As you are aware from your study of chapter 7, WORKABILITY is the relative ease or difficult y of placing and consolidating concrete. When placed, all concrete should be as stiff as possible, yet maintain a homogeneous, voidless mass. Too much stiffness, however, makes it too difficult or impossible to work the concrete into the forms and around reinforcing steel. On the other hand, too fluid a mixture is also detrimental. The mea-sure of the workability or consistency of concrete is its slump, which is a design consideration that is inversely proportional to the stiffness of the mix. As shown in table 15-4, the recommended values for slump vary for different types of construction. To measure slump, either during the preparation of concrete trial batches or as a quality control check during construction, testers perform slump tests. The procedures for performing slump tests will be explained later in this chapter.

In the design of concrete structures, the design engineer specifies given strengths that the final concrete products must be capable of attaining. When trial batches are prepared during mix design or as a quality control measure to ensure that concrete mixed or delivered in the field satisfies those specified strengths, the following tests are performed.

COMPRESSION TEST. Compression tests are conducted to determine the compressive strength of concrete (or its ability to resist a crushing force). In this test, a standard test load is applied parallel to the longitudinal axis of a premolded and properly cured concrete cylinder of a standard size. When the testis properly con-ducted, a maximum load is obtained at the point at which the cylinder ruptures. With this maximum load, the compressive strength, measured in pounds per square inch (psi), can be easily calculated. Although the test procedures will be covered at the EA2 level, the procedures used to prepare the cylinders for testing will be discussed later in this chapter.

FLEXURAL STRENGTH TEST. The flexural strength (modulus of rupture) test determines the flexural strength of concrete (or its ability to resist a breaking force). In this test, a standard

to the test load is applied perpendicular longitudinal axis of a standard size, premolded, and-properly cured concrete beam. From this test, the flexural strength, expressed in terms of modulus of rupture and given in psi, can be readily calculated. As with the compression test, only the procedures to prepare the test beams correctly will be discussed in this TRAMAN.

The slump test is performed on newly mixed concrete. To perform the test, you need a slump cone and a tamping rod. The slump cone (fig. 15-40) should be made of galvanized steel, 12 in. in height, with a base opening 8 in. in diameter and the top opening 4 in. in diameter. Both the top and bottom openings are perpendicular to the vertical axis of the cone. The tamping rod is a straight, steel rod that is 5/8 in. in diameter and approximately 24 in. in length. One end of the rod is rounded to a diameter of 5/8 in. (Do not substitute a piece of rebar.)