bluuebunny/crossref_metadata_2025 huggingface dataset:
The permeability of concrete in a marine environment
Authors: Buenfeld N. R. , Newman J. B.
Published: October 2009

Synopsis

Permeability is a fundamental property governing the durability of concrete in the marine environment and it has only recently been appreciated that some concretes exhibit a very significant reduction in permeability on immersion in sea-water. To investigate this phenomenon further, a resistivity technique has been developed and used to monitor the changes in permeability of five mortars and one concrete on exposure to sea-water. All mixes studied showed a fall in permeability (increase in resistance) on immersion in sea- water. Indeed 25 mm thick specimens of the two most permeable mortars showed a fall equivalent to over 20 mm of additional thickness after only 10 weeks. This increase is due to the formation of an aragonite-brucite layer on the surface of the mortar plus a more widespread progressive constriction of the cement paste pore system. This effect also has a considerable influence upon the absorption capacity of concrete after a period of drying, which is important in the tidal and splash zones.
Superplasticized flowing concrete: durability properties
Authors: Dhir Ravindra K. , Yap Andrew W. F.
Published: October 2009

Synopsis

This second paper reporting the investigation undertaken into the properties of superplasticized highworkability concrete describes the freezing-and-thawing and wetting-and-drying durability of strictly flowing concrete. It is shown that flowing concrete with a cement content equal to or greater than the minimum specified for a structural concrete under these exposures is more susceptible to deterioration than normal concrete having the same mix composition and hence the same strength. This was much more apparent in the case of freezing and thawing where, to attain a comparable response, flowing concrete would require an additional cement content of the order of 100 kglm3 or air-entrainment, the latter being found to be a better remedy.
Permeability of normal and lightweight mortars
Authors: Nyame B. K.
Published: October 2009

Synopsis

Permeabilities of normal and lightweight mortars have been determined and related to pore structure from mercury porosimeter analyses. It was found that lightweight mortar is only about twice as permeable as sand mortar of equal aggregate volume concentration. Permeability of mortar increases as porosity is reduced by the addition of aggregates of lower porosity. In principle, the decision to use dry aggregates is safer for the durability control of concrete mortars.
Measurement of internal concrete strains using embedment strain gauges
Authors: Scott R. H. , Gill P. A. T.
Published: October 2009

Synopsis

The use of embedment strain gauges—electric resistance strain gauges encapsulated in acrylic resin—for the measurement of internal concrete strain distributions infive tension specimens is described. Results are presented to illustrate the performance of the technique.
Shear displacement in open-sandwich reinforced concrete beams
Authors: Wong Carlos C. K. , Vardy Alan E. , Lark Robert J.
Published: October 2009

Synopsis

The deflections of open-sandwich, reinforced concrete beams under load are analysed to verifv that shear deformation of the adhesive layer between the concrete beam and its steel reinforcing plate does not cause an unacceptable loss of structural interaction. The cumulative effect of the shear deformation, i.e. the ‘shear displacement’, is loosely analogous to the slip of steel bars in a conventionally reinforced concrete beam.

The partial interaction theory introduced by Newmark et al and extended by Goodman and Popov and others is further extended to include thermal expansions, cracked concrete, and unsymmetrical loading. The accuracy of the resulting analysis is assessed by comparisons with finite element predictions and with experimental results.

It is found that thin layers of adhesives with a large shear modulus are preferable from an in-service structural point of view. They lead to small shear displacements and to good control of the influence of flexural cracks.
A water absorption test for concrete
Authors: Kelham S.
Published: October 2009

Synopsis

The durability of concrete near an exposed surface is largely determined by the rate at which harmful agents can penetrate into the concrete. This paper presents a simple analysis of the penetration of' water into unsaturated concrete and describes a method by which the material constant that determines the rate of penetration, the sportively, can be measured, together with the effective porosity. The sorptivity is found to depend on the permeability and porosity of the concrete and the strength of capillary forces. The experimental results conform well to the predictions of the analysis and significant variations in sorptivity have been observed for concrete containing cement replacement materials and subject to different curing regimes.
Bond between steel strand and cement grout in ground anchorages
Authors: Laldji S. , Young A. G.
Published: October 2009

Synopsis

Bond action between prestressing strand and cement grout is examined both theoretically and experimentally. Variations in strand type, embedment length, grout strength and lateral compression are investigated using a modified pull-out specimen and a simple test rig. The results support the recommended value of ultimate bond stress of 2 N/mm2proposed in the British Standard Draft for Development DD81: 1982 but show that this value may be trebled by the introduction of biaxial lateral pressure. A theoretical expression is derived, based on a frictional force mechanism, which predicts the bond strength of normal and Dyform strand. The bond characteristics of indented strand are found to be similar to those of deformed bars. A wedge action, which is attributed to “lack ofjit” of strand within the grout channel, is shown to play an important role in the establishment of a stable load/slip bond characteristic.
Blastfurnace slag as coarse aggregate for concrete stored in sulphate solutions for 22 years
Authors: Gutt W. H. , Russell A. D.
Published: October 2009

