Common Defects in Concrete:
What is Defective Concrete?
Defective concrete is one which does not meet the specification requirements of strength, and generally, acceptance criteria are laid down based on the percentage of defectives. i.e. specimens are falling below the stipulated strength. A concrete which shows defects on the surface during construction or within a short time after completion of concrete which shows cracking either before hardening or after hardening is also called defective.
How do we know?
The concrete is not meeting the strength requirements as indicated by the low compressive Strength but which is apparently good will not often get detected until when it cracks or fails under service conditions. After detection, investigations are undertaken, and costly repairs are planned.
Concrete surface defects:
Generally, concrete surface defects are
- Air pockets land bolt holes
- rock pockets and grovels streaks
- sand streaking
- rust stains
- surface scaling.
The above defects are usually caused by poor quality materials, improper mix design, and lack of proper placing, compacting and curing procedures. The repairs of the surface defect are both difficult and costly, and even the best repair work will not be as good as an original properly finished surface.
Causes, prevention and repair of Concrete surface defects:
Honeycombing in Concrete:
This is caused by a harsh dry mix or a non-cohesive mix of a watery consistency, improperly placed and not properly consolidated. Attention to proper mix design, good aggregate grading, proper placing and systematic vibration or rodding will avoid the occurrence of honeycombed patches. Removing the defective concrete and replacing it with good concrete is the only effective method of repair.
Air pockets and Bolt holes in Concrete:
Air pockets on the formed surface occur due to application of the excessive amount of form oils or due to the use of over-sanded mixes. Excess oil on the form causes the air bubbles to stick to the surface, and the over-sanded mix makes it more difficult for the bubbles to escape upward through mortar.
Here also the use of well-designed mix and proper placing procedures controlling the lift heights – not more than 50 cm – and consolidation of each layer by vibration with precaution to packing along the forms should remove the air pockets. Air pockets and bolt holes can be filled by dry pack mortar. This should be carefully packed in place in small amounts.
Rock pockets and gravel streaks in Concrete:
The loss of cement or mortar through cracks in the forms causes such rock pockets. This is more likely to occur when the concrete is consolidated by vibration. This surface defect is very difficult to repair.
Sand streaking in concrete:
The use of wet, over-sanded mix that bleeds excessively is the major cause of sand streaking on the formed surface of the concrete. Stiffer mixes properly vibrated, or air-entrained concrete which is more cohesive and bleeds less do not develop this defect. Such a defect is difficult to repair.
Laitance in Concrete:
It is a mixture of cement, fine sand and water that accumulates on the surface when wet concrete mix that bleeds excessively are used. Laitance leaves a light coloured streak of poor concrete between successive lifts; it has high permeability and is vulnerable to freezing and thawing. Laitance on the top of the concrete lift should be completely removed by wet sandblasting before the next lift is placed.
Rust stains in Concrete:
These are caused by reinforcing steel or some iron-bearing mineral substances in aggregates. In the process of tying by binding wire or in welding reinforcing bars in place, pieces of wires or weld rods of molten weld metal are likely to drop in the forms, which on rusting may cause stains. Reinforcing rods with little or no cover would start staining on oxidation. Rust stains can be removed by washing the surface with oxalic acid.
Dusting in concrete:
Presence of excess clay or silt will cause dusting. Premature floating and trowelling will bring an excess of fine materials and water to the surface to produce al soft cement paste. Spreading dry cement on the concrete surface for early finishing will cause dusting. Inadequate curing also causes dusting. Surface treatment with al solution of magnesium or zinc fluosilicates or sodium silicate will harden the surface and reduce dusting.
Surface scaling in concrete:
This is caused by improper finishing and curing operations or by application of salts for snow and ice removal in case of non-air entrained concrete. The materials and finishing and curing operations that cause dusting also cause thin scaling.
Cracks in Concrete:
The other type of defect in concrete is cracking. There are many types of cracks, e.g., surface, shallow deep and thorough. Regarding pattern, there are two types
(1) map cracks
(2) single continuous cracks.
Map cracks are uniformly distributed short cracks running all over roughly in a hexagonal pattern, whereas continuous cracks run indefinite direction often parallel at the definite interval. They indicate restraint in the direction perpendicular to them. Besides the above types, there are internal cracks around larger stone pieces, or unsoundness cracks form chemical reaction or cracks along reinforcing bars due to corrosion.
prevention and repair of Concrete Cracks:
Factors affecting cracking have been discussed earlier. Proper understanding of the various reasons is very much essential while designing, selecting materials and methods of construction to prevent cracking or at least keeping it down to an acceptable level. Cracks once appeared cannot be repaired until they assume a serious form resulting in the spoiling of concrete or widespread damage. Cracking should, therefore, be prevented by careful attention to various details of design and construction, ensuring good quality of concrete and by proper design.