How to Bond New Concrete or Plaster to Old Surfaces? - lceted

BONDING OF NEW CONCRETE TO OLD

How to Bond New Concrete to Old Concrete?

 

The main points to be noted in the bonding of new to old concrete are the following:

If the bonding of old and new concrete is not satisfactory, with differences in shrinkage of the old and new materials, the joint separates. Faulty joints result in the weakening of the whole structure and make it possible for water to percolate through these joints. Oil, grease, dirt, clay, scum, etc. must not be allowed to come into contact with the surface of the older concrete against which the new is to be placed, as these substances prevent good adhesion. As a rule, concrete should be carried out as continuously as possible but, as interruptions are inevitable, the following points should be observed.

 

Case (i). Where the concrete against which the new concrete is to be placed is not more than four hours old. At the end, where work is stopped as a layer a "laitance" film is usually formed, this being a chalky mass of non-cohesive material, loose in texture. It is actually very fine cement in the presence of an excess of water and it results in poor adhesion of new to old work.

It is, therefore, essential that this shall be removed before placing the new concrete against the old. One of the methods is to fill the forms slightly more than required and the excess is removed before the next concreting. This results in the removal of the poor concrete. This should be done not later than 4 hours after it has been placed in position.

An alternate method for joining to the fresh concrete surface, when the concrete has set but not hardened (is not more than 4 hours old), is by spraying the surface with a fine spray of water and brushing with a stiff brush to remove the outer mortar and expose the larger aggregates without disturbing them.

The new concrete is then added, the mix being sufficiently plastic to enable it to flow sluggishly in position when tamped. If the mix is too dry, the concrete at the bottom of each layer will be porous and if it is too wet, there may be segregation of the aggregates and the formation of laitance, and the work will shrink excessively on setting and hardening.

 

Case (ii). Where the old concrete has been in position for more than four hours but less than three days. In this case, the laitance should be removed as before; the old work brushed with a steel wire brush and thoroughly washed with clean water to remove loose particles, dirt, dust, etc. Cement mortar of composition similar to that in the concrete mix, excluding the large aggregate, should now be applied in a plastic consistency and thickness of about 12 mm (1/2 inch).

The new concrete is then placed upon this mortar grout and well punned towards the joint. Some prefer to use a neat cement slurry at the rate of 2.75 kg of cement per sq.m. instead of the above mortar.

 

Case If the concrete is too hard and where the above treatment is not practicable, sandblasting or a needle gun must be used to remove the surface skin and laitance. Hacking of the hardened surface should be avoided as much as possible.

 

Notes:

1. In the case of rapid hardening cement, the same methods can be adopted, but, owing to the rapidity with which hardening takes place, chipping is necessary for bonding to concrete over 24 hours old. On no account, must aluminous cement be bonded to ordinary cement concrete which has only partially hardened or vice versa, since interaction may occur which will greatly reduce the strength. If, however, the old work has hardened thoroughly, there is no objection to bonding it with a different type of cement.

 

2. Nowadays there are latex-based concrete chemicals that will assist in bonding new concrete to old concrete as used in the repair of concrete.

How to Bond New Concrete or Plaster to Old Surfaces

The old practice of bonding new concrete or plaster to old concrete or plaster surface was to chip the surface at close intervals to ensure bonding as in plastering of the ceiling in under the surface of slabs. This is not only laborious but can also damage the old structures. Instead, we can use the latex-based concrete bonding chemicals by making a slurry of cement with these chemicals and applying it to the surface to be repaired to act as a bonding surface.

Minor leakages in small water tanks and sumps can also be repaired by coating the walls with a slurry made of waterproofing agents, cement and latex-bonding chemicals.

 

3. Another method of designing two-stage works (such as concreting a fixture to a slab after the slab has been cast and cured) is to concrete the initial part using the lost formwork

 

What is Lost Shuttering?

Lost shuttering is different from temporary shuttering is left with the cast concrete and not removed and reused after concreting. It is a valuable device when holes are to be left in members such as slabs which later may or may not have to be filled up with special devices.

In such cases, steel mesh of close spacing, which can retain the aggregate, lines the holes. The mesh retains wet concrete and offers a rough key surface for the subsequent pours without the need for scabbing or other surface preparation. The technique in a modified form is used in countries where labour is costly in removing and re-oiling temporary formwork.

What is the Procedure to Join New Concrete To Old Concrete Slab