Effect of cement and admixture on the utilization of

Construction and Building Materials 149 (2017) 91–102

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Effect of cement and admixture on the utilization of recycled aggregates in concrete Zine-el-abidine Tahar a, Tien-Tung Ngo a, El Hadj Kadri a,⇑, Adrien Bouvet a, Farid Debieb b, Salima Aggoun a a b

L2MGC Laboratory, University of Cergy-Pontoise, France LME Laboratory, University of Medea, Algeria

h i g h l i g h t s Recycled aggregates concrete (RAC) with recycled concrete sand (RS) and recycled concrete gravel (RG). No reduction in slump of recycled aggregates concrete (RAC) up to 30% of recycled aggregates (RA). The workability of recycled aggregates concrete (RAC) depend on the combination of cement-concrete. Decrease in density and strength of recycled aggregates concrete (RAC). Increase in air content of recycled aggregates concrete (RAC).

a r t i c l e

i n f o

Article history: Received 5 December 2016 Received in revised form 2 April 2017 Accepted 15 April 2017

Keywords: Concrete Recycled aggregates Workability Mechanical strength Admixture

a b s t r a c t In this paper, results of an investigation on the effect of type of cement and admixture on fresh and hardened properties of concrete with coarse and fine recycled aggregates (RA) are presented. Natural sand (NS) and natural gravel (NG) were replaced respectively by (15, 30, 70, and 100%) of recycled concrete sand (RS) and recycled concrete gravel (RG). Two types of cement/admixture were used. The fresh (slump, air content and density) and hardened (compressive strength and elastic modulus) properties of recycled aggregates concrete (RAC) are analyzed and compared with those of natural aggregates concrete (NAC). The results indicate that the workability of RAC depends on the combination of cementadmixture and precisely the nature of the admixture and the amount of C3A. RAC can be used with up to 30% of RS and more than 30% of RG. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Large quantities of waste materials are produced by the construction and demolition (C&D) each year. The volume of such materials reaches alarming proportions. The issue of use of environmental friendly construction materials is gaining ground as awareness of the need for sustainability in design grows. Therefore, it becomes necessary to find ways to make concrete a more environmentally conscious material. Utilization of RAC substantially reduces the demand for quarried stone, thereby decreasing transportation costs and emissions related issues, further, the use of RA in the manufacture of concrete and cementitious materials is a way to meet the needs, while preserving the environment in a sustainable approach. With respect to natural aggregates, the recent studies show that fresh concrete ⇑ Corresponding author at: L2MGC Laboratory, University of Cergy-Pontoise, 5, Mail Gay Lussac, Neuville-sur-Oise 95031, France. E-mail address: [email protected] (E.H. Kadri). http://dx.doi.org/10.1016/j.conbuildmat.2017.04.152 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

behavior depends on the type and dosage of admixture as well as the type and dimensions of aggregates [1–4]. Concretes made with RG have been subjected to numerous studies. Most of them focus on the effect of the quantity of used RA on the concrete strength at early age and long term. The results shown that the compressive strength of RAC was not much significant for 30% replacement of NG by RG but it was lower for 100% replacement [5–10]. For recycled aggregates mortars (RAM), Shi Cong Kou et al. [11] studied properties of cement mortars containing RS, and observed that cement mortars always have better mechanical properties than the corresponding cement–lime mortars, and this could possibly arise from a synergic effect of lime hydraulicity and the filler effect due to the fine fraction of RS within the mix, that lead to better densification of the lime mortars by blocking the capillary pores. In framework of mortar, we can find some authors worked on mortars that contain recycle sand and they showed statistically significant differences for replacement ratios up to 25%. These studies focus on the effect of RG

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Zine-el-abidine Tahar et al. / Construction and Building Materials 149 (2017) 91–102

Table 1 Chemical analysis and physical properties of cements used. Cement

C1

C2

Nomination Finesse Blaine (cm2/g) Median diameter (mm) Water demand (%) Initial setting time (min) Hydration heat at 41 h (j/g) SiO2 Al2O3 Fe2O3 CaO SO3 MgO K2O Na2O ClLoss on ignition Clinker (%) Alite (C3S) (%) Belite (C2S) (%) Aluminate (C3A) (%) Ferrite (C4AF) (%)

CEM I 52.5R CE CP2 4520 9.7 27.2 120 328 19.54 5.70 3.06 60.10 3.71 1.85 0.86 0.19 0.07 0.33 99 53.4 15.8 9.9 9.2

CEM I 52.5R CE 3250 16.5 26.0 165 300 20.19 3.81 2.99 61.50 3.31 1.96 0.81 0.17 0.02 0.68 98 67.0 7.4 5.0 9.1

100 90

NS (0/4)

Cumulative passing (%)

80

RS (0/4)

70 60 50 40 30 20 10 0 0,063

0,63

6,3

Sieve size (mm) Fig. 1. Grading of natural and recycled sand.

