Cement Stabilization of Conventional Granular Base and Recycled Crushed portland Cement Concrete

Transcription

1 Cement Stabilization of Conventional Granular Base and Recycled Crushed portland Cement Concrete Rielle Haichert, E.I.T, PSI Technologies Inc, 221 Jessop Avenue, Saskatoon, SK, Canada, S7N 1Y3, Phone: (306) , Fax: (306) , Roanne Kelln, E.I.T., PSI Technologies Inc, 221 Jessop Avenue, Saskatoon, SK, Canada, S7N 1Y3, Phone: (306) , Fax: (306) , Colin Wandzura, E.I.T., M.Sc. Candidate, Department of Civil and Geological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, S7N 5A9, , Curtis Berthelot, Ph.D., P. Eng., PSI Technologies Inc., 221 Jessop Ave., Saskatoon, SK, Canada, S7N 1Y3, Phone: (306) , Fax: (306) , Duane Guenther, P.Eng., City of Saskatoon, rd Avenue North, Saskatoon, SK, Canada, S7K 0J5, Phone: (306) , Fax: (306) , duane.guenther@saskatoon.ca. Word Count: 3853 Table and Figures: 8 x 250 = 2000 word equivalents Total Words: 5853 Submitted To: Transportation Research Board Transportation Research Record November 15, 2011

2 Haichert, Kelln, Wandzura, Berthelot, Guenther 2 ABSTRACT: Challenges in finding quality natural aggregate sources had led Saskatchewan road agencies to explore alternative solutions to meet aggregate demands. The use of recycled materials such as recycled portland cement concrete (PCC), while traditionally limited to low quality applications such as subbase or backfill materials, shows promise to be a technically-viable solution that also offers economic and environmental advantages. In this study, the effects of cement stabilization on traditional granular base and on two impact-crushed recycled portland cement concrete (PCC) materials from different locations were examined through mechanistic material testing. Unstabilized PCC materials had mechanistic material properties substantially better than the unstabilized conventional granular base material, indicating the suitability of using PCC materials in high quality applications such as base course layers, rather than limiting their use to low quality applications such as utility and embankment fills. This study also showed cement stabilization substantially improved the mechanistic properties of the conventional granular base material yet had a much less pronounced effect on the material properties of the PCC materials. This may be due to poor absorption of the cement by the PCC or a lack of rehydration of the PCC. There was minimal variability in the PCC specimen mechanical behaviour, despite a difference in stockpile location. Both PCC materials were processed and crushed with the same technique and equipment.

3 Haichert, Kelln, Wandzura, Berthelot, Guenther 3 INTRODUCTION Flexible pavement systems depend on the load-carrying capacity of all layers of the road structure. Granular base courses are an important component of road structural designs, both to protect the subgrade and to provide drainage for the pavement structure (1). Selecting high quality aggregates for granular base courses is important to ensure pavement durability and performance (1). Areas with concentrated truck haul and poor quality aggregate have experienced road performance that falls short of design expectations (2). Modern field conditions are becoming increasingly challenging for designing road structures and maintaining road networks. Heavy traffic loading, severe climatic conditions of extreme temperatures and increased precipitation due to climate change, poor subgrade conditions, and the aged state of road infrastructure all contribute to the accelerated degradation of roads (3). Finding high quality aggregate materials in Saskatchewan, Canada is challenging as quality aggregate sources are becoming increasingly depleted, especially in urban areas (2,4). These challenges necessitate haul distances as far as 100 km and result in higher costs, energy consumption, and haul road damage (4). Optimizing the use of locally available aggregate sources is economically and environmentally beneficial (1,2,4). Challenges in finding quality natural aggregate sources combined with growing environmental concerns related to the depletion of nonrenewable virgin aggregates and landfill capacities have led road agencies to search out alternative solutions to meet aggregate demands for road infrastructure projects (4). Recycled concrete aggregate is generated through infrastructure renewal including building, sidewalk, curb and gutter, and pavement removal/demolition (3-7,8). Processed and crushed concrete aggregate is used by the road construction industry in highway applications including backfill, drainage/subbase, and base materials (3-8). Recycled concrete aggregate has many environmental and economic benefits: conserves virgin aggregates, reduces impacts on landfills, limits haul distance if crushed locally, reduces disposal and transportation costs, and has an overall cost savings compared to procurement of virgin aggregates (3-8). In many studies, the performance of recycled PCC in a base layer has been comparable to that of virgin granular aggregate (5,6,7). The use of crushed PCC as recycled concrete aggregate has many technical benefits (6,7.8). Crushed PCC typically has a high percentage of fractured faces, which offers increased internal frictional resistance, aggregate interlock, and load bearing capacity (6,8). In the past few years, Saskatchewan agencies have undertaken efforts to recycle and reuse concrete aggregates. Major urban centers such as Saskatoon and Regina stockpile concrete rubble and debris in lieu of landfill disposal. Recycled concrete aggregates are produced by removing deleterious materials, processing the rubble, and crushing it to subbase, base, and rock gradations, as pictured in Figure 1. In Saskatchewan, crushed concrete has been used as backfill material, drainage rock, and subbase material. The effect of cement stabilization was investigated to evaluate the performance of crushed recycled concrete aggregate as a base material. Saskatchewan pavements are susceptible to rigorous moisture and freeze-thaw effects. In the past, cement additives have been used to mitigate climatic susceptibility of granular bases and subgrades and cement stabilization benefits have been well documented (8,9,10). FIGURE 1 Typical Recycled Concrete Aggregate Stockpile in Saskatchewan