Concretes made with six blasrfurnace slags as coarse aggregate, with ordinary Portlandcement or sulphate resisting Portland cement, have been tested for strength after storage in aggressive sulphate solutions for 22 years. The results show that concretes made with blastfurnace slag aggregate which complies with BS 1047 performed as well as equivalent gravel aggregate concretes in aggressive sulphate environments.
Long-term performance of mild steel bars embedded in concrete containing dense blastfurnace slag aggregate
Authors: Davies H.
Published: October 2009

Concrete prisms made with chloride-free concrete containing air-cooled blastfurnace slag aggregates have been destructively examined after 24 years’ exposure in an industrial environment. The reinforcement was almost entirely free of corrosion at all depths of cover. The concrete had carbonated to a depth of 2–3 mm. Concrete made with such slag aggregate is considered to provide deflective long-term protection for steel reinforcement.
Acoustic emission from reinforced concrete beams
Authors: Lim M. K. , Koo T. K.
Published: October 2009

The acoustic emission behaviour of reinforced concrete beams under flexure load was studied. Useful correlation was established between the acoustic emission activity under loading state and crack growth in the reinforced concrete beams. Three different acoustic emission rate regions were observed during the flexural loading. Crack initiation and crack propagation were detected monitoring the acoustice mission. The zone ef crack initiation and development can be identified from the acoustic emission event-location plot, showing major acoustic emission concentration ureas. Repetitive loading showed that the Kaiser effect was valid only up to 80% of the ultimate load level, with norelaxation phenomenon , for test intervals less than 2h. Crack growth acoustic emission burst event signals were found to be of higher amplitude compared with those due to deformation. The accumulated number of burst events of these high-amplitude emissions, monitored by the use of a high threshold, showed good correlation with the total cracked surface.
Fracture mechanism and fracture parameters of autoclaved lime–sand mortar and concrete
Authors: Lixian Wu , Xincheng Pu
Published: October 2009

The fracture process of autoclaved lime–sand mortar and concrete was investigated by means of acoustic emission technology. The results have shown that the fracture process of this mortar and concrete can be divided into three stages: microcracks widen, micro–cracks propagate, and the main crack propagates. A new parameter specific fracture energy Gsf is introduced to describe the brittleness of concrete. K1c Gf and the notch sensitivity of autoclaved lime—sand mortar and concrete were determined by a three–point bending test. The results have shown that the autoclaved lime—sand mortar has notch sensitivity, and concrete with super–fine sand and crushed stone as aggregate has no sensitivity.
On the long–term strength losses of silica–fume high–strength concretes
Authors: de Larrard F. , Bostvironnois J.-L.
Published: October 2009

This Paper presents studies and tests performed at the Laboratoire Central des Ponts et Chaussées with a view to a better understanding of the long–term mechanical properties of silica–fume high–strength concretes (HSC) and very–high–strength (VHSC) concretes. After noting the discrepancies among the various references in the literature, we propose to explain the drop in strength observed by some authors after three months by the hypothesis of dryig–related effects. To check the soundness of this hypothesis, mechanical tests were carried out on three different concretes, their drying was investigated by gammadensime try, and accelerated tests were performed on mortars. The homogeneous drying to which the specimens are subjected has no negative effect on their mechanical strengths. The losses of strength observed on some concretes are therefore probably not caused by a local effect of the drying. Examination of the water content fields of VHSC specimens four years old shows that the drying occurs with a high gradient and affects only a small thickness. Mechanical tests in this case show a non–negligible drop in compressive strength. Calculation of the self–stress field due to drying shrinkage indicates that the compression at the core of the specimen is of the same order as the drop in strength observed. To conclude, it is shown that, given some hypotheses concerning the water content field, the drop in strength linked to the structural effects of drying can be not more than twice the tensile strength.
Possible new method for the assessment of fire–damaged concrete
Authors: Riley M.A.
Published: October 2009

Five cylindrical flint aggregate concrete specimens from existing structures were subjected to temperatures in excess of 800°C for two hours. The use of speccally designed laboratory equipment allowed the determination of the final stabilized temperature profile in each specimen. After exposure, thin sections were taken longitudinally at the diameters of the specimens and examined petrographically. The method of thin section preparation allowed examination of the full range of concrete distress from the outer exposed surface inwards. Certain petrographic features were associated with particular zones of the final temperature profile which may be of use in establishing temperatures reached in concrete subjected to heat in uncontrolled conditions. A prime objective was the identfication of features that may be used to locate the structurally significant 300°C isotherm within damaged specimens.
The effects of permeable formwork on the surface properties of concrete
Authors: Price W. F. , Widdows S. J.
Published: October 2009

