SELF CURING CONCRETE. Garje Rajesh Kumar, Professor of Civil Engineering, National Institute of Technology Warangal , Andhra Pradesh, India

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1 SELF CURING CONCRETE Garje Rajesh Kumar, Professor of Civil Engineering, National Institute of Technology Warangal , Andhra Pradesh, India ABSTRACT: There are number of ways in which internal curing for concrete can be accomplished. One of the techniques of self curing is by using hydrophilic materials in concrete. The use of hydrophilic materials in concrete minimizes the loss of water and also attracts moisture from the atmosphere and help in continuous curing of concrete. Polyethylene Glycol, Paraffin Wax, Acrylic acid are some of the commonly available hydrophilic materials in market. This paper discusses the experimental investigation to arrive at the optimum dosage of Polyethylene Glycol (PEG) in concrete. The parameters in the study are two grades of concrete, PEG of two molecular weights, different dosages of PEG and two curing conditions. The variation of internal moisture and compressive strength was studied using standard concrete cubes of dimensions 150 mm 150 mm 150 mm. The variation of internal moisture was measured by weighing the cubes at regular intervals. It is found that, for low dosages of PEG the variation of weight and compressive strength was not significant when the concrete is cured by indoor curing and conventional curing. 1. INTRODUCTION Curing of concrete plays a major role in developing the concrete microstructure and pore structure and hence improves durability and performance. It is necessary to provide water for concrete so that continuous curing can be accomplished. This is done by providing water externally or by embedding the water inside the materials used to make concrete. It is found that for 1 m 3 of concrete requires about 3 m 3 of water for construction most of which is for curing. Providing water externally has its own problems viz., depleting sources of good quality water, lack of accessibility of the structure, limited water cement ratio used in high strength / high performance concrete. In the absence of providing quality water for cuing, the concrete structures are subjected to premature deterioration. To prevent the above problem, it is advantageous to provide embedded water for curing. There are number of methods for providing embedded water for curing viz., using light weight aggregate, using hydrophilic chemicals in concrete etc., The use of Poly Ethylene Glycol (PEG) is one such method (Liang et al., 2002, El Dieb 2007, Jau, 2008). Liang et al, has conducted experiments on self curing concrete using a combination of PEG and Paraffin Wax in different proportions. PEG of molecular weight 200 was used in this investigation. Water retentivity, porosity and compressive strength were found to prove the efficiency of self curing. The experimental program revealed, the best combination of PEG and Paraffin Wax. However, neither the use of PEG of higher molecular weight was studied nor the reasons for using PEG of lower molecular weight were discussed in this paper.

2 El Dieb studied the use of PEG as a self curing agent. The parameters in this study were the cement content and water cement ratio. The concrete weight loss and internal relative humidity measurements were studied with respect to time. Two types of curing viz., continuous moist curing under water and air curing was considered. The molecular weight of PEG was not mentioned in this paper; however, the dosage of PEG was maintained constant at 0.02 % by weight of cement. Jau listed different types of self curing agents which can be used in concrete. The dosage at the rate of 0.1% to 5% by weight of cement was considered in this investigation. The weight loss and the compressive strength were found for different dosages of self curing agents and at different relative humidity. However, the molecular weight of the self curing agents was not mentioned in this paper. 2. RESEARCH SIGNIFICANCE From the review of literature it is found that the influence of molecular weight and dosage of self curing agents was not given due significance to study the efficiency of self curing. In all the investigations mentioned above the loss of weight of the specimen was considered to study the self desiccation of concrete, which has direct bearing on the hydration of cement. Compressive strength is one simple parameter which is used to study the level of hydration of cement in concrete. This paper describes an experimental program to find the optimum dosage and the molecular weight of PEG to accomplish efficient self curing of concrete by studying the water retentivity by loss / gain of weight of the specimen and compressive strength of concrete subjected to two curing regimes. 3. SCOPE OF THE STUDY This study is conducted to develop self curing concrete by using a hydrophilic material viz., Poly Ethylene Glycol (PEG). Two grades of concrete to obtain about 30 MPa and 40 MPa was considered in this study. Poly Ethylene Glycol (PEG) of two molecular weights 600 and 6000 was added in three dosages viz., 0.1%, 1.0% and 3.0% by weight of cement. Two types of curing methods viz., indoor curing and conventional curing by immersing the specimens in water were adopted in the study. The comparison in the behavior was made with a companion specimen without PEG due to indoor curing and conventional curing. Amount of water retained in concrete for the purpose of curing was monitored for specimens subjected to indoor curing and conventional curing, by weighing them at an interval of 3days up to 27 days. The variation of compressive strength due to above mentioned curing types and dosages of PEG were studied. The ratio of 28 days compressive strength for concrete subjected to indoor curing and conventional curing is studied to compare the curing efficiency of concrete due to different dosages of PEG. The test for compressive strength of concrete was performed as per IS MATERIALS USED Ordinary Portland cement of 53 grade conforming to IS , procured in a single lot and stored properly was used in this investigation. Natural sand conforming to zone II and 20 mm nominal size, well graded crushed granite chips as per IS were used as fine aggregate and coarse aggregate respectively. PEG of molecular weights 600 and 6000 were used in the concrete. PEG of molecular weight 600 was available in the liquid form and PEG of molecular weight 6000 was available in the form of crystals. PEG was mixed with the water which was used in mixing concrete.

