Effect of Temperature and Asphalt Binder Type on the Properties of Foamed Asphalt Mix

In this study, an effect of temperature on both indirect tensile strength (ITS) test and Marshall Stability Test of foam asphalt mixture using Shell #70 A asphalt is firstly investigated. A detailed investigation of the mechanical properties of foam asphalt mixture was then carried out using two different asphalt binders. The experimental work compares the mechanical properties of foam asphalt mixes including water stability test, indirect tensile strength (ITS) test, and freeze-thaw splitting test. The test results indicate that the foam asphalt mixture is temperature dependent and also temperature sensitive material. The foaming properties of asphalt have an important effect on the properties of the foam asphalt mixture for the strength and water stability of the foam asphalt mixture.


Introduction
With the rapid development of transport infrastructure in China [1], the use of foamed bitumen as a stabilizing agent in pavement since 1956 [2] is improving steadfastly and internationally. Foamed technology has been used by numerous road and highway stabilization and recycling projects [3][4][5][6] after its production process is modified by injecting steam to cold water into hot bitumen in 1976 and also introducing new machineries in the market in mid-1990 [7].
The use of foam bitumen stabilization is imperative because of its high field performance properties and its environmentally friendly benefits [8][9][10][11]. Many factors such as bitumen type, grade, and additives; in particular, antifoaming agents often are added to bitumen produced by solvent precipitation processes affected its foaming characteristics.
Asphalt can be foamed at high temperature by adding approximately 2% amount of water to hot bitumen resulting in the formation of liquid asphalt binders, which is called foam asphalt [12]. Foamed bitumen advantages have grown in recent years and consist of (a) improving mix strength and durability, (b) increasing spectrum of aggregates usable, (c) reducing binder content, (d) improving workability, (e) propensity of using labor-intensive methods, (f) energy savings during the mix processing and others [13][14][15][16]. Foam asphalt technology has thus been widely used in asphalt pavement structure during the last decade all over the world [17][18][19][20]. China has mainly used it to use to recycle asphalt pavement [21]. The effects of asphalt binder properties on foamed asphalt mix properties were studied and reported by in references [22] and [23] respectively. They have concluded through their investigation that asphalt binders with higher expansion ratio and longer half-life tend to produce better quality foamed asphalt mixes Use of new aggregate in foam asphalt mixture is also a good choice for new asphalt pavement structure. However, there is a difference between foam asphalt mixtures with foam asphalt recycled mixture in terms of the strong performance, it is imperative to research the strength characteristic in the laboratory. Mechanical performance of cold foamed bitumen mixes was investigated through laboratory [24] using bitumen source, foaming additive, fiber reinforcement and cement effect. The mechanical properties included dynamic modulus and indirect direct tensile strength of dry and conditioned specimens. The results show that bitumen with low foamability provided better mechanical performance than that with the better foamability.
The objectives of this study were to first investigate the effect of temperature on both indirect tensile strength (ITS) test and Marshall Test of foam asphalt mixture using Shell #70 A asphalt and secondly to see the effect of two different asphalt binder type on the mechanical properties of foam asphalt mixture.

Materials and test program
Zhonghai # 90 A grade asphalt and Shell #70 A bitumen currently used in China were investigated. For each source, penetration, viscosity, and softening point were measured. They used to produce foam asphalt at 150℃. Water content is 4% of asphalt mass when producing foam asphalt. Physical properties of base asphalt for the two sources and their respective foaming characteristics are shown in Table 1. The mixture design gradation is given in Table 2. The optimal water content of foam asphalt mixture is determined by Marshall 75 times compaction test. The optimal foam asphalt content is 3.5%.

Water Stability
The residual Marshall stability is used as the index to evaluate the water stability according to the Chinese Technical Specification for Construction of Highway Asphalt Pavement. Standard Marshall Test and immersion Marshall Test were conducted according to Chinese specification T0709-2011, and then the residual stability can be determined from the Marshall stability and the immersion Marshall Stability according to formula (1) where S is the residual stability, %; 1 is the immersion Marshall stability, kN; and 0 is the Marshall stability, kN.

