High-velocity impact behaviour of a new hybrid fibre-reinforced cementitious composite

Abstract

In this article, a new engineered cementitious composite reinforced with 0.6% volume steel fibres and 1.5% volume polyvinyl-alcohol fibres is developed aiming for enhanced impact resistance compared to other construction materials. Fundamental mechanical properties of the new composite including the compressive strength, Young’s modulus, tensile strength and flexural behaviour were tested. To calibrate the impact resistance of the new composite, high-velocity impact tests of panels made of the new material were conducted when subjected to impact from a standard 7.62 mm round in-service bullet fired from a knight armament SR-25 military rifle. For comparison, plain concrete panels and concrete panels reinforced with 2% volume steel fibres were also tested. The post-impact responses of the panels in terms of crater sizes, damage failure mode, fragmentation size, weight and regress velocity are analysed and compared to characterize the impact resistance of the new engineered cementitious composite. The test results demonstrate significantly enhanced impact and shatter resistance of the new hybrid fibre-reinforced cementitious composite with reduced spalling and fragmentation, localized damage areas and improved cracking resistance.

Keywords

engineered cementitious composite impact impact resistance polyvinyl-alcohol fibres steel fibres

Introduction

Recent terrorist attacks on infrastructures and the need to more efficiently protect against high-velocity projectiles and explosives on the battlefields call for the development of construction materials with excellent impact resistance. Steel-reinforced or prestressed concrete has been widely used in defensive structures since the 19th century, which are usually constructed with bulk wall thickness to resist impact and blasts. However, concrete, which is a quasi-brittle material, has an inherent weakness in resisting tensile stress, and it is inclined to exhibit extensive cracking and suffer brittle failure under impact loading. Additionally, conventional concrete has low shatter resistance and is prone to scabbing and spalling, which pose severe risks to people and/or equipment nearby.

Fibre-reinforced concrete (FRC) composed of cement, water, sand, fly ash and some chemical additives with moderate volume fraction short fibres randomly distributed in the mix has been considered to be a good substitute for conventional concrete materials in protective structures due to its high resistance to shatter with reduced scabbing, spalling, fragmentation and zone of damage, and increased energy absorption characteristics due to distributed micro-cracking (Li, 2003, 2007). Research and development of high-performance fibre-reinforced cement composites including those with excellent impact resistance capability have attracted great interests in recent years. Engineered cementitious composite (ECC) which is a special type of high-performance fibre-reinforced cement composite, with usually fibre volume fraction of around 2%, is a promising engineering material for protective structures due to its significantly improved mechanical properties over regular FRCs such as high tensile strength, large pseudo strain-hardening capacity, presence of micro-cracking and high energy absorption (Fischer and Li, 2007; Li et al., 2001).

The type, geometry and volume fraction of fibres play an important role in tailoring the ECC mix and strongly affects its material properties. High modulus fibres such as steel (SE) fibres, glass fibre and carbon fibre, and low modulus fibres such as polypropylene (PP), polyethylene (PE) and polyvinyl-alcohol (PVA) fibres have been used to reinforce the ECC materials. High modulus fibres could increase the bulk strength and toughness of the material; however, their intrinsic brittle behaviour does not allow for ductility or strain hardening. Low modulus fibres have been found to be able to improve ductility of the concrete mix significantly as well as reduce cracking. Thus, a hybrid fibre-reinforced ECC, with a proper volume of both high and low modulus fibres, is expected to improve in both the tensile strength and the strain capacity, which are both important for energy absorption and impact resistance of a material. A study on the mechanical behaviour of a hybrid fibre ECC reinforced with both SE and PE fibres (Ahmed and Maaleg, 2009) demonstrated that it exhibited better tensile strength than that reinforced with only PE fibres and greater tensile strain capacity than that reinforced with only SE fibres. Experimental studies on the high-velocity impact behaviour of hybrid fibre ECCs demonstrated the promising potential of hybrid FRC structures in impact resistance with improved shatter resistance and reduced scabbing, spalling, fragmentation and extent of damage (Maalej et al., 2005; Soe et al., 2013; Zhang et al., 2007).

