension of the surface cracks has recently used [24] in quantifying/determining the crack tortuosity that could be an indicator of energy absorption. Table 10 demonstrates fracture energy dimensions (Gd/Gf) of the surface cracks continually increased with applied load increasing (stress increased). Same behavior was noted with Recycled Aggregate (RA) increase for all applied loads except when the loading reached 70 kN. Such behavior could be justified as with load increasing the cracks propagating and elongating in addition to increase the cracks width which leads to absorb the applied energy. This confirms the D results as seen in Table 9, as explained in Section 6.4. It is stated that when D increases the cracks are elongating and propagating [24]; however, the crack opening mouth increase also increase the absorb energy before the failure. This means that the applied load (energy) should be increased in order to cause failure to the tested concrete member in addition to the gained stress relaxation. Same results was noted for the dimensionless fracture energy parameter i.e. fracture profile (Gf /ɤs), as demonstrated in Table 10. It shows that a linear relationship between (Gd /Gf) and (Gf /ɤs). This previously attributed to their dependency on the modified beam height (a*) based on the fractal theory that presented in Eq. (2), Section 5.5, for steel fiber self-compacting concrete [24]. However, steel-fibers works to elongate/propagate the cracks path while the RCA could also doing same job but in different mechanisms as previously explained. Due to the questionable weakness between the RCA and the matrix as a result of the double ITZ and other previously mentioned reasons, the cracks could propagate around the RCA rather that concrete matrix
7. Conclusions It was concluded that the incorporation RCA caused a decrease in the mechanical strength including compressive, splitting and flexural strengths in addition to modulus of elasticity that slightly decreased with adding RCA. However, this reduction did not affect the potential use of the tested concrete as a structural concrete in terms of the required mechanical strength. Where, even with 100% RAC replacement the compressive strength is 39 MPa which is adequate to be structurally used. The structural behavior of the tested beams that containing RCA was noted to be deteriorated in comparison with the SCC beams. The first-crack load and ultimate load capacity decreased with RCA
Table 10 Surface cracks fracture energy dimension (Gd /Gf) mm, and fracture energy based on fracture profile (Gf /ɤs) values with loading stages. Mix.
S0 Sop Sm Sw
30 kN
40 kN
50 kN
60 kN
70 kN
Gd/ Gf
Gf / γs
Gd/ Gf
Gf / γs
Gd/ Gf
Gf / γs
Gd/ Gf
Gf / γs
Gd/ Gf
Gf / γs
61.94 70.97 81.32 126.54
0.82 0.94 1.08 1.68
99.73 114.26 140.13 166.11
1.32 1.52 1.86 2.21
126.54 140.13 177.8 210.76
1.68 1.86 2.37 2.81
190.32 225.6 233.4 267.42
2.53 3.00 3.11 3.56
267.42 249.83 241.48 219.54
3.56 3.33 3.21 3.00
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increasing. This coupled with the increasing of the deflection that corresponding to the first-crack and ultimate loads. However, these decreases could be qualitatively counted within the permissible range from the structural engineering point of view. The results of this study show that both the tested concrete mechanical properties and the tested behavior of the studied beams were deteriorated in parallel with RCA incorporation. Nevertheless, the examined RCA can be used in producing structural Self-compacting concrete as the strength/structural behavior satisfied the requirements to be structurally used. However, more research is needed to be carried out in order to examine the use of RCA that produced from the demolished building and security concrete precast panels to generalize this conclusion in addition to test the durability of the produced concrete which is ongoing.
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