Table 2 Water absorption and density of aggregates used. Type of aggregate

Water absorption (%)

Density (%)

NS (0/4 mm) RS (0/4 mm) NG (4/10 mm) RG (4/10 mm) NG (10/20 mm) RG (10/20 mm)

0.9 10.0 0.5 5.1 0.4 5.7

2.59 2.71 2.71 2.17 2.31 2.29

but there have been few studies on the effect brought by RS on the mechanical properties of concrete. And in previous studies, the analysis on properties of concretes in fresh state was not given importance. To complete this lack, a recent study [12] showed that the rheology and strength of RAC are negatively influenced by the substitution in coarse and fine recycled aggregates. Use of RAC in high performance concrete is not a common practice, yet simply due to the reductions in mechanical properties as well as durability properties. Further, it has been found that cement paste in RAC contributes to a lowered relative density and higher water absorption than virgin aggregates, while higher shrinkage and creep strains were also observed. There is

reduction in the mechanical properties and significant reduction in the porosity of the concrete when natural aggregate is replaced by recycled aggregate concrete [5]. Unit weight, workability, and durability of concrete containing 30–100% of waste aggregate decrease when subjected to freezing and thawing cycles [13]. In reality, a large number of experiments are required as to decide a suitable mixture for obtaining the desired requirements for concrete made with recycled concrete coarse/fine aggregate. Some authors as Lin et al. [7] used Taguchi’s approach to reduce the numbers of experiment. Ryou et al. [8] studied the durability of recycled aggregate concrete incorporating pozzolanic materials pulverized fuel ash and ground granulated blast furnace slag. Based on the results, It was concluded that (i) 30% PFA and 65% GGBS concretes increased the compressive strength to the level of control specimens made with natural granite gravel, but the tensile strength was still lowered at 28 days; (ii) improved the chloride ion permeability; (iii) kept corrosion rate of 30% PFA and 65% GGBS concretes at a lower level after corrosion initiation, compared to the control specimens, presumably due to the restriction of oxygen and water access. Juan [9] indicated that attached mortar content has an impact on recycled aggregate concrete properties, and it aversely influences the resulting concretes main properties in different applications. Debieb et al. [10] reported that contaminated recycled aggregate are much sensitive to chlorides than sulfates and are rapidly leached when soaked into water. Significant differences were observed between the properties of original and new concrete and the results clearly show the necessity of taking these contaminations into account. Chakradhara Rao et al. [14] introduced a new term called ‘‘coarse aggregate replacement ratio ’’ and defined as the ratio of weight of recycled coarse aggregate to the total weight of coarse aggregate in a concrete mix. Fonseca et al. [15] concluded that mechanical performance of RAC is affected by curing conditions roughly in the same way as conventional concrete (CC). Tegguer [16] developed a new approach of water absorption measurement using hydrostatic weighing in order to observe the kinetic absorption of aggregates, and concluded that the model of capillarity processes of aggregates can be obtained using Hall’s model. Silva et al. [17] advocated the use of recycled aggregates from construction and demolition waste in concrete production if proper procedure of ensuring the quality of recycled aggregates is maintained. Kou et al. [11] reported that at higher temperatures, concretes made with recycled aggregates suffered less deterioration in mechanical and durability properties than the concrete made with natural aggregates. Zhao et al. [18] investigated the influence of fine recycled aggregates concrete (FRAC) on the properties of mortars and concluded that (i) slump of mortars containing dried FRCA is always larger than that of mortars containing saturated FRAC; (ii) the compressive strength of mortars containing dried FRAC is always larger than that of mortars made with saturated FRAC, which is attributed to a thinner interfacial transition zone improving its mechanical properties. Raeis Samiei et al. [19] reported deterioration in mechanical properties with increased recycled aggregates in the cement mortars, whereas cement–lime mortars exhibited improvement in mechanical properties were up to 60% when increasing the amount of recycled aggregates. To explain this improvement, the same explication as that of Shi Cong Kou et al. [11] has been given by Raeis Samiei et al. [19]. In this context, the objective of this research is to study the effect of the cement-admixture combination on the behavior of RAC with different substitution percentages (15, 30, 70, and 100%) of coarse and/or fine NA by RA. Since they are dependent on time in order to interpret concrete rheological properties, four different time – T0, T30, T60, T90 corresponding to the output of mixer and 30 min, 60 min, 90 min afterwards – were tested.


