bluuebunny/crossref_metadata_2025 huggingface dataset:
Bursting forces in concrete panels resulting from in-plane concentrated loads
Authors: Foster S. J. , Rogowsky D. M.
Published: October 2009

The present state of practice for the design of bearing zones is mixed between solutions based on empirical equations, linear-elastic stress analyses and strut and tie models. If, however, the design requires large redistribution of stresses, the panel may perform poorly at service and/or overload. In this paper the results of a plane stress finite-element investigation of concentrated loads on reinforced concrete panels are presented. The numerical investigation suggests that the bursting force distribution is substantially different from that calculated using elastic design methods currently used in some codes ofpractice. The distribution of bursting stresses were found to be flatter and occur over a longer disturbed region than is predicted by the linear solution. The optimum solution for a uniformly reinforced bursting region was found to be with the reinforcement distributed from approximately 0·2 times the overall depth of the member (0·2D) to between 1·2D and 1·6D. A strut and tie model has been proposed based on the finite element analyses reported herein. The model has been shown to produce satisfactory transverse strain fields, for both strength and serviceability, when detailed in accordance with the static requirements proposed.
Neural network modelling of chloride binding
Authors: Glass G. K. , Hassanein N. M. , Buenfeld N. R.
Published: October 2009

The capacity of cement to bind chloride is considered to be one of the controlling parameters in the process of chloride-induced corrosion of steel in concrete. In this work, a literature review has been undertaken to identify factors affecting chloride binding. Data from 21 previously published works have been collated and used to develop a neural network model to predict the free chloride concentration as a function of 18 input variables. These predictions are relatively free of the indiscriminate variations present in individual measurements. The influence of factors not yet fully evaluated is identified. In addition, the relative importance of a wide range of factors is quantified. The neural network model provides a powerful tool for generating binding isotherms for use in models ofchloride ingress into concrete.
Use of fly ash or silica fume to increase the resistance of concrete to feed acids
Authors: De Belie N. , De Coster V. , Van Nieuwenburg D.
Published: October 2009

Concrete in animal houses is subject to aggressive substances from feed and manure. Chemical attack by the most important feed acids, lactic and acetic acids, and abrasion caused by animals and cleaning were simulated and studied using accelerated corrosion tests. The resistance of concrete prisms with different percentages of fly ash or silica fume to simulation liquids with different pH values was investigated. The decrease in volume in terms of percentage and the mass loss per unit area were measured, as well as the pH change and calcium content of the liquids. It appeared that the addition of fly ash and, even more so, the addition of silica fume helped to increase the resistance of concrete to highly and very highly aggressive simulation liquids. A percentage offly ash below 30% by weight of cement seemed to be best. The difference between using silica fume as an addition or as a cement replacement was small. A correlation was found between the mass of the dissolved material and the calcium content of the liquid.
Effects of silica fume on dry-process sprayed concrete
Authors: Austin S. A. , Robins P. J. , Peaston C. H.
Published: October 2009

This paper presents and discusses data on the effects of using silica fume (in powdered form) in dry-process sprayed concrete. A series of spraying trials were carried out using 3:1 and 4:1 (aggregate:cement) mixes, with up to 15% of the cement being replaced by silica fume on a weight-for-weight basis. Measurements of material rebound were made, and in situ mix proportions were determined. Compressive strength, flexural strength and water absorption tests were carried out on specimens extracted from the hardened test panels. The presence of silica fume in the dryprocess sprayed concrete mixes produced substantial (up to 50%) reductions in rebound losses; this in itself is a very significant advantage in this production process, and this fact alone may be adequate justification for use of silica fume in dry-process sprayed concrete. The reduction in rebound appears to be largely a result of the increased water demand caused by the inclusion of silica fume powder, as opposed to its cohesive properties. The work also shows that the benefits (in terms of strength and durability) normally associated with the use of silica fume in more conventional cast concrete should not be expected automatically with dry-process sprayed concrete due to the effects of water demand.
Enhancement of rebar (smooth surface)-concrete bond properties by matrix modification and surface coatings
Authors: Zongjin Li , Maobo Xu , Chui Ngai Chung
Published: October 2009

The interface between reinforcing steel (rebar) and cement-based matrices plays an important role in determining the behaviour of reinforced concrete. Enhancement of rebar–concrete bond properties can improve the performance of reinforced concrete structures as whole. In this study, several methods, including matrix modification and surface modifications, have been investigated on their effects in improving the bond properties. The matrix modification was achieved by incorporation of polyvinyl alcohol (PVA) powder into the matrix. The surface modification were achieved by coating the cement, cement plus PVA, and cement plus silica fume powders on to the rebar surface. The enhancement of the mechanical properties of the interface by these modifications was evaluated by the pushout test, in which a closed loop controlled set-up was used to continuously measure force and displacement during loading. The experimental data were analysed by using a simplified theoretical model. The interfacial properties of the transition zone, including the stiffness, strength, friction and fracture energy of the interface, were obtained by using this model. It was found that the mechanical interfacial properties can be improved by the above-mentioned treatment methods. The interface mechanical behaviour was also investigated by microhardness measurement and scanning electron microscopy (SEM). The experimental results showed that the matrix and surface modification did increase the interfacial microhardness and reduce the porous region of interface, which were consistent with macromechanical property measurements.
Autogenous volume change of concrete at very early age
Authors: Radocea A.
Published: October 2009

