Relationship between Modulus of Elasticity and Strength of Brick Aggregate Concrete

Transcription

1 Paper ID: CBM International Conference on Recent Innovation in Civil Engineering for Sustainable Development (IICSD-2015) Department of Civil Engineering DUET - Gazipur, Bangladesh Relationship between Modulus of Elasticity and Strength of Brick Aggregate Concrete M.S. Mia 1, M.K. Miah 2, M. Hasan 3 Abstract Modulus of elasticity of concrete is expressed in terms of compressive strength. This study carried out an investigation on relationship between modulus of elasticity and compressive strength of brick aggregate concrete. Trial mixes were prepared using crushed bricks as coarse aggregate with various aggregate size, water cement ratios and mixing ratios which results different concrete strength. Concrete cylinders were prepared and unit weight, compressive strength and modulus of elasticity were tested at 7, 14 and 28 days. The ACI Code relations for determining the modulus of elasticity for brick aggregate concrete was found higher than the test values. To introduce a relation to estimate the modulus of elasticity also a series of data have been collected from many researchers. The results of this study shows that the modulus of elasticity for brick aggregate concrete is about 23% lower than the expression suggested by the Code. This study proposed a relation to estimate the modulus of elasticity and correction factor for brick aggregate concrete. Keywords: Brick aggregate concrete, compressive strength, modulus of elasticity. 1. Introduction Modulus of Elasticity of concrete is a key factor for determining the deformation of structural elements, as well as a fundamental factor for determining the modular ratio, n, which is used for design of reinforced and prestressed concrete structures. Based on the relationship of modulus of elasticity of concrete, that is proportional to the square root of compressive strength in the range of normal concrete strength. In Bangladesh, India and part of west Bengal, burnt bricks are used as an alternative source of coarse aggregate(ca) and the performance of concrete made with brick aggregate as coarse aggregate are quite extensive and satisfactory. Concrete of moderate to high strength, are made by using crushed stone and broken bricks as coarse aggregate in this area. In Bangladesh crushed stone are not available and so most of the concrete works are constructed with brick aggregate. The cost of the brick aggregate is much lower than stone aggregate. Some studies are found literature. Akhtaruzzaman and Hasnat [1] investigated the modulus of elasticity of concrete using crushed brick as coarse aggregate. Mansur, M. A. et al. [2] comparing the modulus of elasticity of brick aggregate concrete obtained by replacing stone aggregate. Rashid, M. A. [4] investigate to achieve higher strength concrete using brick as coarse aggregate and proposed an equation to estimate the modulus of elasticity. Rashid, M. A. and Salam, M. A. [5] investigate the effect of replacing natural coarse aggregate by brick aggregate and compare the modulus of elasticity. The present study reports the variation of modulus of elasticity with compressive strength of concrete using brick as coarse aggregate. 1 Lecturer, Department of Civil Engineering, DUET, sumanduet105@gmail.com 2 Professor, Department of Civil Engineering, DUET, mkhasro@duet.ac.bd 3 Lecturer, Department of Civil Engineering, DUET, hasan.m.m.ce@gmail.com

2 Experimental investigation In this study, manually crushed brick aggregates having 12.5mm, 19mm and 25mm with well graded chips were used. A mixture of coarse sand (Sylhet sand) and locally available fine sand in the ratio of 1:1 was used as fine aggregate. The fineness modulus of the mix sand was Absorption capacities of coarse and fine aggregate were 18.5% and 2.32% respectively. The physical properties of brick aggregate and sand are given in Table 1. Four w/c ratios of 0.45, 0.54, 0.63 and 0.72 are used and three mixing ratios of 1:2:4, 1:1.5:3 and 1:1:2 were fixed for this study. In all cases Composite Portland Cement (Shah Cement Special) was used. Table 1: Properties of concrete ingredient Properties Fine aggregate Local sand Sylhet sand 12.5 mm Coarse aggregate Bulk specific gravity (SSD) Water absorption (percent of dry weight) Fineness Modulus Unit weight, kg/m 3 (dry, compacted) Casting of specimens The graded aggregates (both fine and coarse aggregates) were soaked in water for 24 hours and then air-dried to saturated surface dry (SSD) condition before mixing with other ingredients. For each mix, all of the ingredients with appropriate proportions were added in the mixture machine, and then mixing was done for about 2 minutes. The test specimens were cast in steel moulds and compacted with a tamping rod. They were demoulded 24 hours after casting and were cured under water until 24 hours before the test. For all the cases standard cylindrical ( 150mmx300mm) specimen were used Testing of specimens The specimen were taken out of water approximately 24 hours before testing and were kept in the air dry condition in the laboratory. The concrete cylinders of different variable were tested for compressive strength and deformation for determining modulus of elasticity. There are total 108 numbers of specimen were tested following appropriate ASTM standard [8]. 3. Test Result and discussions The comparison is conducted by the variation of compressive strength, which is stand for different mix proportion, aggregate size, water cement ratios and age of concrete. On this study it has found that the average unit weight of brick aggregate concrete is w c = 132 lb/ft 3. Hence the ACI Code [3] relationship is E c = 33 (132) 1.5 (psi) = (psi) = 4150 (MPa). = w 1.5 (MPa) (1) Where, w = kg/m 3, = MPa Experimental values of modulus of elasticity were plotted in figure that is shown below. The results are presented in figures and discussed categorically. The results include compressive strength f c and modulus of elasticity E c Variation of modulus of elasticity with age The relation between of the modulus of elasticity and the corresponding square root of compressive strength for 7 days as shown in Fig. 1. It is seen the modulus of elasticity increase with the increase of compressive strength. It is obvious that the 7 days modulus of elasticity of brick aggregate concrete is about 87% of 28 days and 68% of ACI Code relationship mm 25.0 mm

