1. Introduction

1.1. Physical Properties of Cementitious Materials with PE Fibers

There are many literatures about the physical properties of PE fibers, which were listed in Table 1 ^[1][2][3].

Table 1. The physical properties of polyethylene (PE) fibers ^[1][2][3].

The production process of PE-ECC is now highly mechanized ^[1][2][3]. All mixtures (e.g., sand, cement, fly ash, ground granulated blast furnace slag, silica fume, water reducer, fibers) were prepared in a mixer. The solid dry raw materials were mixed all together for 2 min. Subsequently, water and water reducer were added and mixed to reach a proper fluidity of the matrix. Finally, fibers were added slowly into the mortar by hands and mixed for 3 min to ensure good dispersion. The fresh mixture was cast into steel molds covered with plastic sheets and demolded after one day curing. All the specimens were cured in air for a period of time before testing.

Previous research revealed that PE fibers decreased the slump of cementitious materials. PE fibers reduced the slump, which led to uneven fiber distribution. The cementitious composites had trouble in homogeneity and workability ^[4]. PE fibers increased the volume fraction of air voids. The air void increased from 0.4% to 1.2% when the PE fibers were incorporated. Our research group is developing an innovative vibration-aided method to solve the problem. The results show that the new mixing method is conducive to the reduction of air void and the dispersion of PE fibers ^[5]. The fibers under vibration can be distributed more uniformly.

The addition of PE fibers can reduce the drying shrinkage of cementitious materials. More than 10% reduction of the drying shrinkage of cementitious materials was witnessed ^[4]. The crack width and crack area decreased with the content of PE fibers ^[6].

1.2. Static Mechanical Properties of Cementitious Materials with PE Fibers

1.2.1. Tensile Performance

It is known that plain concrete exhibits brittle collapse when the developed tensile stress exceeds the limited strength of concrete under tension. The use of non-conventional mass reinforcement such as discontinuous fibers has been proven to be a promising alternative.

1.2.2. Flexural Performance

Two different types of PE fibers were mixed in cementitious materials to improve the bending and impact behavior. Meanwhile, the negative effects of PE fibers on compressive strength were removed by using the hybrid system ^[7].

The bending test was conducted on PE-ECC by Said et al. ^[8]. The results showed that the PE fibers raised the ultimate load and deflection of the PE-ECC slab greatly. Multiple cracks were observed in the middle part of the PE-ECC slab. However, the fibers cannot be distributed uniformly in the PE-ECC, which restricted the bending behavior of it. Further, they compared the PE-ECC and PP-ECC used in a concrete slab. The result proved that PE-ECC had a better flexural performance than PP-ECC. The interface between PP fibers and the matrix was weak. Meanwhile, the tensile strength and stiffness of PP fibers were low. The PE-ECC slab showed a higher flexural strength due to its high tensile strength and elastic modulus. Hence, PE-ECC was preferred in the concrete slab.

By using PE fibers, the modulus of cementitious composites improved. The PE-ECC beam had a great modulus of rupture 27.68 MPa with a COV of 4% under bending tests. Further, the flexural deformation reached 2.5% of the span length, which showed great ductility ^[9]. With the bending process, a first crack appeared on the tension side at first. Afterwards, the crack number rose with the bending load while the crack width kept almost constant. Then, multiple cracks developed at the peak load. Finally, the tensile side failed since the fibers were ruptured or pulled out.

The flexural strength of PE-ECC without fly ash was 30.8 MPa, while the flexural strength decreased from 27.6 MPa to 11.5 MPa if the fly ash was incorporated. The midspan deflection was hardly affected by the amount of fly ash. Deflection went up from 4.6% to 6.3% of the span length with the increase of fly ash ^[10].

Flexural loading was imposed on PE-ECC and ECC with steel fibers. Results illustrated that the coarse and wide cracks were shown on ECC with steel fibers. Since the steel fiber had great stiffness, the matrix was easily broken. However, the PE fibers had low stiffness. The cementitious matrix would not break, which increased ductility. More small cracks were initiated in PE-ECC under bending ^[11].

1.2.3. Compressive Performance

The compressive strength of PE-ECC was lower than that of plain concrete. It decreased when the fiber usage increased ^[12]. The average compressive strength was 166 MPa and the corresponding compressive elastic modulus was 51.2 GPa ^[13]. Said et al. ^[14] found a similar compressive strength decrease by incorporating PE fibers.

With the addition of PE fibers, the compressive strength decreased because of the rose of porosity. Ductile fracture mode under uniaxial compression was witnessed. Young’s modulus and strain capability increased with compressive strength, while toughness decreased. The Poisson ratio had almost no relationship with the compressive strength. The compressive strength increased with the reduction of water/binder (w/b) ratio since a lowered w/b ratio decreased porosity and led to a more compacted structure ^[15].

PE-ECC’s compressive strength achieved 121.5 MPa when the volume fraction of PE fibers was 2%. The average Young’s modulus was 44.26 GPa with a COV of 0.7%. It had a great modulus 44.3 GPa with a COV of 0.7%. If the aspect ratio was large and the cementitious matrix was strong, the PE fiber fractured ^[9].

