PURKIS DAVID ALAN (US)
PURKIS DAVID ALAN (US)
| US4708516A | 1987-11-24 |
| 1. | 3 (Material 16 in Figure 1 of Patent No. 4,708,516) be replaced with Portland Cement treated dense graded aggregate for certain constructions. This usage as described herein is depicted on Drawing No. 1, which accompanies this application. The usage of Portland Cement treated aggregate material would be: a) where the construction has a dense graded aggregate of marginal and/or lesser character then as described in Patent No. 4,708,516; b) where the aggregate material is of acceptable character, but where very high highway loading is incurred due to excessive wheel weights on axles and/or the combination of excessive wheel weights and traffic patterns; and c) where the aggregate material is of acceptable character, but where pavement loadings are very high such as for airport, port or rail yards where individual wheel loads are in the range of ten times greater than for standard or legal highway loadings and where in many cases these pavement loadings are slow moving or static as occurs in container storagetransfer yards and in airport taxiways and aprons. The applicant has discovered that the greater stiffness incurred by the Portland Cement treatment of suitable aggregate as further described herein causes an important increase in stiffness of the beamlike action of the pavement structural section of Patent No. 4,708,516 over that developed by the untreated aggregate. A second benefit of the Portland Cement treatment is a better ability to distribute the shear stresses through this member and the underlying aggregate layer than develops in beamlike action of the pavement structural section of Patent No.4,708516, where the upper intermediate layer in Claim 3 is comprised of untreated dense graded aggregate. This results in a significantly greater reduction of the localized tensile stress development in the surface layer. With the inclusion of Portland Cement treatment, greater resistance to permanent straining, both shear and compression, exists in the intermediate layer than exists when the invention utilizes untreated aggregate. An important discovery made by the applicant regarding the use of the Portland Cement treatment as further described herein has to do with reflective cracking in asphalt concrete. In asphalt pavements of the prior art as described in Patent No. 4,708,516 that have utilized Portland Cement treated aggregate as a base layer beneath the asphalt concrete, which is used only as a surface layer, it is well known that reflective cracking of die surface will occur. In these prior art pavements, the cement treated aggregate layers are generally between 10 and 20 inches in thickness and the Portland Cement treatment is generally between 4 and 10 percent cement content by dry weight. The asphalt surface course is generally greater than 4 inches in thickness. The cause of these cracks is a combination of stresses from thermal conditions and traffic loadings acting on a brittle material, the Portland Cement treated aggregate which is much stiffer in an intact state than asphalt concrete but similar in it's tensile strength under typical traffic loadings. In these prior art pavements, the portion of the pavement adjacent to these cracks is subjected to edge loading, a condition that is substantially more severe than interior loading. This condition in a prior art pavement is now compounded by the ready infiltration of surface waters at these locations, which softens the subgrade where the support is most needed. These prior art pavements then tend to display significant cracking and faulting or offsetting of the intact materials adjacent to the cracks. When mixing is completed, the percentage of moisture in the mixture, on the basis of weight, shall be such that immediately prior to compaction, it is within the limits of approximately two percent below to two percent above the optimum moisture content. The Portland Cement treated aggregate mixture shall be transported from the mixing area to the placement area in clean equipment provided with suitable protective devices in unfavorable weather. The total elapsed time between the addition of water to the mixture and the start of compaction shall not exceed a specified time, typically 60 minutes, and the mixture is not to be left undisturbed for longer than 30 minutes during this period. All necessarily precautions shall be taken to avoid damage to completed cement treated soil by the equipment, and to avoid the deposition of untreated material or foreign materials between layers of cement treated soil. The mixture shall be placed with spreading equipment that will produce layers of such widths and thicknesses as are necessary for compaction to the required dimensions of the completed cement treated soil layers or in intermediate layers approved by the design engineer. Portland Cement treated aggregate shall not be mixed or placed when the air temperature is below 45 degrees F. unless the temperature is at least 40 degrees F. and rising. Portland Cement treated aggregate shall be uniformly compacted in a specified manner and to a specified density, typically as a composite lift with the open graded aggregate interlayer to achieve at least 90 percent of maximum density as determined by optimum moisture and maximum density test ASTM D1557. At the start of compaction, the mixture shall be in a uniform, loose condition throughout its full depth. |
| 2. | Final compaction of the mixture to the specified density shall be completed within a specified time, typically. |
| 3. | hours after the application of water during the mixing operation. The Portland Cement treated aggregate layer shall be further shaped if necessary to the required lines, grades and crosssections and rolled to a reasonable smooth surface. Surface compaction and finishing of each layer shall be done in such a manner as to produce, in the required time, a dense surface free of compaction planes or loose material. Temporarily exposed surfaces shall be kept moist for curing purposes for a specified period of time, typically one week or until covered with a curing compound or the surface asphalt layer. |
With this letter, We would like to file an improvement to the "Improved Asphalt Pavement" as described in United States Patent Number 4,708-516 (Edward James Miller). The inventors of this improvement are Dr. E. J. Miller and David A. Purkis.
