Determination of Indirect Tensile Strength of Bituminous Concrete Mix Prepared Using Stone Dust and Cement as Filler Materials

Abstract

Pavement engineers are too much concerned about the tensile properties of bituminous mix because of the problems associated with cracking. Hence tensile strength of bituminous concrete mix is important in pavement applications. The indirect tensile strength test is used to assess the tensile properties of the bituminous mix. The present research work, essentially emphasize to study the effect of filler materials, test temperature on the tensile properties and moisture susceptibility characteristics of bituminous concrete mix prepared using stone dust and cement as filler materials. For the preparation of bituminous concrete mix specimens using stone dust and cement as filler materials, optimum bitumen content was determined by adopting Marshall method of bituminous mix design. Bituminous mix properties were determined at optimum bitumen content. Indirect tensile strength (ITS) and Tensile strength ratio (TSR) of bituminous concrete mix were evaluated by varying test temperatures at 15 °C, 20 °C, 25 °C, 30 °C and 35 °C. Prediction model for Indirect tensile strength was developed by considering the temperature, Marshall stability and optimum bitumen content as independent variable for each filler material. As the test temperature increases the ITS and TSR values of bituminous concrete mix decreases irrespective of type of filler material. Based on the analysis of data, it was observed that at any test temperature, ITS and TSR values of bituminous concrete mix prepared using cement as filler material were higher when compared to bituminous concrete mix prepared using stone dust as filler material. It may be concluded that the behavior of bituminous concrete mix prepared using cement as filler material is superior in terms of mix properties, ITS and TSR.

1 Introduction

The resistance of bituminous mixtures to fatigue cracking is dependent upon its tensile properties, notably its tensile strength and extensibility characteristics. The layers in a flexible pavement structure are subjected to continuous flexing as a result of the traffic loads that they carry, resulting in tensile stresses and strains at the bottom of the bituminous layers of the pavement. The magnitude of the strain is dependent on the overall stiffness of the pavement. Indirect tensile strength test is an indicator of strength and adherence against fatigue, temperature cracking and rutting. Tensile strength is difficult to measure directly because of secondary stresses induced by gripping a specimen so that it may be pulled apart. Therefore, tensile stresses are typically measured indirectly by a splitting tensile test. As per the study of (Breen and Stephens 1966), the split cylinder test is preferred to assess the tensile strength of bituminous concrete mix at temperatures from 0 to 40 F. Many researchers (Ross et al. 1989, Sabnis et al. 1979, Chen et al. 1980) attest that tensile strength is inversely proportional to diameter of specimen (while a study made by Hondros (1959) represents vice versa that tensile strength increases with the increase in diameter of specimen. (Tunnicliff 1962) The application of filler material in a bituminous mix is primarily preferred to improve the characteristics of mix by minimizing the temperature susceptibility of binder.

Filler is a prime constituent of bituminous concrete mix and imparts a significant effect on its characteristics and performance (Anderson et al. 1992). A well-built backbone for the mixture is provided by the good packing of the coarse aggregate, fine aggregate and filler (Vavrik et al. 2002; Qiu 2006). The filler can improve the resistance of particle to move within the mix matrix and/or works as an active material when it interacts with the asphalt binder to change the properties of the mastic (Kalkattawi 1993). Zulkati et al. (2012) represents that performance of asphalt concrete mix is affected by the involvement of filler material in three traditions i.e. Firstly the amount of asphalt content in a mix largely depends upon filler material, Secondly workability during mixing and compaction, and lastly the resultant properties of asphalt-filler mastic contribute to the performance of mix.

