Abstract:
A hollow cylindrical tester uses an inflatable membrane to apply hoop stress to a hollow cylindrical sample. The compact device includes a frame that holds a sample around the inflatable membrane. The membrane is preferably inflated via fluid pressure and the fluid pressure preferably is monitored to determined pressure at critical points in testing procedures. Pressures of fluids within the membrane are also monitored. In a preferred structure, a piston and cylinder pressure injector operatively connected to the membrane is monitored for amount of piston travel and a pressure meter monitors fluid pressure. The preferred structure includes shaped opposing platens to seal the membrane within a cylinder sample being tested, a post failure restraint cylinder, and a cooling fluid bath.

Description:
This application claims priority from Provisional Application Ser. No. 60/184,966, filed on Feb. 25, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to tensile testing of various material samples, to determine the suitability of modern pavement mixtures. 
     BACKGROUND OF THE INVENTION 
     Road building and repair is an extremely expensive and disruptive task. In response to such concerns, Congress funded, in 1987, a research program meant to improve the durability and performance of United States roads. Longevity and safety are the primary directives of the research program. Obviously, roads which have a longer useful life reduce maintenance and reconstruction expenses. Roads which resist cracking, buckling, rutting and holing also have clear safety advantages. 
     Superpave™ (a trademark of the Strategic Highway Research Program) is a product of research funded through this program. The pavement system optimizes asphaltic concrete pavement mixes to control undesired rutting, low temperature cracking and fatigue cracking. Development and installation of such pavements requires testing to obtain fundamental properties of the materials. Other construction techniques also require extensive material testing. Sampling of various concretes and other materials used in building construction ensures that engineering requirements and code requirements for the materials are maintained through building construction. 
     Tensile creep compliance, a temperature and loading time dependent stress-strain property, and tensile strength are important qualities of the asphaltic concrete mixtures developed under the highway initiative program. These properties are used with commercially available software to design and proportion asphalt concrete mixtures to be resistant to thermally-induced cracking. An Indirect Tensile Tester (IDT) has been approved by the Federal Highway Administration (FHWA), and has been the sole available test to provide required material property inputs for available thermal cracking performance prediction models contained in the FHWA&#39;s Superpave™ system and specification for designing high-quality asphalt paving mixtures. 
     The IDT is an expensive machine, costing approximately $140,000. This makes it one of the more expensive of the machines used to determine other characteristics associated with the design and installation of Superpave™ mixtures. Related machinery such as a gyratory compactor, bending beam rheometer, dynamic shear rheometer and rotary viscometer, (all devices used to measure other various portions of mixture design specifications), have costs in the approximate $10,000 to $40,000 price range. The high price of the IDT is therefore a substantial barrier to its widespread use by contractors seeking to install Superpave™ concrete mixtures. 
     Price concerns might eventually be addressed using the IDT, but a more significant difficulty associated with it is its large and cumbersome nature, and the relatively high level of expertise necessary to operate it properly. The IDT works under indirect tension test principles, where compressive loads are applied along thin loading strips on opposite sides of a cylindrical specimen being tested. This compressive loading creates tensile stress on opposite sides of the test specimen. As a result, up to 700 pounds per square inch of tensile stress is required to break asphalt concrete specimens at low temperatures. The need to generate that level of stress makes use of an approximately 22,000 pound capacity load frame and controller necessary. While these contribute significantly to the expense of the device, they also cause it to be large and cumbersome. The IDT frame weights 22,000 lbs. by itself, and is over seven feet tall. This effectively limits its use to a stationary laboratory, as opposed to transportation to a relevant field location. In the laboratory, it occupies a significant amount of space. 
     The IDT also has operational difficulties. Its associated test procedure requires the careful mounting of four high sensitivity transducers on every specimen. Every specimen must also be carefully aligned in the loading frame. A refrigeration based cooling system typically requires about fifteen minutes to restabilize before each test. Mounting the specimen, mounting and electrically balancing the transducers, and allowing appropriate time intervals between tests has shown that the testing is limited to about 1 sample per hour or less. In addition, a skilled technician is required for specimen mounting, transducer mounting and electrical balancing. 
