Before discussing some basic Do's and Don'ts of good concrete practice, it is useful to have a general definition of concrete. ("Concrete" as discussed here is taken to mean Portland cement concrete, as distinct from asphaltic or biluminous concrete, which is mostly used for road pavements, Portland cement concrete is also suitable for making roads, airport runways, and such the like).
Concrete may be seen as a man made rock, similar to so called conglomerates (cemented gravels) which occur in nature. it is made up of a system of rock particles or fragments of different sizes (the so called "aggregate") which are packed together, and fastened (cemented) by a hardened paste of a hydraulic cement (usually Portland cement with or without other materials) and water. In one form or the other, cementitious concrete is the material most used in structural (i.e. Load bearing) applications in construction, sometimes competing with steel, sometimes working together with it, in so called reinforced concrete is known for its Compressive Strength (which is much better than its tensile strength) and for its Durability (some existing Roman concretes are nearly 2,000 years old and are still in good condition) and for its ability to be molded into nearly any desired shape, on or off the construction site. This latter quality makes it adaptable to an extremely wide range of construction applications.
The effects of materials and processes on concrete
The main factors which determine the character and performance of concrete are:
- The properties of the materials which are combined to make it;
- The proportions in which these materials are mixed together, and;
- The effect of any processes which are applied to the mixture so produced.
These points will now be briefly examined: The final properties of hardened concrete are greatly affected by its behaviour in the "wet' or freshly mixed condition and by the processes it undergoes while in that state; it is therefore essential that the making and handling of concrete be guided by a proper understanding of this behaviour, if its full potential is to be realized in practice. The properties and performance of concrete can be affected by any or all of the following *
Choice of Ingredients:
The choice of materials greatly influences the type and character of the concrete which is produced; some of the main considerations are the following:
The Type(s) and Origin(s) of the aggregate(s) selected, this affects the strength, durability and reactivity of the aggregates, which typically make up between 65% and 75% of the volume of an "average" concrete, and so exert a considerable influence on the concrete in both the freshly mixed and the hardened states.
The Grading Of The Aggregates (this is a measure of how much of each of the different sizes of stone particles exists in the particular aggregates, and depends on the aggregates chosen and in what amounts they are mixed). Grading affects how much water and cement ("cement paste") is required to cover all the stone particles and enable a practical concrete to be made.
The Prevailing Shape And Texture Of The Aggregate Particles. This is usually a result of two things: (a) the type of (so called 'parent") stone or rock from which the aggregate was removed; and (b) The natural as well as man made processes to which these stone fragments were subjected.
These factors considerably affect how the aggregate will behave while the fresh concrete is being handled and molded.
The Porosity of the Aggregate depends largely on its type and origin, and can greatly affect its strength and durability; depending on its state of dryness, it can also greatly affect the so called "workability" (the ease or difficulty with which the concrete can be "worked" or molded into various shapes) of the fresh concrete.