bluuebunny/crossref_metadata_2025 huggingface dataset:
Variation of differential strain ratio in normal-and high-strength concrete under short-term uniaxial compression
Authors: Loo Y. H. , Tam C. T. , Zhong H.
Published: October 2009

The creep behaviour of concrete test specimens of three different mix proportions and strength levels was studied at various sustained stress levels. The experimental results of short-term sustained loading tests were analysed to provide a quantitative description of the observed behaviour. The creep behaviour of normal- and high-strength concrete and the companion mortar specimens was compared. A method based on the variation of differential strain ratio (the rate of change of transverse strain with respect to axial strain) for determining whether a concrete specimen has reached the point of creep fracture is proposed. Test results showed that the stress–strength ratio of the creep strength of concrete increased with the strength of concrete. The transverse and axial strains of concrete at this critical stress–strength ratio did not vary much with the strength of concrete, but averaged about 320–350 and 1430–1470 microstrain respectively.
Investigation of framed shearwall behaviour with neural networks
Authors: Mo Y.L. , Lin S.S.
Published: October 2009

To date, concrete structure modelling has involved the development of mathematical models of concrete structure behaviour derived from human observation of and reasoning with, experimental data. An alternative, discussed in this Paper, is to use a computation and knowledge representation paradigm, called neural networks, developed by researchers in connectionism (a sub-jeld of artificial intelligence) to model concrete structure behaviour. m e main benefits in using a neural-network approach are that all behaviour can be represented within a unified environment of a neural network and the network is built directly from experimental data using the self-organizing capabilities of the neural network, i.e. the network is presented with the experimental data and learns the relationships between stresses and strains. Such a strategy has important implications for modelling the behaviour of modern, complex concrete structures. In this Paper, the behaviour of reinforced concrete framed shearwalls is modelled with a back-propagation neural network. Preliminary results of the use of networks to model concrete structure look promising.
Investigation of factors influencing the expansive behaviour, compressive strength and modulus of rupture of alkali–silica reactive concrete using laboratory concrete mixes
Authors: Rigden S. R. , Majlesi Y. , Burley E.
Published: October 2009

Alkali–silica reaction (ASR) in concrete results in expansion of the concrete and possibly severe cracking. The cracking leads to a reduction in the compressive strength and the modulus of rupture of the concrete. A test programme is under way on model structural elements that are suffering from ASR. As a prelude to this, a series of ASR concrete mixes were developed. This Paper examines the performance of these ASR mixes with respect to normal mixes, and comments on factors that were found to influence their compressive strength, expansion and modulus of rupture. Time-dependent variation in these factors is identified and assessed. The mixes used consisted of flint gravel, ordinary Portland cement, fused silica as a reactive aggregate and potassium hydroxide to increase the supply of alkalis in the mix. As well as the quantities of these materials, the particle size distribution of the fused silica was varied, as was the environment in which the concrete was cured and stored.
Analysis of the time-dependent volume reduction of concrete containing silica fume
Authors: Al-Sugair F. H.
Published: October 2009

The use of silica fume (SF) to produce high-performance concretes is gaining acceptance in the construction industry. Concretes of compressive strength above 70 MPa with improved resistance to aggressive chemicals have been produced successfully using SF. The time dependent volume reduction of concrete caused by moisture loss, known as drying shrinkage, is known to have a profound influence on cracking, long-term deflection and losses in prestressing. It is therefore necessary to have a clear understanding of these deformations in concrete containing SF in order to assess its long-term properties. In this Paper, results of an extensive test programme to evaluate the drying shrinkage of concrete containing different amounts of SF are presented and discussed. The results are analysed to help explain the mechanisms of drying shrinkage in concrete containing SF.
Behaviour of strengthened RC T-beams
Authors: Ong K. C. G. , Tan K. H. , Mansur M. A. , Natarajan S. M.
Published: October 2009

Twelve reinforced concrete T-beams were each strengthened in shear by the incorporation of additional stirrups in a thickened section cast using non-shrink grout. Another beam was strengthened by thickening the existing section only, without any additional reinforcement. The beams were simply supported and tested under a point load at midspan and their performance was compared with respect to cracking, deflection and ultimate strength. Separate tests were conducted to assess the contribution of grout to the shear strength of the beam, and accordingly a semi-empirical equation is proposed for the estimation of shear strength of the strengthened beams. The shear strengths of the tested beams compared well with the proposed equation, while they were generally underestimated by equations according to the codes of practice ACI 318 and BS 8110.
High-strength condensed silica fume concrete
Authors: Sabir B. B.
Published: October 2009