In this work, a new hybrid fibre-reinforced ECC is developed aiming for enhanced impact resistance. The cementitious composite is designed based on the ECC-M45 mix (Li, 2003), and it is reinforced with 0.6% volume steel fibres and 1.5% volume low modulus PVA fibres. Fundamental mechanical properties of the new composite including compressive strength, Young’s modulus, tensile strength and flexural behaviour were studied experimentally. To characterize the impact resistance of the new construction material, high-velocity impact tests on panels were tested. To compare the impact resistance of the new mix to other conventional construction materials, plain concrete panels and concrete panels reinforced with 2% volume steel fibres were also tested. The panels are of a dimension of 400 mm × 400 mm with two thicknesses, that is, 55 mm and 75 mm, which is larger than the panels tested under high-velocity impact reported in the literatures so as to avoid the edge effect. Although several research works on high-velocity impact test on ECC were conducted in the lab environment using gas gun facilities, no impact test has been conducted using real ammunitions in lab environments according to the authors’ investigation. In this research, the panels were subjected to impact from the most commonly used in-service bullets, that is, the standard lead, steel jacket 7.62 mm round with an impact velocity ranging from 740 to 780 m/s fired from a knight armament SR-25 military rifle. High-velocity camera and velocity radar and velocity screen were used to determine the ingress impact velocity of the projectile and the regress velocity of the fragmentation. The post-impact responses of the panels in terms of crater sizes, damage failure mode and the potential threat to the surroundings from the fragmentation in terms of size, weight and regress velocity are analysed and compared. The findings from the experimental results are summarized finally.

A new hybrid fibre-reinforced ECC

A new ECC reinforced with 0.6% volume steel fibres and 1.5% volume PVA fibres is developed aiming for enhanced impact resistance. Steel fibres are used for good strength capacity of the new mix, and PVA fibres are used for ductility. Comparing with other low modulus fibres, such as the PP fibres and PE fibres, PVA fibres exhibit significantly improved mechanical properties such as tensile strength and Young’s modulus than PP fibres and are cheaper than PE fibres in micro-scales (Teng et al., 2008). In addition, PVA fibres are capable of creating molecular bond in the matrix during the dehydration process and improving ductility and tensile strength of the matrix dramatically (Li et al., 2001). PVA fibre was recommended as one of the most suitable polymeric fibres for ECC materials (Wang and Li, 2005). Therefore, PVA fibres are selected as the low modulus fibres of the new composite. As for the volume fraction of the fibres, a study on impact resistance of concrete with steel fibres showed that the best range of steel fibres for impact resistance was between 0.5% and 0.75% and that SE fibre volume less than 0.5% resulted in failure due to rupture and more than 0.75% showed failure due to fibre pull out (Wang et al., 1996). It was also found that 2% of volume of fraction of PVA fibre in the mono fibre-reinforced ECC matrix achieved excellent strain capacities, but the strength needs to be improved (Li, 2007). Therefore, in this new composite, the fraction of volume for SE fibre and PVA fibres are determined to be 0.6% and 1.5%, respectively, to represent the most potential impact resistance with a total fibre ratio around 2%.

The mix design is based on the ECC-M45 mix (Li, 2003), and the ingredients and their weight proportion are shown in Table 1. The steel fibres utilized in this mixture are coppered fibres produced by Ganzhou Daye Metallic Fibres Co Ltd, and the PVA fibres are Kuralon RECS15 produced by Nycon Corp. The physical properties of the fibres are shown in Table 2. The mixing procedures suggested in the ECC mix design (Li et al., 2002; Soe et al., 2013) are adopted for the new ECC material.

Table 1.

The mixture of the new ECC.

Material	Weight ratio
Cement	1
Fly ash	1.2
Sand	0.8
Water	0.56
HRWR (Grace ADVA 142)	0.012
PVA fibre (1.5%)	0.034
Steel fibre (0.6%)	0.081

ECC: engineered cementitious composite; PVA: polyvinyl alcohol.

