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INTRODUCTION TO CONCRETE AND FERROCEMENT
Introduction to Concrete and Ferro cement
Scope
Concrete is a basic tool in water and sanitation projects. Having an understanding of how it
works and its uses is fundamental to developing infrastructure – even if your primary task is
management or working with the community. This unit provides both a theoretical and
practical introduction to concrete. It is not a full concrete course and does not cover
structural design.
Intended learning outcomes
After studying this unit you should:
• know the main constituents of concrete
• understand how to mix and lay concrete
• know the importance of compaction and curing
• experience a variety of practical tasks involved in both concrete and ferrocement.
This course is not examined – until you get out on a real project!
1. Introduction
The product of many infrastructure projects is construction. Many materials can be used –
stone, brick, wood, bamboo, clay, metal – but concrete is a widespread and useful material.
However it does require a level of skill and knowledge to be used effectively and efficiently.
This has implications for budgets, timescales, human resources and logistics. The
constituents of concrete can often be sourced locally, reducing transport costs. Concrete can
be made into a variety of shapes, to suit the intended use. It can be used for roads, beams,
walls, well linings, pipes and pipe surrounds, roofs, surrounds to handpumps, water tanks
and reservoirs, building blocks, channels, slabs, containers and even sculptures.
Concrete is strong in compression (ÆÅ), but weaker in tension (ÅÆ), so reinforcement is
sometimes added to make is stronger.
2. Constituents,
The four basic ingredients of concrete are cement, fine aggregate (sand) (<5mm), coarse
aggregate (stones) (>5mm and usually <40mm) and water which, when mixed together, set
to form a hard stone-like material. Its strength depends mainly on the proportions of the
material in the mix, but also partly on the shape, surface texture, crushing strength and
grading (range of sizes) of the course and fine aggregates.
The proportions of the dry materials are always listed as a ratio based on volumes of
material:
Cement: fine aggregate: coarse aggregate
The higher the proportion of cement, the stronger the mix, so 1:2:4 has one part of cement
to six parts of aggregate, whilst 1:3:6 has one part to nine parts of aggregate, so has less
cement and is therefore not as strong.
Because volume batching is usually based on 1 as the measure of cement, for mixing large
volumes of concrete it is useful to make a ‘gauge box’ equal to the volume of one bag of
cement. Often cement comes in 50 kg bags which contain loose cement that will occupy 34
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litres (0.034 m3) so a box can be made this size and be filled level (or to the halfway mark)
the right number of times with the coarse or fine aggregates to suit the number of bag of
cement used.
2.1
Aggregate
Ideally the coarse and fine aggregate should both be graded so that the smaller sizes fit
exactly into the spaces between the larger ones. The resulting mixture will then be as dense
as possible and the cement and water slurry can be used to coat every particle and fills any
remaining very small spaces. Thin flat stones are not suitable; stones with shapes that are
approximately cubical or spherical are better.
The coarse aggregate needs to be of clean stones that are resistant to crushing and are nonporous. Gravel from riverbeds is often ideal as long as it is free from clay. Pieces of rock
from outcrops can be broken into smaller sizes with hammers but care needs to be taken to
avoid including weak and dirty rocks. Coral, laterite, calcite or even broken burnt bricks can
all be used successfully. The individual pieces should usually be between 20mm and 5mm in
size although for mass concrete the sizes can be larger. A good piece of coarse aggregate
may fracture when hit with a hammer but it should not crush. For strong and waterproof
concrete all crushed rock powder and dirt should be removed from the coarse aggregate by
sieving. Sometimes to remove dirt from the stones it may be necessary to wash the
aggregate.
The fine aggregate also needs to be clean, and in particular it should not contain clay. For
concrete the best sand is much coarser than what is normally used for bricklaying mortar.
Clean river sands are often ideal. As with the coarse aggregate, the sand may need sieving
or washing to remove organic matter, silt and clay.
Sometimes ‘all-in’ aggregates may be available which have the coarse and fine aggregate
already mixed in the correct ratio, so that only cement and water need to be added. Good
mixing is essential to obtain good strength concrete.
1.1.1 Bulking of Sand
When dry sand becomes damp it 'bulks' or occupies more space than it did when it was dry.
This bulking increases as the water content increases, but the bulking reaches a maximum at
a water content of about 5 or 6% water by mass. A further increase in moisture content
above this optimum gradually reduces the 'bulking' until the sand is completely saturated at
which point the sand will occupy almost the same volume as when it is dry. A moisture
content of 2 - 5% will typically increase the volume by 15 - 30% and sometimes up to 40%.