The surface properties of concretes cast in both conventional (impermeable) formwork and in permeable formwork have been compared. The effect of subsequent curing after stripping the formwork has also been examined. Three grades of concrete (C20, C30 and C50) have been tested. The laboratory test programme has demonstrated that the use of permeable formwork reduces surface absorption, water permeability and chloride diffusion, while increasing surface hardness. The incidence of blowholes on the formed surfaces is greatly reduced even for high–strength concrete. Results from both laboratory and site trials show a reduced carbonation depth for concrete cast in permeable formwork. A reduction in the near surface water/cement ratio can also be demonstrated. The overall conclusion drawn from this work is that the surface properties of concrete can be considerably improved by the use of permeable formwork. A similar performance may be expected from a grade C30 concrete cast in permeable formwork relative to a grade C50 concrete cast in conventional formwork.
Durability design and repair of concrete structures: chloride corrosion of reinforcing steel and alkali–aggregate reaction
Authors: Miyagawa T.
Published: October 2009

Problems related to the fundamentals of durability design and repair design of concrete structures are discussed with reference to damage caused by chloride corrosion and alkali–aggregate reaction. A unified approach is presented, based on the mechanism of deterioration, which is believed to offer potential for further practical development.
Characteristics of corrosion inhibition admixtures in OPC paste with chloride additions: Part I: Chemistry and electrochemistry
Authors: Jin S. X. , Sagoe–Crentsil K. K. , Glasser F. P.
Published: October 2009

The chemical modfications to OPC paste and pore fluid compositions produced by 1–5% nitrite, chromate and phosphate–ased admixes have been studied. Admixture partitioning and modes of corrosion inhibition have been determined. Constitutional differences between the solid paste with and without chloride additions were analysed, as well as pH, admixture anion and chloride ion concentrations of the extracted aqueous phase. Electrochemical rest potential measurements and linear polarization methods were employed to assess inhibitor efficiency, using mild steel electrodes embedded in cement paste cylinders in accelerated experiments at 50°C and 100% humidity. Admixture anion partitioning between the solid paste and pore fluid phase is variable. The CrO4 ion concentration in the pore fluids remains low; under 10% of the total is soluble, as compared to about 30% for nitrite and phosphate ions. Free CI ion levels in the pore fluid with and without admixture are found to range between 55 and 75% of the total after 75 days cure but the [Cl]/[OH] and [Cl]/[admix] ratios tend to be relatively constant after 28 days. Corrosion rate determinations indicate that phosphate and nitrite–based admixtures become effective as inhibitors during setting, but chromate begins to be effective only after about 28 days. This delayed action of the chromate ions may be linked to the initially high [Cl]/CrO4] ratio, which was about seven. he apparent differences in modes of inhibition result from the extent to which admixture partitioning modifies the chemical and electrochemical environment surrounding the steel reinforcement.
Factors influencing pull-off tests on concrete
Authors: Bungey J. H. , Madandoust R.
Published: October 2009

The pull-off technique has grown in popularity for testing the surface zone strength of concrete, and when used in conjunction with partial coring is particularly suitable for the bond testing of repairs. Theoretical and laboratory experimental studies have been undertaken to examine a number of important practical factors which influence the results obtained during surface and partially cored pull-off tests. Finite-element methods were used to analyse failure zone stress distributions. Disc material and proportions were shown to be particularly significant in both types of test, and in some circumstances the results are also affected by the elastic modulus of the concrete. These factors may influence correlations with compressive strength, demonstrating the importance of standardization of disc characteristics to permit direct comparisons of results. This will also be an important consideration when comparing measured test results with specified values. A very wide range of loading rates is provided by currently available test equipment, and this feature merits further study, together with other factors relating to partially cored tests as applied to repairs.
Differential equation of concrete behaviour under uniaxial short-term compression in terms of atrophy (degeneration) and its solution
Authors: Blechman I.
Published: October 2009

The Paper deals with the problem of the differential equation of concrete behaviour under load in the non linear stage of the stress-strain curve (SSC). The approach is based on following principles: the nonlinear behaviour of concrete is a consequence of the atrophying process of microcracking in it, because of its heterogeneity; the intrinsic elastic modulus ought to be taken constant up to peak stress; the specific stresses remain linearly related to the strains, also up to peak stress. The nonlinear behaviour of plain concrete both up to and beyond the strength limit is described by differential equations, which relate the response of the concrete to the strains in terms of atrophy (degradation) and the specific stresses in it, and represent SSCs for material affected by microcracking but without initial cracks comparable in size to the specimen dimensions. The obtained primary equation represents the total differential of concrete survival as a function of the strains in it up to the peak stress, and its solution is independent of the pattern of the survival function. For the descending branch the primary equation has to be supplemented by the confinement factor and by a function of macrodestruction. Solving this equation, a descriptive model for the nonlinear domain of the ascending branch of the SSC, called the central function, was obtained, using a gaussian survival function. It gives the relationship between the nominal stresses and the strains and atrophy of concrete based on the main parameters: intrinsic elastic modulus E, atrophy threshold εa, peak strain and the scattering factor d. A redefinition of strength in terms of the limiting atrophy and the strength formula is given, based on the balance between the energy added to the live part of the cross-section and the energy loss due to the increment of concrete atrophy. It is shown that the peak point strain εp not the primary parameter but is a function of εa, and d. The total similarity of the SSC for concrete and rock can be interpreted as proof of the validity of the proposed differential equations for brittle heterogeneous solids.
Residual compressive and bond strengths of limestone aggregate concrete subjected to elevated temperatures
Authors: Ahmed A. E. , Al-Shaikh A. H. , Arafat T. I.
Published: October 2009

