bluuebunny/crossref_metadata_2025 huggingface dataset:
Core point-load test for steel-fibre-reinforced concrete
Authors: Robins P. J. , Austin S. A.
Published: October 2009

Synopsis

The point-load test, more commonly used in geotechnical practice, is evaluated for application in concrete testing as a potential method for estimating the strength of steel-fibre-reinforced concrete. Both diametral and axial test configurations are investigated and the effect of load direction in relation to fibre orientation is explained. Whilst the diametral test is suitable for measuring the strength of random three-dimensionally orientated fibre concrete, the axial test is shown to be more appropriate for assessing the properties of an anisotropic material such as sprayed steel-fibre concrete.
The effect of a Pfa with a high total alkali content on pore solution composition and alkali silica reaction
Authors: Nixon P J , Page C L , Bollinghaus R , Canham I
Published: October 2009

Synopsis

The effectiveness of a pfa with a high total alkali content in suppressing expansion caused by alkali silica reaction has been investigated using concrete specimens containing flint sand. The observed behaviour has been related to the effect of the pfa on pore solution composition. With such a slowly reacting aggregate, typical of those found in concrete affected by ASR in the UK, 20 or 30% of the pfa stopped any deleterious expansion. The effect of the pfa on the pore solution depended on the age and the alkalinity of the cement. At 28 days there was a net contribution to the alkalinity of moderate or high alkali cement pastes but at longer ages the pfa reduced the alkalinity below that produced by dilution with an inert material. In very low lkali cement pastes however there was a net contribution to the alkalinity at all ages up to one year.
Post heat exposure behaviour of reinforced concrete beams
Authors: Khan M. R. , Royles R
Published: October 2009

Synopsis

Consideration is given to the behaviour of reinforced concrete beams after subjection to elevated temperatures in the range 20 to 800°C. Prismatic concrete beams reinforced with plain mild steel and tor bars were cast using the same concrete mix and water: cement ratio. Each beam was heated to a particular temperature and held at that level for one hour prior to coolingfreely in air to ambient temperature and then testing to failure under four point loading. Temperature distributions within the beam sections were monitored during the heating cycle, and bond-slip as well as load-deflection responses were examined in the residual heat treated condition. Crack pattern development was studied arising from both thermal and loading effects and the influence of bar type and grouping were examined.
Determination of specimen-size independent fracture toughness of plain concrete
Authors: Nallathambi P. , Karihaloo B. L.
Published: October 2009

Synopsis

Simple analytical expressions are proposed for determining the critical stress intensity factor Kc, and the critical energy release rate Gc, of plain concretes in three-point bending. These expressions take into account the slow crack growth preceding fracture and the complex state of stress existing at a propagating crack front. The fracture toughness so determined is shown to be essentially independent of the test specimen dimensions and to depend only on the mix variables. It is also argued that unlike Kc, or Gc, the fracture energy GF of plain concretes is strongly dependent on the test specimen dimensions. It is therefore advisable to express the fracture toughness of plain concretes through the specimen-size independent parameters Kc, or Gc.
A method for determining the relative permeability of concrete using gas
Authors: Martin G. R.
Published: October 2009

Synopsis

This paper describes a method of measuring the relative permeability of concrete specimens to gases by passing nitrogen through them. The specimen is a 100 mm cube sealed on two opposite faces and with a central 6 mm hole running between them. Nitrogen from a sealed reservoir at a standard initial pressure is passed into the hole. The reservoir pressure is automatically recorded at suitable intervals, and the rate at which it falls is a measure of the permeability. The test has been shown to be quick and simple to carry out, and to have good repeatability provided specimens are allowed to dry suflciently before testing. The apparatus has also been used to pass CO2 through a specimen, causing its permeability to fall as a result of carbonation. Suggestions for future work to make use of the method are given.
Development of mechanical properties of ordinary Portland and Oilwell B cement grouts
Authors: Domone P. L. , Thurairatnam H.
Published: October 2009

Synopsis

The compressive strength and dynamic elastic modulus of ordinary Portland cement (OPC) grouts typical of those used for structural connections in steel jacket offshore platforms has been measured. The main aim of these tests has been to provide baseline data for grout selection and development.

Variables tested include water/cement ratio (0·3 to 0·5). OPC and Oilwell B cement, fresh and sea-water for mixing, curing temperatures of 8 and 20°C and the incorporation of superplasticisers. Properties have been measured at ages of I day to 1 year.

OPC grouts show a significantly more rapid development of properties at early ages than Oilwell B cement grouts, but the differences are minimal after 7–28 days. Other variables have expected effects on the grout properties.

Good correlations of both strength and modulus with ultrasonic pulse velocity have been obtained, and hence greater use can be made of this non-destructive test in future work.

