The Field Test And Mechanism Research Of The Shrinkage Cracks Of Cat-in-situ Slab In Brick Masonry Structure

The shrinkage cracks of the masonry structure can affect the durability of the concrete component, and also make the users feel unsafe. The shrinkage stress distribution at different position was got through field test, theoretical analysis and numerical simulation. The mechanism of the shrinkage cracks were studied to provide a theoretical basis for the prevention and cure of the shrinkage cracks.A calculating formula at any location when the mid-span had aâ–³deformation was got considering the constraint of the walls and the reinforcements. The mid-span has the biggest shrinkage stress and the corner of the slab also very big through the derivation.A simple 3-D model was established in ANSYS to do the numerical analysis under the effect of shrinkage.the equivalent temperature was used to achieve the shrinkage effect. The conclusions through numerical analysis were as follow:(1) At the location of the bearer, the top of the slab had the biggest tensile stress; the biggest tensile stress occurred at the bottom of the slab at the location of the mid-span under deadweight and normal service load. Compared with the tensile stress caused by shrinkage, the tensile stress caused by deadweight and normal service load is too small to be counted.(2) The tensile stress of the cast-in-situ concrete slab caused by shrinkage mainly distribute in the big corner of the slab and the place where the walls contact the slab.(3) The shrinkage stress become bigger from the end room to the middle room, and the rate of the increase is only 5-7%. The difference between the longitudinal shrinkage stresse and the horizontal shrinkage stress is small.(4) The biggest tensile stress occurred at the place where the walls contact the slab when there is no threading pipe in the slab mid-span; and the stress concentration occurred at the mid-span when there is threading pipe. (5) The shrinkage stress is determined by the lateral stiffness of the walls, and the lateral stiffness of the walls is proportional with the horizontal cross-section of the walls. The lateral stiffness of the walls can be reduced by increasing the distance between the walls, so the shrinkage stress can also be reduced.(6) The reinforcement at the negative moment area can make he stress distribute more uniform.A field test lasted for 54 days was set on the fifth multi-span continuous cast-in-situ slab of a masonry structure dormitory, and the rate of the shrinkage was got. Through the analysis the conclusion was got as follows:(1)The deformation of the reinforced concrete slab experienced expansion stage, sustainable shrinkage stage and stable shrinkage stage three stages.(2)The four big corners of the slab have the biggest shrinkage stress; therefore, the shrinkage cracking is most likely occurred at the corner of the slab.(3) The longitudinal shrinkage stress of the slab increasing from the end room to the middle room of the slab.(4)The horizontal shrinkage stress occurred at the slab closed to the wall the end room is bigger than the middle room.(5) The longitudinal shrinkage stress is bigger than the horizontal shrinkage stress, but the difference is small.The shrinkage cracks caused by shrinkage deformation are most likely occurred at the corner and the mid-span of the slab through the result of the field test, the theoretical analysis and the ANSYS analysis. The longitudinal shrinkage stress is bigger than the horizontal shrinkage stress, but the difference is small.The measures that prevent and control the concrete cracking were proposed through the analysis and the experiences of the practical projects. (1) The radioactive reinforcement should be posited at the big corner, internal angle and the corner of big slab; (2) Steel net should be used at the place where there were threading pipe; (3) The guiding cracks can be used and so on. One or more measure can be used to prevent and control the concrete cracking at the practical projects.