Al-Sayed (1988) has done comprehensive investigation on the use of filler material type and recommended the utilization of the crushed limestone (fines passing than 0.975 mm) that improves the characteristics of asphalt mixes and accordingly, reduces cracking and rutting of pavements. The adding of filler material improves the resilient modulus of asphalt concrete mix (Anderson 1987). Excessive amount of filler material may result in a harsh mix as the high amount of asphalt would be needed to cover the aggregates (Elliot et al. 1991; Kandhal et al. 1998), which affect the workability of the mixture as well. According to the Craus et al. (1978), the interactive physio chemical features between the filler and bitumen are correlated to adsorption intensity at the filler-bitumen interface, and surface activity considerably contributes to better bonds at the filler-bitumen interface. It can be ascertain that the interactive role related with the physio chemical reaction, which is influenced by the type of bitumen and filler as well as the selection of filler material type in bituminous mix that would also modify its properties and improve the behavior of the bituminous mix. Wu et al. (2011) Filler material can improve the temperature susceptibility and durability performance of binder and asphalt concrete mixture.

2 Objective of Present Study

The objectives of the present study are-

• To study the effect of stone dust and cement as filler materials on the indirect tensile strength and tensile strength ratio of bituminous concrete mix at different temperatures viz., 15 °C, 20 °C, 25 °C, 30 °C and 35 °C.

• To develop a mathematical model for predicting the indirect tensile strength of bituminous concrete mix prepared using cement and stone dust as filler materials.

3 Material Characterization

Coarse aggregates, fine aggregates, mineral filler and binder are the general constituents of a bituminous mix. In the present study VG-30 as binder while Stone dust and cement are used as mineral filler.

3.1 Aggregates

Aggregates offers compressive and shear strength and shows good interlocking properties. To assess the properties of aggregates i.e. sufficient strength, hardness, toughness, specific gravity and shape, tests were conducted on aggregates in the laboratory and the test results are presented in Table 1. Aggregate gradation for Bituminous concrete mix (Grading-2) is adopted for the present study as per MORT&H (V^th Revision) and is presented in Table 4.

Table 1. Aggregate test results

Table 2. Gradation requirements of filler material

Table 3. Bitumen (VG-30) test results

Table 4. Aggregate gradation for bituminous concrete mix (Grading-2)

3.2 Mineral Filler

Mineral filler fills the voids, stiffens the binder and offers permeability. In the present study, stone dust and cement is used as mineral fillers. The specific gravity test and gradation requirement were tested in the laboratory and results obtained are presented in Table 2. Specific gravity of stone dust and cement are 2.60 and 3.09.

3.3 Bitumen

In the present study viscosity grade (VG-30) bitumen is used as binder material. In order to check the suitability of bitumen as binder material, tests were conducted in laboratory. The test results satisfy the requirements as per IS 73:2013 and are presented in Table 3. The specific gravity of bitumen (VG-30) is 1.01.

4 Test Programme

4.1 Indirect Tensile Strength Test (ITS)

The Indirect Tensile Strength test is conducted in the laboratory by loading a cylindrical specimen with a single or repeated compressive load, which acts parallel to and along the vertical diametric plane. This loading configuration develops a relatively uniform tensile stress perpendicular to the direction of the applied load and along the vertical diametric plane, which ultimately causes the specimen to fail by splitting along the vertical diameter. Indirect tensile strength test mechanism is shown in Fig. 1.

Fig. 1.

Indirect tensile strength test mechanism

Test Procedure for Conducting Indirect Tensile Strength test is-

• The test specimens are prepared at the optimum bitumen content using Marshall method of mix design for bituminous concrete mix (Grade-2) as per MoRT&H (V revision) specifications.

• The height of the specimens is recorded. Each set of specimens was tested at test temperature of 25 °C to determine their indirect tensile strength. This was achieved by using breaking head under a load applied at a rate of 50 mm per minute.

Load at failure is recorded and the indirect tensile strength is computed using the relation given below.

$$ \sigma_{x} = \left( {2 \times P} \right)/\left( {\pi \times D \times t} \right) $$

(1)

Where:

σ_x:

= Horizontal tensile stress/tensile strength, N/mm²

P:

= Failure load, N

D:

= Diameter of the specimen, mm

t:

= Height of the specimen, mm.

