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

TITLE PAGE Abstract 4 1.0 Introduction and Objectives 5 1.1Hypothesis 6 1.2 Experiment procedure 6 1.3 Results and calculations 7 1.4 Discussion 10 1.5 Conclusion 11 References 11 Abstract The objective of this experiment was to assess the effect of changes in water cement ratio for basic concrete mix upon compressive strength and density in maturity period of 7 days. From the experiment it was found that the 035w/c cube had the highest strength of 15.75 kN /mm2 while the 0.75w/c cube had the least strength of 8.49 kN /mm2. In terms of porosity the 0.75w/c cubes recorded the highest porosity of 10.5% by volume and 4.9 by mass. It was concluded from the experiment that high w/c will result to lower strength and high porosity. 1.0 Introduction and Objectives Concrete production is an important component in quality control in many construction projects. The process of concrete formation involves hydration of cement (an anhydrate product) (ASTM C 702-98, 2003). Aggregates, water and cement (bonding agent) are the three components of concrete. In a country like UK ordinary portland cement is most commonly used (Babor, et al, 2009; Laskar, 2009). Portland cement is a mix of calcium and aluminium silicates whose formation involves heating and fusion of limestone and shale. Through reaction process of water and calcium silicates there is formation of fine fibres or crystals which cement together the aggregate components resulting in the formation of a stiff building material. The hydration process is very sensitive to the volume of water added to the mix. In case there is addition of too little water the hydration process will be hindered from reacting to the desired optimal strength, while excess water will form voids that reduces the density and thus strength (Murdock, L. J. and Brook, K. M., 1979). This therefore makes water content of the mix to be one of the basic steps involved in concrete design and control process. In normal practice, structural concrete is designed from characteristic design strengths (Naik, 2005). In this experiment mix details were given and this was to be used in the assessment of water cement ratio and the effect it has on the strength and density of the concrete. The objective of the experiment was to introduce the material and some fundamentals in production and design of material. Through the experiment there was assessment of the effect of changes in water cement ratio for basic concrete mix upon compressive strength and density in maturity period of 7 days. The calculation involved in the experiment were as follows NB: 1 N mm-2 = 1Mpa The experiment involved the preparation of mixes with water cement ratios of 0.3, 0.55 and 0.7 and production three 100mm test cubes each mix. A standard mix was produced that comprised of 1 part OPC: 2 parts of fine aggregate: 4 parts coarse aggregate (mass basis). In order to produce three cubes for one batch the quantities of materials required were: 1.5kg ordinary Portland cement OPC; 3.0 kg of and 6kg of coarse aggregate. 1.1Hypothesis HO there is no difference in variation in strength and density for the different w/c H1 there is a variation in strength and density for the different w/c 1.2 Experiment procedure Coarse aggregate, the fine aggregate and cement were dry mixed thoroughly with care being taken to avoid production of dust. Water was added gradually as the concrete was being mixed to a point where the mix was of uniform colour and consistency. A slump test was done for each of the three mixes with the first step being the placing of the cone on a tray and securing it in place. A representative sample of concrete was obtained and the cone was filled in three layers with each layer being tampered 25 times. Surplus concrete was struck off to ensure that concrete was level with the top of the cone. About 5 to 10 seconds was taken as the cone was being carefully lifted straight up, the cone was then inverted and placed next to the mound of concrete. The tamping bar was laid across the top of the inverted cone in a way that it passed across the top of the concrete mound at its highest point. The distance between the underside of the bar and the highest point the concrete was measured to the nearest 5mm, this being the measure of slumping in the concrete. The concrete was then returned to the original tray and was mixed with the remaining concrete. The 100mm standard cube mould was