Monitoring moisture in the cross laminated timber panels of the UA Stadium Drive Residence Halls
PI: Tahar Messadi (University of Arkansas Fay Jones School of Architecture + Design), Co-PI: Cameron Murray
At the 2nd North American Mass Timber Research Workshop, the moisture behavior in cross laminated timber (CLT) was recognized as a major topic for the research community to clarify and understand. Very little data exist on the long-term movement of the moisture in CLT during a building’s construction and operation. In addition, the wetting of mass timber during construction raises serious concerns about high moisture content in CLT that could lead to decay and mold (compromising the air quality) or decrease the strength and stiffness of the material.
For a better comprehension of the moisture conditions present in mass timber constructions, a long-term moisture monitoring program will be implemented on a mass timber framed building located on the University of Arkansas campus. The University of Arkansas Stadium Drive Residence Halls building that is currently under construction is built with CLT and will be monitored in this project. The building will be monitored with an array of Omnisense wireless moisture meters to track moisture content throughout the building’s construction and post-occupancy operation. Moisture has been monitored in this fashion in other research projects, but these buildings will be the first studies on moisture in CLT in the southern United States.
ARDOT TRC 1902: Capillary Pressure Sensor Testing to Identify Curing Regimen in Freshly Placed Bridge Decks
PI: Cameron Murray, Co-PI: Micah Hale
The overall performance of concrete in bridge decks can be affected by the curing regimen. The Department now recognizes and allows concrete curing compounds that are lithium based. But, do they outperform the standard curing regimens? With the advent of new testing equipment for freshly placed concrete, Capillary Pressure Sensor System (CPSS), the evaporation effects can be measured. By measuring the capillary pressure during the initial set period after finishing, shrinkage cracks can be avoided by adding moisture and/or curing compound to the concrete surface when the alarm is triggered by threshold pressure limit. Additional curing compound and/or moisture can be added to reduce or mitigate the effects of the evaporation. ARDOT currently allows the use of lithium cure under a special provision when the contractor requests it. This project would use the CPSS to evaluate the different curing regimens the contractor uses and determine which product and/or method works best.
This project aims to investigate the use of a CPSS to monitor the development of capillary pressures in the surface of fresh concrete bridge decks or pavements. This capillary pressure can be used to determine if plastic shrinkage cracking is likely to occur and alert the user when moisture should be added to the surface to prevent cracking. The sensor will be tested in the lab to verify its ability to measure plastic shrinkage pressures, then lab testing will be performed to compare curing techniques. A field study will help determine if the sensor is useful in practice to ARDOT and to contractors.
ARDOT TRC 1903: Investigating Concrete Deck Cracking in Continuous Steel Bridges
PI: Ernie Heymsfield, Co-PI: Cameron Murray
Concrete bridge deck cracking can cause serious serviceability issues during a bridge’s design life and compromise a bridge’s structural strength. Cracks allow water and chemical ingress, which accelerate road surface and structure damage. ARDOT has identified bridge deck cracking shortly after decks are placed and prior to applying live loads. The causes of bridge deck cracking are uncertain. Many contractors are currently using continuous deck pours at Arkansas bridges. However, this construction approach restricts concrete slab movement during shrinkage. After concrete cracking is initiated, cracks may enlarge due to excessive service load stresses.
The interaction between the concrete deck and girder flange restricts concrete slab shrinkage and therefore induces tensile stresses. Stresses developing in the concrete deck and bridge girder are a function of the bridge girder stiffness. The transition from Allowable Stress Design (ASD) to Load Factor Design (LFD) to Load and Resistance Factor Design (LRFD) has resulted in bridge girders designed with a lower stiffness. The impact of bridge pouring procedure, shrinkage, and girder stiffness need to be investigated to determine causes for bridge deck cracking at Arkansas continuous steel bridges. After determining the cause for bridge deck cracking, long term corrective measures will be recommended to ARDOT for implementation to prevent future cases of bridge deck cracking. Regardless of what causes bridge deck cracking, it is important to limit cracking in bridge decks to ensure a long lasting structure with minimal maintenance needs.
Arkansas/Oklahoma Chapter American Concrete Pavement Association Gift
The Oklahoma/Arkansas chapter of the ACPA pledged to give $100,000 per year for 5 years to support concrete pavement related initiatives. This money is being used to support students who are interested in studying concrete pavements.
The first students to be supported by this generous gift are Casey Jones (advised by Dr. Micah Hale), and Yancy Schrader. Currently they are working on understanding calcium oxychloride formation and mitigation in concrete pavements, and improving upon test methods to categorize fly ashes based on their susceptibility to de-train air from concrete.
ODOT: Development of Rating Tool for Prestressed Concrete Bridges Vulnerable to Shear
PI: Royce Floyd (University of Oklahoma), Co-PI: Jin-Song Pei (University of Oklahoma), Co-PI: Cameron Murray
A large number of bridges in Oklahoma were designed and put into service between 1960 and 1990 using the quarter point rule for shear design from the AASHTO Standard Specifications (e.g AASHTO 1973). This method considered the applied shear at the quarter-span point to be the critical value for the design demand, which often resulted in larger shear reinforcement spacings near the beam ends than what is typical for new construction. The current AASHTO LRFD Specifications (2015) consider the critical location for shear to be much closer to the support, which can result in a larger design demand and smaller shear reinforcement spacings. The methods for calculating shear capacity included in the AASHTO LRFD Specifications have also evolved considerably over time and a number of additional methods have been proposed by researchers. According to ODOT engineers, as many as 1000 bridges in Oklahoma may have been designed using the quarter point rule for shear, potentially leaving these bridges vulnerable to a lower load rating compared to newer bridges when evaluated using the current LRFD Specifications. This problem can potentially be exacerbated when larger axle loads are required, such as for implements of husbandry or emergency situations. In addition to the differences in design criteria, long-term exposure to the often harsh climate of Oklahoma can cause deterioration of the mechanical properties and structural behavior of the girders. An accurate understanding of the effects of deterioration should be included in capacity calculations for the girders. As the state of Oklahoma pushes to get the number of structurally deficient bridges down to less than 1% of all highway bridges in Oklahoma by the end of the decade, it is important that additional bridges are not labeled structurally deficient or load posted unnecessarily. In rating a bridge, all available information should be collected and examined to ensure that safety and economy are effectively balanced