Development of sustainable concrete using recycled coarse aggregate and ground granulated blast furnace slag

Construction and Building Materials 159 (2018) 417–430

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Development of sustainable concrete using recycled coarse aggregate and ground granulated blast furnace slag R.K. Majhi, A.N. Nayak ⇑, B.B. Mukharjee Department of Civil Engineering, Veer Surendra Sai University of Technology, Burla 768018, Odisha, India

h i g h l i g h t s
Strength of concrete decreases with increase in RCA/GGBFS/RCA & GGBFS contents.
Less reduction in tensile strength is observed with the use of RCA and GGBFS.
Use of GGBFS shows superior performance in RAC as compared to NAC.
Concrete mix with 50% RCA and 40% GGBFS achieves the desired mechanical properties.

a r t i c l e

i n f o

Article history: Received 21 February 2017 Received in revised form 21 October 2017 Accepted 27 October 2017

Keywords: Recycled coarse aggregate (RCA) Ground granulated blast furnace slag (GGBFS) Recycled aggregate concrete (RAC) Compressive strength Flexural strength Split tensile strength Non destructive test

a b s t r a c t The present investigation aims to use recycled coarse aggregate (RCA) and ground granulated blast furnace slag (GGBFS) as replacement of natural coarse aggregate (NCA) and ordinary Portland cement (OPC) for developing sustainable concrete. Sixteen numbers of concrete mixes are prepared with 0%, 25%, 50% and 100% replacement of NCA by RCA for each 0%, 20%, 40% and 60% replacement of cement by GGBFS. The effects of RCA and GGBFS on fresh and hardened concrete properties such as workability, compressive strength, split tensile strength, flexural strength, rebound number, density, water absorption and volume of voids are experimentally investigated. The test results obtained in the present investigation show that the workability increases with the use of RCA or GGBFS or both of these two. The compressive, split tensile and flexural strength decrease with increase in the percentages of RCA or GGBFS or both. The reduction in split tensile and flexural strength of the concrete mixes containing RCA or GGBFS or both RCA and GGBFS is less pronounced unlike its compressive strength. The values of rebound number obtained from non destructive test (NDT) show the similar trend with the results of compressive strength. Water absorption and volume of voids of the concrete mixes increases with increase in RCA content. However, the use of GGBFS improves the quality of the concrete mixes by improving the ITZ and bond between mortar and RCA. The concrete mix with 50% RCA and 40% GGBFS achieves values of these properties closer to those of the concrete mix without RCA and GGBFS. Finally, the concrete mix with 50% RCA and 40% GGBFS is considered as the optimum mix which is satisfying the target mean strength of the mix design and producing sustainable concrete by saving 40% of cement and 50% of NCA and utilizing maximum waste products such as GGBFS and RCA. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, concrete is an important material in the construction industries due to its significant contribution towards accelerated civilization. However, with the advancement in industrialization and urbanization, most of the developed regions suffer from major environmental problems like depletion of natural resources and sustainable waste management [1,2]. Moreover, ⇑ Corresponding author. E-mail address: [email protected] (A.N. Nayak). https://doi.org/10.1016/j.conbuildmat.2017.10.118 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

activities like rehabilitations, reconstruction and destruction of existing concrete structures produce large amount of construction and demolition (C & D) waste every year. To minimize the various environmental problems, many countries have been using the C & D waste as an alternative to the construction materials like natural fine and coarse aggregates (NFA and NCA) for making concrete. Aggregates prepared through screening, crushing and sieving of waste pieces of concrete is often called recycled coarse aggregates (RCA). Although the use of RCA as an alternative to NCA reduces the energy consumption and environment pollution, but the attached mortar of RCA make its physical and mechanical

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character inferior to that of NCA [3]. Previous review works reflect a number of studies related to the use of RCA in the concrete [4–8]. The two important factors related to RCA that influences the behaviour of recycled aggregate concrete (RAC) are content of RCA in the concrete and the strength of parent concrete from which the RCA have been obtained. Most of the previous studies observed no significant reduction in compressive strength of concrete when NCA is replaced up to 30% by RCA in making RAC [7,9–15]. However, compressive, split tensile and flexural strength of concrete with replacement of different percentages of NCA even up to 100% with RCA are reduced up to 30%, 10% and 23%, respectively in comparison to the normal aggregate concrete (NAC) [9,16–20]. Moreover, the reduction in elastic modulus is more pronounced (even up to 45%) than the other mechanical properties due to poor performance of RCA and weak transition between old and new mortar [8,21]. It has been also observed that water absorption of RCA is considerably higher than the NCA [9,14,22–24] which negatively affects the mechanical and durability of RAC as the substitution level of RCA increases in the concrete. Properties like drying shrinkage and creep coefficients also vary directly with variation in the content of RCA [21,25]. Similarly, many industrial by-products such as fly ash, GGBFS, metakaolin, silica fume and rice husk ash have been used as mineral admixtures for improvement in the properties of concrete. Among the available mineral admixtures, GGBFS has been potentially used in concrete as a replacement of cement [26–34] because of its latent hydraulic property which enhances the long term compressive strength and sometimes raises the early and later age flexural strength of concrete. Moreover, the use of GGBFS also improves the workability [35] and durability properties of concrete [31,36,37]. The previous studies reflect the poor mechanical and durability aspects of RAC in comparison to NAC. Therefore, to mitigate the deficiencies, different techniques have been proposed in the literature to enhance the RCA quality and the performance of RAC, such as treatment of adhered mortar of the RCA [38,39], strengthening of adhered mortar of RCA [40–45], two stage mixing approach [46,47] and use of mineral admixtures such as metakaolin, silica fume, fly ash, GGBFS and micronized biomass silica as replacements of cement [48–53]. Moreover, use of GGBFS in RAC improved the chloride penetration resistance and other durability properties of RAC [48,52]. As discussed above, limited research works were conducted to study the mechanical and durability properties of concrete using RCA and GGBFS [48,52,53]. Kou et al. [48] studied the properties, such as compressive strength, split tensile strength, ultrasonic pulse velocity (UPV), drying shrinkage and chloride penetration, of concrete using only two types of concrete mixes, i.e. concrete mix with 50% RCA and 55% GGBFS and 100% RCA and 55% GGBFS. Ann et al. [52] also investigated the compressive strength, split tensile strength, permeability and corrosion resistance of concrete made with 100% RCA and 65% GGBFS only. Cakir [53] considered the concrete mixes with 25%, 50%, 75% and 100% RCA for only two different percentages of 30% and 60% of GGBFS in order to study the properties like compressive strength, split tensile strength, density and water absorption. Since, the physical, chemical and mechanical properties of RCA and GGBFS have a wide range of variations depending upon their sources and hence, it is very difficult to reach a final conclusion with the above limited research. Moreover, flexural strength, non destructive test (NDT) parameter, such as rebound hammer, and other durability properties are yet to be addressed. This needs further extensive research on concrete using RCA and GGBFS available as waste products at different locations. Therefore, the present work aims to have a systematic study for assessing the influence of locally available RCA and GGBFS on various properties of concrete as mentioned above

