Showing papers in "Materials and Structures in 1987"

Influence of age of loading, water-cement ratio and rate of loading on fracture energy of concrete

Abstract: The applicability of linear elastic fracture mechanics to a composite material such as concrete has always been questioned. Recently, a new approach to describe failure of concrete has been developed. In this context a material is characterized by its fracture energy and the shape of the descending branch of the strain softening diagram. So far little work has been done to determine experimentally major influences on these material parameters. In this contribution results of test series to study the influence of age of loading, water-cement ratio, and rate of loading are presented. It is shown that a detailed evaluation of the test data necessitates appropriate computer programs. Essentials of these modules are briefly described. It is shown that failure of a beam under three-point bending mode can be predicted in a realistic way if valid material parameters are incorporated in a numerical analysis. Finally, it is pointed out that further studies are needed before a general application of the new approach can be recommended.

Impact behaviour of concrete beams

Abstract: An instrumented impact machine was used to carry out impact tests on concrete beams, 100×125 mm in cross-section and 1400 mm long. The simply supported beams were struck at their midpoints by a 345 kg mass impact hammer, dropped from various heights. The instrumentation included strain gauges mounted on the striking end of the hammer, strain gauges mounted on one support anvil, and three accelerometers placed at various locations along the beam. The data were collected using a 5-channel data acquisition system. Normal strength, high strength, and fibre reinforced concrete beams were tested. In general, it was found that the properties of concrete under the high stress rates associated with impact loading could not be predicted from conventional static tests.

Effect of steel fibres at two different stages: The material and the structure

Abstract: The addition of steel fibres to plain concrete is intended both to improve the material's ductility and that of the structure in which it is used. These two effects of steel fibres are clearly distinguishable through an analysis of cracking both at a local and a global level. This analysis is essential if the use of steel fibres is to be optimized.

Examination of three methods for studying ion diffusion in cement pastes, mortars and concrete

Abstract: Three laboratory techniques for studying ion diffusion in hydrated cement pastes, mortars and concrete are described and compared. In the first method, concentration profiles are produced by analysing material sampled at various distances from the surface exposed to the selected ion, after various periods of exposure. The second technique employs a diffusion cell and calculation of an effective diffusion coefficient based on the diffusion of the selected ion through a thin slice of the material. Thirdly, the electrical conductivity technique is introduced as a useful means of instantly identifying changes in the rate of ion diffusion.

Numerical modelling of concrete cracking based on a stochastic approach

Abstract: Concrete is a highly heterogeneous material, because of its composite structure, but also because of the physical phenomena that take place during hardening (initial stresses, drying shrinkage, heat exchanges). This heterogeneity can explain some aspects of the complex mechanical behaviour of concrete, particularly the transition from uniform to localized cracking and the important size effect. A numerical procedure taking the statistical aspects of this heterogeneity into account has been developed and implemented. It permits us to reproduce and explain the principal experimental results for the behaviour of concrete under tension.

Permissible crack widths in steel fibre reinforced marine concrete

Abstract: The paper presents some results from a continuing study of the marine durability of steel fibre reinforced concrete. The overall aim of the investigation is to develop the material for marine applications. The results reported here pertain to pre-cracked specimens of steel fibre reinforced concrete which were exposed to wet-dry cycles of marine spray in the laboratory simulating tidal zone conditions of exposure. Two types of concrete mixes were used in the investigation—one with standard concrete constituents and OPC and the second replacing about 26% of cement with pfa. The cement content of the mixes was 590 and 435 kg m−3, respectively. Fibre reinforcement was provided by means of low carbon steel fibres and melt extract steel fibres at a vf l/d ratio of 100 and 147. Prism specimens were manufactured and these were precracked to induce cracks of width ranging between 0.03 and 1.73 mm. After cracking, both sealed and unsealed specimens were exposed to laboratory marine spray cycles using sea water. Some control specimens were cured in the laboratory air throughout. Tests were carried out after 650 marine cycles (450 days) and 1450 marine cycles (900 days). Based on data on flexural strength, energy absorption capacity, stiffness and state of corrosion of the fibres, recommendations are made regarding suitable permissible crack widths for the design of steel fibre reinforced concrete for marine applications. The results indicate that a permissible crack width of 0.2 mm is satisfactory for concrete reinforced with melt extract fibres. A smaller value is recommended for concrete reinforced with low carbon steel fibres. Complete healing of open cracks of small widths is observed under exposure to marine cycles.

