A Superb Strengthening Additive to all Cement Work....

Other test data included, which involves original research work and testing, is taken from papers that have been selected for formal presentation to the American Concrete Institute.

Inhibiting early cracking, even at the micro-level.

The advantageous effects of the fibres are shown by tests undertaken when the concrete is: -

• Freshly placed and before it begins to set.
• After setting and before hardening, i.e. the plastics stage.
• After hardening, i.e. the mature concrete stage.

The ideal fibre maximises the advantageous characteristics and minimises any adverse effects. One of the main applications of the fibres is as an alternative to steel fabric for Crack Control purposes in fully supported ground slabs. The above are discussed in detail below and the advantages achieved by the use of fibres are confirmed by reference to comparative testing undertaken to verify the effects.

Crackstop fibres are the most advanced available being having being developed for the optimum achievement of the specific functions summarised above and described in detail later. The fibres produced are protected in Europe and the United States by patents.

The primary characteristics of fibres to optimise the results in this application are: -

• High specific fibre surface achieved by using very fine fibres.
• Reliable dispersion achieved by fibre bundles and wetting agents.
• Tough, durable, inert fibres achieved by using polypropylene.

The Crackstop fibres effect the rheology of the concrete and this reduces settlement, bleeding and segregation. The following comparative test results indicate the effect of the fibres on the fresh concrete.

The slump of the concrete containing Crackstop fibres will be reduced by about 10%. This does not indicate a reduction in workability since the slump tests is not a test for workability. The reason for the reduced slump is that the fibres create a desirable thixotropic effect, which will effect a static test such as that for slump. The design slump for Crackstop fibre concrete should be based on the mix prior to adding the fibres, if checked after the fibres have been added an allowance should be made for up to a 10% reduction.

The reduced settlement of the concrete is confirmed by tests on bleeding where Crackstop fibre concrete shows a reduction in the percentage bleeding from 5.8 to 4.87. Concrete is particularly prone to cracking as it changes from effectively a liquid to a solid material as the chemical hydration commences and setting occurs. The onset of the chemical reaction also results in a relatively large reduction in volume, greater in fact than the shrinkage, which occurs through the subsequent life of the concrete.

As with the tests undertaken in the foregoing paper plastic shrinkage tests were also undertaken under the supervision of the BBA using the modified Kraii test. Essentially the Kraii test involves the casting of a concrete slab, which is then subjected to drying by directing a stream of air over the surface. The Kraii test has been developed with various modifications to show the comparative tendency of different concrete mixes to develop cracks at this vulnerable early age. This test, with it's various modifications, is the most widely used and is particularly relevant as it involves concrete slabs rather than the more artificial test using restrained concrete rings.

In the BBA test 3 slabs each 600mm square and 50mm thick were used for each test. The concrete in the slabs was subjected to early drying by passing a stream of air, (with a temperature of 28C and a relative humidity of 40%), over the surface at a speed of 7 metres per second, to theoretical rate of evaporation of at least 1.5kg. /metre squared/hour. In the tests these drying conditions were maintained for 24 hours.

In these tests, in order to increase the propensity to cracking, the cementitious content of the concrete was increased and in addition silica fume and PFA were added to the mix.

As has been discussed the incorporation of Crackstop fibres into concrete ground slabs provides a more effective method of Crack control than is achieved with the use of steel wire mesh. Ground slabs are designed on the inherent strength of the concrete and it is far better to improve the reliability of the basic material rather than having to rely on a means of simply reducing the adverse effects after failure.

Improving the characteristics of the basic material, concrete, also improves the overall durability of the concrete slab. The tests supervised by the BBA have shown the following improvements to be achievable in the durability of the concrete.

The Initial Surface Absorption Test, (ISAT), is a well-established test, which, as the name implies, measures the water absorption of a concrete surface. This is obviously important for the durability of the concrete.