bluuebunny/crossref_metadata_2025 huggingface dataset:
Physical properties of the precast concrete masonry unit
Authors: Harvey R. J. , Hughes T. G.
Published: October 2009

The physical characteristics of two commercially manufactured dense aggregate precast concrete masonry units are examined with the aim of providing further information for the development of prototype unit descriptions of the sort used in masonry design codes. Quantitative information of this type and strategies for its interpretation and generalization grow more important as the process of harmonization of European construction standards continues, and the ability to normalize properties between countries and regions becomes a prerequisite for the design process. The effects of varying moisture content on the measured compressive and tensile strength of the units are considered, as well as the influence of the testing method itself, and also the response of the unloaded units to temperature change.
Influence of rib geometry on strength of lapped joints: an experimental and analytical study
Authors: Cairns J. , Jones K.
Published: October 2009

Bond action of a ribbed reinforcing bar generates forces that tend to split the concrete cover around the bar. In the majority of practical circumstances, bond strength is limited by the resistance of the member section to these bursting forces. An analysis of the bond strength of ribbed reinforcing bars is presented in which bond failure is considered as a bearing failure of the ribs on the concrete. The analysis suggests that bond strength is only partially dependent on the splitting resistance of the beam section, and that a substantial component of bond strength is independent of splitting action. Results fiom tests on over 100 pairs of lapped joints of bars with varying rib geometries are analysed, and bond strengths predicted by the analysis are compared with measured values. Two rib geometry parameters are found to exert a significant influence on lapped joint strength, namely relative rib area and the shape of the ribs relative to the splitting failure plane. The study shows that bond strength can be increased by enhancing the parameters that contribute to the non-splitting component of bond strength.
A new quantitative method of assessing fire damage to concrete structures
Authors: Nassif A. Y. , Burley E. , Ridgen S.
Published: October 2009

This paper describes a laboratory investigation into the adoption of the stiffness damage test to assess fire-damaged concrete structures. Laboratory-prepared concrete cores (75 mm diameter, 175 mm long) were fired under different heating regimes and their uniaxial compression stress-strain response at low stress level was determined. The area of hysteresis of the load-unload loops and other characteristics of the response such as the degree of concavity, the loading chord modulus, the unloading modulus and the residual plastic strain provide a quantitative measure of the extent of structural damage caused by thermal exposure. Fire-damaged specimens were also monitored by measuring the ultrasonic pulse velocity. The microstructure of the damaged specimens was studied using scanning electron microscopy (SEM) and X-ray diffraction. 320°C marked the onset of significant modification in the characteristics of the stress-strain response loops, with a sudden increase in the damage index (area of hysteresis loops). The ultrasonic pulse velocity in the fired concrete showed similar variation with temperature to that of the elastic properties. At temperatures higher than 320°C, SEM photographs showed significant cracks in the cement paste, especially in the interfacial zone.
Derivation of the complete stress – strain curves for concrete in compression
Authors: Mansur M. A. , Wee T. H. , Chin M. S.
Published: October 2009

A closed-loop servo-controlled hydraulic testing machine can be used to generate experimentally the complete stress–strain curves for concrete in compression. However, the deformation readings obtained from the built-in low-voltage displacement transducers or other transducers placed between the machine platens include not only deformations of the concrete but also those due to machine flexibility and end-restraint of the specimen. In this paper, a correction factor is proposed to account for these effects. This requires the use of a compressometer fixed directly to the test specimen and transducers placed between the machine platens. More realistic stress–strain curves for concrete with cylinder (100 × 200mm) compressive strengths of up to 130 MPa have thus been obtained for the entire loading range.
Strengthening of shear-damaged RC beams by external bonding of steel plates
Authors: Sharif A. , Al-Sulaimani G. J. , Basunbul I. A. , Baluch M. H. , Husain M.
Published: October 2009

The repair and strengthening of damaged reinforced concrete (RC) beams using externally bonded steel plates has gained universal acceptance. This study presents test results and their interpretation for shear-damaged RC beams with deficient shear strength, strengthened by externally bonded steel plates. Different arrangements of steel plates were used in order to eliminate shear failure and develop ductile behaviour. The strength of all repaired beams was increased and the degraded stifiess of the beams was restored. However, the failure was sudden due to plate separation for most repaired beams, with the exception of beams repaired with full encasement at the shear zone. Such jacket-type repair enhanced the shear capacity and was so effective thatJIexural failure occurred for these beams.
Shear strength of plated RC beams
Authors: Baluch M. H. , Ziraba Y. N. , Azad A. K. , Sharif A. M. , Al-Sulaimani G. J. , Basunbul I. A.
Published: October 2009