Detection of crack growth in concrete from ultrasonic intensity measurements

Abstract: Results of monitoring crack growth in concrete during uniaxial compression using ultrasonic methods offer the possibility of determining the internal properties of a concrete member both during and after loading without causing any damage. The ultrasonic transducers were designed to monitor pulse transmission in both the axial and lateral directions throughout a uniaxial compression test. The waveforms received at various stages of loading were digitized, stored and analysed after the test. The crack growth was inferred from the intensity of these ultrasonic waveforms. Axial stresses/strains and transverse strains were also recorded by the use of a data acquisition system. Four different mix proportions were tested and the results obtained are discussed and compared with other available results.

The effect of the shape of the strain-softening diagram on the bearing capacity of concrete beams

Abstract: The effect of the shape of the strain-softening diagram on the bearing capacity of three-point loaded beams, as calculated by the “fictitious crack model”, is analysed. It is shown that if, at peak load, the openings at the nodes along the fracture zone are smaller than the displacement corresponding to the break-point of the bilinear softening approximation, the maximum load only depends on the slope of its initial linear portion. A “limit break-point” (L) is defined such that bilinear strain-softening diagrams with the same initial slope, and whose break-points are located beyond Point L, will lead to the same maximum load of bent beams. The location of that “limit break-point” as a function of initial slope, beam size and notch depth is also studied; the effect of these variables is explained in terms of notch sensitivity.

Validity of using methanol for studying the microstructure of cement paste

Abstract: Experiments were conducted to assess the extent of interaction of methanol with calcium hydroxide at 22°C. The following techniques were used: X-ray diffraction, infrared spectroscopy, surface-area determination, length change, calorimetry, differential thermal analysis and thermogravimetric analysis. It was concluded that either calcium methoxide or a methylated complex is formed when calcium hydroxide is mixed with methanol.

Attempts to improve the bond between cement paste and aggregate

Abstract: Because there are many coatings on the surface of coarse aggregate particles that reduce the concrete strengths through the reduction of the bond between cement paste and aggregate, a laboratory work is presented whose objective was to find such aggregate coating that improves the concrete strength. To achieve this, coarse aggregate particles were coated with a thin epoxy layer, with or without an adhesive promoter, in which either fine sand particles, or unhydrated Portland cement particles were embedded to make the surface rougher, or chemically more reactive, or both. After the hardening of the epoxy, concrete was made with the coated aggregate and concrete strengths were tested. The test results obtained show that the attempts to increase strength were unsuccessful because even the most effective coating, that with unhydrated Portland cement, produced only small increases in strength.

Microcracking of concrete under compression and its influence on tensile strength

Abstract: A new parameter representing damage of concrete due to compressive stresses is investigated. Very important tensile strength losses are obtained after compressive loading over 40% of the compressive strength. Microcracking of concrete takes place under such load leading to tensile strength losses depending also on time under load, water content and type of aggregate. Under maximum compressive load a tensile strength reduction of about 50% is obtained.

Long-term properties of steel fibre reinforced marine concrete

Abstract: The paper presents some results from a continuing research programme which aims to develop steel fibre reinforced concrete (sfrc) for marine applications and to investigate its durability

Model concrete beam-column joints subjected to cyclic loading at two rates

Abstract: Small-scale models of reinforced concrete beam-column joint were subjected to large cyclic displacements at two rates: 2.5×10−3 and 1.0 Hz. To assess damage, free-vibration tests were conducted. The reliability of the modelling techniques was established by comparison of the results for the slower rate with those obtained from the full-scale tests on prototype. The higher rate of loading caused a greater damage than that at the lower rate. This was evidenced by the measurements of the energy absorbed in the hysteresis loops, equivalent damping, and the stiffness and damping obtained from the free-vibration test. The relatively greater extent of damage appears to result from the different bond behaviour at different rates of loading.

Strength of concrete in warm and dry environment

Abstract: This study is an investigation of the effect of simultaneous changes in the mixing and curing temperature (17 and 37°C) as well as the relative humidity of the curing medium (saturation, 75% and 33%) on the strength development in concrete (up to one year). The results indicate, in general, that the deficiency in the humidity of the curing medium, rather than the increase in its temperature, is the dominant factor affecting the strength development. The compressive strengths of the specimens kept in low humidity are reduced by 30 to 46% as compared to those cured in water. However, the warm specimens attain greater strength than the cool ones when all are kept in low humidity. Under field curing conditions, the effect of the temperature difference between the specimens made and kept under direct sunlight and those protected in the shade is not as significant and not as detrimental as the effect of inadequate curing methods.