including flexural strength and NDT characteristics for potential use of these materials in the field applications. 2. Experimental program 2.1. Materials 2.1.1. Binders The binding materials used in this investigation were 43 grade Ordinary Portland cement in an agreement to IS: 8112 [54] and GGBFS collected from Jindal Steel and Power Limited, Angul, India. Standard tests have been conducted in laboratory for determination of various physical and mechanical properties of the binders as per BIS specifications [55–59] and those results are furnished in Table 1. From Table 1, it is found that the Blaine surface area of GGBFS (5000 cm2/g) is higher than that of OPC (3200 cm2/g) which implies GGBFS is finer than that of OPC. Fineness of GGBFS is a major factor influencing the reactivity of GGBFS and early strength development of concrete [60]. It is reported by Swamy [26] that an increase in fineness of GGBFS to 2–3 times that of OPC benefits on engineering properties of concrete, such as setting time, bleeding, heat of evolution and durability. In order to obtain satisfactory performance the surface of GGBFS may be in the range of 4000–6000 cm2/g [61]. The surface area of the presently used GGBFS is within the range. The scanning electron microscopy (SEM) analysis of GGBFS was done and presented in Fig. 1. It is seen that the GGBFS consists of particles of random sizes and geometry and also the surface of the particles are relatively smooth. It is due to the interimpacting and interrubbing of granulated blast furnace slag between the steel balls in the ball mill. The X- ray diffraction (XRD) analysis of cement and GGBFS was also made and results were shown in Figs. 2 and 3, respectively. The results of XRD of cement (Fig. 2) show that the chemical compounds present in OPC are in crystalline form, which are visible by a number of sharp peaks. It is seen that the calcium silicate (Alite and Belite) is the main chemical constituents of OPC, which are mainly responsible for strength development. Similar type of XRD of OPC is also reported in the literature [19]. In addition to calcium silicate, the presence of tri calcium aluminate (C3A) and gypsum (CaSiO42H2O) are also observed in the XRD of OPC. Fig. 3 shows the XRD of GGBFS in which the XRD peaks are hardly identified. In addition, a wide diffusive hump between the angles 23° and 35° is also observed in the diffractogram of GGBFS. The above facts indicate that the GGBFS is mostly having amorphous character. However, small traces of gehlenite (mililite), merwinite and diopside, which are the major mineral composition of GGBFS, are identified. Apart from that the traces of calcite, wollastonite and quartz, which are minor components of GGBFS, are also observed. The chemical properties of the binders were obtained from the test and furnished in Table 2. From the chemical properties of the binders (Table 2), it is seen that in GGBFS, the lime (CaO) content

Table 1 Physical and mechanical properties of binders. Properties

Test method

OPC

GGBFS

Specific gravity Fineness (cm2/g) Consistency (%) Setting times Initial setting time (min) Final setting time (min) Mortar strength (MPa), 3 days 7 days 28 days

IS: IS: IS: IS:

3.11 3200 32

2.82 5000 –

140 300

– –

29.96 35.20 46.02

– – –

4031(Part-11) [55] 4031(Part-2) [56] 4031(Part-4) [57] 4031(Part-5) [58]

IS: 4031(Part-6) [59]

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Fig. 1. SEM photograph of GGBFS.

Fig. 2. XRD analysis of ordinary Portland cement.

Fig. 3. XRD analysis of GGBFS.

Contents

OPC

GGBFS

SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) SO3 (%) Loss of ignition (%)

19.11 8.61 1.79 64.39 1.62 2.1 1.6

34 14 4 33 7 – 0.8

respectively), which ensures its pozzolanic behaviour. Moreover, the ratios of lime/silica and alumina/silica of GGBFS are 0.97 (0.5–2) and 0.41 (0.1–0.6), respectively, which makes it best suited as binder [62]. Previous studies [28,60,61,63] reveal that GGBFS have mainly hydraulic activity (cementitious property) in addition to certain pozzolanic activity. The GGBFS can have cementitious property with some activators like lime, Portland cement, alkalis such as sodium carbonate or sulphate, calcium or magnesium. The hydraulic activity of GGBFS mainly depends on their production system and chemical compositions. The hydraulic activity of GGBFS has been determined from the chemical compositions using the formulae proposed by Smolezyk [64], which indicates that the GGBFS used in the present study posses low hydraulic activity. Moreover, ASTM C 989-06 [65] has recommended slag activity index (SAI) as a basic criterion for assessing the relative cementitious potential of GGBFS. Accordingly, GGBFS are classified into three grades such as grade 80, grade 100 and grade 120 depending upon their slag activity indices. From Table 3, it is seen that this GGBFS used here is fulfilling the requirements of grade 80 i.e. having low slag activity index. 2.1.2. Aggregates Locally collected siliceous type river sand and 20 mm crushed granite type aggregate were used as NFA and NCA, respectively. Similarly, 20 mm granite type aggregate with attached cement mortar obtained by crushing 10 years old precast railway sleepers from NALCO, Angul, India, was used as RCA. The grading analysis of natural fine aggregate (NFA) and coarse aggregates (NCA and RCA) were done in accordance to IS: 2386 (part-I) [66] and the results of the same were presented in Figs. 4 and 5, respectively along with the requirements as per IS: 383 [67]. The other physical and mechanical properties such as bulk density, specific gravity, water absorption, impact value, abrasion value and crushing value were determined as per IS: 2386 (Part-3) and IS: 2386 (Part-4) [68,69]. The results of the above physical and mechanical properties are furnished in Table 4 along with test methods. From Figs. 4 and 5, it is observed that the grading of natural fine aggregate (NFA) and coarse aggregates (NCA and RCA) are satisfying the grading limits as per IS: 383 [67]. However, RCA is finer than NCA. From the physical properties of NCA and RCA (Table 4), it is seen that the dry density (both loose and compact) of RCA is less than that of NCA. The physical observation of RCA used also reveals that the surface texture is more porous and rough due to porous nature of old mortar attached to RCA. Hence, the specific gravity of RCA is also less than that of NCA. The water absorption capacity of RCA is very high (4%) in comparison to that of NCA

Table 3 Slag activity index of GGBFS as per ASTM C 989-06 [65]. Slag activity index

(33%) is found to be lesser than that of cement (64.39%). However, the silica (SiO2) content (34%) and alumina (Al2O3) content (14%) of GGBFS is more than those of cement (19.11% and 8.61%,

7 days index 28 days index

ASTM C 989-06, min Grade 80

Grade 100

Grade 120

– 75

75 95

95 115

GGBFS used in the present study – 87

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Percentage Passing

120 100

Min IS: 383 [67] (Zone-III) NFA

80

Max IS: 383[67] (Zone-III)

2.2. Concrete mixes

60 40 20 0

0.15

0.3

0.6 1.18 Sieve size (mm)

2.36

4.75

Fig. 4. Grading analysis of NFA.

Percentage passing

120 100 80 60

Min- IS: 383 [67] for 20mm nominal size NCA Max- IS: 383 [67] for 20mm nominal size RCA

40 20 0

4.75

10 20 Sieve size (mm)

40

Fig. 5. Grading analysis of 20 mm NCA and RCA.

(0.5%) as the pores of RCA absorb more water. RCA is derived from C&D wastes generally consisting of NCA and adhered mortar. The old clinging mortar mainly contains NFA, hydrated and unhydrated cement particles. The most important feature of RCA is its old cement mortar which makes it porous due to high mortar content, inhomogeneous and less dense. Therefore, the properties of RCA are found to be inferior to those of NCA [9,15,21]. Though the mechanical properties of NCA and RCA, such as impact value, abrasion value and crushing value (Table 4) are within specified limits as per IS: 383 [67], these values of RCA are slightly higher than to those of NCA. This shows that RCA is slightly less resistant against mechanical action as compared to NCA. These inferior properties of RCA may be due to weak old attached mortar and poor bond between old mortar and coarse aggregate [9].

2.1.3. Water Normal municipal water free from deleterious materials conforming to IS: 10500 [70] was used in this experiment to produce concrete.