Values of dynamic Poisson's ratio calculated from the other properties show a dependence on both strength and water/cement ratio.
Equilibrium element analysis of plain concrete and steel-concrete composite cylinders subjected to external pressure
Authors: Choate P. R. , Montague P.
Published: October 2009

Synopsis

An equilibrium element method is derived for the axisymmetric analysis of the mid-length region of thick- walled cylinders of non-linear material construction. The method is used to examine the behaviour of plain concrete and steel-concrete composite cylinders subjected to externalfluid loading. The incremental form of a nonlinear elasticity model is adopted for the concrete and the steel is described be elasto-plasticity.
Relaxation of concrete and its relation to creep under uniaxial cyclic compression at various frequencies
Authors: Brooks J. J. , Forsyth P.
Published: October 2009

Synopsis

Following an earlier creep investigation, relaxation of five-day cyclic compressive stress under constant cyclic strain is reported for frequencies ranging from 1 cycle per day to 1·2 Hz. As with creep, relaxation has to be defined carefully because of the shape of the stress–strain curve. On the other hand, in contrast with creep, there is no increase in relaxation from a frequency of 30 cycles per hour to 1·2 Hz. Relaxation under a cyclic strain can be related to creep but the established methods of estimating stress from strain require modification. Using a modified Effective Modulus approach, the full stress cycle after relaxation can be predicted from static creep.
Reinforced concrete deep beams some test results
Authors: Subedi N. K. , Vardy Alan E. , Kubotat N
Published: October 2009

Synopsis

thirteen simply supported reinforced concrete deep beams with span/depth ratios of 1,2 and 3 are loaded to failure. The percentage of main reinforcement varies from 0·22 to 1·16 and three modes of failure are reported, namely flexure, diagonal splitting and local crushing. Detailed experimental evidence including some strain measurements is presented to enable the development of the failure mechanism to be monitored. The failure loads are compared with values predicted by CIRIA Guide 2, which is shown to be safe provided that failure is by diagonal splitting (the commonest case in practice). It is argued that a need exists for design recommendations for deep beams with larger span/depth ratios than those covered by the Guide.
Non-linear moisture profiles and shrinkage in concrete members
Authors: Terrill J. M. , Richardson M. , Selby A. R.
Published: October 2009

Synopsis

A simple approach to the measurement of moisture variations in concrete prisms is used to produce profiles such as might occur in a waterproofed bridge deck slab. The method relies on segmentation of a number of companion blocks containing crack initiators.

Shrinkage/weight loss curves are produced for small concrete cylinders with varied curing times. Cylinders cured for less than about four weeks show a bi-linear relation between shrinkage and weight loss, with little shrinkage due to early moisture loss and more rapid shrinkage with later losses.

Combination of non-linear moisture profiles with the shrinkage/weight loss relations into a numerical analysis allows tentative estimates of axial shrinkage, curvature and self-equilibrating stresses in a cracked or uncracked reinforced concrete section with or without allowance for creep.
Tension stiffening in transverse sections of circular-voided reinforced concrete slabs
Authors: Oduyemi T. O. S. , Clark L. A.
Published: October 2009

Synopsis

For an accurate assessment of the service load response of a concrete structure, the tension stiffening effect of concrete between major cracks needs to be taken into account. Previous studies of tension stiffening have been mainly concerned with solid beams and slabs. At present there is no generally accepted method for determining tension stiffening in voided slabs. This paper presents the results of tests on fifteen 1: 3·33 scale model transverse strips of practical reinforced concrete voided slabs. A method of predicting tension stiffening in transverse strips of voided slabs is presented, and a simplified procedure suitable for design purposes is proposed.
Prediction of crack widths in circular-voided reintorced concrete slabs subjected transverse to bending
Authors: Oduyemi T. O. S. , Clark L. A.
Published: October 2009

Synopsis

When concrete slabs containing longitudinal circular voids are subjected to transverse bending, a complex stress pattern occurs and the first cracks appear at the inverts of the voids. If sufficient reinforcement is provided in the tension flange, subsequent outer-face cracks appear at about a quarter of the void diameter each side of the first cracks. Previous studies of the cracking behaviour of transverse strips of voided slabs have shown that, due to the predetermined and highly-concentrated type of cracking, very wide and uncontrollable cracks sometimes occur. Since the equations which are currently used for predicting crack widths in solid slabs do not take into consideration the effect of the presence of voids on the crack formation process, a new method is needed for predicting the widths of cracks in voided slabs subjected to transverse bending. This paper presents a suitable method which gives reasonable predictions of the widths of such cracks. The method was evolved from both theoretical and experimental investigations involving tests on fourteen 1: 3·33 scale model transverse strips of practical voided slabs. Consideration is also given to the amounts of reinforcement which are needed (a) to prevent premature yielding of the steel, (b) to achieve a controlled crack pattern, and (c) to limit the widths of the cracks to acceptable levels as specified in BS 5400, the British Standard Code of Practice for concrete bridges.
Near-surface characteristics of concrete: assessment and development of in situ test methods
Authors: Dhir R. K. , Hewlett P. C. , Chan Y. N.
Published: October 2009

