Foundation, Concrete and Earthquake Engineering

Surface Preparation for Commencing Concrete Repair Work

To make a composite concrete member having composite action, this bonding is very important and to achieve this it is very important to prepare the surface of concrete to be bonded with foreign repair materials. It is also important to provide bonding with reinforcement present in old concrete member. In this post we will learn about surface preparation techniques. We will discuss these in four consecutive posts.

In the figure below we described a typical removal area of spalled concrete to be repaired. In this figure we provide delaminated or loosen concrete boundary in the left side and recommended preparation layout for commencing repair work.

Fig-1: A typical layout for surface repair as an example
Typical Surface Preparation layout for concrete repair

For the convenience of the reader we will provide some sketch for example and images of some pectical work performed before repair work and finished concrete surface after repair work. For these repair positioning and placement of formwork is very important.

To provide perfect bond between old and new concrete the engineers and supervisors should carefully inspect the reinforcement that are exposed due to either corrosion or come out during removing loose and delaminated concrete. These include:

Exposing reinforcing bar before repair work of a concrete column
a. Access clearance

b. Cross-sectional area of reinforcement, if the section is corroded.

c. Location of reinforcement

When reinforcing steel is not properly bonded with the prepared concrete (dismantled delaminated and spalled concrete free surface) then the reinforcing bar has to be exposed to a further depth. Normally the further exposing is done when the prepared surface does not reach to half of the perimeter of reinforcing bar. When it reached to more than half of the perimeter of reinforcing bars, the bar is exposed completely. 

Normally a disc type mechanical cutter is used to expose bar without reducing or cutting the remaining reinforcing bars. Now we will discuss the remaining task to be done for removing loose concrete around reinforcing bars. 
Concrete repair: Corroded reinforcement, sectional reduction more than 15%

When fully exposing is concerned, a question arises how much gap between reinforcement and prepared concrete should be kept. The clearance is

Average clearance=25 m

Average clearance= (Maximum aggregate size+5) mm

Greater of the two are provided as a clearance between reinforcement and concrete surrounding it.

So far we learned about removing loose concrete and making clearance between them. Now we have to observe the condition of reinforcing bars. The things to be noticed that

a. For individual bar, cross-sectional area must not reduced ≥15%

b. For adjacent two bars, cross-sectional area must not reduced ≥10%

c. Loose reinforcement and its location in out of plane should rearrange and relocated.

For the above cases a structural engineer should be appointed to justify the further progress can be carried out. Generally sand blasting the concrete surfaces is adopted to prepare a concrete surface and that of reinforcement to have optimum bonding. Then debris and dust are removed by following procedure:

Anti-corrosive coating to reinforcement before concrete repair work
a. Booming or

b. Air blasting or

c. Water blasting with low pressure

In case of exposed concrete surface, a coating that suit best is applied. This should keep in mind that the rust from the surface of reinforcement must be removed thoroughly. The consideration of coating is taken where corrosion environment is found around the concrete member or existing reinforcement in concrete is found corroded. We have already discussed about allowance of degree of corrosion. With all types of measures undertaken carefully, finally engineer should inspect for any other deficiencies that may be overlooked before and take remedial action instantly to start next phase of repair work.