Large-Scale Concrete Strength Test
Dr. Giorgio T. Proestos and doctoral student Dhanushka Palipana deployed the VIC-3D digital image correlation (DIC) system from Correlated Solutions in their work with large-scale structural testing of concrete beams in the Department of Civil, Construction, and Environmental Engineering in the College of Engineering at North Carolina State University. The researchers used six high-resolution cameras to capture images of 16 ft by 4 ft concrete beams under load. The NCSU team chose the VIC-3D system because of its full-field ability to measure high-resolution strains over the entire surface of a large test subject.
The project involved conducting a pilot series of three experiments with full-scale reinforced concrete models of elements used in civil infrastructure, including bridge pier caps and transfer girders found in high-rise buildings. In structural concrete, there is a size effect; therefore, it is important to test full-scale members. The concrete members tested were each 16 ft by 4 ft (5.3 m x 1.3 m), were 1 ft (305 mm) thick, and weighed approximately 9,600 lbs (4.8 tons). Some specimens reached peak loads exceeding 585 kips (2,600 kN). The main variable being examined in this pilot series was the shear-span-to-depth ratio (i.e., the distance between the applied load and the supports), which varied between 1.80 and 2.25. Specifically of interest was how the crack behavior changed because of the change in member loading arrangement. While it is known the angle of the cracks will change for varying shear-span-to-depth ratios, the detailed kinematic and constitutive response of the cracks remained unknown. For the first time, measurements of the kinematics of shear cracks in large-scale deep beams were made possible by the DIC system from Correlated Solutions.
The purpose of the tests was to investigate the response of shear critical deep beams. Shear failures in such members are brittle, occur without warning, and are therefore considered extremely dangerous. A better understanding of the failure of such members is important to improving the resilience of critical infrastructure. It should be noted that in large-scale structural testing, a series of three large-scale concrete tests is significant and often conducted as a part of larger multi-year research projects. The test articles were loaded with a hydraulic actuator and the members were tested to failure. The test articles and the experimental setup are shown in Figure 1 (above).
Throughout loading, the cracks were manually marked and measured on the non-DIC side of the specimen. Similarly, on this non-DIC side of the specimen, a 3D position-tracking Optotrak system was used to obtain data for comparison with the DIC measurements. The Optotrak data was used to verify and validate the new DIC system. The results of this verification and validation were successful.
Table 1 shows the DIC results from the pilot PQP series of tests. The principal tensile strains and the principal compressive strains at the peak load as measured by the VIC-3D system are shown. It is remarkable that in the DIC-measured principal tensile strain data, essentially every structural crack is visible and identifiable.
While Table 1 shows a summary of the DIC data at failure, the VIC-3D system captured data throughout loading and was, therefore, able to measure the progression of cracking and straining. To demonstrate this, Table 2 shows the principal tensile strains at eight load stages for the PQP3 experiment as measured by the DIC system. The progression of the flexural cracks and the formation of large diagonal shear cracks can be observed. Once the crack pattern is fully formed, the cracks then widen substantially and slip across their interfaces. This crack response is critical in understanding the failure mechanisms of deep beams and will be the focus of future journal articles based on these results, used in journal papers for comparison to nonlinear finite element model predictions, and other numerical simulations. More importantly, the data is detailed enough that it can be used to develop new constitutive and kinematic theories for the behavior of structural concrete subjected to shear.
Prior to this research, assumptions were made about how cracks behave throughout loading. Now, no speculation is needed since the measurements can be used to directly observe the kinematics and strain fields. This data will contribute significantly toward improving the structural safety of critical infrastructure. The PQP pilot series from Dr. Proestos and his team at NCSU has demonstrated the quality and value of the data collected using the digital image correlation method and the effectiveness of VIC-3D from Correlated Solutions in large-scale, full-field applications.
“While it is known the angle of the cracks will change for varying shear-span-to depth ratios, the detailed kinematic and constitutive response of the cracks remains unknown. For the first time, measurements of the kinematics of shear cracks in large-scale deep beams have been made possible by the DIC system from Correlated Solutions.”
“It is remarkable that in the DIC-measured principal tensile strain data, essentially every structural crack is visible and identifiable.”
“Prior to this research, assumptions were made of how cracks behave throughout loading. Now, no speculation is needed since the measurements can be used to directly observe the kinematics and strain fields. This data will be the first of its kind and will contribute significantly towards improving structural safety of critical infrastructure.”
Find out more about the potential of a Multi-System VIC-3D setup like the one used in this test by CLICKING HERE.