Beyond the Numbers: Concrete is More than Compressive Strength

Concrete is commonly qualified by its compressive strength; it is frequently the primary figure of merit for engineers, architects, and inspectors alike. Since concrete’s mechanical properties are generally utilized in compression, this focus makes sense – at least for engineering design considerations. Yet, concrete performance and durability depend on far more than just compressive strength. From aggregate selection to mix design, placement methods to finishing techniques, there are a variety of concrete characteristics that can mean the difference between success and failure in concrete placements. Responsibility for failure can rest with the concrete supplier, general contractor, finisher, rebar supplier or even the engineer of record depending on the root cause(s) of the encountered problem(s).

Influence of Time on Compressive Strength

Concrete holds a unique place in the pantheon of materials science because it gets stronger with time since the hydration of the cement occurs gradually over an extended period. Typically, engineering specifications cite the compressive strength at 28 days or at shorter time intervals (1, 3, 7, 14 days) where early strength is particularly important. However, concrete can continue to strengthen to 56 days and beyond. The quality of delivered concrete is typically measured through the compressive testing of poured cylinders collected at the time of delivery, preserved, cured and tested at the specified time interval. And yet, compressive strength will only be maintained if the concrete has structural stability – making durability critically important to performance.

Common Causes for Concrete Degradation

Many concrete failures are not due to compressive crushing. Rather, cracking often occurs in response to externally applied tensile forces or localized tensile stresses from a variety of internal mechanisms.  Since a common rule of thumb is that the tensile strength of concrete is roughly 10% or less of the compressive strength, the sensitivity of concrete to the following degradation mechanisms can be understood.

Reactive Aggregate

If the aggregate used contains potentially reactive minerals, it can undergo a reaction with the alkali hydroxides in the pore solution that results in a volume expansion that generates tensile stress internal to the aggregate and surrounding paste causing localized stress cracking. This can lead to disintegration of the aggregate (Figure 1) and can lead to more widespread cracking allowing for water ingress and further deterioration. Terminology for these types of conditions includes Alkali Silica Reaction (ASR) and Alkali Carbonate Reaction (ACR). Deterioration of the bond between the aggregate and the cement paste also reduces the ability for the aggregate to effectively prevent crack propagation and destroys the composite nature of the material.  

Figure 1.  Petrographic micrographs of ASR induced microcracking extending into paste from reacted aggregate (left, middle), SEM image of ASR gel filling air voids (right)

Reinforcing Bond Loss

The installation of reinforcing structures like steel bars or cages are typically used in construction to reduce the possibility of tensile stresses developing in the concrete structure. In many circumstances, this reinforcing steel is tensioned either before or after concrete placement to keep as much of the structure as possible under compression, though this requires a good bond with the surrounding concrete. The quality of the paste/steel interface – established through mechanical load transfer by paste adhesion to the wire – determines its strength. Poor bond strength can result from an embossed steel grip pattern that is insufficiently or incorrectly rolled or from low paste strength caused by excessive water content (Figure 2). Corrosion of steel is also a path to bond loss and localized stress cracking given its expansive nature.

Figure 2. Rebar Corrosion (left), Reinforcing wire (top right), Wire Pattern (bottom right) (Source: corrosion.ksc.nasa.gov) 

Environmental Degradation

Concrete placements may appear monolithic or impenetrable, but concrete contains air voids into which water can migrate. Freeze thaw damage occurs when the water that migrates into the concrete freezes and causes cracking, paste disintegration and aggregate spallation (Figure 3). To avoid cracking, air voids can be intentionally incorporated using admixtures to accommodate the volumetric expansion associated with ice formation. The effectiveness of air entrainment can be determined by the combination of the total volume and the spacing of air voids, both of which can be measured as part of quality assurance.

Figure 3. Freeze Thaw Spallation (left), Rapid Air Void Analysis Using Blackout Preparation Method (right)

Improper Surface Finishing

Improper finishing techniques can also be problematic, with excessive water added to the mix during placement, overworking or prematurely working bleed water back into the concrete. This can result in increased paste porosity or trapping rising bleed water and air just beneath the concrete surface creating zones of weakness resulting in premature surface scaling and disintegration (Figure 4).

Figure 4. Surface scaling (left), Petrographic cross section showing surface weakness (middle), SEM image of cross section highlighting porosity at surface (right)

Petrography as the Solution

Petrography is a field within geology that involves the detailed description and classification of rocks and minerals and has been extended to concrete and concrete raw materials.  This analysis is used as an important diagnostic tool for identifying basic components within the concrete, assessing the quality of workmanship and adherence to mix design, and detecting any deterioration mechanisms present.  Petrographic analysis involves a variety of microscopic techniques, including reflected light, polarized light and scanning electron microscopy (SEM). Experienced petrographers can gather valuable information about common deterioration mechanisms in concrete, such as chemical attack, ASR, delayed ettringite formation (DEF) and damage caused by freezing and thawing cycles.  This detailed analysis also helps to determine the nature and extent of deterioration and predict whether it is likely to progress in the future. Importantly, construction professionals such as engineers and general contractors can use petrographic analysis to make informed decisions about effective replacement or rehabilitation strategies, making it an essential part of their overall assessment.  

Importance of Specifications

Engineering specifications and quality assurance procedures – from plant to delivery to placement – are essential tools in avoiding concrete problems and reducing the risk of performance and durability issues. Materials qualification testing and mix design evaluations are commonly required to demonstrate a supplier’s ability to meet specification requirements prior to delivery. On site quality testing captures information (slump, workability, density, poured cylinders) on the concrete as delivered. Some government contracts even include requirements for evaluation of as-placed concrete through the extraction of cores to certify the expected service life of the material as built.

When Things Don’t Go as Planned

If failure occurs, intense scrutiny of mixing parameters, placement procedures, materials quality, engineering design, and post placement operation and maintenance is sure to follow. Root cause determination will be informed by quality documentation and forensic field investigations. As built compressive strength is only one factor of this evaluation and may be difficult or impossible to accurately assess from cores removed from a damaged or deteriorated structure. Significant care should be taken when selecting and collecting specimens for evaluation with attention to both representativeness and appropriateness relative to the intended analyses. Petrography is a key component in evaluation of failure and/or degradation mechanisms but the interpretation can be limited by the provenance of the material being analyzed. Collaboration of experts in both structural engineering and materials science/petrography is often essential to reliably determine root cause of failure and assess questions of liability.

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Author Matthew Perricone, Ph.D., is a Principal Investigator and the Technical Consulting Group Manager at RJ Lee Group, Inc., where he directs and performs root cause failure analyses for multiple industrial sectors in support of product development, performance optimization, and manufacturing quality assurance. His projects also include addressing warranty, insurance and product liability claims.

Author Michael Baker serves as the Department Manager of the Concrete Materials Laboratory and has nearly a decade of experience in concrete petrographic studies at RJ Lee Group, Inc. In addition to managing the concrete materials laboratory operations, he directs and conducts petrographic evaluations, in combination with chemical and physical testing, to identify failure and distress mechanisms in concrete and cementitious materials.

Editor Thanh Do, Ph.D., PE, is a Structural Forensic Engineer and Forensic Visualization Manager with Thornton Tomasetti, Inc. He specializes in investigations of construction/design defects and collapses, standard of care assessment, and visualization/storytelling for litigation.