bluuebunny/crossref_metadata_2025 huggingface dataset:
The use of beams with a single edge notch to study the sulphate resistance of OPC and OPC/pfa pastes
Authors: Hughes D. C. , Grounds T.
Published: October 2009

Synopsis

The diffusion of the sulphate ion into hardened cement paste is relatively slow and a gradient of degradation is observed. The stress distribution at the tip of a notch is sufficiently concentrated for changes in physical properties at the surface of a specimen to be monitored. The use of beams with a single edge notch is described and shown to be more sensitive to sulphate ingress than the conventional expansion test, The earlier indication of sulphate attack from the notched beams highlights the mechanical restraint imposed by the relatively unaffected core of an expansion prism. The enhanced sulphate resistance of OPC/pfa pastes is shown to be strongly dependent upon the degree of curing prior to immersion in sulphate solution.
Influence of frequency of cyclic load on creep of concrete
Authors: Brooks J. J. , Forsyth P.
Published: October 2009

Synopsis

Short-term creep of sealed and drying concrete under a sinusoidal compressive load has been determined at various frequencies ranging from 1 cycle per day to 1 Hz, Under cyclic loading, creep has to be dejined carefully because of the changing shape of the stress-strain curve. When compared with static creep under a stress equal to the mean cyclic stress, creep under a cyclic stress is greater by an amount which decreases with time under load. Between frequencies of 1 cycle per day and 30 cycles per hour, the increase in creep under a cyclic load is approximately equal but, at higher frequencies, there is a further increase in creep. Compared with drying static creep under a stress equal to the mean cyclic stress, drying creep measured at the mean cyclic stress is similar and unaflected by frequency of loading. Expressions are given which give estimates of the full strain cycle when the static creep is known.
Setting behaviour of cement pastes containing admixtures determined from shear modulus tests
Authors: Hannant D. J. , Dunn T.
Published: October 2009

Synopsis

The rates of development of shear modulus of three fresh Portland cement pastes, two containing accelerating admixtures and one containing a retarding admixture, were compared with the shear modulus of a control mix during the, first few hours after mixing. It was found that the shear modulus equipment used was sensitive enough to detect differences in structure development between the mixes.

The initial setting times were determined using the Vicat apparatus and it was observed that the mixes had diferent shear modulus values at their Vicat initial setting time. However, average shear moduli of at least 1970 k Pa had been reached in all cases by the Vicat initial setting time, with the maximum shear modulus of 3270 k Pa being measured for the control mix without additives.
Research on long-term properties of high alumina cement concrete
Authors: Collins R. J. , Gutt W.
Published: October 2009

Synopsis

This paper describes in detail the, final results of a research programme started in 1964 to study the longterm characteristics of high alumina cement (HAC) concrete and reported briefly in a recent Building Research Establishment Information Paper. Current guidelines, for assessing the condition of buildings containing HAC concrete are discussed in the light of these and other laboratory results which have recently become availahle. The main conclusions relate to wet HAC concrete which is shown to continue reducing in strength even when highly converted and to reach lower minimum strength than concrete under dry conditions. This effect of wet storage is additional to the known vulnerability of HAC concrete, when wet, to chemical attack.
Near-surface characteristics of concrete: prediction of carbonation resistance
Authors: Dhir R. K. , Hewlett P. C. , Chan Y. N.
Published: October 2009

The relationship between the intrinsic permeability and the Carbonation of concrete is examined. It is shown that the Figg air index test, which is portable, cheap and simple to operate, can be used to predict directly the potential resistance of concrete to carbonation. The water/cement ratio and degree of moist curing are the main factors influencing the quality of concrete in terms of its carbonation resistance.
The effect of differences in the composition of Portland cement on the properties of hardened concrete
Authors: Ben-Bassat M. , Nixon P. J. , Hardcastlet J.
Published: October 2009

The effects of differences in the composition of Portland cements on the properties of concrete have been investigated using three cements in which the C3S (i.e. 3CaO · SiO2) contents ranged from 33·5 to 67%. It was found that the composition of the cement did have an effect but not as great as did differences in the curing regime. The concretes made with the higher C3S/C2S ratio cements had properties which were as good as, or in some respects superior to, those made with cements with a low C3S/C2S ratio. In good curing conditions the differences were not marked but in adverse curing conditions the faster hydration of the higher C3S/C2S ratio cements were a definite advantage and concretes made with these cements performed better in almost every respect. The results suggest that concerns about the reduced durability of concrete specified by strength, and therefore containing a lower cement content or higher water/cement ratio when a high strength cement is used, may have been over-emphasized. However, it should be noted that the effects of differences in heat release have not been assessed in this study.
A rational model for the stress-strain relationship of circular hoop-confined concrete
Authors: Ahmad Shuaib H. , Abdel-Fattah Hisham
Published: October 2009

