Chipping occurs on external slabs when water trapped in concrete goes through multiple freezes and thaws. When water freezes, it expands by about 9%, which creates tremendous pressure inside the slab. Eventually, microcracks begin to form, and the surface begins to separate from the body of the slab. One of the many reasons why concrete flaking occurs is due to the natural process called carbonation.
Water immediately seeps into concrete and causes other elements, especially reinforcing bars embedded in concrete, to corrode. Thus, it creates bulges and cracks on the surface of the concrete. Steel reinforcing bars are often used in concrete to provide durability. In addition, poor quality steel can lead to chipping.
Sealants are a great way to protect concrete from chipping deterioration. A good quality sealant designed for use on outdoor concrete will help minimize water saturation and protect against salt damage. Even with a good sealant, concrete itself is still the most important component to prevent flaking (see The correct mix of concrete ingress). In most cases, peeling concrete is caused by poor finishing and the use of water on the surface to aid in the finishing process.
Excess water and overfinish create a weak surface that cannot withstand freeze-thaw expansion and contraction. You should always get a signed contract with a concrete installer with a specific warranty language that deals with chipping and cracking. To fix chipping, you may need to re-coat the doorway with a layer (see How to Repair Chipped Concrete). Concrete peeling is a common concern in older concrete structures.
Alkalinity, a normal corrosion mechanism, causes the steel bar inserted into concrete to corrode over time. Concrete peeling is the deterioration of concrete due to environmental factors, poor installation and other reasons. While sealing it would not completely prevent peeling, it will help decrease the chances of it occurring. It's not uncommon for concrete builders and contractors to receive calls from customers asking why their concrete is pitted and peeled.
If properly poured and cared for, concrete can last a long time, enough to survive a couple of generations of bloody knees and chalk art. The concrete that has been peeled must be ground not only by repairing, but also by identifying and correcting the root cause. Delamination and flaking of a concrete member are undesirable conditions; they not only represent a potential impact hazard in the scenario where chipped concrete falls and hits a person, but also reduces the cross-sectional area of the concrete member and decreases its load-bearing ability imposed. These variations in coverage recognize the fact that concrete is porous and that greater concrete cover will significantly decrease the rate of corrosion of steel and subsequent degradation of concrete.
However, despite best intentions, concrete patch repairs often fail prematurely due to ongoing corrosion around the periphery of repairs. Rainwater or water from any other external source enters the capillaries of the concrete first. De-icing chemicals only aggravate already stressed concrete by allowing more water to migrate into concrete, increasing the size and depth of husking failures when freezing occurs. Signs of chipping are a chipped and scaly surface, pieces of concrete coming out of the facility, and concrete that begins to crack.
However, the protection afforded by high alkalinity can be compromised by the ingress of acidic atmospheric gases, a process commonly known as carbonation, or by the presence of salts in concrete, typically from marine environments or by the use of de-icing salts. In addition, the location of the concrete structure, type and other related factors are also considered. Chipping can have far-reaching consequences in terms of health and safety, structural integrity and asset value. It only takes two to four hours to dry and creates an invisible glass-like barrier deep in the pores of the concrete.