The concrete used in the manufacture of prestressed elements must meet certain requirements to ensure that the quality of these products enables them to be used in applications such as floors and precast walls.
The concrete mix must be constantly controlled during the casting of the elements and if necessary its components modified to achieve the best characteristics in terms of workability and strength.
The production of prestressed concrete elements requires concrete of extremely high quality and its cubic strength must be over 550 Kg/sqcm.
Therefore a careful choice of the components is essential as it is impossible to make prestressed elements using low quality concrete.
The composition of concrete cannot be left to the inexperience of the workforce but must be thoroughly tested and studied in the laboratory to find the correct components and dosages.
Even on site the quality of ancillary concrete must always be checked.
Various formulae of granulometric curves can be followed for the composition of a good granulometric aggregate mixture.
These curves are generally shown on graphic tables, where the diameters of the aggregates are draw on the abscissae and the percentage of their weight on the ordinates on a logarithmic scale, the finer aggregates must be checked more frequently.
The type and quantity of cement plays an extremely important part in the strength of prestressed concrete elements however it is wrong to consider this as the only factor and besides being a mistake it can be uneconomical.
320 - 350 kg/m3 of cement is considered to be the best dosage to obtain very high quality concrete bearing in mind that the aggregates have been previously proportioned and selected.
If non-selected aggregates are used a good strength can be achieved by increasing the proportion of cement to 400-420 kg / m³ or more but as the proportion of cement is increased, any increase in strength becomes less and less. Indeed a dose of over 500 Kg / m³ has very little effect on strength and can even be detrimental to the final concrete quality.
The amount of water used should be the minimum possible compatible with the casting requirements, but it will always be more than that which is strictly necessary for the chemical reaction which is about 0.27 to the weight of cement.
However, theoretical requirements have to adapt to practical ones and the usual amount of water used to obtain good workable concrete is between 0.35-0.42 of the cement weight, this is inclusive of water contained in the aggregates. Under no circumstances must the water exceed 0.5.
Aggregates must be clean, free from clay and other organic material.
The sand should be come from hard rock and not be calcareous. It should be siliceous if possible and to be “squeaky” when rubbed between fingers. The gravel or crushed stone should not be excessively hard and indeed if it is harder than the overall strength of finished concrete could cause a concentration of mass which lowers the concrete strength. The use of soft, chalky stones is unadvisable.
It is important that the gravel and crushed stone have a rough surface as the mortar has a better adherence and even the shape of the aggregate has an influence on the strength. A flat or needle-shaped grain is inadvisable and it is better to use a round grain especially when using crushed stone.
When two or, even better, three aggregates are available it is not always necessary to use sieved aggregates in order to have the theoretical curve similar to the granulometric curve, ,the only problem is to find the best proportion.
It is recommended that each category of aggregates has dimensions that fall within a narrow range.
It is evident that when using 2 types of sand they should have different characteristics, e.g. one fine-grained and the other coarse or medium grained. In addition there should only be a very small amount of less than 0.1mm “fine flour” sand.
Elements produced with the Slipformer machine are thin and relatively small so the maximum diameter of the aggregates should not exceed 12-15 mm. While elements produced by the Extruder machine can have larger sized aggregates up to 16-20 mm.
Cement must be of a high strength and of a good consistent quality.
The choice between normal drying cement and quick drying cement should be made according to the local climate conditions and the working cycle being implemented.
The water used for mixing, in addition to water contained in the aggregates, must be clean and clear, without any harmful or impure substances in it.
Additives are generally products of organic origin which if introduced in small quantities to the concrete mixture modify the properties of the concrete.
Additives are classified according to their function.
The most commonly used additives are:
Plasticizers (These improve workability. Decreasing the surface tension of the mixing water reduces the water requirement necessary to achieve the desired consistency (reducing the water/cement ratio) and thereby contributing to an increase in compressive strength.
Superplasticizers As above but about four times more effective.
Retarders These slow down the drying out of the concrete at the beginning of the hardening process especially in hot climates where high temperatures accelerate the drying out with subsequent loss of workability.