Towards Ultra High-Performance Concrete Structural Design Guidance in the United States
Presented By: Rafic El-Helou
Affiliation: Genex Systems
Description: Structural design guidance for members made with ultra-high-performance concrete (UHPC) is proposed in this presentation. This guidance is being developed through a research program at the U.S. Federal Highway Administration whose scope focuses on delivering a draft guide specification to the appropriate AASHTO bridge design committee. The proposed framework delineates the testing methods for UHPC in uniaxial compression and tension and sets threshold values for the key mechanical and durability properties that define a UHPC-class material. The mechanical properties are derived directly from experimental data and utilized to develop constitutive models for use in the structural design of UHPC components. Adopting a strain-based design approach, the mechanical models are applied to describe the flexural behavior and potential failure modes of non-prestressed and prestressed UHPC beams. The shear design model is based on the strain compatibility and force equilibrium principles of the Modified Compression Field Theory (MCFT) and proposes changes to the material constitutive relationships applied in the MCFT for conventional concrete to reflect the behavior of UHPC. The predictive capabilities of the flexure and shear design methods are validated against the experimental results of large-scale UHPC girders tested at Turner-Fairbank Highway Research Center. The proposed guidance is expected to address major aspects of structural design, including axial behavior, transfer and development length of reinforcing steel, approximate estimates of time-dependent losses based on creep and shrinkage data, reinforcing details, durability, and key construction considerations.
Structural Response of UHPC Columns
Presented By: Mahmoud Aboukifa
Affiliation: University of Nevada, Reno
Description: The use of UHPC is currently expanding worldwide from bridge deck joints and connections to full components and larger applications. To improve structural design of UHPC members for different applications in bridge or buildings, a better understanding of the damage mechanism and sections capacity is needed. With focus on columns, and as a result of the enhanced mechanical characteristics of UHPC and high compressive strength, more slender sections are expected relative to conventional reinforced concrete. The objective of this presentation is to provide an overview of the structural response of UHPC columns with varying reinforcement details and slenderness effects. The current ACI 318 procedure for slender columns and second-order moment analysis is assessed and design recommendations are provided for UHPC axial and slender columns.
Carbon Nano Fibre Infused Ultra-High-Performance Fibre Reinforced Concrete: Tensile Strength Characterization by Four-Point Bending with Inverse Analysis
Presented By: Marwa Ibrahim
Affiliation: University of Calgary
Description: Ultra-High-Performance Fibre Reinforced Concrete (UHPFRC) is a special purpose cementitious mix with enhanced durability and mechanical strength. It traditionally contains steel microfibres which increase the material’s fracture toughness throughout their different energy-consuming mechanisms: crack bridging, fibre bending, and fibre pull-out. As a result, UHPFRC has an improved post-cracking ductility and possesses a steel-like high strain hardening behaviour under the uniaxial tension test. Concrete’s post-cracking tensile strength is consequently taken into account in the design process. However, the quantification of this property remains a challenge, hindering the broad deployment of this class of concretes. Two methods are mainly used in the literature: Direct Tension Tests (DTT) and bending tests with point-to-point inverse analysis. DTT imposes fewer bending stresses on the specimen but is hard to conduct. On the other hand, bending tests are easier to conduct but require sophisticated post-processing. A simplified unnotched four-point bending test with a five-point inverse analysis method is proposed in the literature, which guarantees simplified post-processing without lessening the result’s accuracy. The researchers applied this method to thirty prisms from three different batches, which is also adopted by Canadian Standard Association (CSA) code. Results of these tests are presented herein with an overview of the method. On the other hand, DTT is to be performed on the same material soon, allowing a rhetoric comparison between the two abovementioned methods.