Use of externally bonded plates to strengthen or repair damaged reinforced concrete (RC) beams and slabs has gained universal acceptance. However, questions remain regarding the possibility of a highly brittle mode of failure, referred to in the literature as shear failure for certain combinations of plate geometry and other parameters. The paper surveys available experimental evidence from laboratories in the UK and Saudi Arabia and identifies parameters that play a primary role in shear or concrete rip-off failure. It is shown that ultimate load for certain plated RC beams failing in shear is overestimated by the ACI expression for shear strength of unplated RC beams, because crack profile renders stirrups ineflcacious. An expression is suggested for predicting the degree of eflciency of stirrups for use in the design of plated RC beams.
Optimization of flexure environment of concrete beams reinforced with fibre-reinforced plastic rebars
Authors: Alsayed S. H. , Al-Salloum Y. A.
Published: October 2009

This study presents the optimum design and behavioural assessment of simply supported beams subjected to uniformly distributed loads and reinforced with fibre-reinforced plastic rebars. A formal optimization model is developed in which the cost per unit length of the beam represents the objective function to be minimized, and is expressed in terms of two design variables (the effective depth and the reinforcement ratio). The constraints on the optimization model are in accordance with the ACI code provisions. They include flexural demand due to the imposed loads and the serviceability requirements that specify an upper limit on the deflection of a beam under service loads. The optimization process is illustrated with the help of a beam example reinforced with glass fibre-reinforced plastic (GFRP) rebars. The results are compared with those of its counterpart reinforced with conventional steel bars. A parametric study to assess the influence of parameters such as the ultimate compressive strength of the concrete, span of the beam and cost of GFRP rebars is presented.
Cement binders for organic waste
Authors: Owens J. W. , Stewart S.
Published: October 2009

Three cyclic hydrocarbons (benzene, toluene and ortho-xylene (0-xylene)), each of which is a known volatile organic compound(VOC) produced in large quantities by the synthetic chemical manufacturing industry were loaded into Portland cement matrices and stored in a similar organic solvent, o-dichlorobenzene, for two months. The leaching characteristics of these three VOCs were measured using gas chromatography and a flame ionization detector: The objective of this study was to assess the comparative leaching characteristics of these organic contaminants in cement matrices stored in another organic solvent compared with storage in air: The leaching characteristics of benzene, toluene and o-xylene from Portland cement stored in o-dichlorobenzene were compared with reported values for similar organic contaminants loaded into cement matrices that were stored in water: Results indicate that the leaching of benzene, toluene and o-xylene from Portland cement is erroneously high when measured using a weight loss method.
Significance of property mismatch in the patch repair of structural concrete. Part 3: Reinforced concrete members in flexure
Authors: Emberson N. K. , Mays G. C.
Published: October 2009

Approximately £500m is spent in the UK each year on the repair of concrete members. Inrecent years researchers have been paying increasing attention to the structural implications of property mismatch between patch repair materials and the substrate reinforced concrete. Parts 1 and 2of this study, published in 1990, discussed the measurement of relevant mechanical and physical properties of repair systems and applied these results to the study of axial load transfer through simple patch repairs. This paper extends the approach to reinforced concrete beams repaired in either the flexural compression or tension zones and subjected to short-term static, long-term creep and cyclic loading. The research has shown that the performance of patch repaired reinforced concrete beams can be predicted from a knowledge of the basic physical and mechanical properties of the repair material and the substrate concrete. Given suitable repair materials and mode of application, satisfactory structural performance may be achieved in both the short and the long term.
Durability of concrete with addition of limestone powder
Authors: Sawicz Z. , Heng S. S.
Published: October 2009

The paper describes the influence of powdered limestone and water-cement (w/c) ratio on the durability of concrete in 5% Na2SO4. Before immersion in sulphate solution the compressive strength, absorbability of concretes and phase composition of pastes were tested. Sulphate durability was assessed by means of a linear deformation of concrete and X-ray diffraction analysis of pastes with and without addition of limestone powder: The results show that a beneficial influence of powdered limestone on concrete durability was observed when w/c <0·60. Above this value, the powdered limestone has almost no essential effect on sulphate resistance. The addition of limestone powder to cement prevents the transformation of ettringite to monosulphate and hemisulphate. Monocarbonate and hemicarbonate were formed instead of those phases.
Electrically accelerated methods for determining chloride diffusivity in concrete—current development
Authors: Tang L.
Published: October 2009

In this paper, the theoretical background of electrically accelerated methods is reviewed, and the current development of the methods at Chalmers University of Technology is described. The rapid method based on the non-steady state process is emphasized and the advantages of this non-steady state method are summarized. A study of the method based on the steady state process is reported and some factors which influence the measurement are discussed. Some applications of these new methods in the research work on cement and concrete durability are mentioned. Parallel tests show that the new rapid method corresponds well with the conventional immersion test, although fiwther study is needed to explain this coincidence.
Present technology of ready-mixed concrete and future prospects
Authors: Takeyama M.
Published: October 2009

