Welded floor-to-floor connections are widely employed in dry-assembled precast concrete decks with the main aims to stiffen the diaphragmatic action under horizontal loads and increase robustness of the construction. Bars or slugs are often preferred to plates for their smaller room, better allowance, and cost-effectiveness. Despite welded bar connections are mentioned in many official design codes, their design rules are not therein specified, which was only recently done in ISO Standard 20987:2019 on simplified design guidelines for mechanical connections between precast concrete structural elements in buildings. This standard is the result of EU’s Safecast Research Projects FP7-SME-2007-2 and GA 218417:2009, in the framework of which many structural connections of precast structures were tested and investigated at different scale levels, although specific local testing of welded bar connections was not carried out. Thus, the quoted ISO formulation lacked from direct experimental check. This paper aims at filling this gap by presenting the results of shear tests on floor-to-floor connections made by bars welded to rebar-anchored inclined angles. The tests were carried out considering anchorage rebars having different shape and diameter. Moreover, the validity of the currently available ISO design rule is commented, highlighting a disproportioned degree of conservativeness referring to concrete spalling failure. An updated formulation is proposed based on a novel mechanical behaviour model matching the experimental observations and results.
ShearThis paper utilizes double-K fracture criterion to investigate the effect of aggregate volume fraction on the Mode I and II fracture behaviors of concrete experimentally. Wedge splitting tests and Mode II fracture tests for non-notched specimens were carried out simultaneously for 60 specimens which were prepared with coarse aggregate in different volume fractions of 19%, 25%, 31% and 37%. It was found from the experimental results that as the volume fraction of coarse aggregate increases from 19% to 37%, both the Mode I and II fracture toughness increase and the crack trajectory becomes more tortuous. Based on the double-K fracture criterion, the ratio of KIIC/KIC is herein clarified as KIIC/Kini IC and KIIC/Kun IC. It is found that the values of KIIC/Kini IC and KIIC/Kun IC are not constants for concrete, instead, they are logarithmically related to the aggregate volume fraction. With the increase of the coarse aggregate volume fraction from 19% to 37%, the values of KIIC/Kini IC decrease sharply from 18.9 to 6.29, whereas the values of KIIC/Kun IC increase from 2.15 to 2.84. Finally, from the maximum distance between adjacent aggregates in concrete, a general formulation was proposed and verified by the published data and our tests to reasonably estimate Mode I and II fracture toughness with different volume fractions of coarse aggregate.
Because they contain residual water, cementitious materials produce radiolytic dihydrogen under irradiation, which can pose a safety concern for some nuclear applications. The estimation of H2 emissions by current models takes into account the contribution of pore water but not that of solid hydrates, which is considered negligible. Gamma irradiation (dose rate of 359 Gy h-1 up to 6 months in a closed system under Ar gas) of hydrated tricalcium silicate pastes, the main constituent of Portland cement, shows however an initial contribution of solid hydrates to H2 formation. A systemic study combining the source term of solid hydrates and the pore water radiolysis using simulation shows that the primary radiolytic yield devoted to portlandite and cementitious C-S-H (together) is of the order of 2.8 × 10-8 mol J-1, and that the contribution of the solid phases is decreasing and becomes negligible in the long term, suggesting a saturation effect with dose. Even if limited in time, the effect of the additional H2 source term is very sensitive on the radiolysis of the pore water with the very strong solicitation of the recycling reaction chain (equivalent to the Allen chain in basic medium) and leads to a frankly reducing medium (Eh ≈ − 470 mV NHE).
In Japan, RC bridges slabs constructed during the high economic growth period are suffering from severe deteriorate due to fatigue. In cold and snowy regions, in addition to fatigue, the deterioration is accelerated and aggravated simultaneously by freeze-thaw cycles and water penetration simultaneously. Hence, in this study, aiming at investigating the deterioration due to this combined action, three RC beams were artificially damaged on the top surface by introducing expansion agents to simulate the freeze-thaw cycle damages beforehand, and then subjected to moving wheel load under either dry or wet condition. In the experiment, a scaling-off and a disintegration were observed on the top surface due to the existence of water. Besides, to facilitate the elucidation of the mechanisms, image analysis was employed to obtain the displacement distributions and strain distributions on the side surfaces of the specimens during the test. The results of image analysis clearly manifested that the water-induced deteriorations lead to a localized compressive strain on the upper part of the beams as well as detrimentally affected the propagation of the shear and bending cracks under the moving wheel load. In summary, the mechanical reasons of the remarkably shortened fatigue life of artificially damaged RC bridge slabs due to combined fatigue loads and water penetration were uncovered in this study taking advantage of the strengths of image analysis, which also provides reference and inspiration for further related studies.
The use of high chemical admixture dosages in ultra-high-performance concrete (UHPC) mixtures to achieve adequate water demand can slow down early cement hydration and prolong the setting time. In this study, the effects of nanosilica (Ns) with high chemical admixture dosages on the rheological properties of UHPC was investigated. A factorial design approach was employed to predict and optimise the Ns content, water-binder ratio (W/B), and sand-binder (s/b) ratio to obtain the best flowability, setting time, and compressive strength. This study represents an attempt to modelling and optimise eighteen UHPC mixtures containing various proportions of water, cement, and sand, with the Ns powder as a possible property enhancer to achieve the best rheological properties. Response surface analyses revealed the significant effect of Ns in controlling the prolonged setting time and improving the compressive strength. Based on the applied criterion conditions, the optimisation results indicated two mixtures targeting either the maximum compressive strength or cost effective materials. The use of a 1.12 s/b ratio with a controlling level of 0.8% Ns content was suitable to fulfil the compressive strength, flow, and setting time limit values.
The use of low heat Portland cement has been promoted in recent years. However, there is no general method to predict the strength development of the concrete using cement that is composed of a wide range of minerals and the application of the ‘maturity method (accumulated temperature)” has proved difficult for low heat Portland cement. Therefore, the authors consider the application of the equivalent age to predict the strength development of concrete using various type of cement. The strength development of mortar using cement that is composed of a wide range of minerals under different curing temperatures are then examined. As a result, the temperature dependency of strength development of various cements is explained by the equivalent age. The values of the apparent activation energy of cement are dependent on its mineral composition. This paper is the English translation from the authors’ previous work [Taniguchi, M., Katsura, O., Sagawa, T. and Hama, Y., (2011). “Effects of the mineral composition of cement on the temperature dependency of strength development.” Journal of Structural and Construction Engineering (Transactions of AIJ), 76(661), 443-448. (in Japanese)].