Small sized gravel can bulk in a similar way but only up to about 10%.
If the mix ratios are measured by volume this bulking phenomena will obviously affect the
actual mix ratios that result in the concrete. The standard 1:2:4 and 1:3:6 mixes are by
volume, presuming that the sand is dry. Hence if the sand is damp a greater volume of it will
have to be added to the mix to get the correct ratio.
The amount of bulking that has taken place can be determined using the fact that fully
saturated sand has virtually the same volume as the sand when it is dry. This test will be
carried out as part of the forthcoming practical exercise and detailed instructions will be
given at that time.
Errors due to bulking of sand can be entirely eliminated by using 'weigh-batching' instead of
volume batching. This is better because the weight of sand is only affected a small amount
by dampness so the error will be less (usually not exceeding 5%). However the volume
ratios need first to be converted to weight ratios. For example a 1:2:4 mix by volume may
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become 1:2.12:4.71 mix by weight. Weigh batching is the best method if heavy-duty scales
are available but these are not usually practical on site.
2.2
Cement
Cement is a chemical that is used as a fine powder. It reacts with water in a chemical
reaction; it forms a solid because of this reaction, not because it dries out like clay.
The cement should be as fresh as possible because unless it is stored in airtight containers it
is likely to absorb water from the air and some of it will begin to set before it is used so the
concrete mix will be weaker. Store cement bags clear of the ground in a dry place; stack the
bags close and cover them with polythene or a tarpaulin to prevent circulating air giving up
moisture to the cement.
Health and safety
Cement is a chemical; it is alkaline and can burn skin. Avoid touching it where possible; wear
gloves and other protective clothing. Wash hands after concreting; do not smoke or eat
whilst working with cement.
50 kg bags of cement are heavy, especially for women and people who are not healthy (e.g.
refugees). Try and buy 25 kg bags, or divide up bags into smaller containers.
Any building site is dangerous; keep things tidy to avoid other people falling over your tools.
Watch out for what other people are doing – you are responsible for their safety.
1.1.2 Getting the mix right
Watt and Wood (1976) suggest a 1:2.5:5 mix for lining hand dug wells. For the porous
concrete needed below groundwater level they suggest a 1:1:4 mix but great care will be
needed to produce strong concrete from this mix. However the mix can be used as a band
just in the middle portion of a precast well ring so that the normal concrete at each end
gives more strength to the ring. A 1:2:4 mix is a common mix used for structural concrete
whilst a weaker mix (e.g. 1:3:6) can be used for non-structural purposes, such as paths.
It should be noted that one measure of cement plus 2.5 of sand plus 5 of stones do not
produce 8.5 measures of concrete. As sand enters the spaces between the gravel particles
and the cement fills the gaps between the sand particles 1 + 2.5 + 5 = approximately 5
measures of concrete. The added water has little effect on the volume.
2.3
Water
The water needs to be clean, free from clay, mud, oil or salts. If it is not fit for drinking it is
usually not suitable for making good concrete. Measure quantities accurately to ensure that
the correct w/c ratio and/or workability are obtained.
When water is added to cement a chemical reaction takes place, causing the cement to act
like a glue as it 'sets'. It hardens from the chemical reaction and not because the concrete
dries out, hence concrete carefully placed under water (so that it does not segregate) will
harden just like normal concrete.
Only water amounting to about 25% of the weight of the cement (i.e. a water/cement ratio
of 0.25) is required in the mix to complete the chemical reaction which causes the concrete
to set, but usually at least twice this amount is required to lubricate the mixture of stones,
sand and cement to make the concrete sufficiently fluid ('workable') to fill the moulds
('shuttering'). The w/c ratio does not normally exceed 0.6 and can be as low as 0.35 if
mechanical compaction in being used. The water not used in the chemical reaction
eventually evaporates to leave small voids.
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1.1.3 Getting the correct water/cement (w/c) ratio
Unlike the mix ratios which traditionally are measured by volume (e.g. 1:2:4 for
cement:sand:stones) the w/c ratio is by mass. Usually cement comes in 50 kg bags which
are usually assumed to contain a volume of 35 litres (since the density of cement is about
1400 kg/m3). Now the density of water is 1 kg/litre (1000 kg/m3), so to obtain a w/c ratio of
0.55 it will be necessary to have (0.55 x 50) = 27.5 kg of water per 50 kg bag of cement.
