Kampung Baharu-Kampung Teluk Bridge in Manjung-Perak and Batu 6 Bridge in Gerik-Perak, Malaysia, are constructed with the Ultra-High-Performance Concrete (UPHC). This material, which has been commercialised by an Ipoh-based company, Dura, not only plays a key role in the construction of bridges in Malaysia, but is also making its way to the international market.
WHY UHPC IS NOT LIKE ORDINARY CONCRETE
UHPC premixes are powders, superplasticiser and organic fibres, with Portland cement being the main ingredient. In its current form UHPC is mainly used for bridges and bridge components as it could, among other things, address the connection challenge of precast concrete construction. For example, a study on the UPHC structural loading shows that field-cast UHPC connection could facilitate the construction of an emulative bridge deck system with characteristics meeting or even exceeding those of a conventional cast-in-place bridge deck.
The compressive strength is 10 times more than traditional concrete so it will not bend easily under a load or in compression. To illustrate, a normal concrete used in bridges has a compressive strength of 3,000 to 5,000 psi; whereas UHPC has a compressive strength of 18,000 to 35,000 psi. While traditional concrete has a tensile strength of 400 to 700 psi, UHPC has a tensile strength of 1,400 psi. UHPC is also durable under extreme weather conditions; when its surface is abraded, its resistance is nearly twice as high as normal concrete.
UHPC can also accelerate construction and minimise traffic impact in the construction process. Prefabricated bridge components must be connected with materials that are strong, durable and easy to install. UHPC not only has such a necessary composite material quality, but also simplicity in its application.
HOW DOES IT CHANGE CONSTRUCTION METHODS?
In a way, UHPC will speed up the process and improve the longevity of bridges, but from the contractors’ perspective, the question is how much it will change construction methods and techniques. According to J. P Binard, as published in Precast/Prestressed Concrete Institute, the use of UHPC in a bridge construction may mean the following:
There is essentially no change from the general contractor’s perspective if UHPC is selected as the material for a component in lieu of another material. In terms of means and methods, the status quo may continue for the contractor aside from potentially having to drill holes, which may require an increase in the number of drill bits used in the task.
Casting and cleaning
The activity of casting, independent of mixing and delivery, is similar to that of regular concrete. Cleaning UHPC from a smooth surface is relatively easy. A simple knife can work; but if there is anything for the UHPC to bond to, such as bolts, nuts and dimpled concrete surfaces, then jackhammers may be required.
Forming with UHPC has an additional dimension because the autogenous shrinkage, which occurs before form removal, is high. This can prohibit removing forms or cause cracking so attention to free shrinkage during initial curing is necessary.
No active steam is required for the initial curing. Within 14 days of casting, typically all of the individually batched, discretised segments for a given span of a bridge undergo a secondary cure at 90 degrees Celsius for 48 hours. For UHPC, high-temperature curing is not deleterious to the matrix or composition of the concrete. In addition, it produces additional reactions, further hydrating the cementitious material and densifying the system so that maximum strengths can be achieved.
We may have the knowledge and the know-hows to utilise UHPC and to make it a mainstream technology available for everyday use. The reason why it has not been used in a larger scale, like other new technologies, is probably the high initial investment and the lack of design codes. Thus far, the main use of UHPC is for bridges.
The increase in the construction activities in Asia is tremendous, not only in transportation infrastructure but also housing and institutional developments, which results in the increase in material demand. There will also be demands for the renovation and maintenance of ageing infrastructure in developed countries.
The question remains whether design codes will be advanced so that the adoption rate can follow suit. Countries with stringent regulation such as Singapore, for example, may be reluctant to use the material for structural components in buildings just yet, and limit the use to non-structural components such as Prefabricated Bathroom Units (PBUs) in its residential projects.
Innovation, therefore, is needed, to extend the use of UHPC beyond bridges.
Anisa Pinatih – Construction+ Online
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