4.2 Tensile Strength Ratio (TSR)

Continuous contact of water with pavement causes premature pavement damage. Moisture induced distress may be considered as one of the most significant problem for bituminous pavement. Moisture susceptibility of bituminous mixes can be quantified by tensile strength ratio. Similar to Indirect Tensile Strength test procedure, Marshall specimens are prepared at optimum bitumen content. Procedure for evaluating the tensile strength as per ASTM D-1075 is-

Samples were divided into two groups i.e. Group-1 and group-2.

• The Group-1 of specimens are tested in a dry condition (unconditioned state), while the Group-2 specimen are tested in soaked condition.

• For the Group-1, specimens are treated as control specimens without any conditioning and then all specimens are tested for ITS at test temperature of 15 °C, 20 °C, 25 °C, 30 °C and 35 °C (by keeping them in a water bath maintained at test temperature for not less than 2 h) under the loading rate of 50 mm per minute. The load at failure is recorded and the indirect tensile strength is computed using the Eq. (1).

• For the Group-2, specimens were kept in water bath maintained at 60 °C for 24 h. The specimens were transferred to the second water bath maintained at 15 °C, 20 °C, 25 °C, 30 °C and 35 °C and kept for 2 h and then tested for ITS.

• Tensile strength ratio (TSR) can be determined using the following relation-

$$ \text{TSR = Sn / St} $$

(2)

Where

TSR::

Indirect Tensile Strength Ratio

St::

Average Indirect tensile strength of Group-1 specimens

Sn::

Average Indirect tensile strength of Group-2 specimens

5 Results and Discussion

Indirect Tensile Strength test is conducted on bituminous concrete mix (Grading-2) prepared using stone dust (1%), stone dust (2%), cement (1%) and cement (2%) as filler materials at optimum bitumen content. Specimens are conditioned at 15 °C, 20 °C, 25 °C, 30 °C, 35 °C in water bath for a duration of 2 h. The test results are presented in Tables 5, 6 and 7, Figs. 2 and 3.

Table 5. Indirect tensile strength of bituminous concrete mix prepared using stone dust and cement as filler materials

Table 6. Indirect tensile strength (Conditioned Specimen @ 60 °C) of bituminous concrete mix prepared using stone dust and cement as filler materials

Table 7. Tensile strength ratio of bituminous concrete mix prepared using stone dust and cement as filler materials

Fig. 2.

Indirect tensile strength of bituminous concrete mixes prepared using stone dust and cement as filler materials.

Fig. 3.

Tensile strength ratio of bituminous concrete mixes prepared using stone dust and cement as filler materials.

5.1 Indirect Tensile Strength (ITS)

5.1.1 Effect of Filler Material

It is observed from Table 5 that the indirect tensile strength (ITS) of bituminous concrete mix depends on filler content and also vary depending on filler type. At 25 °C test temperature, there is substantial increase in the indirect tensile strength value of bituminous concrete mix prepared using Cement 2% and Cement 1% by about 15% and 11% when compared to bituminous concrete mix prepared using stone dust 2% and stone dust 1% respectively. Similar trend can also be observed at 15 °C, 20 °C, 30 °C and 35 °C test temperature.

Further the ITS value of bituminous concrete mix prepared using stone dust as filler material is always lesser than cement as filler material. The difference among the results may be due to cement as filler material causes better stiffening or reinforcing effect in bituminous concrete mix as well as acting as an antistriping agent.

5.1.2 Effect of Test Temperature

In Fig. 2 it is observed that as the temperature increases from 15 °C to 35 °C, there is substantial reduction in ITS value of bituminous concrete mix prepared using cement 2%, stone dust 2%, cement 1% and stone dust 1% as filler material by about 16%, 14%, 6% and 11% respectively.