In order to fulfil the objective of the investigation, total 16 numbers of concrete mixes were prepared including one control mix. The control concrete mix was designed for M 25 grade concrete as per the guidelines of IS: 10262 [71] taking various ingredients such as cement, water, NFA and NCA, but without RCA and GGBFS. Other concrete mixes contained 25%, 50% and 100% of RCA in replacement of NCA for each 20%, 40% and 60% replacement of cement by GGBFS. All mix designs were carried out in accordance with the above guidelines [71] assuming the aggregates were at saturated surface dry (SSD) condition. Hence, the effective water content for all the mixes was constant. Further adjustment was made according to the water absorption and free surface moisture content of the aggregates used in each mix. All aggregates including RCA used in preparing concrete were air dried. The water content of control mix was obtained considering the water absorption and free surface moisture of NFA and NCA. But, water absorption of RCA is very high (4%) in comparison to that of NCA (0.5%). Therefore, to make them surface saturated dry, additional water on the basis of water absorption capacity and free surface moisture for different substitution level of RCA was added in every mix. Table 5 presents the mix designations, detailed mix proportions of concrete incorporating different percentages of RCA and GGBFS as a substitute of NCA and OPC, respectively, and extra water added in each mix to the water content of the control mix. 2.3. Casting of specimen and curing For preparation of concrete mixes, firstly the binder was prepared by mixing cement and GGBFS thoroughly. Then the binder, fine aggregate and coarse aggregates were mixed for two minutes at a slow rate in a concrete mixer. After that, required water was slowly poured into the solid mixture and the concrete mixer was rotated again for two minutes to get the desired workable concrete. After having a proper mix, the fresh concrete was poured in the slump cone and the slump value was measured. Thereafter, the fresh concrete was placed in specified moulds such as cube, cylinder and prism and kept for 24 h. After 24 h, the specimens were taken out from the moulds and placed in water in fully submerged condition at 27 ± 2 °C temperature for curing up to the desired age. 2.4. Testing of specimens In order to evaluate various properties of concrete for each mix, such as compressive strength, split tensile strength, flexural strength, rebound number (NDT), density, water absorption and volume of voids, the tests were performed on various sizes of specimens at different curing ages in accordance to Indian standard codes of practice and ASTM [72–75]. For carrying the compressive strength test as per IS: 516 [72], compressive load was applied to standard 150 mm size cubes at a constant loading rate of 14 N/

Table 4 Physical and mechanical properties of aggregates. Properties

Test method 3

Loose bulk density (kg/m ) Compact bulk density (kg/m3) Specific gravity Water absorption (%) Free surface moisture (%) Impact value (%) Abrasion value (%) Crushing value (%)

IS: IS: IS: IS: IS: IS: IS: IS:

2386 2386 2386 2386 2386 2386 2386 2386

(part-3) (part-3) (part-3) (part-3) (part-3) (part-4) (part-4) (part-4)

[68] [68] [68] [68] [68] [69] [69] [69]

NFA

RCA

NCA

IS: 383 limit [67]

1455 1578 2.63 0.8 0 – – –

1311 1450 2.53 4 1.95 25 27 22

1440 1575 2.77 0.5 0.4 22.03 25.1 19.41

– – – – 45 50 45

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Replacement of cement and NCA with GGBFS and RCA (%)

Cement (kg)

NCA (kg)

RCA (kg)

NFA (kg)

GGBFS (kg)

Water (kg)

Extra water (%)

RGC-00 RGC-01 RGC-02 RGC-03 RGC-10 RGC-11 RGC-12 RGC-13 RGC-20 RGC-21 RGC-22 RGC-23 RGC-30 RGC-31 RGC-32 RGC-33

0&0 0 & 20 0 & 40 0 & 60 25 & 0 25 & 20 25 & 40 25 & 60 50 & 0 50 & 20 50 & 40 50 & 60 100 & 0 100 & 20 100 & 40 100 & 60

390 312 234 156 390 312 234 156 390 312 234 156 390 312 234 156

1200 1200 1200 1200 900 900 900 900 600 600 600 600 – – – –

– – – – 274 274 274 274 548 548 548 548 1096 1096 1096 1096

710 710 710 710 710 710 710 710 710 710 710 710 710 710 710 710

– 71 142 212 – 71 142 212 – 71 142 212 – 71 142 212

195 195 195 195 203 203 203 203 207 207 207 207 215 215 215 215

– – – – 4 4 4 4 6 6 6 6 10 10 10 10

mm2/min in a 2000 kN capacity compression testing machine. The results of compressive strength were obtained after 7, 28 and 90 days, respectively. Split tensile strength test of concrete was carried out after 28 days by using cylindrical specimens of 150 mm diameter and 300 mm height in 2000 kN capacity compression testing machine in accordance to IS: 5816 [73]. Similarly, flexural strength test was conducted after 28 days on 100  100  500 mm size prisms according to IS: 516 [72] in 1000 kN Universal testing machine. The rebound number test was performed to analyse the non-destructive behaviour of concrete as per the part- 2 of IS: 13311 [74], using Schmidt Hammer (TYPE ND) manufactured by PROCEQ, Switzerland. For this test, minimum 9 readings were taken at different points on each face of 150 mm size cube. Rebound number of each cube was obtained as the average of all rebound numbers taken on four side faces in 150 mm cube placed in compression testing machine. Density, water absorption and volume of voids of standard 100 mm cubes of the concrete mixes were determined in accordance to ASTM C 642–13 [75]. The mean of results was obtained from testing of three specimens in each test and reported in the forms graphs or tables. Fig. 6. Variation of slump of concrete mixes with RCA and GGBFS.

3. Results and discussion 3.1. Workability of concrete The variation of slump of concrete mixes containing varying percentages of RCA and GGBFS is presented in Fig. 6. It is seen from the figure that, the slump value of concrete mixes with 0%, 25%, 50% and 100% RCA are 70, 75, 85 and 95, respectively. It implies that the slump values of concrete mixes increases with the increase in RCA content. It is to mention that the mix designs of all concrete mixes in this study are done based on the surface saturated dry (SSD) condition of the aggregates as per IS: 10262 [71]. Therefore, amount of extra water required for making air dried aggregates (NFA, NCA and RCA) to be SSD condition was calculated considering their water absorption capacity and free surface moisture. The extra amount of free water was required to be added for compensating higher water absorption capacity of RCA leading to higher amount of initial free water. The larger amount of initial free water is mainly responsible for the increase in the slump value in RAC. But later on, RCA absorbs the extra water added. Hence, the above extra water is not available for further reaction of cement. This trend of workability is found to be consistent with the results of previous studies [8,48,76]. Further, the concrete mixes were prepared by adding 0%, 20%, 40% and 60% GGBFS to a particular replacement of RCA, and it is

to be noted that no extra water is added for the inclusion of GGBFS in the mixes. The corresponding slump values are 70 mm, 75 mm, 80 mm and 95 mm for 0% RCA, 75 mm, 75 mm, 85 mm and 90 mm for 25% RCA, 85 mm, 85 mm, 90 mm and 95 mm for 50% RCA, and 95 mm, 95 mm, 110 mm and 115 mm for 100% RCA. It is found from Fig. 6 that in-spite of no extra water added to the concrete mix, the slump value increases with the increase in GGBFS content. This improvement in workability is owing to the smooth surface characteristics and better dispersion of GGBFS particles than the other constitutive materials, such as cement, fine aggregate and coarse aggregate, in the blend [77].

3.2. Compressive strength The variation of compressive strength of concrete containing varying percentages of RCA and GGBFS at 7, 28 and 90 days are furnished in Fig. 7. From this figure, it is evident that for all the three cases, the compressive strength of concrete reduces with the increase in the percentage of RCA. The 7, 28 and 90 days compressive strength of concrete without RCA and GGBFS (control mixRGC00) are 28.37 MPa, 37.77 MPa and 46.1 MPa, respectively. The 7 days compressive strength of RAC with 25%, 50% and 100% replacement of NCA by RCA (RGC-10, RGC-20 and RGC-30)

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Fig. 7. Variation of compressive strength of different concrete mixes.

decreases from 28.37 MPa (100%) to 27 MPa (95.2%), 25 MPa (88.1%) and 22.5 MPa (79.3%), respectively. Similarly, the 28 days compressive strength of above concrete mixes reduces from 37.77 MPa (100%) to 36.5 MPa (96.6%), 33.5 MPa (88.7%) and 30.5 MPa (80.7%). The similar trend is also observed for 90 days compressive strength of the above mixes and the values reduce from 46.1 MPa (100%) to 45 MPa (97.6%), 41.85 MPa (90.8%) and 37.68 MPa (81.7%), respectively. The compressive strength of RAC depends on many parameters like physical and mechanical properties of RCA, replacement level of RCA and w/c ratio [8]. This decrease in the compressive strength is due to the inferior quality of RCA in comparison to NCA, as discussed in sub-Section 2.1.2, and increase in replacement level of RCA. The influence of replacement of RCA on 28 days compressive strength of RAC as reported by several authors [9,10,22,50,51,53] in the literature has been presented in Fig. 8 along with the present results. It is worth mentioning that the past studies including present one used Portland cement as the main binder. From Fig. 8, it is observed that the present trend of decrease in compressive

Fig. 8. Variation of compressive strength of RAC with %RCA as reported in the literature.

strength of RAC with increase in the percentage of RCA is more or less consistent with the trends of the previous studies. However, the decrease is least for Elhakam et al. [10] except with 100% RCA, whereas same is highest for Hani et al. [51] except with 50% and 100% RCA. The performance of RAC with respect to decrease in compressive strength is superior for Elhakam et al. [10] followed by Rao et al. [9], present study, Kou and Poon [50], Kou and Poon [22], Cakir [53] and Hani et al. [51]. Since, this decrease depends upon the properties of RCA (which mainly depend on source of RCA) and w/c ratio in the mix in addition to the percentage of RCA, as already explained, there may be reduction in compressive strength of RAC with change in any above parameter even RCA content constant. It is seen from the literature that the sources of RCA of Hani et al. [51], Cakir et al. [53] and Kou and Poon [22] are laboratory waste cubes of different strength; rubbles of concrete, bricks, and marbles; and stones, old concrete, bricks and tiles, respectively, whereas that of present work is high strength prestressed concrete railway sleepers, due to which the physical and mechanical properties of RCA in the above mentioned past studies are observed to be inferior to those in the present study. Therefore, the performance of RAC in the present study is superior to that in the above studies [22,51,53]. Further, the 7 days compressive strength of the concrete containing 20%, 40% and 60% GGBFS only (RGC-01, RGC-02 and RGC03) reduces from 28.37 MPa (100%) of control mix to 25.25 Mpa (89.0%), 23.65 MPa (83.4%) and 20.2 MPa (71.2%), respectively. The 28 days compressive strength of the concrete with above mixes reduces from 37.77 MPa (100%) to 35.0 MPa (92.7%), 33.5 MPa (88.7%) and 29.0 MPa (76.8%), respectively. Similarly the 90 days compressive strength of above concrete mixes reduces from 46.1 MPa (100%) to 43.85 MPa (95.1%), 42.85 MPa (92.9%) and 39.5 MPa (85.7%), respectively. The above findings clearly reveal that the compressive strength of concrete mixes (RGC-01, RGC02 and RGC-03) at 7, 28 and 90 days decreases from the respective day’s compressive strength of the control mix. This decrease is due to the combined effects of lower strength gain in early days due to increase in GGBFS content and higher strength loss due to decrease in cement content in a particular mix. This is fact that the addition of GGBFS increases the compressive strength of concrete due to the hydraulic activity and slag activity index of GGBFS [28,60,61,63]. Moreover, GGBFS also have certain pozzolanic effect and filler effect in the concrete. The better particle packing due to finer materials and the pozzolanic reaction of GGBFS enhance the strength by making concrete homogeneous due to densification of the interfacial transition zone (ITZ) of old mortar, the weakest link in concrete [29]. The GGBFS presently used is having low hydraulic activity, low slag activity index and pozzolanic character, as discussed in sub-Section 2.1.1. On the other hand, the ordinary Portland cement has high hydraulic activity due to which the compressive strength decreases more due to the reduction of cement content in comparison to the strength gain because of addition of GGBFS. Finally, there is net decrease in the strength with replacement of cement with GGBFS. However, reduction in strength of concrete using GGBFS is least at 90 days (14.3%) followed by 28 days (23.2%) and 7 days (28.8%). This implies that the gain in compressive strength when using GGBFS in concrete is higher at 90 days than at early ages. This is due to the predominant reaction in hydration of GGBFS with Portland cement (PC) at a later age. The previous research work [60] reported that the hydration of GGBFS with PC is generally is a two stage reaction. In the initial hydration at early age, the predominant reaction is with alkaline hydroxide and in subsequent hydration at later age; the predominant reaction is with calcium hydroxide. In this case, the hydration of GGBFS with PC is predominant in the later age due to formation of calcium hydroxide when in contact with water over a long period of time, which is normally termed as latent hydraulic property

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Since the particle size of GGBFS is smaller than that of cement, part of GGBFS penetrate into the pores of RCA, which subsequently improve ITZ bonding between the paste and the aggregates. Secondly, cracks originally presents in RCA are filled by hydration products of GGBFS so the properties of RAC are enhanced as reported in the previous studies [48,53]. Further, it is noticed that with respect to compressive strength of the control mix, the percentage of decreases in compressive strength of concrete with all combination of RCA and GGBFS are least for 90 days, followed by 28 days and 7 days. The superior performance of these concrete at later age is also due to slow rate of hydration of GGBFS. It is reported in the literature [53] that only 18–55% of the GGBFS used as cement replacement has reacted by 28 days. The increase in the age of concrete increases the percentage of GGBFS reacted in the concrete and hence, enhances the compressive strength of concrete. Though there is very limited work in concrete with RCA and GGBFS, a comparison has been made between the results of Cakir [53] and present results on the compressive strength of concrete due to combined influence of RCA and GGBFS. It is reported by the above researchers that the decreases in the 28 days compressive strength of concrete with 60% GGBFS for each 25%, 50% and 100% RCA, with respect to the control concrete, are 33.01%, 37.26% and 48.34%, respectively, where as the corresponding decreases of the present study are 24.54%, 25.86% and 36.50%. This shows same trend for both the cases. However, the better performance is noticed in the present results than those of Cakir [53] due to superior quality of RCA and finer size of GGBFS used in the present study. Moreover, the decreases in the 28 days compressive strength of concrete containing 55% GGBFS for each 50% and 100% RCA, as reported by Kou et al. [48] are 20.68% and 32.75%, which are also comparable to the present one. Further, it is observed that out of nine concrete mixes with varying percentage of RCA and GGBFS, four concrete mixes, i.e. concrete mixes with 25% RCA and 20% GGBFS (RGC-11), 25% RCA and 40% GGBFS (RGC-12), 50% RCA and 20% GGBFS (RGC-21) and 50% RCA and 40% GGBFS (RGC-22) yield 28 days compressive strength of 31.85 MPa, which is more than the target strength of M25 grade concrete considered, i.e. 31.60 MPa. Among the four mixes, RGC-22 may be considered as the optimum mix for sustainable concrete as it utilizes the maximum waste products, i.e. 50% RCA and 40% GGBFS, and saves equal amount of natural resources such as OPC and NCA without sacrificing the strength requirement of the desired concrete. Therefore, it becomes economical and ecofriendly. However, though the other mixes (RGC-11, RGC-12 and RGC-21) are satisfying the strength requirement of the desired concrete, they utilize less quantity of waste products such as RCA and GGBFS in comparison to RGC-22 and thereby lose their importance in producing sustainable concrete. On the other hand, out of other five mixes, RGC-23 and RGC-32 have the 28 days compressive strength more than the target strength of M20 grade concrete (26.6 MPa) utilizing 50% RCA and 60% GGBFS, and 100% RCA and 40% GGBFS, respectively. Hence, these two mixes can be used for M20 grade concrete. Similarly, RGC-33 can be used for M15 grade

