bluuebunny/crossref_metadata_2025 huggingface dataset:
The effect of initial moisture content on water transport in concrete containing a hydrophobic admixture
Authors: Aldred J. M. , Swaddiwudhipong S. , Lee S. L. , Wee T. H.
Published: October 2009

Tests on initially saturated and dried concrete containing a hydrophobic admixture showed that initial moisture content had a significant effect on measured water transport properties. Concrete specimens containing the hydrophobic additive significantly reduced water penetration when the concrete had been initially dried, and had limited or no positive effect on water penetration into initially saturated specimens. The reduction in water transport compared with the control was generally more pronounced in hydrophobic concrete of higher w/c ratio, as well as under conditions of limited curing and greater drying. The hydrophobic admixture had no significant effect on water vapour diffusivity. These results suggest that the ingredients in the admixture only become effective where a clear wet–dry interface is present and tend to influence water transport through larger pores.
Explosive spalling of high-strength concrete columns in fire
Authors: Ali F. A. , O'Connor D. , Abu-Tair A.
Published: October 2009

Current concrete design codes raise concerns about concrete spalling during fire particularly under compressive stresses and high heating rates. High-strength concrete columns would be more prone to explosive spalling due to their low permeability and high brittleness. This paper represents an experimental programme on the behaviour of high strength concrete columns under fire. The research includes testing reinforced high strength concrete columns subjected to various loading levels and heating rates. Eighteen columns were tested under four loading levels and two heating rates. The paper represents the main results including the measured concrete and steel temperatures and axial displacements. Valuable conclusions on the effect of loads and heating on concrete explosive spalling are reached in the paper.
Plane stress tension stiffening effects in reinforced concrete
Authors: Christiansen M. B. , Nielsen M. P.
Published: October 2009

This paper presents a simple model for the prediction of the structural behaviour of reinforced concrete in plane stress. Stresses, strains and crack widths are determined. The model applies the well-known diagonal compression stress field of concrete, amplified to include a so-called developing crack phase, where tension stiffening effects may account for a significant part of the overall stiffness. In this respect, the major assumption of the theory is that the tension stiffening effects are expressible in terms of the mean crack width. A simple linear tensile softening response is assumed introducing a so-called critical crack width, which denotes the crack width level at which tension stiffening effects vanish. The model is compared with tests, and considering the recognised difficulties in the field, a rather good agreement is obtained.
Effect of aggregate volume fraction on the fracture parameters of concrete: a meso-mechanical approach
Authors: Tasdemir M. A. , Karihaloo B. L.
Published: October 2009

It is well known that fine and coarse aggregates play a vital role in the fracture of concrete. There is, however, little quantitative information available in the literature on the effect of the volume fraction of the aggregates on the fracture properties of concrete. The main objective of this work is to gain a better understanding of this effect using appropriate experiment and a meso-mechanical approach. For this, several control mixes ranging from hardened cement paste (HCP) to normal concrete were prepared in which the aggregate grading, w/c ratio, and the maximum particle size of aggregate were kept constant, but the volume fraction of aggregate was varied from 0·0 (HCP) to 0·60 in steps of 0·15. The critical stress intensity factor of the hardened cement paste, for which linear elastic fracture mechanics (LEFM) is valid, was determined from centrally notched disc specimens. The effective stress intensity factors of fine mortar, coarse mortar and concrete were then calculated from that of HCP using the appropriate toughening mechanism in meso-mechanical relations. The specific fracture energies, the macro-tensile strengths and the moduli of elasticity of fine mortar, coarse mortar and concrete were also calculated from meso-mechanical relationships. From these, the ductility of the mixes was calculated in terms of the characteristic length. It is shown how the meso-mechanical modelling approach gives a clear qualitative and quantitative picture of how a brittle matrix (i.e. HCP) progressively transforms into a tougher, stiffer and more ductile composite as the volume fraction of fine and coarse aggregates is increased without altering their grading.
Study on the reduction of tensile strength of concrete due to triaxial compressive loading history
Authors: Lin G. , Lu J. , Wang Z. , Xiao S.
Published: October 2009

Triaxial compressive experiments on 10 cm concrete cubes were performed. The maximum axial strain experienced by the cube associated with lateral confining pressure is defined as the loading history. The reduction of tensile strength and the descent of ultrasonic velocity characterised the damage degree of these cubes. The laws of damage initiation and development of concrete subjected to monotonic, as well as cyclic load, under the condition of varying confinement pressure were investigated. Based on the results of curve fitting of experimental data, the evolution equation of damage was obtained. The main factors affecting the damage development are examined and the causes are analysed. It is concluded that this method is feasible to estimate the damage of concrete in engineering practice.
Effect of fine mineral additions on granular packing of cement mixtures
Authors: Bigas J. P. , Gallias J. L.
Published: October 2009

