Special Methods of Placing Concrete For Repair works

 

Special Methods of Placing Concrete For Repair works

During repair work, in many situations, workers cannot place concrete in the same way as during construction. Here, we consider some special concrete placement methods used in repair work.

 

Self-compacting Concrete Method

As the name suggests, self-compacting concrete is concrete that can be placed in formwork without rodding or vibrators. The concrete slump flow is at least 500 mm-700 mm. The use of this type of concrete has become very popular because it can be pumped like a fluid. This method of placing concrete was developed by the Japanese in 1918. This reduces the noise of placing concrete and helps to place concrete in heavily reinforced sections where vibrations cannot be inserted. Thus, self-compacting concrete is a concrete mix suitable for placing it in a formwork without vibration. It should be a highly flowable mixture which should not separate and it should be able to spread and fill the formwork automatically without any mechanical vibration. These properties are achieved by

 

1. Limiting the maximum size of aggregate to 10mm to 12.5mm instead of the usual size of 20mm which we follow in general works.

 

2. Use good sand with fine aggregate

 

3. Make the concrete mix more homogenous by using an inorganic admixture such as silica fume or fly ash (this is an important addition to compacting itself with ordinary concrete.)

 

4. Use of superplasticizer (The main function of the long molecules of this material is to wrap around the cement particles and give them a negative charge that repels each other, making the mixture more fluid and free-flowing.)

 

5. Use of viscosity-changing agents (viscosity-improving chemicals)

 

6. As there is no guarantee of good performance of a given superplasticizer and a given cement, the trial investigation should always be carried out before finalizing the mix for actual work.

 

Preplaced Aggregate Concrete Method

Preplaced concrete is concrete formed by forcing grout into the voids of clean, graded coarse aggregate pre-placed in a formwork as shown in the figure below. It is very useful for underwater construction. Grout consists of sand, cement, and pozzolana plasticizer/superplasticizer (to improve grout flow).

 

Shotcrete/Gunite Method

Shotcrete or gunite is the sprayed or pneumatically placed Portland cement mortar or concrete, as described in IS 9012–1978. This type of construction is very much used in the repair work of large areas. There are two methods of placing concrete or mortar by this method. They are the dry mix process and the wet mix process, the dry mix process being more extensively used for structural repair. These methods are shown in the Figure below. Both consist of two machines—one is the compressor from which we get compressed air for guniting and the other is the material machine in which the concrete or mortar mix is stored.

 

In the dry mix process, a mixture of dry cement and aggregates is blown through the pipe along with a high volume of high-pressure air and to this, water is added in the form of fine spray at the nozzle. In this case, any admixture like an accelerator can also be added either as a liquid along with the water or preblended with the dry mixture of cement and aggregates. Accelerators with an initial set of 3 minutes and a final set of fewer than 12 minutes are often used in this type of work.

 

In the wet process, the premixed wet concrete or mortar mixture is pumped through a pipeline to the nozzle from where it is forcefully expelled by compressed air.

 

In shotcrete, we can also introduce fibres of steel or other materials in the concrete to make it extra strong.

 

Some use the name shotcrete for sprayed concrete with a maximum size of aggregate 10 mm or greater and gunite for concrete with a maximum size of aggregate less than 10 mm.