Design UHPC Structure with High Deformation Capacity
Presented By: Yi Shao
Affiliation: Stanford University
Description: Compared to conventional concrete, UHPC materials exhibit higher tensile ductility on the material level due to its fiber reinforcement. However, recent studies reveal that reinforced UHPC (R/UHPC) beams may show fewer failure warning signs and lower deformation capacity than reinforced concrete. To guarantee the reliability and performance of UHPC structure, it is necessary to understand the causes of low structural ductility in some R/UHPC beams. This presentation reports the results of a large-scale experimental and numerical program, which characterize the failure mechanism of R/UHPC beams. The experimental program tested R/UHPC beams with different fiber volumes (0.5%, 1.0%, and 2.0%), reinforcing ratios (0.96% and 2.10%), and commercial UHPC materials (Ductal and UP-F). The numerical program validates and expands the knowledge from the experimental program by simulating R/UHPC beams with different UHPC tensile properties, reinforcing ratios, and steel properties. Results reveal that two flexural failure paths exist for R/UHPC flexural members: failure after crack localization or failure after gradual strain hardening of longitudinal reinforcing steel. These two failure paths represent distinct failure mechanisms and ductility ranges. Based on the results of the experimental and numerical program, a simple method is proposed and validated to help design UHPC structure with high deformation capacity. This study provides insights into the selection of appropriate UHPC material properties and the development of future UHPC structures.
Identification of the Tensile Constitutive Relationship for UHPC: Indirect Tension Tests versus Back Analysis of Flexural Tests
Presented By: Liberato Ferrara
Affiliation: Polytechnic University of Milan
Description: In this paper a test methodology is presented for straightforward identification of the main parameters describing the overall behavior in tension of UHPC, to be employed in structural design. Indirect tension tests have been performed via a technique called Double Edge Wedge Splitting (DEWS) together with 4-P Bending Tests (4PBT). Afterwards, starting from the experimental results, an identification procedure has been developed in order to evaluate the effective material behavior in direct tension in terms of stress-strain law. In the paper, the mechanical characterization for the reference mix is reported so to describe the identification procedure adopted.
Strain Capacity of Strain-Hardening UHP-FRC Part I: Steel Fibers
Presented By: Antoine Naaman
Affiliation: University of Michigan
Description: The main focus of this study is to provide a better understanding of the tensile strain capacity of strain-hardening HP- and UHP-FRC composites with the objective to possibly improve it; more precisely, how to increase the value of strain, at the maximum post-cracking stress or tensile strength. In this Part I, composites with steel fibers are examined. Overall, at time of this writing, the maximum post-cracking tensile strength recorded in numerous investigations of UHP-FRC remains mostly below 15 MPa and the corresponding strain below 4/1000. Both values are significantly reduced when the specimen size increases, as is needed for real structural applications. The authors analyze test data on and from close to one hundred series of direct tensile tests carried out in more than thirty investigations. Several factors observed to influence the strain capacity are identified (including the matrix composition and compressive strength, specimen size tested, the length of the fiber relative to the specimen size, fiber orientation, fiber content or volume fraction, fiber parameters (length, diameter, surface deformations, surface coating…), and indirectly the specific surface of fiber reinforcement. However, independently of the numerous parameters encountered, two observations emerged beyond all others, namely: 1) the higher the post-cracking tensile strength is (whichever way it is achieved), the higher the corresponding tensile strain; and 2) fibers with slip-hardening bond characteristics lead to an increase in strain capacity. A rational explanation for these observations is provided. After evaluating desirable strain requirement for structural applications, potential solutions for improving are suggested.
Strain Capacity of Strain-Hardening UHP-FRC Part II: Synthetic Fibers
Presented By: Surendra Shah
Affiliation: The University of Texas at Arlington
Description: Achieving a cement matrix with the highest compressive strength has been a challenge to engineers and scientists since the birth of concrete. While strength exceeding 500 MPa have been reported in the past with very small size specimens, in its present form, UHPC is first to achieve, using conventional processing, strengths in the range of 150 to 250 MPa on a structural scale. It is generally observed in cement-based composites, that the higher the compressive strength the higher the tensile strength which primarily remains in the range of 1/12 to 1/15 f’c. While post-cracking tensile strengths exceeding 35 MPa have been reported in UHP-FRC with steel fibers, the majority of test results reported fall below 15 MPa with a corresponding strain on average below 4/1000; such a relatively small strain capacity represents a severe constraint to wide scale structural applications of the composite. Part I of this study focused on the strain capacity of UHP-FRC with steel fibers. In this Part II, FRC composites with synthetic fibers are evaluated with particular attention to the use of high molecular weight polyethylene (HDPE or HMWPE) fibers and PVA fibers. In theory, everything else being equal, the use of synthetic fibers should lead to higher strain capacities simply because their elastic modulus is smaller than that of steel, and controls crack opening in the multiple cracking phases. However, other important factors driving strain capacity are uncovered and the findings are correlated with those obtained from Part I with steel fibers.