The invention in Patent Number 4,708-516 was described as:
1. A beam-like asphalt pavement structural section for overlying a subgrade comprising in combination: a) at least three layers of material, said layers being arranged in a preselected sequence from said subgrade to upper surface; b) said layers including a bottom layer adjacent to said subgrade, a top layer of bounded material defining said upper surface, and an intermediate layer there between; c) the material of said bottom layer be comprised of a dense graded asphalt concrete having greater tensile strength than the other layers; d) said intermediate layer comprising unbound aggregate filler material of predetermined thickness as to enhance the beam-like action.
2. A beam-like asphalt pavement structural section as described in Claim 1, wherein said intermediate layer comprises open graded aggregate filler material ...
3. A beam-like asphalt pavement structural section as described in Claim 2, wherein said intermediate layer is further characterized by: a) an upper layer; b) a lower layer; and c) said upper layer comprised dense graded aggregate filler material of preselected thickness.
Additional claims are provided in the referenced patent including Claim 14 which describes:
14. A method for constructing a beam-like asphalt pavement structural section over a subgrade comprising the steps of arranging a plurality of layers of material in a preselected sequence and of preselected thickness....
The applicant has found that in contrast to the behavior that occurs in the prior art pavements, the inclusion of Portland Cement treatment of the upper interlayer of the pavement described in Patent No.4,708,516 does not result in the adverse conditions attributed to prior art pavements. This is due to: a) the much thinner layer of Portland Cement treated aggregate in applicant's invention experiences more prevalent traffic and thermal stress induced cracking, thus reducing reflective cracking in the surface course. b) the position of the Portland Cement treated aggregate layer in applicant's invention atop an open graded lower aggregate interlayer and a non or very low permeable asphalt base layer causes the removal of infiltration waters before they reach and soften the subgrade soils. c) the position of the Portland Cement treated aggregate layer in applicant's invention atop a tension bearing member (The asphalt base layer) causes the Portland Cement treated aggregate layer to generally act in a compressive stress environment, similar to that described in Patent No. 4,708,516 for the untreated interlayer; a mode in which the cracks in the Portland Cement treated aggregate layer now have little structural significance.
In Patent No. 4,708,516, the dense graded aggregate materials for the upper interlayer are reasonably well graded as described therein with a generally maximum of 10 percent fines and with a sand equivalency generally above 50. The particles were to be angular, hard and durable as determined by such tests as the Los Angeles Abrasion test. The material to be used for Portland Cement treated aggregate may have the same properties as described in the aforementioned invention with the addition of 3 to 8 percent by dry weight of Portland Cement. However, material of lesser quality may be used when combined with appropriate amounts of Portland Cement to obtain specified unconfined compression strength at a specified age when determined in accordance with certain standard test methods. Typically, this requirement might be an unconfined compression strength of 500 psi when the aggregate material is combined with 5 percent by dry weight of Portland Cement molded with the applicable portions of ASTM D 560 and tested in accordance with ASTM D 1633 after curing for 7 to 28 days.
These materials will have similar gradations to that identified in Patent No.4,708,516 for dense graded materials, but can allow additional fines, up to 20 percent and can allow greater plasticity of the fines, for example a material with a Sand Equivalent of 25 will often be found suitable. The particle angularity, hardness and durability will also generally be less restrictive.
The method of construction of Portland Cement treated aggregate is typically as described in the following:
Portland Cement treated aggregate shall consist of aggregate, Portland Cement, and water uniformly mixed, compacted, finished, and cured to conform to specified lines, grades, and thicknesses.
Mixing of the aggregate, cement and water to produce the Portland Cement treated aggregate shall be accomplished with any mechanical means to the satisfaction of the Design Engineer, typically by either a central plant mixing process or with a traveling mixer.
The mixing time shall be considered as the interval between the time the cement contacts the soil and water, and the time the mixture is being compacted.
All ingredients shall be mixed as required to ensure a thorough, uniform mix of the aggregate, cement, and water.
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