1. Field of the Invention
The present invention relates to a method for the compaction of asphalt and a compaction apparatus. More particularly, the present invention relates to a method and apparatus for compacting hot mix asphalt under conditions which advantageously optimize binder flow within the asphalt during compaction.
2. Description of the Related Art
By the term “binder” as used throughout this specification is meant any thermoplastic visco-elastic material which may be used in hot mix asphalts. Generally the binder will be bitumen or bituminous, that is a bitumen incorporating, for example polymeric modifiers. It is also known for hot mix asphalt to incorporate polymer binders with no bitumen based binders present, and the present invention extends to the compaction of all such hot mix asphalts.
Inherent in modern asphalt mix design for heavy duty applications is the use of components (aggregates and binders) which are purposely selected to resist compaction and loss of shape under heavy traffic. These properties will generally hinder the achievement of the desired compaction during laying of the asphalt.
The principal asphalt mix design element to resist compaction under heavy traffic is the use of aggregates with extremely rugose texture and cuboid shape, aimed at providing high shear resistance within the aggregate skeleton. In simple terms the objective is to ensure the physical properties of the aggregate inhibit particle movement and promote “lock up” in the structure under the applied load stress in operation. Stiffer binders such as polymer modified binders are increasingly being used to augment both the shear strength of the mix and also to improve the flexural or fatigue properties of the mix.
The achievement of lock up of the aggregate and the distribution of air voids in the mix on compaction and during laying determines asphalt durability and overall performance over the entire range of pavement loadings. Lock up of aggregate is advantageously achieved by displacing the aggregate within the binder during compaction of the asphalt mat.
The properties of the asphalt mix are also determined by the visco-elastic properties of its binder. At ambient service temperatures the binder desirably acts as a stiff elastic solid; the response to load in the asphalt mix is very nearly elastic and a rapid load pulse will result in a virtually instant elastic deformation which will recover almost the instant the load is removed. Thus, there is substantially no viscous flow and resultant permanent plastic strain. At the higher temperatures at which asphalt is laid and compacted, the binder in the mix is a visco-elastic fluid. The higher the temperature, the lower the viscosity of the binder and the more readily the binder will deform under any applied stress.
The compaction process begins with the laydown of hot asphalt by a paver on a prepared base, usually followed by pressure on the hot asphalt mat applied by a screed (with or without vibration). The screed is a plate or skid carried by the paver which slides over the surface of the asphalt mat desirably at or close to the temperature at which the mat is laid. The screed applies some initial compaction, but by its sliding action may undesirably cause shear stress in the mat leading to tearing of the mat. Typically the applied static screed pressure is in the order of 10 kPa (1.450 psi) to 20 kPa (2.901 psi) and the load duration may be as long as 10–15 seconds.
Conventionally, asphalt compaction has been carried out using equipment originally intended for compacting granular non-cohesive materials designed to maximize the compaction energy applied to the material, primarily by using large and heavy steel drum rollers, often in combination with high energy oscillation or vibration. Rubber-tired roller compaction is often used in conjunction with steel drum roller compaction, as described hereinafter.
The contact stress between the roller and the asphalt mat generally depends on the stiffness of the asphalt mix which is in turn strongly influenced by the stiffness of the binder. The contact area between the steel drum and the asphalt, that is the length of contact by the width of the roller drum, will diminish as a result of the compaction achievement and the increase in mix stiffness with the cooling of the mat. Typically the mix is at a temperature of about 150° C. (302° F.) when it is laid. In low temperature environments under adverse conditions such as when a strong wind is blowing, it is quite feasible the mix will cool to say 140° C. (284° F.) at the bottom of the layer and 120° C. (248° F.) at the surface before the first compaction pass.
The largest dual steel drum vibratory roller compactor presently in general use has a static mass of about 16 tonne (17.6 ton) with each drum having an axial length of about 2 m (6.56 ft). Assuming a nominal 100 mm (3.94 in) contact length in the roller direction (more in the initial pass, less in the final pass), each drum will apply a contact stress of about 400 kPa (58.015) static and considerably more with vibration. In fact, each drum may apply a contact stress from about 100 kPa (14.504 psi) in a first static breakdown pass to well over 1000 kPa (145.038 psi) as the asphalt mix stiffness and the contact area reduces. Compaction by the roller compactor usually occurs at varying distances, up to several hundred meters, behind the paver and at speeds of about 1.1 m/s (3.61 ft/s) (4 km/h (2.49 mph)) or more. The two drums of the roller compactor each having the above nominal contact length of 100 mm (3.94 in) and therefore the roller will typically be in contact with any part of the asphalt mat for about 0.2 seconds in each pass. Typically, about four steel roller passes are used, giving a total load time of about 0.8 seconds.
