bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of stirrup distribution and support width on the shear strength of reinforced concrete wide beams
Authors: Serna-Ros P. , Fernandez-Prada M. A. , Miguel-Sosa P. , Debb O. A. R.
Published: October 2009

This study reports on the results obtained in an experimental test programme carried out on 18 wide beams of 750 mm in width, 250 mm in depth and a 3 m free span, designed to fail by shear. From the tests, a clear influence of arrangement of vertical shear reinforcement, transversal spacing of stirrup legs and support width on the shear strength is found. The shear strength of the tested beams has been compared with the ones predicted by the codes ACI 318 and Eurocode 2 and several improvements to both codes are proposed in order to take such effects into account. Finally, some design recommendations about distribution of stirrups are included in this paper to optimise the vertical shear reinforcement effectiveness and prevent premature shear failures.
Effects of concrete non-homogeneity on Rayleigh waves dispersion
Authors: Rhazi J. , Hassaim M. , Ballivy G. , Hunaidi O.
Published: October 2009

This article presents the spectral analysis of surface waves (SASW) technique and describes an experimental investigation concerning its ability to non-destructively assess the variation of elastic properties within Portland cement concrete structures. SASW tests were conducted on three concrete slabs, 1·4 m × 1 m × 0·5 m in size, and having different compressive strength (f′c: 17·5, 27 and 53 MPa). Tests were also conducted on a layered concrete slab, 2 m × 2 m × 0·7 m in size, and made up of a base layer, an intermediate layer and a top layer having compressive strength of 53, 27 and 17.5 MPa, respectively. The results show that the propagation velocity of Rayleigh waves is very sensitive to variations in concrete quality and that the SASW technique offers a practical tool to determine the extent of near-surface damage observed in in-service concrete structures.
Study of the alkali reactivity of aggregates used in Beijing
Authors: Feng N.-Q. , Hao T.-Y. , Feng X.-X.
Published: October 2009

Problems due to alkali–silica reaction have occurred in Beijing and there is an urgent need to detect the alkali reactivity of aggregates being used. The typical sands and coarse aggregates of this area were investigated by ASTM mortar bar test and the ASTM accelerated mortar bar test (AMBT). A pebble (rounded coarse aggregate) from Yongding River (YD) and a gravel from Nankou Mountain (NK), exhibited the potential for alkali–aggregate reaction. It was also found that the content, size distribution of aggregate and the size of samples showed influences on the expansion of mortar bars. The mortar bars made with the pessimum composition of aggregates from YD showed the largest expansion.
Influences of shrinkage and construction loading on loss of tension stiffening in slabs
Authors: Vollum R. L.
Published: October 2009

Tension stiffening is shown to be lost more rapidly after loading than predicted by models attributing loss of tension stiffening to shrinkage. Furthermore, contrary to predictions of shrinkage models, tension stiffening is lost quicker in slabs that crack significantly on loading than in slabs that crack mainly after loading due to shrinkage. It is concluded that shrinkage does not contribute significantly to loss of tension stiffening after loading in cracked members. An improved method is proposed to account for loss of tension stiffening in deflection calculations. A simplified method for taking account of construction loading in deflection calculations is shown to give reasonable estimates of deflections in the slabs in the in situ concrete building at Cardington.
Theoretical model for cracked tension zone of reinforced concrete under creep and cyclic load
Authors: Khalim A. R. , Clark L. A.
Published: October 2009

The cracked tension zone model of Tue and Konig is extended to take account of changes in slip between steel and concrete as a result of maintained and/or cyclic loading. The bond stress–slip curve given in CEB Model Code 90 is adapted for the extended analysis, which enables load history to be considered. The predicted results are compared with test results, and found to be in good agreement.
Chloride-induced reinforcement corrosion in blended cement concretes exposed to chloride-sulphate environments
Authors: Dehwah H. A. F. , Austin S. A. , Maslehuddin M.
Published: October 2009

This paper reports the results of a study conducted to investigate the influence of sulphate concentration and associated cation type on chloride-induced reinforcement corrosion in blended cement concretes. Reinforced concrete specimens were exposed to chloride plus sulphate solutions for a period of 1200 days. The exposure solutions contained a fixed concentration of 5% sodium chloride and the sulphate concentration was varied from 0 to 4% SO42−. The effect of cation type associated with sulphate ions, namely Na+ and Mg++, on chloride-induced reinforcement corrosion was also evaluated. Reinforcement corrosion was assessed by measuring corrosion potentials and corrosion current density at regular intervals. The results indicated that the presence of sulphate ions in the chloride solution increased the corrosion current density, but no significant effect on the time to initiation of reinforcement corrosion was noted. Further, the corrosion current density increased with increasing sulphate concentration and the period of exposure. The corrosion current density on steel in the blended cement concrete specimens was much less than that in the plain cement concrete specimens, indicating that the corrosion resistance of blended cements was much better than that of plain cements. The cation type associated with sulphate ions did not significantly influence either the initiation or rate of reinforcement corrosion.
Are existing span-to-depth rules conservative for flat slabs?
Authors: Vollum R. L. , Hossain T. R.
Published: October 2009