Tests to evaluate the compressive strength, tensile strength and static modulus of elasticity of concretes containing various levels of condensed silica fume (CSF) (microsilica) substitutions for Portland cement were conducted at two water curing temperatures. I was found that CSF replacements for cement of up to 28% can have a beneficial effect on the compressive strength. Although high-temperature curing (50°C) accelerates the strength development at early ages, the 91 day strength does not appear to be influenced by temperature. The curing temperature does not affect the observed relationships between the tensile strength, the modulus of elasticity and the compressive strength, There is a marked reduction in the elasticity modulus at the higher levels of CSF substitutions for cement.
Web reinforcement effects on shear capacity of reinforced high-strength concrete beams
Authors: Ahmad S. H. , Park F. , El-Dash K.
Published: October 2009

Thirty-two beams made of normal- and high-strength concrete were tested to study the effects of web reinforcement placement on load-carrying capacity. Sixteen beams had stirrup reinforcement only in the shear span (type A beams); the other 16 had web reinforcement only in the flexural span (type B beams). The shear span-to-depth ratio a/d was varied between 0·6 and 3·7; the concrete strength was varied between 31·7 and 93·1 MPa. Test results indicate that for a/d = 2, high-strength concrete beams have more reserve capacity than normal-strength concrete beams. For beams with web reinforcement in the shear span only, the shear reinforcement is more effective for smaller a/d ratios. The ultimate load capacity of the two types of beam is comparable for a/d < 2. For higher a/d ratios, the ultimate load capacity of type A beams is superior to that of type B beams. The ductility of the two types of beam is comparable for a/d = 0·6, but with increasing a/d the ductility of type A beams becomes superior to that of type B beams. Also, a comparison was carried out with ACI, BS 8110 and CEB-FIP, and it is suggested that some of their recommendations be reconsidered.
The C1−/OH− ratio in chloride-contaminated concrete — a most important criterion
Authors: Kayyali O. A. , Haque M. N.
Published: October 2009

Chloride-contaminated concretes were produced either by admixing sodium chloride with original mixes or as a result of exposure to external sodium chloride solutions. Free chloride and hydroxyl ion concentrations in the pore solutions of the concretes were determined. It was found that the value of free chloride content by itself cannot provide a meaningful indication of the susceptibility of the reinforcement to chloride-initiated corrosion. The ratio of free chloride to hydroxyl ion concentrations in the pore solution and differentiation between admixed and externally applied chloride are found to be factors that must be known if indicative values to predict corrosion are sought. Charts giving safe regions of C1−/OH− values for various concretes are suggested. The use of a superplasticizer in concrete making was found to increase the concentration of free chloride and to raise the value of Cl−/OH− ratio in all the concretes tested. The use of fly ash was found beneficial in immobilizing the chloride ions in non-superplasticized concrete. However, in concrete made with a superplasticizer the inclusion of fly ash was found to cause a greater release of chloride ions into the pore solution.
Prediction of the behaviour of RC beams with exposed reinforcement
Authors: Zhang S. , Raoof M.
Published: October 2009

The paper reports details of a fairly simple theoretical model for estimating the flexural ultimate load of reinforced concrete beam under two-point loading in which the main reinforcement over part of the span is fully exposed in a symmetrical fashion about the mid-span in the course of the patch repair process prior to the application of the repair materials. Encouraging correlations have been found between the theoretical predictions and large-scale experimental data reported by others, which covered a wide range of design parameters. Theoretical numerical results throw some light on the perhaps initially puzzling experimental observations reported previously that the ultimate loads of beams with exposed reinforcement can increase significantly if nominal top steel (in the form of, for example, 2 × 6 mm dia. bars), such as is invariably used for holding stirrups in place, is used. Numerical results are presented that show the influence of various parameters (concrete cube strength, length of exposed main reinforcement, depth of removal of concrete, etc.) on the ultimate loud-bearing capacity of beams with exposed main reinforcement.
Properties of high-strength concrete mixes containing PFA and ggbs
Authors: Domone P. L. , Soutsos M. N.
Published: October 2009

The effects of incorporating pulverized fuel ash (PFA) and ground granulated blastfurnace slag (ggbs) on the workability (slump), adiabatic temperature rise during hydration and long-term (up to 570 days) strength of high-strength concretes have been measured. Binary (PFA/ggbs and Portland cement) and ternary (PFA/ggbs plus microsilica and Portland cement) blends at water–binder ratios from 0·38 to 0·20 have been tested. The results show broadly similar effects to those in lower strength concrete, although of differing magnitude in some cases. Some potential advantages of ternary blends for optimization of properties have been demonstrated.
Accelerated test of ingress of chloride ions in concrete under pressure and concentration gradients
Authors: Lee S. L. , Wong S. F. , Swaddiwudhipong S. , Wee T. H. , Loo Y. H.
Published: October 2009

