Tuncer B Edil

pH-Dependent Leaching of Trace Elements from Recycled Concrete Aggregate

Recycled concrete aggregate (RCA) has excellent mechanical properties and is often used as base course in pavement construction. However, highly alkaline leachate from RCA has been observed in laboratory studies. The associated high-pH leaching patterns can be of concern, especially when compared to the neutral pH environment observed in actual road sections using RCA as base course. In this study, the pH-dependent leaching concentrations of trace elements copper (Cu) and zinc (Zn) and the oxyanion chromium (Cr) were investigated on unfractionated RCA samples and fractionated RCA samples (i.e., fine particles 0.075 mm, and gravel-sized particles 4.75 mm). A pH-buffering plateau was observed between pH 4.9 and 7.0 in the acid neutralization capacity curve. Cu and Zn showed the highest levels of leaching at pH≅2, and the lowest leaching at pH>7.5. Cr showed the lowest level of leaching between pH 5.0 and 6.5, and higher leaching concentrations towards the acid and alkali directions. The fine particles tended to leach more Cu and Zn than sand- and gravel-sized particles at 2

Hydraulic Conductivity of Geosynthetic Clay Liners to Synthetic Coal Combustion Product Leachates

Experiments were conducted to evaluate whether coal combustion product (CCP) leachates adversely affect the hydraulic conductivity of geosynthetic clay liners (GCLs). Chemical properties of CCP leachates were compiled based on a nationwide survey of CCP disposal facilities. Five synthetic leachates were selected from this database to represent a range of conditions encountered in CCP disposal facilities: typical CCP leachate, strongly divalent cation fly ash leachate, flue gas desulfurization (FGD) residual leachate, high ionic strength ash leachate, and trona ash leachate. Five GCLs were tested: two conventional Na-bentonite GCLs, two polymer-modified bentonite GCLs, and one bentonite polymer composite (BPC). Hydraulic conductivity tests were conducted on non-prehydrated GCLs using flexible-wall permeameters. GCLs with Na-bentonite had high hydraulic conductivity (>10 m/s) to trona leachate, whereas the hydraulic conductivity of GCLs with polymer-modified bentonite was variable, ranging from 10 to 10 m/s. For the typical CCP, high ionic strength, FGD, and strongly divalent cation leachates, GCLs with Na-bentonite had moderate to high hydraulic conductivity (10 to 10 m/s). GCLs with polymer-modified bentonite had lower hydraulic conductivity (10 to 10 9 m/s) to FGD and strongly divalent cation leachates, and a wide range of hydraulic conductivities to high ionic strength leachate (10 to 10 m/s). All of the GCLs had low hydraulic conductivity (<10 m/s) to DI water. GCLs with BPC had very low hydraulic conductivity (< 10 m/s) to all leachates.

Utilize Cementitious High Carbon Fly Ash (CHCFA) to Stabilize Cold In-Place Recycled (CIR) Asphalt Pavement as Base Coarse

The purpose of this study was to evaluate the performance of cementitious high carbon fly ash (CHCFA) stabilized recycled asphalt pavement as a base course material in a real world setting. Three test road cells were built at MnROAD facility in Minnesota. These cells have the same asphalt surface layers, subbases, and subgrades, but three different base courses: conventional crushed aggregates, untreated recycled pavement materials (RPM), and CHCFA stabilized RPM materials. During and after the construction of the three cells, laboratory and field tests were carried out to characterize the material properties. The test results were used in the mechanistic-empirical pavement design guide (MEPDG) to predict the pavement performance. Based on the performance prediction, the life cycle analyses of cost, energy consumption, and greenhouse gases were performed. The leaching impacts of these three types of base materials were compared. The laboratory and field tests showed that fly ash stabilized RPM had higher modulus than crushed aggregate and RPM did. Based on the MEPDG performance prediction, the service life of the Cell 79 containing fly ash stabilized RPM, is 23.5 years, which is about twice the service life (11 years) of the Cell 77 with RPM base, and about three times the service life (7.5 years) of the Cell 78 with crushed aggregate base. The life cycle analysis indicated that the usage of the fly ash stabilized RPM as the base of the flexible pavement can significantly reduce the life cycle cost, the energy consumption, the greenhouse gases emission. Concentrations of many trace elements, particularly those with relatively low water quality standards, diminish over time as water flows through the pavement profile. For many elements, concentrations below US water drinking water quality standards are attained at the bottom of the pavement profile within 2-4 pore volumes of flow.

Field Evaluation of Fly Ash Stabilized Subgrade in US 12 Highway

This paper describes a case study where subgrade soils were stabilized with Class C fly ash to create a working platform during reconstruction of a 1.2-km section of rigid pavement in US 12 between Cambridge and Fort Atkinson, Wisconsin. The subgrade soils were blended with cementitious fly ash to increase its bearing resistance and stiffness. Resilient modulus (Mr), unconfined compression strength (qu), soil stiffness gauge and dynamic cone penetrometer tests were conducted on the subgrade alone and fly-ash stabilized subgrade (FASS). Falling weight deflectometer (FWD) tests were conducted on the as-built pavement. After 14 d of curing, Mr of the field-mixed FASS ranged between 60 and 129 MPa and Mr of the laboratory-mixed FASS ranged between 115 and 167 MPa, whereas the Mr of subgrade was between 34 and 42 MPa. qu of the laboratory-mixed FASS was four to nine times the qu of the subgrade, while qu of the field-mixed FASS was up to three times qu of the subgrade. In situ stiffness and dynamic penetration index also illustrated that the addition of the fly ash and compaction increased the strength and stiffness appreciably. Moduli back-calculated from FWD tests showed that the modulus of the FASS varies seasonally.

