Concrete is one of the most common construction materials because it is strong, durable and inexpensive. Although concrete is strong and durable, it is not indestructible. One of the most common issues to impact concrete over time is known as spalling.
This refers to a situation in which concrete breaks, flakes or chips off the surface of the concrete. Spalling can be a structural weakness and a concern for the longevity of the structure.
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Anyone who is active in construction or maintenance must understand the causes, prevention, and methods of repair of concrete spalling. And potential for spalling to impact the performance of the concrete.
In this comprehensive blog, we will discuss the causes of concrete spalling, options to control concrete spalling, repair methods for spalling and how to avoid further spalling of concrete.
What is Concrete Spalling?
Concrete spalling is a widespread issue that happens when the outer layer of concrete deteriorates, causing cracks, flakes, or chunks to break free of the concrete element. Concrete spalling can occur to any concrete element, such as floors, walls, ceilings, columns, or bridges.
Spalling affects the aesthetic of concrete work, but it also can be deemed a safety hazard. Particularly in regard to elements that support a load such as buildings, highways, and bridges.
Spalling severity varies from slight surface damage, as it pertains to aesthetics only, to deep structural damage that reduces the integrity of the concrete.
If spalling continues to deteriorate untreated, it can have significant amounts of damage, expensive repairs, and even total structural failure in very serious conditions. Therefore, taking preventative measures and addressing spalling quickly can prolong the life of concrete structures and provide safety.
Concrete Spalling Causes
There are numerous causes of concrete spalling and understanding these causes is significant for prevention. Below are some of the most common reasons for concrete spalling:
1. Corrosion of Reinforcement Steel
Most concrete structures utilize reinforcement steel (also known as rebar) to aid in imparting strength and durability to concrete.
That said, concrete is a porous material, and as time passes, moisture, air, and chloride ions can permeate inside the concrete member and reach the rebar. When rebar is exposed to moisture, air, and chloride ions, it will corrode and rust.
Rusting expands and produces internal pressures in the concrete and the resulting cracks. As the cracks become larger, pieces of concrete will break off due to spalling.
If rusting leads to spalling, and it is not eliminated early in the process. It may be significant enough to require expensive and time-consuming repairs or strengthening of the concrete.
2. Freeze-Thaw Cycles
In cold weather regions, one of the leading causes of spalling in concrete is the freeze-thaw cycle. When water breaks through the small pores and cracks in concrete, it freezes and expands approximately 9% of its volume as it turns solid.
The cycle continues all winter, causing the material to weaken over time. In the long run, it can cause considerable deterioration on the surface of the concrete, causing it to chip, flake, or bust apart.
3. Alkali-Silica Reaction (ASR)
This is a chemical reaction when the alkalis in cement react with reactive silica in some aggregates. This reaction creates a gel-like product that absorbs water from the air. The gel swells, creating internal pressure, and that pressure can lead to cracking and spalling of the concrete.
Deterioration often takes years to develop before it can be observed, but once it starts, it can be difficult to stop. ASR will typically have significant cracking in broad patterns, usually with white deposits on the surface as well.
4. Poor concrete mix design
The quality of the concrete tends to really matter. A mix with a poor blend of its parts and a poor water-cement ratio can create a weakly held structure.
A mix that has too much water will be more porous upon becoming hardening concrete and the water and chemicals can penetrate the concrete easier.
In the same way, aggregates of low quality or too much of the additives may also impact the strength of the concrete making it susceptible to spalling.
A concrete mix that is balanced with proportions of cement, water, aggregates, and admixtures will benefit durability and reduce the risk of deterioration.
5. Errors on Site
Errors during the construction phase can lead to spalling very early on. Common errors include:
– Curing not done adequately: Concrete needs to be adequately cured before it can achieve its ultimate strength. If the concrete is not cured properly or fully enough the resultant concrete will be weaker and more porous.
– Poor floating: Getting too aggressive with either troweling the concrete surface or overworking the finishing can allow for weak surface skins that can spell.
– Insufficient cover: The depth of reinforcement bars must be sufficient to adequately protect them from the environmental exposure why they are embedded into the concrete. Without sufficient cover, corrosion can occur at a more rapid pace causing spalling.
Proper QA procedures and following industry best practices can help mitigate these errors.
6. Chemical Agents
Concrete that has been exposed to harsh chemical agents such as acids, sulfates, or deicing salts is also at a higher risk rate for experiencing spalling.