(a) Natural Sand

93

(b) Recycled Sand

Fig. 2. Natural and recycled sand aspect.

those of cement C2. The fineness of the cements are rather different.

Cumulative passing (%)

100 90

NG (4/10)

80

RG (4/10)

70

NG (10/20)

60

RG (10/20)

2.2. Fine aggregates

50 40 30 20 10 0 0,063

0,63

6,3

63

Sieve size (mm) Fig. 3. Grading of natural and recycled gravel.

NS of limestone’s crushed rock (0/4 mm) from LAFARGE (Sandracourt in France) and RS (0/4 mm) from Docks of Limeil - Brévannes (DLB) (Gonesse in France) from a recycling plant were used. The grain size distribution of natural and recycled aggregates used is presented in Fig. 1 according to NF 1097-6 and the water absorption is given in Table 2. Fig. 2 shows the morphological appearance of natural and recycled sand. It is observed that NS is smaller than RS. Furthermore, based on the tests, the fineness modulus of NS (2.25) is significantly smaller than that of RS (3.27). Therefore, an increase of segregation and a decrease of workability are expected when natural aggregates are substituted by recycled ones in the recycled concrete [14]. 2.3. Coarse aggregates

Table 3 Chemical analysis and physical properties of used admixtures. Admixtures

A1

A2

Chemical Type Type Solids content Shape Colour pH Recommended Dosage Content Na2O Eq. Content ions Cl-

Polycarboxylate High water reducing 22.5% liquid light yellow 4 up to 6 0.1% up to 3.0% 1% 0.1%

Ether polycarboxylique Water reducing 22.5% liquid light brown 6 up to 8 0.2% up to 1.9% 1% 0.1%

2. Materials and methodology

Two NG fraction of limestone’s crushed rock (4/10 mm and 10/20 mm) from LAFARGE (Givet in France) and two RG fractions (4/10 mm and 10/20 mm) from Docks of Limeil - Brévannes (DLB) (Gonesse in France) from a recycling plant were used. The grain size distribution of natural and recycled aggregates used is presented in Fig. 3 according to the NF 1097-6 and the water absorption and density are given in Table 2. RAC are used in saturated surface dry form. From Table 2 it appears clearly that RA present relatively higher water absorption compared to NA. The higher water absorption of RA is due to the mortar gangue. Visual observations show that RA presents a rough cracked surface compared to a smoother surface for NA: this confirms the high porosity of RA [9,20]. The coefficient of cubicity is higher for NA than for RA.

2.1. Cement 2.4. Admixtures Two type of cement CEM I 52.5 CE CP2 (C1) and CEM I 52.5 CE (C2) from Lafarge in France were used. The different properties are given in Table 1. Cement C1 has a lower C3S and a higher C3A than

Two admixtures (A1 and A2) were used. Chemical analysis and physical properties of admixtures are recapitulated in Table 3.

94


Table 4 Mix proportion. Cement(C)

Mixture

C

NS

RS

NG(4/10)

NG(10/20)

RG(4/10)

RG (10/20)

SP(%)

Water(kg/m3)

w/c

3

(kg/m ) C1

0RS0RG 15RS0RG 30RS0RG 70RS0RG 100RS0RG 0RS0RG 0RS15RG 0RS30RG 0RS70RG 0RS100RG

320 320 320 320 320 320 320 320 320 320

852 724 596 256 0 852 852 852 852 852

0 105 211 492 702 0 0 0 0 0

325 325 325 325 325 325 276 227 97 0

696 696 696 696 696 696 592 487 209 0

0 0 0 0 0 0 42 84 195 279

0 0 0 0 0 0 87 173 404 578

0.40 0.40 0.40 0.40 0.30 0.40 0.40 0.58 0.78 0.78

188 196 206 231 250 188 193 200 217 200

0.59 0.61 0.64 0.72 0.78 0.59 0.61 0.62 0.68 0.72

C2


320 320 320 320 320 320 320 320 320 320

852 724 596 256 0 852 852 852 852 852

0 105 211 492 702 0 0 0 0 0

325 325 325 325 325 325 276 227 97 0

696 696 696 696 696 696 592 487 209 0

0 0 0 0 0 0 42 84 195 279

0 0 0 0 0 0 87 173 404 578

0.20 0.40 0.40 0.40 0.40 0.20 0.67 0.51 0.48 0.42

176 185 195 220 238 176 182 189 206 218

0.55 0.58 0.61 0.69 0.74 0.55 0.57 0.59 0.64 0.68

RS-C1

RS-C2

(a) 240

RG-C1

200 180 160 140 120 100 80 60 40 0

20

40

60

80

RS-C1

220 200 180 160 140 120 100 80 60 40

100

Recycled Sand (%)

