The Effect of Water - Cement Ratio upon the Compressive Strength of Concrete - Research Paper Example

The Effect of Water: Cement Ratio upon the Compressive Strength of Concrete Department of Forensic and Investigative Science Lecturer Name Student ID Date Table of Contents Table of Contents 2 Abstract 4 Introduction 5 Research Hypotheses 7 Results and calculations 9 Discussion of Results 13 Conclusion 14 Recommendations 14 Bibliography 15 Appendix 16 Appendix 1: Determining the amount of water to use for every water-cement ratio 16 Appendix 2: water porosity calculations 16 Figure 1: Effect of water: cement ration on compressive strength 11 Figure 2: Effect of water: cement ration on concrete density 11 Figure 3: Effect of water: cement ration on slump 12 Figure 4: Effect of water: cement ration on concrete porosity 13 Table 1: Results for the slump test 10 Table 2: Filtered results for the slump test 10 Table 3: Porosity results 12 Abstract Concrete is finding extensive application in construction work ranging from simple structures, such as building floors to complicated structures, such as bridges. The compressive strength of concrete and porosity are affected by various issues, such as water-cement ration during concrete preparation, concrete compaction and the relative proportion of coarse and fine aggregates used during concrete preparation. This experiment was aimed at evaluating the impact of cement-water ration on concrete compressive strength and porosity. Results indicated that there is an optimum ratio that provides maximum compressive strength in concrete. Further, increase in water: cement ratio leads to increase in concrete porosity. Key words: water: cement ratio, compressive strength, slump test, porosity. The Effect of Water: Cement Ratio upon the Compressive Strength of Concrete Introduction Concrete is finding extensive application in construction work ranging from simple structures, such as building floors to complicated structures, such as bridges. Concrete, which is made by controlled combination and mixture of aggregate (coarse and fine aggregate), cement and water. Need and amount of reinforcement depends on the application. For example, concrete for basement floors may not require steel reinforcement while concrete for bridges requires reinforcement to enable it to handle extensive loadings. Coarse aggregate used for concrete preparation includes ballast while fine aggregate includes sand. Once prepared, concrete takes the shape of the mould or container. It then forms a solid mass when allowed to dry under controlled conditions of humidity and temperature, which prevents cracks development and propagation. After concrete hardens, it is a brittle structure with high compressive but low tensile strength. A study conducted by Mosley and Bungey (2000) concluded that the compressive strength of concrete is far much more than its tensile strength. Accordingly, concrete strength is often determined with respect its compressive strength while its tensile strength is often neglected when designing buildings. Steel reinforcement is often considered when it is anticipated that application will involve extensive tensile stress (Mosley and Bungey 2000). According to Vu et al. (2009), the compressive strength of concrete is affected by various issues, such as water-cement ration during concrete preparation, concrete compaction and the relative proportion of coarse and fine aggregates used during concrete preparation. Concrete curing during settling and drying process is another factor that has a paramount impact on the compressive strength of concrete (Alawode and Idowu 2011). Extensive research activities have already been conducted on the effect of cement-water ratio on the compressive strength of concrete with a conclusion that there is an optimum amount of water that give maximum strength, which depends on concrete mix under consideration (Vu et al. 2009 & Alawode and Idowu 2011). Less than the optimum water-cement ratio will reduce workability and will result into difficulty in concrete settling. On the contrary, more than the optimum water-cement ration will result into extensive shrinkage, which has a negative impact on concrete compressive strength (Alawode and Idowu 2011). Porosity is another equally important aspect that is used in determining concrete quality. Porosity refers to how well a concrete structure absorbs water, which determines whether a concrete structure will allow water to pass through or not. Excessive porosity is not desirable since it allows concrete to pass water, which renders the concrete structure unusable especially in applications where water is detrimental. Porosity is also affected by water-cement ratio, as well as other minor issues, such as amount of impurities, compaction and the relative size of aggregates used. While compaction should be thoroughly done during construction, an optimum water-cement ratio ensures least porosity (Alawode and Idowu 2011). This experiment was aimed at evaluating the impact of cement-water ration on concrete compressive strength and porosity. The experiment was based on two null hypotheses, which were established upon reviewing available literature. Research Hypotheses H01: Very low or very high water: cement ration leads to low compressive strength concrete structures H01: Very low or very high water: cement ratio leads to high porosity in concrete structures Experiment procedure Coarse aggregate, fine aggregate and cement were obtained and mixed thoroughly using the ration of 1 part of Ordinary Portland Cement, 2 parts of sand and 4 parts of coarse aggregate by weight. To achieve this mixing ratio, 1.5 kg of Ordinary Portland Cement, 3 kg of sand and 6 kg of coarse aggregate were obtained and mixed thoroughly. Extensive care was taken during the mixing process to avoid dust production. 