     Subsequent to the development of the present invention, a new standard was introduced for determining the compliance of various asphaltic mixtures with static creep requirements. Namely, a 4″ cylinder is to be cored from a sample and appropriately tested. This is a radical departure from previous testing standards. Such tests can be performed on samples acquired in the field, however for purposes of testing new or raw materials such tests need to be performed on lab prepared specimens. 
     The need to obtain such specimens in the lab is not a new problem and various machines and methods presently exist that facilitate the creation of asphaltic samples. One such device is the Brovold Gyratory Compactor, manufactured and sold by Pine Instruments. Such a device produces a compacted cylindrical asphalt sample that may be tested for compliance with the various industry standards. The Gyratory Compactor will produce a cylindrical sample having an outer diameter greater than 4″. Thus to perform the above mentioned static creep test a 4″ cylinder is cored, thus leaving a cylindrical hoop. This is beneficial in that the compaction process will affect and modify the material at the outer circumference of the cylinder; thus the smaller cored sample has a more uniform density. 
     The difficulty is that asphalt mixtures are not homogeneous. Thus, the particular sample selected can vary the results. To perform all of the tests required, two samples (tensile creep compliance, static creep compliance) will need to be fabricated in the compactor. Thus, due simply to normal differences in the non-homogeneous asphalt mixture, the samples can be quite different. This makes use of the IDT (which now requires a second cylindrical sample) less desirable. 
     Furthermore, by its very nature the IDT can produce varying results on a given sample. As mentioned above, asphalt is not homogeneous. It is comprised of a plurality of particles ranging in size and configuration that are bonded together. The IDT effectively “samples” selected particles and effectively ignores the remainder. Force is applied along two linear strips that are opposite one another on the cylinder, thus compressing the cylinder. This will cause the cylinder to deform into an oval or elliptical configuration. The mounted transducers monitor the change in diameter along the compressing direction and along the expanding direction. However, this is only done at selected points along the cross section (2 points for each direction). Thus, even though force is applied along the entire height of the cylindrical sample, its effects are only monitored at a few points. Thus, unless such a cylinder universally responds uniformly to an applied force, the IDT results can be affected. Furthermore, differences in the particulate nature of the asphalt can cause unique deformations to occur which will not be observed with the IDT due to its limited number of measurement points. Thus, the points selected for measurement will actually affect the results obtained. In other words, the IDT is incapable of averaging deformation across the whole sample. 
     The IDT thus suffers in that it cannot effectively measure average changes throughout a given sample and such tests cannot be performed on the same sample material used in the various other creep performance tests. 
     SUMMARY OF THE INVENTION 
     Accordingly, there is a need for an improved sample tester which addresses problems encountered in previous testers. It is an object of the present invention to provide such an improved sample tester capable of performing tensile creep compliance and tensile strength testing. The improved tester of the present invention has clear applicability to Superpave™ testing procedures, as well as any construction or engineering analysis in which such material properties are relevant. 
     These and other needs and objects are met or exceeded by the present compact tensile tester. The compact device includes a frame that holds a hollow cylindrical sample around an inflatable membrane. The membrane is inflated via fluid pressure and the fluid pressure preferably is monitored to determine pressure at critical points in the testing procedures. Furthermore, characteristics of a pressure injector used to inflate the membrane are monitored. In a preferred structure, a piston and a cylinder pressure injector are operatively connected to the membrane and the injector is monitored for amount of piston travel. A pressure transducer monitoring fluid pressure, which correlates to the amount of force exerted on the sample is attached to the sample. 
     The structure includes shaped opposing platens to seal the membrane within a hollow cylinder sample being tested, a post failure restraint cylinder, and a cooling fluid bath. A computer can be connected to the pressure transducer to obtain values at specific critical testing points and calculate appropriate sample characteristics based upon the obtained values. 