of GGBFS [63]. Moreover, the pozzolanic character of GGBFS also helps in gaining strength of concrete at later age [29]. As a result of the above, the strength gain in with GGBFS at later ages (90 days) is higher as compare to the concrete without GGBFS. The decrease in the 28 days compressive strength of concrete mixes with GGBFS only, as reported by several investigators [27,30,31,36,37,53] in the literature, has been compared with the present results. It is observed that all the above studies including present one show the similar trend, but the decrease in 28 days compressive strength due to addition of GGBFS is found to be least for Li et al. [37] followed by Lubeck et al. [36], Samad et al. [30], Wang et al. [31], present study, Cakir [53] and Siddique and Kaur [27]. This decrease depends upon the fineness, hydraulic property, slag activity index and pozzolanic character of GGBFS used in the above mixes as explained in sub-Section 2.1.1. From Fig. 7, it is observed that the 7, 28 and 90 days compressive strengths of the concrete with addition of different combination of RCA and GGBFS (RGC-11, RGC-12, RGC-13, RGC-21, RGC22, RGC-23, RGC-31, RGC-32 and RGC-33) decrease from those of the control mix at the corresponding ages. The decreases in the compressive strength of concrete containing 100% RCA and 60% GGBFS (RGC-33) at 7, 28, and 90 days are highest, i.e. 43.6%, 36.5% and 25.5%, respectively, with respect to the compressive strength of the control concrete mix at the corresponding days. It is observed that use of 60% GGBFS in RAC with 100% of RCA shows the worst performance for all three ages. This decrease is due to two reasons, i.e. poor quality of RCA with respect to NCA and lower rate of hydration of concrete containing GGBFS compared to concrete containing OPC [53]. In order to realise the influence of GGBFS on NAC and RAC, the percentages of loss in the compressive strength of NAC and RAC with 20%, 40% and 60% of GGBFS with respect to those without GGBFS are obtained and presented in Table 6. It is found that for 90 days the compressive strength losses of RAC are less than those of NAC with addition of 20%, 40% and 60% of GGBFS except the RAC with 25% RCA and 40% GGBFS (RGC-12) where the compressive strength loss of NAC (7.0%) is less than that of RGC-12 (7.7%). For 28 days, the similar trend is observed except RAC with 25% RCA and 40% GGBFS (RGC-12) and 100% RCA and 40% GGBFS (RGC32) where the loss of compressive strength (11.3%) is less than that of RGC-12 (11.8%) and RGC-32 (13.1%). But, for 7 days the compressive strength losses of NAC are less than those of RAC except RAC with 50% of RCA and 20%/, 40% and 60% of GGBFS (RGC-21, RGC-22 and RGC-33). The reduction in compressive strength of RAC mixes with different GGBFS contents with respect to their respective control RAC mixes is less in comparison to the reduction of compressive strength of NAC mixes with respective GGBFS contents from the control NAC mix at later ages (90 days). Hence, it may be inferred that the addition of GGBFS to RAC increases the rate of gain of compressive strength with age and shows more beneficial effect in RAC in comparison to the addition of GGBFS to NAC at later ages. When concrete is prepared with RCA and GGBFS, the two possible mechanisms enhance the properties of the concrete produced.

Table 6 Percentage of decrease in compressive strength of RAC with addition of GGBFS. RCA

GGBFS 7 days

0% 25% 50% 100%

28 days

90 days

0%

20%

40%

60%

0%

20%

40%

60%

0%

20%

40%

60%

0 0 0 0

11 11.1 7 11.1

16.6 18.5 13 20

28.8 26.0 24 28.8

0 0 0 0

7.3 6.8 3.2 8.2

11.3 11.8 5.0 13.1

23.2 22.0 16.4 21.3

0 0 0 0

4.9 4.4 2.0 4.4

7.0 7.7 2.3 6.8

14.3 12.6 7.5 9.2

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concrete as it is satisfying the target mean strength of above grade concrete utilizing maximum waste products, i.e. 100% RCA and 60% GGBFS and thereby replacing full amount of NCA and 60% of cement. 3.3. Split tensile strength The results of split tensile strength of concrete mixes with varying percentages of RCA and GGBFS at 28 days are presented in Fig. 9. The 28 days results show that the replacement of 25%, 50% and 100% NCA by RCA (RGC-10, RGC-20 and RGC-30) lowers the split tensile strength of concrete from 3.1 MPa (RGC-00) to 3.01 MPa, 2.88 MPa and 2.8 MPa, respectively. The decreases in split tensile strength of RAC, such as RGC-10, RGC-20 and RGC-30, are 2.9%, 7.1% and 9.7%, respectively, with respect to splitting strength of NAC (RGC-00). As reported in the previous studies [9,10,23,48,53], the tensile splitting strength of concretes with RCA are lower than that of concretes with NCA. This may be due inferior quality of RCA and weaker interfacial zone of old attached mortar in comparison to NCA. In this context, it is worthy to mention that the present study shows better results in comparison to the past results as mentioned above due to the use of superior quality of RCA. Further, the replacements of cement with 20%, 40% and 60% GGBFS only in the concrete mixes (RGC-01, RGC-02 and RGC-03) reduce the 28 days split tensile strength by 1.9%, 7.1% and 10.6%, respectively, with respect to that of NAC (RGC-00). This decrease in split tensile strength may be due to lower rate of hydration of concrete containing GGBFS compared to concrete containing OPC [53]. This trend of decrease in the split tensile strength is also observed in the previous studies [27,35,53]. It is seen from Fig. 9 that the 28 days split tensile strengths of the concrete with addition of different combination of RCA and GGBFS (RGC-11, RGC-12, RGC-13, RGC-21, RGC-22, RGC-23, RGC31, RGC-32 and RGC-33) decrease from those of the control mix (RGC-00). The worst performance is observed in RGC-33 with maximum decrease of 24.19%, followed by RGC-23 (18.70%), RGC-13 (17.94%), RGC-32 (17.74%), RGC-31(14.51%), RGC-22 (12.90%), RGC-12 (12.90%), RGC-21 (9.67%) and RGC-11 (8.06%). The spit tensile strength of concrete mixes with RCA and GGBFS mainly depends upon the quality and surface characteristics of RCA, strength of interfacial zone of old attached mortar [8,9,18] and fineness and hydration rate of GGBFS [28,60–62], as discussed

