Durability assessment of self compacting concrete incorporating copper slag as fine aggregates

Construction and Building Materials 155 (2017) 617–629

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Durability assessment of self compacting concrete incorporating copper slag as fine aggregates Rahul Sharma ⇑, Rizwan A. Khan Dr. B R Ambedkar National Institute of Technology, Department of Civil Engineering, Jalandhar, Punjab 144011, India

h i g h l i g h t s
Increase in the content of copper slag enhanced the fresh properties.
Minimum depth of carbonation was noticed for 100% copper slag substitution.
Change in weight and compressive strength were used to assess Sulfate attack.
The maximum electrical resistivity was observed at 20% copper slag content.
Sorptivity and initial surface absorption reduced up to 60% copper slag substitution.

a r t i c l e

i n f o

Article history: Received 22 April 2017 Received in revised form 26 July 2017 Accepted 15 August 2017

Keywords: Self Compacting Concrete Copper slag Fresh properties Accelerated carbonation Sulfate attack Electrical resistivity

a b s t r a c t The present study intends to evaluate the durability of Self Compacting Concrete (SCC) containing copper slag as fine aggregates. A total of six SCC mixes were cast with 0%, 20%, 40%, 60%, 80% and 100% copper slag substitution at constant w/b ratio of 0.45. The various tests conducted on SCC mixes included fresh properties, compressive strength, sulfate attack, accelerated carbonation, electrical resistivity, ultrasonic pulse velocity, initial surface absorption and sorptivity. Results showed that fresh properties enhanced with increment in copper slag substitution. The maximum compressive strength was noticed for 20% copper slag. In sulfate exposure, gain in weight and decrease in compressive strength was observed for concrete mixes. Incorporation of copper slag has significant effect in the reduction of carbonation. The benefit of utilizing copper slag in construction industry bestows as substitute to fine aggregates, preserves natural resources and no land management for disposal of copper slag. This study suggests that 60% copper slag is an optimum content as partial replacement to conventional sand for either enhanced or comparable durability behavior of SCC. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction The construction of large structures and mega projects with highly dense reinforcement has led to the introduction of revolutionary concrete in the construction industry namely Self Compacting Concrete (SCC) in 1988 [1]. SCC does not require any vibrators or external source of energy to fill the formwork, narrow openings and congested reinforcement. SCC flows due to its self weight and high mortar content with lower content of coarse aggregates while maintaining homogeneity without segregation and bleeding. One of the drawbacks related to SCC is its high cost which is due to the utilization of superplasticizer and huge amount of cement content. To overcome the limitation of SCC, supplementary cementitious materials are used as partial replacement of ⇑ Corresponding author. E-mail address: [email protected] (R. Sharma). http://dx.doi.org/10.1016/j.conbuildmat.2017.08.074 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

cement to reduce the cost and improve the durability as well as sustainability. Durability may be defined as resistance of the concrete to the chemical attack, biological attack and physical disintegration [2– 4]. The chemical attack may occur in any form such as sulfate attack [5], acid attack [6], carbonation [7], alkali silica aggregate reaction and many more depending up on the concrete subjected to the environmental conditions. The physical disintegration may be due to overloading of the concrete structures, abrasion, impact, frost attack and natural calamities such as earthquake, flood, fire etc. [8]. The biological attack involves deterioration caused by bacteria, mosses, lichens, marine borers, boring shells and sponges [9,10]. However, chemical attack is mostly responsible for the core destruction of concrete structures [11–13] and durability is the urgent need to be worked by the researchers [14–16]. Increasing infrastructural growth as a result of mounting urbanization, demand for the huge amount of construction materials such

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as cement, fine aggregates and coarse aggregates has been augmented. To meet the need of construction industry, about 48.3 billion tons of construction aggregates were produced globally in 2015 [17]. This has an adverse impact on the construction industry since the amounts of these resources are already limited and the needs of future generations cannot be fulfilled using the scarcely available natural resources. The extraction and processing steps of these materials also create serious environmental concerns and are energy intensive tasks. In developing countries like India and China, there has been continuous development in the construction industry which creates scarcity of fine aggregates. The combined effect of these factors has made the construction extremely unsustainable and there is an urgent need to look for some potential alternatives which can contribute towards sustainability of construction industry. The use and application of various industrial by-products as fine aggregates in most of the countries is severely limited, though generated in huge amount leading to heaps of dump covering larger area of land. The utilization of artificial aggregates or industrial by-products in the construction sector resolves the encumbrance on the consumptions of barely existing virgin resources. This in turn ameliorates in the conservation of natural resources and prevention of extraction of instinctive resources. Copper slag (CS) is an industrial waste generated in huge amount from copper industries and can be exploited as a potential substitute to river sand in concrete production. The CS is obtained as by-product from copper metal either as dense CS or granulated CS depending upon the method of cooling of molten slag discharge from the furnace. The molten slag solidified by pouring into the water results in granulated CS whereas gradually air cooled slag form the dense CS. It was estimated that to yield 1 ton of copper, about 2.2–3 tons of CS is generated as a by-product. CS is rich in iron and contains various other types of oxides, which includes SiO2, Al2O3, CaO and Fe3O4 [18,19]. Globally in the year 2015, approximately 68.7 million tons of CS was generated from the world copper industry. China accounted for one-third of world CS production followed by Japan 9%, Chile 8% and Russia 5%. In India, nearly 2.4 million tons of CS turned out in the same year, which shared about 3.5% of world’s CS [20]. In past, CS has been used in construction industry either alternative part to cement or partial/full replacement of sand. The CS is also used for manufacturing of abrasive tools, tiles, granules, cutting tools, pavements, and sand blasting purposes. The utilization of CS in construction industry is expected to offer several advantages in terms of sustainability as well as performance. The negligible carbon footprints along with minimal energy consumption associated with CS have already attracted the attention of various researchers in the last decade. The constructive properties of CS have made it promising alternative to fine aggregates from both market and technological points of view. The novelty of the research work is utilization of industrial byproduct CS as fine aggregates in the development of SCC. The CS has potential to resolve the scarcity of natural resources in the construction industry for the development of the infrastructure. The previous investigations have either emphasized on the production of normal vibrated concrete or high performance concrete but the development of SCC using CS has been constantly ignored. The prime objective of the current study is to utilize CS to achieve either comparable or better performance of concrete in terms of durability.