A model for predicting the stress-strain relationship of circular spirally-confined concrete is presented. The model utilizes Boussinesq's approach to calculating the beneficial effect of hoop confinement on the strength of concrete. The lateral stress distribution due to each individual hoop is computed and superposition is used to obtain the lateral stress distribution across the height of the confined zone. The strength enhancement factor k is shown to decrease with increasing diameter of the concrete section, and increase with the height of the confined concrete section. This implies that the geometry of the confined section also effects the strength enhancement factor. Once the strength of the spirally-confined concrete is computed using the strength enhancement factor, non-linear elasticity is used in conjunction with a fractional equation to predict the entire stress-strain relationship. The predictions of the model are compared with experimental results reported in the literature and the comparisons are judged to be good.
The behaviour of concrete under Na2 S04 solution attack and sustained compression or bending
Authors: Schneider U. , Piasta W. G.
Published: October 2009

The interaction effect of the simultaneous sustained stress and Na2S04 solution attack on the deterioration rate of concrete has been experimentally noted. Time-dependent deformations, their recovery and residual strength indicate that the effect of compression on the behaviour of concrete under sulphate attack depends on the stress level. The effect seems to be much more significant for severe attack. On the other hand, sulphate corrosion is accelerated by flexural stress of any value.
Materials for small-scale reinforced concrete
Authors: Mo Y. L.
Published: October 2009

The objective of this Paper is to develop model materials, including microconcrete and model reinforcing, with the consideration of bond similitude. The mechanical properties of the developed model materials were compared with those of the normal reinforced concrete found in literature, and were found to be reasonably good. Therefore, these model materials were used to perform small-scale reinforced concrete frame tests, as reported in the companion Paper.
Initial surface absorption of concrete: examination of modified test apparatus for obtaining uniaxial absorption
Authors: Price W. F. , Bamforth P. B.
Published: October 2009

A modification to the initial surface absorption test (ISAT) is examined, which incorporates a water-filled guard ring. lhis produces an absorptionfiw pattern closer to the pure uniaxial situation than the conventional test apparatus. A series of laboratory specimens covering a range of concrete mixes and surface drying conditions have been examined using both the conventional and the modified ISATapparatus. A correlation between the initial 10 min absorption rate and the cumulative absorption over the 2 h test period was observed for both types of apparatus. At higher absorption rates the cumulative absorption, for a given initial su$ace absorption rate, was lower for the modified apparatus, suggesting that a more uniform absorption pattern was achieved using this modification. Althoughthe problem of compensating for the fluctuating state of saturation in concrete structures has yet to be resolved, possible relationships between the modijied ISAT test results and sorptivity are presented.
Direct electric curing of concrete
Authors: Bredenkamp S. , Kruger K. , Bredenkamp G. L.
Published: October 2009

Direct electric curing (DEC) of concrete is a process by which alternating electric current is passed through the bulk of freshly poured Portland cement concrete to acceleratethe curing process by means of direct ohmic heating. Electric JieM strengths, ranging approximately from 300 to 500 V/m, are required. Ris process leads to signiJicantly higher preliminary (8 hours) crushing strengths. The ultimate (28 days) strength is under suspicion, however. The Authors have investighted the influence of the maximum temperature reached, the energy input (kWh/m3), and the rate of energy input (kW/m3)on the preliminary and ultimate strengths of the concrete. It is concluded that the DEC technique is technically and economically viable, and warrants further development.
Structural repair of reinforced concrete portal frame
Authors: Blight G. E. , Alexander M. G. , Lampacher B. J.
Published: October 2009

The Paper describes repairs carried out on a reinforced concrete portal frame that had suffered damage over the previous 25 years as a result of alkali–silica reaction (ASR). This particular structure has been the subject of intense study since 1978 and several previous papers have described its behaviour.3,5 The repair consisted of partly demolishing and rebuilding the upper beam of the portal, while keeping the motorway it supports open to traffic. Results are given for the shrinkage of the replacement concrete, and for depropping movements and surface strain measurements taken on the structure over the year following the repair.
Performance of building structures affected by Scud missile attacks in Riyadh
Authors: Amjad M. A.
Published: October 2009

This Paper reviews the performance of the affected building structures and highlights quantitatively the main structural behaviour due to the Scud missile attack in Riyadh. A general survey of all buildings affected by the Scud in Riyadh was carried out. It was found that almost all the buildings suffered a similar type of damage. As a case study, a three-storey building seriously affected by the Scud missile was chosen and assessed. In the investigation, the actual design stresses as well as blast stresses were considered and studied. It was found that buildings located 300 m from the blast point and beyond are capable of sustaining the imposed lateral blast load (equivalent to weight of charge of 85 kg of TNT). Based on the analysis and discussions it is recommended that hospitals and national defence buildings should be designed to withstand higher stresses than other buildings.
Ionic diffusion across an interface between chloride-free and chloride-containing cementitious materials
Authors: Yu S. W. , Sergi G. , Page C. L.
Published: October 2009

