Striking concrete workers at Hawaiian Cement Co. will be back at the bargaining table Thursday.  The federal mediator has asked Hawaiian Cement and the Teamsters Union to look at new approaches to health benefits. The company’s demand that employees pay a portion of their health insurance premium is a big part of what caused the strike. The unions’ suggestion that both companies negotiate together has been rejected because officials say federal labor and anti-trust laws make joint negotiations too complex. The strike at Hawaiian Cement moved into its fifth day Wednesday.  The strike has forced many local construction companies to start laying off workers because of the halt in cement supplies.