Synopsis

The British Standard initial surface absorption test (ISAT), Figg hypodermic methods (air and water), and a new test, the covercrete absorption test (CAT), have been used to study the permeation properties of the near-surface of concrete. The test methods are critically appraised. It is shown that the moisture condition of the concrete can greatly alter the results obtained. Data on the surface quality of concrete and its dependence on mix proportions and degree of moist curing are presented.
Investigation of the mechanism of shear failure of reinforced concrete beams
Authors: Chana P. S.
Published: October 2009

Synopsis

The mechanism of shear failure in reinforced concrete beams was studied. Crackwidths at critical locations in the shear span were monitored continuously by means of a high speed tape recorder in conjunction with electrical demountable strain transducers and related to the load history approaching failure. It is shown that beam failure is preceded by splitting at the level of the steel; this is identijied to be the primary cause ofshear failure for slender beams. Further tests are described on beams where this splitting is prevented by the presence of links. In these tests, a diflerent failure mechanism isobserved with SignlJcant increase in shear capacity. The implications of theseJindings ondesign methods in codes of practice are discussed.
Decomposition of a concrete floor and ignition of gaseous products due to spillage of molten aluminium
Authors: Shayan A. , Gibbs P , Martin K. G.
Published: October 2009

Synopsis

Spillage of molten aluminium onto the concrete floor of the Bone Island Aluminium Smelters in Queensland, Australia has caused several serious fires which have decomposed the floor and sometimes partially melted the pot assembly, resulting in enormous repair costs and delays in production. A study of the concrete showed that it was made with limestone aggregate. The molten aluminium at about 1050°C formed ponds about 2 m in diameter and 200 mm deep. It decomposed the limestone and produced CO2 gas which was reduced by the molten aluminium to CO gas which ignited. This strongly exothermic reaction produced further heat and melted the reinforcing bars in the concrete. The moisture in the concrete formed steam and agitated the system so that the molten metal was more efectively in contact with the concrete and caused further decomposition of the limestone. Laboratory experiments with limestone and non-calcareous aggregates, and concretes made with them, showed that the limestone was the primary cause of the fires. Based on computer modelling, it was determined that a concrete capping of 75 mm thickness made with non-calcareous aggregate and applied on the existing floor would protect it from decomposition and ignition.
Microstructural studies of Jurassic limestone aggregates with reference to durability in concrete
Authors: Collins R. J.
Published: October 2009

Synopsis

The pore structure of 15 samples of porous limestone has been studied by mercury intrusion porosimetry and optical microscopy. This followed laboratory freeze/ thaw tests in which concretes made with porous lime- stone coarse aggregate performed better on average than control concretes made with low porosity coarse aggregate. Good durability was correlated with relatively low porosity (less than 0·05ml/g) in pores less than 0·2pm nominal radius, together with an adequate volume of pores larger than 0·2pm radius. More of these larger pores are required in weaker aggregates which are less able to withstand internal freezing pressures. These criteria appear to be appropriate only to climates similar to the UK and not to continental or arid climates.
A repeated drop-weight impact testing apparatus for concrete
Authors: Barr B. , Baghli A.
Published: October 2009

Synopsis

The development and use of a versatile instrumented repeated drop-weight impact testing apparatus for concrete is described. Impact resistance is determined in terms of the number of blows required to completely fracture steel-fibre-reinforced concrete test specimens. Parameters investigated included specimen geometry, notch depth, steel-fibre concentration, and mass and height of the dropped weight.

Coefficients of variation of 15–20% were measured. Two load cells were used to monitor the impacting loads and the corresponding loads at the support, from which load-time graphs were plotted. Peak loads developed, impulsive loads and the duration of the impact event could then be measured. Good reproducibility was observed from the peak load results. The graphs also showed that larger masses were more efficient than lighter masses in producing impact failure in the test specimens.
Snap-back and hyperstrength in lightly reinforced concrete beams
Authors: Carpinteri Alberto
Published: October 2009

Synopsis

The phenomena of hyperstrength and snap-back in lightly reinforced concrete beams are interpreted according to the concepts fracture mechanics. The tensile strength and toughness of concrete, usually disregarded, are so high in some cases that the maximum bending moment overcomes the bending moment of limit design (hyperstrength). The drop in load capacity hides a virtual sofiening load-deflection branch with positive slope (snap-back), which can be detected if the loading process is controlled through the crack width.
Early-age strain and temperature measurements in concrete tank walls
Authors: Anson M. , Rowlinson P. M.
Published: October 2009

Synopsis

The paper reports measurements of temperatures induced by heat of hydration in 12 reinforced concrete walls during construction. In-plane movements at early age are also given for one of the walls in which 27 vibrating wire strain gauges were installed. In-plane strains are deduced and estimates made of the restraint to free in-plane movement of the concrete.