This article concerns the influence of the physical characteristics of 18 fine mineral additions on water requirement, agglomeration and packing density of pastes. The main objective is to determine the principal parameters controlling their use in concrete mixtures. A standard Vicat test and, a new and simple test based on agglomeration of fine particles by capillary forces of a water drop, are used to determine the water requirement and the packing density. The influence of high-range water-reducing admixture is also studied. Results show that fine mineral additions unfavourably influence the water requirement and the packing density of cement mixtures in two cases. The first case concerns additions with irregular shaped particles, which lead to loose granular structures and very high water requirement. Both Vicat and ‘single drop’ tests could determine a critical threshold for water requirement. The second case concerns blended mixtures with a low cement content, where the cement–addition particles interaction phenomenon leads to increasing water requirement comparative to other proportioning. A high amount of high-range water-reducing admixtures could attenuate this interaction phenomenon only in water saturated mixtures.
Assessment of concrete structures subjected to fire—the FBTest
Authors: dos Santos J. R. , Branco F. A. , de Brito J.
Published: October 2009

One of the main problems in the assessment of concrete structures that have been subjected to a fire is determining the depth of deteriorated concrete. In order to do that, a new method, the fire behaviour test (FBTest), has been developed. With it the depth of deteriorated concrete is quantified by resorting to the measurement of the water absorption and tensile failure stress characteristics in discs obtained from cores drilled from the structure under analysis.
Radial fictitious cracking of thick-walled cylinder due to bar pull-out
Authors: Nielsen C. V. , Bićanić N.
Published: October 2009

A new analytical model is suggested to describe the radial cracking around a ribbed reinforcement bar embedded in a quasibrittle material such as concrete. The model is based on the partly cracked solution of a thick-walled cylinder, exposed to radial internal pressure. For the partly cracked inner ring of the cylinder an exponential softening model is applied to model the hoop stresses across the fictitious crack. The main advantage of applying an exponential softening relationship instead of a linear or a power-law model, which have been applied in previous investigations, is that the asymptotic behaviour of the exponential softening turns out to be a useful analytical feature. Another difference between the present analytical model and the previous solutions is that the radial displacements in the inner ring are assumed to be equivalent with the linear-elastic displacement field. Furthermore, the solution employs the brittleness number to give a dimensionless description of the problem. Through this brittleness number the size effect of the problem is investigated and it is demonstrated that the maximum capacity of the cylinder has correct small-size and large-size strength behaviour. Finally, the solution is compared with a simplified model, with experimental results and with Tepfers' classic solutions.
Influence of grout proportions on modulus of elasticity of two-stage concrete
Authors: Abdelgader H. S. , Górski J.
Published: October 2009

The modulus of elasticity value is important from the design point of view but it is time-consuming in direct measurement. For normal concrete, it is found that the modulus of elasticity can be satisfactorily predicted from the compressive strength value, which is obtained very easily. In the case of two-stage concrete (TSC) there is no background information for evaluating its elastic modulus. The TSC differs from the conventional concrete in a higher percentage content of the stone aggregate. The aggregate is deposited directly into the moulds with point-to-point contact and a high flow grout is then introduced into the bottom of the mould. The contact stresses are transferred first through the stone skeleton and then after deformations of grains, also through the hardened grout. Therefore the properties of the TSC are more dependent upon the stone aggregate. Thus, the characteristics of the TSC in failure conditions are different from the normal concrete. The objective of this research was to present the influence of the grout proportions on the elastic modulus of the TSC observed during compressive tests.
Crack-based analysis of concrete with brittle reinforcement
Authors: Stratford T. J. , Burgoyne C. J.
Published: October 2009

Brittle reinforcement (such as fibre-reinforced plastic) is being developed as an alternative to traditional steel reinforcement. The lack of ductility in a brittle-reinforced beam means that there is very little potential for stress redistribution, and the lower-bound theorem of plasticity (which allows many of the assumptions made in steel-reinforced concrete analysis) cannot be applied. Analysis of brittle-reinforced concrete must be based on a detailed examination of compatibility requirements within a beam, of which the cracks form an important part. A crack-based model is developed in this article, based on compatibility requirements where reinforcement crosses a crack, and compatibility in the compression-zone concrete. The analysis incorporates dowel-rupture of the reinforcement, and the reduced strength of a corner of a stirrup. It highlights the need for further research into flexure-shear of the compression zone, dowel-splitting, and local failure of the concrete. The crack-based model is used to illustrate the importance of compatibility in both the flexural and shear analysis of brittle reinforced concrete. In particular, the current proposals for shear design (which assume pseudo-plastic reinforcement) are examined, and contrasted with compatibility requirements within the beam.
Factors affecting the plastic shrinkage cracking of high-strength concrete
Authors: Branch J. , Hannant D. J. , Mulheron M.
Published: October 2009