earlier. The results of split tensile strength of concrete with RCA and GGBFS reported by past studies [48,52,53] show the similar trend as above. It is well verified from the similar concrete mixes of the study of Cakir [53] that the reductions in split tensile strength of concrete with 60% GGBFS for each 25%, 50% and 100% RCA, with respect to the control concrete, are 18.18%, 18.18% and 24.24%, respectively, whereas the corresponding reductions of the present study are found to be 17.74%, 18.70% and 24.19%. In order to investigate the relationship between compressive strength and split tensile strength of the concrete mixes, a correlation between both of them has been established as displayed in Fig. 10. The relationship between compressive strength and split tensile strength obtained through the regression analysis is found qffiffiffiffi 0 0 to be f sp ¼ 0:552 f c where, f c is the cylinder compressive strength of concrete. The formula has been proposed to predict the split tensile strength from compressive strength values. The formulae available in the literature [78–84] for predictions of split tensile strength and flexural strength of concrete from the compressive strength are presented in Table 7 for different codes. A comparison of split tensile strength values obtained in experimental program with the values obtained according to different relations given in Table 7, is presented in Fig. 11. The characteristic 0 compressive strength values f c given in Table 7 indicate the 28 day cylinder strength. Therefore, the 28 day cube strength was converted to cylinder strength by using a correlation factor of 0.8 [77]. The comparative study indicates that there is good agreement between the present experimental results of split tensile strength and those obtained from ACI 318 [78]. However, the present split tensile strength values and the same predicted from ACI-318 [78] are found to be higher than the corresponding values obtained from the literature [80,82–84].

3.2 y = 0.5524x R² = 0.8749

3 2.8 fsp (MPa)

424

2.6 2.4 2.2 2 4.00

4.20

4.40

4.60

4.80 5.00 fc' (MPa)

5.20

5.40

5.60

Fig. 10. Relation between split tensile and compressive strength of concrete mixes.

Table 7 Formulae for prediction of split tensile strength (fsp) and flexural strength (ft) from compressive strength value (fc or fc0 ). Splitting tensile strength CEB-FIP [80] EHE [82]

0

ACI 318 [78] GB 10010 [83] NBR 6118 [84] 0

Fig. 9. Variation of Split tensile strength of different concrete mixes.

f sp

Flexural strength

 0 23 8 ¼ 1:56 f c10 2

f sp ¼ 0:21ðf c Þ3 qffiffiffiffiffi 0 f sp ¼ 0:56 f c f sp ¼

0 0:75 0:19ðf c Þ

f sp ¼

0 0:19ðf c Þ3

CEB-FIP [80] IS: 456 [79] ACI 318 [78] DG/TJ [81]

pffiffiffiffiffi f t ¼ 0:81 f c pffiffiffiffiffi f t ¼ 0:70 f c qffiffiffiffiffi 0 f t ¼ 0:62 f c pffiffiffiffiffi f t ¼ 0:75 f c

2

Note: fc and fc are cube and cylinder compressive strength of concrete at 28 days, respectively.

425

Split tensile strength (MPa)

R.K. Majhi et al. / Construction and Building Materials 159 (2018) 417–430

3.5 3 2.5 2 1.5 1 0.5 0

Experimental Result

ACI 318 [78]

CEB-FIP [80]

GB10010 [83]

EHE [82]

NBR 6118 [84]

Concrete mix Fig. 11. Comparison of experimental and predicted split tensile strength of concrete mixes.

3.4. Flexural strength The 28 days flexural strength results of concrete containing varying percentages of RCA and GGBFS are presented in Fig. 12. The figure shows that the flexural strength behaviour is similar to the split tensile strength behaviour. The flexural strength of concrete mixes containing 25%, 50% and 100% of RCA (RGC-10, RGC-20 and RGC-30) decreases by 2.6%, 3.9% and 6.5%, respectively, as compared to the flexural strength of NAC (RGC-00). This reduction in flexural strength may be due to inferior properties and weak ITZ of old attached mortar of RCA as compared to NCA. The reductions in flexural strength for the above replacements of RCA are reported to be 20.26%, 15.48% and 4.97%, respectively, by Rao et al. [9]. Similarly, the reductions of 2.32%, 6.96% and 10.9% are reported by Yang et al. [23] for 30%, 50% and 100% RCA replacements, respectively. It is seen that that the present study and Yang et al. [23] shows same trend of decreasing flexural strength with increase in RCA content. However, the results of Rao et al. [9] indicate a reverse trend with increase in RCA content, which may be due to more rough surface characteristics of RCA used and low water cement ratio as compared to the present study and Yang et al. [23]. Similarly, the replacements of 20%, 40% and 60% of GGBFS only to the concrete mixes (RGC-01, RGC-02 and RGC-03) decrease the flexural strength by 1.8%, 5.2% and 7.5%, respectively, with respect to that of control concrete (RGC-00). The decreasing trend of flexural strength is also observed in the past studies with 20% and 40%

reductions for the use of 40% and 80% GGBFS [33] and 10% and 20% reductions for the use of 20% and 40% GGBFS contents, respectively [34]. This decrease in flexural strength may be due to slower hydration of GGBFS than OPC in the concrete [53]. Moreover, the fineness of GGBFS also has certain effect in improving the microstructure of old attached mortar of RCA. Therefore, the present study having finer GGBFS in comparison to the previous studies [33,34] shows lesser reduction in flexural strength than the previous ones. From Fig. 12, it is clearly seen that 28 days flexural strength of concrete mixes incorporated with both RCA and GGBFS decreases with increase in the percentages of RCA and GGBFS. The decrease in flexural strength of RAC mixes containing 25% RCA with 20%, 40% and 60% GGBFS (RGC-11, RGC-12 and RGC-13) is found to be 4.42%, 7.01% and 10.39%, respectively, whereas, flexural strength of RAC mixes containing 50% of RCA and 20%–60% GGBFS (RGC21, RGC-22 and RGC-23) reduces by 5.19%, 8.05% and 10.39% respectively in comparison to control concrete (RGC-00). Similarly, for concrete mixes with 100% RCA and 20% to 60% GGBFS (RGC-31, RGC-32 and RGC-33), these reductions are found to be 7.79%, 11.69% and 18.18%, respectively. It can be seen from the above results that, RAC mixes such as RGC-11, RGC-12, RGC-21, RGC-22 and RGC-31 show the reductions in flexural strength less than 10% as compared to the control concrete (RGC-00). So, from the above mixes, RGC-22 and RGC-31 can be considered as the sustainable concrete mix with respect to flexural strength as it consumes maximum waste products with minimum reductions. The correlation between compressive strength and flexural strength has been investigated and presented in Fig. 13. A formula for predicting the flexural strength from compressive strength is also obtained by performing regression analysis and indicated in this figure. The formula for predicting the flexural strength of

4 3.9 3.8

y = 0.6408x R² = 0.7813

ft (MPa)

3.7 3.6 3.5 3.4 3.3 3.2 3.1 3 4.50

Fig. 12. Variation of flexural strength of different concrete.

5.00

5.50 fc (MPa)

6.00

6.50

Fig. 13. Relation between flexural and compressive strength of concrete mixes.

