Arnon Bentur

PREVENTION OF AUTOGENOUS SHRINKAGE IN HIGH-STRENGTH CONCRETE BY INTERNAL CURING USING WET LIGHTWEIGHT AGGREGATES

Restrained autogenous shrinkage in high-strength lightweight aggregate concrete was investigated. Effects of a partial replacement of normal-weight aggregate by lightweight aggregate on autogenous shrinkage were also discussed. The concrete with saturated lightweight aggregate exhibited no autogenous shrinkage, whereas the normal-weight concrete with the same matrix exhibited large shrinkage. A partial replacement of normal-weight aggregate by 25% by volume of saturated lightweight aggregate was very effective in eliminating the autogenous shrinkage and restrained stresses of the normal-weight concrete. It should be noted that the internal supply of water from the saturated lightweight aggregate to the high-strength cement matrix caused continuous expansion, which may be related to continuous hydration.

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.

Interfacial interactions in lightweight aggregate concretes and their influence on the concrete strength

Abstract The interactions between sintered fly ash lightweight aggregates and the matrix in portland cement concretes was studied to resolve factors other than aggregate strength which influence the concrete strength. Aggregates of variable properties were produced and concretes of equal effective water cement ratio were prepared and tested for strength and microstructure. It was found that differences in concrete strength could not always be accounted for by differences in the aggregate strength. These trends could be related to physical and chemical interfacial processes, which have an influence on the overall strength beyond that of the aggregate strength. The physical process identified was densification of the interfacial transition zone due to absorption of the aggregates; this process has considerable influence already at early age. The chemical processes were associated with pozzolanic activity of the aggregate and deposition of CH in the pores in the shell of the aggregate; these processes became effective only at later age, beyond 28 days. The enhancement in strength due to these influences ranged between 20 and 40%. Such influences should be taken into account when predicting the concrete strength or in the design of lightweight aggregate of optimal properties.

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.

Effect of lightweight fly ash aggregate microstructure on the strength of concretes

The structure and properties of sintered fly ash lightweight aggregate was modified by heat and polymer treatments to obtain aggregates different in their strength, absorption and pozzolanic activity. These properties of the aggregates were accounted for by changes in their microstructure. The strength of concretes of equal effective water/cement ratio prepared from these aggregates was determined at different ages to resolve the influence of the aggregate properties. It was shown that the strength of the concrete could not be accounted for by the strength of the aggregates only, and it is suggested that the absorption and pozzolanic activity of the aggregates can have an influence on the strength developed.

The microstructure and ageing of cellulose fibre reinforced cement composites cured in a normal environment

Abstract The correlations between microstructural changes after ageing and the mechanical performance of cellulose fibre reinforced cement composites were studied. Ageing conditions which promote carbonation (natural ageing and accelerated ageing in a CO2 rich environment) result in densening of the matrix around the fibres and the petrification of the fibres, leading to an increase in strength and E-modulus. Accelerated ageing in a normal environment leads to densening of the matrix without fibre petrification, resulting in a reduction in strength. Both ageing environments led to a marked reduction in toughness. The processes leading to petrification and matrix densening are discussed with a view of explaining the changes in mechanical properties after ageing.

Geometrical characteristics and efficiency of textile fabrics for reinforcing cement composites

One of the most efficient ways to obtain a high performance cementitious composite is by reinforcement with continuous fibers. Production of such composites can readily be based on the use of textile fabrics, which are impregnated with cement paste or mortar. The present paper discusses the bulk properties and geometrical characteristics of textile fabrics that should be considered in order to predict the performance of cement composites reinforced with fabrics. Geometrical characteristics are the nature of the basic reinforcing unit in the fabric (yarn) and the various geometries by which these yarns are combined together in the fabric (weft insertion warp knitted, short weft warp knitted, and woven fabrics). It was found that the geometry of a given fabric could enhance the bonding and enable one to obtain strain hardening behavior from low modulus yarn fabrics. On the other hand, variations of the geometry in a fabric could drastically reduce the efficiency, resulting in a reduced strengthening effect of the yarns in the fabric relative to single yarns not in a fabric form. The improved bonding in low modulus yarn was found to be mainly the result of the special shape of the yarn induced by the fabric. Therefore, in cement composites, the fabrics cannot be viewed simply as a means for holding together continuous yarns so that they can be readily placed in the matrix.

Fabric structure and its reinforcing efficiency in textile reinforced cement composites

In polymer matrices reinforced with fabrics, the effectiveness of the reinforcement is reduced when the yarns do not maintain a straight geometry. In cement composites, this concept may not be adequate since the nature of the interaction between the cement matrix and the fabric and its individual yarns is more complex, as concluded from pullout tests. The present paper discusses the bulk properties and geometrical characteristics of textile fabrics that need to be considered in order to predict the performance of cement composites reinforced with textile fabrics. It was found that the geometry of a given fabric could enhance the bonding and enable one to obtain strain hardening behavior from low modulus yarn fabrics, due to the special shape of the yarn induced by the fabric. On the other hand, variations of the geometry in a fabric could drastically reduce the efficiency, resulting in a lower strengthening effect of the yarns in the fabric, relative to single yarns not in a fabric form. Therefore, in cement composites the fabrics cannot be viewed simply as a means for holding together continuous yarns to be readily placed in the matrix, as is the case in composites with polymer matrix.

Influence of cement paste matrix properties on the autogenous curing of high-performance concrete

Abstract A novel approach to has been recently proposed mitigate self-desiccation, one of the foremost problems of high-performance concrete (HPC). It is based on incorporation of pre-soaked lightweight aggregate in the concrete mix. Such aggregate acts as an internal water reservoir preventing reduction of relative humidity in the cementitious matrix. This method is known as “autogenous” or “internal” curing. Recent studies demonstrated that this kind of curing could be successfully applied to obtain improved HPC with reduced sensitivity to cracking. However, the content of lightweight aggregate required to completely eliminate autogenous shrinkage was high, and this caused a reduction of compressive strength and an increase in the cost of the concrete. Recently, a work has been conducted to optimize the internal curing strategy by eliminating autogenous shrinkage while using the smallest possible amount of lightweight aggregate. The effect of grain size, pore structure and type of the lightweight aggregate was studied. The next step in this study––the effect of the properties of the cement paste matrix on the effectiveness of internal curing is discussed in this paper.