filled in three equal layers and compacting each of the layers with at least 25 strokes using a tamping bar. Surplus concrete was removed and the top surface was levelled using a trowel and the concrete was then left for 7 days. The corresponding mould numbers were recorded in a table against the appropriate mix After the seven days the samples were retrieved from the curing tank. The dimensions of the cubes were measured, their masses were recorded and their volumes and densities were calculated with the data being entered into a table. One cube for each water cement ratio cube samples was saturated in the vacuum apparatus under a close supervision of the laboratory supervisor or technician. The specimens were placed in a vacuum chamber which was evacuated for 10 minutes. The chamber was then flooded with water that saturated the specimens and upon 10 minutes lapse the specimens were removed. The surplus water was wiped off, the specimens were then weighed and their saturated masses recorded in a table. 1.3 Results and calculations From table 1 it can be seen that the 0.3 w/c cube had a mass 2265g, the 0.55w/c cube had a mass of 2170g while the 0.8w/c cube had a mass of 2135 g which was the lowest of the three. The mass of the 0.3 w/c, 0.55w/c and 0.8w/c when saturated with water were 2267g, 2265g and 2240g respectively. The 08 w/c cube had the highest porosity of 10.5% by volume and 4.9 by mass while 0.55w/c cube had a porosity of 9.5% by volume and 4.4% by mass. The 0.3 w/c cube had the lowest porosity of 0.2 by volume and 0.09 by mass. Table 1: Water Porosity Results Cement Mix 0.3 W:C ratio 0.55 W:C ratio 0.8 W:C ratio Mass of Cube Dry (g) 2265 2170 2135 Cube Volume (cm3) 1000 1000 1000 Mass of Cube Saturated (g) 2267 2265 2240 Mass of water Absorbed (g) 2 95 105 Volume of Water Absorbed (cm3) 2 95 105 Percentage Porosity (By volume- %) 0.2% 9.5 10.5 Percentage Water Absorption (By mass- %) 0.09 4.4% 4.9% From table 2 it can be observed that for the 0.3w/c the failing loads from the highest to the lowest was 180.2 kN , 177.5kN and 115kN and this translated to compressive strength of 18.02, 17.75 and 11.5 kN/mm2 respectively with the average being 15.75kN/mm2 . The 0.55w/c had failure loads of 153.3kN, 147.3kN and 91.4kN which translated to failure strength of 15.33 kN /mm2, 14.73 kN /mm2 and 9.14 kN /mm2 with the average strength being 13.13 kN /mm2. For the 0.75w/c the failure loads were 108.4 kN, 90.6kN and 55.8 kN with the corresponding compressive strength being 10.84 kN /mm2 , 9.06 kN /mm2, and 5.58 kN /mm2 respectively with the average strength being 8.49 kN /mm2. The slump values for the 0.35w/c, 0.55w/c and 0.75w/c were 0.25mm, 45mm and 210mm respectively. From figure 1 it can be seen that the highest of 2188N/m3 was registered by the 0.35 w/c concrete cube while the lowest density was for the 0.75 w/c concrete cube. This graph clearly indicated that the density of the concrete was decreasing with an increase in the w/c. In figure 2 it can be observed that the highest concrete strength was 15.74N/mm2 for the 0.35 w/c while the lowest strength of 8.49 was for the concrete of 0.75 w/c. Table 2 Mix Mould number Slump(mm) Cubecross-sectional area (mm2) Cube volume Cube mass(kg) Cube density Failing load (KN) Compressive strength (Nmm-2) Average compressive strength (Nmm-2) Average cube density (mm3) (m3) 0.35 1 wet 0.25 10000 1000000 0.001 2.249 2249 115 11.5 15.75 2188 2 dry 10000 1000000 0.001 2.176 2176 180.2 18.02 3 dry 10000 1000000 0.001 2.14 2140 177.5 17.75 0.55 1 wet 45mm 10000 1000000 0.001 2.265 2265 91.4 9.14 13.13 2184 2 dry 10000 1000000 0.001 2.105 2105 147.3 14.73 3 dry 10000 1000000 0.001 2.184 2184 153.3 15.33 0.75 1 wet 210 10000 1000000 0.001 2.236 2236 55.8 5.58 8.49 2152 2 dry 10000 1000000 0.001 2.1 2100 90.6 9.06 3 dry 10000 1000000 0.001 2.121 2121 108.4 10.84 1.4 Discussion From the experiment it can be seen that the cubes with the highest w/c (075) had the highest porosity. This can be attributed to the fact that water occupied some space when the brick dries up the space will remain unoccupied. The higher the w/c the higher the amount of space that is unoccupied. The density of the block will depend on the amount of this unoccupied space with high density being recorded for blocks with fewer empty spaces. Blocks with large air space have high capacity of absorbing water hence high porosity. The porosity in terms of volume is higher than in terms of mass. This can be