Compressive test characteristics and constitutive relation of wet polypropylene macrofibers shotcrete

Shotcrete is often subject to poor ductility and cracking problems, particularly under high stresses. To address these issues, we investigated the feasibility of adding polypropylene macrofibres to shotcrete. To evaluate the supporting effect, we used dry shotcrete, wet shotcrete, and wet polypropylene macrofibre shotcrete as samples. We compared and analysed the mechanical response characteristics thereof in uniaxial compression tests by acoustic emission monitoring. The results showed that the three materials were brittle, but the ductility, residual strength, and bearing capacity of polypropylene macrofibre shotcrete were significantly enhanced. The energy absorption value of plain shotcrete was higher in the cracking stage, while that of polypropylene macrofibre shotcrete was higher in the post-peak stage, which indicated that the polypropylene macrofibre shotcrete had the characteristics of a high crack-initiation strength and toughness. Besides, the energy release from fibre shotcrete occurred after the peak stress rather than near the peak stress. The average energy absorbed by polypropylene macrofibre shotcrete was significantly higher than that in dry shotcrete and wet shotcrete, which suggested that polypropylene macrofibre shotcrete could mitigate the brittle instability of a total stress-strain curve of such shotcrete. The results provide a basis for the optimisation of polypropylene macrofibre shotcrete layers.


Introduction
Shotcrete is a commonly used support and sealing method in geotechnical engineering works such as highway lanes and tunnels. Its stress-induced deformation is mainly divided into three stages: An effective way to improve the mechanical properties of concrete is to add steel fibres as an additional basic material, however, it is easy to corrode these in polypropylene macrofibre shotcrete.
It is a material that is difficult to use and it cannot be A new type of composite materials is formed by adding a proper amount of three-dimensionally randomly distributed polypropylene macrofibres to the wet-sprayed concrete, which improves the material properties of the concrete. Compared with the plain concrete, the wet polypropylene macrofibre shotcrete has better mechanical properties and better deformation ability (Bi et al. 2010 The dry shotcrete, wet shotcrete and wet polypropylene macrofibre shotcrete slabs required for the test were taken from a project site. The raw materials were: cement (grade 42.5); medium coarse sand with a fineness modulus greater than 2.5, with the particle size of the stone being 5-12 mm; the dry spray quick-setting accelerator was an ordinary powdered aluminate quick-setting accelerator, and the wet-spray accelerator was an alkali-free liquid accelerator. Water-reducing admixture was added for the wet-spray method, to 0.8% of the mass of cement, which was conducive to improving the strength of the shotcrete; the water-cement ratio was 0.5, and polypropylene macrofibres (1 kg/m 3 ) were added to the wet-spray concrete to make wet polypropylene macrofibre shotcrete (Niu et al. 2017), as shown in Fig. 1 and Tabl. 1.

Compressive stress-strain relationship and characteristics of specimens
The stress-strain relationship of three kinds of concrete under uniaxial compression is shown in Figure 3: before the peak stress is reached, the rate of change of the stress in dry and wet shotcrete is higher than that in the wet polypropylene macrofibre shotcrete under unit strain. When the peak stress is reached, the peak strains of dry shotcrete, wet shotcrete, and wet polypropylene macrofibre shotcrete are 0.245%, 0.270%, and 0.676%. The peak strain in the wet polypropylene macrofibre shotcrete is 1.76 times and 1.50 times than that in the dry shotcrete and wet shotcrete, respectively. This means that, when the test specimen is broken, the wet polypropylene macrofibre shotcrete concrete is less deformed and that the plasticity of the wet polypropylene macrofibre shotcrete exceeds that of the dry and wet shotcrete mixes. It is still a brittle material, despite the addition of the polypropylene macrofibres, but its ductility is significantly enhanced (Liu 2011  and provides a certain residual support, which helps to prevent the spray layer from falling. The stress-strain curve reflects the basic mechanical properties of the three specimens and shows the strength and ductility of the specimens and the energy absorbed in the specimens: however, the energy absorbed by the specimen cannot be obtained from the stress-strain curve alone so we also studied the energy absorption of each test specimen (Table. 4). The energy absorption is obtained by integrating to get the area under the stress-strain curve. For the dry shotcrete, compared with the wet shotcrete, the energy absorbed by the former is always less than that absorbed by the latter ( Figure   4). Therefore, the energy absorption of the concrete after the spraying process increases from the dry spraying process to the wet spraying process: the energy absorbed by the wet shotcrete in the cracking stage is high and that in wet polypropylene macrofibre shotcrete in the post-peak stage is also high. Moreover, the effect of the polypropylene macrofibres increases with increasing strain in the material.  shotcrete, a large amount of energy released from the wet polypropylene macrofibre shotcrete appears after, rather than at, the peak stress. The fracture data are in good agreement with the AE energy and the number of impacts.
(4) AE characteristics of three shotcrete mixes The AE signals from the three different types of concrete can be roughly divided into three stages: a smooth failure stage, severe damage stage, and later damage stage; all of which have a distinct rapid growth period, during which the concrete is destroyed and energy released rapidly.
The AE signals from the three different types of concrete exhibit the following differences: ○ 1 The AE signal of the wet polypropylene macrofibre shotcrete has a significant turning point before the severe failure stage while the AE signal of the ordinary shotcrete appears more uniform before the onset of severe destruction; ○ 2 Compared with ordinary shotcrete, the severe failure stage of wet polypropylene macrofibre shotcrete is delayed until reaching the peak stress.
○ 3 The distribution of the fracture points of the wet polypropylene macrofibre shotcrete is more uniform than that of ordinary shotcrete, which is conducive to improvement of the bearing capacity.
The comparison between the theoretical fitting results and the experimental data is shown in Figure   8.    (Fig. 9). As shown in Fig. 9, the residual strength is zero when the damage coefficient is 1; as the damage coefficient  decreases, the peak strength of the concrete slightly increases, but the residual strength gradually increases. That also complies with the definition of the damage coefficient. When  is 1, it implies complete damage, and the micro-element has no bearing capacity thereafter; when  is less than 1, it implies incomplete damage, and the micro-element can still withstand a certain stress after the onset of damage; the smaller the damage coefficient , the greater the residual strength, which evinces material ductility.
The damage coefficients of dry shotcrete and wet shotcrete are similar (Table. 5 The research results provide an experimental reference for a better alternative to traditional material mix designs in the form of polypropylene macrofibre concrete support.

Data availability statements
Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.

Competing interests
We declare that we have no