Tests were developed to quantify parameters affecting the plastic shrinkage cracking of high-strength concrete of 28-day cube strength in excess of 70 MPa. The parameters measured were tensile stress–strain performance during the first 5 h after mixing and negative pore pressure development and free shrinkage during the first 24 h. Eight high-strength mixes were used containing a variety of supplementary cementing materials such as microsilica, pulverised fuel ash, granulated slag and metakaolin. Two types of superplasticers were included. Plastic shrinkage cracking was assessed using restrained ring tests in which measurements were taken using sealed samples and samples exposed to wind. The research has shown that there is no simple relationship between early age stress–strain curves, negative pore pressure, early age shrinkage and macrocracking in adverse conditions but two factors were always present when plastic cracking was observed, these being microsilica and wind.
Viscometric characterisation of cement grout for prediction of prepacked concrete construction
Authors: Swaddiwudhipong S. , Zhang J. , Lee S. L.
Published: October 2009

The ability of cement grout to penetrate voids of aggregate media, during the injection process of prepacked concrete construction, is strongly dependent on its viscosity. This paper reports experimental work on the viscometric characteristics of cement grout for the prediction of prepacked grouting process. Various factors influencing the measured results have been identified. Shear history dependent viscosity of cement grout requires that an experimental programme should be designed: (a) to ensure repeatability; and (b) to obtain meaningful results for the concerned construction process. Due consideration has been given to establish proper mixing and measuring programmes. Results were obtained through a set of carefully selected parameters to simulate the shear history during the grouting process. The Bingham model was adopted to characterise the shear rate dependency of the apparent viscosity. Time dependency of the Bingham parameters was obtained from a series of tests of the same batch under the specified initial mixing condition and subsequent agitation. Thixotropy loop tests were also carried out to analyse the sensitivity of the internal structure of cement grout to further shearing, thus comparing the mixing effect on rheological property of cement grout.
Monotonic moment–curvature relation of an RC beam
Authors: Kwak H.-G. , Kim S.-P.
Published: October 2009

A monotonic moment–curvature relation to simulate the non-linear behaviour of reinforced concrete (RC) beams is introduced, and material non-linear analyses of RC beams are carried out on the basis of the introduced model. Instead of the sophisticated layered section approach, which has some limitations in application to large structures with many degrees of freedom, we use the moment–curvature relations of RC sections previously constructed through section analysis. To reduce numerical instability according to the finite-element mesh size used, a relation simulating the tension-softening branch is taken into consideration. With the purpose of removing the imprecision in calculation of ultimate resisting capacity, we include the plastic hinge length in finite-element modelling. In addition, the governing equation describing the bond-slip behaviour in beams is derived. Unlike the present bond elements using double nodes, the proposed model uses beam elements representing the structural response by two nodes at both ends simplifies the finite-element modelling and analytical process, along with effectively describing the bond-slip behaviour. Moreover, the developed algorithm is reflected in the moment–curvature relation of the RC section. Finally, correlation studies between analytical and experimental results are conducted with the objective of establishing the validity of the proposed algorithms.
Cyclic moment–curvature relation of an RC beam
Authors: Kwak H.-G. , Kim S.-P.
Published: October 2009

A hysteretic moment–curvature relation to simulate the behaviour of a reinforced concrete (RC) beam under cyclic loading is introduced in this paper, and the non-linear behaviour of RC beams subjected to flexural cyclic loading is analysed by means of the introduced model. Unlike previous moment–curvature models and the layered section approach, the proposed model takes into consideration the bond-slip effect by using the monotonic moment–curvature relation introduced in the companion paper. This model was constructed on the basis of the bond-slip relation and corresponding equilibrium equation at each nodal point. In addition, unlike linearised hysteretic curves, the use of curved unloading and reloading branches inferred from the stress–strain relation of steel considering the Bauschinger effect is employed. This gives more exact structural responses, following a correct assessment of the energy-absorbing capacity of a structure at the large deformation stage. The advantages of the proposed model, compared to the layered section approach, may be on the reduction in calculation time and memory space in application to large-frame structures with many degrees of freedom. The modifications of the moment–curvature relation to reflect fixed-end rotation, axial force and pinching effect are also introduced. Finally, correlation studies between analytical results and experimental studies are conducted to establish the validity of the proposed model.
3D finite-element modelling of splitting crack propagation
Authors: Lura P. , Plizzari G. A. , Riva P.
Published: October 2009

Although many experimental results on bond strength are available and splitting is commonly recognised as an important aspect of bond strength, only a few studies on splitting crack development have been published so far. Although, splitting is a 3D phenomenon, the only measurement easily taken in a test is the splitting crack width on the specimen surface. As a result, the crack development inside the specimen is unmonitored. Such being the limits of testing, finite-element (FE) analysis can be a valuable alternative for the study of splitting crack 3D nature and for a better understanding of bond behaviour. However, FE studies are limited by the intrinsic difficulty in correctly modelling the bond. In fact, the models proposed so far are either too complex or too limited for describing actual anchorages and for studying splitting development.

