For repairing concrete works following methods may be adopted: 1. By the Use of Expanding Cement Method 2. By Dry Pack Method 3. By Replacement of Concrete Method 4. By Mortar Replacement Method or Pneumatically Placed Mortar 5. Pre-Packed Concrete.
1. By the Use of Expanding Cement Method:
This method is specially useful for hydraulic structures. If small holes or cracks have developed in the structure, the hole or crack is cleaned and either concrete made of expanding cement or cement sand mortar is filled in the place. This cement expands upto 1% of its volume and fills space tight against the old surface, resulting water tight joint or repaired surface. It takes about 15 days for full expansion. Further development of shrinkage cracks in concrete can be eliminated by using a small quantity of expanding cement.
2. By Dry Pack Method:
This method is suitable for very small holes usually not deeper than 2.5 cms. The holes are made sharp and square on the surface and the interior portion is roughened and cleaned thoroughly and the dry pack material paste is filled.
The step wise procedure may be adopted as follows:
(a) The hole is prepared well by roughening the interior of the hole and cleaning it by blowing compressed air or by any suitable means.
(b) The hole is washed with the help of a water jet.
(c) The hole is allowed to dry. Over this dry surface a thin layer of cement sand mixture in the proportion of 1:1 is spread.
(d) Before the cement grout layer dries out, the dry packed material is put into the hole. The dry pack material is consisted of 1 part of cement and 2 part of sand. Water should be added in very small quantity to avoid shrinkage.
(e) Now the dry packed material is compacted well so that the compacted layer does not exceed 1 cms in thickness in any case.
(f) To make the good bond with the succeeding layer, the upper surface of the layer is made rough.
(g) The above procedure is repeated till the hole is filled fully and the last layer is finished to match the surrounding surface.
3. By Replacement of Concrete Method:
This method is used for holes 15 cms deep or more in old concrete. The hole is prepared by cutting it square at surface and the bottom and sides should be cut as sharp as possible. At least of 2.5 cms clearance must be available around the hole for proper working. Hence no reinforcement bars should be left partially embedded. The water/cement ratio of the new concrete should be kept same as that of old concrete as it will provide better bond. The shuttering or form work should be removed after 48 hours as during this period concrete will, gain sufficient strength.
i. The hole is prepared by cutting to proper size and roughened.
ii. The hole is washed with a jet of water and allowed to dry.
iii. A layer of sand and cement mixture is spread over the hole and form work adjusted.
iv. Now the concrete is filled and compacted properly.
4. By Mortar Replacement Method or Pneumatically Placed Mortar:
This method is used for repairing holes of moderate depths between 2.5 cms to 10 cms. In this method slurry of sand cement with sufficient water is shot into the hole under pressure through nozzle. The nozzle can be kept at least 1 m away from the defective surface. The hole should be cleaned and washed thoroughly before injecting the mortar. Usually the thickness of layer should not exceed 1.5 cms, but if required the thickness may be increased by applying successive layers at an interval of 45 to 60 minutes.
Advantages of Pneumatically Placed Mortar:
Following advantages of this method have been observed:
i. Ease of placement of mortar.
ii. Minimum need of form work.
iii. It gives high strength and longer durability.
5. Pre-Packed Concrete:
This method is specially useful for the repair of tunnel lining, dams and bridges, under water construction of bridge piers, dry docks etc. In this method the aggregate coarser than 6 mm is filled in the form and a grout mix consisting of 1 part of cement; 1/2 part of finely divided active silicious material, and 1/2 part of fine sand is pumped into the aggregate to fill the voids there. The resulting concrete is strong with practically no shrinkage as the coarse aggregate particles are in direct contact with each other. Due to this fact this system has proved to be an ideal one for patch work in hardened concrete.