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CONCRETE: HARD FACTS. DURABLE STRUCTURES -Part I.
Dr Sirion Robertson
Making concrete: more than just mixing cement, water and earth. Public
science communicator, Dr Sirion Roberston gets down to the real nitty gritty
discussing in Part One of this article some of the chemistry and the art of
making concrete. A must for anyone who has ever had to build or will be building
with concrete. Science for the DIY person!
Concrete is fascinating stuff. The Romans used it at least 500 years before
the time of Christ, and they were probably not the only ones or the earliest
ones. Although concrete technology has advanced a little since those remote
days, it is still used in modern building projects - from foundations for giant
telescope mountings, to nuclear power stations, to sky-scrapers in Manhattan and
Tokyo, to garage and patio floors, roads and railway sleepers, shore protectors
against wave action, dam walls, and settings for the poles of washing lines and
chicken runs. Concrete is also used very successfully in boat building. The
weight of a well-made concrete boat compares favourably with that of a wooden
boat of the same capacity.
The fundamental chemistry is much the same in all cases, although the
concrete itself may be modified in a variety of ways to adapt its properties to
an enormous diversity of modern requirements.
This is the first of a two-part article, written specially for SCIENCE IN
AFRICA. In the first part we deal with important generalities about concrete,
and in the second we will discuss some of the chemistry of concrete, and
"technical" or specialised aspects, including reinforcement, and
various additives for special purposes.
Modern concrete, very much like its earliest form used by our remote ancestors,
is a mixture of cement, or a cement-like substance, with sand, stones and water.
The water is added to the dry components to initiate the chemical changes
leading to hardening, after which the strength and durability of the material is
comparable to some of the hardest rocks. But, unlike the ages-long geochemical
processes involved in rock formation, concrete can be mixed in a few minutes,
and will approach its final hardness within a few weeks - or even a few hours if
certain chemical "accelerators" are added during the mixing stage.
The "heart" of concrete is of course the cement - the substance
that, with water, does the chemical work, and binds the sand and stones into an
astonishingly strong, composite material. We'll revisit the subject of cement,
and "cement-like substances" in Part Two of this article.
The sand and stones are referred to as "aggregate": stones are the
"coarse aggregate" and sand the "fine aggregate". The stones
are usually between about 10 and 20mm in size. Though less important than the
quantity of cement involved, the sizes and proportions of the aggregate
components are amongst the factors that determine the final properties of the
concrete. Both types of aggregate should include particles with widely-varying
sizes. (The difference, incidentally, between "mortar" and concrete is
that mortar has only fine aggregate. Nothing larger than about 5mm particle
size.)
An interesting variety of concrete is sometimes referred to as
"cyclopean concrete", which is made by adding massive rocks to
ordinary concrete. The rocks form a sort of "super coarse aggregate".
This is generally used for the walls of large dams and other massive structures
where enormous volumes of concrete are required.
Proportions and strengths
In determining the proportions of sand and stone, the main considerations are
physical rather than chemical. The spaces between the stones (often refered to
as the "void volume") should be completely filled by the volume of
sand. And the small spaces between the sand grains in turn should be filled by
the very much smaller particles of the cement. Because the cement, when mixed
with water, undergoes the chemical process of changing into a rock-hard, rigid
substance, the amounts of cement and water present in the mixture are the main
determinants of final strength.
The cement, sand and added water make up a "paste", the volume of
which should be slightly more than the void volume of the coarse aggregate.
Typically, the volume of coarse aggregate (including its void volume) represents
about 70-80% of the volume of concrete finally produced. (One of my less
intelligent mistakes, in early work with concrete, was to assume, unthinkingly,
that the final volume of concrete would be roughly equal to the sum of coarse
and fine aggregates. This resulted in major under-production, and prompted me to
approach the subject a little more rationally.)