One end surface of a cylindrical specimen of hardened OPC paste of water/cement ratio 0·5 was cemented to that of a similar specimen which contained 2% chloride ion by weight of cement, added as sodium chloride. The total chloride contents, as well as the concentration profiles across the interface of potassium, sodium, chloride and hydroxyl ions in the pore solution phase, were determined. It was found that the sodium concentration profile could be interpolated by Fick's Second Law to a good approximation. By assuming a Lungmuir-type relationship between the free and bound chloride, the effect of chloride binding on diffusion was incorporated into diffusion equations derived from Fick's First Law and mass conservation. This also yielded a good characterization of the free chloride profile. The concentration profile of hydroxylions in the pore solution was more complex than those for sodium and chlorideions, but it could be modelled by assuming that hydroxyl ions move simply to preserve the charge balance in response to the movements of other difising ions in the system.
Survey of condition of precast high-alumina cement concrete components in internal locations in 14 existing buildings
Authors: Crammond N. J. , Currie R. J.
Published: October 2009

Over the past few years the Building Research Establishment has carried out surveys on 14 buildings between 20 and 35 years old to assess the performance of precast high-alumina cement (HAC) concrete components in service. The single most important finding of these investigations is that the majority of the floor and roof components are carbonated to the depth of the steel. Even though subsequent reinforcement corrosion has not caused disruption to date, the future risk from corrosion in such beams is still unclear, even in nominally dry internal conditions. Chemical attack, which may greatly affect the structural performance of HAC concrete components, has not been identified as a major problem in the 14 buildings investigated, although a few isolated instances have been found in some of the components exposed to persistent water penetration. The implications for structural assessment are also discussed in this Paper. The Building Regulations report by Sub-committee P remains the current guidance on the strength of HAC concrete components produced by the long line casting process, except in situations where chemical attack has been diagnosed.
A plasticity analysis of anchorage zones
Authors: Ibell T. J. , Burgoyne C. J.
Published: October 2009

This Paper is the second of three on the behaviour of anchorage zones for prestressed concrete. Details are presented of a plasticity approach to the ultimate strength analysis of concrete prisms strip-loaded through rigid steel plates. Such a loading arrangement is assumed to represent adequately the transfer of force from tendon to concrete at the anchorage of a prestressed concrete structure. The effects of steel reinforcement are included in the analysis, which is assumed to satisfy plane strain conditions. lke Mod$ed Mohr-Coulomb failure criterion with non-zero tension cut-off is used for the concrete. A model, based on experimental evidence, is used as the basis for the plastic analysis, and good correlation is obtained between the theory developed here and the observed test results. It is concluded that such a plasticity approach is useful in assessing the strength of concrete under concentrated load.
A generalized lower-bound analysis of anchorage zone
Authors: Ibell T. J , Burgoyne C. J.
Published: October 2009

This Paper is the last of three on the behaviour of anchorage zones for prestressed concrete. Details of a lower-bound approach to the ultimate load analysis of concrete prisms strip-loaded through rigid steel plates are presented. The previous papers of the series presented experimental results and analytical plasticity solutions to this problem. This Paper extends the plasticity solution by using a finite element approach, thus enabling three-dimensional failure mechanisms encountered during the experiments to be studied successfully. The analysis is conducted in two stages: the first to determine the cracked state of the anchor block; the second to determine the final failure load. It is shown that such an approach is useful in the design of anchorage zones for prestressed concrete.
Alkali – aggregate reaction and internal release of alkalis
Authors: Gillott J. E. , Rogers C. A.
Published: October 2009

Portland cement concrete may deteriorate because of expansive reactions between certain types of aggregates and alkalis. The alkalis are mainly derived from the cement, but it has also been postulated that some aggregates contain minerals that contribute alkalis by internal reactions and so cause increases in expansion due to alkali–aggregate reaction, Dawsonite, a rare mineral found in volcanic silicocarbonatite dykes and sills in the Montreal area, has been found to react with Ca(OH)2. Expansion of concrete due to alkali–carbonate reaction and alkali–silica reaction increased when the alkali–reactive coarse aggregates were partially replaced by a dawsonite-containing silicocarbonatite. Chemical analysis showed increases in the alkali content of the concrete test-specimens. NaOH is a product of the proposed dawsonite reaction, which also indicates an increase in solid volume that probably accounts for the early age expansion of normally non-expansive control prisms. Comparison also showed that some prisms subjected to accelerated steam curing had higher early-age expansions than fog-room cured prisms, implying that a short exposure to elevated temperature accelerated the dawsonite and alkali–carbonate reaction. Delayed ettringite formation is probably a factor in late-age expansions of samples subjected to accelerated curing.
A method for determining the shor- and long-term behaviour of slender reinforced concrete columns in sway frames
Authors: Lim Y. K. , Bradford M. A. , Gilbert R. I.
Published: October 2009