rate raised expeditiously [21]. The resistance to chloride penetration was maximum and water absorption, as well as sorptivity, decreased in high performance concrete by utilizing 2% colloidal nano-silica as cement replacement with 40% CS as fine aggregates [22]. The use of CS as cement replacement improved the resistance against sulfate attack by reducing the expansion of the specimens exposed to the sulfate exposure [23] whereas increase in the expansion exhibited when copper tailing was incorporated as partial replacement of cement [24]. The use of copper tailings as cement substitution increased the water absorption and permeable voids, although resistance to acid attack was slightly higher and chloride penetration depth was significantly lower with reference to control concrete [24]. The capillary channels and microcracks appear in the microstructure with increase in the CS content [25] and beyond 50% CS substitution, surface absorption proliferated and percentage volume of permeable voids was similar to control concrete [26]. The alkali activated slag concrete subjected to acid attack showed deterioration with increase in CS content but rate of deterioration was still less than ordinary Portland concrete. Conversely, water absorption decreased with increment of CS substitution [27]. The expansion caused by alkali-silica reaction reduced when CS waste was utilized as cement replacement. It was also observed that water and chloride penetration depth decreased with inclusion of 10% CS as cement substitution [28]. The replacement of cement by 20% CS reduced the capillary absorption and carbonation of the concrete at different w/b ratios [29]. The addition of 5% copper tailing in concrete mixes was more effective in delaying the corrosion initiation than using as partial replacement of cement [30]. The copper mine tailing satisfied all the properties of aggregates except water absorption according to standard specification for Malaysia road works [31]. The incorporation of 20% copper mine tailing in asphalt mixture can be used for the construction of roads [32]. The use of 10% CS as fine aggregates in asphalt-concrete mix improved the indirect tensile strength [33]. Utilization of CS as aggregates in the cold in-place recycling (CIR) mixtures enhances resistance to the moisture [34]. The utilization of CS as fine aggregate can improve the permeation properties such as absorption, permeability and diffusion [35]. The use of CS in concrete either as supplementary cementitious material or fine and coarse aggregates showed long term durability [36]. The incorporation of 100% CS as fine aggregates in combination with FA and SF in SCC has better performance in strength and absorption characteristics than control concrete containing 0% CS [37]. An optimum concentration of 40% CS has been recommended as a substitute to sand in concrete [38,39]. The utilization of CS in construction industry serves many benefits such as reduction in the cost of concrete and dosage of superplasticizer, acts as substitute to fine aggregates, reduction in energy consumption and CO2 emission, no land management for disposal of CS and preservation of natural resources and ecological balance. The literature is scanty on SCC containing CS as replacement of sand or cement. The focus of the past research has been on the mechanical properties of normal vibrated concrete with few durability properties using CS mostly as cement and rarely as fine aggregates. Till now, no research has been found to be reported on the durability properties of SCC using CS as fine aggregates to the best of author’s knowledge. The aim of this study is to assess the durability of SCC incorporating CS as fine aggregates.

2. Literature review 3. Experimental program In last decade, research has been carried out on the durability of normal and high performance concrete using CS either as fine aggregates or supplementary cementitious material. The durability of high performance concrete in term of surface water absorption was found to enhance up to the inclusion of 40% CS substitution as fine aggregate, beyond that replacement content, absorption

3.1. Materials Ordinary Portland cement (OPC) of 43 grade equivalent to ASTM Type I cement was used in this study meeting the specifications of IS 8112 and ASTM C 150M-17 [40,41]. Class F fly ash (FA) was used

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3.2. Mixtures The six concrete mixes were prepared with different proportion of CS ranging from 0% to 100% at an increment of 20%. Control mix was designated as OF-CS0 where O stands for OPC, F for Fly ash and CS0 for 0% CS. In the same way, other mixes were designated with change in the percentage of CS. The total cementitious content of 550 kg/m3 was kept constant for all mixes containing fix amount of OPC and FA of 330 kg/m3 and 220 kg/m3 respectively at constant w/b ratio of 0.45. The mix proportions of all concrete mixes are shown in Table 3. The quantities of CS and sand were varied by equivalent volume method. The total coarse aggregate and water content of 700 kg/m3 and 247.5 kg/m3 respectively were fixed for whole mixes. Firstly coarse aggregates, sand and CS were mixed in the power driven laboratory mixer for 1 min with capacity of 140 l and rpm of 22–24. Then binders were added in the mixer and rotated again for 1 min to obtain a uniform mixture of different dry constituents of concrete. Afterwards, 70% water was discharged into the mixer and rotated for around 3 min to achieve homogeneous and uniform mixing. Subsequently, the remaining 30% water was mixed with required dosages of superplasticizer and finally mixed for another 2 min. The prepared mixtures were carried out to test the fresh properties of SCC. The obtained SCC mixes were then poured into the well-oiled moulds and demoulded after 24 h. Samples were water cured in temperature controlled curing tank at 27 ± 2 °C till the age of testing. Table 1 Physical properties of OPC and FA. Characteristic properties

OPC

FA

Standard consistency (%) Initial Setting time (minutes) Final setting time (minutes) Soundness by Le-Chat Expansion (mm) Compressive strength (MPa) 3 days 7-days 28-days Specific gravity Loss on ignition (%)

32 62 270 1.0

– – – – –

Note: OPC- Ordinary Portland Cement, FA- Fly Ash.