Concrete is exposed to elevated temperatures when subjected to accidental fires in buildings, or when is close to furnaces and reactors, as encountered in some industrial applications. Research studies indicate that a serious reduction in concrete strength may result in such cases, and that the type of aggregate used is an important parameter. In thus study, the influence of high temperatures (100–600°C) on the residual compressive and bond strengths of concrete made from limestone aggregates is experimentally investigated. The main test parameters involved were the maximum temperature, the time of exposure at the maximum temperature, the method of cooling, the age of concrete at the testing date, and the cement content. The results showed that exposure of limestone aggregate concrete to such temperatures may result in a noticeable reduction in its strength, especially in the range 400–600°C.
Effects of urea on durability of reinforced concrete
Authors: Sadegzadeh M. , Page C. L. , Vassie P. R. W.
Published: October 2009

This Paper describes a laboratory investigation of the influence of urea on the mechanical properties of concrete and the corrosion of the reinforcing steel. The mechanical properties of concrete cubes, exposed to weekly cycles of drying and submersion in a variety of urea or salt solutions, were investigated by non-destructive and destructive monitoring at regular intervals. The microstructure of concrete was analysed by means of mercury intrusion porosimetry. The resistance to freezing and thawing of concrete prisms was studied by recording weight loss, dynamic modulus of elasticity and ultrasonic pulse velocity. The effects of urea on reinforcement corrosion were investigated by monitoring changes in the corrosion potentials and corrosion rates of steel electrodes, embedded at four depths of cover in concrete slab specimens exposed to various methods of treatment with urea or salt solutions.
Influence of cement type and curing on the drying and air permeability of cover concrete
Authors: Parrott L. J.
Published: October 2009

Experimental results are presented showing the influence of cement type and curing on the relative humidity, initial weight loss and air permeation in cover concrete that has been exposed to drying at 60% relative humidity for at least 18 months. Three curing periods (1,3 and 28 days) and 17 cements were used, the cements differing mainly with respect to their contents of limestone or ground granulated blastfurnace slag (ggbs). The concretes were all made with a water-cement ratio of 0·59 and the 28 day cube strengths ranged from 26 to 46 MPa. The relative humidity at a mean depth of 25 mm below the exposed sudace reduced to about 80% and 70% after 6 and 18 months of drying respectively. Cement type had little on the reduction of relative humidity with drying time and there was only a marginal retardation of drying rate with increased curing time. The initial four day weight loss, an indicator of moisture transport properties in cover concrete, reduced markedly as the curing period was increased. Increasing the ggbs content of the cement increased the initial four day weight loss with one day curing but reduced it with 28 day curing. The initial four day weight loss generally reduced as the strength at the end of the curing period was increased. The air permeability of cover concrete generally increased with drying time, even beyond 18 months; this effect was more pronounced with concretes that had lower pemzeabilities and appeared to be due to slow drying. On average the air permeability of cover concrete increased significantly with increasing drying time and decreasing curing period, but changes of greater magnitude could arise due to cement type. The compressive strength after curing, the initial four day weight loss and the 18 month air permeability were only broadly related. Concretes with 50% or more of ggbs in the cement tended to have a high air permeability for a given strength eight days after the end of curing or for a given initial four day weight loss.
Influence of constituents and composition on autogenous shrinkage of cementitious materials
Authors: Tazawa E. , Miyazawa S.
Published: October 2009

Effects of cement type, water to binder ratio and volume concentration of aggregate on autogenous shrinkage of cement based materials were experimentally investigated. A large amount of autogenous shrinkage was observed in cement paste and concrete with low water to binder ratio. Autogenous shrinkage increased with increasing C3A content of cement. It was also increased by using blast furnace slag with higher fineness. The effect of volunre concentration of aggregate could be estimated by an existing composite law. Some vpes of admixtures were effective in reducing autogenous shrinkage.
Temperature effect on the pore solution chemistry in contaminated cements
Authors: Maslehuddin M. , Paget C. L. , Rasheeduzzafar
Published: October 2009

