Testing concrete: thinking beyond the procedures.

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

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.

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 fulltime 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, port-land 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[degrees]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.

For example, the strength of cement is evaluated by testing mortar cubes in compression, using fixed proportions of sand, cement, and water, and by standardizing the sand source. The resulting mortar is not a replica of the mortar component in an actual job mix, but it does indicate the influence of the cement on the strength of the mortar.

Or consider the typical hand-rodding of a standard 6-inch cylinder compared to the energy imparted to the in-place concrete by a two-inch internal vibrator at 8000 vibrations per minute. The two consolidation methods do not match, nor are they intended to. The hand-rodding protocol just gives us a repeatable field technique, presumably about the same on all sites and among all properly trained technicians.

Note also that these standard techniques and conditions are not necessarily intended to produce "optimum" results, but rather simply repeatable results. Any variation between samples in temperature or moisture conditions when the test sample is curing, or speeding up or slowing down the loading rate during testing, can increase or decrease the measured sample strength, contributing to wide scatter in the results. That also is why innovative, timesaving, or even easier variations of test methods (although these variations may make perfect sense to an individual testing technician), complicate the ability to judge the results in comparison with standardized tests. (Those who have a better idea of how to perform a test are encouraged to propose it to groups like ASTM so that, if the improvement checks out, a new standard can be adopted.)

Standard tests, at the truck

There is tremendous benefit from conducting a standard test on concrete sampled at the point of discharge from the concrete truck. At that point the responsibility for concrete properties is clearly in the hands of the concrete producer, since no one else has yet had the opportunity to perform a "field expedient modification" to the mix (such as adding water). No matter what properties develop at the point of placement, or later in the life of the structure, standardized test results at the point of discharge provide a firm baseline. If subsequent point-of-placement tests (fresh or hardened) point to a problem, then point-of-discharge tests at least provide a clear starting point for investigating the difficulty. Without such data it can be much harder to determine if an in-place problem was caused by materials, placement, consolidation, finishing, temperature, or curing.

But, given that many factors can and will influence the in-place properties of the concrete, we have to recognize that the point-of-discharge properties are not necessarily the same as the in-place properties. And we also have to recognize that knowing one does not always enable us to know the other. For example, the air content of the concrete often drops between the truck chute and its final, consolidated location, regardless of the placing method; air increases are also common. In-place compressive strength also can be higher or lower than the standard lab-cured cylinder made from concrete sampled at the chute, depending on time and temperature of the in-place concrete.

Nonstandard tests, at placement

One common reason for sampling concrete at the point of placement rather than at the truck chute is concern over the effects of placement and handling on air content, and resulting changes in strength. But getting such a sample often involves a nonstandard method. For example, if you want to determine the air content of fresh concrete at the point of placement, it would make sense to grab the sample after the concrete has been vibrated. But how do you dig a Consolidated sample out of the deck in a way that does not influence the results?

Or if you want to sample from the pump hose, do you wait until the concrete hits the deck, or do you make a "clean catch" as it exits the line? This is not a trivial issue, since it is well known that slowing down the pump to make it easier to grab a sample will usually make air loss more severe.

Then once you get your "in-place" air content, what do you do with the result? It is certainly not appropriate to expect the same air content in place as at the truck chute. Pumping, chuting, conveying, dropping from a bucket, vibrating, vibratory screeding, and finishing all tend to knock out the large bubbles. Therefore, it might be more helpful to take into account the reduction in average bubble size, which often results in a reduction in the total air content but also results in a concrete that will still provide frost resistance.

Similar problems are associated with interpreting the results of core tests where the difficulty of making an apples-to-apples comparison of core strength to cylinder strength is a continuing topic of debate. As witness to that, a current provision in AC1318 says that in-place concrete with an average core strength that is at least 85% of the specified compressive strength is acceptable.

Using the information

Regardless of the reason for conducting the tests, or the point in the process where the concrete was sampled, or the reality that standard test conditions are not intended to simulate actual conditions, the test results are far more valuable when used to their full potential. Merely checking to see if cylinder break strengths or air contents exceed the specified values is only part of the story.

The data become much more useful when the results are compiled into a cumulative performance record that enables all parties to keep track of trends. Did strength change when the cement brand changed? Are 28-day lab results increasing now that the hot weather is past and the cylinders are cooler for that first day on the jobsite? Are one-or three-day test results decreasing for the same reason? Was there any change in air content when the fly ash source changed, or when you switched air entraining admixtures?

When monitored continuously, test data are your early warning system for monitoring the construction process. That information allows you to spot trends and make changes before the concrete goes out of compliance with the specifications, or exceeds the specification limits to the point that the mix should be adjusted.

--Kenneth C. Hover, Ph.D., P.E., is a structural/materials engineer and professor of structural engineering at Cornell University, Ithaca, N.Y., and a popular speaker at Hanley Wood's World of Concrete. To learn more about this year's program, visit www.worldofconcrete.com and click on Seminars-at-a-Glance.