     In operation, one of the platens is opened or removed to permit insertion of an appropriately dimensioned hollow cylindrical sample around the relaxed membrane. Due to the nature of the device, no careful alignment of the sample or sensors is required. Instead, the platen is closed, sealing the membrane in the inserted hollow cylindrical sample. While pressure in the membrane is monitored, it is inflated to place even hoop stress on the inner walls of the sample. In a sample failure test, the post failure restraint cylinder contains the sample. The device is quickly cycled for additional tests, in part due to its use of a relatively simple fluid cooling bath, but mainly due to the overall simplicity of the testing protocol and elimination of cumbersome positioning and sensor alignment procedures inherent in the prior art testing protocol. 
     In an alternative embodiment, various sensors are positioned on the hollow cylindrical sample to directly monitor the displacement. Thus, fluid pressure is used to exert a uniformly applied force across the whole of the specimen, but only point measurements are taken. 
     The present invention provides a method and apparatus for determining a sample&#39;s average response to uniformly applied forces. As expected, this provides more accurate and complete data than is obtainable with the IDT. For this reason alone, the HCT becomes more advantageous to use than the IDT. Another advantage is the ability to perform the tensile tests with the HCT as well as the various static tests on a single asphalt sample made from one iteration in a Gyratory Compactor. As explained above, the compactor produces an asphalt cylinder. From this, a 4″ cylinder is cored and used for static testing. What remains is a hollow cylinder ideal for use in the HCT. Though the outer circumference of this hoop has been somewhat modified during the compaction process, this will have little or no effect on the HCT since force is being applied to the inner circumference and this is where a vast majority of the compression occurs. Thus, a single gyratory sample can be used for all of the various required tests. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become apparent upon reading the following detailed description, while referring to the attached drawings, in which: 
     FIG. 1 is a side sectional view of a hollow cylindrical sample tester. 
     FIG. 2 is a sectional view of the hollow cylindrical sample tester including an intensifier within a cylindrical sample. 
     FIG. 3 is a sectional view of an HCT about line III—III in FIG.  2 . 
     FIG. 4 is a sectional view of an HCT within a liquid cooling bath. 
     FIG. 5 is a sectional view of an HCT with external sensors mounted to detect deflection. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention provides a compact hollow cylindrical sample tester (HCT) suitable for Superpave™ testing procedures, as well as other material analysis testing procedures which require tensile stress, creep compliance, failure, and other similar determinations. In operation, an inflatable membrane evenly asserts hoop stress on the sample, and the pressure required to inflate the membrane for the test provides complete and accurate testing data for critical points in a testing procedure. Alternatively, direct measurement of the expanding cylinders are taken. 
     Turning to FIG. 1, a cutaway view of HCT  10  is shown. HCT  10  has a frame  11  composed of an upper platen  14  and a lower platen  12  which are coupled together with a plurality (two or more) of clamping rods  18 . The clamping rods  18  in conjunction with a plurality of locking nuts  20  securely hold the platens  12 , 14  in this configuration. 
     A flexible membrane  16  is disposed between the upper platen  14  and the lower platen  12 . The flexible membrane  16  is preferably made of rubber, however any suitably strong impermeable membrane is acceptable. As will be explained in more detail later, a fluid within the flexible membrane  16  becomes pressurized and exerts a force upon a rigid object, such as cylindrical sample  22 . For purposes of the HCT  10 , fluid is meant to include any liquid, gas or combination of the two. The cylindrical sample  22  will often be a material such as asphalt, or the like. The fluid within the flexible membrane is typically pressurized until the sample is caused to fracture. As such, the flexible membrane  16  must be able to withstand both the internal pressure exerted and the pressurized frictional engagement with the cylindrical sample  22 . A restraint cylinder  32  surrounds the cylindrical sample  22 , contains any fragmentation of the cylindrical sample  22 , and prevents large openings from occurring in the sample  22  after the fracture. 
