The term precast refers to the fact that the cement concrete blocks are formed and hardened before they are brought to the job site. They are manufactured in a factory under controlled conditions ensuring the production of high-quality blocks.
The principle ingredients of precast concrete blocks are cement, sand, aggregate, and water. It is produced in a variety of shapes and sizes, using manual or machine construction technique. Precast cement blocks can be either hollow or solid. The former is most commonly used and helps to reduce the dead load. A typical concrete block weighs around 17.2-19.5 kg (38-43 lb).
Common sizes of precast concrete units are 400, 500 or 600mm length, 200 or 100mm height, and 50, 75, 100, 200, 250 or 300mm width. Tolerances specified by applicable standards shall be considered while these precast blocks are produced.
These units are widely used in construction, for instance: in masonry walls because of their superior properties such as durability and resistance to rain, fire, and inclement environmental conditions. Workmanship, also, plays a very crucial role in the success of structure. Therefore, suitable construction procedure shall be considered while precast units are laid.
Production of Precast Cement Concrete Blocks
1. Batching and mixing
2. Adding water to the mixture
3. Moulding using manual machine, partially manual, or fully mechanized machine.
4. Demolding
5. Cover the block for 24 hours after demoulding
6. Curing blocks for 7 days
Dimensions of Precast Blocks
Concrete masonry building units shall be made in sizes and shapes to fit different construction needs. They include stretcher, corner, double corner or pier, jamb, header, jamb, bull nose, and partition block, and concrete floor units.
Concrete block, hollow (open or closed) or solid, shall be referred to by its nominal dimensions. The nominal dimensions of the block shall be, as follows:
Length : 400, 500 or 600 mm
Height : 200 or 100mm
Width : 50, 75, 100, 200, 250 or 300mm
In addition, block shall be manufactured in half lengths of 200, 250 or 300mm to correspond to the full lengths.
Tolerances
The maximum variation in the length of the units shall not be more than +5mm and maximum variation in height and width of the unit, not more than +3mm.
Precast Block Masonry Construction
For single storey buildings, the hollows of blocks in foundation and basement masonry shall be filled up with sand and only the top foundation course shall be of solid blocks.
However, for two or more storey buildings, solid concrete blocks shall be used in foundation courses, plinth, and basement walls, unless otherwise indicated.
If hollow blocks are used, their hollows shall be filled up with cement concrete 1:3:6 using 12.5 mm nominal size aggregates.
1. Wetting of Blocks
Blocks need not be wetted before or during laying in the walls. In case of hot climate conditions, top and sides of blocks may only be slightly moistened so as to prevent absorption of water from the mortar and ensure the development of the required bond with mortar.
2. Laying
Blocks shall be laid in mortar, as indicated and thoroughly bedded in mortar, spread over the entire top surface of the previous course of blocks to a uniform layer of not less than 10 mm and not more than 12mm in thickness.
All course shall be laid truly horizontal and vertical joints be made truly vertical. Blocks shall break joints with those above and below for not less than the quarter of their length. Precast half-length closer and not cut from full-size blocks shall be used.
For battered face, bedding shall be at right angle to the face unless otherwise directed. Care shall be taken during construction to see that the edges of blocks not damaged.
3. Provision for Door and Window Frames
A course of solid concrete block masonry shall be provided under door and window openings(or a 10 cm thick precast concrete sill block under windows). The solid shall extend for at least 20 cm beyond the opening on either side.
For jambs, very large doors and windows either solid units are used, or the hollows shall be filled in with concrete of mix 1:3:6 using 12.5 mm nominal size aggregates.
4. Intersecting Walls
When two walls meet or intersect and the course is to be laid up at the same time, a true masonry bond between at least 50% of the units at the intersection is necessary.
When such intersecting walls are laid up separately, pockets with 20mm maximum vertical spacing shall be left in the first wall laid. The corresponding course of the second wall shall be built into these pockets.
5. Provisions for roof
The course immediately below the roof slab shall be built with solid blocks. The top of the roof course shall be finished smooth with a layer of cement and coarse sand mortar 1:3 and 10 mm thick. The roof course shall be covered with a thick coat of whitewash or crude oil to ensure free movement of the slab.
Advantages of Precast Concrete Masonry
- High compressive strength
- A Good fire and abrasion resistance
- Very good stability
- Rapid construction
- Hollow units have low self-weight
- Reinforcement can be passed through hollow units and filled with concrete to increase earthquake resistance
- Air-space (hollow units) provides good thermal insulation