3 4.1 Nomenclature of specimens Two grades of concrete are denoted by A and B. PEG of molecular weights 600 and 6000 are denoted by L and H respectively. The dosage is represented by 0, 0.1, 1 and 3% by weight of cement. Indoor curing and Conventional Curing is represented by I and W respectively. For example AH-1(I)1 represents specimen with mix A, PEG with higher molecular weight and dosage of 1% by weight of cement, subjected to indoor curing and 1 st specimen in the series. Three similar specimens were cast and average of three was considered to as the final value. Total of 96 standard cubes of dimensions 150 mm 150 mm 150 mm were cast out of which 48 were subjected to indoor curing and remaining 48 were subjected to conventional curing. 5. RESULTS AND DISCUSSIONS: 5.1 Retention of Water Weight loss / Weight Gain The availability of water in concrete, particularly when subjected to indoor curing is extremely important for the curing of concrete to continue. The retention of water in concrete was monitored by weighing the concrete cubes at regular intervals as mentioned above Weight Loss - Specimens subjected to Indoor curing A total of 48 specimens i.e., 12 specimens each of the following were subjected to indoor curing; Mix A with PEG of low molecular weight AL (I), Mix A with PEG of high molecular weight AH (I), Mix B with PEG of low molecular weight BL (I) and Mix B with PEG of high molecular weight BH (I) with varying dosages of PEG as percentage by weight of cement. The weight of three similar specimens was taken on an electronic balance of accuracy 0.1 grams. The average of three similar specimens was taken for further analysis. It is observed that the initial weight of the specimen was the maximum and all the specimens suffered continuous weight loss with age as they were subjected to indoor curing. The difference between the weight at any age and the initial weight (i.e., weight loss) was plotted with respect to the age of the specimen. Fig. 1 to Fig. 4 shows the variation of loss of weight for the above mentioned specimens. However, it is important to identify the type and dosage of PEG for which the loss of weight is minimum, such that maximum amount of water is available for internal curing. Fig. 1 & 2 Variation of Weight loss (kg) with Age (days) for specimens AL & AH

4 Fig. 3 & 4 Variation of Weight loss (kg) with Age (days) for specimens BL & BH In all the four cases it was observed that the weight loss was lowest for a dosage of 0.1% of PEG (by weight of cement) irrespective of molecular weight of PEG. Further, to study the difference of weight loss of specimens due to the incorporation of PEG with lower and higher molecular weight, the final weight loss of all the specimens (i.e., after 27 days) was recorded as shown in table. 1. The ratio of weight loss of specimen with and without PEG was calculated and recorded in the same table. The ratio of weight loss was plotted with respect to the percentage dosage of PEG and shown in Fig. 5. It can be observed that the ratio of weight loss was lower due to the use of PEG of lower molecular weight and further it can be observed that the ratio of weight loss was lowest for the dosage of 0.1% by weight of cement. With the dosage of PEG beyond 0.1% by weight of cement, the ratio of weight loss was increasing to show that the availability of water for the curing of concrete was less, when the specimen was subjected to indoor curing. Table 1 Final weight loss & ratio of weight loss for indoor cured specimens % PEG WT. LOSS (INDOOR CURING) (kg) RATIO OF WT LOSS (INDOOR CURING) AL(I) AH(I) BL(I) BH(I) AL(I) AH(I) BL(I) BH(I) Fig. 5 Variation of Final Weight loss with dosage of P E G