attributed to the fact that the density of water is lower than the density of the materials used in making the blocks. Figure 1 Figure 2 1.5 Conclusion From the experiment it can be concluded that increaing the w/c will reduce the density and the strenth of the concrete. Therefore, the null hypothesis that there was no relationship between w/c and density and strength was therefore rejected. It is also cleary that concrete of high density will always have high strength. very low w/c may produce the strongest concrete but the concrete may be of very low workability. References ASTM C 702-98(2003), “Standard Practice for Reducing Samples of Aggregate to Testing Size.” ASTM C 637-98a (2003), “Standard Specification for Aggregates for Radiation-Shielding Concrete.” Babor, D., Plian, D. and Judele, L. (2009) “Environmental Impact of Concrete”, Publicat de Universitatea Tehnică „Gheorghe Asachi” din Iaşi Hansen, T. C. and Narud, H. (1983) “Strength of Recycled Concrete Made from Crushed Concrete Coarse Aggregate”, Concrete International, Vol. 5, Iss. 1, pp 79–83. Laskar, A. I. (2009) “Correlating slump, slump flow, vebe and flow tests to rheological parameters of high-performance concrete”, Materials Research, Vol. 12, Iss. 1, pp. 1516–1439. Logic Sphere (n.d.) “Slump Test”, visited on 31-03-2010, available at: http://logicsphere.com/products/firstmix/hlp/html/work5xd0.htm Murdock, L. J. and Brook, K. M. (1979) “Concrete Materials and Practice”, 5th edition, London, Edward Arnold Ltd. Naik, T. R. (2005) “Sustainability of Cement and Concrete Industries”, Presented and Published at the Global Construction: Ultimate Concrete Opportunities, Dundee, Scotland. Available at: http://www4.uwm.edu/cbu/Papers/2004%20CBU%20Reports/CBU-2004-15.pdf, visited on 15/04/2010. Questions 1. Plot two graphs to show: - Strength versus water: cement ratio. Density versus water: cement ratio. 2. How does water: cement ratio affect compressive strength? It can be seen that at a w/c of 0.35 the concrete has the highest compressive strength of 15.75N/mm2. There is a decreasing trend in the strength as the w/c ratio increases with a w/c of 0.75 having the lowest average strength of 8.49 N/mm2. 3. How does water: cement ration affect density? From figure 2 it is observed that at w/c of 0.35 the concrete has the highest density of 2188N/m3 .There is a decreasing trend in density as the w/c increases where the 0.75 w/c had the lowest density of 2152N/m3 4. Why are cubes placed in water to cure? This is done to ensure that there is enough water for the hydration process of the cement. 5. What water: cement ratio gives the greatest compressive strength? A w/c of 0.35 gave the greatest compressive strength of 15.75N/mm2 6. Which water: cement ration has the lowest compressive strength? A w/c of 0.75 gave the lowest compressive strength of 8.49N/mm2 7. What affect does water: cement ratio have on the hydration of cement? When w/c is very low the hydration process will be hindered (will not be completed)but a very high w/c results in excess water which form pores that lowers the strength of concrete. 8. Classify your mixes according to the following slump test results. W:C ratio Slump Value Workability Cohesion 0.30 0.25 Dry mix with very low workability High cohesion 0.55 45mm Medium workability 0.80 210mm Wet , harsh mix Low cohesion 9. How does water: cement ratio affect the workability of the mix and therefore the slump? The workability of concrete will increase with an increase in w/c . 10. Which mix would be the more durable? Why? The 0.35 w/c mix will be the most durable because it has high strength and high density and this make in capable to withstand destructive forces. 11. How does water: cement ratio affect the porosity of the mix? High w/c will increase the porosity of concrete. 12. How would increased porosity increase or decrease durability? Increased porosity will lower durability as the concrete is able to absorb water which may have chemicals that will react with the concrete and thus weakening it. 13. Would you use a concrete of water: cement ratio 0.4 or 0.8 in high sulphate environments? Explain your answer. In a high sulphate environment it would be recommended to use 0.4 w/c concrete. This will ensure that the concrete will not have high porosity. High porosity will cause the sulphates to be absorbed in the pores where there will be a reaction resulting to formation of precipitated that will weaken the concrete. Read More