The effect of redosage of the water to cement ratio on properties of concrete subjected to time lag in concreting

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Date
2019-05
Authors
Bong Yack Huang
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This project is carried out to determine the optimum amount of water redosage to fresh concrete in order to maintain the workability of concrete within the acceptable range for 105 minutes. This project is also aimed to study the effect of adding water to the fresh concrete on its compressive strength. Results of this project are obtained via experimental works. Three sets of the same experiments are carried out to obtain the average result for a more accurate outcome. For each set of experiment, two tests are conducted, namely slump test and concrete cube compression strength test. The slump test was carried out at T1 (0-45mins), T2 (45-90mins), T3 (90-105mins) and T4 (105mins). Meanwhile, concrete cubes were prepared at T1 (0-45mins), T2 (45-90mins) and T4 (105mins). Concrete cube compressive strength test was conducted at days 3 and days 28. Water redosage was carried out at 45 minutes after batching (T2). 0.3% of superplasticizer was added during the mixing of concrete in order to obtain a required slump of 80mm. From the results of experiments, the amount of water redosage that increased the w/c ratio of concrete from 0.45 to 0.50 and 0.55 is not sufficient to restore and maintain the concrete workability within the range of 75-100mm over 105 minutes. The compressive strength of the concrete decreased after water redosage, but the amount of strength deteriorated are small. All of the concrete cubes achieved compressive strength above the minimal requirement of 25MPa, regardless of the amount of water redosage. From this project, the optimum amount of water redosage is unable to be determined since the workability of concrete subjected to a delay of 105 minutes is unable to be restored and maintained within the acceptable range of 75-100mm.
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