Seongcheol Choi

Horizontal Cracking in Continuously Reinforced Concrete Pavements

Horizontal cracking (HC) at the depth of longitudinal steel in continuously reinforced concrete pavements (CRCP) was not known until 1999, when HC was observed in the section on IH 35 in the Waco District. At that time, no prior research was done on this topic and no reports published. Concerned about this type of cracking and its effect on the long-term performance of CRCP, the Texas Department of Transportation (TxDOT) initiated this research study. The primary objective of this study was to identify the mechanism of HC in CRCP due to environmental loading (temperature and moisture variations). To this end, a numerical model was developed to predict the risk of HC in CRCP. Laboratory and field testing was also conducted to evaluate the actual behavior of concrete near longitudinal steel. The measured data was used to develop and validate a numerical model for the prediction of HC potential in CRCP. The mechanism of vertical stress development in concrete near longitudinal steel was investigated with a comprehensive numerical analysis. The results of the study indicate that pavement design, more specifically longitudinal steel design, concrete material properties such as coefficient thermal expansion (CoTE), modulus of elasticity, drying shrinkage and strength, and construction quality such as curing and temperature control, all have effects on HC development. Two-mat steel, lower values of CoTE and drying shrinkage and modulus, higher strength of concrete, good quality curing and temperature control will lower the potential for HC. Since there are almost an infinite number of combinations of design, materials and construction variables during the construction of CRCP, more general guidelines rather than prescriptive ones were developed. The guidelines cover three areas – pavement design, materials and construction quality. The implementation of the guidelines is expected to minimize the occurrence of horizontal cracking in CRCP.

Thermal Strain and Drying Shrinkage of Concrete Structures in the Field

This article describes how substantial changes in temperature and moisture can occur when concrete structures are placed in the field. In order to predict stress and the corresponding risks of thermal and/or drying shrinkage cracking, an estimate is required of thermal strain and drying shrinkage not related to stress. In this study, a proposed method was developed to allow the direct field measurement of thermal strain and drying shrinkage in concrete structures. A nonstress cylinder was designed, which enabled the embedded strain gauge to measure actual thermal strain and drying shrinkage in concrete structures. Sensors, such as vibrating wire strain gauges and temperature and relative humidity (RH) sensors, were incorporated into the developed method. The validity of the method was investigated through laboratory tests as well as field implementation. It is expected that the proposed method can be efficiently used in field testing and allow realistic assessments of time-dependent behavior of concrete structures, including cracking potential.

Variation of Crack Width over Time in Continuously Reinforced Concrete Pavement

The primary objective of this paper is to present the findings of field studies that investigated variations in crack widths over time in three continuously reinforced concrete pavement projects in Texas. Also presented are crack width variations through the slab depth. Vibrating wire gauges were installed at three slab depths—top, middle, and bottom—during concrete placement. Transverse cracks were induced at the locations of the gauges, and concrete strains were measured. The ages of the projects varied from 11 months to 2 years, 10 months. Significant variations existed in crack widths along the slab depth. Crack widths were largest at the top and smallest at the bottom. In the test sections of this study, crack widths at the middepth were about three-quarters of those at the top. Crack widths at the top and middepth were a direct function of temperature variations in the concrete at those depths. However, there was a poor correlation between concrete temperatures and crack widths at the bottom. There was clear evidence that moisture variations in concrete have substantial effects on concrete volume changes and crack widths. An immediate decrease in crack widths was observed after rains. Crack widths decreased over time within the time frame of this study. This finding is not consistent with the current theory on crack width variations over time, and further study is currently under way at the University of Texas at Austin to identify the causes. The findings will be reported as they become available.

Effects of Creep and Built-In Curling on Stress Development of Portland Cement Concrete Pavement under Environmental Loadings

An adequate evaluation of stress developments in concrete is essential to ensure well-performing and long-lasting portland cement concrete (PCC) pavement designs and construction. In this study, the effects of creep and built-in curling (BIC) on the stress history of PCC pavements under environmental loadings were investigated primarily through a series of field tests and numerical data interpretations. To identify the stress-dependent strain component within the in situ measured total strain, a nonstress cylinder (NC) was employed in the field tests. The identified stress-dependent strains from the field tests were converted to stresses using a step-by-step numerical method. To investigate the effect of creep on stress developments, stress histories were computed in two different ways—one with elastic analysis and the other with viscoelastic analysis—and then their difference (stress relaxation) over time was evaluated. The finding indicated that creep may be a key element in the evaluation of long-term stresses and, in turn, the design and analysis of PCC pavements. Furthermore, this study examined the impacts of BIC on the residual stresses of PCC pavements. The result showed that BIC may affect the early-age stress development, but it has little influence on the long-term environmental stress state.