A rigorous model of the short- and long-term behaviour of a slender reinforced column and beam sub-assemblage in a sway frame is developed. The analysis includes cracking at service load levels, creep and shrinkage deformations and the second-order slenderness effects. The analysis shows that second-order moments and dejections may be substantial at service loads. These moments and dejections are accentuated by cracking and the time-dependent creep and shrinkage deformations in the concrete, and are calculated in this Paper without recourse to the contentious effective length factor. This behaviour is illustrated by computer.
Fracture energy and fracture toughness of concrete
Authors: Sabir B. B.
Published: October 2009

The stress intensity factor for a specimen used for fracture toughness testing of cementitious materials was obtained by the finite element method using isoparametric representations and two energy approaches. A range of specimen geometrise were considered and the critical strain energy release rate Gc, and critical value of an energy integral Jc were evaluated using the failure loads obtained from laboratory tests. For linear elastic conditions Gc is equivalent to Jc, and both quantities are regarded as the energy required to produce unit extension of the pre-existing crack. It is found that the fracture energy and the fracture toughness are independent of the specimen geometry. The fracture energy Gc of the concrete employed in the tests is approximately 16 N m/m2. This corresponds to a fracture toughness Kc of 0·7 MN/m3/2, which is 40% greater than that reported in an earlier paper.
Propagation of microcracks in concrete under uniaxial compression
Authors: Loo Y. H.
Published: October 2009

This Paper reports the findings of a series of short-term sustained loading tests on concrete cylinders in which the extent of microcracking in concrete was continuously monitored using a novel non-destructive method. The approximate crack areas within the cross-sections of these concrete cylinders were estimated from the measured transverse and axial strain readings obtained with electrical resistance strain gauges attached to the surface of the cylinders. The experimental results showed that microcracks were initiated at a stress-strength ratio ranging between 0·15 and 0·44 (rather than at a specific value), but they did not propagate under sustained loads unless the stress–strength ratio exceeded a threshold value of about 0·5. Beyond this threshold level, the rate of microcrack propagation generally increased with increasing stress but, up to a certain stress–strength ratio (ranging between 0·8 and 0·9), decreased with time during sustained loading. It was also observed that this particular stress–strength ratio had the same range of values as the stress at which the volume of concrete became a minimum. Beyond this critical stress–strength ratio, the rate of microcracking increased rapidly with stress and increased with time under sustained loading, indicating that failure of concrete was imminent.
Creep loss variation in prestressed concrete beams
Authors: Taher S. E. D. , Sharif A.
Published: October 2009

Stresses due to permanent loading in prestressed concrete beams induce variable creep losses along bonded tendons. The main factors characterizing this variation are outlined and the corresponding effect on mid-span deflection is investigated for several cable profiles commonly used in simply supported concrete beams. Simplified formulae for evaluating maximum deflection taking account of the consequences of creep loss variation are proposed. This study analytically evaluates the creep loss variation along prestressed concrete beams and assesses its influence on deflection. f i e results have shown that the absolute range of variation can be as high as 20·0%. Furthermore, neglecting the creep loss variation along simply supported prestressed concrete beams could cause the total defection due to dead loads and prestressing force including creep losses to be underestimated by 15%.
Electrochemical desalination of cores from a reinforced concrete coastal structure
Authors: Polder R. B. , Walker R. J. , Page C. L.
Published: October 2009

This paper describes a laboratory investigation of the process of electrochemical desalination (chloride removal) applied to cores taken from the splash zone of an eight-year-old reinforced concrete quay wall situated on the North Sea coast. The concrete, which had been made from blastfurnace slag cement in accordance with Rijkswaterstaat specijications, was of high quality and significant levels of chloride contamination were present only in the exposed surface zone of the material at depths well below the thickness of cover to the reinforcement. Segments of steel reinforcement present within cores taken from the structure were subjected to cathodic polarization at current densities in the range 0·6–4 A/m2 for 3–6 weeks. The anodes, which were placed at the chloride-contaminated end surfaces of the cores, were composed of activated titanium meshes in contact with electrolyte solutions of either saturated calcium hydroxide or 1 M sodium carbonate. Changes in the electrical resistance of the concrete, its chloride content and its pore solution composition projile were measured and the corresponding increases in the chloride concentration of the external electrolyte were recorded. The proportion of chloride ions removedfrom the cores was found to depend on the total charge passed and on the nature of the external electrolyte, the eficiency of the desalination process being much higher when saturated calcium hydroxide rather than 1 M sodium carbonate was used. The mechanism of this effect and the associated changes in pore solution chemistry are examined.
The effects of restraint on ASR expansion of reinforced concrete
Authors: Jones A. E. K. , Clark L. A.
Published: October 2009

