The Constructor


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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

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,
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.
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.

Quality Standards for Precast Concrete Blocks:

Indian Standards: IS:9893-2007, IS:15658-2006/,IS: 5751-1984

European Standards: EN 13369

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