Analyses Reveal Viaduct Concrete Failure Not Consistent


An east coast government agency was concerned about the structural integrity of a viaduct on a main north-south highway that handled approximately 75,000 vehicles a day. The structure, which consisted of a 42-foot wide concrete deck of 40 spans and piers, was built in 1953 and was exhibiting signs of general degradation. The agency had hired a civil engineering construction firm to oversee repairs but had contacted RJ Lee Group to provide information on the current state of the concrete, failure analysis, and an estimation of service life. The prospect of dealing with a structure this large in multiple environments was intimidating but our experts took up the challenge and completed the project, generating a comprehensive report in just two months.

Sample Collection

As an industrial forensics laboratory and consulting firm, one of RJ Lee Group’s strengths is our ability to reach out to external partner firms to supplement our capabilities in providing a total solution. In this case, we were able to rely on a joint venture partner to go on site to perform a visual inspection of the viaduct including the deck, concrete piers and expansion joints. During this time, they took 38 core samples from three different sites – 18 from the concrete deck and 20 from the lateral face of the deck. These concrete and steel samples were sent to the RJ Lee Group laboratory for examination and characterization by our expert analysts. Mechanical tests performed on these samples revealed variability in compressive strengths which suggested internal defects in the concrete.

Sample Characterization

Our petrographers examined the concrete cores and determined that the air-void spacing was higher than the recommended value for frost durability in saturated conditions. Their analysis also revealed that the concrete was affected by alkali-silica reaction (ASR) which had been initiated many years previously and was now in a mature state and not likely to continue. They also noted that samples from parts of the viaduct which had been protected from water penetration by the pavement were not affected by ASR and that chloride contamination was variable from one pier to another.

Service Life Simulation

Our scientists observed very little signs of rebar corrosion in the sound concrete samples and so we collaborated with our partnering firm to conduct service life simulations to evaluate the potential for future corrosion. The service life simulation modeling involved various mechanisms such as moisture transport and time-dependent boundary conditions. Results obtained for each pier estimated the time when corrosion would begin based on the types of concrete covers. Including these mechanisms allowed us to make a reliable prediction of long-term service life to assist engineers in the selection of possible solutions for optimum maintenance and repair of concrete structures, as well as providing support for the use of surface protection systems – such as sealers – as an economical solution.

Conclusions

Analysis of the concrete cores determined that the viaduct deck and pier concrete was affected by the ASR, but that the damage was limited to the damp areas near the joints. Deterioration was most likely increased by freeze-thaw cycles in those areas not adequately protected against frost damage. Cracks found at the pier-deck junction were confined to a small section on each side of the deck and most likely a result of restrained movement. Results demonstrated the benefit of covers and protection systems for concrete exposed to harsh environments providing much needed data to the engineering firm tasked with generating and overseeing repairs.