The autogenous volume change of concrete is defined as the external volume reduction caused by cement reactions. This type of volume reduction is often suspected to be the main cause of cracking in high performance concrete at early ages. However, the mechanisms of volume change and cracking are not well understood. The aims of this article are to make a theoretical analysis of the phenomenon, to quantify the most important parameters involved in the process, and to discuss the cases for which this type of shrinkage can cause cracking.
Mechanisms affecting the development of alkali-silica reaction in hardened concretes exposed to saline environments
Authors: Sibbick R. G. , Page C. L.
Published: October 2009

In circumstances where concretes containing UK aggregates with reactive siliceous components are exposed to high concentrations of NaCl there is conflicting evidence as to whether salt ingress enhances susceptibility to alkali-silica reaction (ASR). There is also uncertainty over the mechanisms involved. The research undertaken was intended to elucidate these issues. Expansion tests with concrete prisms and cores immersed in 2 and 7 M NaCl solutions at 20 and 38°C showed that significant ASR expansion and cracking could be induced in specimens containing the more highly reactive UK aggregates at alkali levels well below 3 kg/m3 Na2Oeq, which is the present recommended UK limit for minimizing the risk of ASR. Even concretes containing aggregates of lower reactivity (chert-bearing gravels, etc.) were found to exhibit significant ASR expansion when exposed to NaCl solution in specimens of 3–4 kg/m3 Na2Oeq. The role of NaCl ingress in exacerbating the development of ASR in concretes of varied composition was studied by thermoanalytical techniques and petrography combined with electron probe microanalysis. The mechanisms were found to depend on several features of the environment (NaCl concentration, temperature, etc.) and the composition of the concrete (type of reactive aggregate, alkali content, cement mineralogy, etc.). A simplified reaction scheme was proposed to account for the results obtained.
The initial surface absorption test (ISAT): an analytical approach
Authors: Wilson M. A. , Taylor S. C. , Hoff W. D.
Published: October 2009

The initial surface absorption test (ISAT) is examined in detail. An equation describing water absorption through an ISAT geometry circular absorbing surface is developed. It is shown that the test as currently applied involves the comparison of different parts of the absorption versus time curve for different materials. It is suggested that this results in the test being insensitive to small changes in hydraulic suction, particularly at long times. Two methods of data analysis are applied to experimental water absorption data obtained from stone and concrete materials, and it is shown that it is possible to obtain sorptivity values directly from circular source absorption data.
Concrete mixture design for hot weather: experimental and statistical analyses
Authors: Al-Gahtani H. J. , Abbasi A. G. F. , Al-Amoudi O. S. B.
Published: October 2009

Factorial experimental design was adopted in this investigation to assess the combined effects of the following factors on workability and compressive strength: (a) water to cement ratio (W/C), (b) total aggregate to cement ratio (TA/C), (c) fine to total aggregate ratio (FA/TA), and (d) hot-weather conditions in terms of concrete mixture temperature at placement and curing conditions. The experimental data was thereafter subjected to regression analysis to develop reliable models for predicting workability in terms of the slump and compressive strength of a concrete mixture. Results of this study indicate that lowering the concrete mixture temperature at placement alone, as recommended in the codes of practice, does not eliminate the adverse effect of hot weather on compressive strength.
The use of physical phenomena to predict time effects in cracked reinforced concrete beams
Authors: Rahman M. A. , Ayano T. , Sakata K.
Published: October 2009

A method of calculating the time effects on cracked reinforced concrete (RC) members, based on actual physical phenomena, is presented. The redistribution of stress due to the migration of the neutral axis, and the consequent elastic and creep recovery are included in the calculation using the method of time-discretization. A simplified multiplier derived from the initial/time-dependent curvature, which is a function of the long-term strain factor, is used to calculate the time-dependent deflection. The concrete non-linear creep model established on axially loaded specimens is modified by introducing a parameter to account for the steel restraining effect on the creep deformation of RC flexural member. The modified model also considers the effect of the compression of the steel on the creep of the RC member. The calculated values are compared with laboratory measured values for several test beams of different cross-sectional properties reported in the literature. The proposed method gives results that are in good agreement with experimental results.
Sustained loading effect on the fatigue life of plain concrete
Authors: Zhang B. , Phillips D. V. , Green D. R.
Published: October 2009