0

20

220 200 180 160 140 120 100 80 60 40

Slump (mm) 20

40

60

60

RGC1

(b) 240

0

40

80

100

80

100

80

100

Recycled Gravel (%)

RS-C2

(b) 240

Slump (mm)

RG-C2

(a) 240

Slump (mm)

Slump (mm)

220

80

100

220 200 180 160 140 120 100 80 60 40

Recycled Sand (%)

RGC2

0

20

40

60

Recycled Gravel (%)

RS-C1

RS-C2

240

RG-C1

(c) 240

220 200 180 160

Slump (mm)

Slump (mm)

(c)

140 120 100 80 60 40 0

20

40

60

80

100

Recycled Sand (%) Fig. 4. Variation of slump with percentage of substitution in RS: (a) T30; (b) T60; (c) T90.

RG-C2

220 200 180 160 140 120 100 80 60 40

0

20

40

60

Recycled Gravel (%) Fig. 5. Variation of slump with percentage of substitution in RG: (a) T30; (b) T60; (c) T90.

95


0%RS-C2

15%RS-C1

Slump (mm)

Slump (mm)

0%RS-C1 230 210 190 170 150 130 110 90 70 50 30 0

30

60

90

0

30

Times (minutes) 30%RS-C2

70%RS-C1

230 210 190 170 150 130 110 90 70 50 30 0

30

90

60

90

70%RS-C2

230 210 190 170 150 130 110 90 70 50 30 0

30

Times (minutes)

60

90

Times (minutes)

100%RS-C1

Slump (mm)

60

Times (minutes)

Slump (mm)

Slump (mm)

30%RS-C1

15%RS-C2

230 210 190 170 150 130 110 90 70 50 30

100%RS-C2

230 210 190 170 150 130 110 90 70 50 30 0

30

60

90

Times (minutes) Fig. 6. Variation of slump of concrete with time and percentage of substitution in RS.

3. Experimental program 3.1. Mixture proportioning Keeping the same granular skeleton and the same quantity of cement, two concrete families (with cement C1 and Cement C2), and 20 different concrete mix proportions have been tested. Either natural sand or coarse aggregates were partially replaced (15, 30, 70 and 100%) with recycled aggregates. Table 4 summarizes all the mixed compositions.

Natural aggregates were used original form and their water content was measured and compensated in the mixing water. The compressive strength was measured at the ages of 1, 7 and 28 days, respectively on cylinders (150 300 mm) specimens using a testing machine with a maximum load capacity of 3000 kN (according to NF P 18-455). At the age of 28 days, the modulus of elasticity of concrete was measured on 70 70 280 mm cubic specimen (according to NA 437). 4. Results and discussion

3.2. Casting and testing

4.1. Fresh concrete

The recycled aggregates will be used in a saturated state due to their strong absorption capacity. Hence, it is essential to saturate these aggregates which are stored in dry conditions. The following steps were used to prepare recycled aggregates:

In order to limit the number of mixes and to be able to compare them on a common basis, a constant slump of 200 ± 20 mm for the different percentages of substitution (0, 15, 30, 70 and 100%) has been imposed and the water content and an amount of admixture were varied in consequence.

– Evaluation of the initial water content from an aggregate sample; – The barrel was first pre-humidified and excess water was removed with a sponge, to avoid loss of water by adsorption on the surface of the barrel; – 40 to 80 kg of aggregates were mixed with an amount of water to reach the design moisture content; – Barrel was rolled to homogenize the aggregates; – The barrel stayed in a horizontal position for minimum 2 h.