This paper describes the experimental and numerical work carried out to study the ingress of chloride ions in concrete under pressure and concentration gradients. The proposed accelerated water and chloride permeability tests together with long-term soaking tests are used to develop a pressure gradient model, a concentration gradient model and the combination of the two f or the prediction of ingress of chloride ions in concrete. It is found that the experimental results, based on OPC concrete with a water-cement ratio of 0·6 tested in the pressure range 0·30 MPa, agree well with the finite element solutions derived under similar conditions. The design and application of the specially built testing equipment for the accelerated permeability tests are presented. The repeatability of these tests, expressed as the coefficient of variation, is satisfactory. This confirms that the proposed test apparatus and method can be used with confidence. It is also observed that the applied pressure gradient has little effect on the water permeability coefficient in the experimental range 0–30 MPa.
Properties of concrete in the cover zone: water penetration, sorptivity and ionic ingress
Authors: McCarter W. J. , Ezirim H. , Emerson M.
Published: October 2009

The work described in this paper forms part of a wider research programme being undertaken on water and ionic movement within the cover region of structural concrete. In-situ monitoring techniques described previously were used to measure the depth of penetration of water, cumulative volume gain and sorptivity of concrete samples containing different types of aggregate. The effects of dissolved ions (chlorides) in the invading water were also monitored. As a direct result of the monitoring techniques employed in the testing programme, it is possible to present many of the results in considerably more detail than has previously been the case. Possibilities for evaluating the electrical properties of concrete in the cover zone, and at rebar level, are also discussed. In addition, a method of estimating chloride concentration within the pore water is suggested.
The carbonation of concrete structures in the tropical environment of singapore and a comparison with published data for temperate climates
Authors: Roy S. K. , Beng Poh Kong , Northwood D. O.
Published: October 2009

The carbonation depths and compressive strengths of four reinforced concrete buildings which ranged in age from 7 to 59 years were measured in the tropical environment of Singapore. The average carbonation depths, d, were related to the age o of the building by the relationship d = KVage, where the carbonation constunt, K, Was approximately 7 mm/(year)1/2. For anv given structure the ratio of the maximum to average carbonation depth varied from 2·1 to 2·8. The carbonation depth and K value were also dependent on the quality (compressive strength) of the concrete, concrete with compressive strengths > 20 MPa showing lower carbonation depths and K-values than concrete with compressive strengths < 20 MPa. The K-values measured in this study, 5·5–8·6 mm/(year)1/2 were generally higher than those reported for temperate climates (1–3 mm/(year)1/2). The difirences are attributed to (i) the generallv lower grade (compressive strength) of the concrete used in the construction of the buildings and (ii) the signijicantlv higher mean dai1y temperature in Singapore, 27°C, compared to the temperate climates, 8° to 9°C.
Experimental and analytical investigation of FRP strengthened beam response: Part I
Authors: Quantrill R. J. , Hollaway L. C. , Thorne A. M.
Published: October 2009

Beam tests on 1·0 m long reinforced concrete beams with fibre reinforced plastic plates epoxy-bonded to their soffits are reported. In these tests, a simple parametric study has been carried out in which the plate area, plate aspect ratio, plate material and the method of anchoring the plate ends have been varied. The effects of these variables on overall behaviour are discussed. An iterative analytica1 method which uses the principles of strain compatibility and equilibrium has been used for predicting the strengthened beam response. The use of both a cracked and a partially cracked section including steel strain hardening have been considered in developing the model. The analytica1 and experimental responses are compared and observations noted.
Effect of the quantity of sand on the compressive strength of two-stage concrete
Authors: Abdelgader(Libya) H. S.
Published: October 2009

Traditional concrete is made by mixing all the ingredients in a mixer. Then the mixed concrete is placed in a formwork. Two-stage concrete is constructed by first placing the course aggregate into 30 × 30 × 30 cm moulds and then grouting the voids of the course aggregate with a special grout mixture. Since a large volume of the two-stage concrete is occupied by coarse aggregate, it is likely that variations in coarse aggregate content will significantly influence the strength of the concrete. The stresses are mainly transferred by the coarse aggregate skeleton to the grout. The quanticv of snad does not influence the compressive strength of two-stage concrete, but influences the physical properties of the mixture (grout), e.g. fluidity and sedimentation, because the grout must be sufficiently liquid to fi11 the voids in the aggregate skeleton. The paper describes aspects of the effect of the quantity of sand on the compressive strength of the two-stage concrete method.
Hydrophobic materials, superplasticizer and microsilica effects on setting of cement pastes at various temperatures
Authors: AIshamsi A. M. , Alhosani K. I. , Yousri K. M.
Published: October 2009

The use of hydrophobic materials, superplasticizers and microsilica by the concrete industry in hot weather regions is increasing substantially. This paper investigates the influence of these materials on the setting times of cement pastes. Seven mixes with different component proportions were made to demonstrate the uncombined as well as the combined effects of these materials on both the initial and final setting times at 20, 35 and 50°C. The test results indicated that the hydrophobic material had a retardation effect. The efficiency of either the hydrophobic material or the superplasticizer in retarding setting is lower at high temperatures. Most interesting is that when combining the hydrophobic material with the superplasticizer, the resulting product gave an acceleration effect. The inclusion of microsilica in the mixture as a cement replacement material retarded both the initial and final settings.
Effects of aggregates and microfillers on the flexural properties of concrete
Authors: Penttala V. , Komonen J.
Published: October 2009