The sustained loading effect on the fatigue properties of concrete was investigated. First, flexural sustained loading tests on plain concrete beams were conducted to establish a fundamental relationship between sustained stress and sustained time. A corresponding formula for calculating the equivalent damage under cyclic loading is also proposed. Based on these relations, the fatigue equation previously proposed by the authors is modified to account for the sustained loading effect. The modified fatigue equations agree well with previous experimental results at high stress ratios.
Repair of reinforced concrete portal frame damaged by alkali–silica reaction—strains after 5½ years
Authors: Blight G.E. , Lampacher B.J.
Published: October 2009

In 1991 a two-level reinforced concrete portal frame, damaged by alkali�silica reaction, was repaired by demolishing and reconstructing the upper beam. Strain measurements at the junction of the old and new concretes showed very little differential movement over the next year. This brief article reports on strains that occurred during 4½ years from 1992 to 1996.
Strength and behaviour of steel fibre reinforced concrete corners under opening bending moment
Authors: Abdul-Wahab H. M. S. , Al-Roubai A. A. M.
Published: October 2009

The effect of adding steel fibre to reinforced concrete corner joints under opening bending moment was investigated by tests on 23 specimens containing various amounts of fibres up to 2% (by volume). Two reinforcement details, with and without inclined bars, were used in the full-sized specimens, which were tested to failure. The corner angle was varied from 60° to 180°. The results obtained indicate a considerable improvement in the flexural efficiency and ultimate moment-carrying capacity of the joints with the inclusion of moderate amounts of fibres. By adding 1–2% fibres the risk of diagonal tension failure is considerably reduced, and full flexural capacity is achieved in the weakest corners (corner angles of 75–150°). From the analysis of the results, a simplified empirical relationship was derived for the prediction of the ultimate moment-carrying capacity of joints.
Post-punching resistance of connections between flat slabs and interior columns
Authors: Melo G. S. S. A. , Regan P. E.
Published: October 2009

The paper reports tests designed to study the improvement in the post-punching resistance of slab–column connections by the provision of bottom bars passing through the columns and anchored in the slab. A series of model tests on slabs was used to developan expression for the strength limit governed by the slab concrete, while large-scale tests on abbreviated specimens were used to formulate the criterion dependent on the failure of the reinforcement. It is demonstrated that bottom reinforcement can be of significant benefit and that the resistance given by it can be assessed with reasonable accuracy.
Vertical strength variations in large columns using high-strength concrete incorporating slag
Authors: Sioulas B. , Sanjayan J. G.
Published: October 2009

The paper presents the results of an experimental programme undertaken to investigate the variation of in situ concrete strength along the vertical axis of columns cast using a range of normal- and high-strength concrete incorporating slag-blended cement in the binder. In situ strengths of four columns 800 3 800 mm in cross-section and 1200 mm high with strength grades of 40, 60, 80 and 100 MPa were examined at 28, 56 and 91 days. The estimation of in situ strength was carried out by securing cores throughout the cross-section of the columns at three height levels. The results reported are based on compressive strength tests and density measurements of 156 cores. The results show a definitive and conclusive relationship between vertical strength variation and strength grade. It is also clear from the results that in situ strength–height effects are independent of concrete age after 28 days. The paper also examines the effects of hydrostatic pressure, water loss due to evaporation, bleeding, superplasticizer dosage and slag content on the vertical strength variation.
Measured and design bond strengths of deformed bars, including the effect of lateral compression
Authors: Walker P. R. , Batayneh M. K. , Regan P. E.
Published: October 2009

This paper deals with research undertaken at Oxford Brookes University into the bond strength of deformed bars set in normal-weight concrete. It briefly describes pull-out tests on some 150 specimens (providing 300 results), in which the variables were the position of the bar during casting, lateral pressure, cover, bar diameter, concrete strength and bar spacing. The results of the tests are compared with the values predicted by the British and European Codes of Practice BS 8110, Eurocode 2 and CEB-FIP Model Code 1990. Generally, the British code is shown to be very conservative and to underestimate the bond strength. However, when the bars have low cover-to-diameter ratios or where the bars are spaced in pairs vertically, the code appears to give unsafe values. The European codes provide better predictions of the bond strength because they take account of the majority of the influencing factors. However, they do not adequately account for the variables over the full range and are conservative in many circumstances, particularly when lateral pressure is applied. An alternative design procedure is proposed which is based on the experimental evidence and takes account of the main factors influencing the bond.
Experimental investigation of the optimized use of plastic flakes in normal-weight concrete
Authors: Soroushian P. , Eldarwish A. I. , Tlili A. , Ostowari K.
Published: October 2009

