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-4 The manufacture of concrete

ARNING ELEMENT OBJECTIVES

fter you have learned this element, you should be able to:

explain how the density and final strength of the concrete is influenced by the proportions of the components;

describe various mixtures for different types of concrete;

ay approximately how much concrete of various types can be produced with one bag of cement;

describe how concrete is mixed by hand and by concrete mixer.

he final strength of the finished concrete depends on:

the proportions of the components (i.e. whether the correct quantities of gravel, sand, cement and water have been used)

the quality of the components;

the distribution of the grain sizes of the gravel and sand;

the way the components are mixed;

the way the mixture is transported, placed, compacted and cured.

oportion of the components

hen the mixture is prepared take care that:

the proportions of the aggregates used in the mixture are correct;

the right quantities of cement and water are added.1

The water should be fresh and clean. If the only available water contains a lot of silt, it could be stored in drums to allow the

ttle on the bottom of the drum before use.

nd and gravel contain particles of different sizes with empty spaces in between. Generally, you can assume that sand and g

a dry condition, consist of 60 per cent solid matter and 40 per cent empty spaces.

o obtain a dense concrete the empty spaces need to be filled. The empty spaces between the sand grains will be filled by th

ment while the sand-cement-water paste (mortar) will fill the empty spaces between the gravel particles.

n extra 10 per cent of mortar is necessary to coat the particles completely.

his means that for a certain quantity of gravel we would need 50 per cent wet mortar in order to fill the empty spaces and to

e particles. In theory, therefore, a mixture of 1:2:4 (cement: sand:gravel) will produce a dense concrete. In practice, when aut plastic concrete is required, a mixture of 1:2:3 is often applied mainly because of the variation in the percentage of empty

aces. However, we have already seen (LE-2) that different mixes can be used depending on the purpose for which the conc

ed. Table 2 shows various mixtures producing different types of concrete (volumetric proportions).

able 2:

ype of concrete Mixture

(cement:sand:gravel)

Purpose

an concrete 1 : 4 : 8 Culvert beds, fills

8/11/2019 LE-4 the Manufacture of Concrete (Part1)

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ass concrete 1 : 3 : 6 Non-reinforced structures

ense concrete 1 : 2 : 4 Culvert pipes, light reinforced structures

ense, plastic concrete 1 : 2 : 3

1 : 1.5 : 3

Heavy reinforced structures

OU SHOULD REMEMBER THAT THE ABOVE TABLE PROVIDES RULES OF THUMB ONLY.

a very high quality concrete is required, laboratory tests must be carried out in order to determine the optimum granulome

omposition of each aggregate, i.e. what are the best proportions of particles of different sizes in a particular type of aggrega

emember that the volumes of cement and sand can vary with the degree they are compacted! In addition, the volume of sa

ries with its moisture content. For example, the volume of one kilogramme of sand increases by 15-50 per cent if the water

ontent is increased by 3-12 per cent. For these reasons, volumetric proportioning as shown in table 2 is never used when big

uantities of high quality concrete must be produced. In this case, the materials are weighed and the moisture content is

easured, so that the exact quantities of the different components (gravel, sand, cement and water) can be determined. For

oncrete works carried out on rural roads, however, the above rules of thumb can be applied.

ater/cement ratio

he quantity of water divided by the quantity of cement gives the water/cement ratio. The water/cement ratio varies betwee

nd 0.5 for practically all types of concrete. This gives the minimum amount of water necessary to react with the cement and

ake the mixture workable. To ensure sufficient workability for hand-mixed and hand-placed concrete sometimes more wate

ecessary (water/cement ratio between 0.5 and 0.65) but great care should be taken not to use too much water. The wetter

ix, the weaker the finished concrete !

is very important to remember that much more than 1 m3of components is required to produce 1 m3of concrete. This is ea

nderstood when we realise that the cement fills the empty spaces between the sand particles and the mortar fills the empty

aces between the gravel particles. Table 3 shows the material required to produce one cubic metre of finished concrete.

able 4 shows the approximate yield of concrete per bag of cement for the four different mixtures. The tables assume that on

cement weighs 50 kg and has a volume of 40 litres.

able 3: