Cellulose fibre concrete

It is known that fibres can improve flexural strength, impact resistance, toughness and decrease the risk of brittle failure nature of cement products. The advantage of cellulose and wood-based fibres is their mechanical properties and low weight. In the long-term investigation of fibre concrete, wood-based fibres remain stable in the alkali environment of cement. The cellulose fibres prepared in a sulphate cooking liquid have a hygroscopic surface which absorbs high amounts of water. This can increase the concrete water demand and deteriorate the concrete durability in moist conditions. The solutions to this problem lie in using a special manufacturing technique or fibre treatment. In this investigation concretes and mortars containing different kinds of cellulose fibres were developed. The aim was to select suitable types of fibres for various kinds of concrete products. Four types of cellulose fibres and three manufacturing methods from the concrete industry were studied. The properties of cellulose-fibre-reinforced concrete were compared with those of concrete without fibres. Cellulose-fibre concrete was made using conventional and pressure-casting methods. As cellulose fibres were used only to modify concrete properties, a conventional casting method was employed when using a small fibre dosage. The pressure-casting method is suitable when great amount of fibres is used. Due to the cellular structure of fibres the density of the concrete can be reduced to roughly 700 kg/m3. The mechanical and durability properties of cellulose-fibre concretes were investigated with accelerated laboratory tests simulating the effects of wetting-drying and freezing-thawing. Low-cellulose fibre contents do not essentially increase the volumetric expansion or contraction of concrete in conditions of variable humidity. Cellulose-fibre-reinforced sprayed concrete withstands 1500 freeze-thaw cycles without loss of compressive and flexural strength. Cellulose fibre lime-cement mortar (mortar with higher cement content) adequately withstood the influence of frost without loss of compressive and flexural strength after 146 freeze-thaw cycles. The effect of fungus was studied with respect to the rotting activity of dry-rot fungus and cellar fungus in favourable moist conditions. The studied cellulose fibre concrete did not behave differently from concrete without fibres and did not have a detrimental effect on resistance to biological degradation during accelerated testing. Shown on laboratory test cellulose fibres have been found to reduce internal stresses that develop in the concrete as water evaporates. Cellulose fibres can also be used in products where surface cracking is a problem. In these products a small amount of cellulose fibres improve the workability and cohesion of the fresh mix and reduces the risk of cracking after hardening.