Every other year, the National Ready Mixed Concrete Association asks members to identify their major technical concerns. In its last survey, improper handling and curing of test cylinders headed the list of highest-rated problems.
Producers' concerns are justified. Although several studies show cylinder-strength losses of about 15% due to improper curing, proper jobsite curing is too often ignored, especially during the critical first 24 hours. A 600-psi strength loss for 4000-psi concrete can easily cause out-of-tolerance cylinder test results, making producers the first stop in the blame game.
A 1995 study by the New Mexico Ready Mix Concrete & Aggregates Association (NMRMCAA) was the latest to show that poor curing significantly reduces cylinder strengths. Cylinders cast in plastic molds, sealed with plastic lids and immersed in a tank filled with limewater immediately after casting exhibited the highest strengths. Some were stored in a large tank inside an air-conditioned building, and others were stored outside in a proprietary temperature-controlled box (see related article below). Cylinders exposed to direct sunlight had significantly lower strengths, even when the cylinders were kept moist with wet burlap. And surprisingly, cylinders in molds with the lids on but stored on the floor of an air-conditioned laboratory had significantly lower strengths than immersed cylinders.
Have field curing practices for cylinders improved as a result of this and other studies? Not always. But progress is being made.
Tight quality control helps
Frank Kozeliski, P.E., materials engineer and president, Gallup Sand & Gravel Co., Gallup, N.M., and Tony Montoya, senior engineering technician, Western Mobile, Albuquerque, were actively involved in the NMRMCAA studies. They both note that low cylinder-test results are practically nonexistent on state DOT work. They attribute this to the strict quality control procedures that that DOT personnel follow on these jobs--including immersion of test cylinders in water-filled tanks.
"When inspectors are careful to place the cylinders in either a homemade or manufactured curing box or tank, we just don't get low test results," says Kozeliski. "We often see good results when cylinders are stored in something as simple as a 55-gallon drum that's cut in half, filled with water and kept in the shade. Moisture retention is important because of New Mexico's very dry climate, but the NMRMCAA test results clearly showed that without temperature control, low test results are likely, even for continuously moist-cured cylinders."
Montoya echoes these thoughts. He notes an absence of low breaks for Western Mobile concrete used on a recent freeway project in Albuquerque. "We didn't raise our target strength to a higher level to achieve this," says Montoya. "We're getting the results because state inspectors immerse the cylinders in water in the proprietary temperature-controlled tank right after they're made. The cylinders are then transported to the district laboratory the next day."
The New Mexico DOT is reviewing its concrete specification and will soon require immersion curing of freshly made cylinders, a practice adopted by Bernalillo County officials.
Double-checking the lab
But what can be done on commercial-industrial jobs where testing lab technicians may not be as conscientious? Kozeliski says he first asks the lab manager how many field-cure cylinder boxes they have on the job. If the answer is none, he suggests the lab supply them and explains why they're needed.
"When I see people making tests, I also ask to see their certification card," says Kozeliski. To be in compliance with ASTM C 94, Standard Specification for Ready-Mixed Concrete, concrete tests must be made by a certified ACI Concrete Field Testing Technician, Grade I or equivalent. "Without the certification card," says Kozeliski, "the tests aren't valid, and I tell the testing laboratory managers they need to get a qualified technician on the job."
Western Mobile makes its own 4x8-inch field cylinders on many jobs. ASTM C 31, Standard Practice for Making and Curing Concrete Test Specimens in the Field, requires 6x12-inch cylinders, but Montoya uses the smaller size as a check on test lab results because it permits the use of less-costly field-curing methods.
"We can put four of these cylinders in a 5-gallon plastic pail, fill it with water, insert a high-low thermometer and store the pail in the shade," says Montoya. "In the summer, this usually keeps the cylinder temperature within the 60 to 80 degrees F range required for early curing. During cold weather, we put a lid on the pail and bury it in sand. Heat of hydration keeps the temperature within the required range." Gallup Sand & Gravel also follows this cold-weather procedure.
When field technicians for a test lab have used improper initial curing methods for cylinders and breaks are low, Montoya uses his test results to show that strengths were adequate. "And when we see bad testing practices," he says, "we document them in writing and cite our own results as evidence that low breaks were caused by the bad practices we observed."
Lab accreditation and curing tables
In Georgia, test cylinder field curing has improved due to a laboratory accreditation program run by a Concrete Advisory Board. About 95% of the state's test labs participate in the accreditation effort, which began in 1991 and is patterned after a similar program in Florida.
Roy Keck, president, Georgia Concrete & Products Association, says accredited labs must include the field technician's certification number on all cylinder test reports. When labs are audited, they may be denied accreditation if tests weren't made by an ACI-certified technician. At a preconstruction jobsite meeting, producers emphasize the need for certified field technicians to conduct all fresh concrete tests.
Georgia laboratories usually rely on contractors to provide curing boxes or curing tables--2x3-foot sheets of plywood with 2x2-inch legs and draped with wet burlap. Stan Jaglinski, director of technical services for Atlanta's RMC Allied Readymix Inc., says the tables were first used 20 years ago to improve initial curing of field-cast cylinders. Prior to that time, plastic sheeting was used to cover wet burlap placed over the cylinders. "If you wrap cylinders in wet burlap and cover them with plastic, you'll fry the concrete," says Jaglinski. "The seven-day strengths for hot concrete may look all right, but there's very little strength gain after seven days, especially for high-strength concretes." The wet burlap drapes cool the cylinders and also maintain a moist environment, improving the strength.
For heavy commercial work, most Georgia producers have their own quality control personnel visit the jobsite and observe the cylinder curing methods. If correct methods aren't being used, the QC technician supplies burlap tables and the producer points out the lab's field testing deficiencies at the next jobsite meeting.
The need to educate engineers and architects
"One problem in our area," says Larry Asel, an engineer for CONCO, Springfield, Mo., "is that some engineers and architects still don't understand the difference between lab-cured cylinders used for acceptance testing and field-cured cylinders used to check the adequacy of curing and protection of concrete in a structure."
When he sees poor initial cylinder curing on jobs with tight strength specifications, Asel urges the engineer or architect to require the test lab or contractor to supply horse-watering tanks so cylinders can be immersed immediately. When this is done, low-break problems usually disappear. But some engineers and architects refuse to require the tanks. They reason that if the structure is in the sun, the cylinders also ought to be in the sun so they're more representative of the in-plac