Performance of Continuously Reinforced Concrete Pavement Containing Recycled Concrete Aggregate

This paper presents the performance of continuously reinforced concrete pavement (CRCP) constructed in 1995 that utilized recycled concrete aggregate (RCA) as both coarse and fine aggregates. The project is Houston on a section of IH-10 between Loop 610 W and IH-45. In this project, no virgin aggregates were used. Concerns were raised regarding the performance of CRCP containing RCA. Detailed study was conducted to evaluate concrete material properties containing RCA. The properties of recycled aggregate measured in this study compared with virgin aggregate are consistent with those reported elsewhere: low specific gravity, higher water absorption, sulfate soundness loss, LA abrasion loss, and thermal coefficient. Little variation was observed in the paving operation due to the use of 100 % recycled coarse and fine aggregates. The moisture control of recycled aggregate, especially fine aggregate, is critical in producing consistent and workable concrete. The short-term and long-term performance of the reconstructed CRCP has been excellent, with tight crack widths and little spalling. Between concrete with virgin aggregates and concrete with recycled aggregates, there is no significant difference in thermal coefficient and permeability; however, there are significant differences in modulus of elasticity, compressive and indirect tensile strength, and water absorption. The low modulus of RCA concrete and good bond between recycled coarse aggregates and new mortar appear to be the key ingredients for good pavement performance. After more than 10 years of service under heavy traffic, the CRCP section containing 100% RCA is still providing excellent performance with no single structural distress.

Mechanism of Transverse Crack Development in Continuously Reinforced Concrete Pavement at Early Ages

The transverse crack in continuously reinforced concrete pavement (CRCP), more specifically transverse crack spacing and crack widths, has been cited as one of the most important pavement structural responses determining CRCP performance. Efforts have been made to predict crack spacing and crack widths for given environmental conditions and traffic loading, pavement structure, and material properties, with the primary objective of developing rational CRCP designs. However, that most transverse cracks develop at or near transverse steel implies substantial interactions between transverse steel and other factors causing transverse cracks. These interactions have not been fully incorporated in the theoretical models developed so far to predict transverse crack spacing and crack widths in CRCP. This study investigated the interactions between transverse steel and other factors and identified the mechanisms of transverse crack development at or near the transverse steel. Drying shrinkage and temperature drop in concrete cause concrete volume contractions in all directions (not just transverse and longitudinal, but vertical directions as well). Interactions between concrete volume contraction vertically and transverse steel cause larger concrete tensile stresses at or near transverse steel than at other areas and cause a higher probability of transverse cracks near transverse steel. Traditionally, subgrade drag theory has been used in the design of transverse steel even though current practice is to place just enough transverse steel to support longitudinal steel during concrete placement. If transverse cracks have such substantial effects on CRCP performance as currently thought, interactions between transverse steel and other factors should be considered in the design of optimum transverse steel.

Concept of Film Thickness Applied to a New Approach for Polymer Concrete Mix Design for Airport Pavement Repair

Because of its superior ability to bond to substrate materials and its good resistance to freezing and thawing cycles, polymer concrete has been widely used for repairing airfield pavements. However, polymer concretes used in repairing airport pavements in South Korea have not lasted as long as originally predicted; this premature failure may be attributable to the incompatibility of the coefficient of thermal expansion (CTE) between the polymer concrete and the substrate concrete material. To lower the CTE of the polymer concrete, the amount of polymer must be decreased and the amount of coarse aggregates increased. To determine the optimum amounts of polymer and coarse aggregates, a new concept of film thickness is proposed. In this research, the laboratory evaluation was performed on the mixtures of polymer concrete to measure compressive strength and CTE. On the basis of these test results, it was discovered that a larger amount of coarse aggregates lowered the CTE value and increased the film thickness; these changes resulted in a higher compressive strength than that of a typical polymer concrete with fine aggregates. As expected, the CTE of the polymer concrete decreased as the amount of polymer decreased. On the basis of limited laboratory test results, a film thickness of 33.5 Μm is recommended to satisfy the minimum requirement of compressive strength (21 MPa) for repair material in South Korea. This minimum film thickness can be achieved with a mix of 8.2% polymer and 91.8% sand. However, if coarse aggregates are substituted for 40% of the sand, the same film thickness can be maintained if the amount of polymer is reduced to 6.35%.

