IFCEE 09, American Society of Civil Engineers, 2009, Orlando, Florida

Transcription

1 IFCEE 9, American Society of Civil Engineers, 29, Orlando, Florida Field Evaluation of Fly Ash Stabilized Subgrade in US 12 Highway Lin Li 1, M. ASCE, P.E., Onur Tastan 2, Craig H. Benson 3, F. ASCE, P.E., and Tuncer B. Edil 4, F. ASCE, P.E. 1 Assistant Professor, Department of Civil and Environmental Engineering, Jackson State University, Jackson, MS; lin.li@jsums.edu 2 Formerly graduate student, University of Wisconsin-Madison, Madison, WI; tastan@wisc.edu 3 Professor and Department Chair, Department of Civil and Environmental Engineering, University of Washington, Seattle, WA; chbenson@u.washington.edu 4 Professor, Department of Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, WI, edil@engr.wisc.edu ABSTRACT: 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 (M r ), unconfined compression strength (q u ), 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, M r of the field-mixed FASS ranged between 6 and 129 MPa and M r of the laboratory-mixed FASS ranged between 115 and 167 MPa, whereas the M r of subgrade was between 34 and 42 MPa. q u of the laboratory-mixed FASS was four to nine times the q u of the subgrade, while q u of the field-mixed FASS was up to three times q u 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. INTRODUCTION Fly ash stabilization of subgrade is a developing and promising technology (Edil et al., 22, 26; Bin-Shafique et al., 24; Trzebiatowski et al., 24, Wen et al. 23; Li et al. 28). For example, Edil et al. (22) use fly ash to stabilized finegrained subgrades in two short experimental sections in STH 6 at Wisconsin. Many states have active programs promoting the beneficial reuse of fly ash and other high volume industrial by products. However, the effectiveness of stabilizing subgrades with coal fly ash in full-scale applications is unavailable. The primary objective of Page 1

2 this paper is to evaluate how fly ash increases the bearing strength and stiffness of the subgrade soils during construction and as the pavement is exposed to weather. This paper describes a project where self-cementing Class C fly ash from a coalfired electric power plant was used to stabilize subgrade during reconstruction of a 1.2-km section of US 12 highway between Cambridge and Fort Atkinson in Wisconsin ( 4 km east of Madison). The subgrade soil was graded using motor grader. Class C fly ash was spread uniformly on the surface using truck-mounted lay-down equipment similar to that described in Edil et al. (22). The fly ash was mixed with the subgrade to a depth of 35 mm using the Wirtgen WR 25S road reclaimer, with water being added during mixing using a water truck. This mixture, which contained 12% fly ash by dry weight, was compacted within 1-2 h by a tamping foot compactor followed by a vibratory steel drum compactor. The fly-ash stabilized subgrade (FASS) was cured for 7 d and then overlain with 15 mm of base layer with recycle asphalt and gravel material and 2 mm concrete pavement. Prior to stabilization, samples of the subgrade were collected and tested to determine their index properties and how addition of fly ash would affect the M r and q u of the soil. Tests were also conducted on samples of the in situ stabilized soils to determine if similar improvements in properties were obtained during construction. DCP and SSG tests were used to measure the strength and stiffness near the surface of the stabilized subgrade. FWD tests were conducted to evaluate the overall improvement in stiffness achieved through stabilization. MATERIALS Disturbed samples of subgrade soil ( 2 kg each) were collected from a depth between -.5 m at the fourteen stations during construction (Fig. 1). Tests were conducted on these samples to determine index properties, soil classification, water content and dry unit weight. Samples of the fly ash were also collected. Subgrade A summary of the Cambridge, WI 58+ (Not to scale) Shoulder Line Pavement edge Fort Atkinson properties of the subgrade is shown in Table 1. FIG. 1. Layout of stations at field site in US 12. Particle size distribution curves for the subgrade are shown in Fig. 2. All fourteen soils are broadly graded. The subgrade consists of lean clay (CL), clayey sand (SC), and silty sand (SM) according to the Unified Soil Classification System. According to the AASHTO Soil 12 N Page 2

