Consultant's Corner: Evaluation of Concrete Structures
By Stephen H. Lucy, P.E.
There’s an old adage that says only two things in life are guaranteed - death and taxes. Well, structural engineers can make one more guarantee - concrete will crack. While concrete is a durable building material, it does have its weaknesses and will deteriorate over time if not properly maintained. However, even with a good maintenance program, many concrete structures will eventually deteriorate to the point that a thorough evaluation must be performed to assess the condition and develop a repair strategy to allow the continued use of the structure. The key aspects of structural evaluation include:
• Review of Available Documentation: Before starting any field work, all available documentation related to the construction of the concrete structure should be gathered and reviewed. Information to obtain includes:
• Intended use of the structure. Some structures such as parking garages or wastewater plants have extremely harsh environments and the methods of evaluation used in these structures may be much more involved than structures with less severe exposures.
• Structural systems such as conventionally reinforced versus post-tensioned concrete, precast versus cast-in-place concrete, flat slabs versus pan joists, etc.
• Design loading, both under current conditions and those planned for the future. If the structure has been designed for future vertical or lateral expansion or with a potential change in use envisioned, this must be factored into the evaluation.
• Material properties such as concrete strength, aggregate type and reinforcing steel strength.
• Visual Assessment: The most powerful tools in any evaluation are your own eyes. Visual evaluation should include a thorough inspection of all accessible surfaces for signs of deterioration. This assessment should also try to establish patterns of deterioration which may relate to the use of the structure or the structural system. For example, diagonal cracking in a concrete beam near a support may indicate deficiencies in the shear reinforcing in the beam while vertical cracking at the same location may be more indicative of thermal or shrinkage stresses.
• Testing: A myriad of destructive and non-destructive testing methods are available for use in an evaluation, but they should be selected to provide specific information about a specific item. Testing just for the sake of testing is of no benefit to you or your client. Several testing methods which are widely used include the following:
• Cores: These are cut from the structure and can be utilized to determine concrete strength, bond between concrete layers, and visual assessment of the concrete matrix.
• R-meter or Radiography: Non-destructive tests to locate embedded steel such as reinforcing bars or post-tensioned tendons in concrete. It should be noted that radiography requires restrictive access around the equipment while in use and thus its use should be limited.
• Petrographic analysis: A microscopic review of the concrete matrix to determine the constituent parts of the mix, air content, and concrete quality. This can also be used to determine the presence and effectiveness of sealers and other penetrating materials applied to the concrete surface.
• Acoustics: This can be as simple as sounding with a hammer to detect near surface delaminations to pulse velocity or impact echo testing to detect defects in the concrete at depth.
• Load tests: Although primarily used as a method of determining acceptance of large scale repairs to a structure, load tests can also be used to determine the acceptability of existing conditions so as to avoid implementation of costly repairs.
• Analytical Review: Based on the findings of the document review, visual assessment, and testing, an analytical review can be performed on key structural elements. This analysis can be used to determine deficiencies in the original design as well as determine if the existing structure, even in a deteriorated condition, is still adequate to support the imposed loading.
• Client Involvement: No evaluation can be complete without input from the building owner or maintenance personnel. These individuals live with the building on a daily basis and typically have historical data related to building use, prior maintenance history, change in use of the facility, etc. In addition, a successful repair strategy can only be developed if the expectations of the client are clearly understood. For example, if the structure will be replaced in one year, it is not beneficial to the client to evaluate the structure and develop repairs for a 20-year anticipated life.
Stephen H. Lucy, P.E. is a partner at Jaster-Quintanilla, a structural and civil engineering and land surveying firm with offices in Austin, Dallas, Houston and San Antonio. He can be reached at 214-752-9098 or at firstname.lastname@example.org.