Andrew F. Braham
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Material Production |
Aggregate Testing |
Asphalt Cement Testing |
Portland Cement Concrete Testing |
Asphalt Concrete Testing |
Asphalt Emulsion Mixture Testing |
This webpage briefly describes the laboratory capabilities of pavement materials in the Civil Engineering department at the University of Arkansas. Pavement material research includes the areas of pavement maintenance, pavement rehabilitation, rigid pavement, and flexible pavement. Laboratory capabilities include material production, aggregate testing, asphalt cement testing, Portland cement concrete testing, asphalt concrete testing, and asphalt emulsion mixture testing.
There are several methods of producing pavement samples in the lab at the University of Arkansas. For asphalt concrete, these methods include gyratory compactors, a slab compactor, asphalt foaming devices, an asphalt emulsion mill, and a rock crusher. For Portland cement concrete, three concrete mixers are available to cast laboratory mixtures. The equipment available is described in more detail below.
The lab has seven gyratory compactors in total. Of special interest are a Troxler Model 5850 and a Pine Instrument Company AFG2AS. The Troxler 5850 is capable of compacting samples containing water, with specially designed perforated molds and a catch-basin below the molds to capture any water forced out of the sample during compaction. This is especially important when compacting Cold In-place Recycling or Full-Depth Reclamation samples. The Pine AFG2AS is the latest compactor model from Pine and includes shear measurement instrumentation. The amount of shear energy expended during compaction is becoming an important parameter to capture while evaluating all types of pavement materials that require compaction in the field. Figure 1 below shows the Troxler 5850 and Pine AFG2AS. All gyratory compactors conform to AASHTO T312/ASTM D6925.
Figure 1: Troxler Model 5850 (top) and Pine Instrument Company AFG2AS (bottom)
The lab has a single slab compactor. The slab compactor is a custom made compaction device, capable of producing samples 12x12 inches (30.5x30.5 cm) in width and height, and two to four inches (5.1-10.2 cm) thick. The slab compactor is essential for producing samples for the Single-Edge Notch Beam [SE(B)] fracture test and core-shaped samples that are not produced in a gyratory compactor. Gyratory compactors compact samples in a rotational motion that while extremely convenient to produce, have been known to produce air voids distributions that are not representative of field conditions. However, circular samples can be cored out of the slab compactor, using the lab coring machine and necessary saws. Figure 2 shows a picture of the slab compactor.
Figure 2: Slab compactor
Asphalt foaming has two applications in pavement materials. First, it can be used for in place pavement rehabilitation, including Cold In-place Recycling (CIR) and Full-Depth Reclamation (FDR). Second, it is a type of method to produce Warm Mix Asphalt (WMA). The lab has two foaming machines: the Wirtgen WLB 10S foaming machine and “The Foamer” from Pavement Technology Inc. (PTI). The Wirtgen machine was specifically designed for FDR foaming design, but has also been used in industry and academia to produce CIR and WMA mixtures. The Wirtgen also comes with a dual-shaft pugmill mixer capable of mixing up to 66 lb (30 kg) of material (WLM 30). This mixer is useful for making large batch samples that would not be possible to produce in the standard five gallon (19 liter) paddle mixer. The PTI machine was specifically design for WMA mixtures. These two machines are shown in Figure 3.
Figure 3: Wirtgen WLM 30 mixer & WLB 10S foaming machine (top) and “The Foamer” from PTI (bottom)
The laboratory has a laboratory bench-top scale Herbert Rink asphalt emulsion mill. With this mill, we are able to produce our own asphalt emulsions and allows research into emulsifier chemistries, asphalt cement types, asphalt cement modification, influence of material temperature, pH, and other factors of asphalt emulsion formulation development. Figure 4 shows the mill in the laboratory.
Figure 4: Asphalt Emulsion Laboratory Mill
Often, it is necessary to crush pavement cores from the field in order to break them down for future use. Especially when looking at Cold In-place Recycling or Full Depth Reclamation, it is critical to be able to control the gradation of crushed material. Therefore, the laboratory purchased a RHINO Jaw Crusher in order to further process material. Figure 5 shows the crushing unit.
Figure 5: Jaw Crusher
The laboratory is equipped with three concrete mixers. Two concrete mixers are used to cast laboratory mixtures with batch sizes of 2 cubic feet to 8 cubic feet. A larger mixer is used to cast beams specimens of up to 1.5 cubic yards.
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The Civil Engineering department has all of the common aggregate tests for asphalt concrete and Portland cement concrete. Table 1 summarizes there tests.
Table 1 – Aggregate Testing Capabilities
All of the starred aggregate tests (*) have been certified by the AASHTO Materials Reference Laboratory (AMRL).