Strictly, the proportion of cement in a mix should be specified by mass. This
is because the actual quantity is more reliably specified by mass than by
volume. The same is true of sand and stone, but variations in the quantities of
these components are less important, and it's more convenient and more common to
measure the sand and stone by volume. (In terms of the principle involved, the
argument about mass, as the better measure of quantity, applies also to the
water, because of temperature-related changes of volume. But the effect here is
far too small to have any practical consequence, and quantities of water, like
quantities of aggregate, are conventionally specified in litres, or in any other
convenient unit of volume.)
For ordinary garden work with concrete, and other trivial building projects,
where the final strength need not be accurately known and isn't particularly
important, the quanties of all components are usually measured by volume -
commonly 1:2:3 or 1:3:3.
For more important projects the proportions are expressed on a
mass:volume:volume basis: kg cement to volume of sand to volume of stone. For
"general purpose concrete", local cement manufacturers recommend 100kg
of cement (two standard bags) to three-and-a-half wheelbarrows of sand and
three-and-a-half wheelbarrows of stone. (A standard builder's wheelbarrow has a
capacity of about 40 litres.) Translating this into an all-volume ratio, using
the fact that cement is about 1.4 times heavier than water, it works out to
about 1:2:2.
Another common way of expressing the composition of concrete, which closely
reflects its potential strength, is to specify the mass of cement in the final
volume of concrete. This may vary between about 200 kg and 550kg per cubic metre.
(The mix we've just mentioned is about 285kg/cubic metre.)
Cement is the most expensive component of concrete, so there is often a
tendency to economise by reducing the cement to a very small proportion. In
important building projects the proportions of the components are very carefully
specified and controlled, and samples of the hardened concrete are tested to
ensure that its stength is appropriate to the job.
In addition to the proportions of cement to aggregate, there is a close and
important relationship between the amount of water used in the mix, and the
final strength of the concrete. A sloppy, runny mix produces weak concrete.
Usually the volume of water is about the same as the volume of cement in the
mix. The problem with making too dry a mix is that it isn't easy to work with,
and is likely to have cavities in it when hardened. This of course will greatly
reduce its strength.
The "runniness" or stiffness of the newly-mixed concrete (the best
word here is plasticity) is expressed by a "slump" value. A
cone-shaped, metal container of standard dimensions, open at both ends, is stood
base-down on a solid surface and carefully packed with a sample of the
freshly-mixed concrete. The container is then lifted off the conical mound of
wet concrete. The more runny it is, the more will it collapse into a low-profile
"glob". The slump is found by measuring the loss of height of the
conical pile of concrete after it has "sagged". Normal slump values
are about 7 to 9 cm. The height of the cone itself is about 30 cm.
When "throwing" or "placing" concrete - that is, when
introducing it into foundation trenches or moulds for setting - it's important
to ensure that it occupies all the volume intended for it. "Voids" -
the term given to gaps or cavities in the concrete - can seriously impair the
finished structure. The concrete must be tamped or rammed down into its bed with
a spade or rod. On large projects mechanical vibrators of various sorts are used
to help consolidate the wet, newly-thrown concrete and eliminate voids.
"Curing"
As concrete sets, the water in the mixture enters into a chemical reaction
with the cement, and new chemical substances are formed. Although the concrete
"dries", in the sense that no liquid water remains, the water is still
there, as a very important part of its structure. This is different from the
drying of mud, for example, in which the water simply evaporates and leaves the
remaining solid material, without having changed it chemically. For this reason
we talk of "curing" (or "setting") of concrete rather than
"drying". In fact it's very important to keep concrete wet during the
early stages of curing. This is normally done by spraying it regularly for the
first week or so, and keeping it covered with sacks, leaves or any other
convenient materials.