This paper presents the results of tests relating the expansion of concrete specimens with alkali-silica reaction (ASR) to applied constant stresses, both tensile and compressive, and reinforcement ratio. The results were compared with those found by other researchers and to two proposed mathematical models. Additionally, the effects of rate of expansion, specimen size and shape, and orientation of restraint were investigated. It was concluded that size of specimen and orientation of restraint were important factors, and that the relationship between restraint and expansion could be modelled mathematically but the amount of data required restricted the model's practical use.
Diagonal shear in thin-webbed reinforced concrete beams: fibre and stirrup roles at shear collapse
Authors: di Prisco M. , Romero J. A.
Published: October 2009

The combination of steel or polymeric fibers with stirrups in reinforced concrete (RC) shear-sensitive elements has been recognized as an efficient means of resisting sheal; but so far few models have attempted to describe fibre-stirrup interaction. A recently proposed limit-analysis model for the description of the ultimate behaviour of thin-webbed RC and prestressed concrete (PC) beams subjected to shear is extended here to the study of the role of fibres. A stabilized shear-crack pattern is assumed, with the stirrups at or beyond the yielding threshold, and the end sections of the concrete struts at the onset of failure in shear compression. The shear cracks, either limited to the web (PC beams) or extended to the lower flange (RC beams), are modelled with non-uniform opening and slip distributions, with mode l prevailing at the crack tip and mixed mode along the cracks. The fibres are introduced as fully kinked, randomly oriented, randomly embedded, linear connectors across the shear cracks, and their contribution to shear resistance is analysed in three different types of fibres (crimped and hooked-end or smooth steel fibres, and polyacrylonitrilic (PAN) fibres), different fibre contents, normal and high-strength concretes. In combination with stirrups, the role of crimped fibres and PAN fibres is found to be quite substantial. A comparison with other limit analysis models and 22 fibre-reinforced concrete tested beams is also presented.
Upper bounds in the slant shear testing of perfectly plastic joints in concrete
Authors: Eyre J. R. , Campos E. S.
Published: October 2009

The slant shear test is frequently used to assess the bonding of joints in concrete construction but the information provided when the specimens fail along the joint line is not easily assimilated. When the failure is a ductile yield, Jensen employed an upper bound plastic analysis. However, the restriction of a plane displacement field found in previous work severely limits its application and yields unlikely predictions of upper bound loads for specimens. This paper presents a graphical method of demonstrating the jlow vectors in a yielding medium in the test and, following a plane stress analysis, shows the scope of upper correctly anticipating the space of displacements at failure is bound solutions. In this way the importance of demonstvated.
Study of water suction of concrete with magnetic resonance imaging methods
Authors: Kaufmann J. , Studer W. , Link J. , Schenker K.
Published: October 2009

Magnetic resonance imaging (MRI) methods have been applied by different authors to study water transport into porous media. So far, mostly dry samples of materials with relatively high porosities such as sandstone or limestone as well as mortars have been studied. We report the successful application of MRI techniques to study the water suction of concrete prestored under conditions of varying humidity. The proton MRI technique of SPI (single-point imaging) allowed the study ofpenetrating water in suction tests. A high rest water content made the application of these techniques to samples with higher degrees of saturation difficult. To avoid such problems a deuterium MRI technique with deuterated water as the suction liquid was used. Deuterium MRI allowed the study of the water suction of concrete samples prestored at up to 90% relative humidity. Sharp fronts of inflowing liquid were detected. The front heights were strongly dependent on the prestorage conditions of the samples.
Shear modulus of the interfacial bond between steel fibres and concrete
Authors: Hamoush S. A. , Terro M. J. , Ahmad S. H.
Published: October 2009

An analytical model for determining the interfacial bond shear modulus of steel-fibre-reinforced concrete is presented. The bond shear modulus is a constant that linearly relates the bond shear stress to the fibre slippage at the interface. The analytical model is developed based on the axisymmetric modelling of the pulled-out steel fibre from the surrounding concrete. The model assumes elastic behaviour for both the concrete and the steel fibres. The analytical model evaluates the total displacement of the fibre when pull-out loads are applied at the fibre end. Experimental analyses are used in conjunction with the analytical model to determine the bond shear modulus. The results of the numerical analysis of a few selected experiments are presented. The proposed model is used to evaluate a constant value for the bond shear modulus.
Design of reinforced concrete beams under fire exposure conditions
Authors: Desai S. B.
Published: October 2009