426

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Flexural strength (MPa)

6

CEB-FIP [80]

DJ/TJ [81]

IS: 456 [79]

Experimental Result

ACI 318 [78]

5 4 3 2 1 0

Concrete mix Fig. 14. Comparison of experimental and predicted flexural strength of concrete mixes.

pffiffiffiffi concrete is given by f t ¼ 0:64 f c ; where fc is the 28 days compressive cube strength of concrete. The comparison of the 28 days flexural strength of present experimental investigation with those predicted from the formulae given in Table 7 is shown in Fig. 14. It is to be noted that the 28 days cylinder strength is used in the relation given by ACI 318 [78] and for other relations, cube strength is used. It is observed from Fig. 14 that flexural strength value of present experimental work is found to be lower than the values obtained from IS: 456 [79], DG/TJ [81] and CEB-FIP [80] and higher than ACI 318 [78]. The decrease in 28 days compressive strength, split tensile strength and flexural strength of concrete mixes with varying replacement of RCA and GGBFS with respect to the corresponding values of the control mix is presented in Fig. 15. From this figure, it is clearly evident that the decrease in flexural strength is less followed by split tensile strength and compressive strength. In other words, the concrete mixes with varying replacements of RCA and GGBFS show comparatively better performance in flexural strength and split tensile strength than in compressive strength. This may be due to the improvement of microstructure of ITZ and bond strength between mortar and RCA because of rough surface of RCA and finer particles of GGBFS as discussed earlier. 3.5. Non-destructive test

% Decrease in strength

The rebound number test results obtained after 28 days for all the concrete mixes considered are shown in Fig. 16. It can be seen from the figure that the rebound number values of the concrete mixes shows the similar trend as in case of the compressive strength of these mixes. The rebound number of the concrete mix decreases with the increase in percentage of RCA or GGBFS or both RCA and GGBFS. It is worth mentioning that the rebound number is directly related to the compressive strength and density

40 35 30 25 20 15 10 5 0

Compressive Strength Flexural tensile Strength

Split tensile Strength

Concrete Mix Fig. 15. Comparison of 28 days strength reductions of concrete mixes.

Fig. 16. Variation of Rebound Number of concrete mixes.

of the concrete mix. Since, the addition of GGBFS and RCA reduces both the compressive strength and density, the rebound number of the concrete with RCA and GGBFS is less compared to the control concrete. The correlation between the rebound number and compressive strength of concrete mixes considered is established and shown in Fig. 17 with best fit curve having R2 value of 0.92, which implies good correlation between rebound number values with the compressive strength of concrete mixes.

3.6. Density The variation of 28 days density of concrete mixes with varying percentages of RCA and GGBFS is shown in Fig. 18. The density of control concrete (RGC-00) is 2498 kg/m3, which decreases to 2480 kg/m3, 2446 kg/m3 and 2360 kg/m3 for 25%, 50% and 100% substitution of NCA by RCA (RGC-10, RGC-20 and RGC-30), respectively. It clearly indicates that the density decreases with increase in RCA replacement and the same trend is also reported in the literature [9,53]. This reduction in density is mainly due to the lighter weight and porous nature of the old mortar attached to RCA. It is reported that, for replacement of 25%, 50% and 100% RCA, the decrease in density of RAC is 3.56%, 4.67% and 10%, respectively, for Rao et al. [9] and 7.8%, 10.3% and 20.1%, respectively, for Cakir [53] and, whereas, the present study shows the corresponding reductions as 0.72%, 2.08% and 5.44%. The least reduction in the present study may be due to the use of RCA having comparatively

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Compressive strength (MPa)

40 35 30 25 y = 1.1232x + 1.6328 R² = 0.9201

20 15 10 5 0 18

20

22

24

26

28

30

32

34

Rebound number Fig. 17. Relation between rebound number and compressive strength of concrete mixes.

specific gravity of RCA and GGBFS and for that there is a final decrease in density due to the use of RCA and GGBFS. Cakir [53] has also reported the same trend of density with maximum reduction of 29.68% for concrete mix with 100% RCA and 60% GGBFS followed by 26.59% for 100% RCA and 30% GGBFS, 20.49% for 50% RCA and 60% GGBFS, 18.06% for 50% RCA and 30% GGBFS, 17.74% for 25% RCA and 60% GGBFS and 13.41% for 25% RCA and 30% GGBFS. It can be noticed that, the reduction in density of the present study is less than that of Cakir [53]. Although, the specific gravity of GGBFS used in present study and Cakir [53] is almost same, but the specific gravity of RCA used in the present study (2.53) is more than that of Cakir [53] (2.31) and moreover, the fineness of the GGBFS used in the present study is also higher than that of Cakir [53]. Hence, due to the use of high specific gravity RCA and high fineness GGBFS, the present study shows less reduction in density as compared to Cakir [53]. 3.7. Water absorption

2650 0% RCA

25% RCA

50% RCA

100% RCA

Density (kg/m3)

2550 2500 2450 2400 2350 2300 2250

0

20

40

60

% GGBFS Fig. 18. Variation of density of concrete mixes.

high specific gravity RCA (2.53) in comparison to previous studies [9,53]. Further, the density of control concrete (RGC-00) decreases by 0.72%, 1.24% and 1.88% for 20%, 40% and 60% replacements of GGBFS (RGC-01, RGC-02 and RGC-03), respectively. Same trend with 5.45% and 8.55% reduction in density for 30% and 60% GGBFS replacements, respectively are reported by Cakir [53]. This decrease in density is mainly due to the lower specific gravity GGBFS used in the concrete in comparison to that of OPC. However, the fineness of the GGBFS has also certain effect on density as more quantity of finer GGBFS, which act as a filler material, can penetrate in to the aggregates and make the concrete denser. Although, the present study and Cakir [53] used the GGBFS, which have almost same specific gravity, but the GGBFS used in the former one has more fineness than that of later one which is responsible for making the present concrete denser. Similarly, Fig. 18 clearly indicates that density of concrete incorporated with both RCA and GGBFS decreases with increase in the percentages of RCA and GGBFS. The maximum decrease in density is observed for RGC-33 (8.01%) followed by RGC-32 (7.05%), RGC31 (6.08%), RGC-23 (3.92%), RGC-22 (3.12%), RGC-21(2.60%), RGC13 (2.32%), RGC-12 (1.68%) and RGC-11 (1.12%), respectively. This reduction may be due to lower specific gravity of both RCA (2.53) and GGBFS (2.82) in comparison to NCA (2.77) and OPC (3.11), respectively. However, the higher fineness of GGBFS than OPC has little contribution towards the increasing of density of concrete by filling the pores of RCA and making it denser. But, this effect is quite insignificant in comparison to the effects of lower

The variation of 28 days water absorption capacity of concrete mixes prepared with different percentages of RCA and GGBFS is shown in Fig. 19. It is seen from the figure that the water absorption of concrete mixes increases from 4.88% (RGC-00) to 5.65%, 6.87% and 7.45%, respectively for the substitution of 25%, 50% and 100% NCA by RCA (RGC-10, RGC-20 and RGC-30). Hence, the increments in water absorption capacity of concrete mixes with 25%, 50% and 100% RCA are 0.77%, 1.99% and 2.57%, respectively, with respect to the NAC (RGC-00). This increment in the water absorption capacity of RAC is due to the high absorptive nature of RCA used. Similar trend of increase in water absorption capacity is also reported in the literature for varying percentages of RCA [9,53]. It is also reported in the literature that the increases in water absorption capacity of concrete with 25%, 50% and 100% are 0.86%, 0.99% and 1.82%, respectively, for Rao et al. [9] and 2.1%, 4.5% and 10.5%, respectively, for Cakir [53]. From the above studies, the highest increase in water absorption capacity of concrete due to addition of RCA is observed for Cakir [53] followed by present study and Rao et al. [9]. This can be explained by the fact that water absorption of RCA used by Cakir [53] is 7.4% followed by 4% for present study and 3.92% for Rao et al. [9]. It is observed from Fig. 19 that incorporation of 20%, 40% and 60% GGBFS to control concrete (RGC-01, RGC-02 and RGC-03), reduces the water absorption capacity from 4.88% to 4.84%, 4.70% and 4.65%, respectively, i.e. the corresponding decreases in water absorption of the above concrete mixes are 0.04%, 0.18% and 0.23%, which seem to be marginal. Therefore, the addition of GGBFS has very little effect on NAC in reducing water absorption.