The main purpose of this research work is to develop a 3D FE model in which bond is modelled by a simple kinematic interface. The model developed is useful for establishing rational design criteria for anchorages and splices. Such a model is validated by simulating a series of pull-out tests concerning the interaction between a bar and a stirrup, when the concrete cover is split and the stirrup crosses the splitting plane. The comparison between experimental and numerical results shows that the proposed model describes quite well the behaviour of an anchored bar in terms of both bond–slip response and splitting crack width. In spite of its simplicity, the proposed model can be easily expanded to cover more complex structural cases and to develop more rational criteria for the design of anchorages and splices in RC members.
Experimental investigation of the threshold chloride concentration for corrosion initiation in reinforced concrete structures
Authors: Oh B. H. , Jang S. Y. , Shin Y. S.
Published: October 2009

The corrosion of steel reinforcements in concrete is of great importance when considering the safety and durability of reinforced concrete structures. This study focuses on the corrosion behaviour of steel bars induced by internal chlorides in concrete. The main objective of this study is to determine the threshold chloride concentration causing depassivation and active corrosion of steel reinforcement in concrete. To examine the threshold concentration of the chloride ion, the half-cell potential, the chemical composition of extracted pore solutions of concrete and the extent of corroded area of the specimens were measured. Major test variables include the added amount of chlorides in concrete, type of binder, and water/binder ratios. From the present comprehensive test results, the factors influencing threshold chloride concentration are investigated, and the rational ranges of threshold chloride concentration causing active corrosion of steel are proposed. The present study will enable the specification of a realistic chloride limit in reinforced concrete structures, which can be used effectively in future technical specifications.
A theoretical study of the effect of coatings on concrete members under fire
Authors: Zhou X. , Zhang J.
Published: October 2009

Generally most concrete structures have coatings on their members. Basically the coatings can be classified into two categories: non-combustible and combustible. For non-combustible coating, such as mortar rendering, it is a retardant, which prevents fire heat penetrating to their inner structural members. So the fire safety of concrete structures is improved by this kind of rendering. On other hand, combustible rendering is a fire load that can produce additional heat during fire. And as a result, more damage would be incurred by concrete structures with combustible decorative materials if fire were encountered. In this paper utilising the Eurocode indoor temperature–time relationship under fire condition and concrete non-linear analysis approach, the distribution of temperature within the section of concrete members and the load bearing capacity has been investigated. Based on the analysis of results, factors related to the fire safety of concrete structures have been discussed.
Effect of the interfacial transition zone on the transport and the elastic properties of mortar
Authors: Yang C. C.
Published: October 2009

In order to investigate the effect of aggregate content and interfacial transition zone (ITZ) on the transport properties of mortar, specimens with different fine aggregate volume fractions were cast and tested. The chloride ion migration coefficient and the total chloride content of mortars were obtained using the accelerated chloride migration test and the experimental results were plotted as a function of the fine aggregate volume fraction. The results were analysed using the regression models that represent mortar as a three-phase composite material (cement paste, fine aggregate and ITZ) to estimate the transport properties of ITZ. The elastic modulus of ITZ was obtained using micromechanics method and experimental results. Based on the experimental and regression analytical results, when the assumed thickness of ITZ is 20 μm, the chloride content, chloride migration coefficient and elastic modulus of ITZ are 1·8, 1·6 and 0·2–0·4 times of the matrix properties, respectively.
Lightweight concrete containing thermal power station and stone quarry waste
Authors: Nisnevich M. , Sirotin G. , Eshel Y.
Published: October 2009

Technology for the utilisation of highly porous bottom ash, formed as by-product of thermal power stations using pulverised coal combustion, is under development. The technology is based on the combined use of bottom ash as an aggregate and fly ash as an additive in the production of lightweight concrete. Such technology offers a solution to the persistent ecological and economical problems caused by coal ash. The following problems were addressed: (a) increasing the concrete strength with a low cement content and set resultant density; and (b) increasing the coal ash volume while maintaining rigid control of the concrete radioactivity – radionuclides (226Ra, 232Th, 40K) present in coal ash. To solve both tasks, part of the bottom ash was replaced by a denser aggregate with a minimum radionuclide level; cheap unprocessed crushed sand was utilised. Favourable results suggest a widening of the utilisation of coal ash and quarry waste in lightweight concrete production.
Behaviour of concrete under shear and normal stresses
Authors: Sonnenberg A. M. C. , Al-Mahaidi R. , Taplin G.
Published: October 2009

