SECTION II. TRIAL-BATCH METHOD
3-12. In the trial-batch method of mix design, use actual job materials to obtain mix proportions. The size of the trial batch depends on the equipment you have and how many test specimens you make. Batches using 10 to 20 pounds of cement may be big enough, although larger batches produce more accurate data. Use machine mixing, if possible, because representation is close to the actual job conditions. Always use a machine to mix concrete containing entrained air. Be sure to use representative samples of aggregate, cement, water, and air-entraining admixture in the trial batch. The following steps are setup guidelines:
Step 1. Pre-wet the aggregate and allow it to dry to a saturated, surface-dry condition.
Step 2. Place the aggregate in covered containers to maintain this condition until ready for use. This simplifies calculations and eliminates error caused by variations in aggregate moisture content. When the concrete quality is specified in terms of the W/C ratio, the trial-batch procedure consists of combining paste with the proper amounts of FA and CA to produce the required slump and workability.
Step 3. Calculate the larger quantities per sack and/or per cubic yard.
EXAMPLE USING THE TRIAL-BATCH METHOD
3-13. The trial-batch method determines the mix proportions for a concrete retaining wall exposed to freshwater in a severe climate. The required compressive strength is 3,000 psi at 28 days. The minimum wall thickness is 8 inches, with 2 inches of concrete covering the reinforcement. Enter all trial-mix data in the appropriate blanks on the trial-mix-data work sheets (see Figure 3-1). Table 3-1 indicates that a maximum W/C ratio of 0.50 by weight satisfies the exposure requirements. Using Type IA (air-entraining) portland cement and a compressive strength of 3,000 psi. Table 3-2 shows that a maximum W/C ratio of approximately 0.46 by weight satisfies the strength requirements. To meet both specifications, select a W/C ratio of 0.46. Since the maximum size of the CA must not exceed one-fifth the minimum wall thickness nor three-fourths of the space between the reinforcement and the surfaces, the maximum size of the CA you will choose is 1 1/2 inch. Because of the severe exposure conditions, the concrete should contain entrained air. Table 3-3 shows that the recommended air content is 5.5 +/- 0.5 percent, assuming that vibration will consolidate the concrete. Table 3-4 indicates a recommended slump ranging from 1 to 3 inches. The trial-batch proportions are now determined. For a batch containing 20 pounds of cement, the mixing water required is: 20 x 0.46 = 9.2 pounds
Figure 3-1. Work sheet for concrete trial-mix data
Step 1. Select and weigh representative samples of FA and CA, and record their weights in column 2 of Figure 3-1. Use all the measured quantities of cement, water, and, air-entraining admixture.
NOTE: It is not normal practice to buy air-entraining cement (Type IA) and then add an air-entraining admixture. However, if the only cement available was Type IA and it did not give the needed air content, an addition of an air-entraining admixture would be necessary to achieve frost resistance.
Step 2. Add FA and CA until you produce a workable mixture having the proper slump.
Step 3. Record the weights of all materials in column 4 of Figure 3-1.
Step 4. Calculate the weights for both a 1-bag batch and 1 cubic yard. Record the results in columns 5 and 6.
Step 5. Calculate and record--
The cement factor in pounds per cubic yard as indicated in Figure 3-1 .
The percentage of FA by weight and by volume of the total aggregate.
The yield of concrete in cubic feet per bag.
Step 6. Determine and note the slump, air content, workability, and unit weight of the concrete.
Step 7. Find the most economical proportions, by making more trial batches, varying the percentage of FA.
3-14. In each batch, keep the W/C ratio, aggregate gradations, air content, and slump approximately the same. Table 3-5 summarizes the results of four such trial batches made in the laboratory. Figure 3-2 plots the percentage of FA used in these mixes against the cement factor. The minimum cement factor (538 pounds per cubic yard) occurs at a FA content of about 32 percent of total aggregate. Since the W/C ratio is 0.46 by weight and the unit weight of the concrete for an air content of 5 percent is about 144 pounds per cubic foot, the final quantities for the mix proportions per cubic yard are as follows:
Table 3-5. Results of laboratory trial mixes
Figure 3-2. Relationship between percentage of FA and cement content for a given W/C ratio and slump
|David L. Heiserman, Editor||
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Revised: June 06, 2015