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Three concrete mixes with the same ingredients, except the amount of mixing water, and their slump values were obtained. Three 100 mm × 200 mm concrete cylinders were prepared for each mix. The cylinders were cured for 7 days and then tested for compressive strength. The test results are as shown in Table P7.25. Assume that the aggregate was at the saturated surface-dry condition before adding mixing water.
It is required to do the following:
- a. Plot the relationship between slump and amount of mixing water for all mixes. Comment on the effect of increasing the amount of water on workability.
- b. Determine the compressive strength of each cylinder after 7 days.
- c. Determine the average compressive strength of each mix after 7 days.
- d. Using Figure 7.24, estimate the compressive strength after 28 days (f′C) for each mix.
- e. Determine the w/c ratio for each mix. Plot the average f′C values versus w/c ratios for all mixes. Comment on the effect of increasing the w/c ratio on the compressive strength.
TABLE P7.25
Mix No. | Weight of Cement (kg) | Weight of Water* (kg) | Slump (mm) | Cylinder No. | Maximum Load (kN) | Compressive Strength after 7 Days (MPa) | Average Compressive Strength after 7 Days (MPa) | Estimated Compressive Strength after 28 days (MPa) |
1 | 10 | 5 | 40 | 1 | 138.2 | |||
2 | 175.9 | |||||||
3 | 136.9 | |||||||
2 | 10 | 5.5 | 55 | 1 | 100.7 | |||
2 | 115.2 | |||||||
3 | 113.6 | |||||||
3 | 10 | 6 | 75 | 1 | 73.3 | |||
2 | 78.4 | |||||||
3 | 76.0 |
* Assume that the water listed here is above the saturated surface-dry (SSD) condition of the aggregate.
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