It's hard to imagine the concrete industry without thinking about testing. Concrete producers select aggregates, cement, fly ash, slag, and other admixtures on the basis of test results. Material samples are selected at quarries and batch plants for testing, and concrete test samples are taken at the plant and at the site. Still more tests are conducted after the concrete has hardened.
Amid all this testing activity, a great deal of time and attention is justifiably focused on proper testing procedure. However, variations in operator technique, cleanliness of apparatus, temperature, and sampling method, to name only a few key factors, can make our standard tests nonstandard.
Thousands of committee hours are spent each year hammering out and reviewing test methods. Training and certification programs for testing personnel have become essential activities for sponsoring agencies like the American Concrete Institute. Lab inspections and laboratory certification programs are on the increase, and the test equipment is becoming more sophisticated and more expensive. It's ironic that, with all the attention given to how we test (and even more attention is needed) the critical issue of why we test is discussed far less frequently. But why we test is at the heart of how we interpret test results.
Specifications and testing
Tests can be performed anywhere in the overall concrete-making and construction process. Tests can be used to verify the suitability of raw materials; the uniformity of concrete batching, mixing, and transport; or the acceptability of a product that has been delivered to the site. Beyond the truck chute, fresh concrete can be sampled anywhere that logistics and safety permit, and hardened concrete can be tested to capture the effects of finishing, curing, or environmental exposure. But because each of these testing schemes evaluates concrete at a different point in the overall process, we would fully expect different results from each.
Testing concrete samples taken at the truck chute, as shown here, provides baseline properties of the fresh concrete as delivered.
The specifier has the freedom to require concrete sampling at any stage in the process. Most of our standard testing, however, is based on evaluating the concrete as it is delivered to the site. The supposition is that if the concrete is satisfactory at the chute, and it is placed, consolidated, finished, and cured in accordance with the construction specifications, then the in-place concrete will be satisfactory. That is the essence of a so-called prescriptive concrete specification, which describes materials and construction means and methods rather than final in-place concrete properties. Therefore, the field tests (air content, slump, and making cylinder test samples) are most commonly done using concrete taken at the point of discharge from the concrete truck. We reserve the more difficult and time-consuming in-place tests for instances when the point-of-discharge tests suggest there may be a problem. But the success of this approach depends on at least three powerful assumptions.
First, we assume that the specified requirements for the concrete as delivered will lead to concrete that will develop the in-place properties that the owner and designer really care about. Note that in many cases the desired in-place properties might not be specified at all, and there may be no intention of measuring them.
Second, we assume that the specification has correctly identified the construction techniques and the conditions of temperature and moisture that will allow the concrete to develop the desired properties on schedule.
Finally, we complete the loop by assuming that the concrete will be placed, consolidated, finished, and cured as required in the specifications. In the recent past that was not an assumption—it was a fact, and many commercial construction projects had full-time resident inspectors who made sure that construction operations were conducted as specified. Today, however, full-time inspection of construction operations is the exception and not the rule. It is a testament to the concrete industry that this system works most of the time, and there is a lot more satisfactory concrete in place than there is unsatisfactory concrete in place. If this were not true, portland cement would not be in high demand, and there would not be a cement shortage.
Standard tests and conditions
Test methods are developed such that the results can be replicated by different technicians and different testing labs with a minimum of variation. Without standard tests it would be virtually impossible to compare results from different labs or to compare specimens from different technicians. So we fill slump cones in thirds by volume, and rod 25 times per layer in a spiral fashion using rods with hemispherical tips, and strike-off unit weight buckets with steel plates, and cure cylinders at 73° F and 95% to 100% relative humidity.
But note that standardization and repeatability are the goals here, not replication of the actual construction technique or jobsite environment. If, by standardizing the methods, we can lock in all of the variables of how the specimens are made, cured, and tested, then any variations in the test results will be primarily due to variations in the material itself.