Optimized PCC Mix Desgin

Differential drying shrinkage was believed to be the predominant factor contributing to slab curling and corner cracking observed in CC1. Thus, there was a need to develop an optimized PCC mix design to prevent premature PCC failure in CC2. Specifically, shrinkage and workability were the most important variables considered for the optimized mix design..  The method employed by the U.S. Air Force as discussed by Lafrenze and the State of Iowa were used to optimize mixture proportioning (after Shilstone). Both methods look at obtaining a well graded aggregate blend by optimizing the “coarseness factor” (combined percent retained above 9.5 mm sieve divided by combined percent retained above 2.36 mm sieve) and the “workability factor” (combined percent passing 2.36 mm sieve) among other factors. Therefore, laboratory shrinkage tests were conducted in accordance with ASTM C 157 to evaluate the impacts of coarse aggregate size, aggregate type, aggregate proportioning, and water/cement ratio.

Six laboratory mixes were tested using the dolomite and traprock aggregate at cement contents of 450 lbs. (204 kg.), 475 lbs. (215 kg.), and 500 lbs. (227 kg.), holding water constant to yield water/cement ratios of 0.47, 0.44, and 0.42, respectively.The mixes were tested for shrinkage, slump, air content, flexural strength, and compressive strength.The mixes were batched with and without water reducing agents to optimize the slump at 3 inches to 4 inches (8 cm to 10 cm) for hand placement at the NAPTF.Of these, a candidate mix, designated as Clayton No. 2, was selected for trial batching.This laboratory mix had a water/cement ratio of 0.44, 475 lbs. (215 kg) of cement, 3 inches (8 cm) slump, and 0.05% shrinkage.Although the shrinkage slightly exceeded the set guidelines, it was consistent with results for the other candidate mixes and considered acceptable.The flexural results were near the mid-range of 870 psi (6 MPa) to 1,150 psi (8 MPa) recorded for all the mixes. However, during trial plant batches, the selected mix achieved a maximum slump of 1 inch (2.5 cm). The minimum practical slump considered was 3 inches (7 cm to 8 cm), HRWR was added to the mix to reach 3 inches (8 cm) slump. Trial batches were necessary to optimize the mix design.

Material	Weight, lbs. (kg.)
No. 57 Coarse Aggregate	1,450 (658)
No. 9 Intermediate Aggregate	790 (358)
Concrete Sand	1,120 (508)
Water	231 (105)
Type 1 Cement	525 (238)
Air	4.9%
HRWR	10 oz. per 100 lbs. (0.3 liter per 45 kg.)
Slump	3 inches (7 cm)
Water/Cement Ratio	0.44
Yield	27.1 cubic yards (0.77 cubic meters)
Workability	34.1%
Coarseness	58.4%
Mortar	53%

The outcomes from the material testing program were:

• The maximum size of the coarse aggregate (No. 57 or No. 467) did not influence the shrinkage.
• The use of high range water reducers (HRWR) increased shrinkage.Shrinkage greater than 0.04% in mixes with HRWR, and less than 0.04% in mixes without HRWR.
• The mix used for the original concrete mix (50% coarse aggregate and 50% sand blend; 0.50 water/cement ratio; No.57 coarse aggregate) had measured shrinkage of 0.08%.