     A compressor/controller  24  is coupled to the frame  11  via a connection hose  26 . The compressor  24  provides either the hydraulic or pneumatic force required to pressurize the flexible membrane. 
     In use, one or both of the platens  12 , 14  is separated from the frame  11 . A hollow cylindrical sample  22  is placed about the flexible membrane  16  as shown. The platens  12 , 14  are then re-secured to the frame. The platens  12 , 14  are configured so as to support, but not restrain or exert a force on the cylindrical sample  22 . This allows the cylindrical sample  22  to react freely to the force imparted by the flexible membrane  16 . The primary purpose of the platens  12 , 14  is to prevent the flexible membrane  16  from expanding out beyond the top or bottom of the cylindrical sample  22 . Therefore, in one embodiment, the platens  12 , 14  will not contact any portion of the cylindrical sample  22 , but are simply configured to confine the flexible membrane  16 . As shown in FIG. 1, a pair of O-rings  28  at the top and bottom of the flexible membrane  16  seal any gap that might exist between the membrane  16  and the sample  22 . An additional protective film  30  may optionally be added between the cylindrical sample  22  and the flexible membrane  16 . The protective film  30  is made of a material such as acetate, and serves to further protect the flexible membrane  16  from the abrasive contact generated during use of the HCT  10 . 
     FIG. 2 is a cutaway view of the components within flexible membrane  16 . As mentioned above, the HCT  10  obtains data about the cylindrical sample  22  by expanding the flexible membrane  16  inside the cylinder  22 , and generating an outward force. By measuring the force exerted and the resulting structural changes in the sample  22 , certain fundamental material properties are determined. To exert this force, the flexible membrane  16  is caused to expand by increasing the internal fluid pressure. This could be accomplished by simply pneumatically or hydraulically inflating the flexible membrane  16 . Preferably, the flexible membrane  16  is filled with a (liquid) fluid  40 . Compressed gas could be used instead, however in the event of an accidental rupture, the sudden release of the compressed gas would be more energetic and turbulent than a similar release of pressurized liquid. In either case, the volume of the gas or liquid may undergo substantial compression during the pressurization of the flexible membrane  16 . This compression must be accounted for in any calculation of force or displacement. To minimize this factor, intensifier  34  is disposed within the core of the flexible membrane  16 . 
     Intensifier  34  is a spool shaped rigid structure that is preferably made of metal. The intensifier  34  has a hollow inner chamber  35  which is fluidly connected to the interior of the flexible membrane  16  via drilled port  38 . A pressure injector such as piston  36 , also preferably made of metal, is disposed within the inner chamber  35 . The piston  36  is moveable longitudinally within the inner chamber and is displaced by the compressor  24 . As the piston  36  travels, it pressurizes the fluid  40 . To some extent, the fluid  40  and the flexible membrane  16  will compress, however most of this pressure is translated into a force which is exerted on the cylindrical sample  22 . The intensifier  34  serves to occupy a large percentage of the interior volume of the flexible membrane  16 . This reduces the volume of fluid  40  which is required, thus minimizing the effect of the fluid compression on the final displacement calculations. It is desirable to not have any air present when liquid is being used as the medium. A valve or other release mechanism can be added in intensifier  34 , piston  36  or any other convenient location in order to vent air or other gases that must be present. To facilitate this, the components illustrated in FIG. 2 can be inverted during fabrication or assembly in order to allow rising gases to be vented. 
     The piston  36  has a known surface area that is in contact with the fluid  40 . The particular structure of the intensifier  34  allows for a relatively small (diameter) piston  36  to be used, however the same effect could be achieved without the intensifier  34  and using a larger piston. The movement of the piston  36  is monitored by a sensor, such as LVDT  44  (linear voltage differential transducer). The displacement of the piston  36  is correlated to the give of the cylindrical sample  22 . As such, the pressurizing system of HCT  10  is also a measuring system. In one application, the amount of give allows for the calculation of the creep compliance of an asphalt cylinder. The LVDT  44  need only be a low sensitivity sensor, which allows for a minimization of costs. 