4.2. Workability The variation of slump of the various mixes of concrete with recycled sand is presented in Fig. 4 for T30, T60 and T90 respectively. It clearly appears that in all cases the slump is almost constant for a lower percentage of 30% of RS. However, beyond this limit the slump decreases (55% to 65%) with increase in percentage en RS. This is due to the packing effect of recycled sand, and the

96


0%RG-C2

15%RG-C1

Slump (mm)

Slump (mm)

0%RG-C1 230 210 190 170 150 130 110 90 70 50 30 0

30

60

0

90

30

Slump (mm)

Slump (mm)

70%RG-C1

30%RG-C2

230 210 190 170 150 130 110 90 70 50 30 0

30

90

60

90

70%RG-C2

230 210 190 170 150 130 110 90 70 50 30 0

30

60

90

Times (minutes)

Times (minutes) 100%RG-C1

Slump (mm)

60

Times (minutes)

Times (minutes) 30%RG-C1

15%RGC2

230 210 190 170 150 130 110 90 70 50 30

100%RGC2

230 210 190 170 150 130 110 90 70 50 30 0

30

60

90

Times (minutes) Fig. 7. Variation of slump of concrete with time and percentage of substitution in RG.

surface roughness of the particles. This may be attributed to the adherence of mortar to the recycled aggregates and thereby the surface texture of recycled aggregates is more porous. Similar results were reported by [14,16]. On the other hand, regardless of the couple cement/admixture, Bad retention rheology is observed when the concrete is made with 100% recycled sand as compared to other concretes. This effect is observed because of the packing of recycled sand and surface particle roughness. It is due to the adherence of mortar to the sand particle and the changing of the surface texture of natural sand that is less porous than recycled sand. Hence, the recycled sand has a higher water absorption coefficient. Considering the concrete mixture in a fresh state, when the quantity of recycled sand decreases, less mixing water is absorbed. Therefore, the workability of concrete increases as the results show. The results showed that whatever the percentage of RS used, concrete made with cement C2 have a higher slump than that with cement C1. This is due to the lower amount of C3A in Cement C2. This delays the degree of hydration of the cement and increase the workability of the mixture. This result is similar than that obtained by Kadri et al. [21]. In the same way, Fig. 5 illustrates the variation of slump of the various mixes of concrete with RG. The results showed that whatever the couple cement/admixture used the slump of concrete decreases slightly with the percentage of RG. This fall of concrete slump follows the same kinetics for both types (C1 and C2) of

cement used. The slump of concrete with cement C2 is higher than that of concrete with cement C1. The variations of slump of concrete with time are presented in Fig. 6 and 7. The results show that: (i) The workability of RAC with Cement C2 is better. (ii) The higher the percentage of recycled aggregates, the greater the loss of workability is fast. It can see that up to 30% recycled sand, slump values are similar, and however, reduction in slump values is in the range of 35%. Beyond 30% recycled sand content, decrease was substantial; 80% for 100% of recycled sand. (iii) The loss in workability of RAC with RG is lower than with RS. (iv) Combination cement/admixture influences spreading, and this for the different percentages of recycled, but slightly affects the RAC with 100% of RS and/or. It seems that this is due to the interaction between C3A and superplasticiser [21–23]. 4.2.1. Air content and density The variation of Air content and density of concrete with percentage of substitution in RA are shown in Figs. 8 and 9. The results show that the amount of Air content increase with 30% of RS whatever the cement used. Indeed, natural sand has a lower porosity than recycled sand because air bubbles are not removed during concrete vibration and due to the roughness and shape of


RS-C1

97

RS-C2

(a) 5

Air content (%)

4

3

2

1

0

0

20

40

60

80

100

Recycled Sand (%)

RG-C1

(b)

(a) RAC with RS

RG-C2

5

Air content (%)

4 3 2 1 0

0

20

40

60

80

100

Recycled Gravel (%) Fig. 8. Variation of Air content of concrete with percentage of substitution of RA: (a) RS; (b): RG.

RS-C1

(b) RAC with RG Fig. 10. Compressive strength evolution of RAC with cement C1 and with RA substitution rate.

RS-C2

(a) 2420

Density (Kg m-3 )

2370

2320

2270

2220

2170

0

20

40

60

80

100

80

100

Recycled Sand (%) RG-C1

RG-C2

(b) 2420

Density (Kg m-3 )

2370

2320

2270

2220

0

20

40

60

Recycled Gravel (%) Fig. 9. Variation of density of concrete with percentage of substitution of RA: (a) RS; (b): RG.