The concept of a restraint,factor is already applied in design for crack control. Restraint to free movement us cooling of the new concrete takes place, is provided externally by previously cast elements such as a all foundation, combined with internal effects, which arise from the fact that peak temperatures vary considerably between different parts of the wall itself.

The experimental strain results are compared with the results of a finite element analysis in which base and wall panel are treated as an integral structural unit. Comparisons are also made with codes of practice and other guidance documents.
Effects of microporosity on the compression strength and freezing durability of high-strength concretes
Authors: Penttala V. E.
Published: October 2009

High-strength concretes were produced using four different binder combinations. Curing and heat treatment tests were conducted and the results were used to validate a theory relating binder paste microporosity, concrete compression strength and freezing durability. The porosities of the test concretes were measured by mercury porosimetry and the microporosity of the hardened cement paste by nitrogen adsorption. Thin-section microscopy was also used. Compression and flexural strength values of the test concretes were obtained. The freezing durability of the test concretes were examined by a freeze-thaw test in water and by a salt freeze test, where freezing and thawing took place in a salt solution.
Potential uses of Jordanian spent oil shale ash as a cementive material
Authors: Smadi Mohammad , Yeginobali Asim , Khedaywi Taisir
Published: October 2009

Retort residue ash of El-Lajjun oil shale from central Jordan has been studied for its cementitious and pozzolanic properties and potential uses in construction. During the preliminary phase, properties of the ash were determined and some tests were performed on pastes of ash, cement and their mixtures. According to X-ray diffraction and thermogravimetric studies, the ash mainly consisted of calcite and silica and some lime silicates, providing cementitious character. Its hydration resulted in ettringite and lime silicate gel formation. The ash paste had relatively low setting times and hardened into a lightweight porous structure. Its 28 day strength was about half of the cement paste value for the same water content. The ash had weak pozzolanic activity. In blended pastes, addition of ash up to 30% caused some reductions in 28 day strengths. There seemed to be optimum ash contents which improved compactibility while causing minimum strength reductions. The results indicated that the ash could be used to produce lightweight masonry and building units having medium strengths. It showed potential for use as an admixture in cement and concrete.
Properties of pelletized blastfurnace slag concrete
Authors: Mayfield B. , Louati M.
Published: October 2009

The results presented here were obtained as part of an investigation into the use of pelletized blastfurnace slag (pbfs) as a lightweight aggregate in structural concrete. They cover the basic properties of the concrete: compressive and tensile strengths, bond between concrete and reinforcement, elasticity moduli, ultrasonic pulse velocity and carbonation for both pbfs only and pbfs/ sand mixes. These properties were examined as a function of the age of the specimens with various curing conditions.
Dynamic response of latex modified concrete machine foundations
Authors: Sridharan A. , Nagaraj T. S. , Chandrakaran S.
Published: October 2009

New materials in concrete constructions have been widely used to improve various properties such as impact resistance, strength and durability. Polymer modified concrete is one of the new materials which has been developed for potential application in the construction industry. This Paper describes the use of polymer latex for foundation blocks subjected to dynamic loads. Experiments were conducted using ordinary concrete and latex modified concrete footings of three different thicknesses, for three static loads at four excitation levels. Experimental results have revealed that the amplitude of resonance is reduced considerably in the latex modified concrete footings. It was also observed that the damping factor of the latex footing–soil system is considerably larger than that for ordinary concrete footings. Hence the use of latex in concrete foundations for machines may lead to more economical designs.
The quantitative evaluation of the water/cement ratio using fluorescence microscopy
Authors: Mayfield B.
Published: October 2009

The water/cement ratio is a prime parameter in determining a concrete's strength and durability. The British Standard method for determining the ratio in hardened concrete involves the separate determination of the original cement and water contents. It is acknowledged that the accuracy of this method is strongly dependent upon the availability of separate samples of the original cement and aggregates. A direct method using thin-section optical microscopy has been introduced in Denmark but the method described here uses reflected light fluorescence microscopy. Results are presented for hardened cement pastes and mortars, together with some preliminary results for concrete.
The cylinder splitting test as a method for determining the compressive strength of concrete
Authors: Clayton N.
Published: October 2009

Research into the effects of concrete cracking caused, for instance, by alkali—silica reaction, has shown that the cylinder splitting or Brazilian test does not reflect the tensile strength but rather the compressive strength as would be measured from prisms with a height/width ratio of about 3. This Paper demonstrates that the cylinder splitting test may be used as a compression test using the appropriate bearing width, which is derived to be about one-seventh of the cylinder's diameter. If the bearing width is increased above one-third of the cylinder's diameter with special loading platens, the effective height/width ratio is reduced resulting in the same enhancement of measured compressive strength as occurs in the standard cube test. Practical use of the method in relation to alkali—silica reaction is described and the consequences are stated.
A test method to establish realistic bond stresses
Authors: Chana P. S.
Published: October 2009

