Long-term strains in a two-storey, reinforced lightweight aggregate concrete office

Abstract

Shrinkage and load-induced strains were measured for 27 years in the in-situ concrete first floor and roof slabs of a long, two-storey office, using embedded vibrating wire strain gauges. The floor and roof slabs were cast on precast crossbeams and precast columns supported the crossbeams. Longitudinal precast beams between the columns supported the brick cladding and office windows but they were not connected to the floor or roof slabs. A central staircase provided longitudinal stability and a movement joint was provided at the access corridor to an existing, adjacent stone tower. A lightweight coarse aggregate was employed in the precast and in-situ concretes. The shrinkage and load-induced strains were determined using horizontal strain gauges parallel and transverse to the length of the slab and the direction of the expected flexural stresses. Concrete prisms were cast and instrumented for shrinkage under site exposure and elastic modulus, creep and shrinkage under sealed and dry exposure conditions in a laboratory. The drying shrinkage in the 190-mm-thick floor slab levelled-off around 500 microstrain (μs) at 6 years, a smaller value than the 750 μs from the site prisms at 3 years. The expected flexural load-induced strain profiles in the floor were superposed by a tensile stress component of strain. The tensile stress was attributed to the restraint of the drying shrinkage by the longitudinal precast beams. The deformation properties of the concrete were used in conjunction with the floor slab shrinkage to estimate a maximum tensile stress of 1·3 MPa. This was significantly smaller than the tensile splitting strength of 4·4 MPa that was measured at 10·5 years and suggested that there would be little associated cracking. The drying shrinkage in the 190 mm thick roof slab, sealed on the upper surface, was 450 μs after 27 years but, unlike the floor, had not yet levelled-off. Furthermore, a swelling was observed between 7 and 10 years that was due to water infiltration through leaks in the bituminous roof membrane. Shrinkage resumed after the roof had been repaired. The load-induced flexural strains in the roof, like the floor, were superposed by a tensile stress component of strain that could be attributed to restraint of drying shrinkage by the precast longitudinal beams. The maximum tensile stress in the roof slab was estimated to be 1·5 MPa. The risk of reinforcement corrosion from the roof leak was assessed by measuring the depth of carbonation in a prism stored in the ceiling void below the roof. The 12 mm depth of carbonation was safely smaller than the 20 mm depth of cover to the reinforcement. The longitudinal movement of the first floor caused cracking of the attached ground floor blockwork at 5 years. The mortar joint to the floor slab was replaced with mastic and no further cracking was observed. The floor slab movement also caused cracking of several large windows in the glazed corridor that provides access to the offices.