This paper reports on the current state of the ready-mixed concrete industry in Japan and problems identified in JIS A 5308 (Ready-Mixed Concrete) with reference to European and other standards. Such problems include the following: quality of aggregate, designations of concrete strength, precision of material weighing, performance tests of agitator trucks, carrying capacities, water addition on job site, early judgement of strength, inspection at RMC plants, quality assurance by IS0 9000 and product liability.
The history and development of acoustic emission in concrete engineering
Authors: Ohtsu M.
Published: October 2009

Acoustic emission (AE) is applied to a variety of fields related to concrete engineering. With increasing need for maintenance, the non-destructive evaluation of in-service structures is being actively investigated all over the world. In particular, there is a realization that a number of civil structures are approaching the limit of their service life in Japan. AE techniques look promising as a means of inspecting such structures and evaluating their structural integrity. In addition to the demand for maintenance, the application of AE to construction monitoring is receiving.fresh attention. Research activities related to concrete technology are summarized with a brief review of historical developments.
Predictions of the maximum plate end stresses of FRP strengthened beams: Part II
Authors: Quantrill R. J. , Hollaway L. C. , Thorne A. M.
Published: October 2009

Experimental testing of 1·0 m long reinforced concrete beams with Jibre reinforced plastic plates, epoxy-bonded to their sofits has been carried out as detailed in an accompanying publication. Since collapse due to plate separation was found to be the dominant failure mechanism in these tests, an analytical procedure derived elsewhere has been used in this paper to try to identify the stress levels at the ends of the plate which are thought to be responsible for collapse, and to see the effect of the test parameters on these predicted stress values. This procedure, which has been verified experimentally for steel plated beams, is found to predict maximum shear and normal stress levels in excess of those thought to produce failure in applications in which steel is used as the plating medium.
Experimental research on the determination of the main parameters affecting the behaviour of reinforced concrete columns under fire conditions
Authors: Dotreppe J.-C. , Franssen J.-M. , Bruls A. , Baus R. , Vandevelde P. , Minne R. , van Nieuwenburg D. , Lambotte H.
Published: October 2009

The design of reinforced concrete columns under fire conditions is generally made on the basis of national and international recommendations. The CEB/FIP recommendations have been used for many years. Recently, in the framework of the Eurocodes, the document ENV 1992-1-2 (Eurocode 2-1-2), Structural fire design, has been approved by Commission CEN/TC250/SC2 of the European Committee for Standardization. In the CEB/FIP documents and in Eurocode 2-1-2 the main part is devoted to design from tabulated data containing the dimensions of the cross-sections and values for the concrete cover. The use of simplified calculation methods is not current practice for concrete elements, while they are widely used in the case of steel. The determination of the temperature distribution in the cross-section does not facilitate the development of a simplified method. Furthermore, in columns there is a wide scatter of experimental results. In order to gain additional information, an extensive experimental research programme on the behaviour of reinforced concrete columns under fire conditions, financially supported by the Belgian Funds for Fundamental Collective Research (FRFC-FKFO), has been performed at the Universities of Liege and Ghent. In this article the results of this research are presented and the influence of the main parameters is analysed.
A practical procedure for the analysis ofmoisture transfer within concrete due to drying
Authors: Akita H. , Fujiwara T. , Ozaka Y.
Published: October 2009

The prediction of moisture transfer within concrete is necessary in order to evaluate its durability, because moisture transfer is related to shrinkage cracks and the transfer of chloride ions. Moisture transfer within concrete during drying has been analysed by many investigators using a non-linear diffusion equation. In the present paper, a comprehensive analytical procedure using a diffusion equation is established by comparing calculated results with experimental data for a wide range of concrete mix proportions. These experiments involved observing the effect of one-face drying and six-face drying on the water content profiles and mass decreases in prismatic specimens. The values of the coefficient required for the analysis are reported for a wide range of water/cement ratios, and the effect of these coefficients on the calculated results is discussed.
Strength and fracture toughness of silica fume concrete
Authors: Sabir B. B.
Published: October 2009

This paper gives the results of an investigation which assesses the effects of condensed silica fume on the compressive strength and fracture toughness of concrete. The finite element method was used to determine the stress intensity factor for a range of symmetrically notched concrete cubes. Laboratory tests were conducted to determine the loads which cause the specimens to fail by crack propagation in the opening mode offracture. The test specimens were manufactured from concretes in which several levels of partial cement replacement (up to 12%) by silica fume (microsilica) were used. The fracture energy and fracture toughness of the modified concrete were evaluated through the stress intensity factor, and the loads at failure obtained from the laboratory tests. It was found that whereas the increase in the compressive strength for a cement replacement by silica fume of 12% was around 75%, the corresponding increase in the fracture energy was 15–20%.
Longitudinal splitting in the anchorage zones of post-tensioned members
Authors: Oehlers D. J.
Published: October 2009

