Cracking in Concrete

Learning Basic Knowledge As Civil Engineer

Cracking in concrete

ohayo gozaimasu.  

• As Civil Engineer, cracking in concrete is hard to determine either it can affected the strength of structural or the structural is safe to be use.  
• Based on experience working as Civil Engineer, the crack issue sometime become complicated to resolve between contractor and consultant. 
• So far in Malaysia there no guide line and standard procedure that shall be follow in order to resolve this cracking issue. 
• Please be reminded that crack and honeycomb is not a same. If honeycomb even it seen as severe honeycomb, it still can be repaired by proposing using grout cement material. However, cracking in concrete is sometime more difficult to handle. Even it seen barely visual, it is hard to prove to designer that the integrity of structure is not affected.
• Many cases, when this cracking in structure raised up to top management, normally main consultant will reject the structure with reason the cracking in concrete are not acceptable to be use because the strength and durability of structure is affected and questionable unless the contractor can prove otherwise.
• If the structure such as sump, drainage or any structural as long it not involving the public and live load, it still can be acceptable by main consultant. Normally the cracking in concrete will be come to argument for the pre stressing structure such as Precast Beam, Precast Box Girder etc because of it is main structure and the cost is very expensive.
• Concrete is basically a mixture of three components: aggregates, water and cement. The aggregate component is normally comprised of sand and gravel or crushed stone.
• The cement component is normally comprised of cementing materials, (portland cement with or without supplementary cementing materials), water, and air.

How concrete cracks happened?

Contributing Factors

•      Low-strength concrete: Concrete is designed for compressive strength, and improved or increased tensile strength is generally incidental to improved compressive strength.  Lower strength concrete will have lower tensile strength and be more likely to crack, given equal stress magnitudes. 

•      Poorly consolidated concrete: Poor consolidation causes loss of cross sectional area, and provides opportunities for cracks to initiate. 

•      Unanticipated stresses: Stresses of magnitudes, and especially directions that were not anticipated in the initial design of the structure may cause unanticipated cracks.  Examples of this kind of structural cracking include diagonal cracks at corners of slabs, or at re-entrant corners in building slabs or building facades. 

Type of Crack

•      Plastic Shrinkage Cracking

•      When water evaporates from the surface of freshly placed concrete faster than it is replaced by bleed water, the surface concrete shrinks.

•      Due to the restraint provided by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks of varying depth.

•      These cracks are often fairly wide at the surface.

•      It common happened during construction because of in consistence of delivery of concrete supply.

•      This crack can easily to control by using gunny bag or keep concrete temperature low by spraying water on the surface.

Drying Shrinkage:

•      Because almost all concrete is mixed with more water than is needed to hydrate the cement, much of the remaining water evaporates, causing the concrete to shrink.

•      Restraint to shrinkage, provided by the subgrade, reinforcement, or another part of the structure, causes tensile stresses to develop in the hardened concrete.

•      Restraint to drying shrinkage is the most common cause of concrete cracking. In many applications, drying shrinkage cracking is inevitable.

•      It common happened at jointing between to structure. Therefore, contraction (control) joints are placed in concrete to predetermine the location of drying shrinkage cracks.

 D-cracking

•      It is can be describe as post construction cracking because It is a form of freeze-thaw deterioration that has been observed in some pavements after three or more years of service.

•      Due to the natural accumulation of water in the base and subbase of pavements, the aggregate may eventually become saturated.

•      Then with freezing and thawing cycles, cracking of the concrete starts in the saturated aggregate at the bottom of the slab and progresses upward until it reaches the wearing surface.

•      D-cracking usually starts near pavement joints.

Alkali-aggregate reaction:

     Alkali-aggregate reactivity is a type of concrete deterioration that occurs when the active mineral constituents of some aggregates react with the alkali hydroxides in the concrete.

Alkali-aggregate reactivity occurs in two forms—alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR). 

     Indications of the presence of alkali-aggregate reactivity may be a network of cracks, closed or spalling joints, or displacement of different portions of a structure.

Thermal cracks

     Temperature rise (especially significant in mass concrete) results from the heat of hydration of cementations materials.

      As the interior concrete increases in temperature and expands, the surface concrete may be cooling and contracting. 

     This causes tensile stresses that may result in thermal cracks at the surface if the temperature differential between the surface and center is too great. 

     The width and depth of cracks depends upon the temperature differential, physical properties of the concrete, and the reinforcing steel.

Loss of support 

     Beneath concrete structures, usually caused by settling or washout of soils and sub base materials, can cause a variety of problems in concrete structures, from cracking and performance problems to structural failure. 

      Loss of support can also occur during construction due to inadequate formwork support or premature removal of forms.

 Corrosion: 

     Corrosion of reinforcing steel and other embedded metals is one of the leading causes of deterioration of concrete.

      When steel corrodes, the resulting rust occupies a greater volume than steel. 

     The expansion creates tensile stresses in the concrete, which can eventually cause cracking and spalling.

Crazing 

     Crazing is a pattern of fine cracks that do not penetrate much below the surface and are usually a cosmetic problem only. 

     They are barely visible, except when the concrete is drying after the surface has been wet.

As Engineer, we shall understand about crack type so that we can provide method of rectification. except loss of support crack, there rest of of crack can be repair by using non shrink grout material. but some time, it hard to explain to consultant that some of crack not affected the strength and durability of structure. 

Next time i will sharing more about rectification for crack in concrete. 

sayonara, mata ne