This paper presents a method for estimating the residual flexural and shear capacities of a reinforced concrete beam, corresponding to its exposure to fire for a certain duration of time. The proposed method is based on assessment of reduction in the initial strengths of concrete and reinforcement, resulting from the high temperatures developed inside the beam. The design rules for flexural and shear capacities are validated with the help of tests on beams representing a practical range of parameters: dimensionsof the beam, the strength of concrete and the strength, amount and location of the reinforcement.
Shear behaviour and shear analysis of reinforced concrete beams containing steel fibres
Authors: Oh B. H. , Lim D. H. , Yoo S. W. , Kim E. S.
Published: October 2009

An investigation of the shear behaviour of reinforced concrete beams containing steel fibres is presented. The major test variables were the amount of steel fibres and the volume of shear stirrups. The fibre content was varied from 0% to 2% by volume, and the amount of stirrups from zero to full reinforcement using code-required values. The test results show that the cracking shear strength increases significantly as the fibre content increases, and that improvement in ultimate shear strength is also achieved. It is seen that the steel fibres play an important role in reinforced concrete beams by curbing crack occurrence and reducing the crack width. This effect greatly enhances the serviceability and durability of reinforced concrete structures. The mode of failure changed from shear to flexure when the volume fraction of fibres exceeded 1%. This means that fibre reinforcement greatly increases the shear capacity. The ductility is also enhanced remarkably by the addition of fibres. The present study indicates that fibre reinforcement can reduce the amount of shear stirrups required, and that a combination of fibres and stirrups may be used to achieve both strength and ductility requirements. An analytical method for predicting the shear strength of reinforced concrete beams containing steel fibres is proposed. Good correlation was obtained between calculated and test data. The proposed method allows a more realistic shear analysis of reinforced concrete structural members containing steel fibres.
Statistical evaluations of field concrete strength
Authors: Chmielewski T. , Konopka E.
Published: October 2009

The paper deals with the evaluation of two classes of high strength concrete, C35 and C45, placed on construction sites in Baden-Württemberg in the years 1991–1993. The concrete strength was treated as a random variable. The main purpose of this research was to study the variability of concrete strength produced in different concrete-mixing plants then transported on to sites by means of transit mixer trucks. The components in the concrete were determined by computer analysis. Samples of concrete were taken from each construction site and were cured for 7 days in water and then for up to 28 days to the surrounding atmosphere. The strength of each sample was then tested, providing data from which histograms were plotted, their statistical descriptors were estimated and the validity of the assumed normal probability distribution by two goodness-of-fit tests were appraised. The representative coefficients of variations are suggested for use in reliability analysis of structures by level II methods.
Strength and serviceability of repaired reinforced concrete beams undergoing reinforcement corrosion
Authors: Mangat P. S. , Elgarf M. S.
Published: October 2009

The paper presents the results of an experimental study on 99 under-reinforced concrete beams which were subjected to corrosion before and after the cover zone was repaired with three different generic materials. The repairs represented a wide range of tensile strengths, elastic modulii, ductility and permeabilities. The beams were reinforced with two longitudinal bars, and shear reinforcement was provided externally by steel collars. Different degrees of reinforcement corrosion were induced before and after repair, ranging incrementally from 0% to 5% at a corrosion rate of 3 mA/cm2. The results show that low ultimate flexural strengths and high deflections under service loading result in corroding beams which incorporate repairs made with materials having the following combination of properties: low strain capacity and low tensile strength; high stiffness, high flexural strength and very low permeability. A relatively ductile repair material with a high coarse aggregate content provides significantly superior structural performance. Time-performance functions for the corroding beams have also been derived by non-linear regression analysis of the experimental data. These can be used to assess (a) long-term ultimate flexural strength of repaired beams undergoing known rates of reinforcement corrosion, and (b) the limiting flexural load capacity required of corroding beams in order to prevent excessive deflections. Empirical coefficients representing the different repair materials have been derived for use in these time-performance relationships in order to assess the long-term strength and serviceability (deflection criteria) of repaired beams.
Long-term performance of high-stiffness repairs in highway structures
Authors: Mangat P. S. , O'Flaherty F. J.
Published: October 2009