3 Ec (MPa) y = x R² = (f c) (MPa) Fig. 1 Relationship between f c ' and Ec for 7 days Fig. 2 Relationship between fc' and Ec for 14 days The relationship between f c ' and Ec for 14 days curing period shown in Fig.2. It is observed that the 14 days modulus of elasticity of brick aggregate concrete is about 92% of 28 days and 71% of ACI Code relationship. The relationship between f c ' and Ec for 28 days curing period shown in Fig.3. It is also observed that the 28 days modulus of elasticity of brick aggregate concrete is about 77% of ACI Code relationship. The modulus of elasticity of brick aggregate concrete can be expressed empirically by- E c = 3215 (MPa) = w 1.5 (MPa) (w = kg/m 3, = MPa) (2) Fig. 3 Relationship between f'c and Ec for 28 days 3.2. Variation of modulus of elasticity with aggregate size Fig. 4, Fig. 5 and Fig. 6 have shown that variation of modulus of elasticity with aggregate size. Result shows that the modulus of elasticity is higher when the concrete is made with 19.0 mm downgrade aggregate. Average modulus of elasticity for various aggregate sizes can be express by, E c = 3215 aggregate. (MPa). It is seen that the modulus of elasticity is maximum with 19 mm downgrade Fig. 4 Ec and f'c for 28 days with 12.5 mm CA Fig. 5 Ec and f'c for 28 days with 19 mm CA

4 154 Fig. 6 Relationship between Ec and f'c for 28 days with 25 mm CA 3.3. Variation of modulus of elasticity with water cement ratio Fig. 7, Fig. 8, Fig. 9 and Fig. 10 shows that variation of modulus of elasticity with various w/c ratios. Result shows that the modulus of elasticity is higher when the concrete is made with w/c ratios Average modulus of elasticity for various w/c ratios can express by, E c = 3215 (MPa) Fig.7 Ec and f'c of 28 days with w/c ratio Fig. 8 Ec and f'c of 28 days with 0.54 w/c ratio Fig. 9 Ec and f'c of 28 days with 0.63 w/c ratio Fig. 9 Ec and f'c of 28 days with 0.72 w/c ratio Fig. 11 has shown the relationship between Ec and w/c. It is seen that the modulus of elasticity can be estimated by using the equation, E c = (f c) x (f c) x (f c) (MPa) (3)

5 155 Fig. 11 Relationship between Ec and w/c 3.4. Validation of test data Table 2 shows the test result obtained by Rashid, M. A. [4] at 28 days and comparison of the test result obtained by this test. Table 2 : Comparison of Ec by other researchers with present Study Actual Compressive Strength (MPa) Investigated Ec (MPa) by Rashid, M. A. [4] Ec (MPa) according to the equation proposed by this study Variation with present study % % % % 4. Evaluation of correction factor for brick aggregate Equation (1), introduced the ACI Code relation and equation (2), introduced the proposed relation by the present study to estimate the modulus of elasticity for brick aggregate concrete. By comparing the both equations a correction factor (k = 0.77) is proposed and the equation (1) can be rearranged to estimate the modulus of elasticity for brick aggregate concrete. Ec = 33k w 1.5 (psi) = k w 1.5 (Mpa) (3) Where, w = kg/m 3, = MPa 5. Conclusions This study is conducted by the variation of compressive strength, which is stand for different mix proportion, aggregate size, water cement ratios and age of concrete. From the test results and analysis the following conclusion can be drawn: It was found that the values of modulus of elasticity for brick aggregate concrete is about 23% lower than the ACI Code (1999) expression. The Eq. (2) may be used to estimate the modulus of elasticity for brick aggregate concrete. A correction factor 0.77 may use to estimate the modulus of elasticity for brick aggregate concrete in case of ACI Code relationship. Experimental value of modulus of elasticity is about 68% of ACI Code attained in 7 days and 71% modulus of elasticity of ACI Code attained in 14 days. About 87% modulus of elasticity of 28 days attained in 7 days and 92% (28 days) attained in 14 days.

6 156 The modulus of elasticity is maximum with 19 mm down grade coarse aggregate. The Eq. (3) may be used to estimate the modulus of elasticity for brick aggregate concrete for any water cement ratio. 6. Acknowledgement The authors acknowledge the financial grants provided by the Department of Civil Engineering, Dhaka University of Engineering &Technology, Gazipur for this study. 7. References [1] Akhtaruzzaman, A. A. and Hasnat, A. (1983), Properties of Concrete Using Crushed Brick as Aggregate, Concrete International, Vol. 5, No. 2, pp [2] Mansur, M. A., Wee, T. H. and Cheran, L. S. (1999), Crushed Bricks as Coarse Aggregate for Concrete, ACI Materials Journal, Vol. 96, No. 4, pp [3] ACI 318R-99 (1999), Building code requirements for reinforced concrete and commentary, ACI Committee 318, American Concrete Institute, Farmington Hills, Michigan, pp.391. [4] Rashid, M. A., Hossain, T. and Islam, M. A. (2009) Properties of higher strength concrete made with crushed brick as course aggregate, Journal of Civil Engineering (IEB), 37(1) (2009) [5] Rashid, M. A., Salam, M. A., Shill, S. K., and Hasan, M. k. (2012) Effect of replacing natural coarse aggregate by brick aggregate on the properties of concrete, DUET Journal, Vol. 1, (2012) [6] Neville, A. M. and Brooks, J. J. (2002), Concrete Technology, Pearson Education. [7] Nilson, A. H. and Darwin, D. (1997), Design of Concrete Structures Thirteenth Edition, McGraw-Hill Companies, Inc. [8] Annual Book of ASTM Standards, American Society for Testing and Materials (ASTM), West Conshohocken, PA, USA, Vol , 2001.