The combined effect of chloride and sulphate contamination and temperature on the pore solution chemistry, particularly the OH, Cl and SO42 concentration, in plain and blended cements was evaluated. The cement mortar specimens were contaminated with sodium chloride and sodium chloride plus sodium sulphate and exposed to temperatures in the range 25 to 70°C. Results indicate that the chloride binding capacity of cement is affected by both the temperature and the sulphate contamination. In the chloride contaminated specimens, the chloride binding was influenced by the exposure temperature, while in those contaminated with sodium chloride plus sodium sulphate, it was affected by the presence of sulphate ions. The sulphare concentration in the plain and blended cements was also influenced by the exposure temperature and chloride contamination. The Cl/OH values increased with temperature in both plain and blended cements, indicating the vulnerability to accelerated steel depassivation in concyete exposed to elevated temperature and contaminated with chloride and sulphute ions.
Research on real concrete structures—risks and benefits
Authors: Taylor H. P. J.
Published: October 2009

An apple fell on Isaac Newton’s head. From this ordinary event a great mind fashioned a set of simple laws that have been at the foundations of science and engineering ever since.
Developments of the two-point workability test for high-performance concrete
Authors: Domone P. L. J. , Yongmo Xu , Banfill P. F. G.
Published: October 2009

An improved version of the Tattersall two-point workability apparatus has been developed for the study of the rheology of high-performance concrete mixes. The improvements include a more compact arrangement of motor, impeller and bowl and the computer logging of the speed/torque data so that the flow curve can be obtained in a continuous sweep of about 30 s. Two impeller systems were used—an axial interrupted helix and an H-shaped impeller driven in a planetary motion with an offset gear, that is corresponding to the MH and LM systems on the previous apparatus. Both impeller systems were calibrated to enable the Bingham parameters of yield stress and plastic viscosity to be expressed in fundamental units. The performance of the apparatus was assessed by tests on a range of moderately high-performance mixes incorporating superplasticizers and cement replacement materials. The offset H impeller system was found to be suitable for testing concrete with slumps ranging from about 50 mm up to collapse, and for assessing the development of rheology with time after mixing, and is therefore preferred for high-performance concrete.
Characterization of conditioned pulverized fuel ash for use as a cement component in concrete
Authors: McCarthy M. J. , Tittle P. A. J. , Dhir R. K.
Published: October 2009

This paper considers the influence of moisture addition (conditioning) to PFA of up to 40% by mass on its physical and chemical properties and performance as a cement component in concrete. The work examined conditioned PFA, covering a range of fineness levels and lime contents, stored for up to 18 months, either in the laboratory or in stockpiles established at four UK power stations. Data for laboratory-conditioned PFA indicated progressive particle agglomeration (mainly in the finer fractions) with storage period, which was greatest at moisture levels of 10–20% and for material with total and free lime contents in excess of 3·0% and 0·1%, respectively. Corresponding increases in water demand and reductions in strength factor were also noted with conditioned material. Other changes, including increased LOI, reduced free lime content and formation of hydration products such as bassanite and gypsum, indicated that the effects were due to chemical activity. However, no significant changes to bulk oxide, modal composition and glass contents of PFA were obtained, suggesting that conditioning effects are mainly localized at particle surfaces. In general, similar behaviour to that in the laboratory was noted for PFA stored in power station stockpiles. The workability and strength development to 28 days of conditioned PFA concrete were comparable to that containing dry PFA, with only slight reductions in both properties with long-term conditioned PFA storage.
Workability of concrete incorporating pulverized fuel ash and metakaolin
Authors: Bai J. , Wild S. , Sabir B. B. , Kinuthia J. M.
Published: October 2009

Workability of concrete incorporating combinations of pulverized fuel ash (PFA) and metakaolin (MK) as partial replacements for Portland cement (PC) has been investigated. The aim of the research work is to explore the potential of using PFA and MK as blends with PC in terms of the flow properties of the resulting concrete. Ninety mixtures containing 0, 10, 20, 30 and 40% total replacement of cement with combinations of metakaolin (0–15%) and PFA (0–40%) for concretes with water to binder ratios of 0·4, 0·5 and 0·6 were prepared. The workability of the concrete was measured by the slump, compacting factor and Vebe time tests. Substantial reductions in the workability were obtained when the mixtures contained MK only, with greater reductions being experienced as the MK replacement level increased. In contrast, the mixtures with PFA showed considerable improvement in workability with increase in replacement level. It was found that, for concretes containing blends of PC, PFA and MK, the loss in workability due to the presence of MK can be compensated for by the incorporation of PFA. However, at low cement replacement levels, improvement in workability by the PFA can be achieved only if a superplasticizer is used. The degree of restoration of workability provided by PFA is influenced by the level of cement replacement, MK/PFA ratio and water to binder (w/b) ratio. In particular at low w/b ratio (= 0·4), where superplasticizer is used, there is a critical MKIPFA ratio (∼0·4) above which workability decreases and below which workability increases with increasing replacement level. In general, the incorporation of PFA not only compensates for the loss of workability resulting from the presence of MK but also provides economic benefits.
The effect of alkali—silica reaction on the fatigue behaviour of plain concrete tested in compression, indirect tension and flexure
Authors: Ahmed T. M. A. , Burley E. , Rigden S. R.
Published: October 2009

