The influence of casting position and disturbance induced by reinforcement on the structural concrete strength

Abstract

It is well known that control specimens used to assess the concrete strength of new structures have different casting and curing conditions than those of actual structures. Notably, after pouring of the concrete and before its hardening, a number of phenomena such as concrete bleeding and plastic settlement occur, influencing the in‐situ strength with respect to that of small and homogeneous control specimens (cubes or cylinders). In addition, the development of these phenomena and their structural implications are influenced by the presence of reinforcing bars, disturbing the settlement and bleeding of fresh concrete. In this paper, these aspects, with particular emphasis on the effective structural strength, are investigated by means of a testing program performed with refined measurement techniques such as tomography and Digital Image Correlation. On that basis, consistent design rules are derived to correct the strength of control specimens in order to calculate the resistance of a structural concrete member.

Concrete compressive strength: From material characterization to a structural value

“…For vertical casting conditions (Figure 6c), these voids develop under the hoops, while they develop at a side (with respect to loading direction) of the hoop for column elements cast horizontally (Figure 6d). The presence of these voids disturb the flow of stresses and can contribute to a premature spalling of the cover, with more unfavorable conditions when the voids are located under the hoops when cast vertically 16 . It can be observed that the difference due to casting conditions is limited (7%) and smaller than that for members without reinforcement for crack control (which has been observed to be approximately 10% 16 ).…”

“…The presence of these voids disturb the flow of stresses and can contribute to a premature spalling of the cover, with more unfavorable conditions when the voids are located under the hoops when cast vertically 16 . It can be observed that the difference due to casting conditions is limited (7%) and smaller than that for members without reinforcement for crack control (which has been observed to be approximately 10% 16 ). This is associated to the fact that the disturbances due to the casting conditions are partly covered by η cc , while additional differences and those related to on‐site variability are usually accounted for in γ C for design (this topic is treated in detail in Section 5).…”

“…For members with imposed tensile strains (and thus to transverse cracking), this factor reduces the material strength as acknowledged by Vecchio and Collins 15 while it enhances the strength in case of confinement (transversal compressive stresses applied to the concrete). η is (in‐situ concrete strength factor): This factor accounts for the differences in casting and curing between control specimens and structural members. A typical phenomenon is, for instance, related to material consolidation of fresh concrete (associated to concrete bleeding, plastic settlement and segregation), particularly detrimental close to the free surface, it can be relevant for members without confinement reinforcement or other cracking control means 16 η t (time‐dependence factor): Two effects are normally considered with respect to the influence of time on the concrete strength: the increase of the compressive strength with time due to continuous cement hydration and the decrease of the compressive strength for high levels of sustained loads 17 .…”

“…The pertinence of current design idealizations based on the consideration of a full‐cross section analysis (without cover spalling and without confinement enhancement) or of a reduced cross section (after cover spalling but with enhanced core strength) is investigated in this section with the help of 264 experimental results on reinforced columns subjected to pure compression. The tests are taken from the literature 16,31,33–50 and details of the complete database considered are given in Annex C of this paper. All specimens had low slenderness (negligible second order effects) and presented variable concrete compressive strength (24–200 MPa), cross section (square or circular), longitudinal reinforcement ratio (0.8–6.8%), confinement reinforcement ratio (0.1–4.5%), tie arrangement and spacing, yield strength of the longitudinal reinforcement (260–820 MPa) and yield strength of the confinement reinforcement (300–1,000 MPa).…”

“…The analysis of the test results shows, on average, slightly safer calculations of the strength of specimens cast horizontally than for specimens cast vertically. One source for this difference could be found on the presence of voids developing under reinforcing bars just after casting due to bleeding and plastic settlement of fresh concrete 16 . For vertical casting conditions (Figure 6c), these voids develop under the hoops, while they develop at a side (with respect to loading direction) of the hoop for column elements cast horizontally (Figure 6d).…”