The bond strength of reinforcement in concrete can be measured by means of various test methods. There is a considerable difference of opinion amongst engineers regarding the most appropriate test method relating to practical design. This Paper describes a practical test method to determine the bond strength. It is shown that the bond strength using this test arrangement can be directly compared with bond strengths given in design codes.
Local bond behaviour of deformed bars in steel fibre reinforced concrete joints
Authors: Soroushian P. , Bayasi Z.
Published: October 2009

The local bond behaviour between deformed bars and .steel fibre reinforced concrete in specimens representing reinforced concrete joints was studied experimentally. The effects of fibre reinforcement on local bond behaviour is confined and plain concrete specimens were assessed. Test results indicated significance improvements of bond stress deformation behaviour due to the steel fibre reinforcement. While transverse reinforcing bars were found to have a negligible effect, longitudinal reinforcing bars were effective in enhancing ultimate and post-peak bond resistance of anchored bars at fibre reinforced concrete joints.
Influence of free water in concrete on the cracking process
Authors: Rossi P. , Boulay C.
Published: October 2009

An experimental study has shown that the presence of free water in a microconcrete renders its fracture toughness dependent on crack velocity. This leads to the conclusion that a wet concrete does not have an intrinsic fracture toughness even under quasisteady crack propagation conditions.
Significance of property mismatch in the patch repair of structural concrete Part 1: Properties of repair systems
Authors: Emberson N. K. , Mays G. C.
Published: October 2009

Approximately £500 m is spent in the UK each year on the repair of concrete members. Despite the existence of this large and expanding market, little fundamental attention has been paid to the structural implications of property mismatch between patch repair materials and the substrate reinforced concrete. To pursue this question, it was necessary to identify the desirable characteristics of repair systems for satisfactory structural performance, and then to measure the relevant mechanical and physical properties of the various systems currently on the market. It was found that these repair systems could be conveniently categorized in nine generic types; a representative system from each was selected. Properties measured included strength, modulus, Poisson's ratio, coefficient of thermal expansion, tensile adhesion, early curing shrinkage, long-term creep and shrinkage. Where deemed appropriate, standard test methods have been utilized; otherwise techniques have been developed specifically for the purpose. These techniques, some of which now form the drafts of revisions to British Standards, are described. The range of property values determined are discussed in relation to patch repairs in typical reinforced concrete members and the desirable characteristics previously identified.
Significance of property mismatch in the patch repair of structural concrete Part 2: Axially loaded reinforced concrete members
Authors: Emberson N. K. , Mays G. C.
Published: October 2009

Approximately £500 m is spent in the UK each year on the repair of concrete members. Despite the existence of this large and expanding market, little fundamental attention has been paid to the structural implications of property mismatch between patch repair materials and the substrate reinforced concrete. Part 1 of this Paper has discussed the measurement of mechanical and physical properties of repair systems considered relevant to the subsequent structural performance of a repaired member. Using the range of material properties determined for nine generically different systems, two- and three-dimensional linear elastic finite element techniques have been used to elucidate axial load transfer through simple patch repairs in reinforced concrete members. Areas of localized concentrated stress and requirements for especially high values of adhesion become apparent when the modulus of the repair material differs significantly from that of the concrete substrate. Comparison of the finite element results with those from identical experimental prisms repaired with the nine selected systems showed good agreement within the elastic range, provided that there were no interfacial adhesion failures. Such failures occurred with polyester resin mortars and unmodified cementitious mortars due to early shrinkage.
Rapid estimation of chloride diffusion coefficient in concrete
Authors: Dhir R. K. , Jones M. R. , Ahmed H. E. H. , Seneviratne A. M. G.
Published: October 2009

A simple potential dfference (PD) test is developed that can rapidly (within ∼ 10 days) provide a chloride diffusion index. The test utilizes a small applied potential difference (10 V dc) to accelerate the transport of chloride ions through concrete. Conventional nonperturbative, concentration difference (CD) tests have been carried out in parallel, with typical test periods of up to 12 months. Fick’s First Law of Diffusion is applied to calculate a diffusion index from the PD test, and the coefficient of chloride diffusion D from the CD test. A close relationship between these two parameters is shown, allowing D to be estimated from the rapid PD test.
Development of fracture process zones in concrete
Authors: Raiss M. E. , Dougill J. W. , Newman J. B.
Published: October 2009

