2020 Funded Research
The ACI Foundation is committed to progress in the industry by funding needed research and will fund eight research projects this year. Summaries of each project are below.
Calibration of Simplified Creep and Shrinkage Models Developed Using Solidification Theory
PI: Brock Hedegaard, University of Minnesota Duluth
Co-PI: Mija Hubler, University of Colorado Boulder
Accurate prediction of concrete time-dependent behavior (i.e., creep and shrinkage) is essential for maintaining serviceable and safe structures. This is particularly important for creep-sensitive structures such as high-rise buildings, concrete box-girder bridges, and prestressed beams and floor slab systems. The goal of this research is to develop and calibrate a new concrete time-dependent design model meeting the twin objectives of simplicity in application and theoretical rigor. This work intends to dramatically improve upon and replace the previous ACI-209 model, which has recently been discontinued by that committee, with new provisions that incorporate the most comprehensive data available.
Calibration of the model will begin with preparation of the Northwestern University creep and shrinkage database, sorting this into bins of similar test parameters with respect to the sensitivities of the proposed model. Then the database will be separated into statistically similar training and validation sets. The new model will be calibrated progressively for each of the four primary time-dependent phenomena (autogenous shrinkage, drying shrinkage, basic creep, drying creep), adding additional inputs each iteration. Each iteration will result in a candidate model that will be examined per principles of information theory to balance model complexity with model goodness-of-fit. A single optimal candidate model will then be chosen as the final design model.
This work integrates strongly with current efforts and future goals of ACI Committee 209. The final calibrated model will be incorporated into Report 209.2R documenting currently acceptable creep and shrinkage models. Second, the committee is developing two documents on time-dependent structural analysis: one on using traditional integral-type analysis with compliance functions, the other on modern rate-type analysis. The proposed model will be unique in its suitability for both analysis approaches, and thus may form the basis for robust design guidance in both documents.
Validation of Service Life Prediction for a 28-Year-Old Parking Garage Constructed of Low Permeability Concrete
PI: Amanda Bordelon, Utah Valley University
Co-PI: W. Spencer Guthrie, Brigham Young University
This proposed research leverages a rare opportunity to evaluate a parking structure constructed of low-permeability concrete prior to its demolition at a service life of 28 years in the fall of 2020 as part of an airport expansion project. The data will be valuable for validating service life prediction models for low-permeability concrete regularly exposed to chloride-based salt. The parking structure, which is owned by the Salt Lake City (SLC) Airport, was designed for a service life of 75 years and currently appears to be in excellent condition. It is the only structure in the SLC area with silica fume in the concrete and was one of the first to implement the rapid chloride permeability test as a concrete mixture criterion. Several industry and academic personnel in our ACI Intermountain Chapter have teamed together to evaluate the parking structure using non-destructive testing of the parking garage decks and ramps.
Proposed tests include visual inspection, acoustic impact-echo scanning, vertical electrical impedance scanning, half-cell potential testing, pachometer scanning, chloride concentration measurements, and coring. The chloride concentration data will be analyzed using Life-365 corrosion prediction software. Because the same parking structure was previously analyzed by other researchers at a service life of 13 years (Hooton et al. 2010), this additional testing at a service life of 28 years will enable a unique case study that is expected to 1) validate the benefits of using low-permeability concrete for concrete structures regularly exposed to chloride-based salts and 2) enable a field validation of service-life prediction models for such structures. The total project budget asked of the ACI Foundation is $57,500, with $60,000 additional funding being sought out from Utah Department of Transportation and $11,500 of waived overhead at Utah Valley University.
