A new FHWA study reveals a safer way for testing the flexural strength of concrete.

Flexural strength is an important parameter in the design of concrete pavement because of its significant impact on the cracking behavior of the pavements. Typically, testing the flexural strength of concrete has involved using large specimens, each of which can weigh as much as 65 pounds (29 kilograms). The risk of injury when handling large, cumbersome specimens causes some State departments of transportation to avoid flexural strength testing as part of their construction quality assurance programs.

To address this problem, the Federal Highway Administration conducted a two-phase study. In the first phase, which concluded in 2015, FHWA researchers devised a solution by determining whether a smaller concrete beam would be a viable specimen size. In the second phase, completed in 2018, they determined the precision of the test using standard and smaller beams.

Why Test for Flexural Strength?

Flexural strength, also known as modulus of rupture (MOR), is an important parameter in the design of concrete pavements because of its significant impact on their cracking behavior.

For example, in an estimation of a typical jointed plain concrete slab, using the American Association of State Highway and Transportation Officials’ (AASHTO) Mechanistic-Empirical Pavement Design Guide with 95-percent reliability, when the MOR dropped from 690 to 600 pounds per square inch, psi, (4,760 to 4,100 kilopascals, kPa)—a 13-percent decrease—the estimated predicted cracking dramatically increased from 15 to 60 percent—a 300-percent increase in predicted cracking.

Testing concrete for flexural strength traditionally required big, heavy beams: 6 by 6 by 21 inches (15 by 15 by 53 centimeters, cm) and approximately 65 pounds, lb, (29 kilograms, kg) for concrete with a density of 150 lb per cubic foot (2,400 kg per cubic meter). For this reason, when testing for quality assurance, some State DOTs have replaced testing for flexural strength with compressive strength testing.

Compressive strength, however, may not capture potential issues during the quality assurance testing because different factors affect compressive versus flexural strength. For example, changes in the gradation of the aggregate in the specimen, its mineralogy, or its water-to-cement ratio during construction could lead to an overestimate of its flexural strength. In addition, and more important, concrete pavements primarily fail in bending (as can occur during testing for flexural strength) rather than in compression.

In 2013, to make the flexural strength testing of concrete safer for the testing personnel and to streamline quality assurance testing, FHWA staff at the Turner-Fairbank Highway Research Center (TFHRC) evaluated the use of smaller beams measuring 4 by 4 by 14 inches (10 by 10 by 36 cm) for the third-point load testing. The smaller beam weighs about a third of a standard beam, or approximately 19 lb (9 kg) for a concrete with a density of 150 lb per cubic foot (2,400 kg per cubic meter). This phase 1 research resulted in modification of the ASTM International (formerly the American Society for Testing and Materials) standards and those of AASHTO.

Phase 1: Feasibility Study

During phase 1 of the study, researchers prepared a total of 22 concrete mixtures with water-to-binder ratios varying from 0.37 to 0.47, cementitious content ranging from 521 to 643 lb per cubic yard (309 to 381 kg per cubic meter), coarse aggregate content ranging between 1,699 and 1,823 lb per cubic yard (1,008 to 1,082 kg per cubic meter), and fine aggregate content ranging from 1,089 to 1,581 lb per cubic yard (646 to 938 kg per cubic meter). The coarse aggregates included gravel, limestone, granite, and diabase, with nominal maximum sizes varying from 0.75 in to 1.5 inches (1.9 cm to 3.8 cm). For each mixture, FHWA tested six small beams and six standard beams (totaling 264 beams) in accordance with ASTM C78/C78M.

The results confirmed that the small beam was a viable specimen size, because there is a very good correlation between the flexural strength of the two beam sizes for the same aggregate nominal size and independent of the mixture proportions. Practitioners can use the relationships between the small and the standard specimen sizes presented in this article, or they can determine a relationship specific to the job mixture.

As a result of this study, ASTM and AASHTO modified the Standard Method of Test for Making and Curing Concrete Test Specimens in the Field (AASHTO T23 and ASTM C31/31M) and the Standard Method of Test for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading) (AASHTO T97 and ASTM C78/C78M) to accommodate the use of the small beams for flexural strength testing.

Phase 2: Determination Of the Test Precision

The next step for the implementation of the use of small beams was determining the precision of the MOR test using this specimen size. The precision of a test method is defined as the closeness of agreements between independent test results obtained according to a standard test method (a variability measure), while accuracy (or bias) measures the difference between a mean and a reference value. There are two types of precision: within laboratory (also called repeatability) and between laboratories (also called reproducibility).

Within-laboratory precision is of great importance to determine the validity of the test results obtained by any laboratory and to prevent a high acceptance probability of inadequate materials during the quality assurance process. Within-laboratory precision is also important to determine the number of beams to be tested in order to obtain an average test result, which can help to prevent either an excessive or an insufficient testing program by the State DOT’s contractor.

Between-laboratories precision, on the other hand, helps to determine whether the difference between the test results obtained by two different laboratories is within an acceptable range.

The TFHRC researchers established a collaborative effort with ASTM to determine the precision of the MOR using the small beam (ASTM interlaboratory study #1265). In addition, this FHWA/ASTM interlaboratory study included the determination of the precision of the MOR using a standard beam. The precision statement in the previous versions of the standard involved only a single brand and model of the testing machine, which could have resulted in nonrepresentative precision.

