Dip and strike angles method for yield line analysis of reinforced concrete slabs

Yield line analysis is a useful method for design of reinforced concrete slabs, but has been limited to slabs of simple geometry, because up to now there has been no generally applicable and fully automatic computational procedure for complex-shaped slabs. Herein, a new yield line method that can be applied to any convex polygonal-shaped slab is developed. In this method, the deflections of the slab regions divided by yield lines are measured in terms of the dip and strike angles of the slab surfaces, which can define the geometry of all kinematically admissible collapse mechanisms or yield line patterns. The external work done and the internal energy dissipation at yield lines are evaluated as functions of the dip and strike angles, and the principle of virtual work is used to determine the corresponding load factor. The final solution is obtained by minimising the load factor with respect to the dip and strike angles. A computer program based on this method has been produced. Its correctness is verified by checking against results obtained by others for simple cases, and its versatility is demonstrated by applying it to complicated slabs subjected to point, line, patch and uniformly distributed loads.published_or_final_versio

Effects of CSF content on rheology and cohesiveness of mortar

It is widely known that the addition of condensed silica fume to a concrete mix would improve the cohesiveness and hence segregation stability. However, quite often, this would also impair the flowability. To study the overall effects of condensed silica fume, an experimental programme was launched, whereby 45 mortar samples with different condensed silica fume contents and water/solid ratios, each representing the mortar portion of a concrete mix, were tested for their rheological properties, cohesiveness and packing densities. Their rheological properties were measured in terms of flow spread, flow rate, yield stress and apparent viscosity, while their cohesiveness and packing densities were measured using the sieve segregation and wet packing tests respectively. It was found that the effect of condensed silica fume on flowability could be positive or negative, depending on the water/solid ratio, and when the water/solid ratio is low, the addition of condensed silica fume would improve the flowability by increasing the packing density. It was also found that the effect of condensed silica fume on cohesiveness is always positive. Put together, the overall flowability-cohesiveness performance would be improved by the addition of condensed silica fume. Thomas Telford Ltd © 2011.published_or_final_versio

Testing of coupling beams with equal end rotations maintained and local joint deformation allowed

The strength and ductility of the coupling beams in coupled shear walls can significantly affect the nonlinear behaviour and earthquake resistance of the whole building structure. However, although extensive testing of coupling beams has been performed, the boundary conditions-that the rotations at the two ends of a coupling beam are equal and that local deformation occurs at the beam-wall joints, which could have substantial influence on the test results-have not been correctly simulated. Herein, a new method of testing reinforced concrete coupling beams that ensures equal rotations at the ends of the beam specimen and takes into account local deformation at the beam-wall joints is developed. The method has been successfully applied to test typical reinforced concrete coupling beams with relatively small span/depth ratios and proven to be suitable for studying the post-peak behaviour and failure characteristics of short coupling beams. Test results obtained so far indicate that reinforced concrete coupling beams with small span/depth ratios behave quite differently from ordinary beams in frame structures and that the local deformation at beam-wall joints is quite substantial. Complete load-deflection curves have been acquired and the strength and ductility of the coupling beams evaluated.published_or_final_versio

Effects of various, shape parameters on packing of aggregate particles

The shape of the aggregate particles used has significant effects on the properties of the concrete produced. One major effect is on the packing density of the aggregate which determines the amount of cement paste needed to fill the voids between the aggregate particles. In order to study how the various shape parameters of aggregate particles would affect the packing of aggregate, aggregate samples of different rock types from different sources have been analysed for their shape characteristics using a newly developed digital image processing technique and their packing densities measured in accordance with an existing method given in the British Standard. The packing densities of the aggregate samples are correlated to the shape parameters to evaluate the effects of the various shape parameters on packing. From the results of the correlation, it is found that the shape factor and the convexity ratio are the most important shape parameters affecting the packing of an aggregate. Two alternative formulas revealing the combined effects of these two shape parameters on the packing density of aggregate are proposed.published_or_final_versio

Combined effects of water film thickness and paste film thickness on rheology of mortar

In the mortar portion of a concrete mix, the water must be more than sufficient to fill the voids between the solid particles of cement and fine aggregate whereas the paste volume must be more than sufficient to fill the voids between the solid particles of fine aggregate so that there will be excess water to form water films coating all the solid particles and excess paste to form paste films coating the fine aggregate particles. Hence, it may be postulated that the water film thickness (WFT) and the paste film thickness (PFT) have major effects on the rheology of mortar. In this study, the combined effects of WFT and PFT on the rheology, cohesiveness and adhesiveness of mortar were investigated by testing mortar samples with varying water, cement and aggregate contents. It was found that whilst the WFT is the single most important factor governing the rheology of mortar, the PFT also has significant effects. Particularly, the PFT has certain interesting effects on the cohesiveness and adhesiveness of mortar, which should be duly considered in mortar design. © 2012 The Author(s).published_or_final_versionSpringer Open Choice, 25 May 201