Concrete specimens have been tested in uniaxial tension using a closed loop servo-controlled testing machine. The combination of extension control and stiff testing frame has allowed stable failure to be achieved and the full post-peak portion of the load/extension relationship to be obtained. Full field surface strain of the specimen has been measured using a Moire interferometry technique. The development of fracture process zones has been observed, together with their influence on the surrounding material. Localization in the strain field occurred before the attainment of the maximum load, with subsequent propagation during the post–peak portion of the load/extension relationship. It has been shown that the load/extension relationship obtained from a test is not a material property. A more appropriate method for describing the behaviour of fracture process zones is to relate the opening of the zone at a given location in the specimen with its stress transferring capacity.
Use of the ViscoCorder to study the rheology of fresh mortar
Authors: Banfill P. F. G.
Published: October 2009

The ViscoCorder is a small variable speed consistometer that can be used to determine the rheology of fresh mortar. It shows that the downcurve conforms to the Bingham model, and that two parameters, yield value and plastic viscosity, are required to characterize it. When the instrument is calibrated with model fluids these parameters can be calculated in fundamental units. The range of numerical values obtained agrees well with exiting data.
A torsion test to evaluate the deterioration of concrete due to alkali-aggregate reaction
Authors: Norris P. , Wood J. G. M. , Barr B.
Published: October 2009

A new type of strength test is described in which concrete cores are subjected to torsional loading. The resulting shear stress state, on a representative volume of concrete, is used to evaluate the shear strength properties of the concrete. Two rigs have been developed: one for site use and one for more refined laboratory testing. The rigs and test procedures are described, some preliminary test results are presented, and the significance of the results is discussed. It is suggested that the method may have a wide application in quality control of new construction and assessment of deteriorating concrete strength in existing structures.
Sulphate attack on concrete
Authors: Lawrence C. D.
Published: October 2009

The types of disruption observed during sulphate attack on mortars or concretes, and the chemical processes involved, are summarized. Concrete mix design and binder type have an important influence on concrete performance in sulphate environments. The present UK Codes of Practice (based on BRE Digest 250) reflect the technical information available for various binders currently used in the UK. Sulphate-resisting Portland cement is a purpose-made cement designed to resist sulphates. The chemical factors giving rise to its sulphate resistance are well understood, and its performance has been uniformly good in experimental trials throughout the world. This cement is the standard by which other binders are judged, and should continue to be the preferred binder for concretes expected to withstand sulphate concentrations in ground waters greater than 3000 mg SO4− −/l. Although partial replacement of ordinary Portland cement (OPC) by various latent hydraulic binders can lead to improved sulphate resistance, the mechanism by which these latent hydraulic binders act is not established. There is well-documented evidence of considerable variability of performance for OPC/pulverized fuel ash and OPC/ground granulated blastfurnace slag combinations, and the ability of composite cements to provide a guaranteed performance is suspect. A more rigorous approach towards the selection of composite binder components in the appropriate standards is required before their use can be recommended in ground waters where sulphate levels exceed 3000 mg SO4− −/l.
Compressive and tensile creep of heat-cured ordinary Portland and slag cement concretes
Authors: Brooks J. J. , Wainwright P. J. , Al-Kaisi A. F.
Published: October 2009

Experimental compressive and tensile short-term creep data are reported for ordinary Portland cement (OPC) and slag cement concretes cured under temperature-matched conditions up to peak temperature and then cooled under ambient conditions. The influence of replacement of cement by slag on deformation is analysed, together with that of temperature. An increase of temperature during creep does not appear to cause 'additional' transitional thermal creep and, for the same maturity, creep of slag cement concretes is less than that of OPC concrete. For all concretes, tensile creep is greater than compressive creep, especially after short times under load. Empirical expressions are given that yield creep functions from a knowledge of the strength and temperature history of concrete.
Factors influencing relative humidity in concrete
Authors: Parrott L. J.
Published: October 2009

The effects of water/cement ratio, cement type, curing and exposure condition on time-dependent relative humidity profiles in drying concrete are examined. The water/cement ratio of the concrete and the period of initial moist curing were found to have little effect on the measured relative humidities for laboratory exposure, but the latter dropped more rapidly in concretes made with pulverized fuel ash or ground granulated blastfurnace slag than in those made with ordinary Portland cement. The relative humidities in the concretes subjected to laboratory exposure were represented concisely by a set of simple equations. High relative humidities were measured in concretes exposed outdoors, although these were less high when the exposed surface was sheltered from rain. The effects of moisture on several aspects of concrete performance are briefly reviewed, and the benefits of relative humidity measurements are indicated.
Influence of cracking in the presence of free water on the mechanical behaviour of concrete
Authors: Rossi P.
Published: October 2009

The free water content of concrete underlies the various physical mechanisms that shape its mechanical behaviour. This Paper attempts to show that properties from creep to dynamic behaviour can be explained with reference to the process of cracking. Assumptions are made concerning the physical mechanisms involved, with a view to understanding what happens inside the material rather than to give rise to quantitative predictions.
A preliminary investigation of strains in reinforced concrete beam—column connections due to seismic loading
Authors: Scott R. H.
Published: October 2009