A series of parametric studies is presented to show the influence on deflection of factors including age at striking and peak construction load. The parametric studies suggest that the peak construction load usually governs long-term deflections in slabs struck at 2 days or later. Eurocode 2 is shown to significantly underestimate long-term deflections in slabs under permanent loading if the 28 day concrete tensile strength is used as intended since deflections are usually governed by cracking during construction. This is reflected in existing span-to-depth rules in EC2 and BS 8110 that do not realistically take construction loading into account. Span-to-depth rules in EC2 and BS 8110 are compared for flat slabs and it is shown that EC2 permits significantly thinner slabs. It is shown that deflections can be excessive in slabs dimensioned to the EC2 rules. Consequently, it is proposed that a modified concrete strength is used in the EC2 formula for span-to-depth ratios to allow for cracking during construction. It is concluded that there is scope for reducing slab thickness below BS 8110 guidelines for concrete cylinder strengths greater than 25–30 MPa.
Study of rock–mortar interfaces. Part I: surface roughness of rock aggregates and microstructural characteristics of interface
Authors: Caliskan S. , Karihaloo B. L. , Barr B. I. G.
Published: October 2009

This two-part paper reports the results of an extensive study on the interface between cylindrical rock aggregates and mortar matrix with particular emphasis on the influence of the surface roughness of the rock and the density of the matrix on the thickness of the interface and on its strength. In Part I of the paper the surface roughness of three rock aggregates commonly used in concrete (sandstone, limestone and granite) is measured by means of the Talysurf apparatus on 100 mm long cored cylindrical specimens of various diameters. The roughness is expressed through a roughness index Ra. It is found that the sandstone aggregate has the highest surface roughness, followed by limestone and granite in that order. Investigations are also conducted, using scanning electron microscopy, on how the microstructure of the interface and its thickness are affected by the addition of silica fume to the mortar. A chemical analysis of the constituents in, and in the vicinity of, the interface was also undertaken with a view to identifying the interfacial zone which is distinctly free from sand particles. It was found that the relative thickness of the interfacial zone reduces when silica fume is added to the mortar matrix. For all mortar and rock types the bond was found to be mechanical rather than chemical in nature. Part II of the paper studies experimentally and theoretically the influence of the surface roughness of the rock aggregate and density of the mortar matrix on the strength of the rock–mortar interface.
Study of rock–mortar interfaces. Part II: strength of interface
Authors: Caliskan S. , Karihaloo B. L. , Barr B. I. G.
Published: October 2009

In the second part of this two-part paper, the interfacial strength is determined experimentally using push-out specimens and predicted theoretically using a simple model which ignores the Poisson effect. The interfacial strength is then related to the surface roughness of the aggregate and to the interfacial thickness and the amount of silica in the interface (Part I). It is shown that the higher the surface roughness, the stronger the interface. The interfacial strength is found to decrease with an increase in the aggregate size, irrespective of the type of rock aggregate and mortar. Finally, in order to examine the role that the Poisson effect plays in the push-out test, a finite-element analysis is performed.
Deflection of prestressed concrete beams externally strengthened using Parafil ropes
Authors: Ghallab A. , Beeby A. W.
Published: October 2009

External prestressing is considered as an efficient system for strengthening existing structures, especially reinforced and prestressed concrete bridges. Also, using fibre reinforced plastics (FRP) for external prestressing tendons instead of steel avoids the corrosion that may occur when steel is used outside the concrete. However, the use of this system requires that the behaviour of strengthened beams be well understood. It is also necessary to have available simple equations to determine behaviour. To achieve these goals, thirteen prestressed beams, one with internal prestressing steel only and the rest strengthened externally using Parafil ropes, were tested up to failure to study the effect of the external prestressing force on the deflection of the strengthened beams. Six factors were taken into consideration; the value of the external prestressing force, the number of deviators, the effective depth of the external prestressing force, the load history prior to strengthening, the concrete strength and the span/depth ratio. Also, an analytical investigation was carried out to extend some of the methods currently used in deflection calculations. The comparison between the experimental and the theoretical results show good agreement.
Influence of crack width, cover depth, water–cement ratio and temperature on the formation of electrodeposits on the concrete surface
Authors: Ryu J. S.
Published: October 2009