The potentially influential variables in normal-weight concrete incorporating recycled plastic flakes (recycled plastic content and width of recycled plastic particles) have been investigated. The addition of recyled plastics to concrete was found to help control the drying-shrinkage cracks. An experimental design was formulated and implemented in order to determine the optimum geometry and volume fraction of plastics for the control of restrained drying-shrinkage cracking of concrete. Follow-up experimental investigations were performed to evaluate the effect of the resulting optimum plastic content on some mechanical properties (flexural strength and toughness, compressive strength, and impact resistance) and long-term durability characteristics (permeability, freeze–thaw durability, scaling resistance, corrosion resistance and hot-water durability) of the concrete.
Restrained alkali–aggregate expansion due to steel fibre additions in concrete
Authors: Guo H. , Qian C. , Zhao X. , Lu J. , Stroeven P.
Published: October 2009

The work reported here shows that steel fibres can reduce the expansion caused by alkali–aggregate reaction in concrete, hereby decreasing the resulting strength loss. The addition of 1–1·5% of steel fibres by volume allowed us to increase the alkali content by about 2 kg/m3. Under such conditions, the ultrasonic pulse velocity was found to be stable, and the expansion rate low, indicating only minor effects on the strength of the concrete, even though the concrete contained reactive aggregate and a high alkali content. More specifically, the effliciency of fibres for restricting expansion is governed by the size and geometric parameters of the fibres. The expansion rate was linearly related to the so-called fibre factor or fibre index, for otherwise similar alkali–aggregate reaction conditions.
Flexural behaviour of reinforced concrete beams repaired with styrene—butadiene rubber latex, silica fume and methylcellulose repair formulations
Authors: Ali Y. A.-Z. , Ambalavanan R.
Published: October 2009

The use of plain cement mortar in repair works is limited to unimportant works because of its poor adhesive bond to the substrate and its performance from the point of view of durability. Polymer-modified cement mortar has replaced plain mortar for these reasons. Also, the incorporation of silica fume and methylcellulose in repair systems has been found to provide a better durability and overall performance. This paper presents the results of an experimental investigation of the flexural behaviour of six reinforced concrete beams. Five beams were repaired using five different repair systems, namely plain cement mortar and mortars modified with latex (L), latex and silica fume (L—S), latex silica fume and methylcellulose (L—S—M), and silica fume and methylcellulose (S—M). The sixth beam was an unrepaired control beam. The repair procedure was carried out while loading the beams, to match the on-site practice. The size of the beams was 100 X 150 X 1200 mm and they were reinforced with two 10 mm diameter steel rods with a yield strength of 551 MPa. The test results showed that beams repaired with L, L—S and L—S—M mortars successfully restored the structural capacity of the unrepaired beam, besides improving the durability, crack pattern and toughness characteristics of these beams. The use of plain or S—M mortars showed a poor structural performance since they exhibited early separation along the contact interface with the substrate, as well as reduced ductility and toughness.
The derivation of input data for modelling chloride ingress from eight-year UK coastal exposure trials
Authors: Bamforth P. B.
Published: October 2009

The effects of cement type and concrete composition on the resistance to reinforcement corrosion have been investigated using large reinforced concrete blocks exposed to UK coastal splash zone conditions for eight years. The results have been analysed in conjunction with a substantial volume of published data, particularly in relation to the resistance of the concrete to chloride ingress. Concretes containing either pulverized fuel ash or ground granulated blastfurnace slag were found to exhibit markedly better protection of reinforcement than Portland cement concretes of the same grade and a significant benefit was also apparent when comparisons were made at the same water–binder ratio. Variability of both the surface chloride level and the apparent chloride diffusion coefficient was measured over an area of only 0·15m2 on each of six blocks. The scatter was sufficiently high to indicate that several measurements are needed to achieve reliable predictions of future performance. In addition, time-dependent changes in the rate of chloride ingress were observed.
Equipment for tensile testing of fresh concrete
Authors: Hannant D. J. , Branch J. , Mulheron M.
Published: October 2009

The development of equipment suitable for measuring tensile stress—strain curves on fresh concrete is described. With such weak materials, it is essential to eliminate friction effects and this is achieved by the use of air-bearing plates. The objective of the new equipment is to enable data to be obtained to define the rapidly changing tensile stress—strain performance in the first 6 h after water is added at the mixer. This information is important for understanding plastic shrinkage cracking. The equipment is also being used to investigate early-age internal cracking problems which have been observed with high-strength concrete and which are related to the internal tensile failure strain of the cement paste. Some typical tensile stress—strain curves obtained using the equipment are shown for two types of concrete.
Effect of curing on concrete in hot exposure conditions
Authors: Al-Khaiat H. , Haque M. N.
Published: October 2009

A project was undertaken to improve the quality and performance of concrete structures in the hot weather and severe environment of Kuwait. This paper presents two-year results of specimens exposed to hot inland conditions after 1, 3 and 7 days of initial curing. The results suggest that an initial 7 day curing of both lightweight and normal-weight concrete specimens exposed to hot outdoor conditions enhances considerably their strength and reduces water penetration and carbonation. For the total lightweight concrete tested, both the depth of water penetration and the carbonation depth were found to be higher than in the equivalent-strength normal-weight concrete after 2 years of exposure to hot outdoor conditions.
Strength development in concrete incorporating PFA and metakaolin
Authors: Bai J. , Sabir B. B. , Wild S. , Kinuthia J. M.
Published: October 2009