4 Haichert, Kelln, Wandzura, Berthelot, Guenther 4 This study investigated the effects of cement stabilization on traditional granular base material and two typical, crushed recycled concrete aggregates: one from the City of Saskatoon and one from City of Regina. Unstabilized samples and samples stabilized with one, two, and three percent cement were compared across the different aggregate types in this study. STUDY SCOPE AND TEST PROGRAM The granular base material examined in this study was a conventional City of Saskatoon granular base material from a local sandy pit-run aggregate source. Two PCC materials were investigated in this study: COS well graded PCC and COR well graded PCC. Both COS and COR PCC materials were crushed using the same material processing methodology and impact crusher. Figure 2 illustrates the gradation of each material with respect to the COS granular base specified bandwidth. Both the COS granular base and COS GW PCC met the gradation envelope of the COS base specification (11). The COR PCC was marginally coarse of the COS base specification. It was observed that the impact crushed PCC aggregates had an increased number of fractured faces which can influence aggregate particle interlock during compaction, pictured in Figure 3 (4,7). 100% 90% 80% Percent Finer Than 70% 60% 50% 40% 30% 20% 10% 0% Sieve Size (mm) City of Saskatoon Base Specification Conventional Granular Base, 19 mm COS PCC COR PCC FIGURE 2 Gradation of Base Materials. FIGURE 3 Fractured face comparison: granular base(l) and PCC aggregate (R)

