Institute for Transportation

About the research

An increasing number of concrete overlay projects in Iowa and around the United States have used fiber-reinforced concrete (FRC) mixtures. Fibers provide residual strength to concrete mixtures, and concrete overlay design procedures currently assume that fiber reinforcement enhances fatigue life. A number of studies have suggested that fibers may offer a number of additional performance benefits to concrete overlays. This study conducted a field investigation of six different concrete overlay sites in Iowa. Three of these sites contained test sections with varying thickness and joint spacing designs, and with and without fiber reinforcement. The field investigation performed a variety of tests to measure properties such as joint activation behavior, load transfer, structural response, pavement smoothness, and curling and warping behavior. This test regime allowed for a broad characterization of many aspects of the behavior and performance of concrete overlays, both with and without fiber reinforcement. The results indicated that, to date, fiber reinforcement did not appear to have a significant impact on load transfer, smoothness, or curling and warping at these concrete overlay sties. However, the comprehensive testing regime provided a number of insights into other aspects of concrete overlay design and performance, both with and without fiber reinforcement. The bond between concrete and asphalt was particularly important, even when the overlays were not intentionally designed to bond to the underlying asphalt layer. Finally, the report for this project also contains an appendix detailing a separate investigation of the behavior of FRC pavements placed without transverse joints.

About the research

This research will develop a guide to assist Iowa municipalities, transportation agencies, and others in conducting studies on one-way to two-way street conversions. This guide will compile resources from previous conversion projects in Iowa and other Midwestern states, incorporating data and results from those studies. These will be augmented with the use of other data sets, such as crash data or probe vehicle speed data, for past conversion projects in Iowa for which such data is available. The guide will provide information on several aspects of conversions, including traffic operations, design and implementation, and stakeholder impacts. The guide will also include information to support benefit/cost analyses.

About the research

The main objective of this research was to explore the optimized selection of superabsorbent polymer (SAP) products as internal curing materials in concrete and to examine the effects of the use of eight selected SAPs on properties that affect concrete performance, including shrinkage, hydration, permeability, and strength. Experimental results demonstrated that the use of SAPs improved the shrinkage and hydration of concrete while reducing strength and permeability, although the latter were still within acceptable bounds.

About the research

The project described in the report was developed in response to a documented need for more readily available guidance related to decision-making about roadway cross section reconfigurations. More specifically, there was a need for information that might help during the decision-making process involved in converting four-lane undivided roadway cross sections to three lanes (four- to three-lane conversion) with one through lane in each direction and a two-way left-turn lane.

In response to this need, this project, through consultation with practitioners, identified and developed summary responses to 14 commonly asked questions related to the planning, design, operation, and/or implementation of four- to three-lane conversions. Some of the responses to these questions may also be relevant to the process implemented for other types of conversions and roadway improvements. The summary responses to the questions identified were purposefully kept short and are contained in Appendix A of the report as well as separate standalone documents. References for each summary response, if needed by the user of this guidance, are provided in Appendix B of the report.

Conclusions and recommendations are summarized in the report based on the tasks completed as part of this project. The conclusions are related to the amount and relevancy of material available on four- to three-lane conversions and how the approach to roadway design and operational analysis is changing overall. Recommendations are made about the potential to answer more questions as they are identified, the development of materials specifically for elected officials, and a possible update of the Iowa guidelines for four- to three-lane conversions as an addendum to the national guidelines.

About the research

Live load field tests of bridges were carried out using certain implements of husbandry (IoH) to observe the transverse load distribution and the dynamic impacts. A finite element (FE) analysis of the field-tested bridges was performed. The strain data from the FE analysis were validated with the field test data to establish an FE analysis method for a parametric study. This parametric study was performed to observe the influence of various bridge parameters on the load distribution factors. Observation of the load distribution factors from the parametric study shows that the load distribution factor equations prescribed in the AASHTO LRFD (2020) capture the distribution for these IoH loads. Live load factors for this load type for prestressed concrete (PC) bridges and steel girder bridges were found through a calibration process using reliability theory, which involves the selection of a target safety index. The live load factor for each bridge type was calculated for the Strength I and II limit states. An Iowa-specific legally loaded vehicle (Terragator Max) was established using a conservative axle configuration and axle loads of 25 kips. Calibration of the live load factors yielded the following key findings:

• An update to the AASHTO load and resistance factors (LRFs) is not needed for existing terragator loads as long as the axle loads comply with the legal load limit of 25 kips.
• When a target safety index of 3.5 is considered, the current live load factor of 1.75 for Strength I should be increased to 1.90 if husbandry vehicles of a configuration similar to that of Terragator Max are manufactured.
• When a target safety index of 2.0 is considered, the same case does not suggest an update to the AASHTO live load factor.
• An update to the AASHTO Strength II LRFs is not required.
• The dead load factors were found to be lower than the current AASHTO-prescribed values. Therefore an update to the AASHTO LRFs is not required.