In this paper, the formation of electrodeposits on the concrete surface based on the electrochemical technique is presented. The aim of electrodeposition in a concrete repair method is to fill the cracks in concrete and to coat the concrete surface by electrodeposits of chemical compounds. Laboratory tests were conducted on cylindrical concrete specimens with embedded reinforcing steel bars. After the specimens were cracked by applying splitting tensile load, the samples were immersed in a zinc sulphate solution and a constant current was applied between the reinforcing steel and the external electrode. Afterwards, the weight change of the specimens, the rate of crack closure and the filling depth of crack were examined and evaluated. The results indicate that the formation of electrodeposits on the concrete surface caused an increase in weight of the specimens and varied depending on the crack width, cover depth of concrete, water–cement ratio and temperature of solution.
Advanced analysis of published data on chloride binding in concrete
Authors: Siegwart M. , Lyness J. F. , Cousins W.
Published: October 2009

Chloride attack is a major cause of deterioration of reinforced and prestressed concrete structures. Chloride contamination can occur from the application of de-icing salts, from cast-in chlorides or from sea water as in marine structures. It has been found that chlorides are chemically bound mainly by tricalciumchloro-aluminate (C3A) or Friedel's salt in cement. Chloride binding becomes of particular interest when a critical limit of chlorides in concrete for the initiation of corrosion is to be defined or for diffusion and migration processes of ions in concrete. Traditionally chloride binding is described using Boltzmann, Langmuir or other analytical reaction types. This paper is a survey of existing data on chloride binding in concrete. Attempts to employ empirical data to elucidate chloride binding in concrete are rare and never before have many sources been analysed. Using the data from over 20 different sources an empirical term was derived, which describes chloride binding in terms of total chloride content (TCL) and C3A content (C3A). Chloride binding in concrete is described through C3A and TCL, therefore the term is applicable to any concrete.
Consistent strut-and-tie modelling of deep beams with web openings
Authors: Tan K. H. , Tong K. , Tang C. Y.
Published: October 2009

This paper presents an original and novel approach to analyse the shear strength of deep beams with rectangular openings and with various configurations of web reinforcement. Up to now, no code provision has existed for the design of such beams. Moreover, there are very few published papers that include analysis of deep beams with web openings and with web reinforcement. In most cases, the beneficial effect of web reinforcement is ignored for simplicity. The authors developed five models to cater for different configurations of web reinforcement. These include various combinations of horizontal, vertical and inclined web reinforcement. The main attraction of the approach is that it identifies the most critical nodes in the truss analogy. For simplicity, only the bottom nodal zone is considered. Although this approach cannot be used to predict the failure modes of deep beams, it yields consistent and accurate predictions with three experimental case studies comprising a total of 15 beams.
Determination of size-independent specific fracture energy of concrete from three-point bend and wedge splitting tests
Authors: Abdalla H. M. , Karihaloo B. L.
Published: October 2009

Three-point bend (TPB) and wedge splitting (WS) tests have been conducted on three different concretes and the specific fracture energy GF determined on the basis of the concept of local fracture energy. The latter is influenced by the free back surface of a notched test specimen, as explained by Hu and Wittmann. Tests on three or four specimen sizes with four notch to depth ratios confirm the idea of Hu and Wittmann that the size-independent specific fracture energy GF can be determined from measured fracture energy values that vary with the size of the specimen, W, and notch to depth ratio, a/W. More importantly, it is shown that the same size-independent GF can also be obtained by testing a single size specimen with only two notch to depth ratios, provided they are well separated (a/W = 0·05 and 0·50 in TPB, and 0·2 and 0·5 in WS), thus greatly simplifying the determination of the size-dependent fracture energy GF.
Mechanical properties of old concrete using destructive and ultrasonic non-destructive testing methods
Authors: Prassianakis I. N. , Giokas P.
Published: October 2009

This paper examines the variation in properties of a specific type of 28-year-old concrete, the characteristics of which were known at 7, 28, 36 and 90 days. Beyond the destructive tests (compression, splitting tension and bending), the concrete was studied with non-destructive ultrasound testing. Moreover, the relationship between the compressive strength of this concrete and the ultrasonic longitudinal wave velocity, as well as the dynamic modulus of elasticity were determined. The objective was to control changes in the quality of concrete over a long period of time and to contribute to the development of a non-destructive method for the determination of concrete strength in structures. The presented approach, that seems promising for such development, uses the velocity of ultrasonic waves in concrete.
Estimation of the filler content required to minimise voids ratio in concrete
Authors: Jones M. R. , Zheng L. , Newlands M. D.
Published: October 2009