A flexural strength theory for concrete based on the elastic-plastic theory was derived. In the tests performed the failure criterion of the plain concrete beam was found to be the tensional rupture strain εu which had the value of 0·000 16 irrespective of the different concrete compositions. The elastic-plastic flexural strength ftp was shown to be almost a linear function of the modulus of elasticity of concrete. The problem of enhancing the flexural strength of concrete was transformed into the problem of increasing the modulus of elasticity of concrete. Concrete was modelled as a three-phase material consisting of aggregate, binder matrix and transition zone phases. The transition zone between the binder matrix and the aggregate particulates was reasoned to be around the aggregate particles which had a diameter exceeding 75 μm. The modulus of elasticity of concrete was modelled by an improved law of mixtures in which an adhesion factor was introduced. The adhesion factor takes into account the bond interaction between aggregate particulates and the binder matrix. The results of the theory derived were compared with the test results from concretes in which three different aggregates and two microfiller types, mainly, were used.
Towards a more fundamental non-linear basic creep model for early age concrete
Authors: De Schutter G. , Taerwe L.
Published: October 2009

Based on an extensive experimental programme a new model is proposed for the basic creep of early age concrete in compression. In this model the degree of hydration is an important parameter influencing the creep evolution as well as the final creep value. In this way the fundamental nature of the degree of hydration in the early age basic creep behaviour is illustrated. The non-linearity of the basic creep at early ages is taken into account.
A fractal model for simulating the formation of microcracks in the fracture process zone and a theoretical explanation of the size effect of the fracture energy of concrete
Authors: Ji X. H. , Chan S. Y. N. , Feng N. Q.
Published: October 2009

A fractal model is presented for simulating the formation of microcracks in the fracture process zone of concrete. The model is used to calculate the fractal dimension D of a network of microcracks. It is found that the branching angle of microcracks can greatly affect the value of D, and a lower value of D can be expected for high-strength concrete than for normal-strength concrete. In the light of fractal theory, a formula for calculating the total area of the fracture surface. including the area of microcracks, is proposed. The size effect of the fracture energy of cement-based materials is explained and the quantitative relationship between the fracture energy Gf, the surface energy of the concrete γs, the fractal dimension of the network of microcracks Dc, and the height of the ligament of the concrete specimen H is established as Gf = 2γsC″HDc−1 (where C″ is a constant). Using this formula. the fra ctal dimension Dc, of the network of microcracks in high-strength and normal-strength concrete was determined experimentally.
Use of ammonium nitrate solution to simulate and accelerate the leaching of cement pastes due to deionized water
Authors: Carde C. , Escadeillas G. , François A. H.
Published: October 2009

Studies in the laboratory on the durability of concrete are not easy to perform, particularly in the case ofa linkage between a chemical attack and a mechanical property variation. As a result, these studies require significant experimentation times because on the one hand the kinetics ofchemical degradation are very slow, and on the other hand the sample size has to be adapted to the aggregate size. One of the possibilities for solving these problems consists of developing accelerated tests allowing one to reproduce the real degradation undergone by the concrete during its history. This paper deals with the use of strongly concentrated ammonium nitrate solutions as an accelerated method for the degradation of cement-based materials by deionized water. The direct application of this work is to the durability ofconcrete containers used to store radioactive wastes.
Analysis of the drying shrinkage behaviour of concrete using a micromechanical model based on the micropore structure of concrete
Authors: Shimomura T. , Maekawa K.
Published: October 2009

A micromechanical model for moisture diffusion within concrete and the associated shrinkage of concrete is presented in this paper. The model is based on the micropore structure of the concrete and the thermodynamic behaviour of the water in the pores. The drying shrinkage behaviour, both time-dependent moisture loss and volumetric change, of mortar and concrete specimens is analysed under various conditions. The suitability and applicability ofthe proposed model are discussed on the basis ofthe results.
Drying creep of concrete in terms of the age-adjusted effective modulus method
Authors: Kovler K.
Published: October 2009

