Comment Knowledge for Concrete

WHAT IS CONCRETE

Concrete is the product of mixing, aggregate, cement and water.
The setting of concrete is a chemical reaction between the cement and the water, not a drying process.
This reaction is called hydration, it evolves heat as does any chemical reaction, and the process is irreversible.
There is an initial set when the concrete will cease to be liquid but have little strength (e.g. 6 to 24hrs. old), thereafter the concrete will gradually gain strength over time until it achieves the strength required.
Differing mix proportions and cement types will achieve required strengths in differing time spans.

CONSTITUENTS OF CONCRETE

Cement, Aggregate and Water, (and sometimes additives).

Aggregate
Aggregates are usually distinguished between fine and coarse aggregate.
Aggregates are classed as inert materials, such as washed natural sand (fine); and natural gravel, which can be crushed to produce the appropriate size and grading of aggregate, and similarly crushed, quarried stone (coarse).
The aggregate must have a minimum inherent strength requirement for structural concrete, the coarse aggregate must not be weaker than the concrete paste.
All aggregate must be "clean", i.e. not contaminated with organic matter or clay/silty soils and overburden during extraction and storage.

Cement
Basically a material made by heating limestone and a suitable clay to produce a clinker rich in calcium silicates.
This clinker is ground to produce a fine powder, this is cement.
By using different clinkers, grinding them to differing degrees of fineness and the use of additives many different types of cement are produced with varied properties in their use, e.g. rapid hardening cement, sulphate resisting cement, etc..
Generally speaking the more cement in a mix the stronger more durable the concrete produced will be, but this does have to be related to other factors, primarily the amount of water used in the mix, i.e. water/cement ratio.

Water
Water is an extremely important part of concrete, and drinking quality water is usually required, or water from an approved source free from impurities.

Additives
The most commonly used additive is a "foaming" agent to produce air entrained concrete, mainly for carriageway concrete, but also other exposed situations.
Another common use of an additive is to increase the workability of concrete without adding extra water and thus increasing the water/cement ratio and decreasing the strength of the concrete.

TIME TAKEN TO PLACE CONCRETE AFTER BATCHING

From the time of adding water to the cement the chemical reaction has begun and you only have a limited amount of time to place and compact the concrete, this is usually specified as 90 minutes.
The delivery ticket of the load of concrete will be stamped with the time of batching.

ADDITION OF WATER

Given a set amount of cement and aggregate there is an optimum amount of water to be added to produce a chemical reaction to give the maximum obtainable strength, too little or too much water will produce a weaker concrete.
Unfortunately as in all things, life is not that simple, and the workability of the concrete has to be considered when placing concrete, especially in difficult situations.
These situations can be areas of high density of reinforcing bars, complicated formwork design, or where the concrete needs to be suitable for pumping.
In these situations water content is increased to make the concrete more workable, BUT this increase in water content is calculated at the design stage and the cement content is increased accordingly to retain the strength of the mix.

For every designed concrete mix with a specified strength there is a set WATER:CEMENT RATIO which must be retained in order to achieve the designed strength.

WATER SHOULD NEVER BE ADDED TO CONCRETE, ESPECIALLY ON SITE, WITHOUT THE CONSIDERATION OF ALL THE ABOVE POINTS,
AND THE APPROVAL OF THE ENGINEER

CONCRETE WORKABILITY

An on site simple test for determining workability is the SLUMP TEST.
This consists of a conical mould 300mm. high, with an opening at the top of 100mm. diam., and at the bottom of 200mm. diam..
The mould is filled with concrete in 4 layers and rodded to remove air voids, with the smaller orifice uppermost.
The "slump" is the difference in height between the height of the mould and the height of the concrete column with the mould removed.
The workability of the concrete will depend upon the situation into which the concrete is being placed.
Low workability, i.e. stiff concrete, is needed for carriageway concrete which is laid by a "paving train".
High workability concrete is needed in situations of high density of reinforcing steel to enable the concrete to flow around all the reinforcing without leaving any voids.

SPECIFYING CONCRETE STRENGTH

The strength/grade of concrete is specified and measured in newtons/sq. mm., meganewtons/sq. metre or even megapascals, in fact the numerical figure will be the same in each case.
E.g. a strength of 20 newtons/sq.mm. is the same as 20 meganewtons/sq.metre.

The strength/grade of concrete is normally specified by stating the strength you wish the concrete to achieve after a period of 28 days.

The specifications governing the design, use and testing of concrete have undergone tremendous changes in the lat few years, I will not go into this topic on this page other than to say you may like to be  aware of the introduction

MEASURING CONCRETE STRENGTH

The strength is measured by crushing concrete cubes to failure and recording this strength.

Concrete cubes are made from fresh concrete sampled at the time of pouring by placing correctly sampled concrete into a steel mould and compacting to remove air voids.
The concrete is allowed an initial "set" period of 24 hours, the mould is then stripped and the cube is cured in water at a temperature of 20 deg.c for 28 days prior to crushing.

If you wish to strike shuttering before 28 days, extra cubes will be required to determine that the in-situ concrete has achieved the appropriate strength at the time you wish to strike the shuttering. 
This is usually an arrangement agreed by the contractor, the concrete supplier and the engineer.