Table 2.

Physical properties of steel and PVA fibres.

Fibre	Length (mm)	Diameter (mm)	Density (kg/m³)	Tensile strength (MPa)
Steel	13	0.2	7800	2400
PVA	8	0.038	1300	1600

PVA: polyvinyl alcohol.

Material properties of the new ECC

Fundamental mechanical properties of the new composite including compressive strength, Young’s modulus, tensile strength and flexural behaviour were tested. Cylinders of 200 mm long and a diameter of 100 mm were used as the specimen for uniaxial compression tests and testing of Young’s modulus. For flexural testing, beam specimens with dimensions of 350 mm × 100 mm × 100 mm were employed. For direct tensile test, dog-bone specimens of 330 mm in length, 60 mm in width and 13 mm in depth were used. When the mixing process was complete, the fresh workable and cohesive mixture was poured into the moulds for each specimen, which was then vibrated on a vibrating table. After casting, the specimens were covered by the lids and all the specimens stayed for 24 h before being demoulded except the beam specimens which were demoulded after staying for 48 h. The specimen was placed in a curing chamber with constant temperature of 23°C and 100% relative humidity until the day of testing. Four specimens were tested for tensile tests and bending tests while three specimens were tested for others.

Uniaxial compression and Young’s modulus tests

The cylinder specimens were tested using a 3000 kN compressive testing machine with a loading rate of 0.33 N/mm per second to determine the compressive strength of the ECC mix in a quasi-static condition at the age of 1, 7, 21 and 28 days. The test setup is shown in Figure 1. The tests were conducted in accordance with the Australian Standard AS 1012.8. The compressive strength of the ECC, which is the average of the results of three specimens, is shown in Table 3. A sharp increase in the compressive strength from day 1 to day 7 occurs and then the increase becomes steady. The compressive strength of the ECC at the age of 28 days is 54.5 MPa.

Figure 1.

Young’s modulus test setup.

Table 3.

Compressive strength of the ECC.

Test days	Compressive strength (MPa)
1	5.84
7	39.23
14	47.41
28	54.5

ECC: engineered cementitious composite.

Using the same machine and specimen, Young’s modulus was tested according to the Australian Standard AS 1012.17 (1997). Two linear variation displacement transducers (LVDT) were set up on each side of the cylinder specimens. The Lab-view program was used to record the test data during the testing. The tested Young’s modulus at the age of 7, 14 and 28 days is shown in Table 4. The value of Young’s modulus at the different ages is steady with a slight increase over the time and achieve a value of 23 GPa at the age of 28 days. The less value of Young’s modulus of the new ECC comparing with the conventional concrete is due to the lack of large aggregate and it shows a greater strain ability of the ECC than conventional concrete.

Table 4.

Young’s modulus of the ECC.

Test days	Young’s modulus (GPa)
7	20
14	21.1
28	22.6

ECC: engineered cementitious composite.

Four-point bending test

Four-point bending test of four beam specimens of 350 mm × 100 mm × 100 mm was performed to determine the flexural properties of the hybrid fibre-reinforced ECC mix at the age of 28 days according to the standard AS 1012.11 (2000). Testing was carried out using a 300 kN testing machine with a third point loading frame, which consisted of two supporting rollers and two loading rollers. During testing, load was applied under displacement control to the specimen through the two loading rollers until the specimen had been displaced a minimum of 2 mm according to ASTM C1609. The maximum load was then recorded by the testing machine, and the crack distance from the closest support was recorded. The test setup is shown in Figure 2. The tested average modulus of rupture and the corresponding strain is 13.5 MPa and 2.28%, respectively. The relationship between modulus of rupture and strain obtained for each specimen is shown in Figure 3. The four test results show a high degree of consistency with pseudo strain-hardening behaviour by the increase in load-carrying capacity with increasing strain after the yield strength has been reached. Multicracks occur during the bending loading and strain softening starts after the strain achieving over 2%.