The problem of drying creep of concrete is considered. It is emphasized that there is no single explanation of the paradoxical concrete performance known as the Pickett effect. The existing experimental data cannot clearly distinguish between the different mechanisms of drying creep, namely microcracking and stress-induced shrinkage. The phenomenological approach describing drying creep under tension as a sum of shrinkage-induced creep and creep-induced shrinkage is corrected by taking into account the swelling deformation of sealed concrete in basic creep tests. In such a way, the drying creep strain represents only the extra creep component, and therefore the mechanism of drying creep based on the stress-induced shrinkage is called into question. A new phenomenological approach to the problem of drying creep using the terms of the age-adjusted effective modulus method is demonstrated. The work done by a concrete element under simultaneous loading and drying is represented as restrained shrinkage in addition to basic creep under gradually developed stresses. It is shown that the drying creep strain depends on the values of the ageing coefficients for basic and drying creep and on the individual components of the concrete deformation; instantaneous strain, basic creep and free shrinkage. It is found that the instantaneous strain plays a minor role. The experimental data from the creep tests under tension are used to calculate the ageing coefficient ofconcrete for basic creep.
A study of the application of reliability theory to the design of concrete cover
Authors: Matsushima M. , Tsutsumi T. , Seki H. , Matsui K.
Published: October 2009

If the chloride concentration at the surface of a concrete structure is known, the chloride concentration around reinforcements at year t can be estimated and the initiation time of reinforcement corrosion obtained from the concentration and some associated critical value. The accumulating corrosion products then causes the volume of reinforcement to expand, leading to cracking of the concrete surface. Provided that the structure reaches its critical state when surface cracks appear, the time tL is obtained from when the cracks appear. Taking the parameters associated with deterioration to be random variables, the occurrence probability of deterioration is evaluated. Then the optimum concrete cover thickness is obtained based on the concept of least expected cost.
An upper-bound plastic analysis for shear
Authors: Ibell T. J. , Morley C. T. , Middleton C. R.
Published: October 2009

This paper describes the calibration of a two-dimensional (2D) plasticity-based upper-bound analysis model for the assessment of shear in reinforced concrete beams. This work is part of an ongoing research effort at the University of Cambridge, aimed specifically at the assessment of shear in existing concrete bridges. Due to ductility limitations in concrete, full strength may not be achieved along the assumed lines of discontinuity during collapse, as assumed in a plasticity analysis. For this reason, an ‘effective strength’ of concrete is introduced, which is simply a fraction of the compressive cube strength of the concrete. In order to develop a suitable overall (3D) collapse analysis technique for concrete beam-and-slab bridges, it is necessary to determine this effective strength of concrete in each of the beam and slab portions. Here, a 2D beam shear collapse model, and an associated expression for the effectiveness factor for the concrete, are formulated and calibrated against several test results. Restrictions on the use of this model are established and it is concluded that such an analysis technique is eminently suitable for extension to the more general 3D collapse problem in existing concrete beam-and-slab bridges.
Mechanisms of water retention in cement pastes containing a self-curing agent
Authors: Dhir R. K. , Hewlett P. C. , Dyer T. D.
Published: October 2009

Self-cure concrete contains a chemical agent that reduces the evaporation of water from its surface, primarily by reducing the vapour pressure at the concrete pore solution surface. The self-curing agent developed at the Concrete Technology Unit, University of Dundee, also produces an alteration in cement hydration product microstructure, and it was considered that this may also contribute to the improved water retention properties. To investigate this, weight loss measurements were conducted on both self-cure and ordinary pastes exposed to controlled ambient conditions, whilst thermogravimetric analysis was carried out on identical specimens. It was found that, whilst the evolution of heat of hydration renders the early stages of drying very complex, it was possible to examine the diffusion dependent stage of drying. The diffusion coefficients observed for water vapour passing through the dry region of the self-cure paste surface were much lower than those observed for the control. This has been attributed to two mechanisms: the lower vapour pressure above the pore solution leading to a smaller difference across the dried portion of the paste and lower relative humidities in the cement pores, and the change in microstructure which reduces permeability.
Some considerations on the effect of chloride ions on the corrosion of steel reinforcements embedded in concrete structures
Authors: González J. A. , Otero E. , Feliu S. , Bautista A. , Ramírez E. , Rodríguez P. , López W.
Published: October 2009

The role of chlorides as an initiating factor for corrosion in the steel/concrete system, their mechanism of action, the critical threshold level above which the risk of corrosion of reinforcements in the active state is especially high, the corrosion rates observed under a variety of circumstances and their implications on the durability of reinforced concrete structures are analysed.
Behaviour of steel reinforcement under the effect of strong cathodic or anodic polarizations
Authors: González J. A. , Ramírez E. , Bautista A.
Published: October 2009

The behaviour of highly polarized steel embedded in concrete was analysed using small mortar specimens and concrete beams, as well as slabs of considerable size, in the presence and absence of chloride additions. Electrochemical impedance spectroscopy (EIS) was used for the small samples, and polarization resistance was measured in the large ones. The measurements at corrosion potential provide information about the corrosion kinetics of steel embedded in concrete. The response under high cathodic polarizations provides information about the behaviour of the electrodes subjected to cathodic protection or forming galvanic macro-cells with highly electronegative metals. The response under high anodic polarizations can provide information about the behaviour of reinforcements exposed to erratic currents, or about the effect of galvanic macro-cells with metals much nobler than steel.
Properties of concrete incorporating rubber tyre particles
Authors: Li Z. , Li F. , Li J. S. L.
Published: October 2009

