Abstract
Various types of paving binders have recently been developed for heavy traffic pavements. However, there are only limited studies on the mechanical properties considered from the view point of practical performance. Relationships which paving mixtures must have between mechanical properties and practical performance are as follows;
1) Failure properties and failure envelope of mixtures vs. pavement cracking.
2) Modulus of mixtures vs. load spreading properties and bearing capacity of pavement.
3) Stress Relaxation properties of mixtures vs. pavement cracking and pavement deformation.
4) Deformation properties of mixtures vs, pavement deformation such as waving and rutting.
5) Fatigue properties of mixtures vs, mechanical fatigue failure of pavement. These properties are closely related.
This paper deals with the stress relaxation properties among these mechanical properties of mixtures. It may be convenient to discuss stress relaxation properties from two separate aspects- the deformation resistance at higher temperatures and the thermal crack at lower temperatures. The ease with which stress relaxes corresponds to the ease with which the mixtures can deform. The slower is the stress relaxation of a mix, the less is its deformation at higher temperatures. However, when a mixture is subjected to lower temperatures, thermal crack may appear due to a lack of stress relaxation.
When modifying asphalt cement and using a high quality paving binder such as epoxy resin, the formulator and the highway engineer emphasize: stiffer mixtures and higher modulus (this modulus is defined as a function of temperature and loading time). Overemphasis on stiffer mixtures might lead to such unwelcoming result as pavement cracks caused by shrinkage and expansion of the mixtures.
In this experiment, we chose four kinds of binders; 1) asphalt cement 2) modified asphalt cement (Table 1) 3) epoxy asphalt cement obtained by blending an epoxy resin with asphalt cement containing a hardener 4) epoxy resin (Table 2).
The test mixtures manufactured were examined at the same level of binder content (5.8%) and at the same composition of the aggregates (Table 3).
Four kinds of tests were made: the stress relaxation test, the constant rate of strain test, the creep test and dynamic loading test. The stress relaxation test was used in order to measure relaxation phenomena directly. The other three tests were carried out to calculate the relaxation modulus or Er(t).
The stress relaxation test gave us the following results (Figs. 2-5).
1) Er(t) vs. loading time curve.
2) Master curves of Er(t) obtained by application of superposition principle.
3) Shift factor vs, temperature curves.
The lower the temperature and shorter the loading time, the larger was the Er(t) of each mixture.
The upper limits of Er(t) of straight asphalt mixtures, modified asphalt mixtures, epoxy asphalt mixtures and epoxy resin mixtures were about 1.2×105 (kg/cm2), 1.2×105 (kg/cm2), 1.5×105 (kg/cm2), 1.6×105 (kg/cm2), respectively.
Strain is applied for a very short period, and elastic performance found in the stress-strain relationship under all conditions. The instant elastic modulus, defined by stress-strain in the momental loading corresponds to the maximum of Er(t) obtained under shorter loading times and lower temperatures.
This observation may mean that the mixtures suddenly decrease their values of Er(t) immediately after loading, and this tendency is obvious at higher temperatures. Details of the decrease in Er(t) can be readily understood from the master curve.
Since the values of Er(t) of mixtures defined by the ratio of strain to stress, the Er(t) curve will become stress vs.