This paper reports the findings of a study of the filler content required in concrete to produce a minimum voids ratio (or maximum packing density) according to particle packing models. The calculation of theoretical voids ratio shows that the filler content required to minimise voids ratio depends on the original concrete mix proportions as well as the fineness of the fillers. Concrete mixes with a high original cement content required the inclusion of finer filler to minimise the voids ratio and coarser, low-cost fillers were suitable for mixes with lower cement contents. The study shows the importance of both filler fineness and mix proportions on minimising voids ratio at a cement-sized grain level. Agglomeration of the fine particles and an effective utilisation of filler are also considered and two criteria are proposed to determine the filler content required to minimise the voids ratio in concrete.
Triaxial test results of high-strength concrete subjected to cyclic loading
Authors: Lokuge W. P. , Sanjayan J. G. , Setunge S.
Published: October 2009

High-strength concrete (HSC) has highly desirable structural engineering properties, which can lead to significant cost savings in heavily loaded concrete structures. However, its use is limited by a concern regarding an increased brittleness compared to normal strength concrete (NSC), especially under complex multi-axial loading conditions. The issue of ductility is extremely important in structures subjected to earthquake loading conditions. In order to determine the lateral confinement required for a given level of ductility, the relationship between axial stress and lateral strain of confined HSC needs to be established. This paper describes a testing programme carried out to obtain the relationships between axial stress, axial strain and lateral strain for HSC under cyclic loading and subjected to constant lateral confining pressures. Compressive strengths of concrete tested were 40, 60, 80 and 100 MPa. Uniform confining pressures applied were 4, 8 and 12 MPa. The experimental results are presented and the effects of cyclic loading and confinement on the constitutive behaviour of HSC are described.
Modelling eccentrically loaded high-strength concrete columns
Authors: Lokuge W. P. , Setunge S. , Sanjayan J. G.
Published: October 2009

Based on a confined concrete model for concentric static loading, an analytical method is proposed to predict the behaviour of laterally confined concrete columns subjected to eccentric static loading. Theoretical predictions are verified against the experimental studies reported in the literature. Normal, medium and high-strength concrete were used. Although much research has been reported in the literature concerning eccentric loading of confined concrete columns, there exists a controversial issue about the effect of strain gradient. This method fully incorporates the effect of strain gradient on the lateral confining pressure exerted by transverse reinforcement. It includes modifications for coefficient of effectiveness, and the fact that cover concrete continues to carry load even after spalling. By incorporating the variables, which affect the moment curvature behaviour of laterally confined eccentrically loaded columns, the analytical procedure described here gives a superior alternative to use of a different stress-block for concrete under a strain gradient.
Surface analysis of mineral substrates for repair works: roughness evaluation by profilometry and surfometry analysis
Authors: Courard L. , Nelis M.
Published: October 2009

Existing concrete surfaces need to be roughened to a profile necessary to achieve mechanical interlock with repair material. Surface mechanical morphometry is based on the measurement of surface profile – profilometry – which can be extended to 3D representation with surfometry by means of a stylus registering the profile. Profilometry and surfometry analysis enables the quantification of the profile by means of statistical and amplitude parameters calculated from the total profile of the surface (the waviness (lower frequencies) and roughness (higher frequencies) profiles). These parameters have been used to compare sandblasted and polished concrete surfaces. Arithmetic mean of the profile and flatness coefficient are particularly discriminant. Bearing ratio and Abbott's curve observations are also useful for clear and simple characterisation of surface profiles. Moreover, this investigation led to the calculation of the specific length of the profile and the Wenzel's coefficient.
Influence of mix and environment on plastic shrinkage cracking
Authors: Dias W. P. S.
Published: October 2009

The influence of mix constituents (including corresponding concrete grade), retarding and air entraining admixtures and PFA additives, and the effect of environmental factors on plastic shrinkage cracking of mortar specimens is reported, supplemented by concurrent evaporation and bleeding tests. The dominant effect of environmental factors on evaporation, and that of corresponding concrete grade on plastic shrinkage cracking, were clearly established. Both retarding and air entraining admixtures were found to increase such cracking, as did the inclusion of increasing amounts of PFA in the mix. The test results are discussed in the light of the literature and supported by field observations. The opposing effects on plastic shrinkage cracking caused by various factors are used to explain some inconsistencies among the results of various investigators.
Analytical modelling of confined HSC columns under flexure and axial load
Authors: Konstantinidis D. , Kappos A. J.
Published: October 2009