3 Classification System, most of subgrade soils at this site are A-7-6 with a group index (GI) larger than 1 and A-6 with GI larger than 2. Four of the coarse-grained subgrade soils are classified as A-2-6 (Stations,,, and ) and have GI < 1. TABLE 1. Physical properties and classifications of subgrade soils. Classification Station LL a PI b GI c f g SSG DCP Index w N γ d Number (%) (kn/m 3 Stiffness (DPI) USCS d AASHTO e ) (MN/m) (mm/blow) CL A h SM A SC A SM A CH A SC A CL A CL A SM A SC A SM A SC A SC A SC A a LL = Liquid Limit, b PI = Plasticity Index, c GI = Group Index, d USCS = Unified Soil Classification System, e AASHTO = Association of American State Highway and Transportation Officials, f w N = In situ water content, g γ d = Dry unit weight, h hyphen indicates test was not conducted. Fly Ash Fly ash from Columbia Power Station in Portage, Wisconsin was used for stabilization. Chemical analysis of the fly ash by X-ray fluorescence indicated that the calcium oxide (CaO) content is 23% and the SiO 2 + Al 2 O 3 + Fe 2 O 3 content is 56%. The loss on ignition is.7%. According to ASTM C 618, the Columbia fly ash is a Class C fly ash. Percent Finer (%) Fines Sand.1.1 Particle Diameter (mm).1.1 FIG. 2. Particle size distributions of subgrade. Page 3

4 Fly-Ash Stabilized Subgrade (FASS) FASS was prepared by mixing Class C fly ash (12% by dry weight) with subgrade soils to a depth of 35 mm. Water content and unit weight of the compacted FASS were measured at each station using a nuclear density gage (ASTM D 2922) immediately after compaction was completed. Samples of FASS were also collected immediately after mixing for preparation of test specimens for M r and q u testing. These specimens were compacted to the measured density at each station, sealed in plastic wrap, and cured for 14 d at 1% humidity. Specimens of FASS were also prepared in the laboratory using samples of the subgrade soils and 12% (dry weight) Columbia fly ash. The mixtures, referred as laboratory-mixed specimens, were prepared with optimum water content, and then cured using the same procedures employed for the field-mixed specimens. A similar set of specimens was prepared with subgrade soils only (no fly ash) using the same procedure, except for the curing phase. METHODS Laboratory M r and q u Tests The M r tests were performed in accordance with AASHTO T292 after 14 d of curing (FASS) or immediately after compaction (subgrade). The 14-d curing period is based on recommendations in Trzebiatowski et al. (24), and is intended to reflect the condition when most of the hydration is complete (Edil et al., 26). The loading sequence for cohesive soils was used for the FASS as recommended by Bin-Shafique et al. (24) for soil-fly ash mixtures. Subgrade soils were tested using the loading sequence for cohesive soils. Two specimens of field-mixed FASS split horizontally after curing. These specimens were trimmed to an aspect ratio of 1 prior to testing. All other specimens had an aspect ratio of 2. The q u tests were measured on specimens of FASS after the resilient modulus tests were conducted. Only those specimens having an aspect ratio of 2 were tested. A strain rate of.21%/min was used for the q u tests following the recommendations in ASTM D 512 for compacted soil-lime mixtures. No standard method currently exists for q u test of materials stabilized with fly ash. Field Tests Strength and stiffness of the FASS were measured with SSG, DCP, and FWD. Testing with the SSG and DCP were conducted directly on the FASS after 7 d of curing. The FWD testing was conducted after the concrete pavement. The SSG tests were conducted in accordance with ASTM D 6758 using a Humboldt GeoGauge. DCP testing was conducted at each station in accordance with ASTM D 6951 using a DCP manufactured by Kessler Soils Engineering Products Inc. The dynamic penetration index (DPI) obtained from the DCP was computed as the mean penetration (mm per blow) over a depth of 15 mm. The FWD surveys were conducted using a Dynatest 82E FWD following the method described in Page 4