Two reinforced concrete external beam—column connection specimens were subjected to a series of slow load reversals to simulate seismic effects. Strain distributions were measured using internally strain gauged reinforcement. Specimen C3S, with a 210 mm deep beam, failed after 12 load cycles due to a plastic hinge which developed in the beam at the face of the column; specimen C9S, with a 300 mm deep beam, failed after 10 load cycles due to plastic hinges which developed in the column above and below the beam. Strains of over 17000 microstrain were recorded at the beam hinge in C3S; those in the column hinges of C9S were over 5000 microstrain. Large residual strains were also observed when the load was reversed. Peak bond stresses of∼ 5 N/mm2 were recorded early in each test, but the considerable disintegration of the connection zone caused by the load cycling resulted in an almost complete loss of bond in the connection zones by the end of each test.
The water permeability of concrete and its relationship with strength
Authors: Bamforth P. B.
Published: October 2009

As part of a much broader programme to evaluate the performance of concretes for use in reinforced and pre-stressed containments for liquid gases, seventeen concrete mixes, ranging in strength from 16 to 100N/mm2, were subjected to screening tests by measurement of the water permeability coefficient. Specimens were stored in sealed conditions at 20° C and tests were also carried out to determine the compressive and tensile splitting strengths. The relationships between permeability and strength are discussed, as well as the influence of concreting materials, mix proportions and curing.
the moisture condition in an exposed structure damaged by alskli-silica reaction
Authors: Blight G. E.
Published: October 2009

This study demonstrates the difficulty of attempting to exclude moisture from concrete in structures exposed to the weather, especially if they have been cracked by ASR. Once the concrete has gained moisture it does not easily dry out, even if there is a well-defined dry season. If a waterproofing treatment is applied, the moisture may be sealed into the concrete. If present, leaking internal drainage ducts may exacerbate the problem, and promote the accumulation of moisture.
Durability of cement sheets reinforced with fibrillated polypropylene networks: 10 year results
Authors: Hannant D.J.
Published: October 2009

Cement sheets reinforced with layers of networks of fibrillated polypropylene film have been exposed to natural weathering, underwater storage and storage in laboratory air for periods up to 10 years. The effect of these exposure conditions on the stress-strain performance of the composite in uniaxial tension is described, and different characteristics are apparent for each exposure condition. The bond strength between film and cement has been shown to be independent of time and storage condition, and the film strength also shows little change with time. Important considerations from the potential user's point of view are that 10 years of freezing and thawing under UK winter conditions have not significantly affected the composite as measured by the initial elastic modulus, and the material has remained ductile with a failure strain in excess of 5% after 10 years underwater.
Characteristics of corrosion inhibition admixtures in OPC paste with chloride additions Part II: Microstructures and mechanisms
Authors: Sagoe-Crentsil K. K. , Jin S. X. , Glasser F. P.
Published: October 2009

The changes in ordinary Portland cement paste micro-structure resulting from admixed nitrite, chromate and phosphate-based corrosion inhibitors have been investigated by analytical electron microscopy techniques. The mechanisms by which these admixtures inhibit corrosion are also examined in accelerated experiments, made at 50°C, using pure iron powder embedded in the mix and from the chemical and electrochemical characteristics of the pastes reported in Part 1 of this Paper. Both the morphology and distribution of the principal products of cement hydration, notably calcium hydroxide [Ca(OH)2] and calcium-silicate-hydrate (C-S-H), are found to be dependent on the admix anion type. Thus the admixes condition characteristic differences in paste microstructures. Energy-dispersive elemental X-ray mapping indicates the precipitation of large quantities of chromium salt in the cement matrix and adjacent to the embedded iron particles, while the phosphate admix appears to enhance the formation of lath-shaped crystals of Friedels salt. The effect of microstructural changes on the processes of corrosion inhibition are discussed.
An experimental critique of the BS 8110 method of estimating concrete elastic modulus
Authors: Alexander M. G.
Published: October 2009

The elastic deformation characteristics of concretes made with thirteen common aggregate types are described. The objective of the investigation was to provide data relating to concrete stiffness, and thereby to assist the engineer in making realistic estimates of the elastic modulus E of concrete. The E values of a variety of rock types were measured; aggregates crushed from these rocks were used to make concrete mixes covering a broad range of strengths and tested for E at ages up to six months. The results clearly demonstrate the powerful influence of aggregate type on E. Guidance is given on estimating the E of concrete, and a modification of the BS 8110 approach is suggested.
Mechanical properties of materials used in concrete blockwork construction
Authors: Khalaf F. M. , Hendry A. W. , Fairbairn D. R.
Published: October 2009