10 9

Water absorption (%)

2600

0% RCA

25% RCA

50% RCA

100% RCA

8 7 6 5 4 0

20

40

% GGBFS Fig. 19. Variation of water absorption of concrete mixes.

60

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When both RCA and GGBFS are added as replacements of NCA and OPC in the concrete, there is net increase in the water absorption capacity for all the mixes with respect to the control concrete (RGC-00). The water absorption values of RAC mixes containing 25% RCA with 20%, 40% and 60% GGBFS (RGC-11, RGC-12 and RGC-13) are found to be 5.59%, 5.42% and 5.33%, respectively, whereas, water absorption of RAC mixes containing 50% of RCA and 20% to 60% GGBFS (RGC-21, RGC-22 and RGC-23) are 6.66%, 6.53% and 6.33%, respectively. Similarly, the same for RAC mixes with 100% RCA and 20% to 60% GGBFS (RGC-31, RGC-32 and RGC-33), are 7.42%, 7.32% and 7.14%, respectively. The highest value of water absorption is recorded as 7.45% for concrete with 100% RCA (RGC-30) and the lowest value was recorded as 4.65% for NAC with 60% GGBFS (RGC-03). The addition of GGBFS to both RAC and NAC shows similar trend with respect to water absorption capacity i.e. water absorption capacity of both the concrete reduces marginally with increase in GGBFS content (Fig. 19). However, the water absorption capacity for RAC with addition of both RCA and GGBFS increases primarily due to increase in RCA content only as there is little effect of GGBFS on it. It is reported in the literature that, the water absorption capacity of RAC with 100% RCA and 60% GGBFS is 15.40% [53] whereas the corresponding value for present case is 7.14%. The water absorption capacities of NAC for both the cases are nearly same, i.e. 5.10% and 4.88%, respectively. Hence, the present result of RAC with above RCA and GGBFS content is better than that of Cakir [53] as the quality of RCA used in the former case is superior to that of the later one.

in present study (0.5) is also higher than that (0.43) for Rao et al. [9] in making RAC. The volume of voids of the control mix decreases from 19.10% to 18.95%, 18.64% and 18.46%, for addition of 20%, 40% and 60% GGBFS, respectively (RGC-01, RGC-02 and RGC-03) as observed in Fig. 20. Hence, the corresponding decreases in volume of voids are found to be 0.15%, 0.46% and 0.64%, which appear to be insignificant. Therefore, the addition of GGBFS has very slight effect on NAC in reducing volume of voids. For addition of both RCA and GGBFS in the concrete, there is a final increase in volume of voids for all concrete mixes in comparison to RGC-00. The volume of voids of RAC mixes containing 25% RCA with 20%, 40% and 60% GGBFS (RGC-11, RGC-12 and RGC-13) are found to be 20.39%, 20.03% and 19.74%, respectively, whereas, volume of RAC mixes containing 50% of RCA and 20% to 60% GGBFS (RGC-21, RGC-22 and RGC-23) are 22.39%, 22.02% and 21.53%, respectively. Similarly, the same for RAC mixes with 100% RCA and 20% to 60% GGBFS (RGC-31, RGC-32 and RGC-33), are 23.35%, 23.02% and 22.53%, respectively. The highest value of volume of voids is found to be 23.51% for concrete with 100% RCA (RGC-30) and the lowest value is recorded as 18.46% for NAC with 60% GGBFS (RGC-03). The above results depict that the volume of voids of a particular RAC mix decreases with increase in GGBFS content, as seen in the case of water absorption. This reduction in volume of voids with increase in GGBFS content for a particular RAC mix is due to the use of fine GGBFS that fills the pores of the attached mortar of the RCA and ITZ of RAC mix as discussed earlier.

3.8. Volume of voids 4. Conclusion The results of 28 days volume of voids of concrete mixes containing varying percentages of RCA and GGBFS are presented in Fig. 20. It is seen from the figure that the volume of voids of concrete mixes follows the same trend as in case of water absorption. It increases from 19.10% (RGC-00) to 20.59%, 22.80% and 23.51%, respectively for 25%, 50% and 100% replacement of NCA by RCA (RGC-10, RGC-20 and RGC-30). Hence, the increments of 1.49%, 3.7% and 4.41% in volume of voids are observed for the corresponding mixes in comparison to NAC (RGC-00). This increase in volume of voids of RAC is mainly due to the porous nature of attached mortar of RCA. Rao et al. [9] also observed the same trend of increasing volume of voids with 1.52%, 1.65% and 2.61% increments for 25%, 50% and 100% RCA replacements, respectively. From the above studies, higher increments in volume of voids are observed for present study in comparison to those of Rao et al. [9] due to higher water absorption of RCA used in the present study (4%) than that (3.92%) of Rao et al. [9]. Moreover, the water cement ratio used

Volume of Voids (%)

27 26

0% RCA

25% RCA

25

50% RCA

100% RCA

24 23 22 21 20 19 18 17 0

20

40

% GGBFS Fig. 20. Variation of volume of voids of concrete mixes.

60

The present study attempts to develop sustainable cement based construction materials and sustainable waste management by preserving natural resources and using C&D waste. Accordingly, various concrete mixes with varying percentages of replacement of NCA by RCA and OPC by GGBFS have been made and the effects of RCA and GGBFS on the properties of concrete are studied. Based on the analysis of experimental outcomes, the conclusions can be drawn as follow: 1. The slump of fresh concrete increases with increasing RCA, GGBFS, and both RCA and GGBFS contents. 2. The mechanical properties such as compressive, split tensile and flexural strengths of concrete mixes decrease from the corresponding values of the control mix with increase in the percentage of RCA, GGBFS or both of these two. The concrete mixes with specific combinations of RCA and GGBFS taken in the study (RGC-11, RGC-12, RGC-21 and RGC-22) show marginal decrease in the above strengths with respect to those in control mix. 3. The concrete with 50% RCA and 40% GGBFS (RGC-22) can be considered as optimum one for sustainable concrete of M25 grade based on the parameters of present investigation. Moreover, the concrete mixes with 50% RCA and 60% GGBFS (RGC-23) and 100% RCA and 60% GGBFS (RGC-33) can be considered for the concrete of grade lower than M25 of lower grades concrete, which utilize the maximum waste materials. 4. The effectiveness of GGBFS in RAC increases with the age of concrete in comparison to NAC and is more at 90 days with respect to 7 and 28 days. 5. The concrete mixes containing different percentages of RCA and GGBFS show comparatively better performance in flexural and split tensile strength in comparison to the compressive strength due to the rough surface characteristics of RCA and finer particles of GGBFS.

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6. The values of the split tensile strength of the concrete with varying percentage of RCA and GGBFS obtained from the present experimental investigation are in good agreement with those predicted from ACI 318 [78]. Similarly, the values of the flexural strength obtained from the present study are in between those predicted by ACI 318 [78] and IS 456 [79]. 7. The trend of rebound number, density, water absorption and volume of voids test results conforms to the mechanical properties of RAC. Moreover, further experimental investigation may be done by adding suitable admixture to the concrete containing RCA and GGBFS for reducing water content and enhancing the mechanical and other properties, keeping slump constant. Moreover, the durability properties and microstructure study of the above concrete may be investigated.

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