The roller compactor typically vibrates at about 20 Hz, which at temperatures of 140° C. (284° F.) and 120° C. (248° F.) corresponds to relatively high binder stiffness (shown by Van der Poel's nomograph) of about 0.2 kPa (0.029 psi) and 1 kPa (0.145 psi) respectively (each 20° C. (68° F.)) reduction in temperature has about a 5 fold increase in bitumen stiffness).
As described above, the surface temperature of the mat may fall to temperatures of about 120° C. (248° F.) before the roller compaction process is begun. The compaction process may typically include up to 4 roller compactor passes, by which time the mat surface temperature may be in the range 80 C (176° F.)–90° C. (194° F.). At mat temperatures below about 120° C. (248° F.) cracking of the mat may be initiated in the mat at high contact stresses, particularly at stresses induced using vibration-Mat cracking typically occurs when the applied stress induces strain in the binder in excess of its yield strength. At temperatures considerably above 120° C. (248° F.) conventional roller compaction may lead to significant shear failure in the mat, depending on the asphalt mix type. This may result in the mat being displaced laterally with loss of level and shape and ultimately in de-compaction of the mat.
Roller cracking resulting from low mat temperatures is usually manifest as fine, parallel cracks in the asphalt mat which are transverse to the direction of rolling. A multi-wheeled rubber-tired roller following the vibratory roller compactor is commonly used to apply a kneading/shearing action to at least the surface of the compacted asphalt mat, and thereby complete the compaction of the mat. Such rubber-tired rolling is thought to close steel roller induced cracks, at least at the surface of the asphalt mat, and increases surface texture by compressing the asphalt mortar between any coarse aggregate particles. Water is applied to the tires of the rubber-tired roller during rolling to alleviate material pick-up. However, although the cracks may be closed at the surface this water may inadvertently be injected into the cracks before they are sealed, forming encapsulated water deposits beneath the surface of the asphalt mat. Encapsulated water may inhibit healing or encourage stripping in the asphalt mat.
U.S. Pat. Nos. 4,661,011 and 4,737,050 claim to alleviate roller-induced cracking in the asphalt mat by use of an asphalt compaction machine in which pressure is applied to the asphalt mat through an endless elastomeric belt extending between two rollers. The machine is configured to apply a more uniform pressure over the area of the belt in contact with the asphalt mat.
It has now been recognized in accordance with the present invention that in a visco-elastic fluid, such as the binder in a hot mix asphalt, the response to load is not only temperature dependent but also time dependent. Thus, the application of a load of short duration will result in an asphalt response which is more elastic than viscous as the binder simply does not have time to flow. Therefore, using a vibratory roller compactor at an accepted loading rate in the order of 20 Hz, the binder in the asphalt mix reacts during compaction more as an elastic solid than as a viscous fluid and the compaction attempts to force the aggregate through the binder into a more compact arrangement, rather than causing the binder to flow around the aggregate with consequent movement of the aggregate.
The previously mentioned Van der Poel nomograph provides an estimate of the stiffness of standard bitumen grades at selected rates of load application and temperature. Even though the nomograph is well known to those skilled in the art of asphalt compaction, the disadvantages of applying compaction loads of short duration have not previously been fully recognized and short duration compaction using rollers with both steel and rubber interfaces, with or without vibration, has continued to be the accepted practice.
It may now be recognized that by using the belt compactor of the aforementioned U.S. Patents, improved compaction can be achieved by inducing viscous flow of the binder. Test uses of the belt compactor are described, for example, by Halim OAE et al in “Improving the Properties of Asphalt Pavement Through the Use of AMIR Compactor: Laboratory and Field Verification”, 7th International Conference on Asphalt Pavements, Nottingham, 1992. However, no recognition is given to the advantages of longer load times.
The described belt compactor may apply a load stress of only about 5% of the aforementioned 16 tonne (17.6 ton) roller compactor under static load, but assuming conventional advancement rates are used the load may be applied over a longer duration than a roller compactor due to the increased contact length of the belt. For a contact length of 1.25 m (4.10 ft) as described in the aforementioned paper and atypical compaction speed of about 1.1 m/s (3.61 ft/s), the load duration will be about 1.1 sec. Using Van der Poel's nomograph, this increased load duration can be shown to reduce the binder stiffness at 120° C. (248° F.) from about 1000 Pa (0.145038 psi) for the aforementioned conventional vibrating roller compaction to about 5 Pa (0.000725 psi) for the belt compactor.