Blow holes in concrete – a precast concrete producer’s worst nightmare – can be vastly reduced or virtually eliminated by adding Chryso Fluid Premia 100, a new ‘New Generation’ admixture produced by Chryso Southern Africa, to the concrete mix, says Eddie Correia, Chryso SA General Manager: Technical Services.

Blow holes (also known as “bug holes”), are individual, rounded or irregular cavities generally less than 10mm across in size that appear on vertical surfaces of concrete. The blow holes are usually regarded as a cosmetic problem, but large concentrations of these holes may impair durability in precast concrete structures.

“The tiny holes are surface voids that result from the migration of air entrapped inside moulds and become visible when the moulds are stripped from the concrete. The blow holes generally appear as a result of insufficient vibration, the use of incorrect form-releasing agents, the presence of water in the mould, and – importantly - the formulation and design of the concrete mix,” Correia explains. 

“Chryso SA has indeed found that the concrete mix design can be a significant contributor to blow hole formation. A sticky or stiff mix that does not respond to consolidation causes increases in bubbles and blow holes whereas workable, flowing mixtures are easier to vibrate and reduce the risk of blow hole formation. That is why the use of Chryso Fluid Premia 100 super-plasticiser in the mix to create increased workability can play such an important role in counteracting blow hole formation,” he states.

When added to the concrete mix, Chryso Fluid Premia 100 improves workability retention at all levels of consistency compared with standard additives. “Concrete, mortar and grout, when produced using Chryso Fluid Premia 100 in the mix, are low in viscosity and less sticky. This means surface finish aesthetics are substantially improved due to a reduction of surface bubbles. The concrete’s pumping properties are also much better because of the admixture’s specific formulation, which is based on BET 185-014, a modified phosphonate.”

Correia says the super-plasticiser has been widely welcomed in the South African precast industry.

Strong support of the product has, for example, has recently come from a precast industry leader, GPT Concrete Products South Africa (part of the GPT Group) that designs and manufactures pre-stressed concrete railway sleepers. With its corporate office in Kolkata, India, GPT has been manufacturing concrete sleepers for nearly 30 years and also have plants in other countries like Bangladesh, Sri Lanka, Cambodia, Iraq, Ghana, Mozambique, Myanmar (Burma) and Namibia.

Michael Christopher, General Manager of GPT Concrete Products’ operations in Ladysmith, KZN, says: “Chryso Fluid Premia 100 is an incredibly robust admixture which, when added to our mix, helps to produce less sticky concrete while achieving high early strengths. To release the pre-stressed wires, GPT concrete has to reach at least 35 MPa in strength. This is achieved - together with steam curing - in as little as 11 hours, ensuring greater recycling of moulds and faster sleeper production,” he stated.

“GPT is one of the pioneers in manufacturing pre-stressed concrete sleepers and is committed to producing a quality product with zero tolerance towards blow holes on our sleepers. The Chryso admixture has played a major role in maintaining our high standards,” Christopher added.

The GPT plant in Ladysmith was opened in 2009 and produces around 24 000 concrete sleepers per month.