2021 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.
Standard Critical Chloride Threshold Test Variability due to Material Sources
PI: Ceki Halmen, University of Missouri
The large variability of published CT data and lack of a standard CT test method creates significant difficulties for the concrete industry. Researchers cannot compare CT values obtained from their research to a standard value. Practitioners evaluating condition of reinforced concrete structures for maintenance and rehabilitation cannot make a reliable assessment based on measured chloride contents. Designers cannot estimate the service life of their design without a reliable estimate of critical chloride threshold value. Development of a standard critical chloride threshold test with a complete data set regarding its variability will alleviate these issues.
In addition, researchers are using different test methods in their labs with materials from different sources. To address this issue, ACI Committee 222 established a task group to develop a standard CT test method. This study was supported by the ACI Foundation and a standard critical chloride threshold test, OCcrit, was developed. The results from that study exhibited very low intra‐ and inter‐laboratory variability when used with materials obtained from the same source. However, variability among different sources of materials was not assessed.
The objective of this research is to measure variability due to different sources of materials that meet the requirements of the same ASTM specification. Materials procured from different manufacturers in different parts of the country will be tested in four different labs using the OCcrit test method to quantify variability in critical chloride threshold test results. The data set generated with this study will provide critical data to assist the committee in interpreting results of this test method by better understanding the expected differences due to different material sources. The earlier CRC study and the proposed study combined will provide critical information on CT values that can lead to updated chloride limit values in ACI 222 documents, which subsequently will result in science‐based consistency in other ACI documents (for example, 201, 318, and 365). One of the important parameters used in the service life models developed by ACI Committee 365 is the CT value of a reinforced concrete system. Understanding how much variability in CT should be expected when using material sources from different parts of the country will create less uncertainty in the model outputs.
Considering the large economic impact of early design decisions and maintenance decisions that are made based on these service life models; decreased uncertainty will have a significant and positive impact on our concrete industry. The results of the two CRC studies combined will provide a reliable, timely, inexpensive, and easy‐to‐perform standard critical chloride threshold test for the industry. The results of this study will also provide necessary data for the new test method to be evaluated by ASTM for development as an ASTM standard.
Stress‐Strain Analysis of BCSA Cement for Structural Applications
PI: Cameron D. Murray, University of Arkansas
The concrete industry is under pressure to reduce the environmental impacts caused by concrete construction, especially the large impact of portland cement production. Alternative cements are becoming a hot topic in research because many of them (including BCSA cement) require less energy to produce and result in fewer emissions. If new cement technologies are to be implemented on a larger scale, research must be performed to understand how these technologies differ from portland cement. Alternative cements must be proven to be at least as safe and reliable as portland cement, but another consideration is highlighting design advantages of alternative cements. If alternative cements can perform better in certain applications than portland cement, this will encourage their use and reduce the environmental impact of the industry.
The ACI 318-19 Building Code allows the use of alternative cements for structures. Using alternative cements for structures raises several questions about the applicability of current code provisions. Becker, Holland, and Malits outlined many of these issues for Concrete International. Belitic calcium sulfoaluminate (BCSA) cement concrete is extremely promising for structural repairs or for new precast concrete construction because of its high early strength. The proposer of this research previously tested a series of reinforced concrete beams made with BCSA cement to determine the applicability of the ACI flexural strength provisions, and to compare with portland cement concrete beams. The BCSA beams performed similarly to the portland cement beams, with one major exception: the ductility. The ductility of BCSA beams was higher than portland cement beams, even at lower compressive strengths. This indicates that the ultimate compression strain of BCSA concrete is different than portland cement, and the traditionally assumed value (0.003) should be revised for BCSA. An increased compression strain value has many design implications for beams, columns, and slabs. More steel could safely be used in a section, or shallower sections could be used if this compression strain is increased.
The proposed research will investigate the ultimate compression strain capacity of BCSA cement concrete and suggest a reasonable value for use in structural design. Additionally, the axial stress‐strain behavior will be investigated at varying ages and water to cement ratios and the flexural stress distribution will be modeled. This will provide new and much needed information about an emerging cement technology.
