As an alternative to using factory-made precast elements for major civil works, contractors may elect to cast elements on site. Nine times out of 10, the reason is to save money. So how did site casting come about and is it really the right way to go?
Standard for civil sites gets go-ahead
In late 2015, a new standard for prefabricated concrete elements being used for the construction of buildings – AS 3850 2015 Prefabricated concrete elements – was published. While the standard doesn’t apply to the construction of civil structures, many in the industry have been applying the relevant parts of the new standard to their civil projects.
The decision to do so has been in light of heightened awareness that there exists no similar such standard for using prefabricated concrete elements on civil construction sites, and the risks associated with that.
That has been the exact reason why National Precast submitted a project proposal to Standards Australia – a proposal that has recently been approved – to develop a part 3 of AS 3850. Currently in two parts, a new part 3 will apply to using prefabricated concrete elements in civil construction.
With the standard being published, there is a growing trend to change our terminology. During the development of AS 3850, the BD-066 committee responsible for revising the standard quite rightly reminded industry that tilt-up was referring to a lifting method, rather than a product in its own right. Being lifted by rotation about one edge, in contact with the casting bed until in a vertical or near vertical position, then lifted into position where it is temporarily braced or permanently incorporated into the main structure, is how the term tilt-up came into being. The result: what for so long have been called tilt-up panels are in fact simply prefabricated concrete elements that are cast on site and then tilted. As the standard is being implemented, there is a gradual move away from the term tilt-up, to prefabricated concrete elements that are either site cast or factory cast.
Factory cast, of course, is what industry commonly refers to as precast concrete. Manufactured in purpose-built factories, the manufacturing process really constitutes more than just ‘casting’; it is a highly refined process that is undertaken by skilled workers in a safe factory environment, with outstanding quality outcomes.
In the civil space, we’ve not tended to use the term ‘tilt-up’ for obvious reasons. While site cast panels may occasionally be prefabricated on a civil site, more often than not it is the casting of other prefabricated elements that come into question… but should they be site cast or factory cast?
Site cast civil elements
Occasionally, a contractor will decide to set up a ‘casting yard’ on site to cast civil prefabricated concrete elements. Too often it’s for cost reasons alone. But is that wise? There are a number of considerations, but let’s just take four: finish quality, speed of construction, quality and safety.
It’s a widely accepted fact that the aesthetics of our surrounds contribute to a society’s well-being. How many times do we see an eye-sore of a structure, which is uneven in colour and of a poor quality finish? Site casting may be OK for an off-form finish where the quality of finish isn’t too important – perhaps in a remote location – but if the structure is in a city location or a highly trafficked area, quality of finish is becoming recognised as increasingly important. The forms used in the factory are of better quality than those normally used for site cast elements, hence truer shapes and better finishes are obtained.
More and more, we are seeing form liners or other decorative elements being integrated into the design of civil structures. A variety of decorative finishes can be achieved in a precast factory, achieving both function and form.
Speed of construction
Because precast is manufactured away from site, it can be cast while ground works are being prepared. Delivery can be just in time and in accordance with a project’s schedule, resulting in continuous uninterrupted on site erection of elements. Reducing site congestion by avoiding the need to cast huge elements on site, precast offers faster construction, with no materials deliveries, no formwork, no need to set up expensive stressing beds and no post-tensioning.
This total saving in time equates to lower interest paid on construction finance, earlier commissioning of the structure and a quicker return on the owner’s investment.
The importance of durability can’t be understated in the civil environment. No-one wants bridges which don’t last their intended lifespan, or cracks appearing during the structure’s life. Being manufactured by skilled staff, using advanced concrete mixes and curing methods, plus being subjected to strict quality testing regimes, precast can guarantee a specified strength, quality and durability.
The concrete strengths used in the factory range from 35 – 60 Megapascals, with higher strength preferred to ensure durability and higher cycle production rates. Both conventionally reinforced and prestressed elements provide a higher quality, higher precision product than can be provided with on-site casting.
Every person should be able to go home from work at the end of a day’s work. In that regard, we all have an obligation to take every opportunity to improve safety wherever we can.
This can be achieved by using precast on a civil site instead of site casting. The controlled factory conditions offer a much safer environment for workers; unlike the busy and cluttered construction site.
When one major element of the construction works can be undertaken away from the site in a safer environment, surely the safety considerations override any cost savings. After all, what price can we put on someone’s life?