The influence of cement particle-size distribution on autogenous strains and stresses in cement pastes of identical waterto-cement ratios is examined for cement powders of four different finenesses. Experimental measurements include chemical shrinkage, to quantify degree of hydration; internal relative humidity development; autogenous deformation; and eigenstress development, using a novel embedded spherical stress sensor. Because the latter three measurements are conducted under sealed conditions, whereas chemicalshrinkage measurements are made under "saturated" conditions, the National Institute of Standards and Technology cement hydration and microstructure development model is used to separate the effects of differences in hydration rates (kinetics) from those caused by the different initial spatial arrangement of the cement particles. The initial arrangement of the cement particles controls the initial pore-size distribution of the cement paste, which, in turn, regulates the magnitude of the induced autogenous shrinkage stresses produced by the water/air menisci in the air-filled pores formed throughout the hydration process. The experimental results indicate that a small autogenous expansion (probably the result of ettringite formation), as opposed to a shrinkage, may be produced and early age cracking possibly avoided through the use of coarser cements.
Funding informationDanish Association for Precast Concrete Elements; COWI Foundation. This paper presents a new and construction-friendly shear connection for the assembly of precast reinforced concrete shear wall elements. In the proposed design, the precast elements have indented interfaces and are connected by a narrow zone grouted with mortar and reinforced with overlapping U-bar loops. Contrary to conventional shear connections, the planes of the U-bar loops are here parallel to the plane of the wall elements. This feature enables a constructionfriendly installation of the elements without the risk of rebars clashing. The core of mortar inside each U-bar loop is reinforced with a transverse double T-headed bar to ensure transfer of tension between the overlapping U-bars. Push-off tests show that a significantly ductile load-displacement response can be obtained by the new solution as compared to the performance of the conventional keyed shear connection design. The influence of the interface indentation geometry was investigated experimentally and the failure modes in the push-off tests were identified by use of digital image correlation (DIC). For strength prediction, rigid plastic upper-bound models have been developed with inspiration from the observed failure mechanisms. Satisfactory agreement between tests and calculations has been obtained.
K E Y W O R D Sconcrete plasticity, digital image correlation, ductility, keyed shear connections, robustness
| INTRODUCTIONStructural solutions based on precast concrete elements are often more economically feasible than in situ cast solutions, because precast technology enables a reduction of construction time as well as labor cost. When using precast solutions, the on-site work mostly consists of assembling and connecting the precast elements into an integrated structural system. Hence, connection designs that are constructionfriendly play an important role for the overall cost reduction. It is, however, a challenge to design connections that are easy to construct and at the same time have structural performance (in terms of strength and ductility) which can be compared with that of in situ cast solutions. In cases with unusual structural geometry, it may be necessary to supplement the advantages of precast construction with in situ cast solutions in selected zones. An example of how current precast solutions have been pushed to the limit can be studied in References 1 and 2 that report on the design and construction of a landmark building in Copenhagen, Denmark. The leaning characteristic of the building imposed serious challenges to the design of the shear connections between the precast panels for insurance of overall structural stability.Currently, structural continuity between precast shear panels is established by use of narrow keyed connections containing overlapping U-bars and grouted with mortar (see Figure 1). However, with this conventional solution, which has been used since the 1960s, it is difficult to obtain full structural continuity because the ...
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