Concrete is a composite construction material, composed of cement (commonly Portland cement) and other cementitious materials such as fly ash and slag cement, aggregate (generally a coarse aggregate made of gravels or crushed rocks such as limestone, or granite, plus a fine aggregate such as sand), water, and chemical admixtures.
Concrete solidifies and hardens after mixing with water and placement due to a chemical process known as hydration. The water reacts with the cement, which bonds the other components together, eventually creating a robust stone-like material. Concrete is used to make pavements, pipe, architectural structures, foundations, motorways /roads, bridges/overpasses, parking structures, brick/block walls and footings for gates, fences and poles.
Properties of Concrete
· Compression strength
· Tensile strength
· Protection against corrosion.
Compression Strength: It is commonly known that concrete becomes very hard and can withstand enormous pressures; a property which is called compression strength. This compression strength depends mainly on the properties and quality of the cement paste and the aggregate. If the aggregate consists of a soft or weak material, the concrete will be weak also.
Tensile Strength: The tensile strength of a material means its capability of being stretched to a certain extent without breaking. Although concrete becomes very hard, its tensile strength is very limited. It is so low that in practice. The tensile strength of concrete is regarded as being nonexistent. This is why sometimes concrete members of a structure must be reinforced by steel bars embedded in them.
Protection against Corrosion: Corrosion means a wearing away, a slow destruction caused by a reaction with air, water or chemicals. Reinforcement iron which is left unprotected and exposed to air and humidity will eventually start to corrode on the surface and become rusty. If this process is not halted in time, the rust goes into the bar and it becomes too weak to be used.
Batching, Mixing, Placing and Compaction of Concrete
· Batching
It is the process of measuring concrete mix ingredients either by volume or by mass and introducing them into the mixture. Traditionally batching is done by volume but most specifications require that batching be done by mass rather than volume.
Percentage of accuracy for measurement of concrete materials as follows.
Cement:
When the quantity of cement to be batched exceeds 30% of scale capacity, the measuring accuracy should be within 1% of required mass. If measuring quantity is less than 30% i.e. for smaller batches then the measuring accuracy should be within 4% of the required quantity.
Aggregates:
If the measurement is more than 30% of the scale capacity then the measuring accuracy should be within 1%. If measurement is less than 30% then the measuring accuracy should be within less than 3%.
Water:
Water is measured in volumetric quantity as 1 litre=1kg.
In case of water, the measuring accuracy should be within 1%.
Admixtures:
For mineral admixtures same accuracy as that required for cement. For chemical admixtures same accuracy as that required for water. Mineral admixtures accuracy is same as that of cement because it is used as partial replacement of cement. As chemical admixtures are liquid or added to water therefore its accuracy is same as that of water.
· Mixing
The mixing operation consists of rotation or stirring, the objective being to coat the surface the all aggregate particles with cement paste, and to blind all the ingredients of the concrete into a uniform mass; this uniformity must not be disturbed by the process of discharging from the mixer.
Batch mixer
The usual type of mixer is a batch mixer, which means that one batch of concrete is mixed and discharged before any more materials are put into the mixer. There are four types of batch mixer.
Tilting drum mixer:
A tilting drum mixer is one whose drum in which mixing take place is tilted for discharging. The drum is conical or bowl shaped with internal vanes, and the discharge is rapid and not segregated so that these mixers are suitable for mixes of low workability and for those containing large size aggregate.
Non tilting drum mixer:
A non tilting drum is one in which the axis of the mixer is always horizontal, and discharge take place by inserting a chute into the drum or by reversing the direction or rotation of drum. Because of slow rate of discharge, some segregation may occur.
Pan type mixer:
A pan type mixer is a forced–action mixer, as distinct from drum mixer which relies on the free fall of the concrete inside the drum. The pan mixer consist of a circular pan rotating about its axis with one or two stars paddles rotating about vertical axis of pan.
Dual drum mixer:
A dual drum is sometimes used in highway construction. Here there are two drums in series, concrete being mixed part of the time in one and then transferred to the other for the remainder of the mixing time before discharging.
Continuous mixers:
These are fed automatically by a continuous weigh-batching system.
Charging the mixer:
There are no general rules on the order of feeding the ingredients into the mixer as this depend on the properties of the mixer and mix. Usually a small quantity of water is fed first, followed by all the solids materials. If possible greater part of the water should also be fed during the same time, the remainder being added after the solids. However, when using very dry mixes in drum mixers it is necessary to feed the coarse aggregate just after the small initial water feed in order to ensure that the aggregate surface is sufficiently wetted.
Uniformity of Mixing
In any mixer, it is essential that a sufficient interchange of materials occurs between parts of the chamber, so that a uniform concrete is produced. The efficiency of the mixer can be measured by the variability of the samples from the mix. ASTM prescribes samples to be taken from about points 1/6 and 5/6 of the discharge of the batch and the difference in the properties of the two samples should not exceed any of the following:
1. Density of concrete 1 lb/ft³
2. Air content 1%
3. Slump 1" when average is less than 4"
4. 1.5" when average is less than 4 to 6"
4. % of aggregate retained on 4 No. sieve 6%
5. Compressive strength 7 day, 3 cylinders 7.5%
Mixing time:
It is important to know the minimum mixing time necessary to produce a concrete of uniform composition, and of reliable strength.
The mixing time or period should be measured from time all the cementing materials and aggregates are in mixer drum till taking out the concrete.
Mixing time depends on the type and size of mixer, on the speed of rotation, and on the quality of blending of ingredients during charging of the mixer. Generally, a mixing time of less than 1 to 1.25 minutes produces appreciable non-uniformity in composition and a significant lower strength; mixing beyond 2 minutes causes no significant improvement in these properties.
Prolong mixing:
If mixing take place over a long period, evaporation of water from the mix can occur, with a consequent decrease in workability and an increase in strength. A secondary effect is that of grinding of the aggregate, particularly if soft; the grading thus becomes finer and the workability lower. In case of air entrained concrete, prolong mixing reduces the air content.
