1. Introduction
In recent decades, terrorist attacks on buildings and infrastructure has become a global threat [
1,
2,
3]. To protect civilian lives from possible terrorist attacks, civil infrastructure should provide resistance to extreme loads such as impact and blasts. Ordinary concrete, which is one of the most widely used construction materials, is well-known to be weak under such extreme loadings because of its poor energy absorption capacity and brittle nature [
4,
5,
6]. The addition of fibers is one of the most effective methods to overcome this defect [
7,
8,
9]. The principal benefit of incorporating random evenly distributed fibers into cementitious material is to resist and decay crack propagation [
10,
11,
12]. The fiber contribution to concrete strength begins when concrete micro-cracks initiate and enhance the cracking behavior owing to bridging the cracked sections [
13,
14,
15].
The fibers used for concrete reinforcement are mainly steel fibers, carbon fibers, and polymer fibers [
16,
17]. Among the polymer fibers, polypropylene (PP) and polyvinyl alcohol (PVA) fibers have attracted most attention due to the outstanding toughness for concrete reinforced with them [
18,
19]. Concrete is a complex material with multiple phases which include large amounts of C-S-H gel at micron-scale size, sands at millimeter-scale size, and even coarse aggregates at centimeter-scale size. Thus, the properties of FRC will be improved at a certain level, but not whole levels if reinforced with only one type of fiber [
20]. For instance, steel fibers are supposed to strengthen concrete at the coarse aggregate scale, while PP or PVA fibers are suitable for the fine aggregate-scale crack prevention, and carbon nanotubes are proven to improve the strength at the scale of cement grains [
21]. In practice, hybrid fibers are incorporated in a common cement matrix, and the hybrid fiber-reinforced concrete (HFRC) can offer more attractive engineering properties because the hybrid composite derives benefits from each individual fiber and exhibits a synergetic response [
16,
22,
23,
24]. In addition, HFRC shows improved structural behavior compared to conventional concrete, such as less spalling and scabbing under impact loadings [
3,
25,
26,
27].
Previously, most of the fiber reinforcement research has been carried out to examine tensile strength [
28,
29], flexural strength [
30,
31,
32] and drop-weight impact toughness [
33,
34,
35,
36]. Only some work dealt with the blast or impact resistance performance affected by fiber content and type [
9,
37,
38], since the extreme loading tests are costly and even dangerous.
Yusof et al. [
37] checked the hybrid steel FRC subjected to air-blast loading whereby the hybrid steel fibers may reduce the effect of spalling and scabbing. Lai et al. [
39] evaluated the repeated impact resistance of UHPFRC samples including plain straight steel fibers and hybrid steel-PVA and steel-basalt fibers based on the SHPB test machine. It was concluded that the use of hybrid steel-PVA fibers exhibit greater compressive toughness under repeated impacts than steel-basalt fibers. In addition, the hybrid fiber-reinforced UHPC samples exhibited much smaller cracks than the single steel fiber-reinforced UHPC samples because small-sized PVA and basalt fibers were able to effectively control the micro-cracks.
This work, therefore, aims to investigate the blast-resistant performance of hybrid fiber-reinforced concrete against contact detonation. With reference to control specimens, the HFRC panels are assessed for the blast damage level. The damage coefficient and blast-resistant coefficient are then introduced to evaluate the HFRC panel ability against explosion. Finally, the hybrid effect index is introduced to determine the positive or negative roles played by PP and PVA-fiber.
2. HFRC Preparation and Characterization
Prior to the contact detonation tests, a series of material preparation and characterization work is carried out. The concrete mixture and preparation procedure are first explained with the quasi-static tests results analyzed.
2.1. Mix Proportions
Designed with 70 MPa uniaxial compressive strength, the details of the plain concrete mixture proportions in this study are normalized and listed in
Table 1. Portland cement (P.I 42.5) was used herein as a cementitious material and fly ash was added as a mineral active fine admixture. Ground fine quartz sand worked as fine aggregate. The water-binder ratio and sand-binder ratio were 0.25 and 0.45, respectively. To improve fluidity, a high-performance water-reducing agent, polycarboxylate superplasticizer (DC-WR2), was also added. In this experimental study, polypropylene, polyvinyl alcohol and steel fibers used for HFRC reinforcement were comparatively depicted in
Figure 1. The geometric information and mechanical properties of these three fibers are listed in
Table 2. It is suggested that the steel fiber is stronger and stiffer, while the PP-fiber is finer and more flexible and ductile. To investigate the hybridization of PP, PVA and steel fiber (SF) reinforcement effect on HFRC blast resistance, 12 mixtures (
Table 3) with a single type or hybrid fiber reinforcement at a total content of 2.0% were produced for sequent experimental studies.