Indirect tensile strength test
For the indirect tensile test, Marshall specimens were subjected to compressive loads in equipment fabricated for this purpose. The tests were conducted at 2 ℃, 50 mm/min loading rate. The test procedure adopted was as per Chinese specifications JTJ T0716-2011. The split strength can be obtained according to formula (2).
in which, R T is a split strength (MPa), h: displacement (mm), and P T is the load when specimen destroyed (N).

Comparison of freeze-thaw splitting
The tensile strength ratio is defined as the tensile strength of specimen after freeze-thaw conditioning compared to the tensile strength of specimen at ambient temperature. During the freeze-thaw conditioning, the specimen is immersed in water under vacuum state for 15 min, then froze at −18℃ in the refrigerator for 16 Where TSR: freeze-thaw strength ratio, %, R T1 : strength without freeze and thaw, MPa, R T2 : Strength after freeze and thaw, MPa.

Comparison of test indexes under different temperature
The effect of temperature on both indirect tensile strength (ITS) test and Marshall Test of foam asphalt mixture using Shell #70 A asphalt is investigated. The test results of dry and wet indirect tensile strength test at 2 ℃ are shown in Table 3. Table 4 shows the results of dry wet indirect tensile strength test under different temperatures. The Marshall test results at 40℃ and 60℃ are shown in Table 5.   Through Tables 3 to 5, it can be seen that the properties of the foam asphalt mixture are temperature dependent and are also temperature sensitive materials. The dry-wet ITS ratio of foam asphalt mixture at 1 ℃ increased obviously than that of2 ℃, and the dry ITS increased 40.4% while that of wet ITS increased by 60.6%

Mechanical properties of foam asphalt mixture.
The compared performance properties of foam asphalt mixture from two kinds of asphalt binder including water stability, indirect tensile strength, and freeze-thaw splitting was carried. The results of water stability tests were shown in Table 6. It can be seen from Table 6 that the water stability of these two foam asphalt mixtures meets the specification requirement. And the residual stability of Zhonghai # 90 A asphalt mixture is greater than that of Shell #70 A, which means that the water stability of Zhonghai # 90 A asphalt mixture is better than that of Shell #70 A asphalt mixture. There is no much difference from Shell #70 A asphalt and Zhonghai # 90 A asphalt, after 48 hours of immersion Marshall Attenuation. The above test data demonstrate that the Marshall stability is not sensitive to the performance of the foamed asphalt and that the Marshall stability can meet the specified requirements when the foaming performance is poor. Table 7 presents the results of the indirect tensile test. From the Table 7, it can be seen that the dry splitting strength of Zhonghai # 90 A asphalt is 16.3% higher than shell asphalt while its wet splitting strength is 17.5% higher. Whether it is dry or wet splitting, the Zhonghai # 90 A asphalt has a better strength ability compare to Shell #70 A asphalt. The results of the freeze-thaw splitting test for the two sources of asphalt are shown in Table 8. It can be seen that after a freeze-thaw cycle, the wet splitting strength of both Shell #70 A and Zhonghai # 90 A foam asphalt mixtures decreased in varying degrees. After the freeze-thaw cycle, the water stability of Zhonghai # 90 A foam asphalt mixture is better than that of Shell #70 A foam asphalt mixture, and after a freezing-thawing cycle the water stability of Shell #70 A foamed asphalt mixture was significantly decreased

Conclusion
The aim of this study is to first investigate the effect of temperature on both indirect tensile strength (ITS) test and Marshall Test of foam asphalt mixture using Shell #70 A asphalt and secondly to see the effect of two different asphalt binder type on the mechanical properties of foam asphalt mixture. The main conclusions drawn from the current research work are presented as follows:  Foam asphalt mixture is temperature dependent and are also temperature sensitive materials  The water stability of Zhonghai # 90 A asphalt mixture is better than that of Shell #70 A asphalt mixture.  Foam asphalt mixture with Zhonghai # 90 A asphalt has a better indirect tensile strength compared to that of the Shell #70 A asphalt.  The water stability of Zhonghai # 90 A foam asphalt mixture is better than that of Shell #70 A foam asphalt mixture after the freeze-thaw cycle.