The dispersal of the highly concentrated anchorage forces into the concrete element in post-tensioned members is resisted by large tensile stresses that can cause the concrete element to split in line with the direction of the anchorage force. Extensive research into longitudinal splitting in composite steel and concrete beams has been used to develop a simple hand analysis procedure for predicting the splitting resistances of the anchorage zones of post-tensioned members that allows for the three-dimensional dispersal of the anchorage forces and for inclined anchorage forces. It is suggested that this technique can be used to supplement existing techniques such as three-dimensional finite element models and strut and tie models in order to produce more efficient and safer designs, particularly for complex arrangements of groups of anchorage blocks.
Some results on punching shear in plain and fibre-reinforced micro-concrete slabs
Authors: di Prisco M. , Felicetti R.
Published: October 2009

Punching shear, caused by a concentrated load, induces a complex crack pattern and different failure modes in concrete slabs. In order to study slab behaviour under punching, a series of circular slabs supported along the partially restrained boundary was cast and tested, with various amounts of hooked-end fibres. Although the fibre contribution is certainly affected by the fibre aspect ratio L/D, only a single type of medium-length hooked-end fibres was adopted, as a reasonable compromise among fibre length, slab thickness and maximum aggregate size. Three different volume contents were adopted and, whenever possible, comparisons with plain concrete slabs were carried out. In order to assess the role of the geometry of the punching tip, two different shapes were considered: hemispherical and flat. In fact, different resistant mechanisms are activated and their role depends on the tip shape: bending, an arch action within the slab, punching and, if the tip dimension is small enough, crushing too. The experimental investigation carried out highlights the role ofthe different mechanisms in order to understand the nature of the failure and post-peak behaviour, in which the fibre contribution is expected to be more efficient.
Further research on fatigue properties of plain concrete
Authors: Zhang B. , Phillips D. V. , Wu K.
Published: October 2009

Experimental investigations of the effects of the water-cement ratio, aggregate type and loading sequence on the fatigue properties of concrete are presented. The validity of Palmgren-Miner's hypothesis of linear cumulative damage is examined for variable loading sequences. Also, the effect of the tension-compression strength ratio is discussed. Analysis of the results is based on the fatigue equation previously proposed by the authors.
New method of testing the flowability of grout
Authors: Murata J. , Suzuki K.
Published: October 2009

Various flow-cone methods are widely used to determine the flowability of grout. The methods involve the use of a cone-type viscometer, the purpose of which is to determine indirectly the kinematic viscosity of Newtonian fluids. However, grout is generally accepted to be a Bingham-like fluid. Thus we have investigated whether flow cone methods are appropriate for testing the flowability of grout. That is, an energy equation was applied to the flow condition of the fluid surface in the flow cone and to the outlet of the discharge tube at the bottom of the flow cone. The average outflow velocity offluid was determined both theoretically and experimentally to clarify the effect on the measured values of differences in the shape of the flow cone and in the length of the discharge tube. As a result, an inclined-pipe testing method based on the principle of capillary viscometers was proposed. In this method the discharge tube was lengthened and inclination o/the discharge tube was near to horizontal, in order to obtain a steady flow of grout through the discharge tube. The flow velocities measured using this apparatus agreed with those measured using a rotation viscometer. The inclined-pipe method gave accurate measurements of differences in the physical properties of grouts according to their mix proportions and the ambient temperature. Furthermore, as a practical application of this method, a procedure is proposed for calculating the load on a grout pump when pumping a predetermined quantity of grout through a pumping pipeline in a grouting works.
Drying shrinkage stresses in concrete patch repair systems
Authors: Asad M. , Baluch M. H. , AI-Gadhib A. H.
Published: October 2009

This paper proposes an approach to prediction of the build-up ofstresses due to drying shrinkage in concrete patch repair systems. The technique suggested consists of a twofold sequential approach, requiring the solution of the moisture diffusion equation followed by execution of an appropriate stress analysis program. Solution of the nonlinear moisture diffusion problem is obtained conveniently through a finite element formulation, which outputs the variation ofmoisture content in time and space. Inasmuch as drying shrinkage occurs by virtue of loss ofmoisture, a phenomenological relationship between moisture loss and free shrinkage strain is established and used to convert the finite element output of moisture loss to the free shrinkage strain. Time- and space-dependent values offree shrinkage strain are then fed as input to an appropriate stress analysis program, conceptually similar to a thermal stress analysis program, with the resulting output yielding the stress build-up in the patch repair system due to drying shrinkage. The influence of repair layer variables including the diffusivity Kc. ultimate free shrinkage strain fsh and modulus ofelasticity Er on the build-up ofdrying shrinkage stress is highlighted.
Effect of elevated triaxial cyclic and constant loads on the mechanical properties of plain concrete
Authors: Taliercio A. L. F. , Gobbi E.
Published: October 2009

