Institute for Transportation

About the research

This Phase II project contained two parts. Part A focused on the investigation of the characteristics and thermal properties of commonly used slags in Iowa cementitious materials and their effects on adiabatic temperature rise (ATR) in Iowa mass concrete structures. A new hydration model for slag-containing cementitious materials and a three-parameter ATR prediction model were developed and validated. These models were incorporated into the ConcreteWorks software, significantly improving its thermal predictions for mass concrete with slag. Part B focused on the development of a new seal slab component in the ConcreteWorks software. This new component is simple, user friendly, and well-integrated into the existing ConcreteWorks software and enables the prediction of the temperature profile of a bridge footing placed on a seal slab.

In Part A, the research included the following: (1) characterizing physical and chemical properties of slags and concrete materials, (2) studying slag’s effect on cement hydration using isothermal calorimetry, (3) evaluating temperature rise in mass concrete with various slag types and dosages using semi-adiabatic calorimetry, (4) testing large concrete blocks with various slag replacements, (5) developing and validating the ATR prediction model for slag concrete, and (6) integrating the models into the ConcreteWorks software. In Part B, the research included the following: (1) developing a new seal slab component in the ConcreteWorks software and (2) conducting trial analyses to investigate the effects of the time of footing placement, the sizes of the footing and seal slab, and environmental conditions on the temperature profile of a bridge footing placed on a seal slab.

The updated ConcreteWorks software now features enhanced input parameters and equations for better heat prediction, confirmed through thermal and sensitivity analyses. The improved slag hydration model, which accurately incorporates slag’s chemical properties, enhances temperature prediction accuracy for concrete containing slag. The maximum core temperature of a footing placed on a seal slab was generally found to be higher than that of a footing with no seal slab. In addition, it was found that a seal slab generally enhances the capacity for heat retention in the footing, keeping the core temperature of the footing high for a longer period.

About the research

Low-volume rural roads are generally low funding priorities compared to the roads that are part of the National Highway System (NHS). Therefore, low-volume rural roads tend to deteriorate to a point where traditional pavement preservation and maintenance techniques no longer have the desired effect or sufficient funding is not available.

As a potential solution, the Iowa Department of Transportation (DOT) constructed 10 test sections with various base and surface treatments on a 13 mi low-volume asphalt road segment in northeast Iowa in 2013, which were studied as part of the first phase of this research project. The aim of the project was to develop holding strategies beyond pavement preservation as a solution to low-volume roads that are in poor condition when there are not resources available for a complete rehabilitation. Due to the success of this first phase, a second phase was proposed in 2018.

This second phase study focused on surface treatments and was intended to treat highly distressed composite pavements that have asphalt overlays on portland cement concrete (PCC) pavements. Eight test sections were constructed on US 65, between Hubbard and Zearing in Iowa. The holding strategies evaluated were a combination of cold in-place recycling with various surface mixes, 1 in. profile milling with various surface courses, 2.5 in. profile milling with interlayer and surface course, and double coats of microsurfacing with and without additional spot grinding.

Based on the evaluation of the test sections and follow-up surveys, recommendations are given regarding the selection of the most advantageous strategy for the conditions of the studied pavement.

About the research

Pavement preservation is an essential component of highway asset management, enabling transportation agencies to maintain roadway performance and extend pavement service life while efficiently allocating limited maintenance resources. This study was conducted in collaboration with the Iowa Department of Transportation (Iowa DOT) to evaluate the effectiveness and cost-efficiency of commonly used pavement preservation treatments for both flexible and composite pavements.

Historical pavement management data from the Iowa DOT were analyzed to estimate treatment performance based on two key metrics: life extension, representing the additional service life gained from a treatment, and index benefit, representing the cumulative improvement in pavement condition over time. The analysis considered several preservation treatments, including microsurfacing, patching, slurry seal, crack sealing/filling, seal coat, and hot-mix asphalt (HMA) overlay. To enhance the robustness of the evaluation, national pavement performance data from the Long-Term Pavement Performance (LTPP) dataset were also incorporated to supplement and validate the analysis where appropriate. Historical bid tabulation data from Iowa DOT projects were used to estimate representative treatment costs.

