Suitability of Using Crushed Concrete Adjacent to Geotextiles in Underdrain Systems Second Phase: Field Trial Before Implementation

About the report:

Due to the increasing interest in the use of recycled concrete as a base course aggregate, the Virginia Department of Transportation (VDOT) identified the need for an in-depth research investigate the clogging potential of the geotextile used in highway edgedrains if crushed hydraulic cement concrete (CHCC) is placed adjacent to the drainage fabric geotextile. To answer the research needs of the VDOT and address the discrepancies in the existing literature, George Mason University conducted a laboratory-based study and published their findings (VTRC 21-R12). The study identified two major mechanisms through which CHCC reduces the flowability of the geotextile used in underdrains: physical (fine particles migrating onto the voids in between the filaments of the geotextile) and chemical (precipitation of chemicals within geotextile filaments) phenomena. The findings showed that although the drainage fabric geotextile experienced some level of reduction in its flow capacity, as was expected, the reduction was not significant enough to impact and impede the overall flow of the geotextile/CHCC system. However, the laboratory study had some limitations, e.g., while simulating the physical and chemical phenomena simultaneously, temperature, rain, and humidity variations during seasonal changes were not accounted for, the potential effects of the pavement over the top of CHCC were not included, and drainage pipes were not considered.

The research reported herein is the second phase of the laboratory study and involved constructing a full-scale field test site within VDOT’s Harrisonburg facility. The test site consisted of seven sections, each side of which was constructed to include a 4-inch drainage pipe covered by No. 57 stone wrapped with drainage fabric, per VDOT’s UD-4 edgedrain standards. In all seven sections, an unbound base course was placed in direct contact with drainage fabric. Sections with CHCC content included an unbound base course with (i) 100% CHCC, (ii) 40% CHCC + 60% virgin aggregate (V.A.; Blend 1), and (iii) 20% CHCC + 80% V.A. (Blend 2). Replicates of these sections were constructed as paved (to simulate actual roadway conditions) and unpaved (to provide data comparable with those of the previously completed laboratory study). Additionally, the seventh section was constructed as unpaved with a 100% V.A. section, which served as the control section. The constructed site was evaluated for about 3 years, and performance evaluations were conducted approximately every 6 months by analyzing the exhumed drainage fabric geotextiles and borescope surveying the drainage pipes. The findings confirmed the observations of the previous study by capturing the physical phenomenon. However, the data from the unpaved CHCC section showed approximately 2.5 times less tufa precipitation (chemical phenomenon) on the drainage fabric geotextile than what was previously noted in the one-year laboratory study. For the paved sections, data observed over 3 years were used to predict possible tufa growth on the drainage fabric’s surface in 30 years. This prediction showed that even after 30 years, the surface area of the geotextile that is covered with tufa precipitation could be less than what was observed in the one-year laboratory study. If this prediction holds true, then the drainage fabric geotextile used in this study would continue to function hydraulically in 30 years. Borescope inspections of the drainage pipes showed signs of precipitation in the sections with 100% CHCC and 40% CHCC (Blend 1) but not in those with 20% CHCC (Blend 2) and 100% V.A. Although all the drainage pipes constructed continued to function hydraulically within the duration of this study, the chemical precipitation within the 100% CHCC and 40% CHCC sections of the drainage pipes needs further monitoring. This is because the quantitative estimation of continuing chemical precipitation within the pipes requires more data than what was available within the duration of this study.

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