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Concrete is the composite material that is composed of a mixture of fine and coarse aggregates bonded together with biding material which hardens over time. The binding material mostly includes fluid cement paste, lime putty, lime, etc. In simple language, Concrete is a mixture of cement, sand, aggregates, and water.
1. Pre-Packed Concrete
Generally, concrete is made by mixing all the ingredients in the mixers. Then the mixed concrete is placed in formwork. A concrete member may also be constructed by placing the coarse aggregates in the mold and then grouted with a specially prepared mortar. This process can be employed for both plain concrete and reinforced concrete.
2. Construction of Pre-Packed Concrete
This method of concrete construction is employed where the reinforcement is complicated or where certain arrangements like pipes, and conduits, openings, and other arrangements are required to be incorporated into the concrete. The normal method of concreting may disturb the pre-planned fitments.
This method is also employed in mass concreting, in bridge abutments and piers, well steinning, etc. This is one of the practicable methods of concreting under water wherein aggregate is placed in water and subsequently mortar is grouted in, to displace the water.
There are many proprietary methods in vogue. They are called, intrusion grouting, grouted concrete, pre-packed concrete colcrete process, etc.
3. Advantages of Pre-Packed Concrete
The advantages of pre-packed concrete are that it undergoes little drying shrinkage. As the aggregate is in point contact with each other and the grout only fills the voids. the concrete as a whole doesn’t undergo much drying shrinkage. Vibrating the aggregate before grouting makes the quality of concrete better. Both single-sized or graded aggregates can be used without much difficulty.
4. Requirement of Pre-Packed Concrete
The essential requirement is that the grout should fill the voids fully and develop a full bond with the aggregate. The right extent of pressure is applied so that the grout, just fills the aggregate but doesn’t disturb or lift up the aggregate. Sometimes, the workability agent or expanding admixture such as aluminum powder is also added. The grouting pressure is changed depending upon the depth at which the injection of grout is done.
The grout mixture should be sufficiently fluid for pumping. The fairly thick grout, well mixed in the high-speed machine which has got a pumpable consistency is referred to as colloidal grout. Such grout can travel uniformly into the aggregate voids. The fluidity of grout and keeping its identity while pushing the water out in case of underwater grouting is the property to be looked tor in the grout.
In the concrete process, the mortar grout is made in a special high speed, double drum type of mixer in which cement and water are mixed in one drum and the sand is added and mixed with the cement slurry in the second drum. The high-speed action of the mixers produces a very intimate mix, which is more fluid than a normal grout and is comparatively immiscible with water. The ratio of the grout may range from 1: 5/4 to 1: 4, the rich being used for underwater work or for grouting prestressed concrete members.
5. Method of Grouting
Grouting is done by these methods:
(a) Grout mixture is poured on the packed aggregate and allowed to penetrate. downwards. This method is normally adopted for thin concrete members, such as road pavement slabs and floor slabs. If the depth of the aggregate bed is thicker, it cannot be assumed that the grout has completely traversed through the entire thickness.
b. The mold is partially filled up with grout over which the aggregates are deposited. Vibration at this stage would help distribute the sinking and smearing of paste over the aggregate.
c. The Grout is pumped into the prepacked aggregate at the bottom of the mold. The injection of a pipe full of perforations, one or more in number distributes the grout into the concrete. This injection pipe is extracted upwards by a certain distance and again the grout mixture is sent. In this way, the entire depth of the concrete member is concreted. It should be noted that less pressure is to be used to inject the grout into the top layers of concrete.
Alternatively, perforated horizontal pipes are embedded in the repacked aggregate at different levels. Grout may be sent through these pipes. Finally, it is possible to withdraw the pipe or the pipe may be left in the concrete.
Sometimes, if the grout is forced at high pressure without any regard for the overburden pressure of packed aggregate, the pressure may cause dislodging of the aggregate with the result that excess grout may be consumed. Therefore, the pressure at which the grouting near the top surface. Sometimes, a plywood covering is fixed to the top to obviate the floating effect of the aggregate in the event of high pressure and low viscosity of the grout.
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