An experimental programme was carried out to investigate the effect of elevated cyclic and constant triaxial stresses on the change in the mechanical properties ofplain concrete. Triaxial cyclic tests on cylinders, with the confining pressure and axial load varying in phase opposition at I Hz, were performed in three different deviatoric planes. The maximum stress deviator was 90% of the deviator at failure in a given deviatoric plane; the minimum deviator was 80% of the maximum one. In constant-load tests, the applied stresses were equal to the stresses at the maximum deviator applied in cyclic tests. At the cycle amplitudes selected, the creep effects under constant load are usually larger than the dynamic effects. The results obtained show that: the residual strength is increased with respect to the strength of companion virgin specimens, for any test condition; the increase in residual strength is much more important in triaxial tests than in uniaxial tests and increases with the test duration; the decrease in elastic modulus increases with the isotropic pressure at which tests are performed; and the test duration has a less pronounced effect on the decrease in elastic modulus.
Non-ferrous reinforcement
Authors: Clarke J. L. , Halcrow William
Published: October 2009

Traditionally, concrete structures have been reinforced with steel bars or prestressed with steel wires or strands. Generally, embedded steel is very durable, the concrete providing a suitable alkaline environment. However, for structures in highly aggressive environments, the protection afforded by the concrete is often insufficient to ensure the necessary service life for the structure. In addition to attempts to improve the quality of the concrete itself, there are a number of approaches for protecting the steel directly, such as by means of epoxy coating or the use of cathodic protection. A further alternative is to replace the steel completely by a material that will, hopefully, be more durable. One such option, which has great potential, is the use of fibre-reinforced plastics (FRPs), which consist of continuous fibres, generally of glass, carbon or aramid, set in a suitable resin to form a rod or grid. These materials are well accepted by the aerospace and automotive industries and are starting to find applications in the construction industry.

The mechanical properties of FRPs are chiefly determined by the amount and type of fibre, while the durability will be a function of both the resin and the fibre. The strength of FRP reinforcement will tend to be between that of high-yield reinforcing steel and prestressing stand (say 1000 N/mm2 for glass fibres and 1500 N/mm2 for carbon fibres) but the stiffness will generally be significantly lower (say 45 kN/mm2 for glass fibres and 150 kN/mm2 for carbon fibres). All FRP materials have a straight line response to failure with no plasticity.

The most common method for manufacturing FRP rods is pultrusion, in which the fibres are drawn off storage containers in a controlled pattern, are impregnated with resin and then drawn through a heated die which sets and cures the resin. One limitation at present is that thermoset resins are generally used and, hence, once fully cured, rods cannot be bent into the range of shapes currently used with steel. Thus ‘specials’ are required. Spiral reinforcement, either circular or rectangular in form, is produced by some Japanese manufacturers along with two- or three-dimensional grid. Other manufacturing techniques are being developed in which resin-impregnated fibres are wound on to suitable mandrels to producing closed shapes, such as shear links. As an alternative, thermoplastic resins are being developed, which would allow the fully cured material to be bent.
Study on the suppression effect of natural zeolite on expansion of concrete due to alkali-aggregate reaction
Authors: Naiqian Feng , Hongwei Jia , Enyi Chen
Published: October 2009

The effect and mechanism of natural zeolite on preventing expansion due to alkali-aggregate reaction are studied in this paper. The resultsconfirm that the expansion can be minimized by increasing the fineness of natural zeolite powder (even to superfineness) or by a previous heat treatment for the zeolite additive.
Prediction of total chloride content profile and concentration/time-dependent diffusion coefficients for concrete
Authors: Dhir R. K. , Jones M. R. , Ng S. L. D.
Published: October 2009

This paper presents a new approach to the determination of the total chloride content profile and the concentration/time-dependent diffusion coefficients for water-saturated concrete (i.e. concrete where diffusion is the dominant permeation process) due to the ingress of chloride ions. The mathematical models developed are primarily based on the assumption that the total chloride content profile within a semi-infinite medium can be expressed as an exponential decay function of the Boltzmann variable. This assumption and the simulation results have been found in good agreement with published experimental data. A test period of less than several months for typical structural concrete is believed to be sufficient. The profiles of both the non-steady state and the steady state conditions can easily be predicted. Experimental works involve only a few measurements of the total chloride content of concrete samples at different times plus a conventional penetration test. The advantages of this approach over the conventional methods, using Fick's laws, are that the change in concrete characteristics with time can be accounted for and can produce reliable results in a shorter test period. In addition, the calculated concentration/time-dependent diffusion coefficients can also be used to study the non-linear chloride-binding capacity of concrete.
The achievement of ductility in reinforced concrete beams
Authors: Kemp A. R.
Published: October 2009

