General information about the semi-precast construction method

Building with partly precast Wall and Floor Elements

The semi-precast construction method is based on the use of partly precast wall and floor elements which are made to measure in a precast concrete factory and then supplied to the construction site. The partly precast elements, containing the reinforcement designed for the final structure, provide the formwork for the in-situ concrete poured at the job site. The partly precast elements are erected at the job site using a conventional crane, and after fixing some additional reinforcement at the joints between elements and at wall corners a complete storey comprising the walls and the floor above will be poured in one operation. The site-poured concrete bonds together the various partly precast elements to form a monolithic structure without any joints.

Design Considerations

Similar to reinforced concrete frame construction, where load bearing columns must be arranged one on top of the other, the load bearing concrete walls in the semi-precast construction method must be arranged one on top of the other. The floors are designed to act as a diaphragm which will transfer wind loads to the vertical walls. The load bearing walls will be rigidly connected one to another by right-angle reinforced corners, resulting in a very rigid and monolithic structural system. The non-load bearing partition walls and external walls may be individually designed in accordance with local requirements.

Production of the partly precast Elements

Nowadays it is possible to pre-fabricate wall elements as well as floor slabs completely automatic in precast concrete factories. Integrated production systems for floor slabs and wall elements use multipurpose formwork panels, which are operated in a closed circuit production line and on which one may produce either the one-layer floor panels or the two-layer wall panels in one compact production unit.

In modern curcuit production lines of this kind the operations of placing reinforcement, casting concrete, vibrating concrete and curing concrete always take place at the same specially designed work station. This increases productivity enormeously.

The formwork panels will be automatically stacked on top of each other in a curing chamber up to 14 pallets in one pile. This way the heat resulting from the curing process itself can be utilized. This reduces the cost of energy.

CIM-Computer lntegrated Manufacturing

In many factories, the plate elements are already being produced by the CIM-method. The design office first computerises construction projects, using a special software package (PC-standard). This system first elaborates the detail plan of each plate and the erection plan for the construction site, then it works out the relevant production data for steel and concrete. Via a master computer, the entire factory - plotting, reinforcing, concreting, curing - will be controlled. Owing to this CIM-concept it is possible to erect incividually designed buildings just as economically as mass-produced building units.

Handling and Transport

Totally prefabricated large-size concrete elements have a very high weight and therefore need very strong and expensive lifting devices and special road vehicles. Partly precast elements, on the contrary, can be handled by relatively light portal cranes and the customary site cranes. Road transportation can be effected by normal trucks. The weight of the partly precast elements is only about 25% (floor slabs) or about 50% (double-leaf wall elements) in comparison with fully precast elements. Therefore, the construction "area" that can be shipped in one load is greatly increased.

Erection There is little doubt that the joints in floors and walls of buildings using fully precast elements do create problems. Experienced specialists may tackle the problems, but ordinary construction workers without special experience shy away from these problems. Erecting the partly precast elements is much easier: The site-poured concrete connects all elements in one continuous core without any joints.

Time Factor and Cost Advantages

The essential advantage of the semi-precast construction method results from the sensible distribution of functions between the precast concrete works on the one hand and the construction site on the other hand. The complicated functions of preparing formwork and placing reinforcement are attributed to a well organized precast concrete factory, whilst the bigger part of the heavy concrete is poured at the job site.

The organizational problems of the job site are thus enormeously reduced.

Building with the partly precast construction system means: Erect elements - pour concrete – finished.

An additional and very important time and cost advantage of the system is derived from the fact that both sides of the wall elements and one side of the floor elements are perfectly smooth and even. Traditional rendering therefore is no longer necessary - only the joints require filling in and then the soffit may be "spray gunned" with a plastic bonded paint.

Summary

The partly precast construction system practically eliminates all the problems that industrialized building has suffered from so far.