450 g of clean water was obtained and added gradually as the concrete was being mixed until there was color uniformity and consistency in the concrete mix. Cones for performing slump tests were secured in place on a try. Some concrete, which represented the mix under consideration, was obtained, and this was used for performing the slump test. The slump test involved filing the already secured cones (100 mm height) with concrete in three steps. Concrete was placed in one cone and the cone tapped 25 times to enable the representative sample to settle in the cone. A second layer was added and the cone tapped another 25 times to enable the concrete to settle in the cone. A third, and final, layer was added to the cone and the cone tapped 25 times. Excess concrete was removed using a sharp edge so that the top of the concrete in the cone was in the same level as the top of the cone. The cone was carefully lifted and inverted on the tray so that its contents were left on the tray. The cone was then placed next to the mould of concrete that was left after the inverted cone was removed. The tampering bar was then placed across the top of the inverted cone in such a way that it passed across the highest part of the concrete mould. A ruler was vertically above the highest point of the concrete mould in such a way that it crossed the tampering bar. The distance between the top of the concrete mould and the underside of the tampering bar was read and recorded. This is essentially the height difference between the cone and concrete mould, after the concrete was poured out of the cone, which is the amount by which the concrete has slumped. Another 100 mm cone was filled with the concrete in three layers while tapping every layer 25 times. The same procedure was repeated and data recorded for every test until three cubes had been filled with the 0.3 water-cement ration and tested for lump test. Another set of coarse aggregate, fine aggregate and cement were obtained and mixed thoroughly using the ration of 1 part of Ordinary Portland Cement, 2 parts of sand and 4 parts of coarse aggregate by weight. To achieve this mixing ratio, 1.5 kg of Ordinary Portland Cement, 3 kg of sand and 6 kg of coarse aggregate were obtained and mixed thoroughly. 825 g of clean water was obtained and added gradually as the concrete was being mixed until there was color uniformity and consistency in the concrete mix. The lump test procedure was repeated as when 450 g of was added. Another set of coarse aggregate, fine aggregate and cement were obtained and mixed thoroughly using the ration of 1 part of Ordinary Portland Cement, 2 parts of sand and 4 parts of coarse aggregate by weight. To achieve this mixing ratio, 1.5 kg of Ordinary Portland Cement, 3 kg of sand and 6 kg of coarse aggregate were obtained and mixed thoroughly. 1200 g of clean water was obtained and added gradually as the concrete was being mixed until there was color uniformity and consistency in the concrete mix. The lump test procedure was repeated as when 450 g of was added. For every experiment, mould numbers were recorded against appropriate cement-water mix ratios and the amount by which the concrete had slumped. Cubes were then filled, as aforementioned, using concrete from all the three water-cement ratios. The cubes were left for a day under wet cure after which the concrete, which had already settled, was removed from the cubes. The concrete cubes were then wet cured for another six days, totaling to seven days of wet cure. After seven days, the concrete samples were retrieved from the curing tank and their weights and dimensions determined. Their volumes, hence their densities were also determined and recorded. The cubes were then saturated by placing them in a vacuum chamber that was then evacuated for 10 minutes. Then, water was introduced into the chamber, which flooded the vacuum chamber for ten minutes before the samples were removed. Surplus water was then wiped off using a dry piece of clothing and the samples weight to determine their saturated masses, which was recorded. Porosity (by volume) for each sample was determined by determining the volume of water that was absorbed by the samples. Percentage of water absorption (by mass) was also determined for every sample. Finally, every sample was crushed using a compressive testing machine until it failed. Failing load and compressive strength for every sample was then recorded. Results and calculations Results for slump test and compressive testing machine Cube cross-sectional area: 10 mm2 Cube volume: 1000 mm3 Table 1: Results for the slump test Mix Mould number mass(kg) density Failing load (KN) Compressive strength (N/mm2) 0.35 1 wet 2.249 2249 115 11.5 2 dry 2.176 2176 180.2 18.02 3 dry 2.14 2140 177.5 17.75 0.55 1 wet 2.265 2265 91.4 9.14 2 dry 2.105 2105 147.3 14.73 3 dry 2.184 