INFLUENCE OF CONSTITUENT MIX PROPORTIONS ON CONCRETE WORKABILITY: A PREDICTIVE MODEL USING RSM
Abstract
The existence of voids affects the ultimate strength of concrete. With adequate workability, voids are significantly reduced and hardened properties of concrete are enhanced. The workability of concrete is affected by factors including water content, size, shape and content of aggregate, and use of admixtures. Hence, understanding how the interaction of these factors influence concrete workability is crucial if concrete with desirable properties is sort. This study investigated the workability of fresh concrete using Kuta river gravel as aggregate. Water/cement ratio (W/C), coarse aggregate/total aggregate ratio (CA/TA) and total aggregate/cement ratio (TA/C) were assigned values and used as variables for design. Central Composite Design (CCD) in Minitab 21 was used to generate 20 mixes with different combinations of the design variables. Slump test was used to test workability of the concrete mixes. Further to this, Response Surface Methodology (RSM) was employed to develop a regression model for the slump. It was determined that the aggregate can be used to make concrete with slump ranging between 0-270mm (ranging between no workability concrete to very high workability concrete) depending on proportion of mix constituents. The lowest slump (0mm) was recorded for mixes with W/C ratio of 0.4, CA/TA ratio of 0.65, and TA/C ratio of 6, and W/C ratio of 0.4, CA/TA ratio of 0.55 and TA/C ratio of 6. While the highest slump was recorded for the mix with W/C ratio of 0.5, CA/TA ratio of 0.6 and TA/C ratio of 2.38. The model developed has overall P-value of 0.000, R2 value of 92.34% and Adjusted R2 value of 88.80%. It was concluded that workability of concrete increases with increase in W/C ratio and decrease in TA/C ratio and vice versa, change in the CA/TA ratio has insignificant effect on the workability of concrete, and that the developed model is adequate in predicting slump of concrete.
Keywords: modelling, slump, workability, kuta river gravel, concrete
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