Blowholes in concrete 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.
Blowholes (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 blowholes 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 blowholes 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 blowhole formation. A sticky or stiff mix that does not respond to consolidation causes increases in bubbles and blowholes whereas workable, flowing mixtures are easier to vibrate and reduce the risk of blowhole 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 blowhole 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.
