Economic and Structural Implications of Admixtures in Concreting
Concrete is a very good construction material made by mixing cement, coarse aggregate (gravel or crushed stone); fine aggregate (Sand) and water either in designed or prescribed proportions. It is strong in compression and has some resistance to some chemical and biological attack like termites etc. While steel, which is strong in tension, is incorporated in it, thus becoming “reinforced concrete” a strong and durable material, which can be formed into various sizes and shape. This accounts for its wide spread use in civil engineering structure such as buildings, dams and so on.
However, as a result of the daily increase in the cost of
concrete material most especially cement, crushed stone (coarse aggregate) and
fine aggregate (fine sand). It becomes imperative to introduce and develop
available local materials. This is what has necessitated the use of admixtures
as a substitute for a fraction of cement and aggregates.
ADMIXTURES IN CONCRETE
Admixtures are materials other than cement, aggregates and water that are added, to concrete at the batching stage, to modify the properties of the hardened or plastic concrete. They are usually added in small concentration.
There are a number of different types of admixtures. They are classified according to their functions.
- Water reducing admixtures
- Plasticizing admixtures.
- Air entrainment admixtures
- Setting time retarders
- Setting accelerators
- Shrinkage compensators
- Gas generators
- Water proofing admixtures
- Corrosion inhibitors
- Coloring Admixtures
- Miscellaneous admixtures such as workability, bonding, damp-proofing, permeability reducing, grouting, gas-forming, anti-washout, foaming, and pumping admixtures.
Other forms of Admixtures include:
Fiber is a small piece of reinforcing material possessing certain characteristics which when added to concrete during batching provides it with certain characteristic properties at the plastic and hardened state. They can be circular or flat. Some common types of fibers are polypropylene, alkali resistant glass and steel
Fibers are used in concrete to control cracking due to plastic and drying shrinkage. They also reduce the permeability of concrete thus reducing the rate of formation of bleed water. Some types of fibers produce greater resistance against abrasion and shatter in concrete.
A pozzolana is a natural or artificial material containing silica in a reactive form. By themselves, pozzolans have little or no cementitious value. However, in a finely divided form and in the presence of moisture they will chemically react with alkalis to form cementing compounds. Pozzolanas must be finely divided in order to expose a large surface area to the alkali solutions for the reaction to proceed. In production, they are passed through a 600micrometer sieve size to ensure they are fine enough to expose large surface area for reactions to take place around on the introduction of water. Examples of pozzolanic materials are volcanic ash, pumice, opaline shales, burnt clay Sawdust ash, rice husk ash, palm ash and fly ash.
The major reasons for using admixtures are:
- To reduce the cost of concrete in construction
- To achieve certain properties in concrete more effectively than by other means. These properties could be strength, toughness, abrasion resistance.
- To maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions
- To overcome certain emergencies during concreting operation. These emergencies could be a sudden change in weather condition necessitating setting time alteration. Soil content and nature which might come in form of sulfur deposit will require sulphate inhibitors else concrete will be exposed to, extensive cracking, expansion and lose of bond between the cement paste and aggregate. The effect of these changes is an overall loss of concrete strength.
Concrete should be workable, finishable, strong, durable, watertight, and wear resistant. These qualities can often be obtained easily and economically by the selection
of suitable materials rather than by resorting to admixtures (except air-entraining admixtures when needed). Despite these considerations, it should be borne in mind that no admixture of any type or amount can be considered a substitute for good concreting practice. However, critical conditions may necessitate use of Admixtures
The effectiveness of an admixture depends upon factors such as type, brand, and amount of cementing materials; water content; aggregate shape, gradation, and proportions; mixing time; slump; and temperature of the concrete.
Test conducted on concrete with varying percentage of rice husk ash and sawdust ash after 28days show that the flexural strength of the concrete is relatively adequate when 10% of cement is replaced with these pozzolans. The strength of 10% replacement is 4.378MPa whereas that of conventional concrete is 5.244MPa.
By comparing the two, we notice a slight decline in the strength of the concrete as more pozzolans are added.
Furthermore, bulk density of concrete reduces as more of certain Admixtures are added in concreting. The advantage this has is that we could achieve the construction of multi storey buildings with reduced overall dead load.
A compromise between the strength of concrete, density and cost savings would allow a certain percentage replacement of cement in concrete. The normal strength concrete may be put to uses such as the construction of structural members in buildings and mass concrete casting as in dams due to slow setting rate. Any cement replacement above 10% of pozzolans would be cheaper and friendlier to the environment but would produce low strength concrete. This may be used in works such as paving and minor repairs.