About the research

The design of drilled shafts in Iowa currently relies on the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications. To improve design efficiency at the state level, a series of research projects was conducted to develop the Drilled SHAft Foundation Testing (DSHAFT) database, a regional database facilitating the collection, storage, and efficient access of load test data from Iowa and other states, and to utilize the collected data to establish regional resistance factors that are reflective of the uncertainties associated with predicting drilled shaft capacity under Iowa’s specific geological conditions and construction practices. Resistance factors established in a 2019 study for various resistance prediction methods generally showed improvements over those recommended by AASHTO.

The present research aimed to validate the proposed resistance factors and formulate design recommendations for implementation. To this end, the DSHAFT database was further expanded with additional test data. Additionally, regression analyses were conducted on test data from Iowa to develop local resistance predictions that may provide more accurate estimates of drilled shaft capacity locally. Results from the analysis indicated that a linear correlation between soil parameters and measured unit side resistance was the best fit for most soil types. Moreover, settlement data were collected at several production shafts that were part of a few Iowa DOT bridge replacement projects to evaluate the field performance of drilled shafts designed under the current Iowa DOT guidelines. Various challenges were encountered during the data collection process. Some of the data indicated unexpected negative settlements, and further investigation is needed to develop appropriate conclusions. Design recommendations were formulated based on all findings, and design examples were developed to illustrate the application of the design recommendations.

About the research

The goal of this project is to study the effectiveness of stabilization of granular-surfaced (“gravel”) roads using wicking and non-wicking geosynthetics. The project will continue a newly established cooperative partnership with the Iowa Army National Guard that enables long-term research on granular-surfaced roads at the Camp Dodge base in Johnston, Iowa. The Camp Dodge facility contains several miles of granular-surfaced roads that routinely exhibit various types of damage and distress including rutting, potholes, washboarding, frost boils, insufficient drainage, and other moisture related damage.

About the research

Superheavy loading vehicles, commonly referred to as superloads, exhibit non-standardized loading configurations along with high gross vehicle weights and axle loadings, all of which may cause unexpected distresses on Iowa road infrastructure systems compared to those caused by conventional vehicle class types categorized by the Federal Highway Administration. Superloads encompass various types of vehicles, including implements of husbandry and superheavy loads, prevalent in the Midwestern region of the United States. The determination of critical load factors affecting road damage due to superloads is intricate due to their non-standardized loading configurations and high loading capacities.

This study developed methodologies to quantify superloads and evaluate their impact on Iowa’s road infrastructure, encompassing jointed plain concrete pavements, flexible pavements, and granular roads. It employed extensive mechanistic-based numerical analysis, life-cycle cost analysis, artificial intelligence (AI)-based predictive modeling, forensic investigations, field data analysis, and prototype tool development, with the research aimed at comprehensively evaluating superload impacts on various road types and structures.

Through extensive numerical analyses, incorporating both mechanistic and empirical methodologies, critical findings regarding the effects of different superload types on pavement and granular road distress, associated treatment cost, and service life reduction emerged. Moreover, the Road Infrastructure-Superload Analysis Tool (RISAT) developed in this study has the potential to provide a user-friendly platform for engineers and planners to evaluate structural damages and associated treatment costs induced by superload traffic. The integration of AI-based predictive models into the RISAT enables users to input pavement and superload properties to obtain highly accurate predictions of pavement damages, treatment costs, and service life reductions. Incorporating field data into the RISAT also enhanced its reliability and applicability to pavement management practices, providing engineers and planners with valuable insights for informed decision-making regarding pavement design, maintenance, and rehabilitation strategies.

About the research

The main purpose of this research is to develop a comprehensive asphalt recycling strategy for high RAM mix up to 50% in consultation with surrounding state departments of transportation (DOTs), along with cities and counties, by performing the following tasks:

• Evaluate existing and new high RAP projects and test sections
• Select a cracking test procedure as a performance test
• Conduct performance tests of high RAP mixtures up to 50%
• Develop an approval process for rejuvenators
• Evaluate warm mix asphalt (WMA) with a high RAM content
• Develop a comprehensive asphalt pavement recycling strategy

About the research

The main purpose of the proposed research is to develop a mix design procedure for high RAM mixtures with both rejuvenators and fractionated RAM materials for the Iowa Department of Transportation (DOT) and local public agencies by thoroughly understanding complex interactions between fractionated RAM and rejuvenators. The main objectives of this research are to: (1) examine the effects of various rejuvenators and different methods of RAP stockpile fractionation on the volumetric mix design properties, (2) evaluate long-term oven aging of both laboratory and field rejuvenated/fractionated high RAM mixtures and (3) develop specifications for evaluating asphalt mixtures with rejuvenators and high fractionated RAM contents.