This will have a volume of 27.5 litres. The minimum w/c ratio of 0.25 would need 12.5 litres.
To accurately find the correct amount of water to be added to the mix to get a desired w/c
ratio it is necessary to take into account the amount of water held by the damp sand and on
the aggregate if it is wet. The mass of the water held on and between the particles of sand
and aggregate when they are batched (by mass or volume) needs to be subtracted from the
mass of water that has been calculated from the w/c ratio. The free water in stockpiled
sands normally ranges from 2 - 6% by mass but may reach 8% or more if the sand is
extremely wet after heavy rain. Coarse aggregate seldom holds over 2% free water by mass
unless the stone is porous. The coarser the aggregate the less water it will carry.
3. Strength
Concrete is very strong in compression and structural concrete often has a crushing strength
in the range 25 - 60 N/mm2 although much stronger mixes can be designed. However its
tensile strength is only about 10% of the compressive strength. This weakness in tension
can be overcome by using steel reinforcement bars embedded in the concrete.
Concrete slowly gains strength as indicated by the successively higher lines for 7 days, 28
days and one year on the graph below. About 60% of the strength after one year is gained
over the first month after casting and the strengthening process continues for over a year,
albeit at a slower and slower rate.
Mean compressive strength (N/mm 2)
Relation between water/cement ratio and mean
compressive strength - 100mm cubes
(from Kong and Evans (1987), after Road Note 4 )
90
80
70
60
7 days
50
28 days
40
1 year
30
20
10
0
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Water/cement ratio
(by weight)
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The strength of a particular concrete mix is usually determined by using a large machine to
find the crushing strength of cubes (100mm × 100mm × 100mm, or 150mm × 150mm ×
150mm) made from the concrete in a specific way (see BS 1881:parts 108 and 116). Often
three cubes are made at the same time so at least two cubes can be tested to check that the
result is reliable. These cubes are usually tested 28 days after they are cast, but a test may
first be carried out on the third cube after 7 days to give an early indication of the likely 28
day strength. To predict the 28 day strength the 7 day strength is multiplied by a factor
(based on past experience) to allow for the expected increase in strength over the next 21
days. An alternative to cube crushing is the longitudinal splitting of cylinders made from the
concrete.
Normal cement (Ordinary Portland Cement - OPC) in a concrete will usually start to set
('initial set') between 30 minutes and 1 hour after it is wetted but it will not reach its 'final
set' (when everything is firmly 'glued' together into a weak solid) until several hours later
and occasionally this may take up to 10 hours. Because of the initial set concrete should not
be used more than half an hour after it has been mixed, so it is best not to mix too much at
one time.
Special cements are available which set and harden much quicker than OPC if these
properties are required in special circumstances (such as in in-situ lining seepage zones in
hand dug wells). Other cements are specially designed to resist the deterioration of normal
concrete that can result from sulphates, which are sometimes found in clay soils.
4. Compaction and workability
The voids left when the excess water evaporates and any voids resulting from trapped air
reduce the strength and increase the porosity of the concrete. It is therefore important to
keep the water/cement (w/c) ratio as low as possible consistent with the need to produce a
'workable' mixture. Concrete made with too low a w/c ratio is unworkable and will be hard to
compact so that the finished concrete will be weak and full of holes (‘honeycombed’). In
some instances it is useful to put a very small amount of an 'admixture' in the mix to give
the necessary lubrication and to reduce the amount of water needed to make a workable
mix, but these additives need very careful control.
The ‘workability’ necessary to properly compact the concrete will depend on whether a
mechanical method (e.g. 'poker vibrator') or a hand method (e.g. 'rodding' described below)
is being used to compact the concrete. The workability can be lower if mechanical methods
of compaction are to be used. The required workability will also vary with the type of work
being carried out. For large mass concrete sections the workability will not have to be as
high as when placing concrete through congested steel reinforcement in narrow shutters to
form thin heavily reinforced beams.
4.1
Compaction
The presence of only 5% air voids in concrete can cause a 30% strength loss so the wet
concrete needs to be compacted to reduce to a minimum the air entrapped in the mixture.
The compaction also makes the individual coarse and fine aggregate particles move closer
together to form as dense a mass as possible. Good compaction is important for producing
waterproof concrete.