Figure 2.

Bending test setup.

Figure 3.

Modulus of rupture and strain curve.

Uniaxial tensile test

Uniaxial tensile tests were performed on four dog-bone specimens at the age of 28 days using a 100 kN Shimadzu AG-X material testing machine under a strain rate of 10⁻⁵/s, and the tensile strength and the extension of the specimens were measured using a non-contact extensometer. The dimensions of the dog-bone specimen are shown in Figure 4, and the testing technique developed by Soe et al. (2013) was employed. The test setup is shown in Figure 5. The average value of the tensile strength of the ECC is 5.23 MPa.

Figure 4.

Dimensions of dog-bone specimens (dimensions: mm).

Figure 5.

Uniaxial test setup.

High-velocity impact tests

Specimens

To characterize the impact resistance of the new construction material, high-velocity impact tests on panels made of the new ECC with a dimension of 400 mm × 400 mm with two thicknesses, that is, 55 mm and 75 mm, were conducted. In the reported researches, the largest size of ECC panels subjected to high-velocity impact test is 300 mm × 300 mm. A size of 400 mm × 400 mm was used in this research to avoid the edge effects which may affect the accuracy of the test, and at the same time make sure it is not excessively heavy for easy transportation and setup. To compare the impact resistance of the new mix to other conventional construction materials, plain concrete panels and concrete panels reinforced with 2% volume steel fibres (denoted as FRC) were also tested. Two panels for each material and a total of 12 concrete panels were tested.

Facilities for impact tests

High-velocity impact tests were conducted in the 71-m-long indoor ballistic testing range in Defence Science and technology Organisation (DSTO), South Australia. The panels were subjected to impacts from a standard 7.62 mm NATO round in-service bullet (shown in Figure 6), which is a lead projectile with a steel jacket. The round has a diameter of 7.62 mm, a case length of 51.2 mm and a weight of 9.46–9.65 g with a muzzle velocity of 838–854 m/s (Ness and Williams, 2011). This calibre round was chosen as it is widely used around the world in medium machine guns and rifles and also used by Australian Defence Force. The bullet was fired from a Knight Armament SR-25 sniper rifle (as shown in Figure 7), which is a section sniper rifle that is in service with the Australian Defence Force. The rifle can fire the 7.62 mm NATO round at a velocity of 780 m/s and this weapon is capable of delivering the projectiles in a very accurate zone of impact. A weapon brace was used to hold the rifle in place for testing to increase the precision of the round for impact in the middle of the concrete specimens. A velocity radar and a velocity screen were utilized to determine the ingress velocity of the round. A NAC Image Technology Memrecam HX high-speed camera was used to determine the regress velocity of fragmentation.

Figure 6.

7.62 mm round (left), 5.56 mm round (centre) and 9 mm round (right).

Figure 7.

SR-25 sniper rifle.

Experimental results and analysis

The damage to the panels is a key indicator to the penetration resistance. After impacting test, the crater diameter in the front (impact) face, the scabbing diameter in the back face and the crater depth for each panel representing the sizes of the damage were measured, and the results are shown in Table 5. The weight loss of the panels during impact test is also shown in Table 5.

Table 5.

Damage parameters to the panels.

Specimen no.	Type	Panel thickness (mm)	Ingress velocity (m/s)	Crater diameter (mm)		Crater depth (mm)
				Front	Back	Front	Back
1	ECC	75	770.2	39.4	107.8	Through	19.3
2	ECC	75	752.3	55.2	45.8	Through	20.5
3	ECC	55	768.3	34	63	Through	16.9
4	ECC	55	768.3	44.7	84.1	Through	14.3
5	FRC	75	745.5	38.2	120.8	Through	22.5
6	FRC	75	760	34.3	148	Through	25.8
7	FRC	55	744.6	42	100.3	Through	16.1
8	FRC	55	762.7	65	108.4	Through	21.8
9	Concrete	75	762.7	123.9	0	21.1	0
10	Concrete	75	751.2	121.8	175.7	21.2	16.9
11	Concrete	55	776.1	133	148.8	20.3	20.7
12	Concrete	55	769.6	110	134.5	19.1	21.2

ECC: engineered cementitious composite; FRC: fibre-reinforced concrete.