The 22 participating laboratories included: 3 Federal laboratories, 4 State DOT laboratories, 1 industry laboratory, 1 association laboratory, and 12 commercial laboratories. (One State DOT was counted as two laboratories for statistics purposes because they had two independent technicians using two independent machines to run the tests.) In this interlaboratory study, 16 different testing machines, in terms of brands, models, and loading capacities, were represented.

The interlaboratory researchers designed three mixtures with target flexural strengths of 450, 600, and 800 psi (3,100, 4,100, and 5,500 kPa) at 56 days. Each laboratory tested three replicates per size and per mixture, always using its same testing machine and its same American Concrete Institute (ACI)-certified technician. A total of 658 beams were prepared for this project. For each mixture, all the beams were cast from a single batch of concrete.

Testing Machine Modifications

To accommodate the small specimens, the commercial testing machines used for the study required modifications in terms of spans and distance between loading head and supports. The TFHRC researchers built parts and prepared tutorial videos on how to install these parts for 15 of the participating laboratories. For instructions on the equipment modification, contact Ahmad Ardani at Ahmad.Ardani@dot.gov.

Most of the modifications were simple and involved (a) drilling extra holes on lower and upper platens for positioning the loading support fixtures, and (b) installing 2-inch (50-millimeter)-thick spacers.

On the other hand, hand-operated testing machines with paper charts, required the most significant modifications of all machine types in this study. The modifications included installation of new modified blocks, a top mounting assembly for the support blocks, and recording charts.

Precision Statements

After carrying out an analysis of variance and Bartlett’s test of homogeneity of variances, with a 95-percent confidence level, the researchers calculated the combined single operator standard deviation as 37 psi (255 kPa). Thus, the MOR from two properly conducted determinations by the same operator on the same material is not expected to differ by more than 104 psi (717 kPa), regardless of the beam size and the MOR of the concrete.

On the other hand, the researchers found that the multilaboratory coefficient of variation (COV) was different for the two beam sizes.

The researchers believe that the high multilaboratory COV obtained for the small beams with 1,000 psi (6,900 kPa) MOR is a consequence of the inclusion of laboratories using the hand-operated testing machines with paper charts. These testing machines presented much higher single operator variability, as well as multilaboratory variability for all three mixtures. Some of the reasons include the following:

• Reading accuracy: These testing machines did not have an electronic readout device to indicate the maximum load. They used charts with a subdivision that represents 200 pounds (91 kg) of force, and the pen used by the chart recorders creates a line with a thickness that is almost equivalent to this subdivision. So, in the best-case scenario, considering an experienced technician and good vision, it is impossible to read more accurately than 200 pounds (91 kg) of force.
• Effect of size: The impact of the reading accuracy was greater for small beams, in comparison with the standard beams, because a difference of 200 pounds (91 kg) of force for a lower maximum load (as in smaller beams) is more significant than for a larger load (as in standard beams).
• Calibration: DOTs report that this machine is commonly transported to job sites, and from one laboratory to another, without any verification or calibration after being relocated, which can affect the testing results.

Updated Standards

The research found that third-point flexural strength testing with smaller beams (4 by 4 by 14 inches, 10 by 10 by 36 cm) is viable, and both AASHTO and ASTM standards have been modified, allowing their use. Hand-operated testing machines presented much higher testing variability than other machines when testing smaller beams, especially at higher MOR.

“The research we’ve completed has yielded valuable information that may make it possible for more State DOTs to test the flexural strength of concrete pavement using smaller concrete beams,” says Ahmad Ardani, concrete research program manager at FHWA’s TFHRC. “Doing so increases safety for the testing personnel and helps to streamline quality assurance testing.”

Jussara Tanesi, Ph.D., is a contracted concrete materials researcher and laboratory manager at FHWA’s TFHRC. She is a fellow of ACI and for the past 15 years has been actively involved in ASTM, ACI, and Transportation Research Board committees. She has authored or coauthored more than 50 technical papers, reports, and chapters of books in three different languages. Tanesi received a Ph.D. and B.S. in civil engineering from the State University of Campinas, Brazil, and an M.S. in concrete materials from the University of São Paulo, Brazil.

Haejin Kim, Ph.D., is SES Group & Associates’ concrete laboratory operation manager at TFHRC. Prior to his current position, he was research director at the National Ready Mixed Concrete Association (NRMCA). Kim holds a bachelor’s degree in civil engineering from Kyungnam University, Masan, South Korea, and a master’s and a Ph.D. degree in civil engineering from the University of Maryland, College Park. He has published several papers as a result of his research work at the University of Maryland College Park, Purdue, NRMCA, and TFHRC. Kim is a member of ASTM and ACI.

John Leavitt is a senior materials testing technician with SES Group & Associates working at TFHRC. He possesses several certifications, including NRMCA level III technologist, ACI field technician, and ACI laboratory technician 1. Prior to contracting to FHWA, Leavitt was a quality control technician for DuBrook Ready Mix Concrete. Leavitt, on occasion, proctors for ACI field technician practical testing. He has a bachelor of fine arts degree from Kutztown University of Pennsylvania.

For more information, contact Jussara Tanesi at 202–493–3485 or Jussara.Tanesi.CTR@dot.gov.