24.6 34.3 45.2 3.15 2.04

2.1 3.3

Table 2 Chemical composition of OPC, FA and CS. Component (%)

OPC

FA

CS

SiO2 Al2O3 Fe2O3 CaO MgO SO3 K2O Na2O TiO2 CuO ZnO PbO NiO As2O3 Cr2O3

20.99 5.98 4.10 60.78 0.96 2.86 1.18 0.86 0.25 – – – – – –

57.6 30.5 3.72 1.10 0.38 0.22 1.35 0.10 1.72 0.017 – – – – –

30.53 2.80 57.82 1.60 1.48 1.59 0.71 0.34 0.26 0.64 0.4 0.08 0.008 0.05 0.04

Note: OPC- Ordinary Portland Cement, FA- Fly Ash, CS- Copper Slag.

100 90

Percentage Passing (%)

as partial replacement of cement by 40%. The total sum of SiO2, Al2O3 and Fe2O3 was found to be more than the minimum requirement of 70%, specified for class F fly ash [42]. The physical properties of OPC and FA are shown in Table 1 and their chemical compositions have been shown in Table 2. Locally available natural sand was used in this experimental study, conforming IS 383 and ASTM C 33M-16 [43,44]. The sand was found to lie in the zone II as per IS 383 with water absorption of 0.80%, fineness modulus of 2.79 and specific gravity of 2.6. Granulated CS was used as substitution to natural sand from 0% to 100% at an increment of 20%. CS was procured from Taj Abrasive Industry situated in Sikar, Rajasthan, India. Fig. 1 illustrates particle size distribution of sand and CS. The chemical composition of CS is shown in Table 2. CS has water absorption of 0.36% one-half of sand, fineness modulus of 3.33 and specific gravity of 3.51. CS is large in size and lying in the zone I as per IS 383. Natural coarse aggregate in the saturated surface dry condition was used in the proportion of 40% and 60% of nominal size 12.5 mm and 10 mm respectively. Superplasticizer, Master Glenium SKY 8765 based on polycarboxylic ethers was used in different percentages by weight of binder to achieve fresh properties of SCC. It complied the specification of IS 9103 [45] with relative density of 1.07, chloride content less than 0.2 and pH 6 at 25 °C.

80 70 60 50 40 COPPER SLAG 30

SAND

20 10 0 0.1

1

10

Sieve Size (mm) Fig. 1. Particle size distribution of copper slag and sand.

3.3. Test methods 3.3.1. Fresh properties The slump flow, T500, V-funnel and L-box tests were conducted to assess the fresh properties of SCC. The filling ability of SCC was evaluated by slump flow test whereas viscosity and passing ability were examined by T500, V-funnel and L-box test. Slump flow cone was filled with fresh concrete, lifted upward and then, the spread of concrete was measured in two perpendicular directions. Simultaneously, T50 time was also noticed for concrete to reach spread diameter of 500 mm. The range and class of SCC for slump flow value and T500 value, according to European Federation of National Association Representing Producers and Applications of Special Building Products of Concrete (EFNARC) [46] are shown in Table 4. The V-funnel test was used to measure the viscosity by indirect method depending upon the rate of flow of fresh concrete. Vfunnel was completely filled with fresh concrete and time was recorded to empty the funnel. According to the V-funnel flow time, viscosity of concrete was classified as per the recommendation of EFNARC. L-box was used to check the passing ability by filling the vertical part of the L-box and lifting the sliding gate to flow concrete through bars into the horizontal part. The vertical height was measured at the start and end of the horizontal portion of the box. Blocking ratio was obtained by taking the ratio of two vertical heights. 3.3.2. Compressive strength Compressive strength tests were carried out on cubic specimens of size 100  100  100 mm after 7, 28, 56, 90 and 120 days of curing. For each curing period, triplicates for compressive were tested as per IS 516 [47] on compression testing machine of capacity 2000 tons.

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Table 3 Mix proportion of SCC in kg/m3. Mix No.

Mix description

OPC

FA

CS (%)

S

CS

1 2 3 4 5 6

OF-CS0 OF-CS20 OF-CS40 OF-CS60 OF-CS80 OF-CS100

330 330 330 330 330 330

220 220 220 220 220 220

0 20 40 60 80 100

1020 816 612 408 204 –

– 284.4 568 853.2 1137.6 1422

Coarse Aggregate 10 mm

12.5 mm

420 420 420 420 420 420

280 280 280 280 280 280

Water

SP

247.5 247.5 247.5 247.5 247.5 247.5

4.4 4.4 3.3 2.75 2.2 2.2

Note: OPC- Ordinary Portland Cement, FA- Fly Ash, CS- Copper slag, S- Sand, SP- Superplasticizer.

Table 4 Criteria for SCC [46]. S. No.