Determination of the Curing Efficiency of Externally and Internally Cured Concrete Using Neutron Radiography
PI: Mehdi Khanzadeh Moradllo, Temple University
Co-PI: W. Jason Weiss, Oregon State University
Concrete specifications typically specify curing as a fixed duration of time. The benefits of curing are well known in terms of strength and durability. Currently, State Highway Agencies are actively seeking ways in which they can reduce the time of construction to reduce the inconvenience to the traveling public and to improve safety for the construction workers in work zones . As such, it is often asked if the specified duration of curing can be reduced by implementing new curing practices. This study proposes a new methodology to quantify the effectiveness of external and internal curing with high spatial resolution. This research will develop quantifiable equivalent duration of curing for concrete with conventional curing and internal curing. It is shown that the use of internally cured concrete mixtures can reduce curing time saving contractors both time and money during construction. Additionally, high-early-strength (HES) concrete patches could be made more durable using internal curing; therefore, enabling them to be used more widely. While many specifications require the HES patches to be opened after 4-6 hours, durability concerns associated with cracking due to the short curing duration can be overcome with internal curing. The research findings will be prepared in the form of a technical note, webinar, and ACI journal publication. The immediate findings of this study will provide tools to enable practitioners to implement internal curing for different applications due to its advantages in saving both time and money during construction. In addition, the outcomes of this research can be directly incorporated into a new ACI guide to internal curing (ACI 308-213R).
Full-Scale Testing of a New Double-Beam Coupling Beam (DBCB) with A Simplistic Reinforcing Layout
PI: Shih-Ho Chao, University of Texas at Arlington
The objectives include: (1) conducting full-scale experiments to verify the design procedure developed for double-beam coupling beams (DBCBs) under simulated earthquake forces; (2) investigating the size effect, specifically the spacing of transverse reinforcement, on the performance of DBCBs. The confinement requirements provide essential ductility for DBCBs. Spacing requirements of transverse reinforcement are directly related to the depth of the beam and rebar diameter; therefore, a potential size effect may exist when the specimens are not in full-scale or near full-scale; (3) investigating the performance of ASTM A706 Gr. 100 reinforcement in DBCBs. High-strength rebars allow considerable reduction in rebar quantity; however, they may increase the required development length, and (4) investigating the behavior of DBCBs subjected to a loading protocol simulating a maximum considered earthquake (MCE) ground motion. Cyclic behavior and ductility of structural members are highly dependent on the loading histories. A monotonic testing or a loading protocol simulating an MCE ground motion can have a substantially different envelope, strength degradation rate, and deformation capacity from that of a multiple symmetrical fully reversed cyclic testing commonly used in a lab testing. A coupling beam model used for nonlinear response history analysis needs to be calibrated by responses from different loading protocols. This proposed research will use two loading protocols: one with fully reversed cycles but smaller amplitudes–similar to DBE ground motions, and another one with large amplitudes but, for the most part, not fully reversed, which makes it similar to MCE ground motions. Analytical component models based on the experimental outcomes will be formulated for nonlinear time-history analysis.
Assessing ACI 318 Using Data from the 2015 and 2019 E-Defense 10-Story, Full-Scale Building Tests
PI: John Wallace, University of California, Los Angeles
The proposed research involves investigating results from two, full-scale, ten-story reinforced concrete buildings that were tested at E-Defense in Miki City, Japan. The lateral-force-resisting systems for the two test buildings, which generally complied with ACI 318-19 requirements, consisted of perimeter moment frames and structural walls. The primary difference between the two test buildings involved the beam-column joint design of the perimeter frames, which did not quite satisfy ACI 318-19 requirements for the 2015 test building and were enhanced to satisfied ACI 318-19 requirements for the 2019 test building. Each building was subjected to increasing intensities of shaking at the base of structure to represent low-level to very rare levels of earthquake shaking. Roughly 700 sensors were used to measure responses. These tests provide unique and very detailed data to evaluate US code provisions and gain insight into the expected performance of ACI 318 compliant, or nearly compliant, buildings.
The proposed study will investigate a range of issues, including the ability of linear and nonlinear modeling approaches used in ASCE7, ASCE4141 and ACI318 to capture the distributions of forces and displacements in plan and over the building height, including the role of member stiffness modifiers, torsion, discontinuities, and vertical accelerations. Of particular interest would be the ability to predict moment frame column and wall shear demands, as wall shear amplification was recently added to ACI 318-19, and consideration of moment frame column shear amplification is being considered for adoption in 318-25. Given the variation in the joint designs for the two buildings, a detailed assessment of the impact of column-to-beam strength ratios, joint shear demands, joint detailing, and anchorage of beam reinforcement is proposed. Finally, the detailed instrumentation allows investigation of issues such slab effective widths on column-to-beam strength ratios, column and joint shear demands, and column axial loads.
