Compressive strength is one of the most important measurements in manufacturing concrete. As a producer, I need to know whether this measurement is accurate. How can I be sure my concrete compressive strength results are correct?

Hundreds of thousands of concrete cylinders are tested daily in the United States. Most tests are performed on manually operated concrete compression testing machines which require the operator to continuosly adjust a valve during a test so that the loading rate remains within specification.

ASTM C39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, specifies that the loading rate fall within 0.14 to 0.34 MPa/sec (20 to 50 psi/sec). On average, the compressive strength of cylinders tested at the high load rate limit will be 3% greater than cylinders tested at the low load rate limit. Many testing laboratories are paid by the number of cylinders tested, creating an incentive to test as fast as possible.

A 20% increase in compressive strength of high-strength concrete is possible when loading rates exceed ASTM C39 limits. About half of concrete testing machines are equipped with an analog dial guage or a digital panel meter that has no provision to indicate load rate. An approximation of loading rate can be determined on these machines by using a stop watch and measuring the change in load over 15-, 30-, and 60-second periods. This method of measuring load rate often requiring two people is archaic and is not verifiable.

About 20% of machines used today employ a digital load rate indicator. These indicate load rate but cannot provide verification after the test is complete. More and more manually operated machines are being equipped with state-of-the-art digital indicators that provide live load rate. They also calculate and report the average load rate according to ASTM C39 requirements and can generate load/stress versus time curves.

Both the average load rate calculation and load/stress versus time curves provide verification that a test was performed according to specification. However, none of these solutions eliminates operator loading rate errors on manually operated testing machines capable of loading rates much greater than 0.34 MPa/sec (50 psi/sec.). This, coupled with the incentives to test each cylinder as fast as possible, creates the need for an automatic concrete testing system.

Also, 95% of all concrete testing machines in operation are hydraulically actuated and require oil pressures to 68.9 MPa (10,000 psi). The maximum pressure that conventional servo-hydraulic testing systems operate is 31 MPa (4500 psi). Their inability to operate at oil pressures to 68.9 MPa (10,000 psi) and their high cost preclude them from widespread use in concrete testing.

In the past two to three years, a reliable automatic loading valve for concrete compression machines that operate to 68.9 MPa (10,000 psi) has become available. The simple design results in machine costs 50% to 75% less than comparable servo- controlled testing machines. The new automatic testing system eliminates the operator from controlling the load rate during the testing process and verifies loading rates.

Richard Gedney, founder and president of ADMET, a manufacturer of testing systems, provided the answer. Visit www.admet.com.