Thirty-eight concrete specimens were tested in a servo-controlled shear box test machine. The aim of the tests was to determine the combination of stresses that would cause failure in the concrete and to compare the failure stresses with Mohr–Coulomb theory. It was found that the failure load of specimens tested under a constant normal force could be predicted using Mohr–Coulomb theory with an assumed friction angle of 35°, provided that the normal stress was greater than 0·15fc. For normal stresses less than 0·15fc Mohr–Coulomb theory overestimated the shear capacity of the concrete unless a modified cohesion value and friction angle were used.
Developed method for measuring flexural strength and modulus of elasticity of micro-regions in normal and recycled aggregate concretes
Authors: Otsuki N. , Yodsudjai W. , Nishida T. , Yamane H.
Published: October 2009

Concrete as a material is a heterogeneous system formed by mixing cement, water, aggregate and some admixtures. Each region in concrete has its own distinct properties and structural behaviour. For instance, the interfacial transition zone (ITZ) is considered to be the weakest region of concrete, and its mechanical properties relate to the mechanical properties of concrete. Similarly, the surface layer of concrete is a weak region due to a bleeding of mix water; consequently, the transport of various aggressive substances into concrete depends on the quality of surface layer. Therefore, it is of paramount concern to characterise the properties of concrete in the micro-regions in order to fully understand its complex structure. This research was conducted to evaluate the flexural strength and the modulus of elasticity of the micro-regions in real concrete. By so doing, we would be able to establish quantitatively the strength and performance of concrete. To accomplish this, a newly developed method called the micro-bending method is used in performing the laboratory experiments. This method is specially designed to measure the mechanical properties of concrete specimens located in micro-regions. Moreover, this new method can be applied to characterise and compare the mechanical properties between normal and recycled aggregate concrete.
Ultimate stress of unbonded tendon in continuous members
Authors: Lim J.-H. , Moon J.-H. , Lee L.-H.
Published: October 2009

Mojtahedi and Gamble emphasised the effect of span/depth ratio from the study of continuous shallow specimens, which laid the ground for the revision of ACI 318-77 Code as ACI 318-83 Code. However, the fact that most specimens had failed prematurely before reaching their ultimate flexural capacities was overlooked. Specimens they referred to were not able to deform sufficiently because of improper reinforcing details or failure modes different to the flexural mode. Thus, the purpose of this study is to provide physical evidence for the overemphasis of current ACI Code (318-02) on the unbonded tendon stress of members with a high span/depth ratio. In this study, four new specimens were fabricated to the same specifications as the specimens of Burns et al., but in which bonded reinforcements were provided continuously. As a result, it was revealed that the current ACI Code, revised by the results of Mojtahedi and Gamble's study, overestimates the effect of span/depth ratio on shallow members.
A simple method for determining the true specific fracture energy of concrete
Authors: Karihaloo B. L. , Abdalla H. M. , Imjai T.
Published: October 2009

In a recent paper, Abdalla and Karihaloo confirmed the boundary effect hypothesis of Hu and Wittmann and observed that a size-independent specific fracture energy GF of concrete could be obtained by testing three point bend (TPB) or wedge splitting (WS) specimens containing either a very shallow or a deep starter notch. This observation was based on TPB and WS tests on limited number of specimens. In this paper, we have re-evaluated 26 test data sets on specific fracture energy of concrete published in the literature to assess the validity of this observation. The re-evaluation is found to support this observation. The determination of the true specific fracture energy GF of concrete thus becomes a simple and straightforward task requiring very few specimens of the same dimensions and shape. This re-evaluation also provides guidance for the selection of the specimen dimensions depending on the maximum size of aggregate used in the concrete mix in order to obtain its true GF.
The effect of head plate details on the pull-out behaviour of headed bars
Authors: Park H.-K. , Yoon Y.-S. , Kim Y.-H.
Published: October 2009

This paper presents the pull-out behaviour of headed bars having various head details and the prediction of pull-out failure capacity governed by concrete breakout failure, reinforcing bar failure and pull-through failure. It is becoming increasingly common to anchor reinforcing bars by welding a plate to the end of the bar. This paper presents a study of the influence of the shape and thickness of the plate on the pull-out behaviour of bars anchored in this way. A total of 58 pull-out tests were conducted on four slab-like specimens and 13 beam-like specimens containing headed bars. The test variables were the reinforcing bar diameters connected to the headed plate, the head plate shapes, the dimensions of head plates and the types of welding scheme connecting the reinforcing bars and head plates. Concrete compressive strengths and embedment depths were also taken into consideration. Headed bars were manufactured with varying details of head according to ASTM 970-98 and the embedment depths were based on CSA 23.3-94. Since there is no relevant specification governing the headed bars, pull-out failure capacities through the static tensile load test are compared to the values predicted by the current design methods for anchor systems connected to concrete, such as ACI-349, ACI-349R and the Concrete Capacity Design (CCD) Method. Failure modes and the respective effect of each test variable are discussed.
Autogenous shrinkage of high-performance concrete containing fly ash
Authors: Lee H. K. , Lee K. M. , Kim B. G.
Published: October 2009

