FYCE

1. A precast concrete block is primarily used as a building material in the
construction of walls. It is sometimes called a concrete masonry unit (CMU).
A concrete block is one of several precast concrete products used in
construction. The term precast refers to the fact that the blocks are formed
and hardened before they are brought to the job site. Most concrete blocks
have one or more hollow cavities, and their sides may be cast smooth or with
a design. In use, concrete blocks are stacked one at a time and held together
with fresh concrete mortar to form the desired length and height of the wall.
Concrete mortar was used by the Romans as early as 200 B.C. to bind
shaped stones together in the construction of buildings. During the reign of the
Roman emperor Caligula, in 37-41 A.D., small blocks of precast concrete
were used as a construction material in the region around present-day Naples,
Italy. Much of the concrete technology developed by the Romans was lost
after the fall of the Roman Empire in the fifth century. It was not until 1824 that
the English stonemason Joseph Aspdin developed portland cement, which
became one of the key components of modern concrete.
Fig: Precast Concrete Blocks

2. The first hollow concrete block was designed in 1890 by Harmon S. Palmer in
the United States. After 10 years of experimenting, Palmer patented the
design in 1900. Palmer’s blocks were 8 in (20.3 cm) by 10 in (25.4 cm) by 30
in (76.2 cm), and they were so heavy they had to be lifted into place with a
small crane. By 1905, an estimated 1,500 companies were manufacturing
concrete blocks in the United States.
These early blocks were usually cast by hand, and the average output was
about 10 blocks per person per hour. Today, concrete block manufacturing is
a highly automated process that can produce up to 2,000 blocks per hour.
Raw Materials for precast concrete blocks:
The concrete commonly used to make concrete blocks is a mixture of
powdered portland cement, water, sand, and gravel. This produces a light
gray block with a fine surface texture and a high compressive strength. A
typical concrete block weighs 38-43 lb (17.2-19.5 kg). In general, the concrete
mixture used for blocks has a higher percentage of sand and a lower
percentage of gravel and water than the concrete mixtures used for general
construction purposes. This produces a very dry, stiff mixture that holds its
shape when it is removed from the block mold.
If granulated coal or volcanic cinders are used instead of sand and gravel,
the resulting block is commonly called a cinder block. This produces a dark
gray block with a medium-to-coarse surface texture,

3. good strength, good sound-deadening properties, and a higher thermal
insulating value than a concrete block. A typical cinder block weighs 26-33 lb
(11.8-15.0 kg).
Lightweight concrete blocks are made by replacing the sand and gravel
with expanded clay, shale, or slate. Expanded clay, shale, and slate are
produced by crushing the raw materials and heating them to
about 2000°F (1093°C). At this temperature the material bloats, or puffs up,
because of the rapid generation of gases caused by the combustion of small
quantities of organic material trapped inside. A typical light-weight block
weighs 22-28 lb (10.0-12.7 kg) and is used to build non-load-bearing walls and
partitions. Expanded blast furnace slag, as well as natural volcanic materials
such as pumice and scoria, are also used to
make lightweight blocks.
In addition to the basic components, the concrete mixture used to make
blocks may also contain various chemicals, called admixtures, to alter curing
time, increase compressive strength, or improve workability.

4. The mixture may have pigments added to give the blocks a uniform color
throughout, or the surface of the blocks may be coated with a baked-on glaze
to give a decorative effect or to provide protection against chemical attack.
The glazes are usually made with a thermosetting resinous binder, silica
sand, and color pigments.
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 a great resistance
to rain, fire, and inclement environmental conditions. Workmanship, also,
plays a very crucial role in the success of masonry structure. Therefore,
suitable construction procedure shall be considered while precast units are
laid.

5. Production of Precast Cement Concrete Blocks
1. Batching and mixing
Fig. 1: Batching and Mixing
2. Adding water to the mixture
Fig. 2: Add Water to the Mixture
3. Moulding using manual machine, partially manual, or fully mechanized
machine.
Fig. 3: Moulding

6. 4. Demolding
Fig. 4: Demolding the Block
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 concrete block shall be,
as follows:
Length : 400, 500 or 600 mm
Height : 200 or 100mm

7. 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.
Fig. 5: Hollow Precast Concrete Block
Fig. 6: Solid Precast Concrete
Block
Tolerances

8. 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

9. 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 are not damaged.
Fig. 7:
Masonry Wall Constructed from Precast Blocks
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

10. 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
1. High compressive strength
2. A Good fire and abrasion resistance
3. Very good stability
4. Rapid construction
5. Hollow units have low self-weight
6. Reinforcement can be passed through hollow units and filled with concrete to
increase earthquake resistance
7. Air-space (hollow units) provides good thermal insulation