Existing models for high strength concrete (HSC) confined by rectilinear ties have ignored the effect of axial load eccentricity (strain gradient). An empirical stress–strain model for confined HSC developed by the writers using 108 large-scale specimens tested under uniaxial compression is further verified against recent test results and subsequently extended to include the influence of strain gradient on ductility. This was achieved by introducing the ratio of confined section depth to the neutral axis depth into the equation determining the slope of the descending branch of the stress–strain curve. The effects of the concrete compressive strength, volumetric ratio of transverse reinforcement, yield strength of ties, tie spacing and tie pattern are all explicitly accounted for. The new proposal is found to yield better agreement with experimental results than existing models for uniaxial compression.
A numerical method for the prediction of elastic modulus of concrete
Authors: Li C. Q. , Zheng J. J. , Zhou X. Z. , McCarthy M. J.
Published: October 2009

Concrete is usually considered to be a two-phase material for the prediction of its mechanical properties. More research has, however, shown that there exists an interfacial transition zone (ITZ) between aggregates and cement paste. As such, concrete should be modelled as a three-phase material at a mesoscopic level. This paper presents a numerical method that can predict the elastic modulus of three-phase concrete with reasonable accuracy. In this method, the mesostructure of concrete is simulated and the finite element method is employed for the stress analysis in the concrete. In addition, the method modifies the lattice model to take account of the mechanical properties of each phase constituent of concrete in the finite element analysis. The results produced by the proposed numerical method are compared with those by analytical methods. Finally the numerical results are verified by experimental results obtained from research literature. Based on the results presented in the paper, the effects of the aggregate area fraction, the elastic modulus ratio of aggregate to cement paste and the elastic modulus ratio of ITZ to cement paste on the overall elastic modulus of concrete are examined.
Effects of curing temperature on moisture distribution, drying and water absorption in self-compacting concrete
Authors: Cano Barrita F. de J. , Bremner T. W. , Balcom B. J.
Published: October 2009

Steam curing is normally used in the precast industry to increase the rate of strength development with the aim of reducing the cycle time and thereby increasing productivity. However, there is concern about the durability of steam cured concrete elements, which is mainly determined by the quality of the cover concrete. In an effort to evaluate the effectiveness of curing at 50°C for several hours in producing durable concrete, compared to moist curing at 23 and 38°C for seven days, the moisture distribution of drying self-compacting concrete containing 30% fly ash, as well as the moisture distribution in water uptake experiments were studied using magnetic resonance imaging. The results of drying indicated an increased moisture loss in the cover concrete when the specimens were cured at 50°C. The water uptake experiments showed a higher penetration of the water front, higher sorptivity, and higher moisture diffusivity of concrete cured at 50°C compared to concrete which was moist cured at 38°C.
Reduction of compressive strength of concrete due to triaxial compressive loading history
Authors: Lu J. , Lin G. , Wang Z. , Xiao S.
Published: October 2009

One of the major technical interests for concrete is the damage mechanism under various loading patterns. In this paper the influence of monotonic and cyclic triaxial compressive loading histories upon the compressive strength of concrete cubes is studied experimentally. The maximum axial strain experienced by the cube associated with lateral confining pressure is defined as the loading history. The reduction of compressive strength characterises the degree of damage of these cubes. The laws of damage initiation and development of concrete under the condition of varied confinement pressure are investigated. Based on the results of curve-fitting of experimental data, the evolution equation of damage is obtained. The main factors affecting the damage development are examined and the causes are analysed. It is demonstrated that the method is convenient and applicable to estimation of the damage of concrete in engineering practice.
Reinforcement data for fire safety design
Authors: Hertz K. D.
Published: October 2009

Idealised materials data are derived from a number of test series reported in the literature and made by the author. The data cover a variety of reinforcing steels from mild steel, deformed bars and cold-worked bars to cold-drawn prestressing steels. Processes are described that are responsible for the deterioration of the materials when they are heated and cooled down. A simple expression is established for the description of deterioration curves for fire-exposed materials such as concrete and reinforcement by means of five parameters that may be used as input data for structural fire safety calculations instead of the more troublesome application of curves or tables. This new idealised representation is compared with test data and with other idealised curves for the materials presented, and recommended design values are given. Warnings are given for misleading curves and expressions from the structural codes. The paper serves as a basis for a number of papers presenting calculation methods developed by the author for the loadbearing capacity of constructions of any concrete at any time of any fire exposure. Some of these methods and materials data are adopted in the CEN code and national codes such as the Danish DS411. The paper serves as a part of the documentation for the methods and is therefore also a supporting document for these standards.
Prestress model for shear deformation debonding of FRP- and steel-plated RC beams
Authors: Oehlers D. J. , Liu I. S. T. , Seracino R. , Mohamed Ali M. S.
Published: October 2009