5 5.1.2 Weight Gain - Specimens subjected to Conventional Curing (Immersed in water) A total of 48 specimens i.e., 12 specimens each of the following were subjected to conventional curing; Mix A with PEG of low molecular weight AL (W), Mix A with PEG of high molecular weight AH (W), Mix B with PEG of low molecular weight BL (W) and Mix B with PEG of high molecular weight BH (W) with varying dosages of PEG as percentage by weight of cement. The specimens were taken out of curing tank; surface wiped with dry cloth and allowed to dry for one hour before the weight was taken. The weight of three similar specimens was taken on an electronic balance of accuracy 0.1 grams. The average of three similar specimens was taken for further analysis. It was observed that the initial weight of the specimen was lowest and all the specimens showed continuous weight gain with age as they were subjected to immersion curing. The difference between the weight at any age and the initial weight (i.e., weight gain) was plotted with respect to the age of the specimen. Fig. 6 to Fig. 9 shows the variation of weight gain for the above mentioned specimens. However, it was important to identify the type and dosage of PEG for which the weight gain was maximum, such that maximum amount of water was available for internal curing. In all the four cases it was observed that the weight gain was generally higher for a dosage of 0.1% of PEG (by weight of cement) irrespective of molecular weight of PEG. Further, to study the difference of weight gain of specimens due to the incorporation of PEG with lower and higher molecular weight, the final weight gain of all the specimens (i.e., after 27 days) was recorded as shown in table. 2. The ratio of weight gain of specimens with and without PEG was calculated and recorded in the same table. The ratio of weight gain was plotted with respect to the percentage dosage of PEG and shown in Fig. 10. It can be observed that the ratio of weight gain was higher due to the use of PEG of lower molecular weight and further it can be observed that the ratio of weight gain was highest for the dosage of 0.1% by weight of cement. With the dosage of PEG beyond 0.1% by weight of cement, the ratio of weight gain decreases to show that the available of water for the curing of concrete was less, when the specimen was subjected to conventional curing. Table 2 Final weight gain & ratio of weight gain for conventionally cured specimens % PEG WT GAIN (CONVEN. CURING) (kg) RATIO OF WT GAIN (CONVEN. CURING) AL(W) AH(W) BL(W) BH(W) AL(W) AH(W) BL(W) BH(W) Fig. 6 & 7 Variation of Weight Gain (kg) with Age for specimens AL & AH

6 Fig. 8 & 9 Variation of Weight Gain (kg) with Age for specimens BL & BH Fig. 10 Variation of Final Weight Gain with dosage of P E G 5.2 Variation of Compressive Strength In this experimental investigation a total of 48 specimens were subjected to indoor curing and equal number of specimens were subjected to conventional curing by immersing the specimens in water. The results are tabulated in table 3 and the variation of compression strength due to indoor curing and conventional curing are plotted with respect to the dosage of PEG, which are shown in Fig. 11 to 12. It can be observed from these figures that the compressive strength of specimens subjected to indoor curing is less when compared to the specimens subjected to conventional curing. However, the difference between the compressive strength of specimens subjected to indoor curing and conventional curing is less when PEG of low molecular weight is used. Further it can be observed that the compressive strength of specimens with PEG of dosage 0.1% by weight of cement is closer to the conventionally cured specimens.

7 Fig. 11 &12 Compressive Strength Vs dosage of PEG for Mix A & B Table 3 28 Days Compressive Strength (MPa) of concrete %PEG AL(I) AL(W) AH(I) AH(W) BL(I) BL(W) BH(I) BH(W) To compare the efficiency of curing, the ratio of 28 days compressive strength of specimens subjected to indoor curing and conventional curing was calculated and recorded in table 4. The variation of this ratio with respect to the dosage of PEG was plotted and shown if Fig. 13. Table 4 Ratio of 28 Days Compressive Strength due to indoor and conventional curing %PEG AL(I/W) AH(I/W) BL(I/W) BH(I/W)

8 Fig. 13 Ratio of 28 days compressive strength for specimens subjected to indoor and conventional curing From Fig. 13 it can be observed that the ratio of 28 days compressive strength for specimens subjected to indoor curing and conventional curing with low molecular weight PEG is closer to 1.0. Further it can be seen that for a dosage of 0.1% by weight of cement the ratio is closer to 1.0 when compared to any other dosage. 6. CONCLUSIONS: 1. Poly Ethylene Glycol of lower molecular weight is more efficient as a self curing agent when compared to the PEG of higher molecular weight. 2. Low dosage of Poly Ethylene Glycol is more efficient for achieving self curing concrete when compared to the higher dosages. 7. REFERENCES: Liang et al (2002) United States Patent Publication No. US 6,468,344 B1, Oct 22, A S El. Dieb (2007) Self Curing Concrete: Water retention, hydration and moisture transport, Construction and Building Materials 21 (2007) Wen Chen Jau ( 2008) United States Patent Publication No. US 2008 / A1, March 27, 2008 IS Indian standard specifications for testing for strength of concrete. IS Indian standard specifications for 53 grade ordinary Portland cement. IS Indian standard specifications for coarse and fine aggregate from natural sources for concrete