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Sat 10 May 2008 04:00 AM

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Function over form

Concrete suppliers in the region have to get to grips with a number of issues as they try to keep up with the demand for their product in the construction industry. Hugo Berger looks at the range of additives which are helping them overcome their dilemmas.

Concrete suppliers in the region have to get to grips with a number of issues as they try to keep up with the demand for their product in the construction industry. Hugo Berger looks at the range of additives which are helping them overcome their dilemmas.

As the temperature soars in the GCC summer, ready mix concrete suppliers are facing a battle against the clock.

In Dubai, the race against time to get concrete from plant to site is intensified by the city's infamous traffic gridlock.

The intense Gulf heat speeds up the hydration of concrete, putting pressure on concrete suppliers to ensure their products are delivered to construction sites on time.

As concrete only has a limited lifespan - about two hours in the summer months - if it is not transported from plant to site quickly it is easily spoiled.

In Dubai, the race against time to get concrete from plant to site is intensified by the city's infamous traffic gridlock.

A solution used by many ready mix suppliers is a carefully blended recipe containing additives.

These not only keep the mixture from hardening during transportation, but also improve its workability when it arrives at its destination and is ready for pumping.

Vishal Sharma, general manager, construction chemicals, Conmix, says there are a number of new chemicals which the firm adds to concrete to keep it fresh.

He says controlling temperature during concrete pours was vital to the stability of buildings.

In the case of mass concrete structures there will be an increase in the temperature during the hydration of cement.

As the cement water reaction is exothermic, meaning it releases heat energy, the temperature rise within a large concrete mass where the heat is not easily dissipated, such as the core of the building, can be very high.

If the temperature difference between the core and the surface exceeds 20C then there was a chance of developing thermal cracks in the structure.

Sharma says that because of this, it is vital that the right types of additives are used during the mixing stage.

The use of materials such as fly ash or ground granulated black furnace (GGBS) slag can help control the placement temperature of concrete.

He says: "Water reducing and set-controlling admixtures can be used to keep the concrete plastic in massive blocks longer, so that successive layers can be placed and vibrated before the previous layer sets."

"By using admixtures the water content in the mix can also be reduced, and hence for the same water to cement ratio, the cement content in the mix will be reduced. Low cement content means less heat generation."Many ready mix suppliers in Dubai are also using superplasticisers to help lengthen the lifespan of concrete.

There are additives that increase the plasticity or fluidity of the material to which they are added.

Most concrete mixes allow two hours between batching and placing.

Superplasticisers for cement soften the mix before it hardens, increasing its workability or reducing water.

In ancient times, the Romans used blood as a superplasticiser for their cement mixes, but nowadays cutting edge chemical mixes are used.

They are commonly manufactured from lignosulfonates, a by-product of the paper industry.

And more recently, they have been made from sulfonated naphthalene or polycarboxylate.

When mixing these, the amount of superplasticiser used in the mix is critical.

Usually, it is about one to two per cent. But if too much is used it could have a retarding effect, meaning the cement will take longer to harden.

During the placement of fresh concrete, Sharma insists it is vital to keep the temperature as low as possible - a difficult task in the region.

He says placement temperature should be no higher than 32C.

"This is achieved by using ice in the form of flakes together with the use of chilled water at the readymix batching plants," he says.

But even before any concrete can be poured, there is the challenge of transporting the material from plant to site.

With summer temperatures sometimes exceeding 50C, if concrete trucks are held up in traffic this will affect the workability of concrete, which is measured in slump temps.

In the Gulf winter, when the temperature averages around 25C, a batch of concrete may be sent from the plant with a 180mm slump.

By the time it has reached the site, after an hour for example, the slump will be around 150mm.

But an hour on the road during the summer could lead to the slump falling from 180mm to 120mm.The reaction in the concrete will be more vigorous because of the higher temperatures.

In Dubai, where the traffic problems are notorious across the region, planning of transport becomes even more critical.

In such cases, many ready mix additives can be used to modify the properties of fresh concrete by increasing its workability retention.

"While designing a concrete mix for a particular site, the major factor is the transportation time from the concrete batching plant to the site," says Sharma.

Another key aspect of the ready mix is the water to cement ratio, which has a crucial impact on the overall strength of the concrete.

Extra care need to be taken during the summer when water evaporation occurs much quicker and the reaction of the cement with the water starts earlier.

This poses a dilemma: if water content in the cement is reduced, its workability is affected, but if more is added, then the strength of the cement is reduced.

Because of this, additives are also used to increase the workability of cement without increasing water content.

Most concrete mixes allow two hours between batching and placing.

But in Dubai, where being stuck in traffic for longer this is not uncommon, if the journey exceeds this time it will lead to the concrete being rejected.

If the journey looks likely to exceed this, then there is nothing else to do but for the driver to turn around and head back to the plant with the rejected batch.

Arturo Rodríguez Jalili, technical director of Cemex, says time sensitivity was the biggest headache facing his company.

He says: "There will always be time for the truck to come back and dump the concrete in our dumping yard.

"But if the truck has a major break down, for example, and the concrete sets, then we use high pressure water to remove the concrete from the drum, and sometimes we break the concrete manually from inside."

So a large range of chemical admixtures is available to help prolong the workability of cement.

As summertime approaches, the region's ready mix suppliers are using innovative ways to cope with the cope with the heat.

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