The appearance of a watery scum ('laitance') on the surface of a concrete after compaction
is often an indication that there is too much water in the mix but it can also result from a
deficiency of fine aggregate or because too much ‘tamping’ or ‘trowelling’ (terms defined
below) has been done.
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The higher the cement content in a mix the greater will be its workability and the less the
effect of the grading (range of sizes) of the aggregates.
The workability of concrete for any particular ratio of water, cement, sand and stones will
depend on the size and shape of the stones. Usually the finer the aggregate the less
workable is the mix. A smooth and rounded aggregate will produce a more workable
concrete than if sharp angular aggregate is used. Sometimes segregation, the separating of
the coarse aggregate from the rest of the mix or the separating of the cement-water paste
from the aggregate, may occur. This generally indicates bad mix design, such as poor
aggregate grading, and occurs if there is not enough fine aggregate.
The commonest way of measuring the workability of a concrete mix is the 'slump test'. In
the test the wet concrete is compacted in a standard way into a truncated steel cone with an
open base. The cone is then carefully removed and the change in height (known as the
'slump') of the subsided cone of concrete is measured. The slump test does not in fact give a
complete measure of workability so there are two other tests that are also used, the
‘compacting factor’ and ‘VB consistometer’ tests. However the slump test is an easy and
useful test for monitoring the quality of a particular mix design. Provided no change is made
in the aggregate or its grading, the slump test will indicate whether the correct water and
cement contents are being maintained.
When placing concrete on site, special care should be taken to ensure that the concrete is
worked well into corners, cavities and around reinforcement. It should not be placed in deep
layers since these will be impossible to compact properly; with hand compaction these layers
may have to be as little as 75mm but greater depths are possible when mechanical
compaction is to be used. The mix should not be dropped from a height that causes it to
segregate. Compaction by mechanical vibration permits the use less workable, drier (i.e. with
a lower w/c ratio and therefore stronger) mixes, and a larger proportion of coarse and a
smaller proportion of fine aggregates can be used (giving less shrinkage, which can cause
cracks, and giving more strength).
4.2
Methods of compaction
1.1.4 By hand:
Compaction by hand can be achieved in the
different ways described below.
Rodding: consists of inserting a bar
vertically into the concrete and moving it up
and down until the concrete is thoroughly
worked into place.
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Tamping: is carried out by carefully
thumping the surface of the concrete with
the edge of a flat piece of wood. It is often
used to compact slabs since it is also useful
for finishing the surface.
Ramming: is suitable for deep unreinforced sections. It consists of striking the surface
violently with the end of a piece of timber.
Hammering the shuttering: Striking the outside of the formwork (mould) which contains the
concrete assists with removing air bubbles near to the edge and produces a dense smooth
surface.
Trowelling: Using a hand trowel to smooth and compact the surface of a concrete slab will
improve its water-tightness. For finishing the surface of drier mixes the surface can first be
rammed with a heavy flat-bottomed rammer until a thin film of mortar appears at the
surface. Too much trowelling while the concrete is still plastic should be avoided as the
cement and other fine material will be brought to the surface and cracking might result.
Initially only use sufficient trowelling to provide an even surface, then when the concrete has
taken its initial set it may be trowelled again to produce a hard, dense, smooth surface.
1.1.5 By machine:
Mechanical vibrators can be of three types:
• internal vibrators (such as poker vibrators) which are inserted into the wet concrete;
• surface vibrators (often consisting of vibrators attached to a tamping beam) and
• external vibrators (those which are fixed or held against the shuttering).
Poker vibrators are best suited to deeper concrete pours such as walls or beams whereas the
surface vibrators are best for concrete slabs.
5. Curing
As previously mentioned, the amount of water included in the concrete mix is usually
sufficient for the initial hydration of the cement. However the water that is not immediately
used up in the chemical reaction can evaporate, particularly in hot climates, leaving
insufficient water for the complete hydration of the cement. This continuing hydration, which
takes place over a long period of time leads to the increase in strength already mentioned. It
is therefore important to reduce the water loss due to the sun, wind and heat of hydration.
Providing the best conditions for continued strength gain is known as 'curing'. The strength
of un-cured concrete can be only 50% of that of concrete properly cured for 14 days. Rapid
drying of fresh concrete can cause shrinkage cracks.