Test

Property

Class

Range

1

Slump flow test

Filling ability

2

T500

Viscosity

3

V funnel test

Viscosity

4

L box test

Passing ability

SF1 SF2 SF3 VS1 VS2 VF1 VF2 PA2

550–650 mm 660–750 mm 760–850 mm 2 s >2 s 8 s 9–25 s 0.80

3.3.3. Accelerated carbonation Accelerated carbonation tests were carried out on 28-days cured prism specimens of size 100 mm  100 mm  500 mm after an exposure period of 4, 8, 12 and 16 weeks of CO2. Before CO2 exposure, conditioning of samples was performed by oven drying at temperature of 105 ± 5 °C. All surfaces of the prism were coated with epoxy paint except one face of 500 mm length which was left unpainted. The prisms were placed in the carbonation chamber with an atmosphere of 4% CO2 concentration, temperature 25 ± 2 °C and relative humidity within the range of 40%–70%. At each testing age, three specimen of cross section 100 mm  100 mm of 50 mm length were split from prism of 100 mm  100 mm  500 mm and solution of 1% phenolphthalein was sprayed over them. The carbonated areas were found to remain colorless and non - carbonated area turned pink. The carbonation depth of each specimen was measured according to RILEM CPC-18 [48]. 3.3.4. Sulfate attack The method of sulfate attack was followed according to the ASTM C1012-04 [49]. Sulfate attack tests were conducted on cube specimens of size 150 mm  150 mm  150 mm immersed in 5% Na2SO4 solution. Compressive strength and change in weight of the specimens were noted down at an exposure period of 28, 90 and 120 days in sulfate solution after initial water curing of 28 days. After initial curing, cubes were kept in the room temperature for 24 ± 1 h, then marked and weighed, before immersion in sulfate solution. Sulfate solution was changed periodically after 1 month and pH was maintained between 6 and 8 by adding sulphuric acid (0.1 N H2SO4) at regular intervals. The solution was timely stirred to avoid deposition of salt in the tank. At particular testing age, compressive strength was evaluated and change in weight was noticed after keeping the cubes in the room temperature for 24 ± 1 h. 3.3.5. Electrical resistivity The electrical resistivity was performed on cubes of size 100 mm  100 mm  100 mm at curing period of 28, 56, 90 and 120 days. The bulk electrical resistivity was calculated by using wenner’s four probe method [50]. Four electrodes of copper metal of size 15 mm width, length 60 mm and thickness 1 mm were

inserted at an equal spacing of 20 mm up to half depth of the specimen during casting of the mixes. The 50 mm electrodes were inside the specimen and 10 mm above the surface for supplying the current and to note down the potential drop. At particular testing age, the current is supplied to the extreme external electrodes and potential drop is measured between the inner electrodes. Further, bulk resistivity was measured by using the Eq. (1):

q ¼ R:

A l

ð1Þ

where q is the bulk electrical resistivity in k ohms-cm, R is the resistance in k-ohms, A is the cross-sectional area between electrodes and conductive concrete in cm2, l is the spacing between the electrodes in cm. 3.3.6. Ultrasonic pulse velocity Ultrasonic pulse velocity is a non-destructive test to know the quality of concrete by measuring velocity as per IS 13311 [51]. The tests were conducted on specimen of size 100 mm  100 mm  100 mm at 7, 28, 56, 90 and 120 days of curing. A pair of transducers, one as transmitter and other as receiver was applied across the cube at frequency of 54 kHz. A thin layer of coupling gel was applied between traducers and specimen to facilitate strong signal such that no air remain trapped. Path length was fixed in the automatic Pundit Lab Ultrasonic Instrument and on triggering the pulse, time taken to travel the path and value of velocity were displayed on the instrument. 3.3.7. Initial surface absorption Initial surface absorption was used to assess the ingression of water on the surface of the oven dried concrete with respect to time, by maintaining constant head in the reservoir between 180 and 220 mm. The sample of size 150 mm  150 mm  150 mm was tested at curing age of 28, 56, 90 and 120 days. Before testing, preconditioning of samples was done as per BS 1881-208 [52]. The samples were dried in the oven at temperature of 105 ± 5 °C, until constant mass of 0.1% was obtained between two successive readings over an interval of 24 h. The samples were then kept in desiccators to bring down the temperature within 2 °C of room temperature. The apparatus consists of reservoir, cap and capillary tube mounted with scale. Fig. 2 shows the assembly of

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Fig. 2. Assembly of initial surface absorption test [52].

Fig. 3. Schematic procedure of the sorptivity test [53].

initial surface absorption test used in the current investigation. Reservoir is connected to the inlet with tap to control the movement of water and capillary tube mounted with scale is attached to the outlet. The cap is clamped on the known surface area of concrete to be tested. After wetting the concrete for first 10 min, the tap fixed on the inlet is closed. The water in the capillary tube moves backward and observation of the meniscus on the scale for one minute calculates the rate of initial surface absorption (ISA) in ml/(m2 s) for first 10 min. According to BS 1881-208, readings are observed at an interval of 10, 30 and 60 min, though 10 min reading is considered adequate for conclusion. Since after 10 min, sample gets saturated and rate of absorption becomes negligible. So the value of ISA at 10 min was recorded.

Slump Flow Value (mm)

800

SF 3

750 SF 2

700 650

SF 1

600 550 OF-CS0 OF-CS20 OF-CS40 OF-CS60 OF-CS80 OF-CS100 Mixes

Fig. 4a. Slump flow values of different SCC mixes.

4.00 3.50 VS 2

T-500 (s)

3.00 2.50 2.00 1.50

VS 1

1.00 0.50 0.00 OF-CS0 OF-CS20 OF-CS40 OF-CS60 OF-CS80 OF-CS100 Mixes

Fig. 4b. T500 values for different SCC mixes.

28 24 V- funnel Time (s)

3.3.8. Sorptivity Sorptivity was used to assess the penetration of water through capillary pores from one side of the unsaturated concrete. The size of specimen tested was disc of 100 mm diameter and 50 mm thickness obtained from cylinder of size 100 mm  200 mm by saw cutting machine. The samples were tested at the curing age of 28, 56, 90 and 120 days. Before testing, specimens were dried in the oven at the temperature of 105 ± 5 °C until constant mass of 0.1% was achieved between two successive readings after an interval of 24 h. After attaining the constant mass, samples were kept in the desiccator to cool down over the period of 24 h at temperature of 27 ± 2 °C. The schematic diagram of the sorptivity test is shown in Fig. 3. The circumferential area was sealed by coating with epoxy paint whereas, top surface as well as circumferential area was also sealed with polythene sheet to avoid evaporation from the surface not exposed to water. Initial weight of specimen was recorded before keeping in the pan. After that samples were placed on the support device kept in the pan in such a way that about 1–3 mm depth of water was above the support device. The stop watch was started as soon as the sample touched the water and the reading was noted down by removing the sample from the pan after the interval of 1, 5, 10, 20, 30, 60 min and each hour up to 6 h from the start of the test time. The average of three samples was noticed to calculate sorptivity in mm/s1/2 by using the slope of water absorbed against square root of time as per ASTM 1585-04 [53].