A review is presented of the influence of lateral confinement by stirrups, longitudinal strain gradient, size effects, concrete strength and nature of loading on the strength and ductility of rectangular reinforced concrete beams. Factors influencing the strain-weakening slope of the stress–strain curve for concrete in flexural compression and the critical interaction of this strain-weakening behaviour with the strain hardening of the reinforcement were identified by non-linear analysis of 94 beam tests conducted by various authors. Expressions are presented for the slope of the falling branch and the characteristics of the concrete stress block at maximum moment. Criteria are provided for achieving significant rotation capacity in reinforced concrete beams, and the most important parameters affecting this behaviour are identified.
Transverse reinforcement effects on anchored deformed bars
Authors: Plizzari G. A. , Deldossi M. A. , Massimo S.
Published: October 2009

In the present research project, long anchorages provided with transverse reinforcement were tested with the aim of studying the confining effects of the stirrups. Particular attention was devoted to gathering detailed data on bar-toconcrete slip, splitting crack propagation and stirrup stresses. To carry out the pull-out tests, a special test set-up was built, and the friction between the specimen and the reaction plate was eliminated by resorting to a sophisticated design. The results show that (a) the maximum splitting crack opening increases for decreasing transverse-reinforcement diameter size and, under service loads, reaches remarkable values; (b) anchorage capacity increases with the stirrup index of confinement Ω up to an upper bound; (c) stirrup in specimens with a small amount of transverse reinforcement yielded under service loads. A comparison with the values prescribed by building codes evidence that the bond strength prescribed by both Eurocode 2 and ACI 318-95 code remarkably underestimates the experimental anchorage capacity for large values of the stirrup index of confinement.
Surface modified polypropylene fibres for use in concrete
Authors: Tu L. , Kruger D. , Wagener J. B. , Carstens P. A. B.
Published: October 2009

The research project reported on is concerned with the effect of modification of the surface of polypropylene fibre by a new chemical treatment process, oxyfluorination, on the properties of polypropylene fibre reinforced concrete. As a world first, the interfacial bond of polypropylene fibres with the cementitious matrix is improved by increasing the surface free energy of the fibre surface. The reasons for the poor bonding between untreated polypropylene fibre and cementitious matrix are discussed, using the fibre surface free energy and Lewis acid–base interaction concept. The contact angle of water on the polypropylene fibre surface as well as fibre surface free energy components were measured. This showed reduced contact angles, as well as increased acid–base components of the surface free energy because of oxyfluorination. Mechanical properties such as compressive strength, flexural properties and impact resistance of the fibrous concrete, reinforced with different types of oxyfluorinated polypropylene fibres, were determined and compared with those of untreated polypropylene fibre reinforced concrete. The results confirmed that the surface modification largely improves the mechanical performance of the fibrous concretes. Restrained plastic and drying shrinkage cracking tests, using restrained slab specimens and steel ring restrained specimens, indicated that the surface oxyfluorinated fibres possess a higher shrinkage cracking resistance than do unmodified fibres. The effect of surface oxyfluorination on the fibre/concrete matrix interfacial bondings was investigated using a fibre pull-out test. A mechanism for this interfacial bonding improvement is proposed. Oxyfluorinated polypropylene fibre surfaces and their interfaces with concrete matrix, compared with that of unmodified fibre, were observed using scanning electronic microscopy. Some field application tests were conducted and the good results that were achieved are presented.
Determination of the thermal conductivity of concrete during the early stages of hydration
Authors: Gibbon G. J. , Ballim Y.
Published: October 2009

A knowledge of the thermal conductivity of concrete during the early hydration period (7 days) is essential to obtain an accurate estimate of the thermal stresses that will develop in a concrete structure as a result of the heat produced by the binder during hydration. This paper describes a probe that is capable of measuring the thermal conductivity of a 2 kg concrete or mortar sample during all stages of hydration. A method of calculating the thermal conductivity of concrete with large aggregates from the measured conductivity of the mortar and the conductivity of the aggregates is also discussed.
Fatigue life and change in mechanical properties of plain concrete under triaxial deviatoric cyclic stresses
Authors: Taliercio A. L. F. , Gobbi E.
Published: October 2009

The effects of a lateral confinement on the fatigue behaviour of plain concrete were investigated experimentally. Triaxial cyclic tests on cylinders, with the confining pressure and axial load varying in phase opposition at 1 Hz, were performed in different deviatoric planes. The maximum stress deviator was 90% of the deviator at failure in a given deviatoric plane; the minimum deviator ranged between 70% and 95% of the maximum one. The residual strength and stiffness properties of those specimens that did not fail within a prescribed number of cycles were also assessed. In tests with cycles of equal amplitude (normalized to the triaxial static strength), the fatigue life and residual strength seem to be positively affected by an increase in hydrostatic stress only up to a certain level. With the maximum deviator equal, in a given deviatoric plane the fatigue life usually increases with decreasing cycle amplitude. In one of the deviatoric planes, the cycle amplitude and mean stress were found to have opposite effects upon damage and residual strength.
Determination of the specific fracture energy of AAR-affected concrete using the wedge-splitting method
Authors: Rotter H. M. , Tschegg E. K. , Nadu M. , Schieber M.
Published: October 2009