This paper focuses on the effect of fly ash on autogenous shrinkage of high-performance concrete (HPC). The stresses induced by autogenous shrinkage were calculated using finite element method and a model for predicting autogenous shrinkage including the effect of fly ash was proposed. Test results show that autogenous shrinkage of HPC with water-to-cementitious materials ratio (w/cm) of equal to or less than 0·35 developed more rapidly than that of normal-strength concrete (NSC) with a w/cm of 0·50. For HPC with the same w/cm but containing fly ash, autogenous shrinkage decreased – the higher the fly ash replacement percentage, the lower the level of autogenous shrinkage. Numerical analysis results show that the tensile stress induced by autogenous shrinkage may cause early-age cracking in HPC. In conclusion, although partial replacement of cement by fly ash could effectively reduce autogenous shrinkage of HPC, incorporating fly ash only might not prevent early-age cracking.
Effect of curing on the tensile strength of medium to high strength concrete
Authors: Conroy-Jones G. A. , Barr B. I. G.
Published: October 2009

Curing effects upon the tensile strength of medium- and high-strength concrete are reported. Five concrete grades, with 28-day compressive cube strengths ranging from 40 N/mm2 to 110 N/mm2, were cured in two environments: air, and in water. Tensile strengths were measured using three indirect tests (flexural strength, split cylinder and torsional tensile). Strengths were measured at 28, 84 and 365 days. The objective of the study was to investigate the relative effects of curing on the indirect tensile strength measurement. Various strength-modifying mechanisms are identified, which are related to the observed results. The flexural and torsional surface failure tests exhibit air-cured strengths that approach and in some instances surpass their water-cured comparisons. This is thought to occur because of positive (drying moisture gradient prestressing) and negative (swelling gradient, self-desiccation) effects on the measured air-cured and water-cured strengths respectively. The split cylinder air-cured/water-cured ratio (0·78) compared well with other compressive strength studies (0·81).
Flexural toughness and impact resistance of steel-fibre-reinforced lightweight concrete
Authors: Zhang M. H. , Li L. , Paramasivam P.
Published: October 2009

This paper presents the flexural toughness, impact resistance, and other properties such as density, compressive and tensile strength, and modulus of elasticity, of both plain and fibre-reinforced lightweight aggregate concrete. It also discusses the effect of the density of coarse and fine aggregate on the flexural toughness and impact resistance. The impact resistance of the concrete was determined using instrumented drop-weight testing equipment. An expanded clay type of lightweight aggregate and steel fibres were used for the concrete. The compressive strength of the lightweight aggregate concrete ranged from approximately 40 MPa to 80 MPa. The results indicated that high compressive strength and density are desirable for good impact resistance of plain concrete, and the incorporation of the steel fibres improved the impact resistance substantially.
Reinforced concrete edge beam—column—slab connections subjected to earthquake loading
Authors: Shin M. , LaFave J. M.
Published: October 2009

Four two-thirds-scale reinforced concrete edge beam–column–slab subassemblies (two concentric and two eccentric connections) were tested under quasi-static cyclic lateral loading. Each subassembly represented a cruciform connection in an exterior moment-resisting frame with a monolithic floor slab on one side only, loaded in the longitudinal direction of the edge-beams. The tests explored the effect of eccentricity between beam and column centrelines, and the effect of floor slabs, on the structural performance of edge beam–column–slab connections subjected to earthquake loading. Performance of the specimens was evaluated in terms of overall strength and stiffness, energy dissipation, beam plastic hinge development, joint shear deformation, and joint shear strength. All specimens underwent some beam hinging at the beam/column interfaces. However, both eccentric specimens, and one concentric specimen with a heavily reinforced floor slab, eventually failed as a result of joint shear, whereas the other concentric specimen exhibited more ductile load–displacement response. The eccentric specimens (with different eccentricities and edge-beam widths) underwent similar behaviour before they started to break down, and they also reached similar joint shear strengths. Slab participation was evaluated using slab bar strain gauge data with respect to storey drift. Actual effective slab widths were much larger than the ones typically used in design, especially for the specimens with a column wider than the edge-beams. Finally, floor slabs imposed significant joint shear demand, but they also increased joint shear capacity by expanding effective joint width.
Theoretical study on effect of confinement on flexural ductility of normal and high-strength concrete beams
Authors: Kwan A. K. H. , Au F. T. K. , Chau S. L.
Published: October 2009