The fatigue strength of concrete is an important factor in the design of many structures. However, the behaviour of concrete under indirect tensile repeated load has never been clarified to date by researchers. Therefore one of the objectives of this investigation was to study the effect of alkali—silica reaction (ASR) on the fatigue behaviour of concrete tested in flexure as well as under repeated indirect tensile load. The other objective of this investigation was to study the effect of ASR on the fatigue behaviour of plain concrete tested in compression. The latter is of particular importance to the structural engineer. Its importance lies in that it is a quality control test as well as a basic physical property for use in theoretical analysis of concrete structures. The lack of understanding of the basic nature of the fatigue behaviour of unreinforced alkali—silica–reacted concrete has led to this part of the investigation, which aims to relate the effect of the reaction not only to the performance of small specimens which are used to obtain basic design information but also to the lifetime of complete structures. The data obtained in relation to the fatigue performance of ASR–affected concrete could allow engineers to maintain and repair such structures with greater confidence.
Strut and tie models for analysis/design of external beam—column joints
Authors: Vollum R. L. , Newman J. B.
Published: October 2009

Strut and tie models have been developed for external beam–column joints with and without joint stirrups. The modelling is fraught with difficulties that include determining forces at joint boundaries and strut dimensions. In view of these difficulties, it was found necessary to define strut widths empirically. Test data were used to show that stirrups can increase joint shear strength by less than their yield capacity. The model accounts for this by using a stiffness analysis to determine the proportion of joint shear force resisted by the stirrups at failure. The resulting model predicts joint shear strength more realistically than existing non–finite–element methods and some finite–element techniques. It is necessarily complex but capable of incorporation in spreadsheet–based design techniques. The authors believe that the behaviour of beam—column joints is too complex to be modelled realistically with simple strut and tie models. If a simple design method is required, the authors recommend their simplified empirical method.
Air permeability versus sorptivity: effects of field curing on cover concrete after one year of field exposure
Authors: Austin S. A. , Al-Kindy A. A.
Published: October 2009

There is substantial interest in the air permeability and sorptivity tests as potential indicators of concrete quality. This paper reports on part of a research programme that investigated the effects of field exposure and curing on the penetrability properties of the cover region of concrete. The air permeability and sorptivity profiles indicated that curing generally affected the penetrability properties of concrete to a depth of 20 mm from the exposed surface; however, the curing-affected zone varied widely depending on concrete type and strength as well as exposure conditions. Curing with wet hessian improved the penetrability properties of the cover region of the concrete. The air permeability test was more sensitive to variations in curing than the sorptivity; however, the difference between the two measured parameters was insignificant for concretes characterized by coarser pore systems (young and poorer-quality concrete). Slag concrete had higher sensitivity to inadequate curing and exhibited higher carbonation depths than plain Portland cement concrete. There was a good agreement of the air permeability and sorptivity results with the carbonation results, with the air permeability displaying better correlation.
Corrosion inhibitors in concrete repair systems
Authors: Page C. L. , Ngala V. T. , Page M. M.
Published: October 2009

Corrosion of reinforcing steel represents the most widespread form of deterioration of concrete structures. It affects reinforced concrete which is exposed to the effects of carbonation and/or to significant concentrations of chloride salts. Effective procedures employed in the past to combat reinforcement corrosion have required either extensive cutting out and replacement of contaminated but otherwise sound concrete at the level of the embedded steel or the use of electrochemical measures such as impressed-current cathodic protection. Over the past decade, however, the concrete repair industry has developed novel techniques that are claimed to restore the protective character of cover concrete by introducing corrosion inhibitors into the carbonated and/or chloride-contaminated material. As this approach requries only physically unsound or heavily contaminated regions of the concrete to be removed and reinstated it can involve reduced costs, pollution and inconvenience to owners and users of affected structures. This paper presents a critical review of current knowledge of various types of corrosion inhibitor treatment that are in use, or under consideration for use, in repair systems for concrete structures with corrosion problems resulting from carbonation and/or chloride salts.
Effect of SFRC on shear strength of prestressed concrete beams
Authors: Abdul-Wahab Hashim M. S. , Al-Kadhimi Sabah G.
Published: October 2009

