TYPES OF CONCRETE FAILURES
Concrete failures occur for a variety of reasons. Some types of failure are more frequently encountered and will be discussed briefly here. A more complete discussion of concrete failures and repair techniques may be found in ACI 302.1R-89, Guide to concrete Floor and Slab construction, ACI 503 R-89, Use of Epoxy Compounds with Concrete and various reprints from IACRS, International Association of Concrete Repair Specialists.
These are the result of either:
Shrinkage cracks occur when the water exits the concrete too quickly or where there was too much water in the mix. Compaction cracks occur when the substrate below the slab is insufficiently compacted and settles leaving a hollow under the slab. Stress cracks occur when there are extreme loads placed on the slab for which it was not designed for
CRAZING AND MAP CRACKING
Often referred to as alligatoring, this type of cracking rarely presents structural difficulties or bonding difficulties, as it does not penetrate more than a few millimeters below the surface. The predominant causes are improper finishing practices and improper curing practices.
LOW RESISTANCE TO WEAR
Simply stated the concrete’s mortar at the surface did not gain sufficient strength. Low cement content, high w/c ratio, overworking the surface during finishing, improper curing, carbonation, and freezing prior to sufficient strength development are the primary cause of this type of failure. Subsequent application of polymer coatings and toppings will fail unless the weak surface is removed to sound concrete.
This is a condition attributed to freezing and thawing of concrete that already suffered from the conditions listed for low resistance to wear above. Here the concrete simply delaminates at a depth of about 1/8”. The only remedy is to completely remove the top delamination and prepare the surface for application of the desired finish.
Pop outs are cone shaped pits in the surface approximately 3/8” to 2” in diameter, caused by expansion of alkali reactive aggregates and clays, exposed to moisture or chemical attack. The repair and subsequent application of polymer coatings and toppings is conducted to a routine basis with the following caution:
Pop outs may occur for a prolonged period of time and will not be stopped by the application of coatings and toppings.
Blisters are the result of entrapped air and or free bleed water being trapped beneath the top 1/16” to 1/8” mortar on the surface of the concrete during finishing operations. The primary cause is overworking the surface and attempting to close the surface with a hard steel trowel too early. Application of polymer products to this type of surface will require breaking the skin of the blister, making repairs and/or simply incorporating the repair into the application of the topping.
Spalling is a term used to describe the breaking away of concrete at joints or steel reinforcing, and may be structurally significant. If may be caused by moisture attack of the reinforcing steel creating build up of iron oxide (rust), which exerts pressure on the concrete, causing it to fail in tensile stress. It may also be caused by freeze thaw forces discussed later in 1.6.12. Another mode of failure occurs when inferior concrete is subjected to compression shear or flexural stress at the joints or perimeters. Repair of spalled concrete should be undertaken with the advise and council of the certified coating inspector and preformed by a highly skilled professional installer to insure structural integrity and prevent reoccurrence.
PONDING AND INADEQUATE SLOPE TO DRAIN
The application of polymer coatings and toppings are usually for finished floor surfaces. As such, the required finish conditions of proper slope to drain and the absence of ponding areas, often referred to as birdbaths, is the responsibility of the concrete installer. Depending on the relative condition of the concrete, this could present a series of complicated and expensive problems. Inspection of the concrete levelness and or slope prior to acceptance of the surface for application of finishes should be conducted on routine basis for every project. Coatings will not repair slope to drain or birdbath issues.
CURLING AND WARPING
All concrete goes through a phase of expansion during the plastic state followed by a phase of shrinkage during the drying state. Unless shrinkage compensating Type “K” cements are used, the shrinkage will always result in a final volume that is less that the original volume. Even with Type “K” cements, the final volume will be less than the volume to the expansion phase. As concrete dries, it will dry faster and therefore lose volume faster than exposed surfaces. For concrete slabs, this means that the surface is shrinking faster than the under side and the edges faster then the center. This causes upward curling of the slab at its perimeters and corners and control joints, leaving small voids beneath the concrete. Because of concrete’s low flexural and tensile strength, when loads are applied to these unsupported edges by forklifts, hand trucks, pallet jacks, etc., they fail by cracking. Proper repair requires providing adequate support and repairing or replacing the affected concrete.
Carbonation is a process that can cause accelerated corrosion of reinforcing steel. It occurs when carbon dioxide in the air reacts with calcium hydroxide in the concrete to form calcium carbonate. This lowers the pH of concrete and robs it of its protective alkalinity, thus allowing for the corrosive attack of steel. The process of steel corrosion as the result of carbonation is the transformation formation of steel, water and oxygen into ferrous oxide (rust). Rust has a volume 3 to 4 times that of steel. As the rust builds it creates tremendous pressure on concrete, eventually resulting in spalling of the concrete. In this process, the reaction is dependent on the amount of carbon dioxide in the environment surrounding the concrete and the amount of concrete cover over the steel reinforcement.
FREEZE / THAW
Because of concrete’s porosity, it will absorb water. When water freezes it increase its solid volume by up to 25% of its original volume. When water within concrete freezes it can create forces strong enough to cause spalling and scaling addressed earlier. Proper use of air entraining admixtures are used to produce microscopic air pockets of proper size, quantity and distribution will prevent this type of damage, by providing room for expansion of the moisture. Refer to ACI 211.1R-77, Guide to Durable Concrete, and ACI 306R-88, Cold Weather Concreting.