Fly ash and PE fibers were added to the PE-ECC. The compressive strength dropped from 100.9 MPa to 44 MPa with a strength loss of more than 56.4% ^[10]. With the increase of PE fibers, the compressive strength of PE-ECC decreased since the PE fibers were difficult to be dispersed uniformly ^[16].

The reduction of the compressive strength of PE-ECC was observed by Li et al. (2020). 1% PE fibers decreased 7% compressive strength of PE-ECC compared with plain ECC. The reason was that the incorporation of PE fibers brought air voids. Hence, steel fibers were introduced in the PE-ECC to balance the negative impacts since steel fibers can prevent crack propagation, which increased the compressive strength ^[17].

1.2.4. Shear Performance

The advantageous characteristics of fiber-reinforced ECC under tension tests are also very important for the shear response of concrete structural members which is governed by the tensile response of the fibrous material.

ECC’s strain hardening and multiple cracking behavior on the shear capacity of beams loaded in shear were investigated by experiments ^[18]. Results exhibited that ECC was similar to steel reinforcements for improving the shear capacity of beams. Improved shear resistance, better control of crack sizes and a more ductile shear failure were provided by ECC compared with concrete. Fibers can reduce the shear crack width and increase the crack number. The crack opening and crack sliding at failure in ECC were 20–25% of the crack size in concrete, which showed the potential of ECC exposed to moisture and other aggressive substances. ECC contributed to the shear behavior including: (1) Fiber bridging of shear cracks, increasing the shear ability; (2) Traditional shear reinforcement was activated at smaller crack deformations; and (3) Crack opening was restricted by fiber bridging mechanism and by activating traditional shear reinforcement at smaller crack deformations. The shear crack development mechanism of reinforced ECC included 5 stages: (1) Crack initiation without noticeable sliding, which was restricted due to fiber transferring stresses across the crack; (2) Crack sliding and opening, which was restrained by fiber bridging, aggregate interlock and stirrups. When the crack opening was greater than one-half of the maximum aggregate size, the aggregate interlock stopped. The limited crack deformation was caused by the enhanced crack control by fibers, which transferred stresses over the crack with a small crack opening. However, traditional reinforcement needed greater deformation to activate the stress transferring over the crack; (3) Crack opening and sliding developed slowly due to the combined bridging effect by fibers and stirrups. The aggregate interlock was reduced since the crack opening was great in this stage. The stirrups and longitudinal reinforcement fully worked. Additional transverse reinforcement prevented cracking due to the increased cross-sectional area bridging the crack; (4) Increased rate of crack deformation. Crack opening and sliding were limited mainly by stirrups. The stirrups carried increasing stress and ultimately yielded. Fibers reached maximum bridging stress and failed. At the end of this stage, force led to pullout and rupture of the fibers at the weakest crack; (5) Rupture of stirrups and fibers in ECC resulted in the final failure of specimens.

2. The Self-Healing Effect of Cementitious Composites with PE Fibers

In concrete structures, cracks cannot be avoided from damage, shrinkage, creep and corrosion. Fortunately, PE-ECC has the property of self-healing, which can remove these cracks automatically. Using a self-healing effect can lower maintenance costs, increase service life and reduce manual repair. Self-healing concrete contains two categories: autogenous healing and autonomous healing. PE-ECC belongs to autogenous healing due to the carbonation of calcium hydroxide and continuing hydration of cement grains, while PE fibers can restrict the crack width ^[19].

Li and coworkers firstly developed PE-ECC that can heal cracks itself. The PE fibers can decrease the crack width, and further hydration can cause the self-healing effect of PE-ECC ^[20]. The average crack width was less than 50 μm in the PE-ECC, which facilitated the self-healing effect.

Later, many other scholars confirmed the self-healing behavior of PE-ECC and investigated the healing efficiency of PE-ECC ^[21]. PE-ECC was found to have high ductility and multiple cracks, which was different from plain concrete. The self-healing products on the surface of cracks were calcium carbonate, which recovered tensile strength and decreased water permeability.

Mihashi et al. ^[22] investigated the durability of PE-ECC and ECC with PE and steel fibers. The experimental results proved that hybrid fiber reinforced concrete was better than plain concrete and PE-ECC due to a small crack width which reduced steel corrosion and led to the self-healing effect.

Koda et al. ^[23] tested the self-healing effect of ECC with different kinds of fibers. The results showed that when the crack width was less than 100 micrometers, the self-healing efficiency caused by PE fibers and PVA fibers were similar.

Nishiwaki et al. ^[24] investigated the self-healing effect of PE-ECC. Water-tightness and mechanical properties were tested. The hybrid fibers (i.e., steel fibers and PE fibers) proved to be effective in self-healing considering that mechanical properties and water-tightness increased. Fine cracks were observed as healed using hybrid fibers.

Kunieda et al. ^[25] used a high strength matrix and 88 GPa high-stiffness PE fibers to produce PE-ECC, which exhibited a good self-healing effect. The crack width in the research was less than that of normal ECC due to the strong restriction by PE fibers.

Previous research compared the self-healing ratio by alkali-activated slag-based composites and cementitious composites using PE fibers. The results illustrated that cementitious materials had a higher healing recovery than alkali-activated slag-based materials. Meanwhile, calcium carbonate was the filling material in cracks of PE-ECC ^[26].