Compared with normal concrete, high-strength concrete has higher strength but is generally more brittle. Its use in a reinforced concrete structure, if not properly controlled, could lead to an unsustainable reduction in ductility. However, confinement could be provided to improve the ductility of the structure. In this study, the effects of concrete strength and confinement on the flexural ductility of reinforced concrete beams have been evaluated by means of complete moment–curvature analysis of beam sections cast in different concretes and provided with different confinements. The results reveal that the use of high-strength concrete at a constant tension steel ratio would increase the flexural ductility, but at a constant tension to balanced steel ratio would decrease the flexural ductility. In contrast, the provision of confinement would always increase the flexural ductility. It does this in two ways: first, it increases the balanced steel ratio so that, at the same tension steel ratio, the tension to balanced steel ratio is decreased; and second, it increases the residual strength and ductility of the concrete so that, at the same tension to balanced steel ratio, the flexural ductility of the beam section is increased.
Shear strength predictions of pierced deep beams with inclined web reinforcement
Authors: Tan K. H. , Tang C. Y. , Tong K.
Published: October 2009

Experiments have shown that web reinforcement has a moderate effect on the ultimate strength of solid deep beams. Furthermore, inclined web reinforcement is most effective in resisting diagonal cracks in solid deep beams. In the presence of deep beams with rectangular web openings (pierced deep beams in short), the beneficial effect of inclined web reinforcement is even more prominent. The provision of such web reinforcement may double the shear strength of an otherwise similar pierced deep beam. Up to now, current design codes have provided little guidance on the design of such beams. Also, there are very few published papers that include pierced deep beams with inclined web reinforcement. In this paper, the authors present an original and novel approach to analyse the shear strengths of such beams. This approach is based on a simple strut-and-tie model and has been verified for solid deep beams with various types of web reinforcement. The approach yields consistent and accurate shear strength predictions with published test results.
Effect of chemical admixtures on rheological parameters and stability of fresh lightweight aggregate concrete
Authors: Chia K. S. , Zhang M. H.
Published: October 2009

This paper presents results of an experimental study on the effect of a superplasticiser and an air-entraining admixture (AEA) on the rheological behaviour of fresh lightweight aggregate concrete and the stability of the concrete under vibration. The results indicated that an increase in superplasticiser content in the concrete reduced the yield stress, but did not have a significant effect on the plastic viscosity. The increase in air content in the concrete by using AEA reduced the plastic viscosity. The yield stress, however, was reduced only when the AEA was first introduced. Further increase in the AEA content did not have a significant effect on the yield stress of the fresh concrete. When the concrete experienced vibration, the segregation resistance decreased with a decrease in both the yield stress and the plastic viscosity. However, the effect was more pronounced with the decrease in yield stress.
Long-term strains in a two-storey, reinforced lightweight aggregate concrete office
Authors: Parrott L.
Published: October 2009

Shrinkage and load-induced strains were measured for 27 years in the in-situ concrete first floor and roof slabs of a long, two-storey office, using embedded vibrating wire strain gauges. The floor and roof slabs were cast on precast crossbeams and precast columns supported the crossbeams. Longitudinal precast beams between the columns supported the brick cladding and office windows but they were not connected to the floor or roof slabs. A central staircase provided longitudinal stability and a movement joint was provided at the access corridor to an existing, adjacent stone tower. A lightweight coarse aggregate was employed in the precast and in-situ concretes. The shrinkage and load-induced strains were determined using horizontal strain gauges parallel and transverse to the length of the slab and the direction of the expected flexural stresses. Concrete prisms were cast and instrumented for shrinkage under site exposure and elastic modulus, creep and shrinkage under sealed and dry exposure conditions in a laboratory. The drying shrinkage in the 190-mm-thick floor slab levelled-off around 500 microstrain (μs) at 6 years, a smaller value than the 750 μs from the site prisms at 3 years. The expected flexural load-induced strain profiles in the floor were superposed by a tensile stress component of strain. The tensile stress was attributed to the restraint of the drying shrinkage by the longitudinal precast beams. The deformation properties of the concrete were used in conjunction with the floor slab shrinkage to estimate a maximum tensile stress of 1·3 MPa. This was significantly smaller than the tensile splitting strength of 4·4 MPa that was measured at 10·5 years and suggested that there would be little associated cracking. The drying shrinkage in the 190 mm thick roof slab, sealed on the upper surface, was 450 μs after 27 years but, unlike the floor, had not yet levelled-off. Furthermore, a swelling was observed between 7 and 10 years that was due to water infiltration through leaks in the bituminous roof membrane. Shrinkage resumed after the roof had been repaired. The load-induced flexural strains in the roof, like the floor, were superposed by a tensile stress component of strain that could be attributed to restraint of drying shrinkage by the precast longitudinal beams. The maximum tensile stress in the roof slab was estimated to be 1·5 MPa. The risk of reinforcement corrosion from the roof leak was assessed by measuring the depth of carbonation in a prism stored in the ceiling void below the roof. The 12 mm depth of carbonation was safely smaller than the 20 mm depth of cover to the reinforcement. The longitudinal movement of the first floor caused cracking of the attached ground floor blockwork at 5 years. The mortar joint to the floor slab was replaced with mastic and no further cracking was observed. The floor slab movement also caused cracking of several large windows in the glazed corridor that provides access to the offices.
Use of an ultra-accelerated concrete prism expansion test for alkali-silica reactivity assessment
Authors: Berra M. , Mangialardi T. , Paolini A. E.
Published: October 2009

