ArticleIn this article, civil engineer, Gregory Pestine, P.E., provides an introductory lesson on spalling. He covers the common causes, why spalling is a problem, and discusses the processes for remediation.
The condition shown in the image below is common in bridges, parking garages, and other concrete structures. The term “spalling” is used to describe this condition, and specifically refers to the delaminating or fragmenting of the concrete surface.
Concrete spalling can be triggered by several causes:
- The most prominent cause in the Northern states is the use of road salt or sodium chloride. Salt acts as a catalyst to enhance the reaction between the oxygen in water and the iron in reinforcing steel. The resultant oxidation (rusting) causes the reinforcing steel to enlarge which creates high internal stresses within the concrete and, ultimately, fracturing of the concrete surface.
- Surface cracking can be generated by the reaction between atmospheric carbon dioxide and the alkalis present in the concrete mix itself. These surface cracks can then admit rainwater or snow melt. Water expands when it freezes and then contracts when it thaws. These repetitive freeze-thaw cycles cause stresses which can break off surface concrete as well.
- Improper concrete finishing operations can contribute to the premature spalling of concrete surfaces. If water is added to the freshly laid concrete surface to increase its workability, it could result in a weakening of that surface concrete. This could lead to the weakened surface section delaminating from the stronger concrete below and thus admitting more water to react with the reinforcing steel. If a concrete surface is trowelled while the initial bleed water is present, a similar result could occur. Entrapping the concrete mix water with a liquid-applied curing membrane could be problematic for concrete placed in the late fall where the entrapped water is subject to freezing and thawing before it more fully reacts with the cement in the mix – a period of normally 28 days.
SPALLING: Why it is a problem
Concrete spalling can result in a variety of undesirable outcomes:
- Spalls on the underside of bridge decks can cause damage to cars below as well as unsafe roadway conditions. Falling concrete debris is also unsafe where pedestrians travel, where spectators view sporting events, and in parking facilities.
- Spalled surfaces can lead to rapid degradation of reinforcing steel which would jeopardize the integrity of structures and necessitate costly repairs. Spalls are also unsightly and indicate a lack of maintenance on the part of facility owners.
Remediation of spalling is a tedious and expensive process:
Typically, an engineer is called in to design and oversee remediation. This includes identifying the areas to be repaired. Delaminated areas which have not yet broken loose can be detected by sounding methods, both mechanical and electronic. These areas as well as visible areas of spalls are marked for partial demolition. Possible shoring of existing structures must be evaluated and put in place before any demolition is performed. Demolition consists of sawcutting the perimeter of damaged areas taking care not to cut any existing rebar. Small chipping guns are used to remove the damaged concrete, usually deep enough to provide about one inch of clearance behind existing reinforcing steel so that the new patching material will have a good bond to the reinforcing steel.
The steel is then sandblasted to remove all rust. Replacement of bars with some section loss may be required. After the bars are prepared, an epoxy coating may be required by the engineer. The existing concrete surface requires preparation in the form of cleaning and application of a bonding agent or keeping continuously wet for a specified period of time. Several patching compounds are available including specialty concrete materials or standard concrete mixes. Application will depend on the size, thickness and location of the area to be repaired. It may require a standard formed and poured concrete repair or a drypacked or trowelled-on patch. The area above should be waterproofed and maintained in a waterproof condition to prevent recurrence of the spalls.
SPALLING: Expert Witness Involvement
We see spalling in many of our forensic investigations:
In these cases, our experts are typically tasked with determining, within reasonable technical certainty, the cause or causes of the spalled concrete, and the liability of any parties associated with the construction and/or maintenance of the facility who were responsible for preventing the cause(s) of the spall.
Our experts are available nationwide to assist in your investigation and with 17 locations, we can often provide an expert who is local to your case.
Greg is a Civil Engineer with over 30 years of diverse experience in engineering including construction management, quality control, and construction safety. He has extensive experience in the inspection, design, replacement, evaluation and forensic analysis of buildings and structures of wood, concrete and steel construction.
Greg has broad construction experience, on a wide variety of projects, in many different roles. As a superintendent, quality control manager, and resident engineer he has been responsible for site safety, quality, and budget. Greg has additional expertise in scheduling, construction cost estimating, and site specific safety and quality control plans. Greg is a Professional Engineer in Illinois and Wisconsin and is a member of the American Society of Civil Engineers and NCEES.
Greg is a civil engineer in Robson Forensic’s Chicago office and investigates construction matters across the midwest. He can also put you in touch with other Robson Forensic civil engineers who are located locally to your investigation.