Laboratory and field performance of recycled aggregate base in a seasonally cold region

The objective of this project was to characterize the freeze-thaw properties of recycled concrete (RCA) and asphalt (RAP) as unbound base and to assess how they behaved in the field for nearly 8 years. This paper includes an examination of existing information, laboratory studies of freeze-thaw behavior, and evaluation of data from MnROAD field-test sections in a seasonally cold region, i.e. , in Minnesota, USA. Test sections were constructed using recycled materials in the granular base layers at the MnROAD test facility. One test section included 100% RAP, another 100% RCA, a third one a 50/50 blend of RCA/natural aggregate, and a fourth one only natural aggregate (Class 5) as a control. The stiffness ( i.e. , elastic modulus) was monitored during construction and throughout the pavement life by the Minnesota Department of Transportation, along with the variation of temperatures and moisture regimes in the pavement to determine their effects on pavement performance. The resilient modulus of each material was determined by bench-scale testing in accordance with NCHRP 1-28a, as well as by field-scale tests incorporating a falling-weight deflectometer. Specimens were subjected to as many as 20 cycles of freeze-thaw in the laboratory, and the change in their resilient modulus was measured. In the field-test sections constructed with the same materials as the base course, temperature, moisture, and field modulus (from falling-weight deflectometer tests) were monitored seasonally for nearly 8 years. From the temperatures in the base course layer, the number of freeze-thaw cycles experienced in the field was determined for each test section. Inferences were made relative to modulus change versus freeze-thaw cycles. Conclusions were drawn for long-term field performances of the recycled base (RAB) in comparison to natural aggregate.

Freeze-Thaw Cycling Effect on the Constrained Modulus and UCS of Cementitiously Stabilized Materials

This study evaluates the effect of freeze-thaw (F-T) cycling on the mechanical properties of cementitiously stabilized materials (CSM). Ultrasonic testing was applied to monitor the P-wave velocity (constrained modulus) change during the F-T cycling. Unconfined compression strength (UCS) test was performed at the end of F-T cycling. It was found that F-T cycling could be detrimental to CSM as the constrained modulus decreased to 7% to 96% depending on the CSMs. The residual UCS was 35% to 84% of the initial UCS at the end of the test. It was found that the greater the cement content, the stronger the anticipated durability. Clay-lime and silt-fly ash showed limited resistance to F-T cycling.

Investigation of White Bluffs Landslides in Washington State

The objective of this study was to conduct experimental and numerical investigations to evaluate potential failure mechanisms of the White Bluffs landslides. Increased landslide activity in the 1970s and 1980s has been attributed to irrigation activities behind the bluffs, which altered moisture conditions and caused seepage and piping. Soil instability is believed to originate in weakly cemented clays and silts of the Ringold Formation. The experimental analysis included triaxial compression tests on undisturbed and remolded specimens from the Ringold Formation, to evaluate anisotropy, wetting, and remolding effects. Strength anisotropy manifested an effective cohesion intercept (c) that is approximately 2.7 times larger for the vertical orientation (46 kPa) compared to the horizontal; however, similar effective stress friction angles (φ ≈ 20-21°) were determined in both orientations. Larger φ and c were determined for air-dried specimens compared to saturated specimens in both orientations. A larger φ and smaller c were determined for the remolded saturated specimens, indicating that remolding altered the soil fabric. The numerical slope stability analyses indicate that landslide activity may be attributed to a combination of factors, i.e., strength reduction and pore pressure development with soil wetting, coupled with anisotropic shear strength.

Trace Element Leaching From Recycled Pavement Materials Stabilized With Fly Ash

Percolation rates and concentrations of trace elements are presented from leachate collected in pan lysimeters installed beneath roadway sections where fly ash was used to stabilize subgrade or base course. Data from control sections are also presented. Percolation rates from the base of a pavement profile vary seasonally in response to seasonal variations in meteorological conditions. Percolation rates typically range between 0.1-0.5 mm/d, with the average percolation rate falling between 0.1-0.2 mm/d depending on site conditions. Concentrations of six elements (B, Cd, Cr, Cu, Mo, and Zn) in leachate from roadway sections stabilized with fly ash have been elevated compared to concentrations from control sections at all field sites with control sections. Four elements from fly-ash-stabilized materials have been elevated in concentration relative to control section at some sites (As, B, Cd, Cr, and Mo) and have also exceeded MCLs. Both B and Mo persistently exceed MCLs. In contrast, concentrations of Cd and Cr have only exceeded MCLs in the first samples collected (PVF < 0.25), and then have remained well below MCLs.