The results provide insights into the relative performance of preservation treatments under different pavement conditions and traffic levels and demonstrate how treatment effectiveness varies depending on the timing of application. These findings were combined with cost information to conduct benefit-cost evaluations and to develop decision-support matrices for flexible and composite pavements. This study provides a data-driven framework to support pavement preservation decision-making. The results and tools developed through this research can assist the Iowa DOT in selecting cost-effective treatments and establishing long-term pavement preservation strategies across the state’s highway network.

About the research

Snow and ice removal operations in winter road maintenance are essential for the Iowa Department of Transportation (DOT) as well as counties and cities in Iowa to ensure the safety, mobility, and efficiency of their transportation infrastructure systems. As an innovative snow and ice removal alternative, the researchers at Iowa State University (ISU) have developed (1) new mix design and production methods of heated concrete (dubbed electrically conductive concrete [ECON], hereafter) and (2) new structural and system design approaches for a heated pavement system (HPS) using the new ECON developed. Based on successful real-world implementation of ECON HPS recently demonstrated by the ISU research team for airport pavement and roadway construction projects, the City of Iowa City has expressed strong interest to the Iowa DOT and ISU research team in implementing ECON HPS for their new bus stop stations as well as the bus stop loading area under an upcoming pedestrian crossing and bus stop enhancement project at Muscatine Avenue in the City of Iowa City. This research is proposed in response to such keen interest by the City of Iowa City. A set of proposed research tasks (to implement ECON HPS for the bus stop station application) include the development of ECON mix design and ECON HPS system design options for construction, comprehensive performance monitoring and evaluation, as well as economic analysis of the constructed full-scale ECON HPS at least over two consecutive winter cycles, and a survey and interview on the implementation of heated pavements for winter maintenance and management practices. The primary outcome of this research will be the draft of a technical guide/specification (comparable with the current Iowa DOT specifications and Iowa Statewide Urban Design and Specifications [SUDAS]) that Iowa DOT and Iowa’s counties and cities could use for future ECON HPS implementation projects under their public works departments for enhancing sustainable and resilient winter maintenance and management practices.

About the research

Iowa State University (ISU) has assembled a team of experts in Civil, Chemical, and Industrial and Manufacturing Systems Engineering to address the challenges of integrating scrap and end-of-life rubbers to supplant the use of up to 140,000 tons/year of virgin polymer used in asphalt pavements. The team proposes the use off-specification polybutadiene to modify ground tire rubber (GTR) particles, from recycled tires, and use them as an asphalt modifier to replace widely used SBS elastomers. GTR is currently being used as a modifier, which offers comparable improvements with respect to SBS polymers such as rutting resistance and elastic recovery, with advantages such as improved traction and reduced noise. However, because of the difference in density with asphalt it suffers from inadequate storage stability, rendering it an unpreferred material in asphalt modification. ISU has developed a technology that density matches GTR with asphalt with simple compounding techniques, producing a GTR product that meets storage stability specifications and would be accepted by the market. If 100% of virgin polymer were replaced with recycled GTR, up to 140,000 tons/year could be reclaimed nationwide. Moreover, asphalt pavements are 100% recyclable, including as a major component of new roadways, which means that GTR used in pavements will continue to be reused.

About the research

Cold in-place recycling (CIR) involves cold milling 3 to 4 in. of a deteriorated asphalt pavement, processing and stabilizing the millings, and relaying, compacting, curing, and covering with a hot-mix asphalt (HMA) overlay or (rarely in Iowa) a bituminous surface treatment. This report documents an investigation of CIR projects in Iowa with the objective of understanding CIR pavement deterioration processes so that improvements can be made in project selection, design, construction, and maintenance. Multiple research methods were used, including a literature review, a meta-analysis of the literature, database analysis, recovery of field pavement cores and milling samples, laboratory testing, and development of conclusions and recommendations.

