Peter J. Robins

Shear bond testing of concrete repairs

Good adhesion of a repair material to concrete is of vital importance in the application and performance of concrete repairs. This paper reviews and compares techniques and results of bond strength test methods that induce shear, including a tensile slant-shear test. The effect of surface preparation (particularly as it affects roughness and soundness) and modulus mismatch between repair and substrate are illustrated by experimental and theoretical data. While these tests can provide individually useful information on bond strength and a limited picture of bond characteristics, they can, taken in isolation, result in a misunderstanding of the behaviour of bonded cementitious materials. A more complete appreciation can be obtained by consideration of a bond failure envelope that encompasses all possible normal/shear stress states. Such an envelope is presented in the context of Mohr-Coulomb and Griffiths fracture criteria.

Flexural strain and crack width measurement of steel-fibre-reinforced concrete by optical grid and electrical gauge methods

A research programme is discussed, which has investigated the fracture of steel-fibre-reinforced sprayed concrete under flexural loading, with the aim of developing a stress-block model to predict flexural behaviour in the form of a load–deflection response. This paper reports the work associated with establishing the strain and crack width profiles in relation to mid-span beam deflection. A strain analysis technique is described, which combines the use of electrical strain gauges with a semiautomated grid method (using digital image processing) for measuring and monitoring the strain and crack width profile over the depth of a fibre-reinforced beam during a flexural test. This novel strain analysis technique has established strain/crack width data, which forms a key part of a stress-block approach for predicting residual flexural strength, an essential requirement of a much needed design rationale for steel-fibre-reinforced concrete.

Suitability of GGBFS as a cement replacement for concrete in hot arid climates

Three ground granulated blast-furnace slag (GGBFS) concretes (30, 50 and 70% cement replacement) together with an OPC control, all designed for equal workability and 28-day water-cured strength, are compared when subjected to a variety of curing methods and exposure in both a temperate and a hot arid climate. The effect of replacement level on cube and core strengths, ultrasonic pulse velocity, surface hardness, water absorption and permeability are reported. The test showed conclusively that the 50%, replacement level was best and that a GGBFS concrete can be superior to an equivalent all-OPC concrete in a hot climate, provided that proper curing is provided.ResumeOn compare trois bétons de laitiers granulaires broyés (GGBFS) (remplaçant le ciment dans une proportion de 30, 50 et 70%) et un béton de ciment Portland ordinaire (OPC); ces bétons, d’une ouvrabilité identique et d’une résistance à 28 jours en conservation humide, ont été soumis à diverses méthodes de cure et d’exposition, à la fois en climat tempéré et en climat chaud et aride simulé.Les moyens de conservation utilisés étaient une toile d’emballage humide, une feuille de polyéthylène, une membrane de conservation et l’air (c’est à dire sans protection). Immédiatement après le moulage, on a placé les éprouvettes dans une pièce climatique dans laquelle le cycle diurne atteignait 43±2°C et 25 à 30% d’humidité relative, tandis que le cycle nocturne se situait entre 10 et 13°C et 70–75% d’humidité relative. On rapporte l’effet du pourcentage de substituant sur la résistance sur cube et au carottage, sur la vitesse d’auscultation dynamique, la dureté superficielle l’absorption d’eau et la perméabilité à 7, 14 et 28 jours.Les essais ont montré que le pourcentage optimal de laitier était de 50% et qu’un béton GGBFS peut être supérieur sous climat chaud, à un béton OPC équivalent fait uniquement de ciment Portland, pourvu qu’il ait subi la cure appropriée.Dans des conditions arides, certains mélanges de laitiers conservés en atmosphère humide ont montré des résistances à 28 jours plus élevées que le mélange OPC, mais sans conservation appropriée, la résistance a diminué avec l’augmentation de la quantité de substituant d’une manière similaire à celle observée dans des conditions tempérées. L’essai d’absorption d’eau et de perméabilité a indiqué que, dans un climat chaud, le GGBFS est, en général, moins perméable que le béton OPC en cure humide. Toutefois, sans une cure humide, le béton GGBFS a davantage souffert que le béton OPC et a été très perméable. Dans une ambiance tempérée, on a observé des diminutions de la perméabilité pour des taux de laitier s’élevant à 50% et davantage.Non seulement la résistance, mais aussi la vitesse d’auscultation dynamique, l’absorption d’eau et la perméabilité à l’air ont clairement montré que la toile d’emballage humide était le meilleur conservateur, que l’enveloppe de polyéthylène ou la membrane étaient également efficaces (légère supériorité, en général, du polyéthylène), alors qu’en l’absence de protection on obtenait un béton de médiocre qualité.

