英语翻译For structures that require flexural strength for resist

英语翻译For structures that require flexural strength for resistance to bending mo-ments,reinforcing steel--rebar--is embedded in the concrete.In some cases,tensioned cables are placed in the concrete to make prestressed or post-tensionedconcrete.The tension to resist the bending moment comes from the reinforcingsteel or the stressed cables.For a concrete mix to reach its ultimate strength,a long period of time isneeded.The ultimate strength of concrete and its 28-day strength are not thesame.For design purposes,engineers use the 28-day strength,and the concreteshould be subjected to only minimum loads until the 28 days have passed.Thatrule is very rigidly enforced in heavy construction.It is not at all uncommon forwork in residential and medium-rise commercial building construction to be pro-cessed on a 3-day floor cycle.Place concrete one day,form and shore the nextfloor over the next two days,this puts minimum load on the new slab,and thenplace concrete on the third day.This placement increases the load on the previ-ously placed floor,but the load is carried primarily by the shoring,which pas-ses continually down for several floors,thereby shifting the load to concrete thathas had time to reach its ultimate strength.Note the shoring and reshoring inFig.6.5 is in place to a depth of four floors below the slab that is currently be-ing placed.（ultimate strength ofconcreteThe compressivestrength of concretecontinues to increaseslowly for manymonths.The ultimatestrength may be higherthan the 28-daystrength,which isused for designcalculations.）