the recycled sand [13], (a 64% higher air content is measured when using 100% recycled sand compared to the reference concrete). This causes an increase of the air content for a high per-

centage of RS. The figure also shows that the cement mix/ admixture also affect the air content of RAC. Concretes manufactured with cement C1 has higher air content than those made with the cement C2. Similarly, the results show that the Air content increases with an increase in RG but the amount is marginal compared to that obtained for concrete with RS. This results shown that the fine portion of RA influence negatively on the behavior of RAC. For different combination of cement/admixture, densities of RAC were found to be lower than those of NAC (reference concrete). The density of RAC with RS decreases up of 30% of substitution and remains constant below this percentage. It is because of the content of adhered old mortar included in recycled aggregates surrounding the naturals aggregates. These results verify observations based on the air content tests; when air content decreases, the density increases. The results also show that the density decreases with the augmentation of RG content. This is because of the light weight and more porous nature of adhered old cements mortar to the recycled aggregates. The density of sand aggregate reduction is within 6% and 4% of the gravel aggregate when compared with normal aggregate. This implies that we can use the recycled aggregate where light weight concrete is needed and where the heavy dead weight is a problem such as in bridge constructions where the self-weight is very important on the structure. Therefore, for using the recycled aggregate of low density and with adequate strength, we can use 30% recycled sand. We can also use the gravel aggregate with different percentages to reduce the section of columns and foundation size for concrete building constructions.

98


(a)

60

Compressive strenght (MPa)

50

(a) Recycled Sand (RS)

40

30

RS-C2 RS-C1 fc28(1) fc28(2)

20

10

y = 0,0574x -95,449 R² = 0,84

0 2150

2200

2250

2300

2350

2400

2450

Density (kg/m3 )


(b)

60

50

40

30

RG-C2 RG-C1 fc28(1) fc28(2)

20

10

y = 0,0289x -26,429 R² = 0,93

(b) RAC with RG Fig. 11. Compressive strength evolution of concrete with cement C2 and with RA substitution rate.

0 2200

2250

2300

2350

2400

Density (kg/m3 )

4.3. Hardened concrete

Fig. 12. Relationship between compressive strength and density at 28 days: (a) concrete with RS; (b) concrete with RG.

4.3.1. Compressive strength The compressive strengths of the various concrete are presented in Figs. 10 and 11 for mixes with cement C1 and C2, respectively. It clearly appears that the compressive strength is lower for

RAC: the higher the rate of substitution in RA, the lower the compressive strength. There are two possible reasons: first is that the recycled sand adhered paste contains un-hydrated cement that contributes to strength lose [24]; and the second is the increased

Table 5 Compressive strength vs. and density of RAC with RS and RG. Cement

Mixture

q(kg/m3)

fc1

fc7

fc28

fc28(1)

fc28(2)

(MPa) C1


2405 2418 2375 2320 2278 2384 2370 2350 2326 2255

12 11 11 9 7 12 13 14 13 14

34 34 31 26 21 34 32 30 32 31

44 45 40 35 31 44 42 43 40 39

49 50 47 43 40 49 47 46 45 41

49 49 48 46 45 49 48 48 47 46

C2


2405 2418 2375 2320 2278 2384 2370 2350 2326 2255

19 18 16 15 13 19 18 18 18 18

34 33 31 29 26 34 32 32 32 31

41 41 39 35 32 41 41 41 40 38

49 50 47 43 40 49 47 46 45 41

49 49 48 46 45 49 48 48 47 46

99


40

(a) 40

35

35



(a)

30 25 20 15 10 5 0 2150

RS-C1 y = 0,059x -107,881 R² = 0,91

RS-C2

30

2250

2300

2350

2400

RS-C2

20 15 10 5

2200

2450

35

35


(b) 40


(b) 40

25 20 15

y = 0,009x + 29,147 R² = 0,92

RG-C1 RG-C2

10

2250

2300

2350

2400

2450

Density (kg/m 3 )

Density (kg/m3 )

30

RS-C1

25

0 2150

2200

y = 0,042x -85,352 R² = 0,61

RG-C1

y = 0,0102x -7,8688 R² = 0,33

RG-C2

30 25 20 15 10 5

5 0 2200

0 2200

2250

2300

2350

2250

2400

2300

2350

2400

Density (kg/m3 )

Density (kg/m3 ) Fig. 13. Relationship between compressive strength and density at 7 days: (a) concrete with RS; (b) concrete with RG.