An experimental investigation into the behaviour and ultimate shear strength of 13 large-scale fully prestressed T-beams containing steel fibres is presented. The main parameters considered were the fibre content, shear span/effective depth ratio and web reinforcement. The inclusion of steel fibres resulted in a significant improvement in the ductility and ultimate shear capacity of the tested beams. The mode of failure was also changed in most cases from shear to flexural failure. For design purposes, the concept of modifying BS or ACI shear formulae by incorporating the properties of the fibrous concrete is also examined. Results from this study, as well as those available in the literature, were compared with the predicted ultimate shear values to test the validity of the modified formulae.
Stress–strain model to predict the full-range moment curvature behaviour of high-strength concrete sections
Authors: Mendis P. , Pendyala R. , Setunge S.
Published: October 2009

The use of high-strength concrete (HSC) with compressive strengths exceeding 50 MPa is becoming very popular around the world. However, HSC is a more brittle material than normal-strength concrete, showing a steeper softening slope beyond the peak of the stress–strain curve. Therefore the stress–strain models developed for normal-strength concrete are not suitable to determine the full-range moment–curvature behaviour including the softening portion of a high-strength concrete section. A new stress–strain model to predict the full-range behaviour of HSC is presented in this paper. This model, termed the ‘modified Scott model’, is based on modifying the important parameters of the widely used Scott model, which is applicable only for normal strength concrete.
Durability of high-strength concrete incorporating carrier fluidifying agent
Authors: Chan S. Y. N. , Feng N. Q. , Tsang M. K. C.
Published: October 2009

Carrier fluidifying agent (CFA) was developed to maintain the workability of high-strength concrete (HSC). It retained the workability of HSC for a period of up to about 90 min. In the present study, the permeation characteristics, carbonation, and chloride diffusion of HSC incorporating CFA were studied. The experimental results showed that the use of CFA can maintain or slightly improve the permeation characteristics of HSC and its resistance to carbonation and chloride diffusion.
A numerical approach to the study of chloride ion penetration into concrete
Authors: Onyejekwe O. O. , Reddy N.
Published: October 2009

This paper deals with a key aspect of concrete durability, namely, the ingress of chloride ions into concrete exposed to a saline environment. The work numerically studies the movement of chloride ions in the pore structure by diffusion. This is achieved by treating the porous concrete material as a continuum and adopting a mass transport governing differential equation to serve as the basis of a numerical model. Some of the factors which affect the behaviour of chloride ions in concrete are accounted for by considering the diffusivity functions obtained from earlier studies. Good correlations are observed between numerical results obtained herein and those from studies of laboratory and field specimens. This confirms that, with some efforts, the model can be seen upon as a reliable tool.
Gas permeability of unsaturated cement-based materials: application of a multi-scale network model
Authors: Gallé C. , Daian J.-F.
Published: October 2009

A multi-scale network model based on the percolation theory and using a renormalization method was applied to describe the porous space of cement-based materials and to determine their gas transport properties. Gas permeability was measured on hardened cement pastes prepared with French OPC (CEM I) and BFS–PFA (CEM V) cements currently used in the nuclear waste industry. These experiments were carried out to study the influence of the degree of water saturation and of the microstructure on materials gas transport properties. Between 10 and 100% relative humidity, the cement pastes' average gas permeability varies over five orders of magnitude (10−16 –10−21 m2). Results show that in the range of water saturation considered, there is no linear relationship between the gas permeability and the degree of water saturation in percentage terms. Permeability change was well observed for cement pastes characterized by w/c ratios of 0·30, 0·40 and 0·50, in the low water saturation domain (<60%). CEM V pastes are globally more permeable to gas than CEM I pastes in the range of one order of magnitude. For the modelling aspect, the application of the XDQ multi-scale model made it possible to model a porous network equivalent to the one obtained by the mercury intrusion porosimetry test. Each class of pores is associated with an elementary cubic random network. The whole porous system is rebuilt by a recurrent process involving superposition and rescaling of all the elementary networks. Then, an equivalent conductivity of the multi-scale network is computed and extended to gas permeability. Modelling and experimental results are in agreement and show that mercury intrusion is a relevant method to characterize a porous space in order to estimate the gas permeability of cement-based materials with a conceptual model.
Effects of compression reinforcement on the shear strength of reinforced concrete bridge beams
Authors: Gale L. , Ibell T. J.
Published: October 2009

The predicted shear capacity of existing reinforced concrete bridge beams is an important subject that needs to be addressed in more detail. Concern has been expressed about whether the current bridge assessment code for the United Kingdom is too conservative when evaluating the shear resistance of existing concrete beams that contain considerable amounts of top (compression) steel, ignored during assessment. This paper looks at the beneficial effects of such compression steel on the shear strength of reinforced concrete beams. The results of laboratory testing are compared with the current code predictions for the shear strength of beams assumed to contain solely tension steel. Further comparisons are made with an upper-bound plasticity solution which is able to consider all steel reinforcement in a concrete beam. There are several reasons why bridges exhibit hidden reserves of strength, with compressive membrane action probably the most important. However, reasons such as the presence of compressive steel will contribute to hidden strength, so that research of this kind is required to enable realistic assessments and strength predictions to be carried out. It is shown that the presence of top (compression) steel has a significant effect on the shear capacity of reinforced concrete bridge beams which contain shear stirrup reinforcement.
Reuse of wastewater generated at concrete plants in Florida in the production of fresh concrete
Authors: Chini A. R. , Muszynski L. C. , Bergin M. , Ellis B. S.
Published: October 2009

