Abstract

A wide range of different precast concrete products is manufactured by dry cast production process with immediate demoulding. Therefore so called dry cast, zero-slump or negative-slump concretes are used. In this case, the freshly compacted products are able to undergo the demolding procedure without sustaining any forms of damage, e.g. pop-outs or cracks, and to maintain their geometrical behavior until hardening is achieved. Given the lack of hydration-mediated strength development in the cement at such early stages, achievement of sufficient stability in such concrete-based elements is solely limited to the adhesive forces between the solids, e.g. cement, fly ash, sand and gravel. To this day, there is no available method for the measurement of workability and consistency of zero-slump concretes that can provide proper information on their behavior throughout the casting process.
Hence, comparative investigation of the characteristics of different types of concrete has hitherto remained elusive. In line with such need, the current work has sought to provide a detailed theoretical analysis of the adhesion forces among fine particles of less than 100 μm. This fraction mainly includes cement and other additive materials, and is most influential for the transmission of adhesion forces to coarser fractions. The current study found the intraparticulate adhesion to comprise a plethora of different forces such as those resulted from the adsorbed water layers, liquid bridge bindings, Van-der-Waals forces, plastic Van-der-Waals forces, friction and interlocking. The individual components were thus shown to be strongly influenced by the roughness of the particle surface and particle shape as well as the concrete’s water content. The findings also indicated that all components have numerical shares in the total adhesion. Obviously, the main challenge that has thus far hindered the arithmetical prediction of the expected tensile strains in a moist particle matrix such as zero-slump cement slurry is the lack of exact information on the interparticlulate level, e.g. the shape and surface texture of the particles, which can affect the adhesion forces over several orders of magnitude.
Within the current study, a suitable testing procedure was developed, enabling the detection of differences in zero-slump cement pastes by measuring the tensile force-strain-behavior.
Experimental work on cement pastes with various starting materials and water to cement ratios confirmed the existence and measurability of differences in the tensile strain. It also became evident that even optically indistinguishable compositions behave differently during the testing procedure in terms of the tensile force and/ or the expansion behavior. Similarly, experimentation on zero-slump concrete with 8 mm coarse gravel could successfully detect noticeable changes in graphs of tensile force-strain behavior. Accordingly, the results of the present work can build a solid basis for the future optimization of zero-slump formulae. This developed and validated testing procedure can suitably facilitate making simple and small-scaled investigations on zero-slump cement pastes and concretes immediately after mixing in order to reliably identify the composition-mediated differences, e.g. those arising from the type of cements and additives as well as differences in water to cement ratio.
Therefore, the developed method can serve as a valuable tool both to supplement factory production control of the industrial manufacturing of zero-slump concrete elements, and the development of new concrete recipes e.g. when changing any kind of raw materials.