Chapter 7, Problem 7.25QP

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:

1. 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.
2. b. Determine the compressive strength of each cylinder after 7 days.
3. c. Determine the average compressive strength of each mix after 7 days.
4. d. Using Figure 7.24, estimate the compressive strength after 28 days (f′_C) for each mix.
5. 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.