GOALS For the classification of cracks in the wet and hardened concrete. To diagnose cracks in concrete structures and determine their types.

Introduction : All concrete has a natural tendency to crack due to either internal or external factors, generally influenced by materials, design, construction, service loads and exposure conditions either individually or in combination. Cracks may represent the total extent of the damage, or they may point to problems of greater magnitude.

Classification according ACI 224.1R-07  Cracking of plastic concrete 1 - Plastic shrinkage cracking When moisture evaporates from the surface of freshly placed concrete faster than it is replaced by bleed water, the surface concrete shrinks. 2 - Settlement cracking Concrete has a tendency to continue to consolidate after initial placement, vibration, and finishing.

 Cracking of hardened concrete 1.Drying shrinkage Drying shrinkage is caused by the loss of moisture from the cement paste constituent, which can shrink by as much as 1%. 2 - Thermal stresses Temperature differences within a concrete structure may be caused by portions of the structure losing heat of hydration at different rates or by the weather conditions cooling or heating one portion of the structure to a different degree or at a different rate than another portion of the structure. 3 - Chemical reaction Deleterious chemical reactions may cause cracking of concrete Classification according ACI 224.1R-07

4 - Weathering The weathering processes that can cause cracking include freezing and thawing, wetting and drying, and heating and cooling. 5 - Corrosion of reinforcement Corrosion of a metal is an electrochemical process that requires an oxidizing agent, moisture, and electron flow within the metal; a series of chemical reactions takes place on and adjacent to the surface of the metal 6 - Poor construction practices A wide variety of poor construction practices can result in cracking in concrete structures.

Classification according ACI 224.1R Construction overloads Construction loads can often be more severe than those experienced in service. 8 - Errors in design and detailing The effects of improper design or detailing range from poor appearance to lack of serviceability to catastrophic failure. 9 - Externally applied loads It is well known that load induced tensile stresses result in cracks in concrete members.

Classification according to loaded Structural Cracks 1.Flexural Cracks Cracking in reinforced concrete flexural members subjected to bending starts in the tensile zone, e.g: at the soffit of beams. 2 - Shear Cracks These are caused by structural loading or movement after the concrete has hardened. Shear cracks are better described as diagonal tension cracks due to the combined effects of bending and shearing action. Beams and columns are generally prone to such cracking. 3- Internal Micro-Cracks Micro cracking can occur in severe stress zones, due to large differential cooling rates, or due to compressive loading.

Non- Structural Cracks  Pre-hardening (Plastic) Cracks These cracks occur within a few hours after the placement and compaction of concrete, but before the concrete has fully hardened. 1.Plastic Shrinkage Cracks Caused by rapid drying of the concrete surface, within the first six hours (even within minutes) after placement, as a result of large moisture losses from the surface 2- Plastic Settlement Cracks Caused by concrete settling under its own weight, especially when there is excessive bleeding and the settlement is impeded by a local restraint. 3- Cracks Caused by Formwork Movement Movement of formwork after the concrete has started to stiffen but before it has gained enough strength to support its own weight, can cause cracking

 Cracks in Hardened Concrete Cracking in hardened concrete can be attributed to drying shrinkage (loss of moisture), early thermal contraction (movement) and structural and chemical effects. 1 - Craze Cracking Characterized by a series of very fine closely spaced map pattern cracks which are caused by the shrinkage of the cementitious material of the surface layer of concrete. 2 - Drying Shrinkage Cracks Occur when concrete reduces in volume as a result of moisture losses into the atmosphere in its hardened state. 3 - Early Thermal Contraction (Movement) Cracks All immature concrete elements are subject to thermal contraction or movement for up to 14 days after placement, due to temperature rise from the heat of hydration of the cementitious material. Non- Structural Cracks

 Cracks due to Chemical Effects The expansive effects of chemical reaction products from corrosion of steel reinforcement on alkali- aggregate reaction can also cause cracking in hardened concrete. 1- Corrosion of Steel Reinforcement Some cracks are induced by the expansive forces associated with corrosion of the steel reinforcement which crack and subsequently spall the concrete 2 - Alkali -Silica Reaction Cracks The chemical reaction between the alkali hydroxide in the concrete and reactive aggregates produces an expansive gel, causing map cracking or directional cracking (pre-stressed members) in the structure