5 Haichert, Kelln, Wandzura, Berthelot, Guenther 5 Samples of each material were prepared in a gyratory compactor with the following stabilization systems: unstabilized (0 percent cement) and stabilized with one percent, two percent, and three percent cement. This laboratory program was limited in that only one sample of each material and stabilization system was created. Gyratory compaction was used for specimen preparation in lieu of standard Proctor compaction. In past studies, standard Proctor compaction did not compact recycled aggregates to maximum dry density (4,12,13). Also, gyratory compaction better replicates field compaction (12,13). Specimens were compacted in the gyratory at optimum moisture contents which resulted in the maximum dry density, listed in Table 1. Vertical traction of 600 kpa was applied to compact the samples, with a 1.25 degree angle of gyration and a rotational gyratory speed of 30 revolutions per minute. Samples were compacted in 150 mm diameter moulds to a height of 150 mm. The specimens were compacted to maximum dry density (115 gyrations) and did not fall apart when extracted from the gyratory and placed in moist cure. TABLE 1 Optimum Moisture of Base Materials. Sample Specimen and Stabilizer Optimum Moisture Content (%) Granular Base, 0 % cement 6.5 Granular Base, 1 % cement 6.8 Granular Base, 2 % cement 7.1 Granular Base, 3 % cement 7.5 COS PCC, 0 % cement 5.2 COS PCC, 1 % cement 5.5 COS PCC, 2 % cement 5.9 COS PCC, 3 % cement 6.2 COR PCC, 0 % cement 6.0 COR PCC, 1 % cement 6.5 COR PCC, 2 % cement 7.0 COR PCC, 3 % cement 7.5 Triaxial frequency sweep testing was used to characterize the mechanistic material properties of the specimens across various load frequencies and stress states typical of Saskatchewan field state conditions. Triaxial frequency sweep has been used in Saskatchewan over the past ten years to examine the performance of pavement structure materials including hot mix asphalt concrete, granular materials, subgrade soils, recycled materials, and stabilization systems (2,3,4,13,10,14,15). Test frequencies and stress states used emulate those experienced by Saskatchewan roads (2,3,4,13,15). Testing frequencies, from 10 Hz (representative of highway speed traffic), to 0.5 Hz (representative of slow moving traffic) are employed to simulate load frequencies to which materials are subjected in the field. Various stress states are employed and represent typical Saskatchewan field stress conditions (15). Table 2 lists the stress states examined in this study: a low stress state and a high stress state. The low stress state is emulative of well confined conditions deeper in the road structure, applying a hydrostatic (confining) stress of 250 kpa and a peak deviatoric stress of 200 kpa (15). The high stress state is emulative of low confined conditions closer to the surface of the road structure (15). The high stress state employed a hydrostatic stress of 100 kpa and a peak deviatoric stress of 550 kpa. Further detail on rapid triaxial testing procedures and methodology, as employed in this research, is provided elsewhere (15, 16). TABLE 2 Triaxial Frequency Sweet Test Stress States. Stress State Hydrostatic (Confining) Stress Deviatoric Stress Low 250 kpa 200 kpa High 100 kpa 550 kpa

6 Haichert, Kelln, Wandzura, Berthelot, Guenther 6 RESULTS AND DISCUSSION Mechanistic material properties of the base materials with various concentrations of cement were evaluated using triaxial frequency sweep analysis. Dynamic modulus, Poisson s ratio, and radial microstrain were measured across varying load frequencies and low and high stress states. Presented in Figure 4 through Figure 6 and discussed herein are the results across test frequencies and high and low stress states. Dynamic Modulus Results Figure 4 illustrates the dynamic modulus of the base materials at low and high stress states. The dynamic modulus is a measure of the stiffness of a material (15). The unstabilized PCC specimens were stiffer compared to the unstabilized granular base specimen. The PCC materials were coarser than the granular base material and were crushed using an impact crusher. Impact crushing improves the number of fractures faces on particles, which in turns betters particle interlock during compaction can improve stiffness (4,6,7,14). The unstabilized granular base specimen failed at a high stress state. The dynamic modulus under this stress state reflects the materials stiffness under a high deviatoric stress state and shows that a typical Saskatchewan granular base has greater potential to permanently deform under a high stress state. Across all cement-stabilized specimens, the conventional granular base showed the most improvement in stiffness with increasing cement content. At one and two percent cement, the conventional granular base performed comparable to or better than the PCC materials examined in this study. The stiffness achieved at three percent cement stabilization may be considered too high for granular base materials. Caution should be exercised to avoid over-stabilizing a material. Higher cement concentrations can make a material since a bound base layer that is too stiff can result in severe cracking and may impact the performance of the base course in the field. Improved stiffness with increasing cement content was minimal for the COS PCC and the COR PCC. The cement stabilized PCC materials did not improve in stiffness to the extent of the cement stabilized granular base material. This may be due to poor absorption of the cement by the PCC or a lack of rehydration of the PCC. It is not hypothesized that this is due to a lack of fines content in the PCC gradations, as the fines content was on the upper end of the base specification. Variability in COS and COR PCC stockpiles may have an effect on the absorption of the cement stabilization. Dynamic Modulus (MPa): Low Stress State Dynamic Modulus (MPa): High Stress State FIGURE 4 Dynamic modulus across various load frequencies

