2023 Funded Research
The ACI Foundation is committed to progress in the industry by funding needed research and will fund 11 research projects this year. Summaries of each project are below.
Alternative End-Specimen Conditions to Characterize Compressive Strength of Ultra-High-Performance Concretes
PI: Kinsey Skillen, Texas A&M University
This research will evaluate new protocols to characterize compressive strength of ultra-high-performance concrete (UHPC) mixes applicable to field production. Currently, ASTM C1856/C1856M only allows the use of fixed-end grinding for cylinder end conditions, which requires costly grinding equipment and time-consuming specimen preparation protocols that can be unpractical for quality control. Preliminary results indicate that strengths obtained by fixed-end grinding might effectively be obtained using certain classes of unbonded pads. In the proposed research, cylinder-end conditions consisting of fixed-end grinding and capping with unbonded pads of varying stiffness and thickness will be experimentally evaluated to test concrete mixtures with compressive strengths up to 250 MPa. Unbonded pads that generate a stress state that resembles the fixed-end grinding condition will be proposed as alternative end-specimen conditions to that listed in ASTM C1856. Additionally, a finite element based parametric study will be performed to study how the inclination of the loaded cylinder surface and cylinder-end boundary conditions affect the stress state in the cylinders. Expected outcomes from this study include an alternative, recommended end-specimen condition (pad material, hardness, thickness, and number of reuses) for eventual modification of ASTM C1856. This could facilitate current testing protocols and help promote engagement and research development in the UHPC industry. ACI 363 will also benefit from the outcomes of this study given that, in the process of testing UHPC specimens at early ages, appropriate pads can also be identified for concretes with strengths between 80-120 MPa.
Behavior of Slab-Column Connections under Wind Demand
PI: Christopher J. Motter, Washington State University
Reinforced concrete slab-column framing is commonly used to resist gravity loads in buildings. The slab-column connections must undergo lateral deformation induced from earthquakes and wind without two-way (i.e., punching) shear failure. Previous research has focused on behavior under seismic demand, while the focus of this study will be behavior under wind demand. The recently published ASCE/SEI Pre-standard for Performance Based Wind Design describes design for modest nonlinear response of select structural members, which will result in increased drift demand on the building.
ACI Committee 318-1W was formed at the onset of the 2019-2025 code cycle and was tasked with developing a new Appendix B in ACI 318-25 for performance-based wind design of reinforced concrete buildings. The behavior of slab-column connections at advanced deformation demand from wind was identified by Committee 318-1W as the top research priority, as the applicability of ACI 318-19 seismic provisions for slab column connections and slab stiffness remains a topic of debate in Committee 318-1W.
Recent research on coupling beam behavior under nonlinear wind demand has shown significantly increased stiffness degradation and a resulting increase in yield displacement relative to behavior under seismic demands. Characterization of cyclic degradation of slab-column framing under wind demands is needed. This research program will be conducted on slab-column connections subjected to wind loading demands. The level of two-way shear demand from gravity loading and the reinforcement in the slab will be varied in the testing program. The test results will be used to formulate provisions in ACI 318-25 Appendix B for slab-column punching shear capacity and slab effective stiffness under wind demand.
Direct Tension Test Results and In-Situ Response of Reinforced UHPC Beams: Relationship and Design Implications
PI: Sherif El-Tawil, University of Michigan
Ultra High-Performance Concrete (UHPC) exhibits pseudo-ductility. Its tensile fracture strain (known as the crack localization strain, εt,loc) is two orders of magnitude greater than that of regular concrete. Ironically, enhanced material ductility does not translate into structural ductility in reinforced UHPC (R/UHPC) beams because damage tends to localize in one large crack. This places high strain demands on the reinforcing bars bridging the localized crack and leads to premature rupture. The curtailment in structural ductility is currently handled by specifying a strength reduction factor (ϕ) that is a function of εt,loc. There are two problems with the current approach. First, the relationship between εt,loc, which is determined from the direct tension test, and the in-situ response in R/UHPC beams is unclear for several reasons: a) the effects of strain gradient and bar reinforcement on the in situ value of εt,loc are not known, b) the stochastic nature of εt,loc has not been well studied. Second, the ϕ factor is empirical and has no rational basis. An experimental and computational research program is proposed to address the identified gaps in knowledge. Direct tension, tension stiffening and four point bending tests will be conducted to characterize εt,loc as a function of the steel fiber volume fraction ratio (Vf = 1.5% and 2.0%). Computational simulation will be used to assess the effect of variability in εt,loc on structural behavior.