The work presented in this paper forms part of an ongoing investigation examining the potential of using metakaolin pulverized fuel ash (MK–PFA) blends for cements in concrete. The programme of research involves the examination of the effects of the blends on the workability, strength development and factors affecting durability, including chloride penetration, carbonation and water transport properties. The influence on the workability was reported previously and this paper gives the results for compressive-strength development. Several blend compositions were employed at water : binder ratios of 0·4, 0·5 and 0·6 to produce concretes, the compressive strengths of which were evaluated at 7, 28 and 90 days. It is found that the contrasting influences on the strength, particularly at early curing times, effected by PFA and MK when used in isolation as blends for cement can be combined effectively by the employment of various ratios of MK and PFA. Early strengths surpassing that of the control are obtained with low partial cement replacements and high MK/PFA ratios. Higher cement replacements with low MK/PFA ratios result in 90 day strengths that are also greater than the control strength. Careful choice of composition can enable reduced cement content without reduction in the compressive strength. The reduction in cement content leads to environmental benefits, and with increased utilization of these blends the relative cost of the pozzolanic blend will reduce and overall savings will result.
Tensile strain capacity of concrete under various states of stress
Authors: Wee T. H. , Lu H. R. , Swaddiwudhipong S.
Published: October 2009

The direct-tension-test method and the mechanical properties of concrete in compression, flexure and tension have been investigated. The mould used for preparing the specimens in the direct tension test has been modified to overcome the difficulties in centralizing and aligning the two embedded bars in the specimens. The method adopted improves the weak bond strength between the embedded bar and concrete and reduces the stress concentration at the end of the embedded bar. Concrete is most brittle under tension. The state of stress affects the failure characteristics of concrete. The tensile strain capacity, which is used to evaluate cracking in concrete, should be chosen judiciously depending on the state of stress in the concrete. The tensile strain capacity obtained from the flexure test has a higher standard deviation, indicating wider variability of the results. Relationships between tensile strength and flexural strength and between compressive strength and tensile or flexural strength are also presented in this paper. The modulus of elasticity from the tension test is similar to that obtained from the compression test for low-grade concrete, but the values obtained from the tension test are lower than those from the compression test for concrete of high grade. The values of Poisson's ratio in tension and flexure decrease when the tensile or flexural stress in the concrete is raised. The values of Poisson's ratio in compression vary insignificantly before reaching the discontinuity point, and increase sharply afterwards.
An experimental study of the behaviour of high-strength reinforced concrete columns subjected to reversed cyclic shear under constant axial compression
Authors: Ahn J. M. , Lee J. Y. , Bahn B. Y. , Shin S. W.
Published: October 2009

An experimental study was conducted to examine the behaviour of high-strength reinforced concrete (R/C) columns subjected to a reversed cyclic load and axial compression, and to find the relationship between the amount of lateral reinforcement and the axial load ratio. Twenty column specimens were tested. The test parameters of the column specimens were the compressive strength of concrete (f′c = 25, 52 and 70 MPa), the axial load ratio (0·3 and 0·5 f′cAg) and the volumetric ratio of lateral reinforcement (ρs = 1·05, 2·1 and 3·15%). The test results indicated that the axial load ratio and the shape of the lateral reinforcement could significantly affect and alter the behaviour of high-strength R/C columns under inelastic cyclic loading.
Effect of concrete curing on chloride ion ingress
Authors: Hillier S. R. , Sangha C. M. , Plunkett B. A. , Walden P. J.
Published: October 2009

The effect of curing on the ingress of chlorides into concrete was investigated. In addition to determining chloride profiles on samples prepared in the standard manner, a parallel series of tests was carried out on concrete which had previously been subjected to 14 days of leaching in an aqueous environment. The results obtained highlighted two important conclusions. Firstly, chloride ingress was found to increase with improvement in curing for the leached concrete. Similar trends were observed up to a depth of approximately 10 mm for unleached samples. Thus it appears that the conventional assumption that improved curing results in a more durable concrete may not always be true. Secondly, the existence of a new mechanism for the ingress of chloride ions in saturated concrete, termed ‘leaching absorption’, was discovered. This mechanism results from the increased leaching of Ca(OH)2 with increasing duration of the initial moist curing period.
Influences of conditioned pulverized-fuel ash as a cement component on the properties of concrete
Authors: McCarthy M. J. , Tittle P. A. J. , Dhir R. K.
Published: October 2009