The properties of concrete incorporating scrap rubber tyre particles were investigated. Properties studied included the compressive, flexural properties as well as the vibration isolation capability. The rubber particles were either plain particles or pre-processed using one of two coating methods (coating with cement paste and coating with METHOCEL cellulose ethers solution) before mixing in concrete. The concrete mixtures incorporating rubber tyre particles exhibited lower compressive and flexural strength than normal concrete. However, the mixtures demonstrated a ductile failure, and had the capability of absorbing a large amount of energy under compressive and flexural loads. Moreover, they demonstrated excellent vibration isolation capability. These results indicate that the mixtures might be suitable for applications such as driveways, sidewalks or road constructions where strength is not a high priority but greater toughness is preferred, and where vibration reduction is required (e.g. base isolated structures and machine foundations).
Tensile behaviour and moment–curvature relationship of steel fibre reinforced concrete
Authors: Lok T. S. , Xiao J. R.
Published: October 2009

Conventional concrete reinforced with discrete steel fibres exhibits some unusual behaviour which is not displayed by plain concrete when it is subjected to flexural loading. In this paper, a modified constitutive model for defining the tensile characteristics of the fibrous composite is proposed. Parameters that influence the tensile behaviour of steel fibre reinforced (SFR) concrete are examined. Composite material properties, such as the compressive and tensile strengths (f′c and ft), fibre volume concentration (Vf), fibre aspect ratio (L/d) and fibre–concrete matrix bond stress (τd), affect the numerical coefficients in the model. Depending on the relative magnitudes of these parameters, the behaviour may exhibit a post-cracking softening or strengthening characteristic. Based on the proposed constitutive model, the full flexural moment–curvature response of SFR concrete is derived, and the relationship between tensile and flexural behaviour developed. The calculated flexural response is verified by comparison with published experimental results. Good agreement is obtained for a number of concrete specimens reinforced with different types of steel fibre and different fibre concentrations.
Application of new ultrasound and sound generation methods for testing concrete structures
Authors: Popovics J. S. , Achenbach J. D. , Song Won-Joon
Published: October 2009

Transient stress waves offer a powerful approach for non-destructive condition evaluation of concrete structures. However, currently used stress-wave-generating techniques are limited by poor controllability and/or penetrating ability. Stress wave generation must therefore be improved so that the application of existing and new ultrasonic and sonic non-destructive testing techniques to concrete structures will be more effective. This paper first summarizes the existing, traditional stress-wave-generating techniques for testing concrete structures. A method by which electromagnetic and piezoelectric-based sources are driven by AM burst signals is then introduced, fully detailed and shown to be appropriate for concrete tests in place of traditional sources. The performance of these sources is demonstrated through the results of impact-echo tests and spectral analysis of surface waves performed on a Portland cement concrete structure.
A study of dowel action in reinforced concrete beams
Authors: Jeli I. , Pavlović M. N. , Kotsovos M. D.
Published: October 2009

This paper reopens the question concerning the contribution of dowel action to the shear capacity of reinforced concrete beams. First, a literature survey on the subject reveals that, despite the implicit belief in much of the current design thinking that dowel action can play a significant role in the mechanism through which shear is carried in a beam, opinion on the matter is divided and often contradictory. Secondly, a programme of beam tests in which the total amount of main steel remains constant but the number of bars (and hence their diameter) is varied is reported, the results showing that the bar diameter has no effect on the load-carrying capacity of the members, suggesting, therefore, that dowel action is unlikely to be a significant factor in the internal load-transfer mechanism leading to final collapse. Faunally, these experimental findings are confirmed by an established finite-element package which faithfully reproduced the experimental results, on the basis that the numerical model makes no allowances for the so-called dowel-action effect.
Transfer bond stresses generated between FRP tendons and concrete
Authors: Lees J. M. , Burgoyne C. J.
Published: October 2009

There is a growing interest in the use of new materials in construction. Materials such as fibre-reinforced plastics (FRPs) are lightweight, strong, and, for the most part, non-corrodible. Hence, the use of FRPs as prestressing tendons for concrete represents a viable, durable alternative to conventional practice using prestressing steel. This paper addresses the nature of the bond between an FRP tendon and concrete in the transfer zone of a pretensions concrete beam. In particular, two different types of aramid fibre-reinforced plastics (AFRPs) are considers. In the experimental investigation, the tendon pull-in behaviour during detensioning of a number of small-scale (100 X 200 X 2800 mm) pretensions concrete beams was monitored. These results were then used to make inferences about the bond stress distribution through the transfer zone. Both a constant and a non-linear relationship between the bond shear stress and the tendon slip were considers and the transfer behaviour of the two types of AFRP and steel were compared. It was found that by assuming a non-uniform bond stress distribution some interesting results about the distinctly different natures of the bond behaviour of the three materials were obtained.
Plastic shrinkage cracking of blended cement concretes in hot environments
Authors: Almussalam A. A. , Maslehuddin M. , Abdul-Waris M. , Dakhil F. H. , Al-Amoudi O. S. B.
Published: October 2009

