What Are The Types Of Defects In Concrete And Its Repair | Causes | Lceted

 

WHAT ARE THE TYPES OF DEFECTS IN CONCRETE AND ITS REPAIR

What Are The Post Concrete Defects?

Post concrete inspection is a tool to identify the possible general defects in newly hardened concrete. The typical defects are: -

a.   Occurrence of surface blemishes.

b.   Honeycomb.

c.    Cavity holes are due to tie rods.

d.   Various types of cracks in concrete.

             i.        Plastic shrinkage cracks.

           ii.        Plastic settlement cracks.

          iii.        Drying shrinkage cracks etc.

 

A. OCCURRENCE OF SURFACE BLEMISHES.

The exposed surface area of concrete is blemished when spots or area vary noticeably and detract from the appearance of surrounding surface areas. Before treatment of blemishes, the contract documents should be referred to for identifying the acceptable level of blemishes for the various surface to be produced under the terms of given in the contract.  Any repair process for the treatment of blemishes so chosen should also be  evaluated to determine if it would produce a less apparent and more  pleasing appearance than the original blemish

Repair

When a concrete surface is noticed with air bug holes:

Site engineers can follow either of the following methods for corrective action.

1)  Sack rubbing to repair the blemishes and the fill air bug holes.

2)  Action should be such that the treatment is a less apparent and more pleasing appearance.

3)  Such treated surface should be cured properly

 

HONEYCOMB

During concreting if the voids left in the concrete due to the failure of mortar to effectively fill the space among coarse aggregate then the structure is said to have the defect of honeycomb. Such  defects occur due to:

·       Congested reinforcement.

·       Improper vibration.

·       Difficult construction.

·       Improper mix i.e. improper aggregate ratio of fine aggregate to total aggregate.

·       Leakage of slurry from formwork gaps or damaged formwork.

Site engineers can follow either of the methods to minimize the defects:

·       Ensure proper vibration.

·       Ensure proper consistency using water-reducing admixture to increase slump.

·       Ensure no gaps in the formwork to prevent slurry leakage.

·       Using polymer modified cement sand mortar – by the hand of towelling – this method can be used if honeycomb is less than the concrete cover.

·       If it is more than concrete cover that the area must be chipped off and concrete must be replaced with normal in-situ concrete of the same grade.

 

CAVITY HOLES DUE TO TIE RODS.

Tie rods, made of reinforcement steel, are often used to ensure that the form assembly remains intact when subjected to concrete pressure during concreting process. These tie rods are fixed with cones at both ends. These cones are open from the outer end hence when the formwork is removed, they are visible as holes. As this rod is made of reinforcement and its ends are open to the atmosphere, if left expose for a long period, they are susceptible to correction.

Repair Method: By Dry Pack Mortar

 

CRACKS IN CONCRETE

This defect permits the ingress of the aggressive agent in the structure and adversely affect it.

A.  The durability.

B.   Water tightness.

C.  The integrity of structure i.e. strength

Sound transmission: Crack through NDT techniques

Various types of cracks in concrete

PLASTIC SHRINKAGE CRACKS: This caused due to repaid loss of moisture due to the temperature of ambient air and concrete, relative humidity and prevailing wind velocity at the surface of the concrete.

Site engineers should take the following precautions to prevent rapid moisture loss due to hot weather and dry wind.

·       Covering the concrete with a plastic sheet.

·       To provide sunshade during concreting.

·       Use fog nozzles to saturate air above the green concrete surface.

·       Use of windbreaker to reduce wind velocity.

 

PLASTIC SETTLEMENT CRACKS: concrete is consolidated after initial placement vibration and finishing. The plastic concrete generally gives local retrain by reinforcement etc. and result from cracks or known as plastic settlement cracks.

The site engineer should follow the following preventive measure to minimize the plastic settlement cracks.

·       Maintain proper cover in concrete.

·       Provision of the time interval between placement of concrete in slabs and beams.

·       Use or minimum slump concrete.

 

Drying shrinkage cracks: When concrete is wet, it tends to expand.  It reduces its volume when loss of moisture from cement paste occurs.  It can be explained in simple words. If this shrinkage can occur without any restrain no cracks will appear. But a combination of shrinkage and restrain provided by another part of a structure or by subgrade develops the tensile stress in concrete. This is resulting in a crack known as drying shrinkage crack.

Site engineers should follow the following methods to reduce these effects:

·       Use less w/c ratio in a concrete mix.

·       Use a large amount of aggregate.

·       Proper consolidation of concrete.

 

REPAIRING OF CRACKS:  In spite of precautions observed some times cracks are seen in the concrete surface and need repair. The  repairing of cracks mainly depends on:

·       Location and width of cracks.

·       Extent of crack

·       General condition of the concrete.

 

Following methods can be used for the repair of cracks: -

1)  GROUTING: Wide crack can be filled by either cement grout or various chemical grouts.

2)  Dry packing: Method used for minor dormant cracks but not effective for active cracks.

Cracks are widened to a slot of 15-20mm wide and 20-25 deep with base width slightly more than surface width. This is packed by cement mortar with a low w/c ratio.

3)  Gravity filling: Cracks can be filled up with low viscosity epoxies.

4)  Routing and sealing: - This method is used where structural repair is not necessary. This procedure is more accurate for flat and horizontal surfaces like floors/pavements. These methods can be commonly used for waterproofing by sealing cracks on the concrete surface a concrete surface, where water stands or hydrostatic pressure applied.

5)  Injection grouting: - Repairing of narrow crack width up to 0.05  mm can be done. In this method, GI or PVC nozzle is drilled in the cracked over up to the depth of the thickness of half the member and grouting is done.

 

Every Engineer has to deliver the output based on the requirement and facing the challenges to meet the quality standards with optimum use of manpower. It is needless to say that in a competitive environment, the restricted and compressed time schedule, the structural adequacies for the various problems need to be reviewed as listed below: -

·       overall strength and stability of the structure.

·       Surface finish and appearance of the structure i.e. crack leaching discolouration etc.

·       Effect of leakage /seepage and effect of un-stability due to local parameters like improper backfilling/ undulations etc.

·       Durability with respect to corrosion, carbonation and sulphate attack.

Various type of defect and damages in structures have been noticed over a  period of time in various localities this has resulted from a need for an  Engineer to understand these defect and they are respective possible causes. Responsibility is not only limited to identification of the damages but requires to a recommendation of necessary corrective measures including investigations if necessary.

 

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