roughness of the recycled sand that contributes to a diminution of the maximal compactness of the granular skeleton. This results in increased porosity and thus a reduction in the density and strength of the concrete. On the other hand, this compressive strength of RAC decreases slightly with the increase in RG percentage. This small diminution in strength is because of a slight variation in air content and density of concrete when the content of recycled gravel increases. Furthermore, the structure of the mixture with recycled sands is weakened by the small amount of the old cement stuck in the recycled gravel. Also, a low amount of pores facilitating rupture are observed when using recycled gravel as well as interlocking between the cement paste and the recycled aggregates themselves [25]. According to other researchers [26,27], the compressive strength depends upon the type of cement. Thus, the combination C2 whose the C3S dosage is higher than that of C1 provides improved early strength, but there is a 45% reduction in compressive strength with 100% RS in comparison 0% RS. This is quite logical in keeping the increase in air content and the density of concrete when the percentage of RS increases. Moreover, the presence of the cement mortar content in RS lead to weaken the structure of the mixture, and secondly the presence of pore facilitates rupture. From Figs. 10 and 11 it appears that compressive strength is differently influenced by the combination cement/admixture at early age of 1 and 7 days regardless of the percentage of RA. The RAC with cement C2 has a better compressive strength at 1 and

Fig. 14. Relationship between compressive strength and density at 1 day: (a) concrete with RS; (b) concrete with RG.

7 days than with cement C1. It is because of the higher amount of C3S in cement C2 than that of in cement C1. This one results a higher resistance of the combination C2 at 1 and 7 days. However, the compressive strength at 28 days is very little influenced by the combination cement/admixture. Analysis of the concrete compressive strength values we found the following remarks: all combinations of the types and recycled sand and recycle gravel aggregate have approximately the same compressive strength development with time. Both combination types and recycled gravel aggregate have at 28 days, compressive strengths that are higher than 40 MPa. Both combination types and recycled sand have at 28 days compressive strengths that are larger than 40 MPa until 30% of recycled sand, and they decrease until 30 MPa at 100% of recycled sand. The difference between compressive strengths of concrete at different percentages of recycled aggregate is negligible for the same combination. 4.3.2. Compressive strength and density Studies on resistance of concrete on recycled gravel demonstrate that there is a linear relationship between compressive strength (fc28) in MPa and the density of concrete (q) in kg/m3. Xiao et al. [28] have studied concrete with different percentages of RG (0, 30, 50, 70 and 100%) and dimensions 5–3.5 mm. They established the following relationship:

f c28ð1Þ ¼ 0:069q 116:1 Where q (kg/m3): density of concrete

ð1Þ

100


RSC1

f c28ð2Þ ¼ 0:026q 13:85

RSC2

Module elasticity 28d (GPa)

35 32 29 26 23 20 17

0

20

40

60

80

100

80

100

RS (%)

(a) Recycled Sand (RS) RGC1

RGC2

Module elasticity 28d (GPa) [Gpa]

35 32 29 26 23 20 17 0

20

40

60

RG (%)

(b) Recycled Gravel (RG) Fig. 15. Elasticity modulus variation of concrete at 28 days.

Elasticity modulus 28d (GPa)

(a) 45

C1 C2 Ravindrarajah and Tam [33] Kakizaki [34] IS: 456 [35] ACI 318 [36] Ravindrarajah and Tam [37]

40 35 30 25 20 0

20

40

60

80

100

Recycled Sand (%)

Elasticity modulus 28d (GPa)

(b) 45

C1 C2 Ravindrarajah and Tam [33] Kakizaki [34] IS: 456 [35] ACI 318 [36] Ravindrarajah and Tam [37]

40 35

ð2Þ

Where q (kg/m3): density of concrete fc28(2) (MPa): compressive strength (at 28 days of cubic samples 100x100x100 mm) and. In order to verify whether Eqs. (1) and (2) are applicable in this paper for concrete with different content of recycled sand and gravel, a comparison is made between the values of the resistances calculated by Eq. (1) (fc28(1)) and Eq. (2) (fc28(2)) and experimental results (fc28) based on cylindrical samples of 150 300 mm (Table 5). Fig. 12 (a and b) show the comparison between strength calculated by Eq. (1) (fc28(1)) and Eq. (2) (fc28(2)) and those measured for RAC with RS and RG. The results indicated that compressive strength at 28 days of concretes containing recycle aggregates behaves linearly in function of concrete density. This result is confirmed by other researchers [14,28]. Fig. 12 (a) shows for concrete with RS, the measurements do not well fit with the model, because measurements were performed on different specimens. These two models have been established using recycled gravel but not recycled sand. Fig. 12 (b) shows the relationship between compressive strength and density at 28 days in case of concrete with recycled gravel. Experimental values in this case align more closely with the resistances estimated by these two models. However, it can be seen that there are still deviations in the measurements that may be due to the use of different cement and admixture in this study. Based on the experimental results mentioned above and using an error optimization method, the relationship between compressive strength of cylindrical samples at 28 days and concrete density with recycled sand or with recycled gravel is proposed. These relationships are described in Eqs. (3) and (4): For diver percentage of RS in the concrete:

