Concrete in the initial 6-12h after pouring, if the moisture contained in the rapid evaporation is very easy to produce cracks, which is mainly due to still in the plastic state of the concrete due to water loss and shrinkage caused by contraction, in the case of shrinkage is limited, the tensile stress generated in the concrete exceeds its tensile strength limit (plastic state of the concrete of the value of the repair is very low), then the concrete inevitably will crack, usually referred to as the “plastic shrinkage cracking”. Plastic shrinkage cracking can be reduced or prevented if the concrete used has the right mix ratio, the construction method is formalized and the temperature and humidity during the curing period are ensured. However, under unfavorable construction conditions, especially in harsher climates, the addition of a small amount of polypropylene fine fibers to the concrete is an effective technical measure to reduce or prevent plastic shrinkage cracks!!!!

Polypropylene fine fiber density is very low, small diameter, the direct diameter of 48 microns, length of 19mm polypropylene monofilament fibers with a volume rate of 0.1% mixed into the mix, 1m3 concrete in the number of fibers can be up to 29 million, the number of such a large number of fibers uniformly distributed in the concrete can be formed in three dimensions of the “support network! ”, in the concrete after pouring to play the following three aspects of the role:

1 to prevent the segregation of concrete, improve the homogeneity of concrete. After pouring small concrete due to the settlement of aggregates and segregation phenomenon, reducing the homogeneity of concrete and continuity, in serious cases, the concrete surface will appear “settlement cracks”. The existence of polypropylene fine fiber three-dimensional network to prevent such segregation phenomenon, and thus prevent the generation of plastic settlement cracks.

2 Reduce and prevent early mixed degree shrinkage cracks in mass concrete. Since the late 1980s, the international “mass concrete” to be a new definition, where the minimum size of the concrete structure entity is equal to or greater than 1mm, or expected to be caused by the heat of cement water generation of concrete inside and outside the temperature difference is too large and lead to cracks in the concrete are mass concrete. When the tensile stress due to the temperature difference between the inside and outside of the mass concrete exceeds its early tensile strength limit value after the mass concrete is poured, early cracks are bound to occur. Polypropylene fine fiber three-dimensional network can withstand the tensile stress caused by the temperature difference, which can reduce or prevent the generation of such cracks.

3 Reduce and inhibit plastic shrinkage cracks in concrete and improve the durability of concrete. As mentioned above, in unfavorable construction conditions, due to the rapid evaporation of water contained in the concrete, it is very easy to produce the generation of plastic shrinkage cracks, due to the reduction of the number of such cracks and the scale of refinement, so that the permeability of the concrete and frost resistance and so on can be improved, thus improving the durability of concrete.

Concrete Branch of Guangzhou Municipal Housing Construction Development Co., Ltd. conducted a comparative test on the plastic shrinkage cracks of mortar without fiber and with different volume rates of Dura fiber. The test basically followed the method suggested by Kraai, with only slight differences in specimen size. The mass ratio of mortar used for the test was cement : sand : water = 1 : 3.3 : 0.55 and the specimen size was 200mm x 200mm x 50mm. The specimen size was 200mm x 200mm x 50mm. The poured specimen in the restricted shrinkage state was first exposed to the air in the test chamber and placed under a fan to accelerate the air flow to promote rapid evaporation of water from the specimen surface. The room temperature was 20-25°C and the relative humidity of the air under the fan was 23-25%. 3 h later, the fan was closed, and the specimens were transferred to the outdoor atmosphere for a further exposure of 21 h. The outdoor temperature was 26-31°C and the relative humidity was 56%-72%. After the specimens reached the age of 24h, the surface cracks of each group of specimens were evaluated by the weighting method.

The addition of a small amount of polypropylene fibers to the mortar significantly reduced the number of plastic shrinkage shrinkage cracks and early drying shrinkage cracks, and resulted in crack refinement. When the method suggested by kraai is used to determine plastic shrinkage cracks in mortar, the data obtained can vary considerably due to differences in mortar ester ratio, fiber morphology and length, air flow rate, and entering air temperature and humidity. The method is more suitable for the comparison of different specimens under the same test conditions.