5 ASTM D The FWD data were inverted using the program MODULUS v6. from the Texas Transportation Institute. RESULTS Resilient Modulus Resilient moduli of subgrade, field-mixed FASS, and laboratory-mixed FASS are shown in Fig. 3a. These M r correspond to a deviator stress of 21 kpa, which represents typical conditions within the base course of a pavement structure (Trzebiatowski et al. 24). There is no systematic variation in M r along the alignment, suggesting that the variability in M r is more likely due to heterogeneity in material rather than systematic variations in site conditions or construction methods. Comparison of the M r for subgrade and FASS in Fig. 3a indicates that adding fly ash increased the M r. For the subgrade, the M r ranged between 34 and 42 MPa (mean = 38 MPa), where the field-mixed FASS had M r 6~129 MPa (mean = 82 MPa) and the M r of laboratory-mix FASS ranged 115~167 MPa (mean = 139 MPa). In the laboratory-mixed FASS, there are only three specimens selected from Station 58, 586 and 616 to represent lean clay and clayed sand. Interaction of the two different soils with fly ash may vary depending on the mineralogical compositions (Edil et al. 26). The cementitious reactions occurring in lean clay is slightly different from the reactivity of clayed sand. The range of measured M r in the three laboratory-mixed FASS (115~167 kpa) showed that the variability of the M r of the laboratory-mix FASS. The M r of the field-mix FASS also was more variable than the M r of the laboratory-mix FASS. The M r of the field-mix FASS is lower, on average, than the M r of the laboratorymixed FASS (82 MPa vs 139 MPa). This is consistent with the investigator s experience, which has shown that M r of field mixtures of fly-ash stabilized materials typically lower than M r of mixtures prepared in the laboratory (Li et al. 28, Bin- Shafique et al. 24). The effect of cementation of fly ash depends on the mineralogical compositions of soil, mixing water content, type and amount of fly ash, type of mixing, and curing period (Edil et al. 26). The difference between the M r of field-mixed FASS and laboratory-mixed FASS in this study should be related to the mixing condition and type of mixing. Bin-Shafique et al. (24) attribute these differences in M r to more thorough blending of soil and fly ash in the laboratory compared to the field, resulting in more uniform distribution of cements within the mixture. Unconfined Compressive Strengths (q u ) The q u of the subgrade, field-mixed FASS, and laboratory-mixed FASS are shown in Fig. 3b. As observed for M r, there is no systematic variation in q u along the alignment. The q u of the subgrade ranges from 92 to 138 kpa (mean = 112 kpa), the laboratory-mix FASS has q u between 36 and 112 kpa (mean = 663 kpa), and the q u of field-mix FASS ranges 11~41 kpa (mean = 183 kpa). Thus, adding fly ash to Page 5

6 the subgrade increased the q u appreciably, although the q u in the field was 72% lower, on average, than the q u of the laboratory-mix FASS. As observed for M r, the range of measured q u in the three laboratory-mixed FASS (36~112 kpa) showed that the variability of the q u of the laboratory-mix FASS. The q u of the field-mix FASS also was more variable than the q u of the laboratorymix FASS. The relative low q u of field-mixed FASS compared to q u of laboratorymixed FASS should be caused by a more thorough blending of soil and fly ash in the laboratory compared to the field, resulting in more uniform distribution of cements within the mixture. Resilient Modulus (MPa) (a) Subgrade Field-mixed FASS Lab-mixed FASS Unconfined Compressive Strengths, q u (kpa) (b) Subgrade Field-mixed FASS Lab-mixed FASS FIG. 3. Stiffness and strength subgrade and FASS (laboratory-mix and fieldmix) after 14 d of curing: (a) M r, and (b) q u. In situ Stiffness In situ gained stiffness measured with the SSG and DPI measured with the DCP are shown in Fig. 4 for the subgrade and FASS after 7 d of curing. Addition of the fly ash and compaction increased the strength and stiffness appreciably, with the DPI decreasing from 33 to 15 mm/blow, on average, and the stiffness increasing from 8 to 2 MN/m, on average. The DPI and stiffness of the stabilized and compacted subgrade are also less variable than those of the subgrade. FWD Deflections and Moduli Maximum deflections from the FWD tests conducted with a 4-kN load are shown in Fig. 5a. Maximum deflection, which is measured at the center of the loading plate, is a gross indicator of pavement response to dynamic load. Tests were conducted in August 24 (one month after construction), and the following spring (May 25). Similar deflections were measured during both surveys, suggesting that the FASS had maintained its integrity even after exposure to freezing and thawing. Elastic moduli of the FASS obtained by inversion of the FWD data are shown in Fig. 5b. Page 6

