Experimental Analysis on Flexural-tensile Performance of Polyester Fiber Asphalt Concrete

The cracking of asphalt concrete pavement may occur due to the lack of flexural-tensile strength or the accumulation and development of meso-structure deformation.  In this case, water will penetrate through the cracks and erode the interface between the asphalt and the aggregate. Prolonged exposure to moisture can de-bond the aggregate from the asphalt, which is a major cause of early damage of asphalt concrete pavement.

The asphalt concrete beam bending test offers an important way to study the low-temperature crack resistance of asphalt concrete [1, 2]. A comprehensive index system has been developed for the bending tests on intact and pre-cracked asphalt concrete beam at different temperatures, and adopted to explore how the low-temperature crack resistance of asphalt concrete is affected by temperature, grading, asphalt-aggregate ratio and loading rate. The index system mainly involves indices like the flexural-tensile strength, the strain and stiffness modulus at flexural-tensile failure (hereinafter referred to as failure stiffness modulus and failure strain), the J-integral, and the critical strain energy density [3-5]. With the development of computer technology, the bending test results have been simulated on finite-element, discrete-element and boundary element analysis software, aiming to disclose the correlation between the meso-structure and macroscopic mechanical performance of asphalt concrete [6].

Fiber asphalt concrete has received more and more attention, thanks to its excellent pavement performance and mechanical performance. Some progress has been made in the research of the flexural performance of this type of asphalt concrete. For instance, Reference [7] examines the impact of fiber type on the low-temperature crack resistance of fiber asphalt concrete through beam bending test. Reference [8] evaluates the resistance of glass fiber reinforced asphalt overlay to cracking and crack propagation, in light of fracture potential and critical strength. Reference [9] carries out a low-temperature beam bending test to disclose the effects of fiber content on low-temperature performance of fiber asphalt concrete. The mechanism for the low-temperature crack resistance of fiber asphalt concrete has also been discussed, and the low-temperature performance of fiber asphalt concrete has been evaluated based on the bending test index system [10-16].

Nevertheless, the existing studies have not tackled the following issues: the temperature induced-variations in the load at flexural-tensile failure (hereinafter referred to as the failure load), flexure-tensile strength, failure stiffness modulus and failure strain of fiber asphalt concrete; the relationship between bending test parameters of fiber asphalt concrete and the ratio of fiber volume to the volume of asphalt mixture (fiber volume ratio, V_f); the impact from the ratio of mean fiber length to mean fiber diameter (fiber aspect ratio R_a) to the flexural performance of asphalt concrete; the combined effect of fiber volume ratio and fiber aspect ratio on the flexural performance of asphalt concrete.

Taking the AC-13F mixture as the matrix, this paper carries out beam bending tests on fiber asphalt concrete, using the optimal asphalt content (OAC) determined by Marshall test. The test parameters include temperature, fiber volume ratio and fiber aspect ratio. Based on the test results, the author systematically explored how the three parameters affect the flexural performance of asphalt concrete. In addition, the fiber content feature parameter (FCFP) was introduced to describe the combined effect of fiber volume ratio and fiber aspect ratio on the flexural performance of asphalt concrete, and an FCFP-based model was set up to calculate the flexural performance of polyester fiber reinforced concrete beams at different temperatures.

3.1 Flexural-tensile strength

It can be seen from Table 1 that, as the temperature changed from -30 ℃ to 15 ℃, the flexural-tensile strengths of the base asphalt concrete (AC-13F) and polyester fiber asphalt concrete (PFAC-13F) both increased first and then decreased, and peaked at the temperature of 0 ℃. Under this temperature, both types of asphalt concretes obviously softened under stress [18]. This is because temperature can change the mechanical performance of the asphalt, and thus determine the flexural-tensile failure mode of asphalt concrete. With the temperature growth from -30 ℃ to 15 ℃, the brittle asphalt becomes viscoelastic and then plastic. The 0 ℃ is the division point between the brittle phase and the viscoelastic phase.

