Prestressed concrete prefabricates are composed of two materials working together, each of them providing its characteristic properties to the final elements: concrete and steel.
The ability of these materials to complement their mechanical characteristics is what determines the advantageous properties of prestressed concrete elements, the result of such a peculiar symbiosis.
Therefore, it is important to understand the basics of each of these raw materials.
For the molding of prefabricated buildings, a high-strength concrete must be used, above 550 kg/m2. In addition, it is of crucial importance to properly dose the main components: water, aggregates, cement and admixtures.
To optimize the resistance, it is advisable to reduce the water supplied as much as possible, although it will not always be possible to achieve the desired shape without a certain degree of water. Normally, around 30-35% of the cement weight will be added, to find a good balance between strength and moldability. In any case, 40% content should never be exceeded.
It is in this aspect that extrusion technology stands out, by admitting concretes with a significantly lower degree of water, not suitable for molding by vibro-compression.
The dosing of cement is crucial to obtain good results under a good economic performance. Normally, with a good selection of aggregates, a good mixture can be obtained using dosages below 350 kg/m3.
If the final resistance is not sufficient, you can choose to gradually increase this figure. However, if 450 kg/m3 is exceeded, there are most likely other more cost-effective ways of acting, such as improving the dosage of aggregates.
Aggregates play a vital role in the results obtained, so their selection, often subject to geographical limitations, is a key aspect, which is often not given the required attention and resources.
It is interesting to look for an optimal granulometry, which allows to adjust the amount of cement to be provided in the mass to the maximum. The ideal is to adjust, approximately, to the following distribution of calibers:
If extrusionis used, it is possible to use larger-caliber aggregates, up to 20mm, also increasing the proportion of coarse aggregates. In any case, it is advisable that the grain shape be as rounded as possible, to facilitate geometric shaping and achieve better results.
Granitic aggregates, because they are especially abrasive, can be harmful to the mechanical components of the machines, being especially discouraged for the use of extrusion molding.
The field of admixtures for concrete is of enormous breadth, with constant novelties on the market that invite permanent innovation.
The objective of the admixtures is to alter some of the properties of the concrete, to improve both the molding process and the final properties of the pieces. Some of the most widespread in this area are:
The steel used in the prestressing wires is a high strength steel, capable of reaching tensions of 14000 – 19000 kg/cm2 before breaking.
According to the conformal morphology, it can be used:
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