Wastewater produced at ready-mixed concrete plants from the cleaning of the concrete truck's drum has recently been banned from direct disposal to ground or surface waters. Recent environmental regulations require ready-mixed concrete plants to manage and contain wastewater by the use of a reclamation system until it is environmentally acceptable for discharge. The objective of this study was to investigate water quality standards in Florida and the possibility of reusing concrete wastewater as aggregate irrigation and/or batch mixing water in the production of fresh concrete. The results indicate that wastewater used in this study, which did not meet the Florida Department of Transportation (FDOT) water quality specifications but did comply with the water quality standards of AASHTO M 157, had no detrimental effects on concrete properties. Based on the data produced during this project, it is recommended that the FDOT water quality specification be supplemented to address the use of wastewater as aggregate irrigation and/or batch mixing water in the production of fresh concrete.
A model of confined concrete in high-strength reinforced concrete tied columns
Authors: Han B.-S. , Shin S.-W. , Bahn B.-Y.
Published: October 2009

An experimental and analytical study was conducted to develop the confined concrete model of reinforced high-strength concrete tied columns subjected to monotonically increasing concentric axial compression. Twenty-one large-scale columns (260 × 260 × 1200 mm) of high-strength concrete of 50 MPa and 85 MPa were fabricated to simulate full-size structural members. The effects of the main variables such as the concrete compressive strengths, tie configurations, amount of the transverse reinforcement, tie spacing and the spalling of the concrete cover were studied in this research programme. More confinement is required in columns of high-strength concrete than in columns of low-strength concrete to achieve the same degree of strength enhancement. The proposed model satisfactorily predicted the experimental stress–strain behaviours for the confined high-strength concrete up to 100 MPa.
Steel–concrete bond deterioration due to corrosion: finite-element analysis for different confinement levels
Authors: Berra M. , Castellani A. , Coronelli D. , Zanni S. , Zhang G.
Published: October 2009

The effect of corrosion on bond strength of ribbed bars in reinforced concrete (RC) is studied by axisymmetric finite-element analysis, modelling different levels of confinement given by the transverse steel reinforcement. The importance of this parameter is highlighted by a summary of the experimental results in the literature. An approach is developed to model the corrosion product expansion causing concrete-cover cracking. The numerical analyses reproduce the experimental results of the bond tests performed in two different studies. A parametric study shows the effect of the varying transverse steel percentage on the residual bond strength, and the influence of different arrangements of confinement bars. These results lead to a discussion of the different findings of several experimental researches. The conclusions regard the main parameters to be included in a model to predict the bond strength of corroded bars. Some indications for the assessment of corroding structures are given.
Proposed revision on CIRIA design equation for normal and high strength concrete deep beams
Authors: Leong C. L. , Tan K. H.
Published: October 2009

The popularity of high strength concrete in the construction industry propels the need for a revision in the current shear design equations for deep beams in EC2. This paper proposes shear design equations that are applicable for both normal and high strength concrete beams. The comparison study involves a total of 151 beams, out of which 24 were the ‘ideal’ beams. The idea of simulating ‘ideal’ beams using finite element modelling (FEM) was to provide more data points in a controlled manner to benchmark the proposed shear design equations. Additionally, parametric variations are introduced in the FEM simulation. It is noteworthy that the proposed shear design equations adequately predict the ultimate shear strengths of deep beams of various shear span-to-depth ratios (0·27–2·70), main reinforcement contents (1·23–5·80%), web reinforcement contents and concrete grades (covering both normal and high strength concrete from 25–100 MPa).
A methodology for durability-based service life design of reinforced concrete flexural members
Authors: Anoop M. B. , Balaji Rao K. , Appa Rao T. V. S. R.
Published: October 2009

This paper proposes a methodology for durability-based service life design of reinforced concrete (RC) flexural members that ensures acceptable performance under service loads. The performance measures considered are safety (or collapse) and serviceability (or cracking) of the RC member with respect to chloride-induced corrosion of reinforcement. In the proposed methodology, the uncertainties arising due to the use of linguistic terms for describing the exposure conditions and quality of construction are taken into account by considering the time for corrosion initiation and the variables affecting the rate of corrosion as fuzzy variables. The efficacy of the proposed methodology in predicting the corrosion damage to the structural members is demonstrated through a practical case study. An example is provided to illustrate usefulness of the proposed methodology in making design decisions.