2.jpg

Figure 2. Ordinary asphalt concrete bending process

As shown in Figure 2, there was no obvious descending segment in the load-deflection curve and the stress-strain curve, when the temperature was below 0 ℃. During flexural-bending failure, brittle fractures appeared on the asphalt concrete. The lower the temperature, the more obvious the brittleness of the asphalt, and the poorer the resistance of asphalt concrete beam to flexural-tensile deformation. That is why fiber asphalt concrete beam had low flexural-tensile strength and failure load, when the temperature was at a low level. When the temperature was above 0 ℃, the asphalt became viscous, and the asphalt concrete suffered from bending plastic failure, as evidenced by the clear descending segments in the load-deflection curve and the stress-strain curve at the temperature of 15 ℃. The higher the temperature, the weaker the fiber asphalt concrete in the viscoelastic phase, and the smaller the flexural-tensile strength and failure load.

As the temperature changed from -30 ℃ to 15 ℃, the flexural-tensile strength of the fiber asphalt concrete increased first and then decreased. This trend can be illustrated as a quadratic curve:

${{R}_{B}}={{a}_{0}}+{{a}_{1}}T+{{a}_{2}}{{T}^{2}}$           (4)

The bending test data on fiber asphalt concretes were fitted nonlinearly to obtain the model parameters of formula (4) (Table 2) for fiber asphalt concretes of different fiber volume ratios and fiber aspect ratios. Next, the brittle temperatures of fiber asphalt concretes were derived from these model parameters. It can be seen that fiber asphalt concretes had a lower brittle temperature than base asphalt concretes; the lower the brittle temperature, the stronger the low-temperature crack resistance of fiber asphalt concrete; the optimal low-temperature crack resistance was observed on the fiber asphalt concrete with the fiber aspect ratio of 324 and the fiber volume ratio of 0.35 %.

Table 2. Model parameters of formula (4)

Mixture type	$R_{a}$	$V_{f} / \%$	$a_{0}$	$a_{1}$	$a_{2}$	Peak point temperature /℃
AC-13F	0	0	8.90	-0.067	-0.0097	-3
PFAC-13F	162	0.35	9.19	-0.093	-0.0065	-7
	324	0.17	9.28	-0.088	-0.0087	-5
		0.35	10.55	-0.093	-0.0061	-8
		0.52	10.26	-0.103	-0.0078	-7
		0.69	9.77	-0.105	-0.0088	-6
	486	0.35	9.52	-0.088	-0.0075	-6
	649	0.35	9.21	-0.072	-0.0071	-5

 

3.2 Failure strain

It can be seen from Table 1 that the failure strain of fiber asphalt concrete increased with temperature. This trend can be attributed to the following facts: When the temperature is below 0 ℃, the asphalt is highly brittle, and the fiber asphalt concrete exhibits obvious elasticity. The lower the temperature, the more elastic the fiber asphalt concrete, and the smaller the deflection and strain at flexural-tensile failure. When the temperature is above 0 ℃, the asphalt is highly viscous, and the fiber asphalt concrete exhibits obvious viscoelasticity. The higher the temperature, the more viscoelastic the fiber asphalt concrete, and the greater the deflection and strain at flexural-tensile failure.

When the temperature was equal to or smaller than 0 ℃, the failure strain of fiber asphalt concrete increased linearly with the rising temperature. The linear relationship can be expressed as:

${{\varepsilon }_{B}}=a+bT$           (5)

where a and b are two parameters about the relationship between the failure strain of fiber asphalt concrete and temperature.

The bending test data on polyester fiber asphalt concretes were fitted nonlinearly to obtain the model parameters of formula (5) for fiber asphalt concretes. The results show that a and b first increased and then decreased with the growth in fiber volume ratio and fiber aspect ratio. The two parameters minimized at the fiber volume ratio of 0.35 % and the fiber aspect ratio of 324. In this case, the time-failure strain curve of polyester fiber asphalt concrete remained at a low level, indicating that the polyester fibers fully restrain deformation at low temperatures and that polyester fiber asphalt concrete is not sensitive to temperature changes.