Analysis of 44 CIR pavement projects from the Iowa Pavement Management Information System database showed that rutting was the distress that triggered performance concerns first, followed by longitudinal wheel path cracking and transverse cracking. Pavement distress indices typically dropped below trigger values 10 to 15 years after CIR rehabilitation, and thicker CIR and HMA layers were associated with better performance outcomes.

Laboratory testing of field cores recovered from an Iowa CIR pavement with performance issues confirmed that the flexibility of the CIR pavement layers was greater than that of the overlaid HMA layers. Since CIR layers have a relatively high air void ratio, a possible deterioration process was postulated where heavy wheel loads cause compaction rutting in the CIR layer, which in turn causes the less flexible HMA layer to longitudinally crack in the wheel path after it is forced to conform to the rutted CIR cross section. Resulting recommendations are to specify a more flexible HMA overlay, use rut filling treatments after ruts appear, and avoid selecting CIR for roads that experience many heavy wheel loads.

This report also documents a forensic case study of two low-volume roads that did not meet performance expectations and a comparison of asphalt milling gradations from CIR versus hot in-place recycling (HIR) projects. The forensic study showed that pavement thicknesses were less than planned, the pavement structure was insufficient for typical local subgrade strengths, and gradations were finer than expected. In the gradation comparison, HIR millings were confirmed to be coarser than CIR millings.

About the research

Earth pressure on concrete box culverts is a key component of design and load rating decisions. Currently, the Iowa Department of Transportation (DOT) Office of Bridges and Structures uses the maximum and minimum lateral earth pressure of 36/18 lb/ft³ specified for load factor design (LFD) and allowable stress design (ASD) and 60/30 lb/ft³ for load and resistance factor design (LRFD).

The objective of this project was to analyze collected data to determine the actual earth pressures associated with Iowa soil conditions to determine which load pressures are more realistic for typical Iowa soil conditions and construction methods for concrete box culverts.

A brief literature review was conducted to identify potential factors related to the vertical and lateral earth pressures. To gain a better understanding of actual pressures on concrete box culverts in Iowa, a culvert in Ida County was instrumented in 2016. Two concrete box culverts—the one in Ida County and the other in Crawford County—were monitored for more than 2.5 years and 1 year, respectively, to identify the realistic design soil pressure for Iowa soil conditions.

The captured pressure, strain, and temperature data were analyzed to find the relation between the temperature and earth pressure experienced by the culverts. The measured vertical and lateral pressures were compared with specified design pressure loading.

The monitoring results from both culverts led to the consistent conclusion that the earth pressure experienced by the culverts were 2 to 6 times that of the design values with the LRFD and LFD/ASD methods. Extensive longitudinal cracks (parallel to the flow) were observed at the bottom surface of the top slab on both culverts.

Given the findings from this research, further research is recommended, as outlined at the end of this report.

About the research

The objective of this project was to determine the optimum joint spacing for thin concrete overlays based on different concrete overlay thicknesses, support systems, and concrete overlay types with and without structural macro-fibers.

In thin concrete overlays, field observations have sometimes shown that not all contraction joints activate initially and, in some cases, do not activate until many years after construction. Contraction joints that do not activate may be considered an inefficient design that may lead to unnecessary maintenance efforts and costs. The optimum joint spacing design may need to be determined based on factors other than those that are currently considered.

This project included an analysis for recommended joint spacing using pavement design software as well as a field review of joint activation in existing concrete overlays using nondestructive testing. Test sections were also constructed to analyze a wider range of variables and to study early-age joint activation behavior.

The data showed that joint spacing was the most significant factor affecting joint activation. A design parameter, slab length over the radius of relative stiffness (L/ℓ), was identified to have a correlation with joint activation percentage and timing.