Fracture mechanical tests were done on specimens of concrete affected by the alkali—aggregate reaction (AAR). The influence of deterioration of the concrete due to the AAR on the fracture mechanical behaviour was determined using the wedge splitting method. Cubic samples were taken from standard concrete prisms, and a load—displacement diagram was recorded for the stable separation of the specimen. The results show that for comparable variants the specific fracture energy GF of AAR-affected concrete exhibits a marked drop according to the induced damage. The calculated nominal notch tensile strength behaves similarly to GF. The mechanical parameters GF, Fhmax and ó are more relevant than the classical lengthchange measurements to the quantitative assessment of the deterioration of concrete affected by the AAR.
Fire resistance of concrete: prediction using artificial neural networks
Authors: Chan, Y. N. , Jin P. , Anson M. , Wang J. S.
Published: October 2009

A model based on an artificial neural network (ANN) for predicting the loss of strength of concrete under high temperatures (75–1200°C) is presented. The ANN is a recent development and is particularly useful for evaluating non-linear systems with several variables the relationships between which cannot be defined explicitly by mathematical equations. It has been reported in the literature that the fire resistance of concrete is related to several material and environmental factors, but this relationship is difficult to quantify mathematically. The advantage of the ANN is that it can be trained, using experimental data assembled from published research, to recognize the relationship between these influencing factors and the fire resistance of concrete measured by the loss of strength. After training, the ANN is applied to predict the loss of strength of concrete with new material and environmental factors. In the tests conducted the prediction errors between the network outputs and the actual experimental results were less than 15%.
Size-effect tests in unreinforced concrete columns
Authors: Sener S. , Barr B. I. G. , Abusiaf H. F.
Published: October 2009

Test results obtained as part of a size-effect study on axially loaded plain concrete columns are presented. The test specimens were geometrically similar concrete columns (with dimensions in the ratio 1:2:4) having slenderness ratios of 9·7, 18·0 and 34·7. The columns were of square cross-section with sides of 50, 100 and 200 mm and varied in length from 145 mm to 2080 mm. Although such columns are at the bottom end of practical column sizes, the test results extend existing research information on size effects in columns. It was observed that for all slenderness ratios, the failure loads exhibited a size effect. The nominal stress at maximum load (maximum load divided by nominal cross-sectional area) decreased as the size of the columns increased. This behaviour is not taken into account in current design codes and needs further investigation to include practical column sizes. The results are not inconsistent with the 'size-effect law' proposed by Bazant.
Pulverized fuel ash concrete: air entrainment and freeze/thaw durability
Authors: Dhir R. K. , McCarthy M. J. , Limbachiya M. C. , El Sayad I. , Zhang D. S.
Published: October 2009

The paper describes a study undertaken to determine the effect of pulverized fuel ash (PFA) and its characteristics on air entrainment, the air void system and the freeze/thaw durability of concrete. The results demonstrate that the admixture demand of PFA concrete was higher than that of Portland cement (PC) concrete and greatly influenced by the type of air-entraining admixture, the level of air required and the characteristics of the PFA used. While PFA fineness had little influence on admixture demand, PFA with high loss on ignition required dosages in excess of two times that of PC. However, PFA was found to have little influence on the rate of air loss with handling and reduced the variability of air content at a given admixture dosage when combined with PC from different sources. Tests to assess the reliability of the ASTM method for examining the air void system confirmed this. The results indicate that improvements in air void parameters were obtained with increasing air content in the concrete. However, similar or slightly enhanced parameters were measured for PFA concrete compared to those of PC concrete. In this case, the characteristics of the PFA had no effect. Following on from this, tests for freeze/thaw durability (ASTM C666: Procedure A) indicated that the critical factors influencing deterioration were the air content and design strength of the concrete. In this respect, no difference was observed between PC and PC/PFA concrete, and all concretes, irrespective of design strength, exhibited very good freeze/thaw resistance above an air content of 3·5%. A nomogram was developed to demonstrate possible routes to material selection/admixture dosages for the practical achievement of durable concrete in freeze/thaw conditions.
Punching strength of slabs reinforced for shear with offcuts of rolled steel I-section beams
Authors: Gomes R. , Regan P.
Published: October 2009

Punching shear failure may occur at columns supporting flat slabs or at concentrated loads. The rupture is brittle and may occur clearly below the flexural capacity of the slab. This work aims to analyse the symmetric punching resistance of reinforced concrete flat slabs with shear reinforcement. Twelve 200 mm thick slabs representing regions around columns, loaded at the centre and reacted at the edges, were tested up to failure. Short offcuts of steel I-beams were used as shear reinforcement. The principal variables considered in the tests were the area, number and distribution of the shear steel elements. All the slabs failed by punching, and the failure load of the specimen with the most reinforcement was more than twice that of the slabs without shear reinforcement.
Effect of microcracking and cracking on the development of corrosion in reinforced concrete members
Authors: François R. , Arliguie G.
Published: October 2009