Actually temperature is inversely proportional to viscosity. Therefore, with the elevation in temperature, the viscosity of bitumen reduces. This may be one of the factor that, as the test temperature increases from 15 °C to 35 °C, the ITS value of bituminous concrete mix reduces irrespective of type of filler material and filler content.

5.2 Tensile Strength Ratio (TSR)

Figure 3 shows the relationship between the tensile strength ratio (TSR) and different test temperature ranging from 15 °C to 35 °C for stone dust and cement as filler material. At 25 °C test temperature, there is marginal increase in tensile strength ratio of bituminous concrete mix prepared using cement 2% and cement 1% by about 6% and 5% when compared to bituminous concrete mix prepared using stone dust 2% and stone dust 1% respectively. Similar trend can also be observed at 15 °C, 20 °C, 30 °C and 35 °C test temperature.

As the test temperature increases from 15 °C to 35 °C, the TSR value of bituminous concrete mix reduces irrespective of type of filler material and filler content. This is observed in Table 7.

6 Prediction Model

Prediction model for indirect tensile strength (ITS) is developed by considering the temperature, Marshall stability and optimum bitumen content as independent variables for each filler material i.e. cement 2%, stone dust 2%, cement 1% and stone dust 1%. Prediction model is developed in the form of-

$$ {\text{ITS}}_{{{X\_n}}} \text{ = }{\text{A}}_{0} \text{ + }{\text{A}}_{\text{1}} {\text{T}}\text{ + A}_{2} {\text{S}}\text{ + A}_{3} \text{OBC} $$

(3)

Where-

ITS_{X_n}:

= Indirect Tensile Strength, N/mm²

X:

= Filler Material, Stone dust or Cement

N:

= Filler Content, 1% or 2%

A_o:

= Constant

A_1, A_2, A₃:

= Regression Coefficient

T:

= Temperature, °C

S:

= Marshall Stability, Kg

OBC:

= Optimum Bitumen Content, %

6.1 Model for Stone Dust 1%

Summary of prediction model for indirect tensile strength (ITS) of bituminous concrete mix prepared using stone dust 1% is presented in Table 8. Model developed for stone dust 1% is as-

Table 8. Summary of prediction model for ITS value of bituminous concrete mix prepared using stone dust 1%

$$ {\mathbf{ITS}}_{{{\mathbf{S\_1}}}} = - {\mathbf{15}}.{\mathbf{548}} - {\mathbf{0}}.{\mathbf{005}}{\mathbf{T}} + {\mathbf{0}}.{\mathbf{01}}{\mathbf{S}} + {\mathbf{0}}{\mathbf{.822OBC}} $$

(4)

Coefficient of correlation and adjusted R square (R²) values are 0.990 and 0.978 respectively. R² is the coefficient of determination. Table 8 illustrate that independent variables i.e. temperature, Marshall stability and optimum bitumen content are able to explain the variation in indirect tensile strength (ITS) up to an extent of 97.8% (R² = 0.978). Further analysis also indicates that temperature, Marshall stability and optimum bitumen content are statistically significant associated with the ITS value (p = 0.0000) as p value is less than the predetermined α value i.e. 0.05 (5%).

6.2 Model for Stone Dust 2%

Summary of prediction model for indirect tensile strength (ITS) of bituminous concrete mix prepared using stone dust 2% is presented in Table 9. Model developed for stone dust 2% is as-

Table 9. Summary of prediction model for ITS value of bituminous concrete mix prepared using stone dust 2%

$$ {\mathbf{ITS}}_{{{\mathbf{S}}\_{\mathbf{2}}}} = - {\mathbf{29}}.{\mathbf{248}} - {\mathbf{0}}.{\mathbf{006}}{\mathbf{T}} + {\mathbf{0}}.{\mathbf{025}}\;{\mathbf{S}} - {\mathbf{0}}.{\mathbf{183}}\;{\mathbf{OBC}} $$

(5)

Coefficient of correlation and adjusted R square (R²) values are 0.988 and 0.972 respectively. R² is the coefficient of determination. Table 8 illustrate that independent variables i.e. temperature, Marshall stability and optimum bitumen content are able to explain the variation in indirect tensile strength (ITS) up to an extent of 97.2% (R² = 0.972). Further analysis also indicates that temperature, Marshall stability and optimum bitumen content are statistically significant associated with the ITS value (p = 0.0000) as p value is less than the predetermined α value i.e. 0.05 (5%).

