Learn A Bit More About Concrete

Introduction

This section has been written to explain the most important factors to consider when making and using concrete, mortar and plaster. And is given to you with our compliments to perhaps help you:

Save Yourself Countless Hours of Needless Hassle, Worry, and Unexpected Expenses That You or Inexperienced or Careless Contractors Can Easily Create With Your Building Project.

Please bear in mind that it is not easy to pass years and years of learning and experience along while still trying to keep everything simple and short. So, remember that this is basic stuff. It should help - but it is far from perfect.

We will be discussing how and why choosing the best materials, using the correct mix proportions and ensuring good site practice affects the strength, durability and economy of the finished concrete. We will also briefly touch on the various processes used by the small builder, DIY enthusiast or handyman using concrete, mortar and plaster for building projects.

One must understand that there's nothing new about concrete. The Romans were using it at least 500 BC, and they were in all probability not the only ones or the earliest ones. Although concrete technology has advanced a little since then, 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.

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 the section Concrete Materials.

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 referred 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 a layman's most common mistakes, is to assume that the final volume of concrete would be roughly equal to the sum of the coarse and fine aggregates. This results in major under ordering: and affects the budget somewhat!)

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 quantities 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 200kg 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 strength 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 minimized 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 strength, 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. But, a University Architect indicated a minimum of three days. Small local building contractors very often only allow 24 hours. 

So views on acceptable curing times vary quite widely - and actual building practices 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 later on.

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