Fracture energy for three point bend tests on single edge notched beams: Proposed evaluation

Abstract: The research in fracture mechanics of concrete is reviewed in an attempt to reconcile the different, and sometimes contradictory, approaches for evaluation of the fracture energy, Gf. An improved method to measure Gf for beams in three point bending is presented. This method is expected to provide a more reliable characterization of the fracture toughness of concrete.

Porosity of concrete in relation to the nature of the paste—aggregate interface

Abstract: Mercury intrusion porosimetry was performed on hardned-cement-paste-ground-aggregate mixes (concretes). The results were compared with pore size distribution curves obtained for the aggregates and the hardened cement paste, separately. Considerable modification in total porosity and pore size distribution was observed to take place in the combined state of aggregate and paste. The observed development is regarded as indicative of the formation at the aggregate surfaces of an interfacial zone in which porosity is low. This zone is believed to mask the intrudable pores of the aggregates.

Relationships between mechanical properties of cement stabilized materials

Abstract: A laboratory testing programme of granular materials stabilized with cement has been implemented in order to determine the influence of changes in mix composition on their mechanical characteristics. The elements of the composition of stabilized mixes such as gradation, cement content, cement type, density and curing time were varied. The results obtained were analysed statistically and three types of correlation were determined. It is concluded that the relationship between the dynamic modulus of elasticity and the pulse velocity is almost unaffected by individual composition parameters, while the relationship between compressive and tensile strength, and especially that of compressive strength and dynamic modulus of elasticity, is dependent on these parameters.

Probabilistic analysis of the cracking of RC beams

Abstract: Three beams of rectangular cross-section and having same cross-sectional dimensions were tested in two-point bending, over an effective span of 4.2 m. All three beams contained steel only in the tension zone and the tension steel was distributed in three different ways. A deterministic analysis of strains, crack spacings and crack widths for all three beams is carried out and the results are compared with the respective experimental values. A probabilistic analysis of strains, crack spacings and crack widths is performed for all the three beams at different stages of loading.

Structural restoration of wooden beams by means of epoxy resin

Abstract: The structural restoration of deteriorated beam ends by means of epoxy mortar is discussed. The results of a test programme are presented: determination of bond strength as a function of the degree of humidity of the wood; anchoring strength of glass-fibre reinforced polyester bars and steel bars, glued in wood or in epoxy mortar. A comparison is made between the results of these elementary tests, and the behaviour of restored full-size beams. The phenomena leading to rupture of the structural element are described, in order to define the direction of future research.

A new tensile test for concrete

Abstract: A new tensile-test arrangement for concrete is presented, featuring a simple loading frame and an equally simple specimen shape.

Etude de l'extrusion et de la stabilisation par le ciment d'argiles monominerales

Abstract: L'etude presentee porte sur trois argiles monominerales, choisies de facon a individualiser les parametres qui peuvent intervenir dans l'evolution des proprietes de terres stabilisees au ciment. Ces terres sont mises en forme par extrusion sous vide dans le but d'obtenir un materiau de construction economiquement interessant. La montmorillonite ne peut pas etre stabilisee par ce procede car le produit obtenu n'a pas de resistance mecanique et ne presente aucune tenue a l'eau. La palygorskite donne un materiau qui possede une certaine resistance a la compression et a la traction mais qui n'offre qu'une mauvaise tenue a l'eau. Seule la kaolinite permet, apres stabilisation par le ciment, de satisfaire a des exigences de fabrication, de resistances mecaniques, de tenue a l'eau, et de stabilite dimensionnelle. Il est important de souligner l'influence des caracteristiques physico-chimiques (limites d'Atterberg, capacite d'echange cationique) sur ces comportements.

