Precast concrete technology - Architectural finishing
Posted by
mashrabiya
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construction
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Posted on
5:33 AM
Hedorf's residence Hall, Denmark (KHR Architect)
Being in Saudi Arabia for the past 3 years enable me to conclude that besides steel, glass enveloped building, precast concrete façade is widely use here as it is somehow practical. Besides been used as a structural member, its also use as final architectural finishing elements. What is precast concrete?
Precast concrete is a form of construction, where concrete is cast in a reusable mould or "form" which is then cured in a controlled environment, transported to the construction site and lifted into place. (http://en.wikipedia.org/wiki/Precast)
Why I said it is practical here?
When we discuss about practicality in construction, we cannot avoid to relate it with cost, time and process/method.
CLIMATE - First of all, lets take a look into Riyadh climate being in the central region of Saudi Arabia. This city has 2 types of climate which are summer (April-Oct) and winter (Nov-Mar). The temperature during summer can go as high as 50 degree C while during winter it may go as low as -5 degree C. The difference in temperature is about 55 degree which is considered extreme. Compare to Malaysia, The difference in climate is about 15 degree C. The extreme climate condition in Riyadh does affect the performance of building materials as building elements tend to contract in cold and expand in hot condition.
In term of method, precast concrete as define earlier, is developed in the controlled environment. It saves the hassle of curing consideration on climate. However the design of the precast panel should incorporate building movement into consideration. Example ; soft joints, movement joint, expansion joint.
Unlike ‘in-situ’ concrete casting or plastering works, one should take consideration on the curing issue especially during summer. Fail to do so may result failure in concrete strength or debonding substrate. Certain chemical also need to be implies to enhance bonding ability and expand/contract reaction. And if the concrete did fail due to climatic response, the main contractor has to bear the cost of rectifying the works. In this case, cost implies.
QUALITY – Precast concrete panel is constructed using mold. The quality of the outcome mainly depends on the sub contractor / specialist in precast concrete. The QA/QC should check the mold prior to casting process. The thing that the main contractor has to take care of is the erection process unless it was done by other parties. The final finishing such as paint either can be applied later after erecting or could be pigmented during the molding process. Pigmented concrete is good for long term cost saving in term of maintenance. The sub contractor for precast concrete is held responsible for the quality issue which the main contractor could highlight before acceptance of the precast unit.
For non precast work, more on site QA/QC and supervision is needed before, during and after the casting work. The formwork, setting out, the reinforcement, the materials, grade of concrete, etc have to be properly supervised and executed. It requires more materials to finish it such as plastering accessories, paint works, concrete preparation and repair. Again, cost implies in the matter of rectification works and the methods involved along the way.
SITE PERSONNEL AND LABOUR - labour usage for precast concrete process may be minimal from the main contractor’s side as they just emphasize on supervision and QA/QC. The delivery of the precast panel can be regarded into sub contractor’s responsibility.
In comparison with in situ casting, the labour may expand from the preparation of formwork and reinforcement, concrete pouring, post concreting process and finally finishing. Supervision in hourly basis also increases throughout every stage.
TIME – In term of timing, it may be affected by certain factors such as the complexity of the precast panel, mobilization of precast into site and technology availability. However, provided that the precast sub contractor is reliable, from the design of the precast to the inspection period, in my opinion precast construction is faster than in situ concrete casting. As the quality take place, it sure save the time of major repairing of the concrete surface. If the panels are repetitive, it also saves the cost and time of rebuilding formwork as well as labour wise.
Considering the nature of the construction industry in Saudi Arabia, I may suggest that it is best to go for precast solution where it is possible. Precast method also better in dealing with curvy design and organic form where it reduces the uneven and undulation of concrete surface which easily exposed under the sun by the shades it creates. However, the main contractor may have to invest more on the short run but it does save a lot on the long run as we don’t want to be as a saying ‘save a penny, lose a pound’ or ‘save a halalah lose a riyal’.
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