On some jobs, concrete test cylinders “don’t get no respect.”

Sometimes we forget to let the testing company know we are pouring. Sometimes between the truck chute, the pump, and stacks of materials there isn’t much space for the testing technician to work. Sometimes the technician’s work area isn’t level, and sometimes it is just plain unsafe. Sometimes the technician has to carry the freshly cast cylinders a good distance to store them for the first 24 hours onsite, and we rarely use a curing box to physically protect the cylinders and maintain the specified air temperature. We argue that the test cylinders are not a fair representation of the actual concrete in the structure: Differences can include water and air adjustments, consolidation, concrete and air temperature, and moisture control. And we like to complain that the cylinders are not handled gently enough between the site and the lab.

Although many of these concerns are justifiable, and while properly making and handling cylinders is not easy, we have to face the facts: By the time the cylinders are broken, the dust settles, and the results printed and distributed to the owner, designers, contractor, and concrete producer, those test results generally are considered to legally indicate the actual strength of the concrete. Test results are the stand-ins for actual concrete performance, especially in the early stages of the project before the concrete in place has had an opportunity to speak for itself. It is during these early days in the life of the concrete test results are accepted as the primary indicators of concrete quality, and in these same early days, the owner is making the decision to pay or not pay, or to release or withhold the retainer. When the cash flow stops, the test cylinders suddenly get a lot of respect and attention!

  • The safer and more comfortable the technician, the more he or she can concentrate on your tests.

    Credit: Portland Cement Association

    The safer and more comfortable the technician, the more he or she can concentrate on your tests.

The importance of tests

As an analogy, consider the task of hiring someone fresh out of school or a training program. If that job candidate has no actual experience to date, you might place a lot of importance on things like grades or standardized test scores (even though we might question the relationship between test scores and real-world, productive capability). When evaluating a job applicant with several years of relevant professional experience, however, their actual performance counts far more than standardized test scores.

Similarly with concrete, until the structure has been in service for awhile and experienced a few seasons of freezing or thawing, or been loaded to a significant fraction of its design load, the actual long-term performance of the concrete is unknown—we need test scores such as air content, unit weight, and compressive strength to give us confidence. Once that same structure has performed its desired function, carried its intended load, and survived its expected environment for awhile, those test scores are no longer looked upon as the primary evidence of acceptability.

Of course, we can always fall back on the ACI 318 Building Code provision that core tests can be authorized “if the likelihood of low-strength concrete is confirmed,” and this often gets us out of trouble, especially given the acceptance criteria that “the average of three cores is equal to at least 85% of f´c and if no single core is less than 75% of f´c.” But even if the cores turn out OK, how much time and money was lost in the process? It costs a lot more to extract a core than to make a cylinder—wet-coring is messy, and all of this takes time to organize, drill, cure, test, report, and then wait for the green light to get back on schedule. Wouldn’t it pay to encourage reliable compression tests in the first place?

Accounting for the technician

Let’s start by planning a convenient work zone for the test technician. Everybody wins when it’s easier for that person to properly sample concrete, perform air and slump tests, make cylinders, and store them in a safe, temperature-controlled environment. Uneven, unlevel surfaces are miserable for slump and air tests, and bad conditions usually increase the apparent slump. The same bad ground makes it harder to consolidate a cylinder. If certification programs wanted to test applicants under real-life conditions, they would place the candidate between the truck and pump, on wet rocky ground, balancing on a nasty piece of plywood on the edge of the excavation.

Of course the actual conduct of the tests is in the hands of the testing technician, which is why most of our standard specifications require certification. Top-grade testing companies do a great job of making sure their people are well trained and certified, but it does not hurt the contractor to reinforce this need and even to verify certification.

  • According to the commonly required ASTM C172, Standard Practice for Sampling Fresh Concrete, testing starts by collecting the sample from the truck chute.

    Credit: Portland Cement Association

    According to the commonly required ASTM C172, Standard Practice for Sampling Fresh Concrete, testing starts by collecting the sample from the truck chute.

