Modifying the Composition of Hollow-core Slab Concrete

The questions of the extruded concrete composition and the possibilities of modifying it are very important for the technology for the extruded concrete. The gained experience of working with the extruders of hollow core slabs shows that the operators of such equipment frequently choose an improper strategy for the production process. The main drawbacks are as follows: a) the use of fairly stiff mixture that is far above the necessary Vebe consistency class V2 for this technology; b) the over saturation of the mixture with coarse aggregates which determines a low compaction factor of the mixture; c) the rejection of using concrete admixtures which causes equipment overloads or led to its exploitation in the limitary conditions. Besides, the experience of using extruders proves that all parameters predicted by standards and other norms (for example, concrete strength class C40/50 or C50/60 including water cement ratio W/C<0,45) are obtainable without large efforts. Therefore, the main criteria for the suitability of such concrete modifi cation have rather technological character including the lowest energy consumption during shaping and the compaction of the semi manufacture of the reinforced article as well as the highest structural strength or stability of the fresh concrete slab. Whereas the mechanical properties (mainly strength) of such hardened concrete are mostly within acceptable values, it cannot be treated as the main criterion for optimizing the extruded concrete composition. Extruded concrete compositions used for producing hollow-core slab were chosen for technological and laboratory scale investigations. The amount of Portland cement in the concrete mixture was 335…370 kg/m³, sand made 0/2 mm grade – 330…440kg/m³ and 0/4 mm grade – 680…510 kg/m³; the amount of coarse aggregates 200…325 kg/m³ and 755…825 kg/m³ for grades 2/8 mm and 11/16 mm respectively; W/C ratio 0,34…0,39. The crushing strength of the extruded concrete was within 57…68 MPa and more (the results of technological trials). The character of the structure and the compaction level of the extruded mixture are the indicators of its technological suitability. Adding common lignosulfonate-based plasticizer (up to 1% of the cement mass) or a very small dose (0,2…0,3%) of the new generation super plasticizer with or without air entrainment agent could improve the structural and technological properties of such concrete. The experience obtained during laboratory scale investigations and on trials for the manufacturing lines of hollow core slabs shows that the main cause of such improvements is a better dispersion of cement particles in the stiff concrete mixture while the cohesion of the mixture of the freshly extruded article rests near unchanged. The effectiveness of such improvement was proven within the process of observing the level of the consumption of compaction energy – it was registered by the control console of extruders. After improvements in the concrete mixture were carried out, the consumption of compaction energy was reduced by 20…25 %. Investigations into concrete cores drilled-out from the hardened articles prove the apparently better structure of the modifi ed concrete while concrete strength and other physical properties rests rather unchanged (if the mixture W/C ratio rests unchanged). Improvements to concrete structure manifests by the absence of ‘air pockets' (large pores of irregular form, air gaps trapped during mixture extrusion and compaction), more gradually distributed and comparatively small pores, the absence of internal concrete structure zones with cleft aggregates and loose sand particles and evenly coloured concrete (which proves a very good distribution of cement particles). The use of an air entrainment agent in the stiff concrete mixture allows reducing the dose of the plasticizer (super plasticizer) and reduces the density and crushing strength of the extruded concrete. Such was negligible during investigation and technological trials (up to 1,5 % of concrete density and about 3 % of strength), it was concluded that the air entrainment agent could be used in the extruded concrete for articles the exploitation conditions of which are severe, for example, for class XF2 etc.