Hot mix asphalt load simulator

This application claims priority from provisional application 60/483,674 filed Jun. 30, 2003 and entitled HOT MIX ASPHALT LOAD SIMULATOR.

The present invention relates to a device that can be used to simulate a hot asphalt load placed on an asphalt gyratory compactor dynamic angle validator (DAV).

Asphalt is a heterogeneous mixture of aggregate and asphalt binders and has attributes that can vary widely with factors such as aggregate size, binder quality, and air content. For example, if the air content of a batch of asphalt is too low, such as less than 4%, the asphalt starts to exhibit pressure transmission qualities similar to that of a liquid. Thus, forces placed upon the asphalt are transmitted through the asphalt, rather than through the aggregate structure, and can cause the asphalt to buckle. Considering the material and construction costs of building an asphalt roadway, it becomes easy to see how crucial it is to use accurate asphalt quality control equipment to ensure asphalt mixes meet minimum standards before they are applied to a roadway.

One widely used piece of quality control equipment is the gyratory compactor. Used to measure compaction and other composition characteristics, the gyratory compactor includes a cylindrical mold placed inside a compactor and filled with hot asphalt. A plate is placed above and below the hot asphalt in the mold, and a piston is used to compress one plate toward the other, thereby compressing the asphalt. In order to more accurately simulate the types of pressures that the asphalt will be subjected to when exposed to vehicle traffic, the cylindrical mold is tilted slightly and gyrated around a vertical axis while maintaining the tilt angle. Doing so allows the aggregate to shift and settle during compression.

Through exhaustive experimentation, it has been determined that the desired tilt angle is 1.25 degrees. For many years, gyratory compactors were calibrated by measuring the difference between the tilt angle of an outside wall of the mold in relation to a vertical axis, represented by an inside wall of the gyratory compactor. Later, it was determined that this simple measurement did not accurately represent the complexities of the angle of the force being placed on the asphalt sample. The plates placed above and below the asphalt sample transmit nearly all of the vertical compaction force to the asphalt. If these plates do not remain parallel to each other, and perpendicular to the vertical centerline of the gyratory compactor, the angle between the outside wall of the mold and the inside wall of the gyratory compactor (representing vertical) is not accurately related to the angle between the forces applied by the plates on the mix and the internal walls of the tilted mold.

The shortcomings of measuring the external angle of the mold as the sole indicator of tilt angle lead to the development of the Dynamic Angle Validator (DAV), shown and described in U.S. Pat. No. 6,477,783 and incorporated by reference herein. The DAV is an angle measurement device that measures the angle between one of the two plates acting on the asphalt and an internal wall of the mold. Due to the positioning of the DAV on the plate during measurements, deflections of the plates are accounted for in the measurement of the angle.

In order to accurately measure the reaction of mold and plates to the compression of hot asphalt, it has been heretofore required that hot asphalt be used in conjunction with the DAV during calibration. However, using a DAV with hot asphalt presents problems and challenges. Asphalt varies widely in its composition. Thus, each batch of hot asphalt is going to transmit different forces on the inside walls of the mold, and on the plates. Thus, calibrating a gyratory compactor using hot asphalt becomes a less-than-precise method of performing a calibration, which is by its nature supposed to be a very accurate exercise. The heat of the asphalt presents DAV design challenges. Prior to compaction, the asphalt, mold, and plates are heated to 300 F, just as the asphalt mixture is when produced at the plant to allow the asphalt to shift during compaction and to prevent the asphalt from solidifying in the mold. The DAV must therefore be able to operate in a 300 F environment and under approximately 600 kpa of compaction force. Some DAV designs are able to operate in these environments, while others have electronics that fail at elevated temperatures.

Calibrating a gyratory compactor with asphalt is also very time consuming. When placed in the mold, the hot mix is completely loose. The mix must be compacted for three minutes while readings are taken on the internal tilt angle. Considering that a calibration requires two runs for each sample (one where the DAV is placed at the top of the mold and one where the DAV is placed at the bottom of the mold) and at least two samples are used so that data can be interpolated or extrapolated linearly, a minimum of four three minute runs, plus data retrieval and DAV cooling time, is necessary with each calibration.