The influence of autocorrelation on OC-lines of compliance criteria for concrete strength

Abstract: For the calculation of Operating Chacteristic (OC) lines of compliance criteria, it is necessary to introduce a model that reflects the stochastic nature of the relevant variable as realistically as possible. As far as concrete is concerned, it was found that a significant autocorrelation exists between consecutive strength values. As shown in the paper, this aspect has important consequences for the resulting probabilities of acceptance as its effect may be represented as a reduction of the effective number of observations. In order to stimulate the consideration of autocorrelation by people concerned with compliance control of concrete, both exact and approximate methods that take into account autocorrelation are presented and the practical application is illustrated.

Aggregates in Saudi Arabia: a survey of their properties and suitability for concrete

Abstract: In Saudi Arabia, aggregates for concrete structures have often been used by the contractors without knowing their properties and requirements. Consequently, damage caused by the use of unsuitable aggregates has been observed on structures to an increased extent. Given the variety of natural aggregates exploited in the different regions, a research programme was established, aimed at providing a general overview of the properties of aggregates in Saudi Arabia.

Use of gel composition as a criterion for diagnosis of alkali-aggregate reactivity in concrete containing siliceous limestone aggregate

Abstract: The silica gel found within alkali reactive limestone aggregates from Montreal, Quebec, Trois-Rivieres and Temiscaming areas was submitted to quantitative chemical analyses with a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). The composition of the silica gel found in the form of small veins within the aggregate particles is almost constant: 51% SiO2, 12% CaO, 8% K2O, 6% Na2O and 2% FeO. The missing 21% is attributed to water of hydration. The uniform composition of the gel permits analysis of it to be used as a means of diagnosing alkali-aggregate reactivity in some concrete structures. Rapid diagnosis is possible by using a SEM (scanning electron microscope) equipped with EDXA (energy dispersive X-ray analyser) for the analysis.

Field investigations of abrasion resistance

Abstract: An extensive field investigation has been undertaken to assess the abrasion resistance of concrete floors in industrial premises. The performance was expressed in terms of the depth of wear produced by an accelerated abrasion machine, which simulated the damage mechanisms in a typical warehouse or factory. The results were used to compare site and laboratory practices, and this demonstrated that results obtained solely with laboratory methods may not be applicable to industrial concrete floor slabs.

Sables calcaires a fines calcaires et argileuses : influence et nocivite dans les mortiers de ciment

Abstract: On a realise un programme de recherches assez exhaustif dans le but de rassembler des donnees pour une prochaine revision de la Norme Espagnole du Beton sur la teneur en fines des sables, actuellement fixee, sans exception, a 5%. On presente les resultats de l'influence des differentes teneurs en fines calcaires et argileuses sur les proprietes mecaniques des mortiers, prelablement a l'etude de cette influence sur des betons confectionnes avec des granulats calcaires de broyage. Cette phase est en cours d'execution a l'heure actuelle. Avant de se livrer a ces travaux experimentaux, on a realise une prospection generale de toutes les carrieres de broyage de calcaire dans la zone nord-est de la peninsule correspondant au Pays Basque. D'apres les resultats obtenus, il semble que la limite pourra s'elever jusqu'a 12 a 15%, a condition d'assurer la presence d'argile a un taux defini par des essais. To obtain data for the next revision of the Spanish Concrete Standard, with regard to the content of sand fines, presently set at 5%, an exhaustive research project was undertaken. The results show the influence of different contents of calcareous and clay fines in the mechanical properties of mortars made with crushed limestone sand, prior to studying the corresponding influence on concrete prepared with this type of aggregate, which is presently being examined. Prior to this research work, a general survey was carried out of all the quarries with crushed limestone in the northwest of the peninsula corresponding to the Basque Country. From these analyses carried out in 44 quarries, an average value of 13%, taken from a range between 7 and 23%, was deduced for its contents in fines, the most frequent value being between 11 and 12%. In this study, we have seen that for clay contents in sand below 2%, 20% of total fines can be achieved with a tendency to increase resistance in cement mortars of equal plasticity. For any set contents of total fines, the increase in proportion of clay in the sand is what really makes the resistance to mortar compression drop noticeably and linearly. It has been verified for example, that a jump of 2% in the contents of clay in sand influences the resistance just as much as a modification in the contents of fines from 3 to 20%. In short, it is important to limit the contents of clay in sand, either by the sand equivalent or methylene blue (analogous to NF P 18-301, Dec. 1983). In this way, calcareous crushed sands with fines much higher than the present 5% specified could be allowed. We think that this amount could be set between 12 and 15% after carrying out the present tests on concrete.