850

20

VF2

16 12 8

VF1

4 0 OF-CS0

OF-CS20

OF-CS40 OF-CS60 Mixes

OF-CS80 OF-CS100

Fig. 4c. V-funnel time for different SCC mixes.

4. Results and discussions 4.1. Fresh properties The results of fresh properties i.e. Slump flow, T500, V-funnel, Lbox ratio of all SCC mixes are shown in Figs. 4(a)–(d). The slump

flow value of SCC mixes increased with increment of CS in the concrete. The highest slump flow value of 735 mm was obtained for OF-CS100 without any bleeding and segregation whereas lowest value of 705 mm was recorded for OF-CS0. According to the

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L-box Ratio

0.95 0.90

PA 2

0.85 0.80 0.75 0.70 OF-CS0 OF-CS20 OF-CS40 OF-CS60 OF-CS80 OF-CS100 Mixes

Compressive Strength (MPa)

622

45 40 35 30

7 days

25

28 days

20

56 days

15

90 days

10

120 days

5 0 OF-CS0

OF-CS20 OF-CS40 OF-CS60 OF-CS80 OF-CS100

SCC Mixes Fig. 4d. L-box ratio of different SCC mixes. Fig. 5. Compressive strength at different curing ages.

EFNARC, the slump flow values of whole mixes fall in the category classified as SF2 is shown in Fig. 4(a). It is evident from the Fig. 4(b) that T500 times for all SCC mixes were within the permissible limits as suggested by EFNARC. The results indicate that T500 time varied from 2.18 s to 3.8 s and all mixes were in the class VS2 on the basis of viscosity. In addition, on the basis of viscosity, V-funnel time for two mixes i.e. OF-CS0 and OF-CS20 with the values of 8.55 s and 8.12 s respectively were in the upper limit of viscosity class VF1. The value of viscosity class VF1 is equal to and less than 8 s, whereas the value of VF2 is between 9 and 25 s. The other four mixes were within the range of class VF1 as shown in Fig. 4(c). It can be noticed from Fig. 4(d) that blocking ratio varied from 0.87 to 0.96 for entire mixes. As passing ability test conducted include 3 rebars with blocking ratio over 0.8 were classified as PA2. It was observed that viscosity decreased whereas filling and passing ability of SCC mixes excelled with increase in substitution of CS over the conventional sand without any bleeding and segregation. This may be attributed to the non porous and low absorption characteristics of CS particle. The inclusion of 100% CS in the concrete mix reduced the requirement of superplasticizer to half of 0% CS substitution for achieving the desired consistency properties of fresh SCC at similar w/b ratio. The dosage of superplasticizer was reduced with increment in CS substitution to avoid bleeding and segregation. The utilization of 100% CS substitution in SCC can easily eliminate the problem of flowability and compaction in highly dense reinforced concrete structures such as foundations, dams, tall structures, bridges etc. without any skilled labor. The previous investigations on the fresh state of high performance concrete and normal concrete in terms of workability are entirely in agreement with the fresh properties of the present study. The workability of high performance concrete was observed to be remarkably enhanced with increment in the substitution level of CS at constant dosage of superplasticizer [21]. In normal concrete, slump value of 65.5 mm was observed for concrete containing 0% CS while 200 mm of slump exhibited for 100% CS substitution [26]. With constant w/b ratio and superplasticizer, slump values were found to be escalating as the replacement level of sand increased by CS [25]. 4.2. Compressive strength The development of compressive strength with duration of curing age for different proportion of CS is shown in Fig. 5. The results show decrease in the strength of all SCC mixes containing CS in comparison to the control concrete at an early curing age of 7 days. This may be attributed to either the presence of heavy metals in CS which delayed the hydration process of the concrete mixes [54] or increase in free water content with increment in CS substitution. The compressive strength of control concrete i.e. OF-CS0 was found to be 22.89, 31.43, 35.47, 38.07 and 40.15 MPa at curing age of 7, 28, 56, 90 and 120 days respectively. The scanning electronic microscopy (SEM) micrograph of OF-CS0 at 120 days is shown in

Fig. 6(a) which depicts some cracks and presence of a few needle-shaped structure i.e. ettringite, with uneven formation of C-S-H gel. It is clear from the Fig. 5 that concrete mix OF-CS20 exhibited maximum compressive strength among all mixes from 28 to 120 days of curing. An increase of about 12.59%, 9.02%, 7% and 5.2% was noticed for the corresponding curing age of 28, 56, 90 and 120 days. The SEM micrograph of OF-CS20 illustrates homogeneous and dense formation of C-S-H gel at curing age of 120 days, while some semi-spherical voids are perceptible in Fig. 6(b) without occurrence of ettringite crystals. The improvement of 7%, 6.85%, 5.56% and 2.26% was detected in compressive strength corresponding to curing age of 28, 56, 90 and 120 days for OF-CS40. However, insignificant increase in strength was detected on 60% CS substitution. The SCC mixes, OF-CS80 and OF-CS100 was observed with decline in the compressive strength. It is evident from Fig. 5 OF-CS100 has lowest compressive strength among all mixes. The decrease in strength for the OF-CS100 was 11.86%, 7.72%, 3.46% and 3.93% for the ascending curing period of 28, 56, 90 and 120 days respectively. The SEM micrograph of OFCS100 (Fig. 6c) demonstrates the presence of voids and a porous microstructure with agglomeration of ettringite crystal. The decrease in the strength of mixes OF-CS80 and OF-CS100 may be due to the two reasons, firstly CS particles are glassy and smooth with low water absorption characteristics, which increases the free water in the concrete mixes. Secondly, CS particles are heavy in weight compared to sand, which settles down during the fresh state and water rise to the surface resulting into the voids with porous microstructure, cracks, capillary channels and increase in the thickness of interfacial transition zone (ITZ). The larger particle size of CS is also responsible for the increase in the volume of voids resulting into the decrease in the strength of the SCC mixes OFCS80 and OF-CS100. The result of compressive strength in this study suggests optimum content of 60% CS substitution. However, past studies on high performance concrete and conventional concrete showed optimal substitution of CS in the range of 40–50%. The increment in compressive strength for high performance concrete was found up to replacement level of 50% sand by CS [21]. For normal concrete, compressive strength improved up to 60% CS substitution with w/b ratio of 0.50 whereas 22% lower strength was noticed for 100% CS substitution in comparison to control concrete at 28 days of curing [26]. The dynamic compressive strength containing 20% CS was observed to exhibit higher strength and beyond 40% CS substitution, strength declined in comparison to control concrete [25]. For the high strength concrete, the optimum content suggested on the basis of mechanical properties was 40% CS [38]. 4.3. Accelerated carbonation Carbonation depths of all SCC mixes are shown in Fig. 7. The results demonstrate that inclusion of CS reduced the carbonation depth. The carbonation depth for OF-CS0 was maximum at each

