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  ASIAN JOURNAL OF CIVIL ENGINEERING (BHRC) VOL. 14, NO. 5 (2013) PAGES 773-781   Technical Note STUDY ON INTERNAL CURING OF HIGH PERFORMANCE CONCRETE USING SUPER ABSORBENT POLYMERS AND LIGHT WEIGHT AGGREGATES C. Chella Gifta *1 , S. Prabavathy 2  and G. Yuvaraj Kumar  2   1 Department of Civil Engineering, Einstein College of Engineering, Tirunelveli-627012, Tamilnadu 2 Department of Civil Engineering, Mepco Schlenk Engineering College, Sivakasi - 626 005, Tamil Nadu Received: 2 November 2012; Accepted:  14 February 2013 Abstract High performance concrete (HPC) is popular for its low water-to-cementations materials ratio (w/cm). Because of this low w/c ratio and rapid hydration, proper curing is essential at the earliest time. This paper explores the use of Super Absorbent Polymers (SAP) and Light Weight fine Aggregates (LWA) as internal curing material. Mix M2 is achieved by adding SAP at 0.3% weight of cement and in mix M3 is obtained by replacing 25% weight of LWA to fine aggregates. Strength and durability of these HPC are studied experimentally and the results show greater strength with LWA mix. Load carrying capacity of the beams in flexure and shear also greater in LWA mix and the durability study results reveal that mix with SAP is better compared to the other two mixes. Keywords:   Internal curing; high performance concrete (HPC); super absorbent polymers (SAP); light weight fine aggregates (LWA) 1. Introduction Over the last decades, HPC with a low w/c has become widely used and research works on this matter have been increased tremendously. The high strength and durability performance of HPC were considered as key issues for structural use under severe conditions [1]. However, from the practical point of view and laboratory investigations, it has been  proven that HPC is very sensitive to the risk of early age cracking unless special precautions are taken. One of the specific requirements is the curing practice of concrete for the *  E-mail address of the corresponding author: (C. Chella Gifta)  C. Chella Gifta, S. Prabavathy and G. Yuvaraj Kumar 774   development of strentgh.Proper curing of concrete structures is important to ensure that they meet their intended performance and durability requirements [2, 7]. In conventional construction, this is achieved through external curing, applied after mixing, placing and finishing. Internal curing (IC) is a very promising technique that can provide additional moisture in concrete for a more effective hydration of the cement and reduced self-desiccation. Internal curing implies the introduction of a curing agent into concrete that will  provide this additional moisture [3]. Currently there are two major methods available for internal curing of concrete. The first method uses saturated porous lightweight aggregate (LWA) in order to supply an internal source of water, which can replace the water consumed by chemical shrinkage during cement hydration [4]. This internal curing water is naturally drawn during cement hydration from the relatively large pores of the lightweight aggregate into the smaller pores of the cement paste [5]. The second method uses super-absorbent polymers (SAP), as these  particles can absorb a very large quantity of water during concrete mixing and form large inclusions containing free water, thus preventing self-desiccation during cement hydration[3,6].The large absorption characteristics of SAP is shown in Figure 1.Internal Curing distributes extra curing water (uniformly) throughout the entire microstructure of the concrete so that it is more readily available to maintain saturation of the cement paste during hydration, For internal curing to do its job, the individual pores in the internal reservoirs should be much larger than the typical sizes of the capillary pores (micrometers) in hydrating cement paste and should also be well connected for proper percolation [7]. Internal curing is not a substitute for external curing. As a minimum, evaporative moisture loss, after set, should be prevented using conventional external measures like misting, fogging, curing membrane or compound [8]. Figure 1. Super absorbent polymers 2. Experimental Investigation 2.1. Materials High performance concrete with water binder ratio of 0.32 was produced [9]. The cement used was ordinary Portland cement of 53 grade and the silica fume had a specific surface  STUDY ON INTERNAL CURING OF HIGH PERFORMANCE CONCRETE USING ... 