Chloride Desorption Isotherms of Cementitious Systems Exposed to Low‐pH Environments
PI: Mahmoud Shakouri, University of Nebraska Kearney
The surface of concrete pavements and reinforced concrete bridge decks are commonly treated with chloride‐bearing deicing and anti‐icing salts during winter conditions to minimize freezing risk. The frequent application of these salts can result in the ingress of chlorides into the concrete. When enough chlorides reach the surface of the reinforcing bars, localized corrosion can occur. The corrosion of the reinforcing bars can ultimately reduce the structure’s service life and result in significant repair or rehabilitation costs. One of the several reported factors that can delay the corrosion initiation of the embedded reinforcing steel in concrete is the chloride binding capacity of the concrete matrix. Chloride binding mechanisms remove a portion of free chlorides from the pore solution that can participate in corrosion. However, under certain circumstances (for example, carbonation, sulfate attack, and acid attack) and because of a drop in the pH of the concrete pore solution, bound chlorides can disassociate from the hydration products and return to the pore solution, leading to an increased risk of corrosion. The current state‐of‐the‐art in service‐life modeling of concrete structures often incorporates a sink term to account for the role of chloride binding in estimating the remaining service life of a structure. However, a recent review indicates that little is known regarding the disassociation kinetics of chlorides as a function of the pH of the pore solution. More research is needed to incorporate the influence of chloride desorption in service‐life models. Ignoring the desorption of bound chlorides can lead to an overestimation of a structure’s remaining service life, resulting in significant maintenance costs.
The overall objectives of this project are to understand the kinetics of chloride desorption mechanisms and develop empirical chloride desorption isotherms for cementitious systems that contain ordinary portland cement (OPC), fly ash, slag, and silica fume. Given that the knowledge of available free chloride content in the pore solution is essential for a reliable service‐life estimate, this project will fill this knowledge gap by developing empirical desorption isotherms incorporated in service‐service life models. The data from this project is expected to provide the baseline for addressing the disassociation of bound chlorides in ACI 222 and 365 documents. Moreover, a standardized test will be proposed for measuring the chloride desorption capacity of cementitious systems.
Core Compressive Strength and Elastic Modulus Measurements of High‐Strength Concrete
PI: Matthew Sherman, Simpson Gumpertz & Heger, Inc.
Co-PI: Brett Holland, Simpson Gumpertz & Heger, Inc.
The trend towards high-strength concrete and associated high elastic modulus has outpaced specifications, acceptance criteria, and in‐place evaluation guides. Many of the current methods for evaluating in‐place concrete are encountering difficulty when applied to modern high-strength concretes and the emerging requirements for specified elastic moduli are not well defined. Very little is known about the effect of coring on modern high‐strength commercially produced concrete. The researchers and supporters have encountered situations where coring and the application of conventional acceptance rules to in‐place high‐strength concrete has exposed significant differences in expected performance. The researchers suspect that the differences may be due to high self‐generated curing temperatures, differences in curing, or the effect of coring itself, all of which will be investigated by this program.
This research will provide information regarding the relationship between cores and cast cylinders from modern high‐strength production concrete from multiple commercial suppliers, especially as relates to compressive strength and elastic modulus. The information will support the interpretation of core test results and the design and construction community in the implementation of high‐strength concrete.
We expect that the data gathered can be used by ACI Committee 363, High-Strength Concrete, to provide guidance to specifiers, producers, designers, and owners regarding the use of coring to evaluate the strength and elastic modulus of in‐place concrete. This guidance could be in the form of a “technote” and the data and findings could be used for future work and guide documents. We also hope that the work will spur the reporting of similar work done by others and provide data to ultimately allow the incorporation of this knowledge into the ACI 318 design code. We plan to write an article for Concrete International to inform designers, constructors, and owners about the challenges of core testing of high‐strength concrete.
While not a main objective of the work, we expect to gather information regarding the relationship between the thermal history of high‐strength concrete and strength which will be an additional benefit.
Transverse Reinforcement Requirements for UHPC Columns: Assessment of Existing ACI Provisions and New Developments
PI: Mohamed Moustafa, University of Nevada, Reno
Even though the UHPC global market is growing, and more feasible non‐proprietary mixtures are on the rise, UHPC is still relatively expensive, and every effort is needed in the design and construction process to optimize use of the material. Simplifying or relaxing unnecessarily conservative transverse reinforcement requirements for UHPC columns can provide considerable cost as well as time and effort savings.
Current efforts to expand the UHPC market in the United States are focusing on mega bridge deck girders and developing non‐proprietary mixtures for precast/prestressed girders. There has been less focus on UHPC columns and their applications for precast construction. To fill the knowledge gaps here, the PI has recently completed two projects on the behavior of seismic and slender UHPC columns. This new project will expand upon prior work to further assess existing ACI provisions on transverse reinforcement detailing and requirements for applicability for UHPC columns, in both ordinary and special frames.