This paper describes a laboratory-based study carried out to examine the use of conditioned pulverized fuel-ash (PFA) as a cement component in concrete. A range of conditioning variables, including moisture content (10 and 20%), storage period (up to 18 months) and PFA properties (fineness and low and high lime contents), were investigated. Tests were carried out for heat of hydration and calcium hydroxide content of paste to examine Portland cement/PFA reactions, and workability, strength development and permeation properties of concrete to consider influences on practical performance. The results indicate that conditioning of PFA leads to slight changes in rates of heat evolution in fresh and hydrating paste, with reduced pozzolanic activity apparent after setting, compared to dry PFA. In concrete, conditioning of PFA gave slightly reduced slump, but early strength development (to 28 days) and permeation properties were either unchanged or slightly improved. At later test ages, slightly poorer properties, compared to dry PFA concrete, were generally noted, particularly with storage periods of six months or greater. While PFA fineness and conditioning moisture content did not influence these trends, the effects described increased with material having high total/free lime contents. A parallel series of tests with material conditioned and stored at power stations (i.e. exposed to an external environment) showed general agreement with laboratory data. The practical implications of the study covering the use of conditioned PFA are considered.
Structural behaviour of short steel—concrete composite spun tubular columns
Authors: Song Y. L. , Chen J. F.
Published: October 2009

This paper presents an experimental study on the structural behaviour of short spun concrete tubes and short steel–concrete composite spun tubular columns under axial loading. The results showed that high-strength spun concrete tubes are very brittle and work linearly until close to failure. Steel–concrete composite spun tubular columns have much better ductility and integrity than spun concrete tubes. The progress of concrete confinement provided by the steel tube during the whole loading process has been observed by analysing the strain measurements on the steel tubular walls.
A theory for drying creep of concrete
Authors: Brooks J. J.
Published: October 2009

A theory for drying creep is proposed based on the stress concentrations arising from the presence of a rigid inclusion and a hole in a surrounding medium subjected to an external stress. The analogy is made with cement paste where there are some pores filled with water and other pores empty. When full, the average stress acting on the solid gel is less than the nominal external stress but, when empty, the corresponding stress is greater than the external stress so that there is a stress enlargement as the pores empty of water. By implication, there is an increase in creep of the solid gel as drying or water movement occurs. In contrast to most existing theories, the new theory does not require an interaction with drying shrinkage to account for additional creep. The theory is first developed to account for the water–cement ratio influence on basic creep and then the stress enlargement factor for various pore water contents is calculated to simulate different drying conditions. As well as drying creep, the theory explains the additional creep in tension under drying conditions, transitional thermal creep and transient thermal strain.
Dynamic behaviour of frames containing prestressed concrete beams
Authors: Mo Y. L. , Perng S. F. , Hwang W. L.
Published: October 2009

Results from small scale models of frames containing prestressed concrete beams, tested by a shake table simulating earthquake forces, are presented. The dynamic response analysis of such frames are described. A comparison between tests and analysis is made. Basically, there are three parameters in the test programme, namely concrete strength, effective prestress and seismogram. Concrete strength is approximately between 32 and 84 MPa. Two types of effective prestresses are used: 40 and 80% of the yielding stress of the pretressing bars. Two kinds of seismogram are used: 1940 EL Centro acceleration and linear incremental acceleration. It is found that the effects of concrete strength and effective prestress on the ductility are significant because the ductility increases with increasing concrete strength and with decreasing effective prestress. Also, linear incremental seismogram produces smaller ductility.
Finite element analysis of coupled heat and mass transfer in concrete when it is in a fire
Authors: Tenchev R. T. , Li L. Y. , Purkiss J. A. , Khalafallah B. H.
Published: October 2009

This paper presents a comprehensive model for simulating coupled heat and mass transfer in concrete when it is subjected to a fire. The model considers not only the transport of dry air, water vapour and liquid water but also the evaporation of liquid water and the dehydration of bound water. It can be applied to multidimensional problems. The governing equations describing this coupled heat and mass transfer phenomenon are solved using finite element methods with a standard Galerkin approximation. Results related to temperature, pore pressure, moisture content, the level of evaporation and dehydration of water within concrete for a standard fire curve are given. The influence of the liquid water transport on the spatial distributions of temperature and pore pressure is investigated through the use of parametric studies. The present results can be used to predict the potential of concrete spalling.
Looking into concrete
Authors: Karihaloo B. L. , Jefferson A. D.
Published: October 2009

The latest General Electric CTi Helical scanner has been used to reveal the hitherto hidden internal structure of concrete. High quality images have been obtained in which the concrete constituents and defects, such as pores and cracks, are clearly visible. Cube and notched cylindrical specimens, some of which have been subject to temperature or mechanical loading, have been used for the study. The results obtained demonstrate the immense potential of CT imaging for revealing the internal structure of coarse-grained non-homogeneous materials, such as concrete.
Effects of the natural polymer-based durability-enhancing admixture on the frost resistance of concrete
Authors: Li Z. , Chau C. K. , Li F.
Published: October 2009