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623

Carbonation Depth (mm)

Fig. 6. SEM analysis at 120 days (a) OF-CS0, (b) OF-CS20, (c) OF-CS100.

40.00 35.00 30.00 25.00

4 weeks

20.00

8 weeks

15.00

12 weeks

10.00

16 weeks

5.00 0.00 OF-CS0

OF-CS20 OF-CS40 OF-CS60 OF-CS80 OF-CS100

Mixes

The percentage of decrease in carbonation depth with respect to reference concrete was 5.82, 14.81, 25.13, 27.51 and 29.36 in the order of OF-CS20, OF-CS40, OF-CS60, OF-CS80 and OF-CS100 at an exposure period of 16 weeks. At an intermediate exposure period of 8 weeks, about 4.31%, 5.88%, 18%, 18.5% and 24.3% of decline was observed in carbonation depth for OF-CS20, OF-CS40, OF-CS60, OF-CS80 and OF-CS100 respectively. With progress in the exposure period from 4 weeks to 16 weeks, carbonation depth increased remarkably. 4.4. Sulfate attack

Fig. 7. Carbonation depth of SCC mixes at different exposure periods.

exposure periods among all mixes. The carbonation depth of 21.30, 25.50, 31.50 and 37.80 mm was detected at an exposure period corresponding to 4, 8, 12 and 16 weeks. The 100% CS substitution has the lowest carbonation depth with values of 15.40, 19.30, 22.90 and 26.70 mm at an exposure period of 4, 8, 12 and 16 weeks respectively. The incorporation of CS beyond 40% has significant decline in the carbonation. This may be attributed to the presence of higher content of iron oxide in the composition of CS which increases the pH of pore solution and makes concrete matrix to be alkaline in nature. The control mix OF-CS0 contains 100% sand which includes some impurities in the form of silt and clay leading to the decrease in the alkalinity of the pore solution in comparison to concrete mixes containing CS. The passive layer in concrete around the reinforcement is composed of iron oxide with pH value of 13–14 which helps in the protection against corrosion [55,56].

Sulfate attack is assessed by change in mass and compressive strength of the concrete specimens after an exposure of sulfate solution at 28, 90 and 120 days. 4.4.1. Change in mass Fig. 8 demonstrates gain in mass of the concrete mixes with increment in CS content from 0 to 100%. It was also observed that all concrete mixes gained mass with progressive immersion period in sulfate solution. The difference in mass gain was more pronounced between 28 and 90 days of sulfate exposure. The concrete mix OF-CS0 has lowest change in mass among all mixes of about 0.04%, 0.37% and 0.45% at 28, 90 and 120 days of sulfate exposure respectively. A remarkable gain in mass of approximately 0.28%, 0.52% and 0.68% for OF-CS80 and 0.30%, 0.69% and 0.75% for OFCS100 was noticed at 28, 90 and 120 days of sulfate exposure. This gain in mass may be attributed to the formation of needle – shaped ettringite crystals after the reaction between hydrates, Ca(OH)2

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Fig. 8. Change in mass after exposure of sulfate solution.

Fig. 9. Compressive strength after exposure of sulfate solution.

and sodium sulfate. The penetration of sulfate salt through pore solution into concrete samples reacts with Ca(OH)2 and some of the sulfate precipitates into microstructure leading to the gain in mass. However, an additional possible cause in mass gain of concrete mixes may be due to the filling of voids with sulfate solution at higher substitution level of sand by CS.

neous C-S-H gel accompanied with ettringite. Fig. 10(e) and (f) reveals huge amount of ettringite crystals which camouflaged the microcracks and voids in concrete mixes OF-CS80 and OF-CS100 respectively. Fig. 11(a)–(f) illustrates the EDS analysis of all concrete mixes. The EDS analysis displays the dominant peaks of Ca, Si, Al and O in the entire mixes. The various other elements observed are S, Fe, Mg, C, K and Na. The presence of major peaks of Ca and Si confirms the formation of C-S-H gel in respective spectrum. The formation of huge amount of ettringite in OF-CS80 and OF-CS100 is authenticated by the presence of Al and S which are principal elements for the occurrence of ettringite.

