Above: During visual inspection, the investigator notes and documents freeze/thaw damage on a concrete pier cap. Right: The core sample from a bridge deck shows an asphalt layer over the concrete base. The core reveals longitudinal cracking that would not be apparent on surface examination. Photos: CTL Group
Material Sampling And Testing
A thorough concrete condition evaluation usually will analyze at least some concrete cores and powder samples. It may also require sampling and testing steel reinforcement, prestressing tendons, other embedded metals, sheathing, grout, asphalt, or other component materials. Some typical techniques and applications include field observation, laboratory testing of materials, and petrographic analysis.
Field observation is used to verify de-laminations and other conditions of structural elements in situ. For example, an investigator might compare the depth of a core hole to the length of the core extracted to determine if any material was lost, or check to see whether cracks in a core are reflected in the core hole. One might also need to expose underlying materials or layers to verify conditions detected or suggested by nondestructive test methods.
Extracted samples can be laboratory tested to determine actual material properties and conditions such as strength and modulus, permeability, potential alkali-silica reactivity (ASR), chloride content, and chloride profile.
Petrographic analysis is the visual and microscopic inspection of prepared concrete sections. It is used to evaluate concrete composition and a range of conditions such as its air content and air void distribution, the presence or extent of freeze/thaw damage, the depth of carbonation, cement-paste components and condition, aggregate composition, the presence of ASR gel or ASR damage, and other kinds of damage. A skilled petrographer's analysis often can shed light on the likely causes of damage and deterioration.
Such comprehensive condition evaluations give bridge owners the information needed to maximize bridge service life, minimize disruption of traffic flow, and optimize the allocation of maintenance and rehabilitation resources. By requiring thorough evaluations of concrete bridge structures, public works officials can more successfully develop realistic maintenance, repair/rehabilitation, and replacement plans.
— Christopher A. Ligozio is a senior structural engineer with CTL Group in Skokie, Ill.