Introduction to concrete building products
Practical Action
The mc mix typically has a cement to sand ratio of 1:3 by weight, so it is quite a rich mix. Only the
minimum amount of water is used to make the mix just workable enough, and vibration is usually
used to distribute the mortar in the mould. Quality control in production needs to be good or
numerous tiles can get broken during transport and use.
Floor and wall tiles are rectangular in their most basic form, but other shapes, e.g. hexagonal, are
possible for a more decorative appearance. There are few specifications for the sizes of wall and
floor tiles in any of the national standards. A typical size can be 230 by 230 by 16mm. However,
as sawing or splitting equipment for concrete tiles is expensive it is best to choose a size for the
lateral dimensions that they can fit exactly within the typical sized rooms used in an area.
With micro-concrete roofing (mcr) tiles it is not all that common to make flat tiles (slates). More
usually tiles with a curvaceous profile are made as they offer some technical advantages compared
with flat tiles and are easier to fit by builders who are not specialist roofing contractors. The most
common of the curved shapes is the pantile.
Mcr tiles typically have dimensions of 500 by 250 by 6, 8 or 10mm. For further information on
mcr tiles see the companion technical brief - Micro-Concrete Roofing Tile Production.
Mixes for concrete slabs can contain coarse aggregate as well as sand and cement. A typical size
is 600 by 600 by 50mm. Mix proportions of 1:2:6 cement: sand: coarse aggregate by weight
may be used. Similar proportions can be used for fencing panels and pit latrine slabs.
Extended Elements
These comprise long and thin products that are usually reinforced with steel rods or frameworks.
The main types of these products are:
• Lintels over door, window and other openings
• Coping stones to provide protection to the tops of exposed walls
• Structural beams and columns
• Fencing posts
• Concrete pipes
• Roofing channels
• Roadside kerbs.
Note that coping stones and kerbs are usually made thicker than the other types of element and
may not need reinforcement. Concrete pipes are also usually made without reinforcement.
These are quite specialised types of product that may not suit the small to medium scale
producer, except for coping stones that are relatively easy to make. Potential difficulties for non-
specialised producers are the need to know how much reinforcement to use, where to place it in
the mould and how to ensure that the workers in the yard know how to place the reinforcement
and maintain it in position as the concrete is poured. Additionally, medium to high strength
concrete would be needed for structural elements, generally above 20 N/mm2 (MPa) at 28 days
and this presents particular quality control issues at the production yard including the use of
large-scale vibration equipment to ensure that the sections are evenly vibrated throughout their
whole length.
Prestressing is a technique to enable elements loaded in flexure or tension to relieve some of these
stresses by utilizing the high compressive strength of concrete. In this way savings can be
achieved on the quantity of expensive reinforcing steel used. There are essentially two methods of
pre-stressing:-
(i) In pre-tensioning the steel reinforcement rods are stretched by jacks with concrete cast around
the rods. When the concrete has hardened sufficiently the pressure in the rods is released and it is
taken up by increased compression of the concrete.
(ii) In post-tensioning reinforcing rods are passed through hollow ducts in the concrete product
after it has hardened, then tensioned and mortar grout poured around the rods. The grout hardens
and the tension in the rods is subsequently released.
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