f c28;RS ¼ 0:057q 95:449 ðR2 ¼ 0:84Þ

ð3Þ

For diver percentage of RG in the concrete:

f c28;RG ¼ 0:029q 26:429 ðR2 ¼ 0:93Þ

ð4Þ

Knowing that the resistances at early ages are important for the form release phase, Figs. 13 and 14 present the variation of the compressive strength at early age depending on the density. The results of Fig. 13 (a) have shown that the 7 days strength of RAC with RS linearly changes with the density according to the relationship described by Eq. (5) below

f c7;RS ¼ 0:059q 107:881ðR2 ¼ 0:91Þ

ð5Þ

However, the 7 days strength of RAC with RS is not very sensitive to variation of the density of the concrete (Fig. 13 (b)). The trend of the variation of the resistance at 1 day depending on the density is not obvious (Fig. 14).

30 25 20 0

20

40

60

80

100

Recycled Gravel (%) Fig. 16. Comparison of experimental and analytical model results of elasticity modulus of concrete with C1et C2 combination at 28 days.

fc28(1) (MPa): compressive strength (at 28 days of prismatic samples 100x100x300 mm). Rao et al. [14] modified Eq. (1) to fit these concretes with RG of size less than 20 mm. and they recommended the following relationship:

4.3.2. Elasticity modulus Elasticity modulus results at 28 days age of concrete are shown in Fig. 15. The figure shows a linear decrease in the elasticity modulus of concrete with recycled sand and gravel. The reductions in values are estimated from 30 to 100% with recycled sand and from 15 to 100% with recycled gravel. The results obtained are in full agreement with those reported by several authors [14,28–31]. The reference elasticity modulus and recycled aggregate concrete are related to compressive strength. For a given strength of concrete, the elasticity modulus of recycled aggregate concrete is lower than that of the reference concrete. Like for compressive strength the higher the percentage of recycled aggregates increases more the modulus of elasticity decreases. The elasticity modulus is

Zine-el-abidine Tahar et al. / Construction and Building Materials 149 (2017) 91–102 Table 6 Analytical models proposed by various researchers. Methods Ravindrarajah [33] Kakizaki et al. [34] IS: 456 [35] ACI 318 [36] Ravindrarajah et Tam [37]

101

References Elasticity modulus 4,63.Rc0,5 i

E= E = 2,1(ds/2,3)1,5(Rci/200)0,5 E = 5.Rc0,5 i E = 4,127.Rc0,5 i E = 10/(2,8 + 40,1/Rci)

also affected by the aggregate porosity [32]. The porosity of recycled sand causes further reduction in the elasticity modulus of concrete but through lower porosity of recycled gravel the reduction of elasticity modulus is almost constant with the recycled gravel increase. Fig. 16 shows the elasticity modulus results of concrete with C1 and C2 cement combinations compared with results of analytical model given by other researchers [33–37]. The analytical models are recapitulated in Table 6. From Fig. 16 it can be observed that, the elasticity modulus of RAC with the combination of C1 and C2 cement decreases with the increase of RS and not so much for the RG. These results are in accordance with those reported in the literature [25,38,39]. This elasticity modulus decrease could be due to the lower elasticity modulus of RA.

5. Conclusions Based on the results of this experimental investigation, the following conclusions could be drawn: – The slump of RAC decreases when the percentage of RA increases. – The slump of RAC with RS is almost constant when the percentage of RA is lower than 30%. However, there is significant slump loss beyond 30% of substitution. – RAC with 100% of RA has a loss of workability than other concretes regardless the couple cement/admixture. – The combination type of cement affect the slump regardless the percentage of RA. The mix of RAC with the low amount of C3A has a better workability and a better behavior of rheology. That can be caused also by differences in the fineness. – The higher the percentage of RA, the greater the loss of workability is fast. The loss of workability of concrete with RG is less than that of concrete with RS. – The air content of the concrete begins to increase almost linearly beyond 30% of RS in any combination cement / admixture. On the other hand, the increase in air content is very marginal for any type of RG. – The density of RAC decreases with the increase in percentage of RA for different combination of cement and admixture. – RAC with low density and adequate strength is obtained with 30% of RS. – Regardless the couple cement/admixture, the compressive strength of RAC decreases with the increase of RA content. – The elasticity modulus of RAC decreases significantly with the increase of RS but marginally with the increase of RG.

Acknowledgements The authors are grateful for the support of SIGMA BÉTON – France.

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