This paper reports results of a study conducted to evaluate plastic shrinkage cracking of concrete made with varying dosages of several pozzolanic materials, such as fly ash (20, 30 and 40%), silica fume (5, 10 and 15%) and blastfurnace slag (50, 60 and 70%). These concrete specimens were exposed to hot—humid and hot—dry environments. The effect of these environmental conditions on plastic shrinkage cracking was evaluated. The rate of water evaporation in the blended cement concrete specimens was noted to be more than that in the plain cement concrete specimens. Further, bleeding in the blended cement concrete specimens was less than that in plain cement concrete specimens. The cumulative effect of these two parameters resulted in increased plastic shrinkage cracking of the blended cement concretes. Although cracks were observed earlier in the plain cement concrete specimens than in the blended cement concrete specimens, the total area of cracks in the latter cements was more than that in the former cements. The results of this study also indicate that relative humidity influences plastic shrinkage cracking of concrete significantly in comparison with the effect of the type of cement.
Influence of ground granulated blastfurnace slag on the sulphate resistance of lime-stabilized kaolinite
Authors: Wild S. , Tasong W. A.
Published: October 2009

This paper describes the influence of ground granulated blastfurnace slag (GGBS) on the durability of lime-stabilized kaolinite in 4·2% Na2SO4 solution. Sulphate durability was assessed in the laboratory by means of linear expansions measurements, microstructure and mineral phase analysis of compacted cylinders of the lime-stabilized kaolinite with different percentages of lime replacement by GGBS. The cylinders were immersed in 4·2% Na2SO4 solution and also in deionized water for various time periods. The exposure test data showed that the effect of GGBS on reducing expansion in 4·2% Na2SO4 solution is dramatic. The results also showed that high percentages of replacement of lime with slag (in particular 83%), with just enough lime to activate the slag, were the most effective in preventing sulphate solution attack.
A geotechnical description of fresh cement grout—filtration and consolidation behaviour
Authors: McKinley J. D. , Bolton M. D.
Published: October 2009

The removal of water from three Portland cement grouts by pressure filtration is examined, and the consolidation behaviour of the filtered material clarified. The filtration takes place by the laying down of a very stiffs filter cake through the removal of excess water. The behaviour due to further loading resembles that of a reconstituted silt. For stress levels above the filtration pressure the calculated permeability values are similar to those from the filtration phase only if the data sampling rate is sufficiently rapid to discriminate the first portion of the observed primary consolidation curve. The change in void ratio for incremental loading is roughly linear with the change in the logarithm of the vertical effective stress. The characterization of fresh cement paste using standard soil mechanics models is both appropriate and useful, at least during the first few hours after mixing.
Influence of curing on durability performance of concrete
Authors: El-Sakhawy N. R. , El-Dien H. S. , Ahmed M. E. , Bendary K. A.
Published: October 2009

Durable concrete experiences a long service life in most natural environments without deterioration. One of the important factors that influence the durability of concrete is curing. This paper represents a comprehensive investigation to study the effect of method and duration of curing on concrete durability. The techniques of curing used in this study encompass the majority of methods commonly encountered on site. The methods of curing were freshwater, wet burlap, plastic film, liquid membrane and air (no curing). The study considered different durations of curing of 3, 7, 14, 21 and 28 days. Concrete specimens were exposed to various conditions of deterioration: sulphate attack, cycles of wetting and drying, temperature changes, high temperature, fire and corrosion of reinforcing steel. Compression, tensile strength, weight and corrosion of steel were then monitored. Each of these parameters is utilized to evaluate the durability performance. Durability performance is defined as the ratio between a monitored parameter, under one of the aforementioned conditions of curing and deterioration factors, and the counterpart parameter of a standard reference specimen. A corresponding standard reference specimen was cured for 28 days in freshwater and subjected to the same deterioration condition. The study demonstrates that the method and the duration of curing heavily affect the durability performance. For the same method of curing the increase in the duration of curing increases the durability of concrete. The effect of the various curing methods on the increase of the durability of concrete for the same duration can be put into the following hierarchy: air (no curing), liquid membrane, plastic film, wet burlap and finally freshwater. Due to the tremendous impact of curing on the durability of concrete, quality control for proper field curing is of the utmost importance.
Effect of different boundary conditions in direct tensile tests: experimental results
Authors: Cattaneo S. , Rosati G.
Published: October 2009