Adhesive bonding plates to the surfaces of RC structures is a widely accepted form of strengthening or stiffening because of the ease of application and unobtrusive nature of this retrofitting procedure. Design guidelines have identified three major forms of plate debonding, one of which is debonding due to the shear deformation across a critical diagonal crack (CDC). CDC debonding is particularly important in RC beams with stirrups, as they are prone to debond before the shear capacity of the beam is attained. In this paper it is shown that the CDC debonding resistance of a plated RC beam is analogous to the shear resistance of a prestressed beam; a design procedure based on the shear resistance of prestressed beams is developed, and correlates well with 77 plated beam tests. In a companion paper the prestress analogy developed here is taken further: it is shown that code approaches for determining the shear resistance of prestressed beams can be used to predict CDC debonding. This has allowed the development of a simple design procedure for adhesive bonding of plates to any surface of an RC beam.
Dip and strike angles method for yield line analysis of reinforced concrete slabs
Authors: Kwan A. K. H.
Published: October 2009

Yield line analysis is a useful method for design of reinforced concrete slabs, but has been limited to slabs of simple geometry, because up to now there has been no generally applicable and fully automatic computational procedure for complex-shaped slabs. Herein, a new yield line method that can be applied to any convex polygonal-shaped slab is developed. In this method, the deflections of the slab regions divided by yield lines are measured in terms of the dip and strike angles of the slab surfaces, which can define the geometry of all kinematically admissible collapse mechanisms or yield line patterns. The external work done and the internal energy dissipation at yield lines are evaluated as functions of the dip and strike angles, and the principle of virtual work is used to determine the corresponding load factor. The final solution is obtained by minimising the load factor with respect to the dip and strike angles. A computer program based on this method has been produced. Its correctness is verified by checking against results obtained by others for simple cases, and its versatility is demonstrated by applying it to complicated slabs subjected to point, line, patch and uniformly distributed loads.
Preliminary views on the potential of foamed concrete as a structural material
Authors: Jones M. R. , McCarthy A.
Published: October 2009

Low self-weight (800 to 1600 kg/m3), high workability (flowing and self-compacting) and excellent thermal insulating properties (< 0·50 W/mK) make foamed concrete attractive for many construction applications. Indeed, it is now a well-established material in void filling and highway reinstatement uses. However, its wider use in structural applications has been inhibited by its technical and engineering unfamiliarity and a perceived difficulty of achieving sufficiently high strength (say > 25 N/mm2). This paper describes a laboratory study of the development of foamed concrete, utilising two types of fly ash, with the potential for use in structural applications. ‘Fine’ fly ash (i.e. to BS EN 450) was used to partially replace Portland cement and a ‘coarse’ fly ash (i.e. to BS 3892-2) to replace sand fine aggregate. In addition, the potential of polypropylene fibres in foamed concrete to enhance plasticity and tensile strength was examined. The key early age, engineering and durability properties were measured and these data show that foamed concrete is indeed viable for structural uses. To further demonstrate the concept, the results of full-scale pilot tests on conventionally reinforced foamed and normal weight concrete beams are also reported and their performance compared with the BS 8110 and Eurocode 2 requirements for serviceability. However, the characteristics of foamed concrete, particularly high drying shrinkage strain and relatively low tensile strength and stiffness performance, mean that straight substitution for normal weight concrete is not possible and more innovative structural forms will need to be developed.
Effect of concrete surface treatment on adhesion in repair systems
Authors: Garbacz A. , Górka M. , Courard L.
Published: October 2009

Existing concrete surfaces need to be roughened to a profile necessary to achieve mechanical interlocking with any repair material. In this study, different surface treatments (e.g. grinding, sandblasting, shotblasting, hand- and mechanical milling) were performed and the quality of the preparation established on the basis of three main parameters: surface geometry, superficial concrete microcracking and adhesion. Surface geometry was characterised on the basis of the measurement of surface profile—profilometry—and the analysis of statistical and amplitude parameters calculated from the waviness (lower frequencies) and the roughness (higher frequencies) profiles of the surface. Investigations were also performed to assess the quality of the superficial zone of concrete and cracks were systematically observed in relation to surface treatment where both scanning electron microscopy and light microscopy were used for analysis. Finally, a repair mortar, with or without bond coat, was applied to the concrete substrates in order to measure adhesion. Relationships clearly show the effect of roughness on adhesion in the case where no bond coat was used and also the influence of the power of the surface treatment on the waviness shape of the profile and the presence of microcracks in the near-surface layer, related to failure type.
Recommendations for shotcrete on rock subjected to blasting vibrations, based on finite element dynamic analysis
Authors: Ansell A.
Published: October 2009