With so much going on during a major concrete placing operation, it is easy to overlook the post-pour fate of concrete test cylinders, which are to remain onsite for up to 48 hours after casting, and then transported to the testing lab. If those cylinders were made near the point of concrete discharge, it is likely they will be in the way after the trucks and pump depart. So they often are moved to temporary storage, and if this happens a few hours after casting, the concrete might be at its most fragile. It is difficult to lift and transport a concrete cylinder to minimize damage. Plastic caps are not only handy for reducing drying of the top surface, but they also reinforce the mold so it is stays circular during handling.

But even when the cylinders are gently moved to a safer place, ASTM C31-09, “Standard Test Method for Making and Curing Concrete Test Specimens in the Field,” requires “Immediately after molding and finishing, the specimens shall be stored for a period up to 48 hours in a temperature range from 60° F and 80° F [16° C and 27° C] and in an environment preventing moisture loss from the specimens. For concrete mixes with a specified strength of 6000 psi [40 MPa] or greater, the initial curing temperature shall be between 68° F and 78° F [20° C and 26° C].” Although there are a few jobsites in which the air temperature will not dip below 60° F nor rise above 80° F for a couple of days after casting (Waikiki in January comes to mind, but a field trip is required for verification), such limited temperature swings cannot generally be relied upon. Curing boxes therefore are required most of the time, winter and summer.

  • A curing box equipped with a heater and thermostat is useful whenever ambient air temperature will drop below the minimum temperature specified in ASTM C31, (68° F for fc ¥ 6000 psi, 60° F otherwise).

    Credit: Portland Cement Association

    A curing box equipped with a heater and thermostat is useful whenever ambient air temperature will drop below the minimum temperature specified in ASTM C31, (68° F for fc ¥ 6000 psi, 60° F otherwise).

Using curing boxes

For some reason, few in the design-build-supply-test chain like to provide or take responsibility for curing boxes. Part of the problem might be confusion between standard-cured and field-cured cylinders. We have been talking about initial standard curing conditions in the field which are followed by standard lab-curing conditions until the specimen is tested. The purpose of standard curing is to control thermal and moisture conditions and to isolate the inherent properties of the concrete as-delivered from the variable conditions of the jobsite. Sometimes we want to get a handle on the effects of the actual site environment on concrete properties, so we intentionally expose test cylinders to field conditions. However, this doesn’t always work as well as it seems like it ought to, owing to the fact that test cylinders heat up and cool down toward ambient air temperature far faster than the actual structure they are supposed to represent. Cylinder specimens are far more likely to cook in the summer and freeze in the winter than the structure being tested. So unless field-cured cylinders are specified or required by the contractor, the initial curing temperature limits pertain, and a cure box is the best way to stay in compliance.

  • With this highly effective curing box used by the Washington DOT, the cooler is partially filled with water to increase thermal mass and slow temperature changes.

    Credit: Kenneth C. Hover

    With this highly effective curing box used by the Washington DOT, the cooler is partially filled with water to increase thermal mass and slow temperature changes.

Another point of confusion is the weird effect of temperature on concrete strength gain. Although it is true that higher concrete temperature accelerates hydration of cement, faster hydration can lead to poorer quality of hydration products and a reduction in 28-day strength. High temperature curing generally increases strength at an age of up to about three days, but decreases 28-day strength. It is not unusual for reported concrete strengths to drop in the summer, due largely to hot concrete cylinders baking in the sun. I have observed a surface temperature of 124° F for 6x12 concrete cylinders in black plastic molds after a few hours in the afternoon sun in June in upstate New York. The concrete was an ordinary sidewalk mix, and 28 days later no one would have thought to lay the blame for low strength on whoever chose to place the specimens at the base of a west-facing stone wall (an unintentional solar oven). On an earlier project in the same location, I monitored a 4x8 cylinder that froze solid overnight while the well-protected structure stayed toasty warm. Given a curing box helps achieve specified 28-day compressive strength in both summer and winter, it is just silly to not make one a standing part of concrete placing operations.

It makes great sense, therefore, to include testing procedures and logistics as part of your prepour conferences (see 10 Things on Testing to Cover in the Prepour Conference). By increasing the chances of getting acceptable concrete strengths, the retainer you save may be your own.