Concrete is a mixture of Portland cement, water, and fine and coarse mineral aggregates, and sometimes various admixtures. When all of these materials are mixed in the correct proportions, an complex chemical reaction takes place. This reaction is known as cement hydration, the process by which concrete hardens and cure.
Even though it can be a very strong, hard substrate, bare concrete is also subject to deterioration. Concrete can be very porous, so chemicals can penetrates the pores and attack the paste. The paste and aggregate can also be worn down by physical impact and abrasion. Water can penetrate concrete, freeze and expand inside it when the temperature drops, and ultimately weaken the concrete from within. In addition, if the concrete has reinforcing steel bar (rebar) to impact additional strength and other properties, the rebar can corrode if moisture, oxygen and chloride ions penetrate the concrete. Corrosion or rebar contributes to the deterioration of concrete.