The proportions at which you arrive in determining mixtures will vary somewhat depending upon which method you use. The variation is the result of the empirical nature of the methods and does not necessarily imply that one method is better than another. You start each method by assuming certain needs or requirements and then proceed to determine the other variables. Since the methods begin differently and use different procedures, the final proportions vary slightly. This is to be expected and points out further the necessity of trial mixtures in determining the final mixture proportions.

The initial mix design assumes that the aggregates are saturated, surface dry (SSD); that is, neither the fine aggregates nor the coarse aggregates have any free water on the surface that would be available as mixing water. This is a laboratory condition and seldom occurs in the field. The actual amount of water on the sand and gravel can be determined only from the material at the mixing site. Furthermore, the moisture content of the aggregates will change over a short period of time; therefore, their condition must be monitored and appropriate adjustments made as required. Coarse aggregates are free draining and rarely hold more than 2 percent (by weight) of free surface moisture (FSM) even after heavy rains. A good field test for estimating the FSM on fine aggregates is the squeeze test described below.

2. Squeeze a sample of the sand in your hand. Then open your hand and observe the sample. The amount of FSM can be estimated using the following criteria:

a. Damp sand (0- to 2-percent FSM). The sample will tend to fall apart (fig. 17-1). The damper the sand, the more it tends to cling together.

b. Wet sand (2- to 4-percent FSM). The sample clings together without excess water (fig. 17-2).

c. Very wet sand (5- to 8-percent FSM). The sand will ball and glisten or sparkle with water (fig. 17-3). The hand will have moisture on it and may even drip.

The procedure for adjusting the mixing water caused by free surface moisture is as follows:

. Determine the approximate FSM of the fine aggregate by the squeeze test.

. Estimate the FSM of the coarse aggregate by observation. Usually, 2-percent FSM is the maximum amount gravel will hold without actually dripping.

. Multiply the percentages of FSM on the aggregates by their respective weights per cubic yard.

. Divide the total weight of the FSM by 8.33 pounds per gallon to determine the number of gallons of water. Subtract those gallons from the mixing water requirements in the original mix design.

. If you are batching your concrete mix by weight, you need to account for the weight contributed by the FSM by increasing the total weights of the aggregates per cubic yard by the weights of the FSM.