The chemical process involved in curing is called "hydration"
(which simply means combining with water), and keeping plenty of water on the
surface ensures that the concrete doesn't lose water by evaporation. If water is
lost from the wet concrete in this way there may be insufficient to allow the
hydration process to go to completion, and the result will be reduced final
strength. Another, related reason for keeping it wet while curing is that there
is always some contraction of the concrete volume as it cures. This can lead to
small cracks forming. Contraction, and crack-formation, are minimised if plenty
of surface water is present. (Because it is only the cement and water that enter
into chemical reaction, mixes with high concentrations of cement tend to
contract more than do weaker mixtures.)
In addition to drying by evaporation to the air, wet concrete may lose water
to its surroundings - such as dry ground, when poured into trench foundations.
To guard against this, the earth, and any other porous structures that will come
into contact with the concrete should be thoroughly sprayed with water before
the concrete is placed.
While setting, the concrete gains hardness and srength, as the process of
hydration slowly permeates the entire body of material, and new chemical bonds
extend their fingers throughout the structure. Curing should be allowed to
progress for several days before subjecting the new concrete to significant
stress. The rate of curing depends on the temperature (as the rates of all
chemical reactions are dependent on temperature), and for this reason the
"safe curing time" is less in hot weather than in cold, and generally
less in tropical climates than in higher latitudes. In the same way that curing
concrete should be prevented from drying, it should also be protected from
extreme cold. If you have an option, it's better to work with concrete in warm,
humid weather than in cold, windy, dry weather.
Trying to get advice from experts on safe curing times for concrete can be
frustrating. An authoritative (British) textbook on building science says that
trench foundations, for example, should be left for a week before wall-building
begins. Having read this, I asked the Rhodes University Architect whether he
agreed with it. Perhaps not a week, he said, but a minimum of three days,
certainly. A little later I asked a local building contractor whether he left
trench foundations for three days before building on them. Perhaps not as long
as that, he answered, but 24 hours, certainly. You must leave them for 24 hours.
But in the event it didn't happen like that. I subsequently gave him a small
building contract: I wanted my garage extended. His men finished throwing a
trench foundation for the new wall at 5 one afternoon. At 7 the next morning -
14 hours later - they arrived to start building. They weren't especially pleased
when I told them to go away and come back at a later date, but in view of what
the contractor himself had told me I felt justified.
So views on acceptable curing times vary quite widely - and actual building
practises vary more widely still. The problem, of course, is that leaving
concrete for extended curing times - however desirable it may be - can involve
expensive delays. "Time is money". In the same way that builders -
amateur or professional - may be tempted to use too little cement in their
concrete mixes, so too they may be tempted to allow insufficient time for
adequate curing of new concrete. In some cases, where speed is essential, but
standards of strength cannot be compromised, chemical additives are used to
accelerate the process of curing. We'll say more about them in the second part
of this article.
Where practical (such as in the manufacture of concrete bricks) the curing
may be accelerated simply by raising the temperature. An interesting
rate-temperature relationship, well-known to biologists, but with far wider
applications than biology alone, is the so-called "Q10" value. This is
the proportion by which the rate of a chemical reaction is raised by an increase
in temperature of 10 degrees on the Celsius scale. In many cases - including the
rates of enzyme-catalysed reactions in living cells - the Qten value is very
close to 2. In other words, the rate doubles for every 10 degrees increase in
temperature. So, applying this to concrete, if adequate curing is achieved in
six days at 20 degrees, you could reduce this to three days if the temperature
were kept at 30 degrees. (Remembering, in relation to this, that the curing time
recommended in Britain is about a week for trench foundation concrete, this
might very well be equivalent to about three days in the warmer South African
conditions.) In the concrete industry, fully-cured blocks are produced at a very
high rate by subjecting them to temperatures of about 200 degrees Celsius, in a
water-saturated environment to prevent drying. The process is exactly the same
as pressure-cooking food, and in these conditions adequate curing is achieved in
a few hours.
This is the end of Part One. We'll leave our concrete for a leisurely and
thorough curing until the next edition of SCIENCE IN AFRICA!
For PART TWO: 
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