4.4.2. Compressive strength The result of compressive strength of all mixes immersed in sulfate solution after 28 days of initial curing in water is shown in Fig. 9. The compressive strength decreased with progressive immersion time of concrete in sulfate solution. After an exposure of 28 days in sulfate solution, except OF-CS20, each concrete mix has shown a lower compressive strength with respect to control concrete. A decrease of 2.35%, 3.93%, 8.41% and 10.17% was observed corresponding to OF-CS40, OF-CS60, OF-CS80 and OFCS100. It can be observed from the Fig. 9 that OF-CS100 was least resistant to sulfate solution at all exposure periods. At 120 days of sulfate exposure, decline in strength of 4%, 6% and 11% was noticed in the order of OF-CS60, OF-CS80 and OF-CS100. This may be attributed to the formation of ettringite which increases the volume and stress inside the microstructure leading to the microcracks and deterioration of concrete mixes. It was noticed that OFCS20 and OF-CS40 showed slightly superior compressive strength at 90 and 120 days in sodium sulfate solution. An increase of 3.38%, 14.82% for OF-CS20 and 0.25%, 5.84% for OF-CS40 was detected at an exposure of 90 and 120 days of sulfate solution respectively. The compressive strength continues to decrease with increment in immersion period from 28 to 120 days. The microstructural analysis of concrete mixes in terms of scanning electronic microscopy (SEM) and energy dispersive spectroscopy (EDS) was carried out after 120 days of immersion in sulfate solution to correlate with compressive strength. Fig. 10(a) shows the formation of ettringite partly intermixed with some CS-H gel and voids in the concrete mix OF-CS0. The SEM micrograph of concrete mix OF-CS20 (Fig. 10b) exhibited compact structure with rich C-S-H gel and resulted in better performance against sulfate attack. In OF-CS40 (Fig. 10c), microcracks, semi voids and little amount of ettringite are detected with dense microstructure of CS-H gel whereas the microstructure of OF-CS60 (Fig. 10d) seems to be porous containing microcracks along with loose and heteroge-

4.5. Electrical resistivity Electrical resistivity is the measure of transfer of ions in the microstructure of the concrete sample. The results of electrical resistivity for all SCC mixes with different proportion of CS are shown in Fig. 12. The magnitude of electrical resistivity for OFCS0 was 11.57, 13.86, 16.66 and 19.10 k Ohms-cm at curing age of 28, 56, 90 and 120 days respectively. It can be observed that with increase in curing age from 28 to 120 days, electrical resistivity of all mixes increased. This can be attributed to the longer hydration of the concrete mixes leading to the densification of the microstructure. The values of resistivity varied from 11.57 to 19.10 k Ohms-cm for OF-CS0, 14.21 to 24.92 k Ohms-cm for OF-CS20, 13.44 to 23.96 k Ohms-cm for OF-CS40, 12.25 to 21.58 k Ohms-cm for OFCS60, 10.59 to 1848 k Ohms-cm for OF-CS80, 10.19 to 17.84 k Ohms-cm OF-CS100 from curing age of 28 to 120 days. The inclusion of CS in concrete mixes increases the electrical resistivity up to 60% substitution. The concrete mix OF-CS20 has highest resistance to the movement of the ions among all mixes. At curing age of 28 days, OF-CS20 showed remarkable increase of 22.77%, 26%, 28.60% and 30.48% at 28, 56, 90 and 120 days respectively with respect to control concrete. About 22.77%, 16.12% and 5.82% of increment in electrical resistivity were observed corresponding to OF-CS20, OF-CS40 and OF-CS60 at 28 days. An increase of 30.48%, 25.46% and 12.98% was noticed in the order of OF-CS20, OF-CS40 and OF-CS60 at 120 days. A slight decrease in the values of resistivity was found for concrete mixes OF-CS80 and OFCS100 at all curing periods. This may be due to the increase in the porosity of the concrete matrix with higher substitution of

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Fig. 10. SEM analysis after120 days of sulfate exposure (a) OF-CS0, (b) OF-CS20, (c) OF-CS40, (d) OF-CS60, (e) OF-CS80, (f) OF-CS100.

CS. Although with progressive curing, difference in the values of the resistivity decreased. 4.6. Ultrasonic pulse velocity Ultrasonic pulse velocity (UPV) evaluates inner microstructure of concrete in terms of velocity. Higher the velocity represented by the concrete, more is the dense and homogeneous microstructure. Fig. 13 shows the velocity of all concrete mixes at different curing ages. It can be noticed from Fig. 13 that at an early curing period of 7 days, all concrete mixes exhibited good quality of concrete [51]. Although with progressive curing age, velocity increased for entire SCC mixes. At 28 days of curing, OF-CS80 and OF-CS100

showed good quality of concrete whereas remaining mixes exhibited excellent quality of concrete as per IS 13311 part 1. The control concrete have UPV values of 4367, 4525, 4630, 4717 and 4739 m/s at 7, 28, 56, 90 and 120 days respectively. The highest value of UPV among all mixes was observed for OF-CS20 varying from 4405 to 4808 m/s, followed by OF-CS40 within the range of 4365– 4785 m/s correspondingly for 7–120 days. The lowest value of UPV was marked for OF-CS100 with magnitude of 4292, 4464, 4525, 4630 and 4673 m/s for the corresponding curing age 7, 28, 56, 90 and 120. The decrease in the UPV values for OF-CS80 and OF-CS100 as compare to control concrete may be due to the increase in the thickness of the interfacial transition zone and presence of voids along with cracks in the concrete matrix.

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Fig. 11. EDS analysis of all SCC mixes (a) OF-CS0, (b) OF-CS20, (c) OF-CS40, (d) OF-CS60 (e) OF-CS80, (f) OF-CS100.

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R. Sharma, R.A. Khan / Construction and Building Materials 155 (2017) 617–629

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4.7. Initial surface absorption The resistance to the surface absorption by the concrete is an important parameter to determine durability of the concrete. The top surface of the concrete is most vulnerable to the aggressive environments. The exposed surface of the SCC is heterogeneous and porous in nature due to the settling of coarse aggregates during consolidation in the formwork and congested reinforcement. The transport of any foreign matter into the concrete depends upon the characteristics of upper surface. The results of initial surface absorption of all SCC mixes are shown in the Fig. 14. It can be observed from Fig. 14 that substitution up to 60% CS in the concrete had reduced the surface absorption. With progressive curing period, all concrete mixes showed decrease in the surface absorption values. The control concrete OF-CS0 had absorption value of 69  102, 62.5  102, 58.5  102, 54  102 ml/(m2 s) corresponding to 28, 56, 90 and 120 days of curing age. The lowest value of absorption was noticed for OF-CS20 among entire mixes at all curing ages. The decrease in the absorption was 14.93%, 16.57%, 26.36% and 38.79% with respect to control concrete at 28, 56, 90 and 120 days. The resistance to absorption was more than control concrete for OF-CS40 and OF-CS60 varying from 8.29% to 36.13% and 2.24% to 16.84% respectively for different curing ages. This may be attributed to the densification of the mortar matrix by optimum proportion of sand and CS. It is evident from Fig. 14 that beyond 60% CS substitution, absorption increased abruptly and maximum absorption was found to be in OF-CS100 among all mixes. The absorption values of 77.9  102, 69.7  102, 62.1  102, 56.9  102 ml/(m2 s) for OF-CS80 and 80.2  102, 74.4  102,

