Q.: On an average job, how often should slump and air content tests be made? How many test cylinders should be made?

A.: Specifications usually dictate the number of tests made. The ASTM specification for ready mixed concrete (ASTM C 94) says slump, air content, and temperature tests shall be made at the time of placement at the option of the inspector as often as is necessary for control checks. ASTM C 94 also says that these tests shall be made when specified and always when strength specimens are made.

Chapter 16 of Specifications for Structural Concrete (ACI 301-89) tells how often to make tests. Determine slump for each strength test and whenever consistency of concrete appears to vary. Determine air content of normal-weight concrete whenever a strength test is made.

ACI 301 also says to make at least one strength test for each 100 cubic yards, or fraction thereof, of each mixture design of concrete placed in any one day. When the total quantity of concrete with a given mixture design is less than 50 cubic yards, the strength tests may be waived by the engineer if, in his or her judgment, adequate evidence of satisfactory strength is provided.

Make three test specimens from each sample. Two are tested at 28 days for acceptance and one is tested at 7 days for information.