Sidney Mindess

Impact testing of concrete using a drop-weight impact machine

A detailed description of the instrumented dropweight impact machine is presented. The instrumentation, the calibration, the inertial loading correction, and the dynamic analysis of a concrete beam specimen undergoing three-point impact flexural loading are described. Some results, using such an impact testing machine, obtained from tests done on plain concrete, fiber-reinforced concrete, and conventionally reinforced concrete are presented. It is concluded that the use of such a testing machine may be successfully made in order to test cementitious materials under impact.

Rheology of high-performance concrete: Effect of ultrafine particles

The effect of ultrafine particles used as partial replacements for cement on the rheology of high-strength concrete (HSC) was investigated. Various proportions of 3 μm and 0.7 μm mean particle size limestone fillers, a finely ground silica, and silica fume were individually used as partial replacement for cement in 0.33 water/binder (w/b) ratio concrete mixtures. In addition, concrete mixtures were made with triple-blended composite cements containing different proportions of limestone filler and silica fume. A rheometer and the slump test were both used to measure the rheological properties at different ages, and the induced bleeding of the various concrete mixtures was investigated. An effort was made to explain why the production of HSC is made rheologically easier when ultrafine particles are added to the concrete mix. The effect of ultrafine particles on the superplasticizer requirement and the effect of time on the rheological characteristics of HSC were also examined. Likewise, the advantages of using triple-blended binders containing microfillers of various particle sizes were investigated.

Microhardness testing of cementitious materials

Abstract The present paper discusses the underlying principles of microhardness testing, addressing the theoretical background and the testing procedures. The advantage and limitations of this technique are highlighted and on that basis guidelines for its proper use in the research of cementitious systems are presented. For proper micro-structural characterization of restricted zones such as the interfacial transition zone (ITZ), there is a special need for adequate preparation of the surface and choice of the right load. For measurements at the ITZ, this load should be in the range of 0.02 to 0.05 N (2 to 5 gmf). The various microhardness profiles obtained next to inclusion surface can be classified and discussed in terms of the influence of a rigid inclusion that should be superimposed on the influence due to a weak ITZ. In the study of the properties of bulk pastes, linear relations were reported between the microhardness value and the compressive strength. The load sensitivity of the microhardness test might be used to generate additional parameters (n, lnK L ) to quantify microstructural characteristics. However, because the interpretation of such parameters is based on empirical relations, they should not be used their own, but in combination with other test methods.

Impact behaviour of concrete beams

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.

The J-integral as a fracture criterion for fiber reinforced concrete

Abstract The J-integral, a one-parameter average measure of the elastic-plastic field near a crack tip was investigated as a fracture criterion for fiber-reinforced concrete. Cement paste, plain concrete and eight different steel and glass fiber concretes in the form of notched beams were tested in bending, and J IC , as well as the linear elastic fracture mechanics parameters K IC and G IC were evaluated. The results suggest that J IC is a promising fracture criterion for all of these, while K IC (or G IC ) almost certainly are not, except for cement paste. In evaluating the effect of steel fiber additions, it was found that a fiber addition of less than 0.75% did not improve the strength or fracture behaviour of the concrete, nor did J IC change very much. However, at higher fiber contents, J IC showed significant increases, reflecting the changed character of the material; K IC and G IC did not.

The microstructure of the steel fibre-cement interface

The microstructure of the steel fibre-cement paste interface was studied by scanning electron microscopy. The interfacial zone surrounding the fibre was found to be substantially different from the “bulk” paste further away from the fibre surface. The interfacial zone consisted of (a) a thin (1 or 2Μm thick) duplex film in actual contact with the reinforcement, (b) outside of this, a zone of perhaps 10 to 30Μm thickness, which, in reasonably well hydrated systems, is largely occupied by relatively massive calcium hydroxide crystals, with occasional interruptions of more porous regions, and (c) outside of this, a highly porous layer parallel to the interface. The interaction of cracks initiated in the matrix with this interfacial zone was observed.

Optimization of high strength limestone filler cement mortars

Abstract The effect of limestone microfilier replacement of cement on the mechanical performance and cost effectiveness of low w c ratio superplasticized portland cement mortars was investigated. The experiments were designed based on a so-called uniform-precision factorial plan. Cement pastes of different w b ratios and incorporating various proportions of limestone powder and/or silica fume were designed to have a constant flow time. Mortars corresponding to the different cement pastes were made, their 1, 3, 7, 28 and 91 day compressive strengths were measured and their cost effectiveness was analyzed. The statistical approach used permitted the calculation of the isoresponse curves for the parameters under study over the experimental domain and the optimization of their effect.

A review of the cement-aggregate bond

Abstract This paper reviews the basic nature of the cement-aggregate bond and its effect on concrete performance. The bond is a result of mechanical interlocking provided by epitaxic growth of cement hydration products on the aggregate surface and of chemical reactions between the cement paste and aggregate. The degree to which the bond results from each of these processes is not known. The effect of the bond on concrete performance is a subject of controversy; however, there seems to be a relationship between concrete strength and bond strength, and it is suggested that the quality of the bond affects concrete durability.

A preliminary SEM study of crack propagation in mortar

Abstract A device constructed to permit the testing of wedge-loaded compact tension specimens of mortar within the sample chamber of an SEM is described. Using this device, the process of cracking was observed in mortar specimens. It was found that the process of crack extension in mortars is very complicated: the crack is tortuous, there is some branch cracking, discontinuities in the cracks are observed, and there is some tearing away of small bits of material in some areas of cracking. The results suggest that the simple fracture mechanics models oversimplify the geometric features of the crack extension process.

Fibre reinforced concrete beams under impact loading

Abstract Impact tests were carried out on small concrete beams reinforced with different volumes of both polypropylene and steel fibres. The drop height of the instrumented drop-weight impact machine was so chosen that some specimens failed completely under a single drop of the hammer, while others required two blows to bring about complete failure. It was found that, at volume fractions less than 0.5%, polypropylene fibres gave only a modest increase in fracture energy. Steel fibres could bring about much greater increases in fracture energy, with a transition in failure modes occurring between steel fibre volumes of 0.5% and 0.75%. Below 0.5%, fibre breaking was the primary failure mechanism and the increase in fracture energy was also modest; above 0.75% fibre pull-out was the primary mechanism with a large increase in fracture energy.