66.2  102, 58.3  102 ml/(m2 s) for OF-CS100 was observed at 28, 56, 90 and 120 days respectively. At 28 days of curing, 12.94% and 16.33% of increase were noticed corresponding to OF-CS80 and OF-CS100. However, with progressive curing, difference in the absorption decreased to 5.29% and 7.97% for OF-CS80 and OFCS100 at 120 days as compared to control concrete. The previous investigation on high performance concrete showed reduction in the initial surface absorption up to the replacement level of 40% sand by CS [21]. For normal concrete, beyond 40% CS substitution, surface absorption and permeable voids increased [26]. 4.8. Sorptivity Sorptivity of concrete is mainly concerned with complex path of capillary pores in the microstructure of the concrete. Higher the volume of capillary pores, larger is the sorptivity and possibility of deterioration of concrete becomes more pronounced. It can be examined from the Fig. 15 that, in general, sorptivity decreased with incorporation of CS in comparison to control concrete. The sorptivity values of control concrete i.e. OF-CS0 varied from 0.0193 to 0.0123 mm/s1/2 for 28 to 120 days. The lowest values of sorptivity were observed for OF-CS20 with 0.0140, 0.0123, 0.0110 and 0.0100 mm/s1/2 at curing age of 28, 56, 90 and 120 days respectively. At 28 days of curing age, about 27.58%, 18.96%, 15.51%, 8.62% and 6.89% of decrease was observed in the water absorption corresponding to OF-CS20, OF-CS40, OF-CS60, OF-CS80 and OF-CS100 with respect to control concrete. Like initial surface absorption results, sorptivity also increased beyond 60% CS

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substitution. However, OF-CS80 and OF-CS100 has lower sorptivity value at 28 days whereas increase in the sorptivity was observed for remaining curing ages. It is also clear from Fig. 15 that maximum sorptivity was observed for OF-CS100 which varied from 0.0169 to 0.0140 mm/s1/2 correspondingly commencing 56 to 120 days. The durability properties in the present investigation i.e. resistivity, sorptivity, ultrasonic pulse velocity and initial surface absorption are mainly concerned with the transport mechanisms. The sorptivity is related to the absorption of water through capillary pores in the microstructure whereas absorption of water near the surface of the concrete under the gradient of pressure head is clearly demonstrated by initial surface absorption. The electrical resistivity is measure of the resistance offered to the movement of ion through concrete matrix whereas UPV measures quality of concrete in terms of path travelled by the velocity from one end to the other end of the specimens. The result of all these properties collectively provides an idea about the durability of the SCC mixes which are interrelated to each other by the volume of the voids and pore microstructure of the mixes. It was observed that with increment in the substitution of CS, durability of the SCC mixes escalated up to 60% substitution of CS and decline in durability exhibited with further increase in the substitution level. This may be attributed to the lower water absorption characteristics and different gradation of CS from the natural sand which increased the voids, capillary channels and the thickness of the interfacial transition zone. 5. Conclusion This investigation was conducted to assess the durability of SCC containing CS as fine aggregates. The following are the main findings from this study.
The incorporation of CS as fine aggregate replacement escalated the fresh properties. Slump flow values and L-box ratio of all mixes were in the class of SF2 and PA2 respectively. On the basis of viscosity, T500 time and V-funnel time of entire mixes were in the category of VS2 and VF1 respectively.
The compressive strength of control concrete was highest at an early age of 7 days. The maximum compressive strength was observed for 20% CS for remaining curing ages and decline in strength was noticed beyond 60% substitution of CS. The decrease in strength was due to the increase in free water content on higher replacement of sand by CS.
The presence of higher content of iron in the CS decreases the carbonation depth of the concrete mixes. Concrete mix OFCS100 was found to be more resistance to the carbonation whereas OF-CS0 was least resistance to the carbonation at each exposure period among all mixes.
The main reason for the increase in weight and loss of strength was attributed to the expansion and microcracking caused by ettringite. With progressive sulfate exposure, compressive strength decreased and maximum change in weight was observed for OF-CS100. The compressive strength of OF-CS20 and OF-CS40 was more than control concrete at 120 days of sulfate exposure.
The electrical resistivity and UPV of concrete mixes increased up to 60% CS substitution. Beyond 60% CS, porosity of mortar matrix increases resulting into the decrease in the values of electrical resistivity. The UPV values of entire concrete mixes were under the category of good quality of concrete at early age of curing. However, at later age, all concrete mixes exhibited excellent quality of concrete.
The lowest values of initial surface absorption and sorptivity of concrete mixes were found for OF-CS20 whereas highest values of initial surface absorption were detected for OF-CS100 at all

curing ages. For sorptivity, maximum value was observed for OF-CS0 at curing age of 28 days while for remaining curing ages, OF-CS100 showed maximum capillary absorption.
This study suggests 60% optimal substitution of CS over the conventional sand in the development of SCC for comparable or better durability properties. The past researchers have recommended the optimal content of 40%–50% as fine aggregates for normal vibrated concrete and high performance concrete. SCC developed can be used in the construction of various structures such as foundations, dams, high rise buildings, bridges and piers. To utilize CS confidently in the construction industry, research in depth is required with different combination of materials. The future studies are required on the durability properties of SCC such as acid attack, freeze-thawing and alkali-silica reaction. The use of CS as cement replacement in SCC is also an area of future study.

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