Concrete is one of most durable manmade materials, but even this old industry workhorse has its weaknesses.
Exposure to harsh weather, reactions with common elements, and poor construction can all lead to concrete failure.
Michael Brainerd, principal at Boston-based Simpson Gumpertz & Heger, details five common ways concrete can meet its demise, and offers advice on how to avoid them.
1. Corrosion of steel reinforcement is probably the most common form of deterioration in cold climates, and it's one of the nastiest too, says Brainerd. “It's difficult to deal with because once the conditions are right for corrosion, just fixing the damaged areas does not stop the ongoing corrosion,” he says. “It is essentially impossible to stop, you can only slow it down.”
The deterioration occurs because the by-product of this electrochemical process (rust) takes up many times the volume of the original uncorroded steel. The resulting pressure created inside the concrete will cause cracking and severe deterioration to the structure over time.
The most common cause of steel rebar corrosion is exposure to de-icing salt used for roadways. If the concrete and rebar are not protected, the salts will eventually reach the depth of the rebar and cause corrosion. Exposed concrete structures such as parking garages, sidewalks, and bridges in cold climates are most at risk.
Another culprit of reinforcing steel corrosion is “carbonation,” where carbon dioxide and moisture in the air enter the concrete and reduce the pH of the concrete.
“One of the inherent protective features of concrete is its high pH,” says Brainerd. “But as carbonation advances from the outer surface inward, it eventually can lower the pH at the reinforcement, making the reinforcing steel vulnerable to corrosion.”
Luckily, there are several ways to protect steel reinforcement from corrosion. First, make sure to provide at least 1½ to 2 inches of concrete cover over the reinforcement. In addition, create a concrete mix that is highly impermeable by using a mix with a low water-to-cement ratio (typically no greater than 0.40) “so that it takes longer for the chlorides or carbonation to reach the steel,” says Brainerd.
Other internal protection options include adding corrosion inhibitors to the fresh concrete and using epoxy-coated reinforcing steel.
External protection measures such as penetrating sealers or waterproof coatings applied to the exposed concrete can also inhibit ingress of chlorides and moisture.
2. Sulfate attack typically occurs when the concrete is exposed to water that contains a high concentration of dissolved sulfates. “We see this most often where there's sulfate-bearing groundwater,” as in the Western states and the Northern Great Plains, and near industrial areas and seawater, says Brainerd.
The two most common types of sulfate attack are physical attack, where the sulfate-containing water enters the surface of the concrete, crystallizes, and expands, disrupting the hardened concrete; and chemical attack, where the sulfate salts react with the portland cement paste, causing it to dissolve, soften, and erode. Another type of sulfate attack, internal sulfate attack, occurs mainly in precast concrete, and has been attributed to high curing temperatures or cement chemistry.
To minimize the risk of sulfate attack, test the sulfate content of soil or water that will be in contact with the concrete and use the published guidelines to specify a resistant concrete mixture. For buildings with severe sulfate exposure, limit the water/cement ratio to 0.45, require a minimum compressive strength of 4,500 psi, and use Type V cement or Type V cement in combination with alternative cementitious materials such as slag or fly ash.
3. Finish-related delamination can occur when water or air gets trapped and accumulates just below the surface of the concrete. The accumulation of water raises the local water-to-cement ratio, which decreases the concrete strength in that area. In addition, air bubbles can be elongated and interconnected by the finishing process, thereby creating a weakened horizontal plane in that area.
“Any sort of stress on the surface from wheel loads or volume change movement of the concrete can cause the concrete to fully fracture along that weakened plane, and you'll have pieces of the concrete popping up off the surface,” says Brainerd.
Trapped water/air and elongation of air voids typically occur when finishers apply a finish to the slab before all the bleed water can reach the surface. It can also occur when the ambient conditions of temperature, wind, and humidity result in a high evaporation rate for the bleed water, which causes premature drying and stiffening of the near-surface layer and traps bleed water and air below the surface.
Brainerd says delamination is a common problem with floors with a hard steel-trowel finish, such as warehouses, factories, retail stores, and offices with exposed interior concrete floors.
To prevent delamination of slabs receiving a trowel finish, Brainerd advises Building Teams to specify air content of no more than 3% and to test the fresh concrete to assure the maximum air content is not exceeded. Also, contractors should place the concrete when ambient conditions are not conducive to rapid evaporation of bleed water, and should avoid finishing the concrete slab prematurely.
4. Freeze-thaw deterioration occurs when concrete is saturated with moisture while exposed to freezing temperatures. The freezing water within the concrete creates hydraulic pressures within the concrete, causing micro-cracking of the concrete. “The near surface just crumbles away and exposes the aggregate,” says Brainerd. “In extreme cases, over time, the whole body of the concrete can disintegrate.”
To resist freeze-thaw deterioration, the hardened concrete must contain an air-void system that consists of tiny, closely spaced air bubbles, called entrained air. The voids provide space to relieve the pressure of the freezing water, thereby minimizing stress on the concrete. The process involves adding air-entraining admixtures to a fairly strong concrete mix (at least 4,500 psi).
Achieving a proper air-void system can be tricky, says Brainerd. “There's no way to measure the size and distribution of the air voids in the fresh concrete,” says Brainerd. “We'll specify a certain percentage of total air content in the fresh concrete that experience shows can result in a proper air-void system in the hardened concrete, but if the concrete is not handled properly in terms of its mixing, placement, and finishing, we can end up with an improper air-void system.”
For critical structures, Brainerd often requires testing of not only the total air content of the fresh concrete, but also the air-void system in the hardened concrete. The latter involves microscopic examination of concrete samples extracted from the structure.
5. Alkali-silica reaction occurs when certain silica-containing aggregates react to form an expansive gel that causes the concrete to crack. The cracks typically form in a widespread “map cracking” pattern that is typically accompanied by deposits of white or grey gel on the concrete surface or inside air voids. While ASR is less debilitating than the other forms of deterioration, it can cause potholing and localized crumbling in pavements, curbs, driveways, and other structures.
Not all silica-containing aggregates are vulnerable to ASR. Only aggregates containing a disordered crystaline structure, such as strained quartz, chert, opal, or volcanic glass are susceptible.
“When high-alkali cements and silica-bearing aggregate are exposed to humid conditions, a reaction occurs that creates a gel that, when exposed to water, can expand and fracture the aggregates and cement paste,” says Brainerd. “A significant amount of concrete of any age has probably suffered this to some degree.”
To minimize the risk of ASR, require that the concrete producer test the cement-aggregate combination for cement alkalinity limit, expansion, petrographic characteristics, and historical performance. Consider using portland cement in combination with alternative cementitious materials, such as slag or low-alkali fly ash, to decrease permeability and reduce the quantity of alkali in the concrete. Some experimentation has also been done using admixed lithium salts to slow down or stop ASR.