International Concrete Abstracts Portal

Professor Dennis Mertz passed away after a prolonged battle with cancer. He spent a large portion of his professional career working on advancing of the state-of-the-art of bridge engineering. He was a great friend and colleague to many at ACI and ASCE. Joint ACI-ASCE Committee 343, joined with ACI Committees 342 and 348, sponsored four sessions to honor his contributions and achievements in concrete bridge design and evaluation. These sessions highlighted the important work and collaborative efforts that Dr. Mertz had with others at ACI and ASCE on various topics. These sessions also combined the efforts among ACI and ASCE researchers and practitioners in addressing various topics related to the design and evaluation of concrete bridges. The scope and outcome of the sessions are relevant to ACI’s mission. They raise awareness on established design methodologies applied for various limit states covering topics related flexure, shear, fatigue, torsion, etc. They address problems related to emerging design and evaluation approaches and recent development in design practices, code standards, and related applications. The Symposium Publication (SP) is expected to be an important reference in relation to design philosophies and evaluation methods of new and existing concrete bridges and structures.

Dr. Dennis Mertz was involved with the AASHTO LRFD Bridge Design Specifications [1] for 30 years. Starting with the original development of the specifications and continuing with maintenance and related course development and presentations. His last major contribution to the Specifications was to serve as Principal Investigator for the reorganization of Section 5, Concrete Structures. This presentation summarizes the changes to the structure of the Section including the increased emphasis on design of “B” and “D” regions of flexural members and introduces new and expanded material on beam ledges and inverted T-caps, shear and torsion, anchors, strut and tie modeling and durability. The product of this work was included in the 8th Edition of the Specifications as a complete replacement of Section 5.

Service I limit state in the AASHTO LRFD Bridge Design Specifications (BDS) is applied for the control of cracking in reinforced concrete elements in order to maintain its normal functionality and to achieve its design life. There are two methods specified in AASHTO LRFD BDS: 1) equivalent strip design method and 2) empirical method. For the empirical method, no exhaustive design calculation are needed and the reinforcement area is obtained as a percentage of the concrete section. However, usually, the reinforcement area designed using empirical method is less than that designed using the equivalent strip method, which could result in shortened service life and excessive crack width. Albeit arching action effects were considered in the empirical method which improves the flexural resistance of concrete deck after cracking, the effects of arching action on crack control of reinforced concrete deck were not studied. In addition, different exposure conditions and different design sections (positive moment vs. negative moment regions) were not considered in the empirical design method. Thus, it is extremely important to investigate and calibrate the Service I limit State for reinforced concrete decks designed using the AASHTO empirical method. In this study, the Service I limit state function is formulated and the load and resistance models are developed. The arching action effects are integrated into the resistance model. Detailed calibration is performed to ensure uniform target reliability will be achieved for different design parameters including exposure conditions, span lengths, deck thickness, and positive moment and negative moment regions.

In the new generation of design code, safety of structures is provided in form of load and resistance factors. Safety is measured in terms of the reliability index. The acceptability criterion in the selection of load and resistance factors is closeness to the target reliability index which can be different depending on limit state. The paper presents a procedure to determine these factors using the concept of “design point”. The coordinates of design point are equal to factored load or factored resistance. The required input data includes for each load component and resistance: mean values, bias factor (ratio of mean to nominal), standard deviation or coefficient of variation. The procedure is demonstrated on example of bridge design code (AASHTO[1]) and design code for concrete buildings (ACI 318[2]) for prestressed concrete girders and reinforced concrete beams in flexure and shear.

For many decades, latex-modified concrete (LMC) overlays have been successfully used in the United States, inclusive of providing protection for many bridge decks and their steel reinforcements. LMC remains one of the most desirable rehabilitation materials for concrete bridge decks because it is easier to place and requires minimal curing. Nevertheless, as is the case with any cement-based material, LMC overlays are susceptible to plastic shrinkage and delamination. These problems are often solved by proper curing and better surface preparation. Yet, despite these solutions, many questions have been raised regarding the best practices for placing LMC overlays and the proper curing and placement conditions. The current curing practice for LMC in most states simply follows the latex manufacturer’s recommendation because very little information on the proper curing methods is available. There is a need to establish detailed technical specifications regarding curing and placement conditions that will provide more durable LMC overlays. This paper provides an in-depth laboratory-based experimental study of the effect of curing methods and duration on the mechanical properties and durability aspects of LMC. Four different curing methods were examined: (1) dry curing, (2) 3 days of moist curing, (3) 7 days of moist curing, and (4) compound curing. Based on the results from the laboratory tests, technical specifications were developed for field implementation of LMC. Various types of sensors were installed to monitor the behavior of the LMC overlays on bridge deck. Results show that extending the moist-curing duration to a minimum of 3 days (and a maximum of 7 days) significantly improves both the mechanical properties and durability of LMC.