JohannesburgThe South African construction and airport workers went on strike for higher wages yesterday, extending a series of industrial action threatening to slow growth in Africa’s largest economy.The labour unrest poses a risk for President Jacob Zuma’s African National Congress as it heads into elections next year facing increasing criticism that it has not done enough to help the millions of unemployed and working poor almost 20 years after the end of white-minority apartheid rule.The rand last week tumbled to a four-year low after gold miners last week threatened to strike and 30,000 workers in the car manufacturing sector, responsible for 6 per cent of gross domestic product, walked off the job.The National Union of Mineworkers (NUM) said about 90,000 of its members in the construction sector planned to down tools on Monday. More than 50 percent of employers were affected by the labour action, industry group SAFCEC said in a statement.Employment in the construction industry was at just over one million people at the end of June, according to government data.The strike has grazed construction shares, with an index of top companies in the sector down about 0.7 per cent since news broke last week of the walk-out, compared with a 1.2 per cent rise in the broad All Share Index.Firms affected by the strikes include Wilson Bayly Holmes Ovcon, Aveng Ltd and Group Five Ltd.Police reported clashes at a building project in Johannesburg’s financial district of Sandton, where NUM members in the union’s red T-shirts arrived at the site and forced workers to down tools.