proportions, or water content. Table 3-13 gives recommended slump ranges for various types of construction.

COMPRESSIVE TEST

The compressive strength of concrete is the ability to resist a CRUSHING force is, as previously explained, controlled by the water-cement ratio. However, the theoretical compressive strength related to a particular water-cement ratio will be attained only if the actual amount of water added is carefully regulated according to the considerations previously mentioned. Samples cast from the mix being used must be cured and tested to determine what compressive strength was actually attained.

The first step is to obtain a sample of the concrete. The sample should consist of not less than 1 cubic foot when it is to be used for strength tests. Smaller samples may be permitted for routine air content and slump tests.

The procedures used in sampling should include the use of every precaution that will assist in obtaining samples that will be representative of the true nature and condition of the concrete sample as follows:

1. Sampling from stationary mixers except paving mixers. The sample must be obtained by passing a receptacle completely through the discharge stream of the mixer at about the middle of the batch or by diverting the stream completely so that it discharges into a container. Care must be taken not to restrict the flow from the mixer in such a manner as to cause the concrete to segregate. These requirements apply to both tilting and nontilting mixers.

2. Sampling from paving mixers. The contents of the paving mixer must be discharged, and the sample must be collected from at least five different portions of the pile.

3. Sampling from revolving drum truck mixers or agitators. The sample must be taken at three or more regular intervals throughout the discharge of the entire batch, except that samples must not be taken at the beginning or end of the discharge. Sampling must be done by repeatedly passing a receptacle through the entire discharge stream or by diverting the stream completely so that it discharges into a container. The rate of discharge of the batch must be regulated by the rate of revolution of the drum, and not by the size of the gate opening.

The sample must be transported to the place where test specimens are to be molded or where the test is to be made and must be remixed with a shovel at the minimum amount to ensure uniformity. The sample must be protected from sunlight and wind during the period between taking and using which must not exceed 15 minutes.

Tests are made on 6- by 12-inch cylinders, cast in cylindrical molds. For the final test, a cylinder is cured for 28 days; however, the PROBABLE 28-day strength that a mix will attain can be estimated by determining the 7-day strength (which actually runs about 2/3 of the 28-day strength). Therefore, one or more cylinders are tested after 7 days of curing.

Test cylinders are cast in either metal or heavy cardboard molds. For filling, a mold is placed on a metal BASE PLATE. To avoid loss of the mix water, you can seal the bottom of the mold to the base plate with paraffin. A cardboard mold is expendable; that is, for stripping it from the test cylinder, it is simply torn off. A metal mold is hinged so that it can be stripped by opening. Before the mold is filled, the inside surface and base are lightly oiled to prevent the concrete from BONDING (adhering) to the mold and plate.

The test specimens should be formed by placing the concrete in the mold in three layers of approximately equal volume. In placing each scoopful of concrete, the scoop must be moved around the top edge of the mold, as the concrete slides from it to ensure a symmetrical distribution of the concrete within the mold. The concrete must be further distributed by a circular motion of the tamping rod. Each layer must be rodded with 25 strokes of a 5/8-inch-round rod, approximately 24 inches in length and tapered for a distance of 1 inch to a spherically shaped end having a radius of approximately one-fourth inch. The strokes must be distributed uniformly over the cross section of the mold and must penetrate into the underlying layer. The bottom layer must be rodded throughout its depth. Where voids are left by the tamping rod, the sides of the mold must be tapped to close the voids. After the top layer has been rodded, the surface of the concrete must be struck off with a trowel and covered with a glass or metal plate to prevent evaporation.

After about 24 hours of hardening, the mold is stripped off and the cylinder is immersed in either water, moist sand, moist sawdust, or moist earth for curing. At the expiration of the curing period (7 or 28