The suitability of an ultra-accelerated concrete prism expansion test (testing concrete mixes in alkaline solutions at 150°C) for the assessment of alkali-silica reactivity (ASR) was investigated using concrete mixes made with different alkali levels and different typologies of Italian natural aggregates of known field performance. The results of previous concrete prism expansion tests at 38°C and 100% RH were used for comparative purposes. It was found that the ultra-accelerated test is appropriate for discriminating between innocuous and potentially alkali-reactive concrete mixes and may also be used as a rapid method for evaluating the threshold alkali levels of ASR-susceptible aggregates. The results of the ultra-accelerated test may be interpreted on the basis of a percent expansion criterion (0·12% at 21 days) or through the use of a reactivity parameter, ln k, the latter being determined from the correlation of the test results with a kinetic-based method. A value of –3·4 for ln k appears to be a suitable reactivity criterion for interpreting the results of the ultra-accelerated concrete prism test.
Moment redistribution effects in beams
Authors: Scott R. H. , Whittle R. T.
Published: October 2009

Moment redistribution in beams has traditionally been considered as an ultimate limit state (ULS) phenomenon closely associated with considerations of reinforcement ductility. This paper demonstrates that a significant proportion of this redistribution will almost always occur at the serviceability limit state (SLS) because of the mismatch between the flexural stiffnesses assumed when calculating moments for the ULS and those actually occurring at the SLS due to variations in the reinforcement layout along the member and the influence of cracking. This is demonstrated analytically in the paper and comments are made concerning the current recommendations in BS 8110 concerning member stiffness. Tests on 33 two-span beams are then presented, parameters investigated being values of redistribution, beam depth, reinforcement arrangements, concrete strength and the effect of brittle reinforcement. The results confirm that there is significant redistribution at the SLS and that there is scope for increasing the permissible limits for redistribution beyond those currently prescribed in design codes. This is supported by further modelling although it is shown that considerations of crack width may become a limiting factor.
Long-term behaviour under axial service loads of circular columns made from concrete filled steel tubes
Authors: Kwon S. H. , Kim Y. Y. , Kim J. K.
Published: October 2009

This paper presents analytical studies on the long-term behaviour of circular concrete filled steel tubular (CFT) columns under central axial loading. Two loading cases are considered in this study: (1) a load applied only at the inner core concrete of a CFT column; and (2) a load applied simultaneously on both the inner concrete and the steel tube. Analysis methods are formulated based on compatibility, equilibrium conditions, and a number of assumptions for the two loading cases. Bond behaviour between the inner concrete and the steel tube along with confinement created by the steel casing are considered in the formulation. Experiments were performed for CFT column specimens under the aforementioned two loading cases to verify analysis methods. Analysis results exhibit good agreement with test results, and therefore the validity of the assumptions used in the formulation and the accuracy of analysis methods is proven. Furthermore, the following features of the long-term behaviour of circular CFT columns under these loading cases are revealed by test and analysis results. In the case of loading at the inner concrete of the CFT column, the strains of both the concrete and steel tube are non-linearly distributed along the longitudinal axis of the CFT column because of the bond stress and the confinement effect. Slip occurs and becomes larger with time due to the difference between the strain distributions of the concrete and the steel tube, which also increases with time. In the case of loading on the entire section of the CFT column, the strains of both the concrete and steel tube are uniformly distributed along the longitudinal axis of the column. As loading time increases, however, the stress acting on the concrete section decreases, while the stress of the steel tube section increases. The confinement effect of the concrete does not appear at initial loading or in long-term deformation.
Residual capacity of corroded reinforcing bars
Authors: Du Y. G. , Clark L. A. , Chan A. H. C.
Published: October 2009

This paper presents an experimental investigation into the residual capacity of corroded reinforcing bars. By performing both accelerated and simulated corrosion tests on bare bars and on bars embedded in concrete, the mechanism of the reduction of the capacity of corroded reinforcement was investigated. The influence of type and diameter of reinforcement on its residual capacity is discussed. The experimental results show that, due to local attack penetration, the residual cross-section of a corroded bar is no longer round and varies considerably along its circumference and its length. Although the force–extension curves of corroded bars are similar to those of non-corroded bars for up to 16% corrosion, their residual yield and ultimate forces decrease more rapidly than their average cross-sectional area and, therefore, their residual strength decreases significantly. Even though the residual capacity of corroded small diameter and/or plain bars reduces more than that of large diameter or ribbed ones, differences are not significant and can be neglected. Finally, a simple equation is proposed to predict the residual capacity of corroded reinforcing bars in practice.