775 area of 19 m 2 /g. The replacement ratio of silica fume was 10% by weight of cement. Sieved aggregate with a maximum size of 10mm and fine sand with a fineness modulus (FM) of 1.94 were used. The relative proportions of the coarse aggregate and the sand were determined for the reference normal weight concrete to obtain sufficient workability. Commercially available super absorbent polymer was used and the light weight fine aggregates were obtained from waste aerocon blocks by breaking and sieving them through 4.15mm size sieve. The results of water absorption tests are given in Table 1. For the concrete in which part of the normal-weight aggregate was replaced by lightweight aggregate, the replacement ratio of aggregate was 25% of the total volume of aggregates and for the concrete with SAP the percentage of SAP added was 0.3% weight of cement. The dosage of super plasticizer (naphthalene formaldehyde sulfonate type) for each mix was determined to obtain a slump of 100 ± 20 mm. Mix proportions of the concrete are given in Table 2. Table 1: Water absorption S. No Material Dry weight (gms) Saturated weight (gms) Water Absorption 1 Super Absorbent polymers 25 2640 105 times its weight 2 Light weight aggregates 1000 4050 4 times its weight Table 2: Mix Proportion (kg/m 3 ) of concrete   Fine Aggregate Mix Cement River sand LWA Coarse AggregateWater Super  plasticizer (1.5% of Cement) Super Absorbent  polymers M1 547 779.68 --- 941.8 166.8 8.2 --- M2 547 779.68 --- 941.8 166.8 8.2 1.64 M3 547 584.68 195 941.8 166.8 8.2 -- 2.2. Strength tests Strength tests are conducted to find out compressive strength, tensile strength, flexural and shear behaviour of test specimens. Cube specimens of size 150 x150 x 150 mm were cast for compressive strength, cylindrical specimens of size 150 mm x 300 mm were used for tensile test. Similarly the flexural and shear behaviour of beams size 1200 x 100 x 150 mm were studied in this experimental program. Compressive strength tests were carried out at the age of 3, 7, and 28 days. Tensile strength, flexural and shear behavior were calculated at 28 days after casting. The load setup for finding flexural and shear strength are shown in Figure 2.  C. Chella Gifta, S. Prabavathy and G. Yuvaraj Kumar 776   (a) For flexure (b) For shear Figure 2. Schematic representations of load set up in beams 2.3. Durability tests Durability tests that are done in this study are permeability test and rapid chloride  penetration test. Cylindrical moulds of size 100 mm diameter and 150 mm height are cast for  permeability test and specimens of size 100 mm diameter and 50 mm height is used for rapid chloride penetration test. Durability studies are done at the age of 28 days curing. 3. Results and Discussions 3.1 Compressive Strength The compression test is carried out at 3, 7 and 28 days age of curing. The test results given in Figure 3 shows that the 3days compressive strength is greater in mix M1 and it implies that the hydration process has not fully started due to the internal curing of water within 3 days. But the 7and 28 days results proved that the internal cured mix has a greater compressive strength when compared to the control mix. Mix with SAP has attained 6.88% increase and LWA mix has shown 12.35% increases on 28 days compressive strength than the control concrete mix. This predicts that there is an increased hydration; leading to a strong microstructure of concrete gives greater compressive strength.  STUDY ON INTERNAL CURING OF HIGH PERFORMANCE CONCRETE USING ... 777  Figure 3. Compressive strength Figuer 4. Tensile Strength 3.2 Split Tensile Strength The Split tensile strength test is carried out at 28days and strength results are given in Figure.4 shows that mix M3 has better results compared to other two mixes. Mix M3 has 2.43% higher tensile strength than mix M2 and 5.3% greater than mix M1. 3.3 Flexural Behavior The flexural behaviour of the beam specimens are carried out at 28 days curing and the results are tabulated in Table 3. The initial crack for beam M1 srcinated when the load was about 7.5 kN and for beam M2 and M3 was 10 kN. The crack was mainly occurred between the points of loading and initiated from the tension side of the beam and moved towards the neutral axis layer. The stiffness was initially higher and started to decrease for all the beams gradually. The initial stiffness was greater for M3 specimens comparing with the other two specimens. The value of stiffness for M2   was 10 kN/m, which is greater than M1   and it is found to be less than beam M3. The ultimate load carrying capacity of the beams M1   and M2 were more or less equal but M3   had better load carrying capacity of 19.4% greater than M1   and 13.15% greater than M2 .  The pattern of failure cracks under flexure is shown in Figure 5. The experimental results of mid span deflection for all the three mixes are plotted in Figure 6. Table 3: Ultimate load and stiffness in flexure S.No Beam Specimen Initial Crack Load (kN) Ultimate Load (kN) Ultimate Moment (kN/m) Maximum Stiffness (kN/m) 1 M1 7.5 21.6 4.05 8.33 2 M2 10 22.8 4.28 10 3 M3 10 25.8 4.84 13.89 Table 4: Ultimate load and stiffness in shear    S.No Mix details Initial Crack Load (kN) Ultimate Load (kN) Ultimate Moment (kN/m) Maximum Stiffness (kN/m) 1 M 1  1.5 49.8 24.9 10.41
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