Another problem with calibrating a gyratory compactor with hot mix asphalt is that the height of the asphalt column greatly affects the forces placed on the mold by the asphalt. The greater the height, the greater the moment that results from the angle of the compaction force relative to the mold. Gyratory compactors are designed to test 115 mm columns of asphalt. Thus, in order to calibrate a compactor with asphalt, the DAV and 115 mm of asphalt must be placed in the mold. However, most compactors are not tall enough to accommodate the added height of the DAV. Thus, accurate measurements cannot be obtained. Rather, a smaller column of asphalt is placed in the mold and the data is extrapolated. This method may not be as accurate as using a 115 mm column of asphalt.

It is evident that there is a significant need for a device that accurately replicates the loads placed on the inside walls of a mold, as well as on the upper and lower plates, when hot asphalt is compacted in a gyratory compactor. Preferably, this device could be used in a hot or cold environment, and with a variety of different DAV designs.

There is further a need for a device designed with at least one variable that can be selected to simulate different types of asphalt mixes.

There is also a need for a device that accurately replicates the loads placed on the inside walls of a mold by a 150 mm column of hot asphalt, yet short enough to fit in most molds with most DAVs.

The present invention meets the aforementioned needs by providing a device and method that applies the same forces and moments on a mold as a hot asphalt mixture would during compaction. The device includes two pieces that act against each other during compaction.

The first piece has a base surface that contacts the DAV and includes a feature, such as a ridge, détente, or other contour that is configured to mate with a corresponding feature on the DAV. These mating features keep the first piece from moving relative to the DAV. The first piece also has an active surface that is either angled relative to the first surface or convexly curved.

The second piece has a first surface and a second surface. The first surface acts against the active surface of the first piece. The second surface contacts an end surface of the gyratory compactor, such as an end plate or piston.

The second piece is not connected to the first piece. Further, the first surface of the second piece is angled relative to the second surface of the second piece. The first and second pieces are constructed and arranged such that, when they are arranged for use in the gyratory compactor, but prior to being subjected to compaction forces, the contact area between the active surface of the first piece and the first surface of the second piece takes the shape of a complete circle. However, once the compaction forces are applied in combination with the forces necessary to tilt the mold, the first and second pieces maintain their angles relative to each other, but become laterally offset by the tilting forces on the mold. Becoming laterally offset while maintaining their original, no-load angles causes the second piece to “ride up” the first piece, thereby reducing the contact area to a single point. The forces applied through this single point are representative of a column of hot asphalt and are precisely repeatable. These forces are also dependent on the angled first surface of the second piece. By changing the angle of this first surface, a different type of asphalt can be simulated.

These two pieces have a combined height that is less than that of a column of asphalt, and are therefore useable with any gyratory compactor and DAV. Additionally, with some gyratory compactor/DAV combinations, there is enough room to place a DAV, the two-piece load simulator, and a column of asphalt.

FIG. 1 is an exploded perspective view of an embodiment of the present invention;

FIG. 2 is an exploded perspective view of an embodiment of the present invention;

FIG. 3 is a cutaway elevation of an embodiment of the present invention being used in a gyratory compactor with a DAV;

FIG. 4 is a cutaway elevation of an embodiment of the present invention prior to tilting;

FIG. 5 is a cutaway elevation an embodiment of the present invention after tilting;

FIG. 6 is a diagram showing some of the forces acting on a DAV, a mold, and the device of the present invention; and,

FIG. 7 is a diagram showing some of the forces acting on a piece of the present invention.