Adding limestone fines to reduce heat generation of curing concrete

It is well known that the heat generation of curing concrete may be reduced by decreasing the cement paste volume of the concrete. However, as the cement paste must be more than sufficient to fill the voids between aggregate particles, the cement paste volume should not be limitlessly decreased. Herein, it is proposed to add an inert filler, such as limestone fines, to fill into the voids between aggregate particles so that a smaller cement paste volume may be used and the heat generation of the concrete may be further reduced. To study the effectiveness of adding limestone fines in reducing the heat generation of curing concrete, a series of concrete mixes with water/cement ratios ranging from 0.35 to 0.60 and different amounts of limestone fines added were tested for their workability, strength and heat generation. The results revealed that the addition of limestone fines to decrease the cement paste volume would significantly increase the strength and, more importantly, substantially reduce the heat generation of the concrete.published_or_final_versio

Adding steel fibres to improve shock vibration resistance of concrete

A newly developed shock vibration test method was employed to study the effects of shock vibration on steel-fibrereinforced concrete so as to explore the possibility of improving the shock vibration resistance of concrete. In total, 21 batches of concrete with steel fibre contents ranging from 0 to 4% were cast and subjected to the shock vibration test at ages of 12 h, 1 day and 7 days. The results revealed that the effectiveness of adding steel fibres to alleviate the short-term damage caused by shock vibration (in terms of immediate reduction in ultrasonic pulse velocity) was quite low, especially for shock vibration applied at an early age. However, the effectiveness of adding steel fibres to mitigate the long-term damage caused by shock vibration (in terms of reduction in 28-day direct tensile strength) turned out to be much higher for shock vibration applied at age within 1 day than at later age. One probable reason is that, even after vibration damage had been caused, the continuing development of the steel-concrete bond while the concrete was still young could restore part of the reduced tensile strength. Finally, a new set of shock vibration control limits for steel fibre reinforced concrete was established.published_or_final_versio

Effects of packing density, excess water and solid surface area on flowability of cement paste

Although it has been postulated for many years that it is excess water rather than whole water that lubricates the cementitious materials and governs flowability of paste (excess water is water in excess of that needed to fill up voids), there has been no detailed investigations to study the actual effects of excess water. This was due to the lack of a suitable method for measuring the packing density and voids content of cementitious materials, To resolve the problem, the authors have recently developed a new wet packing method that was applied herein to measure the packing densities and voids contents of cementitious materials containing different amounts of cement, pulverised fuel ash and condensed silica fume. The flowability properties of the paste formed of the cementitious materials with different water contents were also measured and correlated to the excess water contents, each determined as water content minus voids content. The results revealed that whereas an improvement in packing density would increase excess water content, flowability is governed mainly by excess water to solid volume and excess water to solid surface area ratios.published_or_final_versio

Cyclic behaviour of deep reinforced concrete coupling beams

Six half-scale models of reinforced concrete coupling beams with span/depth ratios ≤ 2.0 were tested under reversed cyclic load by a newly developed test method that can accurately simulate the boundary conditions of coupling beams in coupled shear wall structures. Five of them were conventionally reinforced and one was diagonally reinforced. Span/depth ratio and reinforcement layout were the main structural variables studied. Test results revealed that the deep conventionally reinforced coupling beams behaved quite differently from the ordinary beams in frame structures. Generally, shear failure was more likely to occur. Moreover, the additional longitudinal reinforcement bars (those placed near the centroidal axis) could contribute significantly to bending strength and therefore lead to an increase in shear demand. Nevertheless, the measured drift ratios of the conventionally reinforced coupling beams still reached 3·6-5·7%, which are not small for deep coupling beams. On the other hand, it was found that the provision of diagonal reinforcement radically changed the load resisting mechanism and significantly improved the energy dissipation capacity of the coupling beam. However, it had not improved the deformability of the coupling beam.published_or_final_versio

Packing density of cementitious materials: Measurement and modelling

Packing density has great effect on the performance of a concrete mix. However, little research has been carried out on the packing density of cementitious materials owing to the lack of an established measurement method. Herein, a new method, called the wet packing method, is presented. With this method, the packing densities of blended cementitious materials, consisting of ordinary Portland cement (OPC), pulverised fuel ash (PFA) and condensed silica fume (CSF), were measured. The results verified the theory that the packing density could be significantly increased by blending two or even three cementitious materials together. Comparison between the measured results and the predicted values by three existing packing models, together with some additional tests, revealed that in the presence of a third-generation superplasticiser, the packing density of CSF is dependent on the lime content. When the lime-containing OPC and PFA contents are low, the CSF particles would flocculate and pack rather loosely, but at higher OPC and PFA contents, or with lime added, the CSF would pack to a higher density. With the effect of lime accounted for, very good agreement between the measured results and the predictions by the packing models was achieved.published_or_final_versio