The effects of the kinematic ‘constrained’ of a direct tensile test on the complete load—displacement response in tension of concrete specimens were examined from tests with imposed uniform displacement (fixed platens) and from experiments with free rotation rotating platens through a closed-loop testing system. Cylindrical specimens and prismatic specimens were considered. The tests were monitored with an interferometric strain measurement (electronic speckle pattern interferometry). A notable outcome of the experiments is that the fixed platens imposed a more uniform strain field with higher peak loads. Also the post peak branch had a shape that depends on the boundary conditions under which the tests were performerd.
Combined effects of silica fume, superplasticizer and temperature on setting behaviour
Authors: Pinto R. C. A. , Hover K. C.
Published: October 2009

Setting time of mortar incorporating both silica fume and superplasticizer in varying amounts is observed to be a function of temperature and composition. Temperature effects are shown to be predictable using conventional maturity techniques. Reductions in cement content and increases in superplasticizer dosage tend to retard setting, while increases in silica fume content tend to accelerate setting. Adjustment factors are developed which enable prediction of initial set time as a function of composition. Practical applications are suggested.
Pressure required to cause cover cracking of concrete due to reinforcement corrosion
Authors: Williamson S. J. , Clark L. A.
Published: October 2009

Tests were carried out to investigate the magnitude of pressure created by corrosion products on steel in reinforced concrete that would cause surface cracking of the concrete cover. Test specimens were 150 mm cubes with 8 mm or 16 mm holes provided at various locations and formed by reinforcing bars. The cracking resistance of cover concrete was measured experimentally by pressurizing the holes, and an attempt was made at its prediction using plastic methods of analysis. Experimental failure pressures were found to be influenced by cover/bar diameter ratio, aggregate size, bar diameter, and bar location. The observed lack of correlation between tensile strength and failure pressure lends support to the suggestion made by other researchers that a fracture mechanics approach may be more appropriate. This suggestion was supported by an unsuccessful attempt to find a relationship between cover thickness and effective plastic strength.
Relationship between coulomb and migration coefficient of chloride ions for concrete in a steady-state chloride migration test
Authors: Sugiyama T. , Tsuji Y. , Bremner T. W.
Published: October 2009

A steady-state chloride migration test with a constant electrical potential drop of 15 V was conducted with a saturated 100 mm diameter × 50 mm long concrete specimen. The migration coefficient of chloride ions for concrete was calculated using the constant flux on the basis of the Nernst–Planck equation. Electrical current was simultaneously monitored throughout the entire period of the migration test. Initial and final coulomb values were separately defined and calculated using the current recorded in non-steady-state and subsequently in steady-state conditions with respect to chloride migration, respectively. A unique relationship existed between the initial coulomb and the migration coefficient depending on the binders of the concrete mixes, that is ordinary Portland cement (OPC) alone, OPC plus fly ash or OPC plus silica fume. However, it appeared that the final coulomb related linearly to the migration coefficient, irrespective of the water:cement ratios, binders and curing periods.
Estimation of stress intensity factor (KI) for an FRP bonded concrete beam using the superposition method
Authors: Lau K.-T. , Shi S.-Q. , Zhou L.-M.
Published: October 2009

The deterioration of civil concrete infrastructures has increased dramatically in recent years. The majority of recent studies have focused on the use of fibre-reinforced plastic (FRP) materials to strengthen or reinforce decayed concrete structures by bonding the FRP plate externally onto the damage surface. These studies have concentrated on the shear and peel-off effects of the adhesive material rather than the failure mechanisms of the concrete itself. However, the stress intensity factor, KI, at the crack-tip of the strengthened cracked concrete structures, cannot be overlooked. Numerical and experimental investigations have prove that KI of the strengthened structure is highly influenced by the modulus and geometry of the reinforced plate. Therefore, the failure of the strengthened specimen may be due to a high KI value in the concrete material. In this paper, a theoretical model is proposed to estimate the value of KI with different reinforced plate properties for a strengthened concrete beam using a simple superposition method. The results from the proposed method compare well with the numerical solutions obtained from the finite-element method. Both results show that the steosintensity value, of the beam decreases as the modulus and thickness of the reinforced plate increase, while the closing force of the beam, induced by the reinforced plate, increases.
Behaviour of high-strength concrete plates under impact loading
Authors: Murtiadi S. , Marzouk H.
Published: October 2009

This research programme was carried out to investigate the behaviour of high-strength concrete plates subjected to impact loading. The research includes an experimental investigation on 16 concrete plates. A rigid projectile was used to apply the impact load to the tested reinforced concrete specimens. The rigid projectile consisted mainly of a solid steel cylinder with 220 kg mass and 304·5 mm dia. contact area, dropped at variable heights of up to 4 m. The structural behaviour with respect to deflection, concrete and steel strains, failure mode and energy absorption was examined. The effects of dynamic loading, concrete strength, reinforcement ratio and support pattern were the test parameters. Based on the experimental test results, it has been found that at a test velocity of 4−7 m/s the impact load is about twice the static punching shear capacity estimated from code provisions.