Shotcrete is sprayed concrete applied pneumatically on, for example, a rock surface to prevent fallout of rock blocks and thereby securing the arch-shape of a tunnel profile. A finite element model, especially adapted to the dynamic analysis of shotcrete on rock that is subjected to vibrations from blasting, has been developed and tested. The model consists of spring and beam elements that are used to simulate the behaviour of an elastic concrete area, bound to a rock surface through adhesion. This facilitates the calculation of a two-dimensional displacement field, using mode superposition and Duhamel's integral. The loads applied on the model are accelerations that give rise to inertia forces on the system. The accelerations are calculated from given weights of explosives, geometrical conditions and rock properties. The presented numerical examples demonstrate the response of shotcrete to vibrations, based on material data from an extensive literature survey. A series of calculations are compiled to give recommendations for how close to shotcrete blasting in rock can be allowed.
Punching shear testing of reinforced concrete slabs, and design implications
Authors: Papanikolaou K. V. , Tegos I. A. , Kappos A. J.
Published: October 2009

The paper presents results from an extensive testing programme involving a total of 30 reinforced concrete (R/C) slabs with and without shear reinforcement, subjected to a concentrated load in the middle. Shear reinforcement consists of either bent-up bars or closed stirrups. Measured punching shear strengths were compared with strengths predicted from two major design codes (the American, ACI 318, and the European, Eurocode 2), as well as from two models from the literature, the plasticity model and a recent multi-parameter empirical model. It was found that predictions by both codes were conservative in the case of slabs without shear reinforcement; Eurocode 2 and ACI 318 predictions are generally similar in this case. Less conservative predictions were found in the case of slabs with shear reinforcement, particularly in the case of Eurocode 2 that overestimated measured strengths of slabs with stirrups. The plasticity model tended to overestimate punching strength, whereas the multi-parameter empirical model gave the best overall predictions. Finally, bent-up bars appeared to be more efficient than stirrups in increasing the punching shear strength.
Numerical calculations of vapour pressure in concrete exposed to fire
Authors: Qian C. , Wang H. , Sun W. , Guo Z. , Stroeven P.
Published: October 2009

According to the diffusion equation and the hypothetical relationship between vapour pressure, moisture density and temperature, a differential file method is used to calculate the inner vapour pressure of concrete at high temperatures. When concrete is heated, a distribution field of vapour pressure will be formed. If the temperature changes, the value and the location of the peak pressure in concrete will change. However, the temperature at which the peak pressure is obtained is nearly the same, ranging from 220°C to 240°C. Calculation results show that the vapour pressure reaches peak values at a distance of less than 10 mm from the heating surface when the heating time is not more than 1 h according to the ISO 834 heating regime. The peak pressure values are from 1 MPa to 2 MPa depending on heating time. Some explanations about concrete spalling are given, which indicate that vapour pressure is the main reason for the spalling of concrete.
Comparative study of confinement models for high-strength concrete columns
Authors: Sharma U. , Bhargava P. , Kaushik S. K.
Published: October 2009

The analysis of structural members requires an analytical model for the full stress-strain relationship of concrete in compression both in confined and unconfined states. Many empirical confinement models have been reported in the literature during the last three decades. The models developed for normal strength concrete are not applicable to high-strength concrete, because they overestimate the ductility of high-strength concrete. This paper critically reviews the various proposed stress-strain models for confined high-strength concrete columns. There are very few models for confined concrete with compressive strength in the range of 60–120 MPa. The main objective of the present study is to examine the capabilities of the various models to predict the actual experimental behaviour of confined concrete columns with concrete strengths falling in this range. To this end, twenty-two test specimens were chosen from the reported experimental studies in the literature and their stress-strain curves were compared with the ones predicted by the various models. It was shown that the Legeron and Paultre model estimates the actual practical curves more closely in comparison with other models employed in this study.
Observation of flexural cracks in loaded concrete beams using MRI
Authors: Marfisi E. , Burgoyne C. J. , Amin M. H. G. , Hall L. D.
Published: October 2009

This paper describes apparatus that enables loading of a concrete beam reinforced with aramid fibre-reinforced plastic (AFRP) inside the magnet of a magnetic resonance imaging (MRI) scanner, and the subsequent measurement in three dimensions of the propagation of fractures within the concrete. These are then correlated with the internal structure of the concrete which had been previously determined by scanning the beam soon after casting. The particular requirements of the test frame for use within the scanner and the methods used to satisfy them are described in detail, as is the test procedure. It is shown that the resultant images can be visualised either as two-dimensional (2-D) slices, or as 3-D data sets; the images can also be post-processed to highlight the particular feature of interest.
Shrinkage and creep response of slender reinforced concrete columns under moment gradient: theory and test results
Authors: Bradford M. A.
Published: October 2009

