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Ready Mix Supplies

Make Sure You Get What You Pay For

All the materials in a mix will contribute to the characteristics of the concrete that arrives at your jobsite. Some qualities, such as workability and ease of finishing, are readily apparent as you place the concrete. But others, such as compressive strength and air content, can't be observed. That's why onsite testing of the delivered concrete is important. These tests will assure you that the material you ordered is the same material that arrives in the truck.

Slump (a measure of consistency), air content, unit weight, and compressive strength tests are the most common field tests for determining the quality of freshly mixed concrete. The following ASTM standards give procedures and acceptable time frames for performing these tests, which should be conducted by certified technicians:

• ASTM C 172: Standard Practice for Sampling Freshly Mixed Concrete
• ASTM C 143: Standard Test Method for Slump of Hydraulic Cement Concrete
• ASTM C 138: Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete
• ASTM C 231: Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method

The unit weight test will also tell whether you're getting the amount of concrete you paid for because it determines the yield of a sample of the ready mixed concrete as delivered, according to PCA. This is a simple calculation, but requires you to know the unit weight of all materials batched. The total weight information may be shown on the delivery ticket or can be provided by your supplier.

Based on the results of these field tests or external factors, such as long delivery times or hot weather conditions, it may be necessary to add air entraining, water reducing, set retarding, or other admixtures to the concrete prior to discharge. Let your ready mix supplier advise you in such circumstances and take responsibility for fine-tuning of the mix. Your supplier can't be held accountable for the quality of concrete that you alter yourself, such as by adding more water.

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