Plastic shrinkage properties of baler twine fibre reinforced concrete

Abstract

The large amount of used polypropylene baler twine generated from the agricultural community may provide a low-cost, environmentally friendly source of fibre reinforcement that can be used to improve the properties of concrete. However, the performance of such fibres for the application has not yet been explored. The effectiveness of using small amounts of chopped baler twine to control the restrained plastic shrinkage cracking of portland cement mortar was investigated in this study. To determine the influence of baler twine fibre type, length and volume fraction on their performance, two types of baler twine ( one composed of strands with circular cross section, the other composed of flat band shape strands) in two lengths (19 mm and 38 mm) and three volume fractions (0.05%, 0.1%, and 0.3%) were evaluated. To compare the performance of baler twine fibre with that of other commercially available synthetic fibres, fibrillated polypropylene fibres at equal lengths and volume fractions was investigated.The restrained plastic shrinkage tests were carried out by subjecting the fibre-reinforced mortar specimens, cast on rough substrate bases, to a wind speed of 2.6 m/s, and relative humidity less than 3% at 35 °C for 22 hours. To evaluate the effectiveness of the fibres, the crack numbers were recorded, and the maximum crack width and total crack area on the surface of each specimen were measured using an image analysis technique. Unrestrained plastic shrinkage tests were also conducted in which fibre-reinforced mortar specimens without the substrate bases were tested under the same environmental conditions.Test results indicate that both types of baler twine are capable of controlling restrained plastic shrinkage cracking to some extent, but are not as effective as fibrillated polypropylene. The baler twine composed of band shape strands performed better than the one composed of strands with circular cross section. Compared with plain specimens, the total crack area was reduced by 95.3, 77.5 and 38.7% when 0.3% volume fraction of 38 mm fibrillated polypropylene, band shape baler twine and circular baler twine fibres, respectively, were added. Similar reductions in maximum crack width were observed. Fibre length did not significantly influence cracking behaviour. Free plastic shrinkage was significantly reduced only when long fibre lengths (38 mm) and high volume fractions (0.3%) were used.