Agents such as these will destroy the cementitious matrix and weaken the bond between cement and aggregates. As time passes, a concrete surface could deteriorate due to chemical exposure by cracking, flaking, and losing significant amounts of material.
For example, deicing salts used for winter maintenance can speed up the spalling of concrete due to chloride-induced corrosion of reinforcement steel.
Concrete surfaces in industrial settings will experience severe spalling over time, if exposed to acids and sulfates, without proper coatings or sealants in place.
Controlling and Preventing Concrete Spalling
Concrete spalling is a very common issue, but it is also easily preventable if the concrete is correctly constructed and maintained.
With proper protective measures, the longevity of concrete will be useful, and costly repairs will be minimized. Here are a few key components to spalling control and prevention:
1. Select High-Quality Building Materials
Durable concrete starts with the materials used. Selecting high-quality cement, well-graded aggregate, and quality building chemicals will increase the strength and lifespan of the concrete.
Further, the use of air-entrained cement is a highly beneficial consideration, as it improves freeze-thaw resistance by allowing for tiny air pockets in the concrete that will allow for expansion and contraction, relieving stress and eliminating spalling.
2. Improve Concrete Mix Design
A properly proportioned concrete mix is key to durability and the structural integrity of your project. The water-cement ratio must be correct, since too much water can lead to a concrete matrix that is too porous, making the concrete more vulnerable to moisture becoming trapped within and leading to cracking.
The use of pozzolanic supplies, including fly ash, silica fume, slag, and others, helps improve the microstructure of the concrete and improve chemical resistance while reducing permeability. This will lead to concrete that has greater strength and durability, with a reduced risk of spalling over time.
3. Use surface Sealers and Coatings
Concrete surfaces will not be protected from the constant onslaught of environmental elements (moisture, salts, chemicals, etc.) and resulting in spalling.
High performance surface sealers, or water repellents based on silane or siloxane, will keep moisture from penetrating beneath the surface while allowing vapors to escape the concrete (breathability).
Epoxy and polyurethane coatings will protect concrete from aggressive chemicals and wear while extending the service life of concrete surfaces.
4. Cure Properly
The process of curing is a critical and essential step in concrete construction, permitting the concrete to achieve full strength and durability. Without proper curing, shrinkage cracks will result, and the porosity of the concrete will increase, causing greater vulnerability to environmental stresses.
Effective methods of curing include water curing (keeping the surface moist for no less than seven days), curing compounds, or wet burlap and plastic sheeting. Using sufficient curing will increase the life of the concrete, and a cured surface will not deteriorate as quickly.
5. Increase Concrete Cover for Reinforcement
To limit moisture, and especially chloride ions, from reaching the steel reinforcement in reinforced concrete structures, you should consider a minimum amount of concrete cover above the steel reinforcement.
Typically, the cover thickness is from 25-50 mm depending on exposure conditions. When possible, you should consider additional concrete cover for marine and other highly corrosive environments. Additional concrete cover will provide additional protection from rusting on the rebar and eventual spalling.
6. Utilize Corrosion Resistant Reinforcement
Reinforcement is required for nearly all concrete structures; however, steel reinforcement is subject to rusting when water or salts are present.
To minimize the risk of spalling caused by the rusting of the reinforcement, you can specify epoxy coated, galvanized, or stainless steel reinforcement bars. You can also use fiber reinforced polymer (FRP) bars which are considered corrosion resistant, and durable.
7. Provide Proper Drainage Systems
Water sitting on the surface of concrete elements can deteriorate the concrete much quicker, especially in freeze-thaw climates. Having properly designed drainage systems in place for buildings, bridges, and pavements will help limit water intrusion and minimize damage.
Sloped surfaces, gutters, and drainage channels should be installed to aid in the separation of water and reduce the potential for spalling.
Repair Methods for Concrete Spalling
Upon the occurrence of spalling, remediation must occur immediately to stop the continuous deterioration of the structure and restore structural integrity. There are a variety of repair methods available for spalling concrete, including:
1. Patching with Repair Mortar
For minor surface spalls, the use of a high-strength polymer-modified repair mortar can be an effective repair approach. The repair process includes:
Thoroughly cleaning the damaged area to remove loose debris, dirt, and weakened concrete.
Applying a bonding agent to the base concrete surface to improve adherence between the existing concrete and the repair material.
Filling the spalled area with the repair material and smoothing it so that it is less than or equal to the surrounding surface.