INFLUENCE OF CURING METHODS ON THE STRENGTH AND PERMEABILITY OF GGBFS CONCRETE IN A SIMULATED ARID CLIMATE

Abstract Problems are frequently encountered in producing good-quality concrete in hot climates. Inadequate curing results in early cracking or porous and permeable concrete, or both; these effects, in turn, make structures prone to reinforcement corrosion and other processes of degradation. This research compares the development of strength and permeability of ordinary Portland cement (OPC) and ground granulated blast furnace slag (GGBFS)-modified concretes which were cured in a simulated arid climate. This was achieved with an environmental room in which temperature and humidity were cycled to imitate a typical Algerian Sahara climate. Four curing regimes were investigated to encompass the range of practical methods encountered on site. Specimens were placed in the hot environment immediately after casting and conditioned for up to 28 days. The strength of the GGBFS concretes was higher than that of the OPC control concrete at all test ages (7, 14 and 28 days) when good curing was provided. Partial cement replacement with GGBFS therefore offers the potential to produce stronger and more durable concrete in hot climates. The disadvantage of GGBFS concretes is that they proved to be more sensitive to poor curing than OPC concrete. In this case, both their strength and permeability, and hence their durability, were seriously impaired. Therefore, special care must be taken when using this type of concrete, especially on site, where the working conditions and the application of curing are not as easy to control as in the laboratory.

Material and fibre losses with fibre reinforced sprayed concrete

Abstract Fibre reinforced sprayed concrete can be an appropriate material for the repair of structures in a variety of situations. These include sites where: casting against shutters is inappropriate; access is difficult or limited in time; and where hand application is uneconomic. This paper reviews the use of this material for repair, including application techniques, typical mixes and descriptions of some previous UK contracts. It then discusses in more detail the factors affecting the losses that occur with sprayed concrete, including rebound, and how these might be minimized. The causes of, and links between material (aggregate plus cementitious) losses and steel fibre losses are also analysed, including the effects of silica fume as a cement replacement. The paper concludes by summarising the implications of material losses for repair applications of fibre reinforced sprayed concrete.

The resistance of steel fibre concrete to VTOL engine jet blast

Abstract A two stage investigation was carried out to assess the suitability of steel fibre concrete as a pavement material for VTOL aircraft. Conventional pavement concretes can spall when subjected to jet blast during take-off, and to a lesser extent landing. Initial laboratory tests produced a fibre mix which complied with PSA pavement quality concrete specifications and which could be suitably finished. In two tests series, twenty-nine slabs were subjected to jet blast simulating Harrier take-off/landing conditions. Thermal imaging and video camera recordings were used to establish the surface temperatures and residence times to first spall for the fibre mix and two control plain mixes under jet exit temperatures in the range 500 to 800°C. The presence of steel fibres marginally increased the residence time to cause spalling for jet temperatures above 650°C. Steel fibres did not appear to influence the rate of increase of surface temperature and resulted in slightly higher spalled areas.

Bond of Lightweight Aggregate Concrete Incorporating Condensed Silica Fume

Synopsis: Condensed silica fume, at up to 30% by weight, was used as a partial cement replacement in lightweight aggregate concrete. The results of round and deformed bar cube pull-out tests, with and without applied lateral stress, show that condensed silica fume increases ultimate bond strength and affects the mechanism of failure. The influence of condensed silica fume on bond stress of round bars was similar at all lateral stresses, producing a 50% increase at 20% by weight replacement of cement. For deformed bars the increase in bond strength was more pronounced at higher levels of lateral stress, producing increases approaching 70% at 20% silica fume content. The improvements in ultimate bond strength with condensed silica fume are shown to only partly result from the associated increases in compressive strength, the greater part resulting from the modified properties of the concrete matrix.

Response of Protective Current to Environmental Conditions During Sacrificial Anode Concrete Repair Treatments

Abstract This work examines the current response of installed “hybrid” (combined impressed current and sacrificial) anodes to changes in environmental conditions and investigates the effect of this...

Toughness Testing of Fibre Reinforced Concrete

An experimental investigation into the effect of beam geometry and test control method on toughness index is described. The results generally support the view that the I 5 , I 10 and I 30 ASTM C1018 toughness indices are unaffected by variation in test spans between 300 and 500 mm. Different post-crack load-deflection behaviour was observed using crosshead and strain (beam deflection) control, resulting in significantly different values of toughness index. Whilst it is common to test under simple crosshead control (which is not in accordance with ASTM C1018), this practice should be reviewed in the light of the different beam behaviour and measured values that result.

Melt Extract Fibre Reinforced Sprayed Concrete

The performance of melt extract and wire fibre reinforced sprayed concretes produced at three field trials in the UK is reported. An emphasis was placed on measuring properties relevant to the applications of the material and in particular the effects of in situ fibre content and fibre type. Flexural strength was found to increase significantly with fibre content in a linear relationship from the unreinforced strength. Core compressive strengths of around 70 MN/m2 were achieved and increased slightly with in situ content. Toughness index, calculated from beam load/deflection curves, increased rapidly with fibre content to over 30 times that of the unreinforced matrix at 7% by weight. Two splitting tests on cores were investigated as possible methods of on site quality control and results indicated linear relationships between flexural and splitting strengths.