This research will provide critical information about the effect of key parameters on the crack localization strain of Ultra high-performance concrete (UHPC), which plays a profound role in the design of UHPC flexural members. It will also provide a rational basis for calibration of the flexural strength reduction factor and inform R/UHPC design guidelines currently under development by ACI Technical Committee 239 on UHPC.
Evaluating Residual Strength of Corrosion-Damaged Reinforced Concrete Members
PI: Anca-Cristina Ferche, The University of Texas at Austin
Co-PI: Serhan Guner, The University of Toledo
Although significant progress has been made in understanding the causes of corrosion, there is a scarcity of structural analysis models for quantifying the effects of corrosion on the residual strength of reinforced concrete (RC) elements. This project will develop a finite element (FE) analysis framework for incorporating corrosion-induced mechanisms into the analysis of RC. This research will develop: (1) a bond strength model, (2) numerical modeling guidelines, and (3) an experimental dataset for benchmarking FE models – all for corrosion-damaged members.
Corrosion of the steel reinforcement is recognized as the major cause of degradation for RC structures. Corrosion leads to major deterioration of bond strength, which in turn greatly affects the overall response of RC elements. However, the lack of experimental studies hinders efforts to develop structural analysis models to account for this phenomenon. This research project will result in advanced numerical modeling procedures for strength evaluation of corrosion-damaged members. This will facilitate the timely repair and rehabilitation of concrete structures with a new generation of concrete and rebar products. In addition, this project will advance ACI standards and the state of science of the concrete repair industry.
This project will produce the following deliverables for the benefit of ACI and the concrete industry:
- A bond strength model and numerical modeling guidelines for corrosion-damaged members for ACI Technical Committee 447.
- Unique experimental dataset for benchmarking FE models of corrosion-damaged members, shared via ACI’s online platform.
- Development-length equations for corrosion-damaged rebars for the ACI 562 Code Requirements for Assessment, Rehabilitation, and Repairs to Concrete Structures.
- ACI Structural Journal paper, Concrete International article, and dissemination at ACI Concrete Conventions.
Nano-Modified Calcined Clay-Based Cement Concrete: A High Modulus Concrete with Low Carbon Footprint
PI: Panagiotis Danoglidis, The University of Texas at Arlington
The research objective is to design sustainable concrete with low embodied carbon emissions by replacing high volumes of ordinary Portland cement (OPC) with calcined clay and reinforcing the cementitious matrix with carbon-based nanomaterials. Replacing OPC at volumes > 10% with calcined clay materials, i.e., metakaolin (MK) is a promising way for significantly reducing CO2 emissions, associated with the production of cement, and improving concrete’s compressive strength due to a densified microstructure. MK-based cement concrete however exhibits brittle behavior with low tensile properties. An effective way of addressing this challenge is to employ the use of 1D carbon nanotubes (CNTs) and nanofibers (CNFs), which have been shown to significantly improve the tensile strain capacity of engineered concrete. The preliminary studies on calcined clay-based concrete reinforced with monodispersed CNTs have shown exceptional improvements of the modulus of elasticity and unexpectedly enhanced tensile load carrying and energy absorption ability.
The key objective of the research is to successfully control and adjust the nanoscale chemical and mechanical properties of the sub-10 nm interfaces between cement hydration products, MK, and nanomaterials; and the nanomodified cement/aggregate Interfacial Transition Zone for the successful design of concrete with optimum bulk mechanical properties. The performance of the proposed engineered concrete product will be validated through prototype demonstrations in laboratory environment. Substitution of high volumes of OPC with clay materials will result in a significantly lower carbon footprint of concrete. Utilizing low cost SCMs will also result in important economic benefits for manufacturing cost-competitive concrete materials for the construction market. The findings will be submitted to ACI Technical Committees 236 and 241 for consideration of inclusion in a new ACI technote and results will also be widely disseminated through presentations at ACI Concrete Conventions and various publications.