Behavior and Design of Concrete Structures under Natural Fires
PI: Thomas Gernay, The Johns Hopkins University
Co-PI: Patrick Bamonte, Politecnico di Milano
This proposal aims to complement the available design methods for concrete structural members exposed to standard fire by developing a method to design for resistance to full burnout under real fires. The novel method will provide enhanced safety and resilience for reinforced and prestressed concrete members because it will capture the effects of fire on the material and structural response throughout the different stages of heating and cooling, including the potential vulnerability to delayed failure. Of straightforward application for design by practitioners, the method is not intended to replace advanced analysis for performance-based assessment, but rather to enable defining a burnout resistance rating in complement to the fire resistance rating in design standards.
The objective will be achieved by combining laboratory experiments and numerical analyses. Experiments will be carried out at Politecnico di Milano to fill identified knowledge gaps as regards the evolution of concrete thermal and mechanical properties during and after cooling down from elevated temperatures. Experimental data will be used to formulate models and recommended standard provisions. The material models will then be used in numerical analyses to evaluate the structural behavior of prototypical RC and PC members under natural fire scenarios through burnout. The analyses will be performed at Johns Hopkins University and Politecnico di Milano using a set of methods including simple rational models and nonlinear finite element analyses. Finally, the numerical dataset will be used to formulate simple design provisions, which could range from tailored adaptation of tabulated data to suitable additional provisions or modifications to section-based calculation methods, for achieving burnout resistance in RC and PC members.
Reliability Evaluation of ACI 318 Strength Reduction Factor for One-Way Shear
PI: Robert Barnes, Auburn University
Co-PI: Andrzej Nowak, Auburn University
Reliability analyses provided a rational basis for increases in the ACI 318 strength reduction factors for flexure in 2002 and 2008, which in turn resulted in more efficient design of concrete structures. However, a corresponding improvement in the strength reduction factor for shear was not possible during this period because of well-founded concerns about the level of safety—particularly for large and lightly reinforced beams and slabs—associated with the ACI 318 one-way shear strength expressions, which were first introduced in 1963. After a dedicated, collaborative effort of several ACI technical committees (318-E, 445, and 446), improved one-way shear strength expressions were adopted in ACI 318-19. These new expressions more reliably describe the collected experimental test data of RC members. This improved accuracy now yields an opportunity for reliability-based re-evaluation of the strength reduction factor for shear. A statistically-justified increase in this factor would result in more competitive concrete structural systems. Therefore, the objectives of the proposed project are to provide an objective, statistical basis for improving the strength reduction factor for one-way shear, and to propose an appropriate new strength reduction for one-way shear, if justified.
These objectives will be achieved by developing a reliability model for one-way shear strength consistent with practices used to develop the current strength reduction factors for flexure and axial force. Mechanical property statistics will be determined using the recent material test data provided by PCA and CRSI. Geometric variability will be incorporated as in previous code calibration efforts. The resulting Monte Carlo-based reliability analyses will be supported by the extensive ACI-ASCE Committee 445 shear test databases. The target reliability index for shear will be taken as 4.0, consistent with long-standing code philosophy that the probability of nonductile shear failure shall be much less than for a flexural failure.
Durability of Anchorage Pour-backs: Evaluating the Link between Surface Preparation and Bond
PI: Natassia Brenkus, The Ohio State University
This work investigates post-tensioning (PT) end anchorage pour-back details susceptibility to moisture intrusion and develop evaluative criteria for acceptance. Post-tensioning anchorages transfer all prestress force to the concrete member. Degradation of this zone can lead to tendon failure and violent energy release. As a measure of corrosion protection, anchorage components -- typically cast iron or machined steel -- are sometimes covered with a cementitious material, also known as a pour-back. Communication and implementation of best practices for these pour-backs has been limited, leading to varied material use and bond quality for the final layer of protection of this critical zone.