7 Haichert, Kelln, Wandzura, Berthelot, Guenther 7 Poisson s Ratio Results Figure 5 illustrates the Poisson s ratio of the base materials at low and high stress states. The Poisson s ratio of a material is the ratio of the strain in the radial coordinate direction to the strain in the axial coordinate direction, and is indicative of the shear stress developed in a material as it defines the lateral strains developed due to an applied load (15). At the low stress state, the unstabilized (0 percent cement) granular base had Poisson s ratios approaching 0.5, which can be an indication of orthotropic material behaviour causing specimen dilation. At the high stress state, the unstabilized conventional granular base failed. Cement stabilized conventional granular base showed the greatest improvement in Poisson s ratio behaviour among the base materials tested. At the high stress state, the unstabilized and one percent cement stabilized PCC specimens had a Poisson s ratio greater than 0.5, which indicates orthotropic material behaviour. The materials stiffness and volume change under the high stress state results in an orthotropic stress-dependent stiffness (14). For the PCC materials, two percent cement stabilization was needed to reduce the Poisson s ratio. Poisson's Ratio: Low Stress State Poisson's Ratio: High Stress State FIGURE 5 Poisson s Ratio across various load frequencies Radial Microstrain Results Figure 6 illustrates the radial microstrain results of the base materials at low and high stress states. Radial microstrain indicates the potential for a material to experience shear failure (15). Of the unstabilized samples (0 percent cement), the conventional granular base had the highest radial microstrain values, followed by COR PCC, and COS PCC. This may be due to the difference in gradation or anomalies within the crushed PCC material itself. The unstabilized granular base failed at the high stress state. Radial microstrain values decreased with an increase in cement stabilization, across both granular and PCC materials. At the low stress state, the decrease in radial microstrain was more pronounced in the granular base material from unstabilized to one percent cement stabilized; the radial microstrain decreased by 89 percent for granular base compared to five percent for COS GW PCC and 37 percent for COR GW PCC. The decrease in radial microstrain was further pronounced at two and three percent cement for the granular base. Although PCC materials did not have a prominent reduction in radial microstrain with an increase in cement concentration, the cement did stabilize the PCC material at both the low and high stress states.

8 Haichert, Kelln, Wandzura, Berthelot, Guenther 8 PCC materials stabilized with cement concentrations of one, two, or three percent demonstrated radial microstrain behaviour comparable to an unstabilized granular base material, at a low stress state. Where unstabilized granular base specimens failed at a high stress state, the cement stabilized PCC specimens were capable of withstanding high stress states. Radial Micro-Strain: Low Stress State Radial Micro-Strain: High Stress State FIGURE 6 Radial microstrain across various load frequencies CONCLUSIONS In this study, the effects of cement stabilization on traditional granular base and on two impact-crushed recycled portland cement concrete (PCC) materials were examined through mechanistic material testing. The following conclusions were drawn from this study. The unstabilized PCC materials had mechanistic material properties substantially better than the unstabilized conventional granular base material. These results indicate the suitability of using PCC materials in high quality applications such as base course layers, rather than limiting their use to low quality applications such as utility and embankment fills. Cement stabilization substantially improved the mechanistic properties of the conventional granular base material. Most notably, while the unstabilized granular base samples failed at the high stress state testing in the rapid triaxial tester (RaTT), the cement-stabilized samples all survived the high stress state. At one and two percent cement, the conventional granular base performed comparable to or better than the PCC materials examined in this study. The stiffness exhibited by the three percent cement stabilized granular base is likely too high for granular base materials and could result in brittle failures in the field. Cement stabilization had a much less pronounced effect on the material properties of the PCC materials. These findings suggest there is a limiting content of cement that will offer improved properties for PCC base materials. COR and COS PCC materials exhibited similar mechanical behavior despite variability in stockpiles due to location differences. This may be due to the processing technique used prior to impact crushing the concrete rubble. While further exploration is warranted into optimizing stabilization designs for PCC materials, the findings of this study suggest that for unstabilized materials, crushed PCC aggregate offers solutions to technical concerns with base courses, in addition to solutions to other economic and environmental concerns facing urban road agencies today.

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