Although it is rare for concrete structures to fail due to lack of intrinsic strength, concrete structures have increasingly failed to survive their specified service lives due to gradual deterioration caused by lack of durability. Any new product that can reduce concrete deterioration will certainly be beneficial to the building and construction industries. Along this line, a new natural polymer-based concrete durability-enhancing admixture and concrete containing such an admixture have been studied. It has been shown by experimental results that the natural polymer-based admixture can greatly improve concrete's durability, especially its frost resistance. In addition, the admixture exhibited a slow reaction to the surrounding temperature variance, which is vital for concrete structures located in the northern part of the world and other places that have freezing and thawing problems. Due to its natural renewability, low cost, ease of manufacture and excellent durability improvement, the new natural polymer-based admixture provides a unique way of protecting concrete from deterioration caused by factors such as freezing and thawing.
Effect of aggregate on shrinkage crackresistance of steam cured concrete
Authors: Freidin Constantin
Published: October 2009

A simple test method to characterize the shrinkage crack-resistance (SCR) of steam cured concrete is advanced. It includes the estimation of time of appearance of first crack on specimens and SCR coefficient as well as parameters of concrete stress state to this period. The SCR of steam cured conventional concrete and micro-concrete of two compositions with compressive strength of approximately 20 MPa was studied. It was observed that the conventional concrete had the highest SCR. The appearance of cracks on specimens from this type concrete was detected after 49 days of drying exposure. The micro-concrete, prepared from sand with a fineness modulus of 0·36, had the lowest SCR: the first crack on specimens appeared after 34 days of their drying. The tensile stress, free shrinkage, tensile strain capacity and SCR coefficient of concretes at the beginning of crack-generation were calculated.
Deflection of RC beams under simultaneous load and steel corrosion
Authors: Ballim Y. , Reid J. C. , Kemp A. R.
Published: October 2009

This paper presents the results of a series of tests aimed at characterising the serviceability and ultimate limit state deflection of reinforced concrete beams subjected to steel corrosion while under load. Reinforcement corrosion was accelerated by impressing a DC current on the main tension steel while the load was applied through a static, spring-loaded support frame. Central deflections of beams subjected to 23% and 34% of the design ultimate load, under 4-point loading over a span of 1050 mm and accelerated corrosion, were monitored over a period of approximately 30 days. These beams were then tested to failure to assess the effects of reinforcement corrosion on performance at the ultimate limit state. Uncorroded beams were used as control samples and tested in parallel with the corroded samples. The results show the importance of assessing the structural effects of reinforcement corrosion under simultaneous load and corrosion conditions, as would occur in situ. In this situation, when 6% of the mass of steel is corroded, beam deflections are increased by between 40% and 70% relative to the deflection of the control samples. Furthermore, depending on the extent of corrosion and the mode of failure, the ultimate bending strength of the beams can be reduced by up to 30%.
Behaviour of concrete with low-strength bricks as lightweight coarse aggregate
Authors: Padmini A. K. , Ramamurthy K. , Mathews M. S.
Published: October 2009

The quantity of masonry demolition waste is higher than that of concrete waste. A review shows that extensive studies have been made of recycled concrete aggregates with only limited studies on recycled brick aggregate. The objective of this paper is to investigate the use of two types of low-strength bricks as lightweight coarse aggregate in concrete. After characterisation of recycled brick aggregates, a study of the relative influence of different parameters on the strength of concrete follows, adopting a five-factor, eight-run two-level fractional factorial experimental design. While cement content, aggregate condition (pre-wet or dry) and aggregate strength influence concrete strength significantly, the contributions of the maximum size of aggregate and sand percentages are relatively very low. Finally, a systematic behavioural study is undertaken adopting the volumetric method of mix proportioning for structural lightweight aggregate concrete. Typical guidelines for mix proportioning of such lightweight aggregates is presented.
Evaluation of bond strength between old and new concrete in structural repairs
Authors: Clímaco J. C. T. S. , Regan P. E.
Published: October 2009

Good bond between old and new concrete is indispensable for many repairs. Following an examination of the factors involved and the methods of testing bond, the results of an experimental programme of slant shear tests are presented. The programme comprised 223 tests designed to study test methods and the effects of bond coats, surface preparation and ageing of the base concrete. The results are analysed in terms of a Coulomb criterion and evidence is produced that good bond can be achieved by casting the repair against mature concrete with no bonding aids, provided the base surface is dry and reasonably rough.
Structural repair of shear-deficient reinforced concrete beams using SIFCON
Authors: Shannag M. J. , Barakat S. , Jaber F.
Published: October 2009

