micro-cracking of concrete

Micro-Cracking Of Concrete

Cracking of concrete can be defined as a separation of the individual components of concrete resulting in a discontinuous material. Depending upon the extent of cracks can be classified as microcracks, microcracks, and semi-microcracks.

According to the location, the cracks can be classified as bond cracks, mortar cracks, and aggregate cracks. the bond cracks are formed at the interface of the aggregate and mortar, wheres the mortar cracks and aggregate cracks are formed through the mortar and the aggregate, respectively.

A knowledge of the micro-cracking of concrete contributes considerably to the understanding of its inelastic nature, the descending portion of the stress-strain curve, the strength under combined, repeated and sustained loading etc. The progressive cracking in concrete with increasing strain has been indirectly determined by measuring the lateral expansion contraction, surface cracking and by sonic methods. The exact nature of cracks and the strains at which they occur give precise information pertinent to the mechanism of cracking.

Micro-cracking effects on the strength of concrete 

Micro-cracking effects on the strength of concrete 

• The bond strength between the aggregate and mortar will be less.
• The compressive strength of concrete will be less but does not effect in tensile strength of concrete.
• Extensive continuous cracks are developed, the load carrying capacity of concrete will be decreased.
• With progressive cracking in concrete, increasing strain with lateral expansion and after sometime stresses will be developed.
• These cracks are very fine microscopic cracks within the concrete to therefore life of concrete will reduce.

                    You should understand something about microcracking for two reasons. First, how watertight a concrete structure is can be affected by the extent of microcracking. Water moving through these small channels obviously goes slowly, but the more microcracking there is, the less watertight the concrete element. That's particularly important where chloride ions in the water might reach reinforcing steel embedded in the concrete and cause corrosion problems.

                      The other reason to know about this phenomenon is that microcracking can act as a starting point for the development of larger cracks. If you develop a deep curiosity in this subject, a thorough explanation can be found in ACI 446.1R-91, “Fracture Mechanics of Concrete.” But the main point is that overloading or demolition activities can lead to compressive microcracking in concrete. That makes it easier for larger cracks to form and for repairs to fail.


                       ACI 546R-04, “Concrete Repair Guide,” describes localized microcracking that can result from concrete removal activities. “Removal of concrete using impact tools may result in small-scale microcracking damage (termed bruising) to the surface of the concrete left in place. Unless this damaged layer is removed, a weakened plane may occur in the parent concrete below the repair material bonding.

                         The repair guide explains that when “aggressive impact methods,” such as a boom-mounted breaker, are used to remove concrete prior to a repair, “the concrete left in place should be prepared by using a secondary method, such as chipping, abrasive blasting, or high-pressure water jetting, to remove any remaining damaged surface material.” It also notes that saw cutting around the area of concrete to be removed can limit microcracking due to removal operations. Section 2.7 provides an explanation of various content removal methods and a table comparing the features, considerations, and limitations o each.

                         Microcracking is also a concern when drilling hardened concrete, such as for dowels or anchors. The commentary provided with ACI 318-08, “Building Code Requirements for Structural Concrete,” specifically notes that “drilling holes for post-installed anchors can cause microcracking,” explaining that this is a factor in the minimum edge distance requirements for installing anchors.