2.2. Concrete Production
The mixing procedure of concrete needs to be rigorously controlled to ensure good workability, particle distribution and compaction, noting that the small particles tend to agglomerate which may break the chunks when the particles are dry. It is suggested to blend all fine dry particles before adding water and superplasticizer. In the climatic chamber with 90% humidity, the concrete samples were prepared with the following mixing procedures. First, the dry cementitious materials (cement, fly ash) and quartz sand were put together simultaneously and mixed for 1 minute at a low speed to achieve the binder–sand mixture. Afterwards, the water and superplasticizer were mixed and gradually poured into the mixture to improve its fluidity. Finally, the fibers were slowly added and mixed for another 5 to 8 min to ensure that all the fibers were evenly distributed in the mortar. 24 h later, the specimens were removed from molds and cured for another 6 and 27 days at room temperature (about 15–25 °C ) with humidity >95%.
It is worth noting that PP and PVA microfibers at 2% volume content negatively affect the fluidity of mixture due to their high specific surface area. Due to the lack of slump-flow test or J-ring test to assess the fluidity of the fresh mixtures, the qualitative evaluation of workability was achieved since the HFRC mixtures exhibit good deformability and proper stability to flow under their own weights. To ensure fluidity, superplasticizer-to-binder ratios were controlled between 0.06% (S1) to 0.5 % (S3). It was observed that HFRC with higher content PP or PVA-fiber exhibited poorer fluidity since more porous microstructure might be yielded due to relatively poor consolidation condition.
2.3. Static Test for Characterization
With the foregoing concrete samples reparation procedure, the quasi-static tests, including uniaxial compression (UC) and 3-point bending, were performed to investigate the effect of fiber reinforcement on the compressive and flexural strength. Since only fine gradation of quartz sand were used as aggregate, we prepare the UC and 3PBT specimens with similar sizes adopted in [
40,
41]. In this section, the experimental program is explained in detail. Then, experimental results will be reported and discussed based on the average values of tests for 3 specimens. To ensure quasi-static condition, a loading rate of 0.5 mm/min for the load cell of MTS machine was used herein to conduct both UC test and 3PBT.
Specimens of 40 mm × 40 mm × 40 mm were cast for quasi-static compressive strength testing. Three samples of each mix were tested to determine the uniaxial compressive strength. Abrasive paper was used to smooth the surface of the specimens. The non-casting surfaces of the cube specimen were used as bottom and top surfaces of the compression test to ensure complete contact with the plates of the universal testing machine.
To analyze the fiber effect on the flexural strength of FRC, 3-point bending tests were conducted herein with specimens of different fiber mixes. The dimensions of the tested beams are 40 mm (width
b) × 40 (depth
d) mm in cross-section, and 160 mm in total length where the span
l is determined as 120 mm. The beams were supported by a fixed rolling base which provides vertical constraints. The peak load value was then recorded for further study. The nominal flexural strength can expressed as
[
36,
42], where
is peak load,
l is span length,
d denotes the beam depth and
b is the beam width.
The UC and 3PBT results were plotted in
Figure 2 and
Figure 3. For 2% hybrid fiber reinforcement, both the 7-d
and 28-d
increase as steel fiber content increases. Although PP and PVA fibers are not expected to increase the compressive strength [
43], it is observed that polyvinyl alcohol fiber could better improve the compressive strength than polypropylene fiber when the SF content remains constant; meanwhile, the flexural strength can be better improved by PP-fiber incorporation. The post-test beam specimens are shown in
Figure 4 where the fibers on the broken surfaces can be clearly observed.
The PVA-Steel and PP-Steel hybrid fiber-reinforced concrete materials are characterized with 0.03 mm smaller-diameter polymer-fiber and 0.22 mm larger-diameter steel fiber. The small-size PP or PVA fibers bridges micro-cracks and therefore controls their coalescence, meanwhile the larger steel fiber tends to arrest the propagation of macro-cracks. Moreover, the steel fiber, which is stronger and stiffer, provides the first crack strength and ultimate strength. It explains why the S4 mix with the more steel fiber content exhibits the highest compressive and flexural strength.