This article reports on the structural behaviour of shear-deficient reinforced concrete beams repaired with Slurry Infiltrated Fibre Concrete (SIFCON), and proposes a simple analytical procedure for predicting the shear capacity of the repaired beams. SIFCON is made by pre-placing short, discontinuous steel fibres into the form, followed by infiltrating the fibres with cement-based slurry. A series of 14 shear-deficient reinforced concrete rectangular beams was tested under third-point loading to study the effectiveness of using externally applied SIFCON jackets as a method of increasing the beams' shear capacity. The parameters investigated included the shear span-to-effective depth ratio (a/d), amount of longitudinal reinforcement (ρ) and the thickness of the jacket (t). For the range of variables tested, the results indicate that all the beams failed in shear tests and those repaired with SIFCON displayed an excellent shear capacity. The use of SIFCON jackets as external shear reinforcement eliminated the brittle shear failure and increased the ultimate shear strength of the repaired beams from 25–55%. This indicates that SIFCON may be considered as a promising material for design–maintain–rehabilitate programme of concrete structures.
Numerical and experimental studies of concrete damage under impact
Authors: Koh C. G. , Liu Z. J. , Quek S. T.
Published: October 2009

This article reports the numerical and experimental studies of concrete damaged under impact loading. In the framework of continuum damage mechanics, a relatively simple damage model is developed by incorporating experimental relations to account for strain rate effects on concrete characteristics, such as tensile and compressive strengths and elastic moduli. The computational study is supplemented by an experimental investigation programme. A test rig with a spring ejector is designed to produce impact on concrete specimens. In conjunction, a thin load cell suitable for measurement of reaction force is specially developed. Comparison of numerical and experimental results show that the proposed model captures well the non-linear behaviour of concrete under impact compressive load.
Strength of reinforced concrete columns bonded with external steel plates
Authors: Uy B.
Published: October 2009

Rehabilitating reinforced concrete elements using external steel plates has been applied to both beams and slabs in bridge and building construction over the last two decades. The effect of applying external steel plates increases the strength of beams and slabs. This technique can also be applied to reinforced concrete columns to increase their strength with a minimal increase in the cross-sectional dimensions of the member. This article presents an experimental programme used to ascertain the behaviour of both short and slender columns. These columns had steel plates bonded to them by the use of both bolting or gluing and bolting. A simplified theoretical model based on composite construction techniques is then presented and calibrated with the experiments to determine the strength increase and recommendations for further research are then made.
An experimental study on the effect of recycled aggregate on concrete properties
Authors: Ryu J. S.
Published: October 2009

In this experimental study, the effect of the recycled aggregate on interfacial property, permeability and strength characteristics of the concrete was evaluated in order to examine the relationship between the recycled aggregate and the performance of the hardened concrete. Five types of recycled aggregate and four levels of w/c ratio were tested. The effect of the recycled aggregate was evaluated using Vickers hardness test of aggregate interface, carbonation depth test, tensile strength test, and compressive strength test. The experimental results showed that the recycled aggregate has a significant effect on the overall properties of concrete.
Water sorptivity and pore structure of wood–cementitious composites
Authors: Bouguerra A. , Amiri O. , Aït-Mokhtar A. , Diop M. B.
Published: October 2009

This article was carried out to evaluate the effects of non-sorptive macroporous inclusions, i.e. wood aggregates on the water sorptivity of lightweight concrete prepared from clay, cement and wood aggregates. A series of seven mixtures of wood composites with various mass fractions of wood aggregates (ranging from 10% to 40% by weight of dry clay + wood aggregates) added to a clay–cement mixture were used in this investigation. The mercury intrusion porosimetry (MIP) has been used to reflect the effect of wood aggregates on the pore size distribution. The water sorptivity of various mixtures was measured. It is shown that the addition of macroporous wood aggregates considerably reduces the capillary absorption inside the material and the cumulative water absorption (per unit area of the inflow surface) increases as the square root of the elapsed time. Tests results also show that the normalised sorptivity decreases with an increase in macroporous wood aggregates. It is also found that the macroporous non-sorptive particles considerably reduce the sorptivity of composite.
Modelling the effect of corrosion on bond in reinforced concrete
Authors: Lundgren Karin
Published: October 2009

Corrosion of reinforcement leads to a volume increase; thus, splitting stresses are induced in the concrete. Thereby, the bond mechanism between the reinforcement and the concrete is influenced. In order to model the splitting stresses of the corrosion, the mechanical behaviour of the corrosion products must be known. Analyses of corrosion cracking tests led to the assumption that the corrosion products behave like a granular material; i.e. their stiffness increases with the stress level. This mechanical behaviour, and the volume of the rust relative to the uncorroded steel, were given as input for a corrosion layer. This layer was used, together with a model developed earlier for the bond mechanism, in finite element analyses of corrosion cracking tests and pull-out tests with corroded reinforcement. Reasonably good agreement between test results and analyses was obtained. The results show that this method of modelling can predict the decrease of bond when splitting of the concrete occurs, due to the combined action of corrosion and the bond mechanism.