Allowing sufficient time and conditions for the repair to properly cure to ensure that the repair adheres and lasts.
2. Concrete Resurfacing
Resurfacing is an affordable option when concrete surfaces have extensive spalling while still retaining their structural integrity. A thin layer of a polymer-modified overlay is applied to reinstate the surface’s appearance and function, and is often performed on sidewalks, driveways, and parking structures to improve wear-resistance and durability.
3. Structural Strengthening and Reinforcement Repairs
Spalling caused by corroded reinforcement requires extensive repair work to ensure long-term stability. The repair process includes the following:
Removing the damaged concrete surrounding the poor rebar to expose the corroded reinforcement. Cleaning the steel reinforcement by removing any rust and debris, or replacing it if it has deteriorated excessively. Applying a corrosion inhibitor or protective coating to the steel reinforcement to mitigate the reoccurrence of rust.
Recast the area with high-strength concrete or repair mortar to restore integrity to the structure.
4. Crack Epoxy Injection
Deeply spalled cracks can be repaired with epoxy injection as a method to restore structural strength and inhibit moisture intrusion. The process involves the following steps: Cleaning out the crack of any loose debris and contaminants.
Injecting an epoxy resin with specialized equipment to fill any voids, and, Allowing the epoxy to cure and create a solid, watertight bond that reinforces the concrete.
5. Shotcrete Application
When large areas of spalling need repair, shotcrete (sprayed concrete) may be the most efficient repair and strongest long term repair for unique situations.
Shotcrete entails spraying a high-strength concrete mix onto the deteriorated surface that will bond and enhance the structural integrity of the component. The shotcrete method is usually applied for scoped repairs of bridge decks, tunnels, retaining walls, and industrial floors.
Long-Term Protection Against Concrete Spalling
To extend the service life of concrete components and reduce the potential for concrete spalling. The best defense is to provide preemptive maintenance and protection services to the concrete in question:
1. Take a proactive approach
To maintenance by creating a routine inspection and maintenance program. Establishing a routine inspection program will assist in identifying the early stages of deterioration and allow the use of maintenance services prior to the concrete spalling becoming extensive damage.
Routine inspections and maintenance of concrete surfaces should require regular assessments of concrete surfaces, especially in areas of high-exposure.
2. Use waterproofing systems
Waterproofing membranes, sealants, and hydrophobic coatings can add another layer of protection against moisture penetration. Waterproofing systems are especially important to use on any and all foundations, basements, and bridges where moisture is present.
3. Apply Anti-Carbonation Coatings
Carbonation of concrete naturally reduces the alkalinity of concrete and is known to accelerate corrosion rates of steel reinforcing.
Anti-carbonation coatings reduce the carbonation of concrete which can extend the durability of reinforced concrete structures. Anti-carbonation coatings act as a shield to prevent the moisture and CO₂’s ability to penetrate the concrete surface.
4. Avoid Using Deicing Salts
Deicing salts (e.g., sodium chloride) helps cause the deterioration of concrete in cold weather. Which happens through an unintentional increase in moisture absorption and acceleration of reinforcement corrosion.
The use of alternative deicing methods, (e.g. sand, calcium magnesium acetate) can help mitigate the effects of chloride penetration and reduce the risk of spalling.
5. Use Advanced Construction Chemicals
Use of modern construction chemicals (e.g. corrosion inhibitors, waterproofing agents, shrinkage-reducing admixtures) may contribute to concrete durability and reduce future environmental damage.
Incorporating these additives into the concrete mix proportion will greatly enhance the performance of the concrete over time while lowering maintenance frequency.
Conclusion
Concrete spalling is a significant problem in terms of both visual appeal and structural integrity in buildings and infrastructure alike.
By understanding the causes of spalling, including corrosion, freeze-thaw cycles, and inadequate mix design, we will be able to implement preventive measures. By using formulated, quality construction chemicals, curing properly, and using protective coatings, concrete structures will be able to last for decades.
as long as steps to address spalling are performed in a timely manner- patching mortars, epoxy injections, and shotcrete coverings can restore the structural integrity of concrete after spalling has occurred.
Preventing spalling in the long term is accomplished through ongoing maintenance and monitoring waterproofing, and the use of corrosion-resistant materials installed.
These components of controlling and preventing spalling can be useful to construction industry professionals as well as private property owners to manage and prevent concrete spalling successfully while ensuring safe and durable structures.