Table 3. Model parameters of formula (5) and brittle temperatures

It can also be seen from Table 1 that the failure brittleness modulus of polyester fiber asphalt concrete is negatively correlated with temperature. The negative correlation was obviously linear when the temperature was below 0 ℃, and was extremely significant when the temperature was above 0 ℃. A possible reason is provided as follows: When the temperature was below 0 ℃, the flexural-tensile failure of fiber asphalt concrete was clearly elastic. The lower the temperature, the smaller the failure deformation of the fiber asphalt concrete in the elastic phase, and the greater the failure brittleness modulus. When the temperature was above 0 ℃, the flexural-tensile failure of fiber asphalt concrete was clearly viscoelastic. The higher the temperature, the greater the failure deformation of the fiber asphalt concrete in the viscoelastic phase, and the smaller the failure brittleness modulus.

5.1 Flexural-tensile strength

As can be seen from Table 1, the flexural-tensile strength of polyester fiber asphalt concrete increased first and then decreased with the growth of fiber aspect ratio, when the temperature remained the same. The maximum flexural-tensile strength appeared at the fiber aspect ratio of 324, under which the fiber asphalt concrete showed good flexural-tensile performance. These results can be explained by the following facts.

When the fiber diameter and fiber volume ratio remain constant, the fibers tend to be longer and point to different directions under a high fiber aspect ratio. In this case, the fibers are highly likely to curl in the asphalt concrete, forming cake-shaped defects [14], which dampens the enhancement effect of fibers. Under the same conditions, a small fiber aspect ratio means each fiber is relatively short, and more fibers exist per unit volume of asphalt concrete. Then, the fibers will become less dispersive and form cake defects in the asphalt concrete, which reduces the enhancement effect of fibers. Under the polyester fiber aspect ratio of 324, the fibers will neither curl into cakes due to the excessive length nor form cake defects due to the numerous fibers per unit volume of concrete. As a result, the fiber asphalt concrete will achieve a good flexural-tensile performance when the fiber aspect ratio is 324.

5.2 Failure strain

It can be seen from Table 1 that, when the temperature remained unchanged below 0 ℃, the failure strain of fiber asphalt concrete decreased first and then increased with the growth in fiber aspect ratio, and reached the minimum at the fiber aspect ratio of 324; in this case, the fiber asphalt concrete enjoyed a good flexural-tensile performance. When the temperature stood at 15 ℃, the failure strain of fiber asphalt concrete increased first and then decreased with the growth in fiber aspect ratio, and reached the maximum at the fiber aspect ratio of 324; in this case, the fiber asphalt concrete also enjoyed a good flexural-tensile performance.

When the temperature is below 0°C, brittle fractures form on the asphalt concrete during the flexural-tensile failure. The flexural-tensile deformation of the asphalt concrete is restrained by the fibers, which bear lots of tensile stress. Under the fixed fiber volume ratio and fiber diameter, each fiber is relatively long at a high fiber aspect ratio. In this case, there will be relatively few fibers per unit volume of asphalt concrete, and thus limited number of longitudinal fibers on the beam section under flexural-tensile failure. Therefore, the fibers’ restraining effect on the flexural-tensile deformation of asphalt concrete will decrease. The high fiber aspect ratio also means the fibers tend to point to different directions, making it easy for fibers in the asphalt concrete to curl into cakes and reduce the restraining effect of the fiber network.

Under the fixed fiber volume ratio and fiber diameter, each fiber is relatively short at a low fiber aspect ratio. In this case, there will be many fibers per unit volume of asphalt concrete, leading to nonuniform dispersion of the fibers. It is highly likely for fibers to accumulate in local areas of the fiber network in the asphalt concrete, which undermines the restraining effect of the fiber network on concrete deformation. In addition, the tensile strength of short fibers often exceeds the interface strength. The fibers are pulled out along the interface, causing slippage damages. Thus, the fibers will become less efficient in restraining concrete deformation. The optimal restraining effect appears at the fiber aspect ratio of 324, which corresponds to the minimum failure strain of fiber asphalt concrete.

When the temperature remains at 15℃, the fiber asphalt concrete beam mainly suffers from plastic damages during flexural-tensile failure, and the fibers are mostly damaged by slippage along the interface. During interface slippage, the fibers’ ability to prevent crack extension is positively correlated with the failure strain of the fiber asphalt concrete. If the fiber aspect ratio is high, each fiber will be relatively long and bear a high tensile stress in interface slippage, and will be more likely to break. If the fiber aspect ratio is low, each fiber will be relatively short and get pulled out early in interface slippage. Once pulled out, the fiber will no longer bear the tensile stress. Then, the cracks will propagate quickly, causing flexural-tensile failure of the asphalt concrete.