Novel Concrete Containments for Nuclear Reactors: Delamination Testing of Curved Wall Sections
PI: Christopher A. Jones, Kansas State University
Co-PI: Thomas Kang, Seoul National University
This research seeks to leverage the unique tensile and shear capacity along with the extreme durability potential of Ultra-High-Performance Concrete (UHPC) for use in nuclear reactor containment vessels. Because of the rigor associated with nuclear construction and the associated review and oversight by regulatory agencies, compliance with consensus standards is critical for obtaining operating licensing and thus project feasibility. This research will evaluate the use of UHPC along with modern construction techniques within the appropriate context of ACI 359/ASME BPVC Section III Division 2 and will importantly not seek to make large changes to the existing code provisions. Instead, the novel concrete containment is predicated on challenging the current design philosophy. In current practice, the post-tensioning system in a concrete containment vessel is designed to resist internal pressurization while mild-steel reinforcement is utilized to resist other loads. By significantly increasing the level of precompression, the rebar requirements are reduced. Furthermore, steel fiber reinforced UHPC can resist much greater shear stress than ordinary concrete, which will further reduce the amount of required reinforcement. By focusing on and evaluating incremental change, the research team believes that short term impact will be realized.
This research will evaluate the delamination capacity of the UHPC wall section in response to radially induced loading from the post tensioning tendons. Understanding the capacity in this load case will support the development of code provisions for novel containment design and will provide further credibility to design concept.
Proposal to Investigate ICF Wall Construction Meeting the Requirements of NFPA 285 – Phase II
PI: Shamim Rashid-Sumar, National Ready Mixed Concrete Association
Co-PI: Douglas Bennion, Quadlock
This research is the second part of a two-Phase study. Phase I which is currently in progress, is focused on developing the test plan. Based on the results, refinements will be made to the make-up of the wall assembly and the design of the window opening header detail for Phase II testing. Phase I is scheduled to conclude by the end of June 2023. Phase II research will commence in July of 2023 and will begin with the construction of a base wall assembly which will then be tested in accordance with NFPA 285.This research will provide test data to confirm ICF flat walls can be constructed to meet the requirements of NFPA 285. Further the data generated will enable engineered evaluations to assess if ICF walls with components other than those tested will meet the code requirement of NFPA 285. The results of the research will confirm existing construction details and/or aid in the development of new construction details to limit lateral and vertical flame spread in ICF buildings.
Architects, design engineers and building officials will use the results of this research to allow ICF walls to be specified in multi-story building construction where surface burning characteristics of the exterior surface of exterior walls is of concern and must be addressed. NRMCA and ICFMA are active in building codes and industry standards advocacy, serving as principal technical committee members and representing members and affiliates in the United States and Canada. It is anticipated that, where appropriate, the outcomes of the research will be used for technical changes to relevant industry codes and standards, including but not limited to ACI, ASTM, ICC, NFPA, and ULC codes and standards. Specifically, the outcomes of the research may be incorporated into new design examples for ACI PRC-560-16 “Report on Design and Construction Insulating Concrete Forms” as well as provisions in ACI/TMS 216.1 “Code Requirements for Determining Fire Resistance of concrete and Masonry Construction Assemblies.”
Rheological Behavior of Fresh Ultra-High-Performance Concrete (UHPC) Enhanced by Nano-Additives and Data-Driven Approaches
PI: Chengcheng Tao, Purdue University
Co-PI: Kamal Khayat, Missouri University of Science and Technology
The objective of this research is to develop a novel framework to select nano-additives for optimal rheological behavior of fresh concrete by integrating nanotechnology, multi-scale modeling, fluid mechanics, and data-driven approaches. This research will enhance interfacial properties, increase flexural and tensile strength and toughness for UHPC with adapted rheology, improved workability, performance, and sustainability. The addition of nanomaterials can significantly improve the workability of UHPC, the interfacial properties with fibers, and enhance hardened properties. The proposed framework will provide a computationally efficient and cost-effective way to achieve user-defined performance criteria for the properties of both fresh and hardened concrete. To achieve our objective, we will develop rheological models for fresh ultra-high-performance concrete (UHPC), validate the models by testing yield stress, viscosity, and thixotropy of the UHPC and integrate the data-driven approaches with computational and experimental data to ultimately determine the constituent and type of nano-additives that will improve rheology as well as intended performance of both fresh and hardened UHPC based on user requirement.
This research will directly benefit the work of ACI Committee 241 - Nanotechnology of Concrete, ACI Committee 238 – Workability of Fresh Concrete, and ACI Committee 135 – Machine Learning-Informed Construction and Design by applying nanomaterials and machine learning algorithms in concrete to improve the workability, rheology, and performance of UHPC. The research results will also provide input for ACI committees to update the ACI technotes and reports. Results will be shared by presentations at ACI Concrete Conventions and through various publications.