This experimental investigation is intended to study and determine the difference in strength of unfilled and filled concrete block specimens compressed in two orthogonal directions. It also studies the effects of using different concrete infill mixes on the block compressive and splitting strengths, and the mechanical properties of the materials used in blockwork masonry construction. The materials involved were the concrete block, concrete infill and mortar; one type of concrete block, three mixes of concrete infill and three types of mortar were used. The characteristics studied were the stress versus strain curves, the modulus of elasticity, the Poisson ratio, the block compressive strength and the splitting strength, the shrinkage after 28 days, the cohesive bond between the block and concrete infill, and the shear strength. The Paper gives values and relations to determine the above characteristics and also compares them with some standardized ones.
A new technique for the measurement of the electrical resistivity of concrete
Authors: Lakshminarayanan V. , Ramesh P. S. , Rajagopalan S. R.
Published: October 2009

The measurement of electrical resistivity is quite important in the study of concrete properties, since it provides information on permeability and hence the durability and strength. In addition, the rate of transport of aggressive ions like chlorides and sulphates, which lead to corrosion of the reinforcing steel, is controlled by permeability. In this Paper a simple and elegant technique for measuring the electrical resistivity of concrete is described. The method consists in the application of a current pulse to the steel–concrete system immersed in an electrolyte, and measuring the amplitude of the potential step, either using an oscilloscope or a digital voltmeter through an interface circuit. Using this technique the variation of the resistivity of concrete has been measured as a function of the curing time. The resulting curve has been found to follow a hyperbolic expression; this is correlated with the curve of the variation of compressive strength with time.
Factors controlling the compressive strength of silica fume concrete in the range 100–150 MPa
Authors: Novokshchenov V.
Published: October 2009

Effects of various factors on the compressive strength of silica fume high-strength concretes were investigated. Trial batches were prepared using commercially available materials and conventional experimental techniques. Tests showed that ASTM Type I and II cements gave strengths higher than Type III cement. The optimum cement content range was 504 to 623 kg/m3. The brand of a silica fume had a significant effect on compressive strength of concrete. The optimum amount of the optimum silica fume was 20% by weight of cement. ASTM Class C fly ash when added at a low dosage rate had a beneficial effect on the strength of concrete. The highest increase in the concrete strength was observed with the naphthalene-based superplasticizer and highly purified lignosulphonate based normal water reducer added at a dosage of 163 and 23cm3/100 kg of cement + silica fume, respectively. The optimum sand content, in terms of the strength and workability was within the range of about 653 to 890 kg/m3. Contrary to a general belief, the gravel was found capable of producing good quality high-strength concretes. An increase of the concrete mixing temperature from 16 to 27°C had a beneficial effect on the strength of hardened concrete. In general, the type and brand of materials were observed to exercise a significant effect on compressive strength of silica fume concretes. Therefore, while obtaining high-strength concrete it is important that all ingredients were selected based on results of laboratory trial mixtures. Tests showed that the compressive strength of silica fume concrete was noticeably affected by type of specimens and testing procedures. It is believed that this strength characteristic will be properly represented if determined on 102 × 203mm cast in steel moulds concrete cylinders, lapped and tested after 56 days of curing under moist conditions.
Diffusion of chloride and hydroxyl ions in cementitious materials exposed to a saline environment
Authors: Sergi G. , Yu S. W. , Page C. L.
Published: October 2009

One end surface of a cylindrical specimen of hardened OPC paste of water/cement ratio (w/c) 0·5 was exposed to 1 mol/l NaCl solution saturated with Ca(OH)2. The free hydroxyl, free chloride and total chloride concentration profiles were determined after 100 days of exposure by analysis of the paste at various depths from the exposed surface. Attempts were made to characterize these concentration profiles by Fick's laws. It was found that the total and free chloride profiles could be interpolated by Fick's second law to a fairly good approximation, but the hydroxyl concentration profile near the exposed surface deviated from such an interpolation. It was also confirmed that the relationship between free and bound chloride could be approximately represented by the Langmuir adsorption isotherm. The effect of chloride binding on chloride diffusion was determined by consideration of mass conservation, and this yielded a good characterisation of the free chloride profile. It is suggested that a reasonably accurate estimation of the experimentally determined hydroxyl concentration may be obtained from charge balance consideration after diffusion of all the other ions besides hydroxyl has been modelled.
Effect of superplasticizers on the plastic shrinkage of concrete
Authors: Cabrera J. G. , Cusens A. R. , Brookes-Wang Y.
Published: October 2009

This Paper presents the results of a study on the effect of five different superplasticizers on plastic shrinkage of concrete. Test specimens of fresh concrete were subjected to a simulated hot dry environmental condition of 35°C, 50% RH and a wind velocity of 3·5mls immediately after casting. Measurements were made of surface tension, bleeding rate and negative capillary porewater pressure. It was found that the time of occurrence of an initial peak or plateau of capillary porewater pressure is related to surface tension and the development of plastic shrinkage.