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Asphalt Cement Testing
Several tests have been obtained in the past two years in order to measure properties of asphalt cement. The equipment includes a Brookfield Rotational Viscometer, a Dynamic Shear Rheometer, and a Rolling Thin Film Oven.
Brookfield Rotational Viscometer
The lab has a Brookfield DV-II+ Pro Viscometer that meets standards AASHTO T316 and ASTM D4402. The viscometer measures the workability of asphalt cement, which is critical for production and construction. With four SC spindles (SC4-21, SC4-27, SC4-28, SC4-29) a full range of products can be tested. In addition, the unit was purchased with a Helipath stand. This allows the viscometer to raise and lower during testing, creating a helical path and eliminating channeling of non-flowing substances. As new products are developed and evaluated for pavement materials, flexibility with testing equipment is critical. Figure 6 shows the viscometer with the thermosel and Helipath stand.
Figure 6: Brookfield Rotational Viscometer
Dynamic Shear Rheometer
For understanding the performance of asphalt cement at high temperatures and ambient temperatures, a Dynamic Shear Rheometer is necessary. For capturing both rutting and fatigue cracking, AASHTO T315/ASTM D7175 is used to measure G* and sin d. A new test, Multiple Stress Creep Recovery, has shown to do a better job of predicting rutting for polymer modified asphalt cements. AASHTO TP70/ASTM D7405 is the procedure for measuring Jnr, the primary measurement from the Multiple Stress Creep Recovery test. The lab has a TA Instruments HR-2 Discovery Hybrid Rheometer (DHR) to capture G*, sin d, and Jnr,. To control testing temperatures, the DHR is equipped with a peltier plate and environmental test chamber. Figure 7 shows the Dynamic Shear Rheometer.
Figure 7: Dynamic Hybrid Rheometer
Rolling Thin Film Oven
The lab has a Rolling Thin Film Oven in order to short-term age the asphalt cement according to AASHTO T240/ASTM D2872. This aging is representative of the aging that occurs during the high temperatures of production and construction. After aging, rutting and cracking characteristics can be measured. Figure 8 shows the lab’s Rolling Thin Film Oven.
Figure 8: Rolling Thin Film Oven
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Portland Cement Concrete Testing
The laboratory houses three Forney compression machines. Two of the machines, 400 kip capacity and 600 kip capacity, are compression only, while the third machine, 600 kip capacity, has both tension and compression capabilities. The machines are used to measure concrete compressive strength (ASTM C39), split tensile strength (ASTM C496), modulus of elasticity (ASTM C469), and modulus of rupture (ASTM C78).
The laboratory also houses several pieces of equipment used to measure concrete durability. It contains two, freeze-thaw chambers (ASTM C666, Procedure B) and the equipment necessary to measure the fundamental transverse frequency (ASTM C215). Concrete permeability can be measured using the Rapid Chloride Ion Penetrability Test (ASTM C1202). Furthermore, concrete contraction or expansion due to chemical attack can be measured using ASTM C157.
An end grinder was recently added to the laboratory. The end grinder is necessary for testing high-strength and ultra-high performance concrete cylinders with compressive strengths that exceed 15 ksi. Another piece of equipment recently added measures the coefficient of thermal expansion of concrete.
All of the standard concrete tests can also be run in the laboratory, including:
• ASTM C31 (AASHTO T23): Making and Curing Concrete Test Specimens in the Field
• ASTM C138 (AASHTO T121): Unit Weight, Yield, and Air Content (Gravimetric) of Concrete
• ASTM C143 (AASHTO T119): Slump of Hydraulic Cement Concrete
• ASTM C172 (AASHTO T141): Sampling Freshly Mixed Concrete
• ASTM C173 (AASHTO T196): Air Content of Freshly Mixed Concrete by the Volumetric Method
• ASTM C231 (AASHTO T152): Air Content of Freshly Mixed Concrete by the Pressure Method
• ASTM C1064: Temperature of Freshly Mixed Portland Cement Concrete
• ASTM C 617 (AASHTO T 231): Capping Cylindrical Concrete Specimens
• ASTM C 1231 (AASHTO T 22): Use of Unbonded Caps in Determination of Compressive Strength of Hardened Concrete Cylinders
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Asphalt Concrete Testing
The University of Arkansas has very comprehensive asphalt concrete testing lab. Equipment is available for the conventional asphalt concrete tests, including Marshal Stability (AASHTO T245/ASTM D6927), moisture torture testing, and tensile strength ratio (AASHTO T283/ASTM D4867). In addition, the lab has the capability to test dynamic modulus, creep compliance, flow number, multiple fracture tests, and torsion bars. To capture data, the lab has Digital Image Correlation equipment that takes pictures during testing, and allows for full-field displacement measurements.
