Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Patent
1994-06-28
1995-03-21
Bell, Mark L.
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
106802, 106819, 106823, 428364, 428374, 428113, 524 2, 524650, C04B 1606
Patent
active
053991958
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The use of various types of fibres in the production of concrete, to provide additional tensile strength and reinforce against impact damage and crack propagation, has been known and practiced for a long time. It is also known that while conventional reinforcement and coarser fibres can reduce the larger visible cracking which tends to occur in concrete, only very fine fibres are really effective in combating the development of smaller cracks. However, the fibres which are generally used in concrete, for example synthetic fibres of materials such as polypropylene, are relatively coarse, due to the fact that it is difficult to achieve a satisfactory dispersion in concrete of very fine fibres, and particularly fibres with high aspect ratios, using conventional mixing procedures and equipment. In fact, the uniform dispersion of even relatively coarse fibres in concrete can also be difficult.
It is common for such fibres to be produced as an integral fibrillated tape, and to rely on extended mixing to break down the fibrillation and to disperse the individual filaments, which are still relatively coarse, within the concrete. This system may not always be reliable, and the fibrillated tape is not always broken down into the desired individual filaments, especially since the degree of extended mixing required is in practice frequently not achieved. Even when effectively separated, the fibres may still be too coarse to achieve maximum effectiveness for crack inhibition, particularly against micro-cracking.
Concrete is prone to self-induced cracking and, as it is a brittle material, these cracks propagate readily under relatively low stresses. Concrete fails in tension by progressive crack development rather than the more usual failure mode of engineering materials.
It is generally assumed that the discrepancy between concrete's actual and theoretical strength can be explained by the presence of flaws (Neville, A.M., Properties of Concrete, 1981). Thus, concrete does not crack because it is weak in tension, but rather it is weak in tension because it already contains cracks. These cracks and flaws vary in size, so that scale is very important when dealing with fracture mechanics, in that the actual strength of the whole is a matter of statistical probability which is dependent upon the crack distribution within the material. The effective strength of the concrete can therefore be increased, and failure, i.e. the development of large-scale cracks or fractures, can be prevented, by inhibiting the development and propagation of cracks.
Self-induced, non-structural cracks occur in large masses of ready-mixed concrete due to small cracks which form early, and these are subsequently propagated by stresses induced by changes in the dimensions of such relatively large structures. Pavement concrete units are typically about 3 m by 10 m by 200 mm thick; small cracks in such concrete can readily propagate, producing a weak link which results in subsequent fracture. This clearly visible cracking is often the only form of cracking which is perceived as being of importance, but it is a direct result of much smaller and probably essentially invisible earlier crack development.
EP-A-0 235 577 discloses agglomerates of fibres having improved dispersability in viscous organic or inorganic matrices, e.g. cement-based matrices, comprising acrylic staple fibres, each fibre having a diameter of less than 50 .mu.m and length of more than 3 mm, the fibres being bonded to each other by a cohesion-conferring agent which is dissolved in, swells or melts in the matrix to be reinforced. The cohesion-conferring agent, e.g. polyvinyl alcohol, is applied in an amount of 1-30% by weight of the fibre. The fibres preferably have a high elastic modulus.
EP-A-0 225 404 discloses a method of manufacturing a fibre-reinforced moulded cement body, which comprises dispersing strands comprising a plurality of fibres into an unhardened cement material and thereafter hardening the material, at least some of the strands being impr
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Davies Derek
Hansen Anders S.
Bell Mark L.
Danaklon A/S
Marcheschi Michael
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