Pangil Choi

Development of mechanical tube feeding method for construction of continuously reinforced concrete pavement (CRCP)

ABSTRACT In South Korea, the performance of continuously reinforced concrete pavement (CRCP) has been better than that of jointed concrete pavement (JCP). However, most of the Portland cement concrete (PCC) pavements in South Korea have been JCP, primarily due to the difficulty in providing extra space along the edge of the pavement needed for concrete delivery in CRCP construction. In a tube-feeding method (TFM), concrete can be supplied from the front of a paver, which could facilitate CRCP construction without extra space needed. In the present study, efforts were made to improve the capability of TFM in placing longitudinal steel at correct elevations, which resulted in the development of tube feeding equipment (TFE). Field testing results showed that the key to ensuring steel placement at correct elevations was to keep longitudinal steel in adequate tension. The distance from the back of the tubes and the front of the mould, up to 1500 mm, did not have substantial effects on the steel settlement. To evaluate the viability of CRCP construction with TFE, a 400 m long CRCP section with 300 mm slab thickness and 8.2-m width was constructed and performance monitored.

Rheological Properties of Wet-Mix Shotcrete Mixtures Made with Crushed Aggregate

AbstractThe recent depletion of high-quality and well-graded natural-aggregate sources is one of the factors compromising the production of good-quality shotcrete. For this reason, the use of vario...

Predicting Pumpability and Shootability of Crushed Aggregate Wet-Mix Shotcrete Based on Rheological Properties

This study aims to estimate the pumpability and shootability of wet-mix shotcrete (WMS) made with crushed aggregates and various admixtures such as silica fume, fly ash, ground granulated blast furnace slag (GGBFS), metakaolin, and steel fiber based on rheological properties. The IBB rheometer was employed as an apparatus to measure the rheological properties of freshly mixed shotcrete such as flow resistance and torque viscosity. Results have shown that the use of silica fume and metakaolin led to satisfactory pumpability, whereas mixtures with fly ash and steel fiber failed to meet the pumping criteria at normal pump pressure. The build-up thickness, an indicator to represent shotcrete shootability, was predicted to vary between 68 and 218 mm, demonstrating that the use of admixtures resulted in a wide spectrum of shootability. In particular, the use of metakaolin was found to substantially increase the predicted build-up thickness only with a small replacement. The findings of this study are expected to be used as an easy-to-use guideline for estimating pumpability and shootability of WMS when no compliance testing data is available.

Evaluation of Pavement Support for Pavement Design

AbstractIt is well-known that the performance of Portland cement concrete (PCC) pavement depends, to a great extent, on the support condition under the slab. The Texas Department of Transportation (TXDOT) recognized the importance of the support condition and requires the use of a stabilized base, either a 10.16-cm hot mix asphalt layer or a minimum 2.54-cm asphalt concrete over a 15.24-cm cement-stabilized layer. For pavement design purposes, 81.4  MPa/m has been used as the default value for the modulus of the subgrade reaction (k value) for the support provided by these bases. The use of a single value of 81.4  MPa/m for the k value in rigid pavement design was partly because of the insensitivity of the required slab thickness to the modulus of the subgrade reaction in the current TXDOT rigid pavement design procedure. To improve rigid pavement design, TXDOT developed a new design procedure for continuously reinforced concrete pavement (CRCP) on the basis of mechanistic-empirical principles and plans t...

Construction Issues and Design Procedure for Transverse Steel in Continuously Reinforced Concrete Pavement (CRCP)

PURPOSES : The objective of this study is to evaluate construction issues and design for transverse steel in continuously reinforced concrete pavement(CRCP). METHODS : The first continuously reinforced concrete pavement(CRCP) design procedure appeared in the 1972 edition of the “AASHTO Interim Guide for Design of Pavement Structures,”which was published in 1981 with Chapter 3 “Guide for the Design of Rigid Pavement” revised. A theory that was accepted at that time for the analysis of steel stress in concrete pavement, called subgrade drag theory(SGDT), was utilized for the design of reinforcement of CRCP tie bar design and transverse steel design in the aforementioned AASHTO Interim Guide. However SGDT has severe limitations due to simple assumptions made in the development of the theory. As a result, any design procedures for reinforcement utilizing SGDT may have intrinsic flaws and limitations. In this paper, CRCP design procedure for transverse steel was introduced and the limitations of assumptions for SGDT were evaluated based on various field testing. RESULTS : Various field tests were conducted to evaluate whether the assumptions of SGDT are reasonable or not. Test results show that 1) temperature variations exist along the concrete slab depth, 2) very little stress in transverse steel, and 3) warping and curling in concrete slab from the field test results. As a result, it is clearly revealed out that the assumptions of SGDT are not valid, and transverse steel and tie bar designs should be based on more reasonable theories. CONCLUSIONS : Since longitudinal joint is provided at 4.1-m spacing in Korea, as long as joint saw-cut is made in accordance with specification requirements, the probability of full-depth longitudinal cracking is extremely small. Hence, for transverse steel, the design should be based on the premise that its function is to keep the longitudinal steel at the correct locations. If longitudinal steel can be placed at the correct locations within tolerance limits, transverse steel is no longer needed.

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.

Autogenous Shrinkage of Very-Early Strength Latex-Modified Concrete with Latex Contents

Very-early strength latex-modified concrete (VES-LMC) was developed for the purpose of fast-track overlay of a concrete bridge deck under heavy traffic, concentrated on the workability, durability, and strength gain so that it can be opened to the traffic only three hours after its placement. The mixture of VES-LMC might accompany very high heat of hydration at early-age because of its inherent rapid hardening property and could have susceptibility to autogenous shrinkage because of its relatively low water-cement ratio. This study evaluated the effect of the latex-cement ratio(L/C) both of the constant and variable slumps on the autogenous shrinkage of VES-LMC by carrying out simple temperature rise test and early-age shrinkage experiment. Test results are as follows: The latex contributes on the enhancement of the concrete durability but has little effect on its hydration and the accompanied heat of hydration in VES-LMC. Autogenous shrinkage increased with the increase in latex-cement ratio at variable slumps and its pattern followed regularly a logarithmic increase. However, the influence of water-cement ratio and latex-cement ratios for the test specimens at constant slump on early-age autogenous shrinkage property was found to be minor due to the simultaneous effect of the two experimental variables.