This paper presents the results of field monitoring of repair patches in two reinforced concrete highway bridges, Lawns Lane Bridge on the MI and Gunthorpe Bridge across the River Trent. The repairs were applied by spraying (guniting) repair materials to compression members of the bridges. The structural members were unpropped during repair and throughout the 60 week monitoring period. The strains in the repair patches were monitored with vibrating-wire gauges. Four different repair materials were investigated whose elastic modulus was greater than that of the substrate concrete (Erm > Esub). The results show that efficient repairs are achieved with Erm > Esub, the optimum relationship being Erm > 1·3Esub. This enables the repair material to shed a significant proportion of its shrinkage strain to the substrate, thereby reducing restrained-shrinkage tension. It also enables the repair to attract externally applied load from the substrate in the long term. The effect of creep and shrinkage on the performance of the repair patch is also determine. Overall, the results show that current repair standards have limitations with respect to repair material specifications.
The rheological performance of wet-process sprayed mortars
Authors: Austin S. A. , Robins P. J. , Goodier C. I.
Published: October 2009

This paper, which reports on part of a three-year research project into wet process sprayed concrete for repair, examines the influence of rheology on the pumping and spraying of mortars. The performance of seven commercially available prepackaged repair mortars and six laboratory-designed fine mortars was examined using the Tattersall two-point and Viskomat rotational viscometers, the pressure bleed test, the slump test, a build test and a vane shear strength test. These tests were used to form a rheological audit of each mortars. The two-point apparatus was successful with low-workability mortars, and their flow resistance and torque viscosities were determine. These parameters were also obtained with the Viskomat, although problems were encountered owing to the low workability of the mortars. The pressure bleed test measured both the rate and the total volume of liquid emitted from the mixes, while the vane shear strength test provided an instantaneous reading of the shear strength of the mortars and is compared with their slump. The mortars were pumped and sprayed through a worm pump to assess their suitability and to measure their adhesion to a substrate by build thickness. This value is a measure of sprayability and is converted into values of maximum shear and bending stress, which are then compared with the workability parameters in order to determine their interrelationship.
Factors influencing the density and compressive strength of aerated concrete
Authors: Ramamurthy K. , Narayanan N.
Published: October 2009

Aerated concrete is a relatively homogeneous material as compared with conventional concrete since it is devoid of the coarse aggregate phase, yet it exhibits vast variation in its properties. Most of the investigations in the past have been confined to autoclaved products. This paper reports the results of a systematic study conducted to ascertain the influence of the lime–cement ratio, fly ash as a partial/complete replacement of sand, the particle size of sand and the dosage of aerating agent on the density and compressive strength of moist-cured and autoclaved aerated concrete. This study establishes that from the viewpoint of density reduction, fly ash can play a pivotal role in aerated concrete, without much compromise on strength. An increased lime–cement ratio is observed to affect adversely the strength, whereas a reduction in the particle size of the sand below a certain level is not justified in the case of non-autoclaved aerated concrete.
The performance of hardened wet-process sprayed mortars
Authors: Austin S. A. , Robins P. J. , Goodier C. I.
Published: October 2009

This paper, which reports on part of a three-year research project into wet-process sprayed mortars and concretes for repair, investigates the hardened performance of wet-process sprayed mortars. Seven commercially available preblended repair mortars were pumped and sprayed through a worm pump, three through a piston pump and two through a dry-spray machine. A laboratory-designed mortar was also worm and piston pumped. The properties measured included compressive and flexural strength, tensile bond strength, hardened density, modulus of elasticity, air permeability, sorptivity, and drying and restrained shrinkage. In situ test specimens were extracted from 500 mm × 500 mm × 100 mm deep sprayed panels. Tests were also conducted on corresponding cast specimens and, where possible, on specimens that had been sprayed directly into a cube or beam mould. A new test to quantify the degree of reinforcement encasement has been developed and an initial investigation into the measurement of the restrained shrinkage of in situ repairs is presented. The compressive and flexural strengths of the laboratory mix were comparable to those of the best of the commercially available preblended mortars. The values for modulus of elasticity, when compared with the compressive strength, were lower than published formulae for this relationship would suggest, especially at lower strengths. The air permeability of most of the mortars was lower than that for normal wet-cured concrete and decreased with an increase in compressive strength. The sorptivity values showed no clear relationship to the compressive strength. The type of wet-process pump was found to have little effect on the in situ compressive and flexural strengths but did affect the bond strength, although this was mainly because of the stream velocity and w/c ratio rather than the pumping process. The pump type also affected the reinforcement encasement, with higher stream velocities producing better encasement. The mixes exhibited a wide range of drying shrinkage, but the data from the restrained specimens suggest an actual repair is influenced as much by ambient conditions as it is by the mix proportions.
Effects of various shape parameters on packing of aggregate particles
Authors: Kwan A. K. H. , Mora C. F.
Published: October 2009