5.3 Failure stiffness modulus

It can also be seen from Table 1 that, when the temperature was below 0 ℃, the failure stiffness modulus of fiber asphalt concrete increased first and then decreased with the growth of fiber aspect ratio, and peaked at the fiber aspect ratio of 324; in this case, the fiber asphalt concrete demonstrated good flexural-tensile performance. When the temperature was at 15 ℃, the failure stiffness modulus of fiber asphalt concrete decreased first and then increased with the growth of fiber aspect ratio, and minimized at the fiber aspect ratio of 324; in this case, the fiber asphalt concrete also displayed good flexural-tensile performance.

The reason is that, when the temperature is below 0 ℃, the asphalt concrete may suffer from low-temperature brittle damages under the flexural-tensile stress. The fibers can bear a part of the stress to restrain the concrete deformation. Within a proper range, the restraining effect increases with the fiber aspect ratio. If the fiber aspect ratio is excessively high, however, the fibers will curl into cake defects, and become less effective in restraining the flexural-tensile deformation. That is why the failure stiffness modulus of fiber asphalt concrete increased first and then decreased with the growth of fiber aspect ratio, when the temperature was below 0 ℃.

When the temperature reaches 15 ℃, the fiber asphalt concrete, which belongs to the viscoelastic phase, may suffer from plastic damages under flexural-tensile stress. In this case, the fibers mainly prevent crack extension with the tensile stress from interface slippage. The greater the prevention ability, the larger the failure strain of fiber asphalt concrete beam. Under the same fiber volume ratio, the fibers tend to slip along the interface at a small fiber aspect ratio and rupture at a large fiber aspect ratio. In either case, the fibers will become less efficient in restraining crack extension, dragging down the resistance of fiber asphalt concrete to flexural-tensile deformation. As a result, the failure stiffness modulus of fiber asphalt concrete decreased first and then increased with the growth of fiber aspect ratio, when the temperature was at 15 ℃.

The following conclusions were drawn from our bending tests on polyester fiber asphalt concretes:

(1) The flexural-tensile strength of fiber asphalt concrete increases first and then decreases with the rising temperature, and peaks at the temperature of 0 ℃. Under this temperature, the fiber asphalt concrete obviously softens under stress. Through nonlinear fitting, the author obtained the relationship between flexural-tensile strength of fiber asphalt concrete and temperature, which shows that the fibers can reduce the brittle temperature of the asphalt concrete.

(2) The failure strain of fiber asphalt concrete increases with temperature. When the temperature is below 0 ℃, the failure strain has an obvious linear relationship with temperature, while the failure stiffness modulus decreases with the rising temperature.

(3) Under different temperatures, the flexural-tensile strength of polyester fiber asphalt concrete increases first and then decreases with the growth in fiber volume ratio or fiber aspect ratio. When the fiber asphalt concrete suffers from low-temperature brittle damages, the failure strain decreases first and then increases with the growth in fiber volume ratio or fiber aspect ratio, while the failure stiffness modulus increases first and then decreases with the growth in fiber volume ratio. When the fiber asphalt concrete suffers from plastic damages under flexural-tensile stress, the failure strain increases first and then decreases with the growth in fiber volume ratio or fiber aspect ratio, while the failure stress modulus decreases first and then increases with the growth in fiber volume ratio.

(4) The FCFP was introduced to describe the combined effect of fiber volume ratio and fiber aspect ratio on the flexural-tensile performance of asphalt concrete. The correlations of the FCFP with the flexural-tensile strength, failure strain and failure stiffness modulus of polyester fiber asphalt concrete can be expressed respectively by formulas (12)-(16), formulas (17)-(21) and formulas (22)-(26). The test results and theoretical analysis show that the optimal FCFP of polyester fibers is 1.13.

The future research will probe deeper into the FCFP, a parameter reflecting the combined effect of fiber volume ratio and fiber aspect ratio on the flexural-tensile performance of asphalt concrete.