Rational Use of Terminal Anchorages in Portland Cement Concrete Pavement

Portland cement concrete (PCC) pavements have long been thought to expand and push bridge structures, with bridge damage resulting. To protect bridge structures from expanding PCC pavements, three terminal systems are currently used in Texas: anchor lug (AL), wide flange (WF), and expansion joint (EJ). Even though the Texas Department of Transportation uses all three systems in continuously reinforced concrete pavement (CRCP), the effectiveness of these three systems has not been fully evaluated. The parameters affecting CRCP movement near bridge terminal areas are investigated: whether thermal expansion of CRCP is damaging bridge structures, and if so, which terminal type is most cost-effective. Extensive field evaluations reveal that slab movement at the end of the CRCP are substantially restrained by subbase friction. Slab length from the end of the CRCP that significantly contributes to slab movement is limited to about 200 ft (60.96 m) for the CRCP system with asphalt base typically used in Texas. Slab movement rates due to seasonal temperature variations are larger than daily slab movement rates. The movement at the end of the CRCP can be accommodated by a simple EJ with subbase friction, which can be achieved with typical asphalt-stabilized base. The use of an AL system is not needed to restrain concrete slab movement. The benefits of WF and AL systems are doubtful when one considers that their cost is higher than that of the simpler EJ system.

Basic Analysis on Fractal Characteristics of Cement Paste Incorporating Ground Granulated Blast Furnace Slag

This study aimed to conduct the basic analysis on the fractal characteristics of cementitious materials. The pore structure of cement paste incorporating ground granulated blast furnace slag (GGBFS) was measured using mercury intrusion porosimetry (MIP) and the fractal characteristics were investigated using different models. Because the pore structure of GGBFS-blended cement paste is an irregular system in the various range from nanometer to millimeter, the characteristics of pore region in the different scale may not be adequately described when the fractal dimension was calculated over the whole scale range. While Zhang and Li model enabled analyzing the fraction dimension of pore structure over the three divided scale ranges of micro, small capillary and macro regions, Ji el al. model refined analysis on the fractal characteristics of micro pore region consisting of micro I region corresponding to gel pores and micro II region corresponding to small capillary pores. As the pore size decreased, both models suggested that the pore surface of micro region became more irregular than macro region and the complexity of pores increased.

Improvements of Full-Depth Repair Practices for Distresses in Continuously Reinforced Concrete Pavement

The Texas Department of Transportation has by far the most continuously reinforced concrete pavement (CRCP) lane miles in the nation, and sections as old as 50 years are still in service. Having served much longer than intended, some sections are showing distress. Full-depth repair (FDR) is one of the methods used to repair CRCP distresses in Texas. Over the years, various FDR methods have been used, and the effectiveness of each method has varied. In the most widely used FDR method, a full-depth cut is made at a minimum of 1.5 ft (0.46 m) inside the transverse repair boundaries, a partial-depth cut is made at repair boundaries, and the concrete between the cuts is removed to expose longitudinal steel. The method has inherent disadvantages, the longer repair time being the primary one. The full-depth cut FDR method, in which a full-depth cut is made at the repair boundaries and longitudinal tie bars are epoxy grouted into the existing concrete, has advantages over other methods, including a faster operation that minimizes roadway closure time. Because CRCP is normally used in areas with high traffic volume, the maximum time the Texas Department of Transportation allows for the FDR operation is usually 9 h, which makes the full-depth cut method the only acceptable repair method. Factors affecting the effectiveness of the full-depth cut method were investigated with laboratory testing and field evaluations. The method of epoxy injection, keeping the epoxy in the holes after insertion of the tie bars, restoring base support, and the length of the embedded tie bar were the most important variables affecting the performance of FDR. Recommendations were made to revise specifications for FDR and epoxy materials on the basis of these research findings. Implementation of the recommendations should result in improved FDR performance of CRCP.