The Components

Referring now to the Figures and first to FIGS. 1 and 2 , two embodiments of a hot mix asphalt load simulator 10 of the present invention are shown. The simulator 10 includes a first piece 12 and a second piece 14 . The first piece 12 has a base surface 16 and an active surface 18 . The base surface 16 includes a feature 20 that is useable to anchor the first piece 12 to a DAV 1 . The particular feature 20 shown in FIG. 1 includes a circular ledge 22 that fits into an indentation 24 in the top of the DAV 1 . The feature 20 of the embodiment of FIG. 2 is a diameter that is sized to fit within the indentation 24 ( FIG. 1 ) in the top of the DAV 1 . One skilled in the art will realize that there are numerous acceptable substations for the feature 20 shown in FIG. 1 . A circular ledge 22 is provided in FIG. 1 as a best mode for purposes of manufacturing ease. However, the purpose of feature 20 is to prevent the first piece 12 from sliding on the DAV 1 and any configuration accomplishing this function would be acceptable. The active surface 18 is opposite the base surface 16 and has a circular cross section along a horizontal plane such as a conical surface or a spherical surface. The active surface 18 of the simulator 10 of FIG. 1 is spherical. The active surface 18 of the simulator 10 of FIG. 2 is conical.

The second piece 14 has a first surface 30 and a second surface 32 . The first surface 30 is angled relative to the second surface 32 , which acts, directly or indirectly, against an end of the gyratory compactor. The first surface 30 is constructed and arranged to act against the active surface 18 of the first piece 12 . The second piece 14 of FIG. 1 is a disk-shaped piece while the second piece 14 of FIG. 2 is ring-shaped.

The first and second pieces 12 and 14 of the device 10 are of sturdy construction; able to withstand the pressures and heat of a gyratory compactor. Preferably, the pieces 12 and 14 are constructed of stainless steel and are substantially solid.

Referring now to FIG. 3 , the device 10 is shown being used with a DAV 1 in a gyratory compactor 2 . The DAV 1 rests on an end plate 3 of the compactor 2 within the cylindrical mold 4 . The first piece 12 of the device 10 is placed on the DAV 1 in such a manner that the feature 20 of the base surface 16 mates with the corresponding feature (indentation 24 , in this case) of the DAV 1 .

The second piece 14 of the device 10 is placed on the first piece 12 such that the active surface 16 of the first piece 12 is received by the first surface 30 of the second piece 14 . A moveable piston or ram 5 of the compactor 2 provides pressure against the second surface 32 of the second piece 14 . The ram 5 may directly contact the second piece 14 or a plate (not shown) may be interposed between the ram 5 and the second piece 14 . Pressure from the ram 5 keeps the second surface 32 parallel to the DAV 1 .

Movement Between the First and Second Pieces

Once the gyratory compactor 2 is started, the mold 4 is tilted and presses against the second piece 14 . This causes the second piece 14 to ride up the first piece 12 as the second piece 14 becomes laterally offset from the first piece 12 . This action is shown in FIG. 2 by comparing the original, pre-tilt positions of the second piece 14 and the mold 4 , drawn in phantom lines, to the tilted positions. The second piece 14 is offset from its original position both laterally and vertically.

FIGS. 4 and 5 provide a more detailed view of this offset. In FIG. 4 , the second piece 14 has not been offset. The second piece 14 rests on top of the active surface 18 of the first piece 12 such that the second piece 14 is centered on the first piece 12 . The first surface 30 of the second piece 14 contacts the active surface 18 of the first piece 12 in such a manner as to form circular ring of contact points 34 . In FIG. 5 , the mold (not shown) has been tilted, causing the second piece 14 to shift laterally and upwardly, thereby reducing the ring of contact points to a single point 36 .

The Forces Imparted by the Simulator

Having described the physical features of the device 10 , and the relative movement of the pieces 12 and 14 when the mold 4 is tilted, discussion will now turn to the forces imparted on the DAV 1 and the compactor 2 by the device during operation and how these forces closely simulate a load of hot mix asphalt.

FIG. 6 shows the various forces at play during a calibration of a gyratory compactor 2 . The two external forces that act on the simulator 10 and the DAV 1 are the resultant force F c from the compression of the ram or piston of the gyrator compactor, and the resultant tilting force F t placed on the outside of the mold 4 by the compactor.

