In this article, the civil and structural engineers at Robson Forensic provide an introductory lesson on concrete spalling. They address the common causes and why spalling is a problem.
There are many causes of concrete spalling; they include the improper placement of concrete and its reinforcing, electrochemical (galvanic) reactions between embedded metals within the concrete matrix, and corrosion of embedded reinforcing steel due to exposure to water and/or chemicals. This article will address concrete spalling as a result of corrosion of its steel reinforcing and how concrete placement and its exposure can affect the corrosion process.
What is concrete spalling and what are its effects?
A spall is defined as flakes of material that are broken off of a larger solid body. Concrete spalling typically begins when the steel reinforcing embedded within the concrete member rusts. Contrary to popular belief, concrete is porous. Rusting of the embedded steel reinforcing occurs when that reinforcing bar is exposed to water and air; without both of these elements, the steel bar does not rust. When exposed to both of those elements, a chemical reaction takes place wherein iron oxide (rust) is produced. The production of iron oxide includes a volumetric expansion of the bar by up to 6 times the original volume, and that increase in volume imposes significant expansive forces upon the surrounding concrete. These expansive forces can cause the concrete to delaminate or to crack, spall, and break off. An illustration of this is shown below:
Delamination and spalling of a concrete member are both undesirable conditions; not only do they represent a potential struck-by hazard in the scenario where the spalled concrete falls and strikes a person, but they also reduce the cross sectional area of the concrete member and decrease its ability to safely carry imposed loads. An additional consideration is that both delamination and spalling offer increased access of air and water to the reinforcing steel within that member; thus creating a cycle of corrosion and increased access of the corrosive elements exacerbating the process with each subsequent cycle.
Beyond the obvious aesthetic issues, a reduction in the cross sectional area due to spalling and delamination is synonymous with a weakened concrete section. Additionally, delamination and spalling require increased maintenance, subsequent higher maintenance costs, and can result in a decreased service life of the concrete member(s).
Effects of concrete/reinforcing placement
When concrete is placed, its steel reinforcing is required to be properly positioned within that concrete member. ACI 318 (Building Code Requirements for Structural Concrete), the industry standard in the US for design and construction of structural concrete elements in buildings and structures, prescribes the minimum concrete “cover” for all steel reinforcing within that concrete element. Concrete cover is the thickness of concrete between the outside edge of the concrete member and the steel bar, which covers or protects the steel from exposure to the outside elements.
Concrete cover varies for different exposures of the concrete member and for the conditions in which the concrete member is formed. For example, greater cover is required for greater exposure conditions, such as where the concrete element is cast against and permanently exposed to the ground, vs. being formed and located in an interior environment. Minimum concrete cover is reduced when the member is cast in plant
vs. cast in the field (better quality control). These variations in cover acknowledge the fact that concrete is porous and that greater concrete cover will significantly decrease the rate of steel corrosion and the subsequent concrete degradation.
Construction activity can affect reinforcing placement:
- Workers often dislodge reinforcing bars by walking on them or working adjacent to them before the concrete is poured and altering their cover.
- The forces imparted by the act of placing concrete can also dislodge reinforcing, altering its concrete cover.
Concrete finish operations influence the corrosion process. A surface with a smooth finish holds less water on the surface and creates a more durable and less porous surface.
Effects of Exposure of the Concrete Member
The environment into which the concrete member is placed has a significant effect on its performance relative to durability. The following are examples of how environmental conditions can accelerate the process of
corrosion and rust in steel members by exposing the concrete member to aggressive chemical forces:
- In coastal regions (areas adjacent to bodies of saltwater), the salt-laden air continuously deposits salt on the surface of the concrete member. These salts dissolve in rainwater and then permeate the concrete matrix along with the water, made chloride-rich and highly caustic by the salt.
- In Northern states the use of deicing salts is a prominent contributor to the cause of spalling. Similar to the salt air, salts used for snow/ice control frequently act as a catalyst to start the reaction between the oxygen in water and the iron in reinforcing steel.
- In areas where significant pollution exists, sulfide and sulfate compounds get deposited on the concrete members and, similar to the chlorides in salt, mix with water that ultimately reaches and acts upon the steel reinforcing bars.
Remediation of Spalling can be a Tedious and Expensive Process
When properly addressed, an engineer is called in to design and oversee the concrete 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, selective demolition. The requirement for shoring must be evaluated and put in place before any demolition is performed. Repair techniques for delaminated and/or spalled concrete are numerous and should be evaluated by an engineer to determine the most efficient and safe repair for the conditions that exist at that site.
Forensic Investigations involving Spalling
In forensic casework involving spalling, our experts are typically tasked with determining, within reasonable technical certainty, the cause or causes of the spalled concrete, and assessing the responsibilities of
the parties associated with the construction and/or maintenance of the facility.
Robson Forensic offers in-house experts across the country who can apply firsthand knowledge and experience surrounding the issues of spalling as it applies to their specific geographic region. To discuss your case with a technical expert, please submit an inquiry or contact one of the authors of this article.
This article was developed with collaboration between members of our Civil and Structural Engineering practices. Both Mark Duckett and Greg Pestine were integral in its development.
Gregory H. Pestine, PE
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, replacement, evaluation and forensic analysis of buildings and structures of wood, concrete and steel construction.
Mark Duckett, PE, SI, SECB
Mark has over three decades of professional experience in structural engineering; in this time he has designed and inspected all types of structures including residential, commercial, marine, industrial, institutional, religious, retail, hi-rise, and warehouse structures. His expertise extends to most building materials, including concrete, steel, wood, masonry and many others.