It should be noted that it is almost impossible to get the same ingress velocity in the shooting, and although the ingress velocity is a bit different in different tests, they are close enough. From Table 5, it can be seen that the impact test shows a range of results for different types of panels and even for the same types of panels of the same thickness. For the ECC panels, the average frontal face crater diameter is 47.3 and 39.4 mm, respectively, for the panel of 75 mm and 55 mm in thickness, and the average crater diameter in the back face is 76.8 and 73.5 mm, respectively. More damages occurred in the 75-mm-thick ECC panels compared to the 55-mm-thick ECC panels. It should be noted that a large difference exists in the scabbing diameter of the back face for the two 75-mm-thick ECC panels, with a value of 107.8 mm in one panel and 45.8 mm in the other.

For the FRC reinforced with 2% steel fibres, the tested results are more consistent. For the 75-mm-thick and 55-mm-thick FRC panels, the average crater diameter in the front face is 36.3 and 53.5 mm, respectively, and average crater diameter in the back face is 134.4 and 104.4 mm, respectively. The crater size in the front face of the 55-mm-thick panel is 47% larger than that of the 75-mm-thick panel, whereas the crater size in the back face of the 55-mm-thick panel is 22% less than that in the 75-mm-thick panel.

For the plain concrete panels, a consistent test result is obtained except the panel No. 9 which was not penetrated with no back face crater. For the 75-mm-thick and 55-mm-thick plain concrete panels, the average crater diameter in the front face is 122.9 and 121.5 mm, respectively, and average crater diameter in the back face is 175.7 and 141.6 mm, respectively. The crater size in the front face of the two panels is very close, while the crater size in the back face in the 75-mm-thick panel is 24% larger than that of the 55-mm-thick panel. Further to the crater damage, the plain concrete panels also suffered large cracking throughout the panel and in examining the high-speed imagery a plain concrete panel can be seen cracking to the edge of the panel.

Comparing the crater sizes of the ECC, FRC and concrete panels of the same thickness, for the 75-mm-thick panels, the average crater size in the front face is 39.4, 53.5 and 121.5 mm, and the average crater size in the back face is 39.4, 134.4 and 175.7 mm, respectively. For the 55-mm-thick panels, the average crater size in the front face is 47.3, 36.3 and 122.9 mm, and the average crater size in the back face is 73.5, 104.4 and 141.6 mm, respectively.

It is obvious that the damage to the new ECC material has the least crater sizes in the front and back face of the 75-mm-thick panels. The crater size in the front face of the FRC and concrete panels is 35.8% and 208% greater than that of the ECC panel, and the crater size in the back face of the FRC and concrete panels is 241% and 346% greater than that in the ECC panel. For the 55-mm-thick panels, the ECC has the least crater size in the back face and the size in the FRC and concrete panels is 42% and 92.7% greater than that of the ECC panel. For the crater size in the front face, the size of the ECC panel is significantly less than that in the concrete panel but slightly greater than the FRC panel.

The post-impact images of the front face and the back face of the 55-mm-thick panels are shown in Figure 8. It can be seen that the damaged area for the ECC panel is localized with very small damage with nearly no cracks around the damaged area. The damage to the FRC panel is increased with a larger damaged area with more cracks developed around the damaged area. The damage area to the plain concrete panel is the largest. It can be clearly seen that the ECC panels perform best among all the panels under high-velocity impact. The same findings can be obtained from the post-impact damage to the 75-mm-thick panels as shown in Figure 9.

Figure 8.

Post-damage images of the 55-mm-thick panels (front face damages on the top and back face damages on the bottom).

Figure 9.

Post-damage images of the 75-mm-thick panels (front face damages on the top and back face damages on the bottom).