The shape of the aggregate particles used has significant effects on the properties of the concrete produced. One major effect is on the packing density of the aggregate which determines the amount of cement paste needed to fill the voids between the aggregate particles. In order to study how the various shape parameters of aggregate particles would affect the packing of aggregate, aggregate samples of different rock types from different sources have been analysed for their shape characteristics using a newly developed digital image processing technique and their packing densities measured in accordance with an existing method given in the British Standard. The packing densities of the aggregate samples are correlated to the shape parameters to evaluate the effects of the various shape parameters on packing. From the results of the correlation, it is found that the shape factor and the convexity ratio are the most important shape parameters affecting the packing of an aggregate. Two alternative formulas revealing the combined effects of these two shape parameters on the packing density of aggregate are proposed.
Concrete—steel bond under repeated loading
Authors: Soh C. K. , Chiew S. P. , Dong Y. X.
Published: October 2009

This article reports an experimental study to characterise the concrete–steel bond under repeated loading. Three loading patterns, namely partial unloading, full unloading and multiple unloading were performed. In addition, the non-linear bond behaviour of the concrete–steel interface was studied whereby the initial elastic bonding, non-linearity and softening behaviour of the interface were simulated in a numerical analysis. The complete tangential traction–slip curves, as well as the unloading/reloading routes in the repeated loading cases, were calculated and compared with the experimental curves. 3D interface elements and a damage model are used in the numerical analysis. The small number of 3D interface elements used and the high accuracy achieved by the numerical simulation ascertained the viability of the proposed approach for analysing composite structures.
Parametric study of bond failure in concrete beams externally strengthened with fibre reinforced polymer plates
Authors: Kim D. , Sebastian W. M.
Published: October 2009

The use of adhesively bonded fibre reinforced polymer (FRP) plates to enhance in-service concrete beams and slabs in flexure is rapidly expanding within the structural rehabilitation industry. Failure of FRP-plated concrete members commonly occurs by brittle rupture of the plate-to-concrete bond. For plated reinforced concrete beams the most commonly reported mode of such bond failure initiates at the ends of the plates and propagates inwards along the beam. In this article, it is shown that two other bond failure modes, which initiate either at midspan with propagation out to the ends of the plates or within the inner end of the shear span with propagation to both the ends and middle of the plates, can also be critical for FRP-plated concrete beams. It is also shown that both failure modes propagated mainly through the concrete near the bondline. One use of the FRP plating in this context is to replace corroded internal steel in the deficient concrete structure. However, the local beam section changes due to the presence of corroded steel rebar generate high local plate-to-beam bond stresses, which trigger bond failure. Therefore in the study reported here, varying extents of steel corrosion were simulated in different plated beams, which were tested to bond failure. The thickness of the adhesive layer was introduced as another test parameter. The effects of these variables on the bond failure loads and modes of the plated beams are reported in this paper. Commercial plates and adhesive were used in the tests. No rupture of the plates was observed. Key video images of the final sudden debonding failure, showing the crack propagations through, and explosive rupture of the cover concrete adjacent to, the plates are presented.
Theoretical model of high temperature effects on uniaxial concrete member under elastic restraint
Authors: Nielsen C. V. , Pearce C. J. , Bićanić N.
Published: October 2009

An analytical solution to a uniaxial benchmark problem of a concrete member restrained by an elastic spring and subject to heating and subsequent cooling is given. The model demonstrates the effect of load-induced thermal strain, which has so far only been described through experiments. Heating a concrete specimen under constant compressive load shows a significant contraction rather than expansion, which would be the case if the specimen were heated without any constraints. The magnitude of this contraction is beyond that of conventional creep and mechanical strain even though the stiffness degradation from high temperature exposure is taken into account. Thus, it is attributed to a special thermo-mechanical coupling, taking place during the first transient heating process.

Furthermore, the analytical model describes the tensile stresses, developing during cooling, including non-linear material behaviour arising from fictitious cracking. Through several examples the effect of the various parameters is evaluated.
Experimental investigation of the strain rate dependent impact behaviour of cementitious composites
Authors: Verleysen P. , Degrieck J. , Taerwe L.
Published: October 2009

As for most construction materials, cementitious composites exhibit a strain rate dependent mechanical behaviour. The frequent occurrence of impact loads during the lifetime of civil engineering constructions makes the experimental research into the impact properties of construction materials sensible. To this purpose split Hopkinson bar tests are often used. However, due to the specific behaviour of plain cementitious materials—low strains and softening—special attention has to be paid to some aspects of the experimental setup, the treatment of the recorded signals and the interpretation of the final results. Some results of impact tensile tests on microconcrete are presented. These results show that the strength, the strain at maximum stress, the ultimate deformation and the energy absorbing capacity are higher than in static test conditions.