6.3 Model for Cement 1%

Summary of prediction model for indirect tensile strength (ITS) of bituminous concrete mix prepared using cement 1% is presented in Table 10. Model developed for cement 1% is as-

Table 10. Summary of prediction model for ITS value of bituminous concrete mix prepared using cement 1%

$$ {\mathbf{ITS}}_{{{\mathbf{C}}\_{\mathbf{1}}}} = - {\mathbf{15}}.{\mathbf{246}} - {\mathbf{0}}.{\mathbf{002}}{\mathbf{T}} + {\mathbf{0}}.{\mathbf{012}}{\mathbf{S}} - {\mathbf{0}}.{\mathbf{159}}\;{\mathbf{OBC}} $$

(6)

Coefficient of correlation and adjusted R square (R²) values are 0.992 and 0.984 respectively. R² is the coefficient of determination. Table 8 illustrate that independent variables i.e. temperature, Marshall stability and optimum bitumen content are able to explain the variation in indirect tensile strength (ITS) up to an extent of 98.4% (R² = 0.984). Further analysis also indicates that temperature, Marshall stability and optimum bitumen content are statistically significantly associated with the ITS value (p = 0.0000) as p value is less than the predetermined α value i.e. 0.05 (5%).

6.4 Model for Cement 2%

Summary of prediction model for indirect tensile strength (ITS) of bituminous concrete mix prepared using cement 2% is presented in Table 11. Model developed for cement 2% is as-

Table 11. Summary of prediction model for ITS value of bituminous concrete mix prepared using cement 2%

$$ {\mathbf{ITS}}_{{{\mathbf{C}}\_{\mathbf{2}} }} = - {\mathbf{64}}.{\mathbf{70}} - {\mathbf{0}}.{\mathbf{009}}{\mathbf{T}} + {\mathbf{0}}.{\mathbf{044}}{\mathbf{S}} + {\mathbf{0}}.{\mathbf{107}}\;{\mathbf{OBC}} $$

(7)

Coefficient of correlation and adjusted R square (R²) values are 0.997 and 0.992 respectively. R² is the coefficient of determination. Table 8 illustrate that independent variables i.e. temperature, Marshall stability and optimum bitumen content are able to explain the variation in indirect tensile strength (ITS) up to an extent of 99.2% (R² = 0.992. Further analysis also indicates that temperature, Marshall stability and optimum bitumen content are statistically significant associated with the ITS value (p = 0.0000) as p value is less than the predetermined α value i.e. 0.05 (5%).

7 Conclusions

On the basis of limited laboratory study carried out in the present study, the following conclusion are drawn-

• At the elevated temperature, indirect tensile strength (ITS) value of the bituminous concrete mix prepared using stone dust and cement as filler material decreases.

• ITS value of viscoelastic material is highly influenced by the test temperature.

• Substantial increase in indirect tensile strength value of bituminous concrete mix prepared using cement (2%) and cement (1%) as filler materials.

• Marginal increase in tensile strength ratio (TSR) of bituminous concrete mix prepared using cement (2%) and cement (1%) as filler materials.

• If the tensile strength of bituminous concrete mix is more and TSR is with in the permissible limit, then it can be concluded that bituminous concrete mix may be safe against fatigue.

• In the prediction model, temperature, Marshall stability and optimum bitumen content are statistically significant with the ITS value.

• It may be concluded that the behavior of bituminous concrete mix prepared using cement as filler material is superior in terms of mix properties, ITS and TSR value.