The fragmentation with big size, weight and velocity created during an impact has the potential to cause damage or injuries to the personnel or equipment nearby. The size, weight and velocity of the fragmentation generated during the high-velocity impact test are listed in Table 6. The velocity was determined using the slow motion imagery, which was captured by the high-speed camera.

Table 6.

Characteristics of fragmentations.

Panel	Thickness (mm)	Mean diameter (mm)	Weight (kg)	Velocity (m/s)
ECC	75	32.1	0.0275	44.5
ECC	55	19.4	0.00465	109
FRC	75	59.5	0.110	12.9
FRC	55	32.7	0.0119	43.9
Concrete	75	52.5	0.084	1
Concrete	55	32.63	0.0214	13.4

ECC: engineered cementitious composite; FRC: fibre-reinforced concrete.

For the ECC panels, fragmentation size is the least with a mean diameter of 19.4 and 32.1 mm for the 55-mm-thick panel and 75-mm-thick panel, respectively. The weight of the fragment for the two panels is 0.00465 and 0.0275 kg, respectively, with a velocity of 109 and 44.5 m/s, respectively. Although some of the fragments were travelling at approximately 100 m/s, the fragmentation is unlikely to cause any injury and damage to personnel or equipment nearby due to the small size, shape and tumbling nature. For the FRC panels, the fragmentation has a mean diameter of 32.7 and 59.5 mm for the 55-mm-thick panel and 75-mm-thick panel, respectively. The weight of the fragment for the two panels is 0.0119 and 0.110 kg, respectively, with a velocity of 43.9 and 12.9 m/s, respectively. Although the fragments from the FRC panel had a diameter of 32.7 mm and 0.0119 kg, which was on average larger than that from the ECC panels, they were travelling at a significantly reduced velocity of 43.9 m/s, hence could cause little to none damages. The main concern from the FRC fragments is the pulled-out steel fibres during the high-velocity impact, which may pose a threat to human. The plain concrete has the similar fragments size as the FRC, with a weight of 0.0214 and 0.084 kg for the 55-mm-thick and 75-mm-thick panels, respectively. The velocity, however, is the least with an average value of 13.4 m/s for the 55-mm-thick panel and 1 m/s for the 75-mm-thick panel. These fragments are not expected to cause injury or damage to personnel or equipment positioned behind the panel.

Summary and conclusion

A new ECC reinforced with 0.6% volume steel fibres and 1.5% volume PVA fibres is developed aiming for enhanced impact resistance. The fundamental material properties of the new ECC material were tested at different ages. The characteristic compressive strength of the hybrid ECC is in the 50 MPa range with an average value of 54.5 MPa at the age of 28 days, with a rapid increase in strength from day 1 to day 7.

Young’s modulus of the new ECC is 22.6 MPa, which is less than that of a concrete. This might be due to the removal of course aggregates and the introduction of discontinuous fibres in the ECC and demonstrates the less stiffness and enhanced ductility of the ECC material. The tensile strength of the new ECC is 5.23 MPa, which is significantly greater than the average value (2–3 MPa) for concrete. The tested average modulus of rupture and ultimate strain is 13.5 MPa and 2.28% respectively, which is significantly greater than the average value of concrete. The relationship between modulus of rupture and strain shows a high pseudo strain-hardening behaviour, multicracks occur during the bending loading and strain softening starts after the strain achieving over 2%.

High-velocity impact tests on panels made of the new material subjected to a standard 7.62 mm round in-service bullet fired from a knight armament SR-25 military rifle were conducted. To compare the impact resistance of the new materials with others, concrete panels reinforced with 2% volume steel fibres and plain concrete panels were also tested. The post-impact responses of the panels in terms of crater sizes, damage failure mode, fragmentation size, weight and velocity are analysed and compared. The test results demonstrate significantly enhanced impact and shatter resistance of the new hybrid fibre-reinforced cementitious composite with localized damage areas, reduced spalling and fragmentation, and improved cracking resistance.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship and/or publication of this article.

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