     A pressure meter such as pressure transducer  42  is mounted within the flexible membrane  16  adjacent to the intensifier  34 . The pressure transducer  42  monitors the pressure exerted by the fluid  40  and from this, the amount of force exerted is calculable. As such, it is possible to determine the force exerted on the cylindrical sample  22  as well as the physical response of the cylindrical sample  22  with a single LVDT  44  and a single pressure transducer  42 . This arrangement is particularly efficient in that the force imparted to the piston  36  is significantly less than the force that will be imparted to the sample  22 . For example, applying 400 lbs. of force to the piston generates 700 lbs. of force (tensile/hoop stress) on the inner circumference of the sample  22 . As is shown in FIGS. 1 &amp; 2, the flexible membrane  16  is not in contact with the entire inner surface of the cylindrical sample  22 . As explained above, this is done to prevent the flexible membrane  16  from expanding beyond the cylindrical sample  22 . As a result, three dimensional finite analysis must be employed to determine the material properties of the cylindrical sample  22  based upon the force/displacement data generated. 
     FIG. 3 is a top planar view of HCT  10  taken about sectional lines III—III. From this view is becomes apparent that the force exerted by the pressurized fluid is evenly distributed about the inner circumference of the cylindrical sample  22 . 
     In FIG. 4, the frame  11  is surrounded by a cooling chamber  50 . The cooling chamber  50  is filled with a liquid  52  that is maintained at a particular temperature. This effectively maintains the HCT  10  and the cylindrical sample  22  at this same temperature. This allows the HCT  10  to be used to measure specific material properties at preselected temperatures. The cooling chamber  50  allows for cylindrical samples  22  to be removed and replaced without a significant increase in thermal energy. That is, the temperature of the liquid  52  in the cooling chamber  50  can be efficiently maintained during the testing of multiples samples  22 . Due to the efficiency of this arrangement, new samples  22  are brought to their preselected temperature relatively quickly, therefore allowing for the rapid testing of a large number of samples  22 . 
     As the flexible membrane  16  expands, it will exert an outward force on the cylindrical sample  22 . As this force increases, the cylindrical sample reacts by compressing (and eventually fracturing, if the force is sufficient). Concurrently, the height of the cylindrical sample  22  will decrease as a direct result of this outward expansion. This decrease in height is related to Poisson&#39;s ratio and may be measured by adding another LVDT  46  or  48  at either the top or bottom of the sample  22 . As discussed later, one of the measurements that the HCT  10  is to obtain is the “creep compliance” of the cylindrical sample  22 . Essentially, this amounts to the outward expansion of the cylindrical sample  22  under a constant force. The decrease in height will add a certain degree of error to the determination of the creep compliance. The LVDT  46  or  48  can measure the change in height and from this data, that error can be eliminated in the creep compliance determination. As a practical matter, the error introduced by the vertical contraction is minor and may be ignored without seriously affecting the results. Thus LVDT&#39;s  46  or  48  are entirely optional. 
     While the HCT  10  can be used to measure the fundamental physical properties of cylinders of any type of material, the primary purpose of the described embodiment is to test asphalt mixtures which are to be used in the Superpave™ program. Within this program, certain standards have been implemented. For example, the various mixtures which are to be used are placed within a gyratory compactor which forms uniform cylinders. These cylinders are typically 115 mm in height and have an outside diameter of 150 mm. The cylindrical wall will usually be about 1 inch thick (about 50 mm). The HCT  10  is sized to accommodate these standard cylinders. 
     In use, the fluid  52  in the cooling chamber  50  is brought to a predetermined temperature (+/−0.2°). For compliance with Superpave™ requirements, the asphalt mixtures must perform adequately under low temperature thermal stresses. Therefore, the mixtures are tested at 0° C., −10° C., and −20° C. for creep compliance and at 20°, 4°, 0°, −10° and −20° C. for tensile strength. As such, the cooling bath needs to have a temperature range of between +25° C. to −20° C. 