2184 153.3 15.33 0.75 1 wet 2.236 2236 55.8 5.58 2 dry 2.1 2100 90.6 9.06 3 dry 2.121 2121 108.4 10.84 Table 2: Filtered results for the slump test Water :Cement ratio Average slump value (mm) Average cube density Average Compressive Strength (N/mm2) 0.30 0.25 2188.3 15.76 0.55 45 2184.7 13.07 0.80 210 2152.3 8.49 Figure 1: Effect of water: cement ration on compressive strength Figure 2: Effect of water: cement ration on concrete density Figure 3: Effect of water: cement ration on slump Table 3: Porosity results Cement : Cement Ratio 0.3 0.55 0.8 Mass of dry cube (g) 2265.00 2170.00 2135.00 Volume of the cube (cm3) 1000.00 1000.00 1000.00 Mass of saturated cube (g) 2267.00 2265.00 2240.00 Mass of absorbed water (g) 2.00 95.00 105.00 Volume of absorbed water (cm3) 2.00 95.00 105.00 cube porosity (% Volume) 0.20 9.50 10.50 Water absorption (% mass) 0.09 4.38 4.92   Figure 4: Effect of water: cement ration on concrete porosity Discussion of Results It is worth noting that the impact of compaction on the compressive strength and porosity of concrete was not taken into consideration. All the samples were assumed to achieve equal compaction since they were given equal (25) number of taps. From figure 1, it is apparent that compressive strength reduces with increase in the water: cement ration. The highest compressive strength was recorded as 15.76 N/mm2 at a ratio of 0.3 while the lowest compressive strength was recorded as 8.49 N/mm2 at a ratio of 0.8. Optimum water: cement ration was not determined. It is possible that the optimum ration is below 0.3 since the shows continuous decrease in compressive strength as ration increases. Referring to figure two, concrete density increases with increase in water: cement ratio up to an optimum ration of about 0.5 from which density decreases. This may be considered the optimum value that would provide maximum compressive strength. From figure 3, there is a rapid change in slump value from a cement: water ration of about 0.5, after which there was a drastic change in slump value as water: cement ration increased. Before this value, slump value did not change drastically. This implies that before this value (about 0.5), concrete shrinkage was not as extensive as after the optimum ratio. Figure 4 shows that increase in water: cement ratio resulted into increase in porosity until an optimum ratio of about 0.7 when porosity was constant at about 10.5%. This implies that increase in water: cement ratio beyond this value does not lead to further increase in porosity. According to Alawode and Idowu (2011), there is an optimum water-cement ratio ensures least porosity. This theory, however, is not being supported by the findings of the experiment since porosity is seen to increase with increase in water: cement ratio. However, Alawode and Idowu (2011) were arguing on the basis that high water: cement ration results to cracks, which contribute towards porosity, a factor that was not considered in the experiment. Conclusion The first hypothesis was not rejected since the experiment indicated that there is an optimum amount of water that gives maximum strength. The second hypothesis was not accepted since porosity was seen to increase with increase in water: cement ratio. Therefore, increase in water: cement ratio leads to increase in concrete porosity. Recommendations As aforementioned, several factors contribute towards concrete compressive strength and porosity. Key factors include compaction, water: cement ratio and relative aggregate sizes. Furthermore, it was noted that the impact of water: cement ration depends with concrete mix under consideration. Therefore, future experiments should take into consideration all these factors, which will enable the determination of every factor for different concrete mix and applications. Bibliography Alawode, O., and Idowu, I. Effects of Water-Cement Ratios on the Compressive Strength and Workability of Concrete and Lateritic Concrete Mixes. The Pacific Journal of Science and Technology, 12(2), 99-105. Mosley, W., and Bungey, J. 2000. Reinforced Concrete Design, Fifth Edition. London, UK: Macmillan Publishers Limited. Vu, X., Malecot, Y., Duadeville, L., and Buzuad, E. 2009. Effect of the Water/Cement Ratio on Concrete Behavior under Extreme Loading. International Journal for Numerical and Analytical Methods in Geomechanics, 33, 1867-1888. Appendix Appendix 1: Determining the amount of water to use for every water-cement ratio It involved multiplying the weight of cement by the water ratio desired (0.3, 0.55 or 0.7). For 0.3 ratio, amount of water = 0.3 water/cement x 1.5 kg cement = 0.45 kg water = 450 g water For 0.55 ratio, amount of water = 0.55 water/cement x 1.5 kg cement = 0.835 kg water = 850 g water For 0.80 ratio, amount of water = 0.50 water/cement x 1.5 kg cement = 1.2 kg water = 1200 g water Appendix 2: water porosity calculations Mass of absorbed water (g) = Mass of saturate cube (g) – mass of dry cube (g) Volume of absorbed water (cm3) = mass of absorbed water/density of water Where density of water = 1g/cm3 Cube porosity (% volume) = 100 x volume of water absorbed (cm3)/volume of dry cube (cm3) Water absorption (% mass) = 100 x mass of absorbed water (g)/mass of dry cube (g) Cube density = cube mass/ cube volume Compressive strength of a cube = Failing load/cube cross sectional area Read More