High-strength concrete is widely used and designers are increasingly specifying strengths exceeding 10,000 psi. Because these concretes are used for critical structural applications, failure to reach the specified strength can cause costly delays while further testing is done. It's especially important to avoid low test results caused by incorrect testing procedures. PRELIMINARY TESTING Preliminary testing of high-strength concrete is often done in two phases: 1. Laboratory-made concrete is tested to develop mixture proportions that will produce the desired strength. 2. Plant-mixed concrete is made to ensure that large batches yield the same test results as small batches. TESTING MACHINE REQUIREMENTS Evaluate both load capacity and rigidity when choosing a testing machine. One manufacturer of testing machines recommends that the target maximum compressive strength should be about 80 percent of the machine capacity. FOLLOW ASTM STANDARDS Standards of the American Society for Testing and Materials (ASTM) should be followed when high-strength concrete is tested.