     It is critical to realize that asphalt behaves differently at lower temperatures than it does at higher temperatures. As such, entirely different tests (using different types of equipment) must be performed for high and low temperatures. The HCT is being utilized at low temperatures to test the creep compliance and tensile strength of asphalt mixtures. 
     When being set up for testing, one platen  12 , 14  is removed and a cylindrical sample  22  is slid over the flexible membrane  16 . Due to the nature of the HCT  10  and the tests which are to be performed, no precise alignment is required. The removed platen  12 , 14  is then re-secured. The fluid  52  in the cooling chamber  50  maintains its temperature during the interchange of cylindrical samples  22 , and serves to rapidly bring each new sample to the appropriate temperature. 
     Once the proper temperature is achieved, two types of tests are performed: a creep compliance test and a tensile strength test. 
     One of the primary concerns with asphalt paving is thermal cracking. When asphalt pavement is cooled, tensile stress develops. That is, the length of asphalt is held constant, and any contraction that occurs as it cools results in a developed strain. Thermal stresses develop because the pavement is forced to contract as the temperature lowers. Generally, pavement will not have joints added and must rely on the flexibility of the materials used to accommodate such contractions. Those tensile stresses can cause two types of problems. Thermal fatigue cracking occurs as a progressive and gradual crack propagation during temperature cycling. This problem can be predicted by testing and measuring the asphalt mixtures&#39; creep compliance. The other problem occurs when asphalt pavement, which has been rapidly cooled, suddenly cracks. The propensity for a given asphalt mixture to behave like this is predicted by a tensile strength test. 
     To perform the creep compliance test, the piston  36  is rapidly actuated until the pressure transducer  42  determines that the pressurized fluid  40  is exerting a predetermined amount of force on the cylindrical sample  22 . The purpose of this test is to exert a constant force on the sample  22  for a predetermined period of time and then to measure the resultant changes in the sample  22 . Such a change will occur in the cylindrical sample  22  by the cylinder expanding outward (the cylinder wall will essentially compress). More specifically, the inner circumference compresses towards the outer circumference and to a lesser extent, the outer circumference may also expand. With no other change, the force exerted by the pressurized fluid  40  will be reduced because of the corresponding increase in volume generated by the cylinder&#39;s expansion. Therefore, as the cylinder  22  expands, the piston  36  must be further actuated to maintain a constant pressure (and hence, a constant force) on the volumetrically dynamic cylinder. This constant force is maintained for some predetermined amount of time. At the completion of that time period, the distance that the piston  36  has traveled, since first establishing the correct amount of force (as a starting point) until the point the piston  36  is at when the time period has expired (the finishing point), is measured. As a practical matter, the entire distance traveled by the piston  36  is monitored, however, only the distance traveled after establishing the predetermined pressure is relevant to this particular test. The additional data acquired may be useful in other calculations. For example, with uniform cylinders, the initial amount of travel will correlate to the pressure generated thus reducing the reliance placed upon the pressure transducer  42 . 
     The distance is accurately measured by LVDT  44 . Since the surface area of the piston  36  is known, the distance traveled corresponds to the change in the volume of the fluid  40  as the cylinder  22  expands. This change in volume is then translated into the corresponding “creep” of the asphalt cylinder. In other words, as the sample  22  deforms under stress, the additional amount of fluid  40  that would be required to maintain a constant pressure is accurately measured. Alternatively, fluid could be injected into the flexible membrane  16  to maintain the constant pressure. The amount of fluid added would be equal to the volumetric change in the sample  22 . In this approach, piston  36  would remain stationary after establishing the starting pressure. 
