All concrete testing laboratories produce essentially the same compressive strength test reports. So, how can a laboratory manager create a distinctive product while boosting profitability?
Concrete exhibits loading-rate sensitivity relative to compressive strength,1,2 so ASTM C39, “Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens,”3 limits loading rate to 0.2 to 0.3MPa/sec (28 to 42 psi/sec). This helps ensure consistency within and among laboratories.
Almost 70% of the testing machines now in service are manually operated. The operator is therefore required to manually adjust a valve to achieve a loading rate within specification. Unfortunately, these adjustments can be inaccurate, particularly because only about half of the concrete testing machines now in service have any provision to indicate load rate. A fifth of testing machines now in service do have digital indicators that provide loading rates, but the attained rates aren’t verifiable after the tests.
A Step in the Right Direction
To address the shortcoming of nonverifiable loading rates, manually operated machines are now being offered that calculate and report the average loading rate according to ASTM C39 requirements and can generate load and stress versus time curves to verify that a test was performed according to specification. They also offer digital indicators that provide a live indication of loading rate. These systems don’t, however, eliminate the possibility that an operator could perform tests at rates exceeding ASTM C39 limits.
There is clearly a need for an automatic concrete testing system that can control loading rate. Control systems used on conventional universal testing machines, however, aren’t appropriate for concrete testing applications. Most concrete testing machines in operation are hydraulically actuated and operate at oil pressures as high as 68.9 MPa (10,000 psi). In contrast, conventional servo-hydraulic testing systems operate at maximum pressures of about 31 MPa (4500 psi). These systems therefore have large-and very expensive-actuators, and their high cost precludes them from widespread use in concrete testing.
Over the past six years, ADMET, Inc. has offered a low-cost, reliable, automatic concrete testing system that addresses these issues. As shown in Fig. 1, the MegaForce II automatic testing system works with compression machines that operate to 68.9 MPa (10,000 psi), prevents the operator from overriding the testing process, and provides verification of loading rates-all for 50 to 75% less cost than for a comparable servo-controlled testing machine. The automatic testing system can be installed on new machines or retrofitted to existing machines, generating further cost savings.
1. Carino, N.J.; Guthrie, W.F.; Lagergren, E.S.; and Mullings, G.M. “Effects of Testing Variables on the Strength of High-Strength (90 MPa) Concrete Cylinders,” High-Performance Concrete: Proceedings, ACI International Conference, Singapore, 1994 (SP-149), V.M. Malhotra, ed., American Concrete Institute, Farmington Hills, MI, 1994, pp. 589-632.
2. Han, N., and Walraven, J.C., “Properties of High-Strength Concrete Subjected to Uniaxial Loading,” High-Performance Concrete: Proceedings, ACI International Conference, Singapore, 1994 (SP-149), V.M. Malhotra, ed., American Concrete Institute, Farmington Hills, MI, 1994, pp. 269-288.
3. ASTM C39/C39M-05e1, “Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens,” ASTM International, West Conshohocken, PA, 2005, 7 pp.
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