Strut-and-Tie Design of Disturbed Regions Utilizing Internal Fiber-Reinforced Polymer Reinforcement
PI: Rudolf Seracino, North Carolina State University
Co-PI: Giorgio T. Proestos, North Carolina State University
Reinforced concrete (RC) deep beams are common structural elements that can be found in a variety of structures including high-rise buildings, industrial facilities, and other situations where large loads are transferred in structures. For situations where fiber-reinforced polymer (FRP) reinforcement is used in place of traditional steel reinforcement, research is needed to confirm that designs based on the strut-and-tie method perform adequately. Recommendations need to be developed for the strut-and-tie design of RC deep beams utilizing internal FRP reinforcement.
This research focuses on developing design requirements for disturbed regions of structural concrete members reinforced with internal FRP bars. In the design of RC deep beams, the use of sectional design methods may result in unnecessarily conservative designs. The application of strut-and-tie models can result in more efficient structural designs that also better represent the load carrying mechanisms of the members. However, these procedures are based on various implicit assumptions that did not originally contemplate the use of FRP reinforcement. This research will investigate the use of internal FRP for both the longitudinal and transverse reinforcement and develop recommended design procedures. The overall goal is to develop code language and commentary for the design of deep FRP reinforced concrete beams that can be incorporated into Chapter 23 of ACI 440.11.
Sustainable and Safe Reinforced Concrete Retaining Walls
PI: Luis B. Fargier-Gabaldon, University of Notre Dame
Two independent studies conducted by ACI Technical Committee 318-F indicated that recent changes in the shear design provisions have a substantial impact on tall retaining walls exceeding 12 feet. Tall retaining walls that meet the ACI 318-19 Building Code may require an increase of the stem thickness by a factor greater than 4 (at the base of the stem) when compared with ACI 318-14, leading to larger CO2 emissions, greater environmental impact, and more costly structures. There are, however, no reports of a single failure in the field attributed to the application of previous code editions. Because an experimental campaign on full-scale retaining walls in the field is an expensive enterprise, it is proposed to investigate the behavior of retaining walls based on a.) available test data on the shear strength of beams with geometry and reinforcement ratios comparable with that encountered in tall walls, current design provisions for retaining walls and reliability-based estimates of earth thrust demands on the retaining walls, taking into account the variability of the soil parameters and soil-structure interaction.
Research objectives are to define upper and lower bound estimates of (i) thrust imposed by the earth fill on retaining walls (demand), and (ii) upper and lower bounds of the unit shear strength of the retaining wall (strength). These estimates can then be used to revise sections that affect the design of retaining walls in Chapter 13 (Foundations) and Chapter 22 (Sectional Strength) of the ACI 318-19 Building Code.
Results from this research will provide data to members of the ACI 318 Building Code to evaluate whether the new provisions for shear design of retaining walls need to be modified. Ideally, code provisions shall produce retaining walls with proportions that are comparable with those observed in the field and that have performed as intended. The new provisions (ACI 318-19), however, suggest the opposite for tall walls. ACI Technical Committee 318-F and ACI Technical Committee 318-E are aware of this issue and support this effort.
The Role of Testing Conditions and Concrete Durability Issues in Chloride Binding and Desorption of Cementitious Systems
PI: Mahmoud Shakouri, Colorado State University
This research is the second phase of an effort with an overarching objective of investigating the role of chloride binding and chloride desorption in the corrosion of steel and service life prediction of concrete structures. The state-of-the-art in chloride-induced corrosion has adequately highlighted the prime role of chloride binding in delaying chloride ingress and the onset of steel reinforcement corrosion. However, in the absence of a standardized test method, the current state of practice in measuring the chloride binding capacity of cementitious systems shows large variability in the chloride binding test results from one study to another. Nevertheless, many gaps in understanding remain regarding the influence of testing conditions, including the influence of the physical properties of the testing specimen and exposure solution chemistry on chloride binding and desorption mechanisms. Moreover, little is known about the role of durability issues in chloride binding and the possible synergetic effects of carbonation and ASR on the mechanisms of chloride binding and chloride desorption in cementitious systems.
The key objective of this phase of research is to determine the optimal procedures and testing parameters required to obtain reliable and repeatable chloride-binding isotherms. It will also address the remaining questions pertinent to chloride desorption in concrete suffering from ASR and carbonation. As the concrete industry is moving away from C150 cement and gaining an increased interest in using C595 cement for sustainability reasons, the results of this project will help determine if blended types of cement, including type IL, can suppress the risk of release of bound chlorides in the face of carbonation and ASR attack. Finally, this research will add to the discussions on the role of chloride binding in mitigating corrosion and extending concrete structures’ service lives in documents such as ACI 222-19 and ACI 365-17.