Evaporation of mixing water through the sides of newly cast concrete members is largely
prevented by the formwork (moulds) used to contain the concrete. Hence these should be
kept in place for as long as possible. Where timber formwork is used care must be taken to
ensure that water is not lost by the wood absorbing water from the concrete. This can be
done by painting the wood with oil (which will also assist with the removal of the formwork)
or by well wetting the forms before concreting. For similar reasons the surface on which
concrete slabs are to be laid should also be wetted before the concrete is laid so that water
is not absorbed from the bottom layer of the concrete.
While the concrete is taking on its first set it is good practice to shade the surface from the
sun. After a few hours, when the concrete has hardened sufficiently, any of the following
methods can be used:-
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• Covering with wet sacking, wet sand etc. and keeping these moist throughout the day.
Sacking can be used on vertical surfaces. Polythene sheeting can be used to cover any of
these wet materials to reduce evaporation. Precautions have to be taken to ensure
coverings are not displaced by wind.
• Ponding water on the surface by constructing earthen mounds in squares over the entire
area and filling each square with water to a depth of 25 - 50 mm. It is commonly used on
the surface of slabs.
• Spraying with water. This is best used together with wet fabric. If there is no fabric or
sand on the surface the spraying will have to be very frequent because the surface alone
can not retain much water.
• Proprietary membrane curing compounds. These coatings are sprayed onto the concrete
surface as soon as it sets to prevent evaporation.
5.1
Thermal cracking
The chemical reaction ('hydration') that takes place when the cement first becomes wet is
accompanied by a rise in temperature. Heat and humidity accelerate hydration but if the
temperature of the concrete is high when it sets then, when the temperature reduces and
the concrete shrinks cracks are likely to develop in the concrete because it has not yet
gained enough strength to resist the tensile forces. These cracks can be limited by using
reinforcement (particularly meshes) in the concrete. Using curing water and shade to reduce
the temperature of the concrete will also help.
6. Reinforcement
Steel rods and meshes are used to strengthen concrete where it will be subject to tension
(for example that which results from bending or shrinkage). Plain bars are usually made
from mild steel (except in the case of meshes where cold drawn bars of higher strength may
be used). Bars with a deformed (‘ribbed’) surface or twisted square bars, sometimes of high
yield steel which is 84% stronger than mild steel, are also available where higher
performance is required.
Reinforcing rods, or rebar as it is sometimes termed, must be completely embedded in
concrete or it is likely to weaken due to rust. So that it bonds well with the concrete the
surface of each bar must be clean, free from mud and oil and any loose rust or scale. Care is
needed to ensure that the bars are positioned correctly and that they do not move out of
place during the placing of the wet concrete. Horizontal bars and meshes in slabs and beams
can be positioned accurately if small precast mortar blocks are tied to them (using wires cast
into the block) to prevent them touching the shuttering or the ground below a slab or beam.
Bars are usually held in place by tying them with soft wire to other bars laid at right angles
to them. As previously mentioned, the wet concrete needs to be well compacted around the
reinforcement so that it bonds well to it and there is a minimum of voids.
6.1
Ferrocement
A special case of reinforcement is ferrocement. Here a thin shell of concrete is reinforced by
wires. As the concrete is thin, large stones cannot be used, so only fine aggregate is used in
the mix to form a coarse mortar. The workability of the mix is important – too stiff and it is
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difficult to place, too fluid and it will not stay in place.
Wires are placed to form a framework in the desired shape. A firm surface is needed on one
side of the wires – this could be made from wood, metal, bamboo or cloth under tension (1).
The mortar is applied to the wires (2) and, once the mortar has gone stiff, the formwork is
removed and a layer of mortar applied to the other side (3). Additional layers can be added
to create the required thickness.
Small containers can be made without any reinforcement.
7. Useful Data
Density of bulk cement ≈ 1450kg/m3
Density of reinforced concrete ≈ 2450 kg/m3
Density of sand and aggregate ≈ 1700 kg/m3
Density of water = 1000 kg/m3
8. References
Watt, S.B. and Wood, W.E., (1976), Hand Dug Wells and their Construction, Intermediate
Technology Publications, London, UK.
Khanna P N (1978), Indian practical Civil Engineers’ Handbook, Engineers’ Publishers, New
Delhi, India
Kong F K and Evans R H (1987), Reinforced and Prestressed Concrete (3rd edition),
Chapman & Hall, UK
Watt, S.B., (1978), Ferrocement water tanks and their construction, Intermediate
Technology Publications, London, UK.
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