Prior to the application of F t , F c falls on the centerline ℄ of the mold and DAV 1 . When F t is applied, the contact point 36 moves laterally a distance d. This shifts F c to provide a countering moment as the mold 4 tilts and equilibrium is achieved. The angle θ between the first surface 30 of the second piece 14 and horizontal is determinative of the distance e to which the force F c will shift from ℄, and thus, the size of the resulting moment. (If using the device 10 of FIG. 2 , the angle θ is measured from horizontal to the active surface 18 of the first piece 12 .) The moment M cDAV on the DAV 1 due to the compressive force F c of the ram can be represented by:

M cDAV =F c ·d+F c ·e

The relationship between F t and F c at equilibrium is dependent on θ as follows:

F t =F c tan θ

This relationship is due to the shifting of the force F c from the centerline ℄ to the angled first surface 30 . The vertical compressive force F c , applied against the angled first surface 30 , results in a lateral force F L that is equal and opposite to the tilting force F t .

The moment M LDAV on the DAV 1 created by the lateral force F L counteracts the moment M cDAV created by the eccentric compressive force F c , and can be represented by:

M LDAV =F L ·H=F c ·d+F c ·e

FIG. 7 shows the balance of the moments on the second piece 14 . Again, F c is the compressive force from the ram of the gyratory compactor 2 . Similarly, F L is the lateral component of F c resulting from the angled first surface 30 and is related to Fc as follows:

F L =F c tan θ

The moments created by the forces F c and F L are dependent on the location of the contact point 36 on the first surface 30 of the second piece 14 . F c creates a moment M c2nd in one direction due to the offset horizontal distance c between the contact point 36 and the resultant force vector F c . This moment M c2nd is calculated:

M c2nd =F c ·c

The countering moment is created by the lateral component force F L and the vertical distance L between the contact point 36 and the force vector F L . This moment M L2nd is calculated:

M L2nd =F L ·L

Referring again to FIG. 6 , a distance H can be defined as the height of the contact point 36 above the base of the DAV 1 . Because the gyratory compactor 2 is calibrated to measure a column of asphalt 115 mm tall, it is desired that the height of the DAV 1 and the device 10 equal 115 mm. Thus:

L+H= 115 mm

The moment M t on the mold 4 due to the tilting force F t is thus:

M t =F t ( L+H )= F t ·115 mm=F c tan θ·115 mm

The countering moment M c on the DAV 1 and device 10 is:

M c =F c ·e

The distance e, between the resultant compressive force F c and the centerline ℄ can be determined using:

e =tan θ·115 mm/ 2

Realizing the relationships between the distances c, d and e are as follows:

d+c=e

allows the following substitutions and reductions to be made in order to derive an alternative formula for e:

=

=

Practical Use of Mathematical Relationships

Knowledge of the aforementioned mathematical relationships allows the device 10 to be used to perform tests previously unavailable. For example, the angle θ of the first surface 30 of the second piece 14 is used to simulate the shear force normally created by an asphalt mix. Changing the angle θ, in other words using a variety of second pieces 14 each having a different angle θ, allows a determination of how a particular gyratory compactor will react to varying asphalt loads.

Furthermore, the device 10 can be used in a quality control capacity for various asphalt mixes. For example, if an asphalt mix is purported to have a shear characteristic that corresponds to a particular angle θ on the simulator device 10 , a run can be made with the DAV 1 and the simulator 10 to determine the corresponding reaction of the mold 4 and the end plates 3 of the compactor 2 . Then, the simulator may be removed and a load of the asphalt can be placed in the mold 4 with the DAV 1 . After the run, the data from the loaded run should match the data from the simulated run. Furthermore, because the angle θ has a known mathematical relationship to the forces created by the simulator, measuring the angle θ with an asphalt load allows the quantification of the moments created by the asphalt.

It is contemplated that features disclosed in this application can be mixed and matched to suit particular circumstances. Various other modifications and changes will be apparent to those of ordinary skill in the art without departing from the spirit and scope of the present invention. Accordingly, reference should be made to the claims to determine the scope of the present invention.