The purpose of the work described here was to determine the effect of both cracking and microcracking on the service life of loaded reinforced concrete. This paper provides a synthesis of the results of experiments carried out over 12 years on reinforced concrete elements kept in a loaded state in a confined salt fogs. The test specimens were beams 3 m long, which is a sufficiency size to be representative of the actual operating conditions of reinforced concrete structures. The interpretation of the results allowed us to conclude that the development of reinforcement corrosion is not influenced by the widths of cracks (for widths less than 0·5 mm) or by the cracks themselves. It is, rather, the tensile microcracking in the concrete due to the service loading which increases the penetration of chloride ions and then reduces significantly the service life of the reinforced concrete. A relationship was established between the tensile stress in the rebar σs and the penetration of chloride; σs seems to be an important parameter of the durability of reinforced concrete.
Enhancements in surface quality of concrete through use of controlled permeability formwork liners
Authors: Cairns J.
Published: October 2009

This report presents results of a study to compare the performance of concrete wall surfaces cast against controlled permeability formwork (CPF) liners with that of concrete surfaces cast against conventional impermeable plywood formwork Two formwork liners were used, namely 'Zemdrain', a proprietary spun-bonded formwork liner manufactured by Dupont, and 'Lotrak FL', a woven geotextile manufactured by Don & Low. A variety of test methods were used to assess performance, including surface hardness, initial surface adsorption, sorptivity, depth of carbonation, chloride penetration and freeze–thaw resistance. In addition, the quality of the surface finish was assessed visually. Two concrete mixes and two different curing regimes were tested. The variation in surface quality of concrete with depth from the top of the pour was also examined. Results demonstrate that surface qualities of concrete can be markedly improved by use of CPF liners. The two liners produced identical enhancements in surface quality. They differed in one aspect of performance only, as cement particles were able to travel through the Lotrak fabric and to bond to the backing plywood, necessitating cleaning of the plywood after each cast. This problem did not arise with Zemdrain. Effectiveness of CPF liners was slightly greater for the lower grade mix and slightly lower close to the top of a pour. It is also observed that measured enhancements in performance may be strongly influenced by test duration.
The effects of rapid cooling by water quenching on the stiffness properties of fire-damaged concrete
Authors: Nassif A. Y. , Rigden S. , Burley E.
Published: October 2009

This paper reports the results of experimental research into assessing the fractured state of fire-damaged concrete under rapid cooling in an attempt to establish the effects of fire-fighting operations on the structural integrity of concrete buildings subject to a fire. The investigation was driven by the need to improve the material models used in predicting the fire resistance of concrete members or redesigning such members after a fire incident. The extent of damage was quantified using the stiffness damage test. The stiffness characteristics of 40 fired limestone concrete cores were determined from a series of quasi-static load—unload cycles. The strain energy dissipated in the specimen during such cycles provides a quantitative measure of the damage. The damage index is defined as the area of the hysteresis loop divided by the stress range. Other stiffness parameters, such as the initial and chord moduli of elasticity, were also determined. The extent of damage was also examined using ultrasonic pulse velocity (UPV) and dynamic modulus methods. Visual examination and scanning electron microscopy were also used to provide qualitative assessment. Quenching hot concrete with water was found to result in a reduction in the stiffness of fire-affected concrete over the entire range of temperatures investigated. The damage index for air-cooled concrete after firing to 320°C is at the same level as that for concrete heated to 220°C and cooled by quenching. The inference here is that the traditionally held view that 300°C marks the onset of significant damage in concrete is debatable. UPV measurements confirmed the damaging effect of rapid cooling on the stiffness properties of fire-affected concrete. A further 10% reduction of the UPVof quenched concrete compared to air-cooled concrete was observed. The dynamic modulus measurement also confirmed the adverse effect of quenching on the modulus of elasticity of fired concrete.
Prevention of diagonal tension failures in beams using a flexure—shear interaction approach
Authors: Ziara M. M. , Haldane D. , Kuttab A. S.
Published: October 2009

The development of a flexure–shear interaction design model for the prevention of the diagonal tension failure in beams is described in this paper. The model is based on the understanding that failure of structural concrete members occurs as a result of the development of critical tensile stresses in the compression region of the beam structure. In this model, confining the concrete compression region with closed stirrups prevented this type of failure. The enhancement of the concrete strength due to the confining influence of the stirrups was utilized to offset the reduction in the flexural capacity of beams due to the effects of shear. The results from tests on 24 specially designed beams were used in the development of the flexure–shear model. The programme of experimental work for the validation of the proposed model consisted of ten full-size rectangular beams, which were selected to cover the entire range of Kani's valley. The test results confirmed the applicability and the efficiency of the proposed design model.