     In theory, the creep compliance of any given asphalt mixture will predict how much a paved surface will give under a relatively constant load (within the predetermined temperature range). The HCT  10  is ideal for performing this test because of its configuration. That is tensile, or hoop, stresses are created evenly along the entire inner circumference of the asphalt cylinder  22 , rather than at individual discrete points. Thus, almost the entire cylinder  22  is evenly subjected to the same stresses, which will produce more accurate and consistent results. 
     The HCT  10  is also used to perform tensile strength tests. This test determines the amount of force required to fracture the asphalt cylinder  22  at specific temperatures. Once a test cylinder  22  has been inserted and brought to the correct temperature, the piston  36  is actuated to increase the fluid pressure within the flexible membrane  16 . Flexible membrane  16  is caused to expand uniformly and correspondingly exerts a force upon the inner circumference of the cylindrical sample  22 . The pressure transducer  42  measures the pressure of the fluid  40 , which corresponds to the force exerted upon the cylinder  22 . The pressure is slowly and evenly increased until the cylinder  22  fractures. When this occurs, the restraint cylinder  32  contains the cylinder  22 , thus preventing any large holes from forming or any debris from being ejected. When the cylinder  22  fractures, there will be a sudden decrease in the pressure recorded by pressure transducer  42 . This indicates the completion of the test and the highest pressure recorded corresponds to the amount of force required to fracture the cylinder  22 . The fluid  40 , the flexible membrane  16 , and any other compressible material disposed within the center of the cylindrical sample  22  can impart an error into the test results. That is, these various materials will undergo some amount of compression under pressure. This compression factor must be determined and removed from the pressure/force calculations. One way to accomplish this is to perform the test with a cylindrical sample  22  (having the same inner diameter but very thick walls) made from a very rigid material, such as steel. In this manner, the compressibility of the various materials is easily measured. 
     As mentioned above, in one embodiment the flexible membrane  16  is shorter than the height of the cylindrical sample  22 . That is, there is a small portion of the interior of the cylindrical sample  22  which is not engaged by the flexible membrane  16 . Due to this, the various force calculations discussed above become more difficult. To arrive at the proper results, three dimensional finite analysis must be employed to extract the fundamental physical properties of the cylindrical sample  22  from the data acquired from the various LVDTs and pressure transducers. Once so established, correction factors are determined which allow for the proper correlation between the closed form solution for the stressed cylinder and the data that is actually acquired. 
     While the present invention is used primarily to determine the tensile proportions of asphalt mixtures, it has many other practical applications. For example, the HCT  10  can be used to measure Poisson&#39;s ratio, resilient modulus, fracture toughness, fatigue resilience, and the moisture sensitivity of various types of materials. The HCT  10  is useful in determining such proportions in a wide variety of materials including, but not limited to: asphalt, ceramic, mortar, composites and polymers. 
     In an alternative embodiment, it is possible to obtain direct measurements of the resultant changes in cylindrical sample  22 . Referring to FIG. 5, the general concept is illustrated. Rather than measuring the change in volume in fluid  40 , various sensors  60  can be positioned to monitor changes in the diameter over sample  22 . Though not separately shown, sensors  60  could also be used to take “before” and “after” measurements of the internal dimensions of cylindrical sample  22 . There is no limit to the number of measurement points that can be obtained. Directly measuring sample  22  will not provide as averaged a result as measuring the volumetric change, but does produce an easier system to assemble and utilize. The results achieved will still be dramatically improved from devices such as the IDT because the force is still uniformly and evenly applied to the whole of the sample. Sensors  60  can be any electronic or mechanical measuring device (such as a LVDT) having a sufficient degree of accuracy. 
     Those skilled in the art will further appreciate that the present invention may be embodied in other specific forms without departing from the spirit or central attributes thereof. In that the foregoing description of the present invention discloses only exemplary embodiments thereof, it is to be understood that other variations are contemplated as being within the scope of the present invention. Accordingly, the present invention is not limited in the particular embodiments which have been described in detail therein. Rather, reference should be made to the appended claims as indicative of the scope and content of the present invention.