Evaluation of Rutting and Stripping of Asphalt (ERSA)
The Evaluation of Rutting and Stripping of Asphalt (ERSA) is very similar to the Hamburg Wheel Tracking test (AASHTO T324). It is designed as a moisture torture test, where submerged samples of asphalt concrete have a steel wheel rolled over until failure. Figure 9 shows the ERSA equipment.
Figure 9: Evaluation of Rutting and Stripping of Asphalt
MTS Load Frames
For many of the performance tests, two MTS load frames are utilized with an environmental chamber. One of the MTS load frames is rated for 100 kip (445 kN) and the other 22 kip (98 kN). These frames can use three load cells: 100 kip (445 kN), 5 kip (22 kN), and 1 kip (4 kN) depending on the required resolution. A custom made environmental chamber is compatible with both load frames with a temperature range from -50C to +60C, allowing for analysis of asphalt concrete across the full service range of temperatures. Figure 10 shows a picture of the two load frames and the environmental chamber.
Figure 10: MTS Load Frames and Environmental Chamber
Many tests can be run on the two MTS load frames, which are also known as universal load frames. The dynamic modulus (AASHTO TP62) measures the strain response to applied stress at multiple frequencies and temperatures. This is a primary input into the Mechanistic-Empirical Design Guide (MEPDG) and quantifies the fundamental linear viscoelastic characteristics of asphalt concrete. Creep compliance (AASTHO T322) and flow number (AASHTO TP79) are asphalt concrete tests that indicate rutting behavior of asphalt concrete mixtures. The dynamic modulus and flow number are the two primary tests of the Asphalt Mixture Performance Tester (AMPT), but can be run on any universal load frames. Most of these tests are run at ambient temperature or above.
For low temperature testing, especially to indicate cracking susceptibility, the lab has the traditional Superpave Indirect Tension Test (IDT, AASHTO T322) and the capability to run two fracture tests: The Semi-Circular Bend [SC(B)] and the Single-Edge Notch Beam [SE(B)]. These fracture tests are able to quantify a fundamental material property of asphalt concrete, the fracture energy, which is a strong indicator of cracking resistance. Figure 11 shows the SC(B) and SE(B) fracture test setup. Note, the SC(B) is shown just on the fixture, while the SE(B) is inside of the environmental chamber.
Figure 11: The Semi-Circular Bend [SC(B)] test (top) and the Single-Edge Notch Beam [SE(B)] fracture test (bottom)
Dynamic Shear Rheometer
A significant drawback to testing asphalt concrete mixtures is the large amount of material required for testing. In order to attempt to reduce the amount of material, the Dynamic Shear Rheometer has been adapted by many researchers to run a torsion bar test. These relatively small samples, only 0.5x0.5x2 inches (12x12x50 mm), are run in the lab’s Dynamic Hybrid Rheometer. With special fixtures, the torsion bar is able to quantify the rutting susceptibility of asphalt concrete. The HR-2 is equipment with an environmental test chamber that, with liquid nitrogen, is able to span the entire service temperature of asphalt concrete, from -50°C to +60°C and even higher if desired. Figure 12 shows a picture of a torsion bar sample and the environmental test chamber.
Figure 12: Torsion bar sample (top) and environmental test chamber on the DHR (bottom)
Digital Image Correlation
The lab has many ways to capture data during testing. For all of the asphalt concrete mixture tests above, linear variable differential transformers (LVDTs), extensometers, and clip gauges are available to capture displacement data. However, often it is necessary to capture data from locations on a sample away from where these gauges are attached to the sample. Therefore, Digital Image Correlation, or DIC, is a versatile tool that is able to take pictures during testing of the sample deforming. After the pictures are collected, computer programs can analyze the image to measure displacements across the entire sample face, called full-field displacements. This allows for quantifying both local and bulk displacements of the sample, critical for analyzing the properties of the asphalt concrete. Figure 13 shows the camera in front of a SC(B) sample in the 22 kip MTS load frame, and the resulting image from the camera.
Figure 13: Digital Image Correlation lab setup (top) and analyzed image (bottom)
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Asphalt Emulsion Mixture Testing
With the emulsion lab mill, it is important to be able to run tests of asphalt emulsion mixtures. Almost all of the tests described in the asphalt concrete section can be run on both Cold In-place Recycling (CIR), cold mix asphalt, and Full-Depth Reclamation (FDR) mixtures. In addition, the raveling test (ASTM D7196) can be run on these three pavement materials. Figure 14 shows the Hobart Mixer along with the raveling fixtures to run the raveling test.
In addition to the raveling test, the Hobart Mixer is able to run the sweep test (ASTM D7000) for chip seals and the wet track abrasion test (ASTM D3910/ISSA TB100) for slurry seals and microsurfacing. This is done by simply removing the raveling fixtures and replacing it with the appropriate fixtures for either the sweep test or the wet track abrasion test.
Figure 14: Hobart HL120 Mixer
Last updated: January 2014