7 The moduli drop between August 24 and May 25 (average mean 1324 MPa vs 725 kpa). These changes in modulus probably reflect changes in the water content of the FASS, which was highest in the spring and lower in the summer and fall. 6 5 (a) 1.5 (b) SSG FASS /SSG subgrade :1 Ratio DPI FASS / DPI subgrade :1 Ratio FIG. 4. In situ stiffness gain after fly ash stabilization after 7 d of curing (a) SSG, and (b) DPI. Maximum Deflections at 4-kN Load (mm).5 (a).4 Aug 24 (Avg. Max. Deflection =.1 mm) May 25 (Avg. Max. Deflection =.1 mm) Modulus (MPa) 25 (b) Aug 24 FWD May 25 FWD FIG. 5. Maximum FWD deflection for 4-kN load (a) and moduli of FASS obtained by inversion of FWD data (b). SUMMARY AND CONCLUSIONS The FASS had significantly higher M r and q u than the subgrade. In situ stiffness measured with the SSG and dynamic penetration index (DPI) measured with the DCP also illustrated that the addition of the fly ash and compaction increased the strength and stiffness appreciably. These finding suggest that fly ash stabilization of subgrade should be beneficial in terms of increasing pavement capacity and service life. However, the M r and q u of FASS mixed in the field were lower than those for FASS Page 7

8 mixed in the laboratory (4% lower for M r, 72% lower for q u ). Similar biases between mixtures prepared in the laboratory and field have been observed by others and need to be considered as one of factors when pavement design is based on data obtained by testing mixtures blended in the laboratory. Moduli back-calculated from falling weight deflectometer tests conducted along the alignment at twice since construction showed that the modulus of the FASS varies seasonally, with the lowest moduli occurring in the spring when the pavement has thawed and has higher water content. Monitoring of the pavement will be conducted in the future to determine how the resistance to damage by freeze-thaw cycling changes. ACKNOWLEDGEMENT Financial support for this study was provided by the Wisconsin Department of Transportation (WisDOT). The conclusions and recommendations in this report are solely those of the authors and do not reflect the opinions or policies of WisDOT. Bulent Hatipoglu, Xiaodong Wang, and Jacob Sauer assisted with the project in the field and laboratory. REFERENCES Bin-Shafique, S., Edil, T.B., Benson, C.H. and Senol, A. (24). Incorporating a fly-ash stabilised layer into pavement design. Geotechnical Engineering-ICE, 157 (GE4): Edil, T.B., Acosta, H.A. and Benson, C.H. (26). Stabilizing soft fine-grained soils with fly ash. Journal of Materials in Civil Engineering, 18 (2): Edil, T.B., Benson, C.H., and Bin-Shafique, S. (22). Field evaluation of construction alternatives for roadways over soft subgrade. Transportation Research Record, 1786: Li, L., Benson, C. H., and Edil, T. B (28) Sustainable construction case history: fly ash stabilization of recycled asphalt pavement material. Journal of Geotechnical and Geological Engineering, 26: Trzebiatowski, B., Edil, T.B. and Benson, C.H. (24). Case study of subgrade stabilization using fly ash: State Highway 32, Port Washington, Wisconsin. Beneficial Reuse of Waste Materials in Geotechnical and Transportation Applications, 127: Wen, H.F., Tharaniyil, M.P. and Ramme, B. (23). Investigation of performance of asphalt pavement with fly-ash stabilized cold in-place recycled base course. Transportation Research Record, 1787: A27-A31. Page 8