Several large‐scale tests will be conducted to accurately capture the interaction between transverse reinforcement and steel fibers. The new tests along with previous work on axial UHPC columns will be used to assess transverse reinforcement detailing and confinement requirements and develop new guidelines to be incorporated into an upcoming ACI Subcommittee 239‐C design guide document if existing ones are found to be unnecessarily conservative for UHPC columns.
Development of FRP Retrofit Guidelines for Deficient Reinforced Concrete Horizontal Lateral Force Resisting Systems
PI: Eric Jacques, Virginia Polytechnic Institute and State University
Co-PI: Matthew Eatherton, Virginia Polytechnic Institute and State University
The objective of this research project is to develop design guidelines for strengthening deficient horizontal lateral force-resisting systems (hLFRS) in older reinforced concrete buildings using externally bonded fiber-reinforced polymer (FRP).
The proposed research focuses on diaphragm strengthening because the critical lack of data has led designers to incorrectly rely on experimental tests of FRP-strengthened shear walls to justify hLFRS strengthening applications. Engineers and manufacturers need proven methods for the use of FRP to address hLFRS deficiencies related to inadequate chords, collectors, and in‐plane shear capacity.
The research plan consists of: (1) the development of retrofit design approaches leveraging the expertise of the coalition and including both conventional and innovative solutions; (2) a series of large‐scale experiments, facilitated by industry co‐funding, to investigate the behavior and optimal arrangement/anchorage of FRP to strengthen deficient reinforced concrete diaphragm zones carrying primarily shear; and (3) development of design recommendations and technical commentary to inform and advise practitioners as part of a future update to ACI Report 440.2R (2017).
The project contributes to increased sustainability by helping reuse and reconfigure existing buildings to meet changing occupant needs while also mitigating structural deficiencies for resilient performance during natural hazards. Producing buildings that are capable of long, economically productive lives reduces the environmental impact of waste generated by demolition and the number of resources consumed by new building construction.
Shear Behavior of Macro‐Synthetic Fiber‐Reinforced Concrete
PI: Travis Thonstad, University of Washington
Co-PI: Paolo M. Calvi, University of Washington
The objective of the proposed research is the development of simple, rational design equations for the contribution of macro‐synthetic fibers to the shear strength of reinforced concrete members containing at least the minimum shear reinforcement required by ACI 318‐19.
Currently, ACI 318 does not recognize the contribution of macro‐synthetic fibers to the shear capacity of structural members that contain conventional transverse reinforcement. Developing provisions for use of a combined system for resisting shear (stirrups and distributed fiber reinforcement) will allow more advanced, taller, and complex construction projects to benefit from the use of macro‐synthetic fiber‐reinforced concrete for its improved durability, ductility, and strength.
The design equations will be based on a rational shear behavior model that will be developed as part of this work using the response of macro‐synthetic fiber‐reinforced concrete (PFRC) panel elements, subjected to in‐plane loads (for example, shear and axial tension or compression).
ACI Committee 544, along with the PIs and industry professionals, will work to incorporate the research into a guide on the use of macro‐synthetic fibers for the enhancement of the shear capacity of structural members that contain stirrups.
Minimum Design Requirements for Insulated Concrete Form Wall Systems
PI: Laura Lowes, University of Washington
Co-PI: Dawn Lehman, University of Washington
Recently, masonry and wood are replacing concrete walls for low‐ and mid-rise construction, in part because of the increased cost of purchasing and placing reinforcing steel. ACI Code minimum reinforcement requirements for cast‐in‐place (CIP) walls, including maximum allowable spacing and minimum reinforcement ratios, are much more stringent than for masonry or precast concrete construction. These conservative requirements are generally understood to be historic and based on engineering judgment, rather than experimental testing.
The objective of this research is to demonstrate that CIP concrete walls with larger spacings (more than 18 in.) than the ACI 318‐19 Code requirements will exhibit acceptable structural response when subjected to gravity and out‐of‐plane loading. Experimental data from this testing will extend finite element analysis results from a current study funded by the Ready‐Mix Concrete Research and Education Foundation (a co‐funder of this research). The results will be used to support an ACI 318 code‐change proposal to increase the spacing limits for CIP reinforced concrete walls. A code change will significantly reduce the cost of CIP reinforced concrete walls, including ICF walls, and enable these systems to be more cost competitive with wood, masonry, and other systems for low‐ and mid‐rise construction in regions of low seismicity.