It started crossing the Channel in bulk in the 1960s when the frantic pace of construction in those booming days made its rapid hardening and early strength seem like the answer to contractors' dreams.Soon Lafarge was selling 30,000 tonnes a year and two thirds of this were going into pre-stressed concrete beams.In 1973 disaster struck. Pre-stressed beams in three different buildings collapsed. HAC was taken off the market with its failure blamed on a process known as conversion.However, Lafarge was convinced that the product itself was not at fault, only the way it was used. So the company started a long-term research programme to prove its worth.Now the company is relaunching HAC on the results of its research and independent tests carried out by the Building Research Establishment, the Royal School of Mines and Imperial College.This has shown that the product goes through a two-stage process of crystallisation which gives high early strength. Then the natural conversion process reduces this strength to more normal levels. A further strength increase comes with time.A company spokesman said: 'It is a highly technical product for use under controlled conditions which, for the right applications, gives contractors advantages they can achieve in no other way.' These include resistance to extremes in temperature, thermal cycling, abrasion and acid.Lafarge is concentrating on road construction, runway repairs, tunnelling, pipelines, foundations and marine applications. But no-one is suggesting it is used in concrete beams.