Slender structural members experience geometric non-linearities under eccentric axial compression. Reinforced concrete columns under sustained compression also experience material non-linearities owing to the viscoelastic effects attributable to shrinkage and creep. This paper presents a technique based on expanding the deflected shape of an eccentrically loaded column as a Fourier series to study the response of rectangular reinforced concrete columns under a sustained compressive force applied at each end with unequal eccentricities. The technique allows for the coupling of geometric non-linearity and viscoelasticity, and includes cracking of the concrete. Test results are reported on five slender columns of square cross-section (with L/D ratios of 33·3) under sustained axial compression applied eccentrically, and it is shown that the numerical scheme predicts the experimental results with good accuracy. These test results also add to the smallish pool of accurately conducted experiments on slender columns under sustained loading, and especially provide benchmark test data for unequal end eccentric loading for which test results seem somewhat scarce.
Propagation of reinforcement corrosion in concrete and its effects on structural deterioration
Authors: Li C. Q. , Zheng J. J.
Published: October 2009

Studies of the most recently published literature reveal that the current state of research on corrosion propagation in reinforced concrete and its effects on structural deterioration remains unsatisfactory. This is in spite of extensive and intensive research on reinforcement corrosion in concrete for the past three decades or so. This paper presents results produced from an experimental programme designed to investigate the propagation of chloride-induced reinforcement corrosion in concrete and its effects on structural deterioration. Factors that are deemed to contribute to the propagation of reinforcement corrosion and affect structural deterioration are studied. Structural behaviour of corrosion-affected concrete flexural members is examined in terms of strength, stiffness and bond. A deterioration factor is proposed in the paper in order to analyse the test results in a more systematic manner and to make maximum use of the data for uncertainty analysis. Also presented in the paper is a comparison of the test results on strength deterioration as obtained from both a destructive load test and non-destructive measurements, and a relationship between structural deterioration as determined from the two methods is derived.
Experimental behaviour of high-strength concrete columns in fire
Authors: Benmarce A. , Guenfoud M.
Published: October 2009

Twelve high-strength concrete columns were tested in fire under two heating rates at different load ratio and restraining levels. Imposing axial restraint against column thermal expansion during fire causes additional axial forces to be generated and develops excessive buckling deformations. Therefore, an early failure can take place which may lead to partial failure of the building during fire. This paper reports an integrated parametric experimental study, in which the influence of load level and degree of restraint are investigated together with their interactions. The paper contains main results and conclusions including measured concrete and reinforcement temperatures, axial deformation, temperature failure and additional restraining forces. It is found that, the maximum value of axial deformation is independent of the rate of heating and it is dependent on the load that has been applied during the test. Furthermore, the failure time of columns tested under low heating rate is almost twice the failure time of columns tested under high heating.
Influence of the interfacial transition zone and microcracking on the diffusivity, permeability and sorptivity of cement-based materials after drying
Authors: Wong H.S. , Zobel M. , Buenfeld N.R. , Zimmerman R.W.
Published: October 2009

Available experimental investigations on the effect of the aggregate–paste ‘interfacial transition zone' (ITZ) on mass transport properties of cement-based materials appear to be ambiguous. While some studies have found a deleterious effect of the ITZ, results from others seem to suggest otherwise. The present study examines the relative influences of ITZ and microcracking on the oxygen diffusivity, oxygen permeability and water sorptivity, to further enhance understanding of the links between microstructure and transport properties. Specimens of several neat cement pastes, mortars and concretes were prepared and tested. Variables include water/cement (w/c) ratio (0·3 and 0·5), binder type (CEM I and CEM II with 8% silica fume), curing period (3 and 90 days), aggregate volume fraction (0–70%) and preconditioning temperature (50°C and 105°C). 105°C drying was adapted to induce microcracking. Backscattered electron microscopy and image analysis were applied to characterise the microstructure, in particular the microcracks. It was observed that in all cases, the transport properties of mortars decreased with increasing ITZ fraction. Permeability was far more sensitive to the presence of microcracking and changes in total porosity, compared with diffusivity or sorptivity. However, no critical threshold sand content linked to an ITZ percolation effect was found, even in the deliberately damaged samples. Concretes have about the same diffusivity and sorptivity, but significantly higher permeability than mortars with the same volume fraction of aggregate, despite a lower ITZ fraction. It is argued that the higher permeability of concrete is attributable to more microcracking, and because the microcracks and paste are less tortuous than the equivalent mortar. It is also shown that conventional permeability testing is affected by the sample thickness-to-maximum aggregate size ratio, which may be another factor explaining the disparity between the measured permeability of concretes and mortars.