The data showed that use of macro-fibers did not affect the percentage or rate of joint activation compared to overlays without macro-fibers.

About the research

Ultra-high performance concrete (UHPC) provides superior properties in strength and durability for the long-term performance of bridges. Despite these desirable properties and the potential to be applicable in the majority of projects, UHPC is still not widely used, mainly because of the cost associated with it.

The report details a study performed on the design of non-proprietary UHPC mixes that provide comparable strength properties to that of commercially available mixtures. A set of base mixtures were explored by varying the ratios for various constituents and investigating their durability, strength, and transport properties, including volume stability and freeze-thaw resistance.

In the later stage of the project, the selected non-proprietary mixes were evaluated for their flexural strength. The flexural strength in UHPC comes mainly from the fibers used in the mix. Bearing in mind the role of fibers, the effects of various types of steel fibers (i.e., variation in shape, size, and dosage) were evaluated. The role of fibers on strength and post-cracking behavior was carefully examined using laboratory testing and image analysis utilizing digital image correlation techniques. The efforts found that an optimal combination of micro- and macrofibers can enhance the flexural strength of UHPC mixtures.

Steel fibers contribute to more than a third of the cost of UHPC mixtures, so the possibility of utilizing less expensive and more environmentally friendly synthetic fibers—polypropylene, polyvinyl alcohol, nylon, alkali resistant glass, or carbon—to partially replace the steel fibers could reduce the cost of UHPC. The steel fibers were partially replaced by the different synthetic fibers to see their effect on the UHPC’s fresh properties and flexural strength. Utilizing digital image correlation, the synthetic fiber contribution to post-cracking behavior was evaluated, especially from the crack width control and crack propagation aspects. The replacement of steel fibers with synthetic fibers showed promise for flexural strength and post-cracking behavior.

The report provides recommendations for the preparation of cost-effective, non-proprietary UHPC mixtures that could be used for various transportation infrastructure applications. Further recommendations are also made for the optimal combination of different types of steel micro- and macrofibers to get the best flexural response. Recommendations are then extended for the use of different types of synthetic fibers and the optimum percentage of dosage replacement for steel fibers.

About the research

This Phase II research project on the shrinkage behavior of high-performance concrete (HPC) used in Iowa bridge decks and overlays evaluated several concrete mixes, building off or modifying mixes developed in Phase I. Based on shrinkage behavior and mechanical properties, the mixes studied in Phase I were characterized as having either high, medium, or low cracking potential. In the Phase II study, three concrete mixes (Mixes 6, 8, and 2, characterized in Phase I as having high, medium, and low cracking potential, respectively) were selected for further investigation. The selected mixes were modified using three shrinkage control technologies: shrinkage-reducing admixtures (SRAs), cementitious materials (CM), and internal curing (IC) agents, respectively. The modification methods were first studied in a laboratory until the optimal shrinkage behavior of each concrete mix was achieved. Two pairs of the tested concrete mixes (Mixes 6 and 8 with and without modification) were then used in a field investigation on the US 20 over I-35 dual bridge. The mixes were placed side by side for the bridge overlays, which were monitored for about one year with strain gages, temperature and moisture sensors, and regular visual examinations.

The laboratory investigation confirmed positive effects for the concrete shrinkage control technologies used. The laboratory test results also provided specific details for the concrete mix modifications, ensuring optimal concrete performance and shrinkage control. The modifications included the addition of 1.0/1.25 gal/yd³ of SRA in Mix 6, the use of 10% CM reduction for Mix 8, and the use of lightweight fine aggregate as an IC material in Mix 2. The results of the field investigation suggest that environmental conditions on the casting day and the first few days of curing play an important role in the development of concrete properties. Future studies could benefit from a comprehensive stress analysis to better understand the long-term effects of the shrinkage control technologies, as well as further field tests and an extended monitoring time.