Causes Of Deterioration

Physical Mechanisms

• Corrosion of embedded steel-Embedded reinforcing steel (rebar) is widely used to give concrete greater strength and even flexibility. If the steel corrodes then the expansion of the corrosion products will cause cracking and spalling of the concrete. Normal concrete is very alkaline and will therefore cause passivation of any embedded steel. If the concrete loses its high pH it will no longer offer protection to the steel.
• Spalling –Spalling is the dislodgment of large or small pieces from a structure. The dislodged pieces often have a conical or bowl shape. Spalling can result from impact, fire damage, or corrosion of embedded steel.
• Shrinkage cracking-During the curing of new concrete the moisture within the concrete will diffuse to the surface where it will evaporate. The concrete at the exterior surfaces will dry and shrink faster than the concrete deeper within the structure resulting in tensile stresses at the surface which if great enough cause the formation of drying shrinkage cracks. Geometry can also affect formation of shrinkage cracking with long spans between joints being most prone to cracking.
• Carbonation-In older concrete structures carbonation occurs at the exterior surfaces. The process of carbonation will cause the carbonated surface layer to shrink and crack. Cracks from carbonation tend to be shallower than cracks from initial drying shrinkage.
• Freeze thaw damage (Frost damage)-Concrete is an inherently porous material although the permeability can vary between different structures. The original water cement ratio is largely responsible for the degree of permeability. When water within the concrete pores is exposed to temperatures below 32 degrees Fahrenheit this water can freeze which can initiate internal cracking. Over time and with multiple exposures to freezing temperatures the internal cracks will grow and the cement matrix will begin to crumble. Water saturated concrete is most susceptible to freeze thaw damage.
• Pop outs-Pop outs are small localized pits in a concrete surface usually caused by the fracturing of a susceptible aggregate just below the surface. The susceptible aggregates are often sedimentary rocks which are inherently weak and prone to moisture absorption. The mechanism of freeze thaw causes the formation of pop outs.
• Vegetation in cracks-Cracks in concrete which fill with dirt and moisture can sometimes promote the growth of vegetation within the cracks. As the plant roots propagate into the cracks they can promote further crack propagation and weaken the structure.
• Erosion-Erosion is the progressive deterioration of the concrete surface resulting from high velocity water flow, cavitation damage in flowing water systems, and mechanical scouring and abrasion by ice, sand and gravel, or other solid materials.

Chemical Mechanisms

• Salt Damage-If dissolved salts are present in the water to which the concrete is exposed, these salts may be absorbed into the concrete where they crystallize and precipitate out of solution. The volumetric expansion caused by crystal formation may result in internal cracking eventually leading to disintegration and surface scaling of the concrete.
• Sulfate attack-Ground water and seawater usually contain sulfates and higher concentrations of sulfates can often be found in industrial wastewaters and mine waters. Sulfates can chemically attack the cement matrix of the concrete leaving behind a soft powdery surface. On other occasions sulfate attack can result in the formation of compounds that cause expansion and spalling of the concrete.
• Structural Cracking-Structural cracking results from external loads imposed upon the structure. Direct impact or earthquakes can cause cracking. These cracks tend to be wide and deep. Differential settlement of a structure can cause cracking. Flexural loading can result in fatigue cracking.
• Efflorescence-Water movement through hardened concrete can result in internal leaching. Calcium ions are dissolved and transported to the surface. At the surface reaction of the leachates with carbon dioxide in the air and drying of the deposits can result in whitish colored encrustations. The process is known as efflorescence. Efflorescence often occurs at cracks in the concrete. Internal crystallization can weaken the concrete.
• Alkali-aggregate reaction-The original cement used in the concrete mix can often contain alkali ions such as sodium and potassium. Alternatively these ions can be introduced from the environment or even be present in the aggregates or from an admixture. Certain types of amorphous silica aggregates, or aggregates containing amorphous silica, are susceptible to chemical reactions with the alkali ions. The resulting reaction products will swell in the presence of moisture leading to internal cracking which will eventually be visible at the surface as map cracking