scholarly journals Discrete Element Modeling and Electron Microscopy Investigation of Fatigue-Induced Microstructural Changes in Ultra-High-Performance Concrete

Materials ◽  
2021 ◽  
Vol 14 (21) ◽  
pp. 6337
Author(s):  
Sebastian Rybczynski ◽  
Gunnar Schaan ◽  
Maksym Dosta ◽  
Martin Ritter ◽  
Frank Schmidt-Döhl
Keyword(s):  
Electron Microscopy ◽  
High Performance ◽  
Lightweight Concrete ◽  
High Performance Concrete ◽  
Fatigue Tests ◽  
Cement Clinker ◽  
Discrete Element Modeling ◽  
Ultra High Performance Concrete ◽  
Microstructural Changes ◽  
The Matrix

In view of the growing demand for sustainable and lightweight concrete structures, the use of ultra-high-performance concrete (UHPC) is becoming increasingly important. However, fatigue loads occur more frequently in nature than static loads. Despite the impressive mechanical properties of UHPC, a reduced tolerance for cyclic loading is known. For this reason, our paper deals with experimental and numerical investigations regarding the main causes for crack initiation on the meso, micro, and nanoscale. After mechanical fatigue tests, we use both scanning (SEM) and transmission electron microscopy (TEM) to characterize microstructural changes. A new rheological model was developed to apply those changes to the mesoscopic scale. The origins of fatigue damaging can be traced back to a transformation of nanoscale ettringite, resulting in a densification of the surrounding binder matrix. Additionally, a higher content of unhydrated cement clinker in the matrix benefits fatigue resistance. On the mesoscale, stress peaks around aggregate grains expand into the surrounding binder with increasing load cycles and lead to higher degradation.

scholarly journals Microstructural characteristics of ultra-high performance concrete by grid nanoindentation and statistical analysis

2021 ◽  
Vol 15 (1) ◽  
pp. 90-101
Author(s):  
Pham Thai Hoan ◽  
Ngo Tri Thuong
Keyword(s):  
Mechanical Properties ◽  
Statistical Analysis ◽  
Calcium Silicate ◽  
High Performance ◽  
High Performance Concrete ◽  
Cement Clinker ◽  
Indentation Modulus ◽  
Ultra High Performance Concrete ◽  
Deconvolution Analysis ◽  
Calcium Silicate Hydrates

In this study, grid nanoindentation and statistical deconvolution analysis were applied into a developed Ultra-high performance concrete (UHPC) to broaden the understanding of the microstructure phases and their mechanical properties. A total of 550 nanoindentation tests was carried out on UHPC and the mechanical properties, including indentation modulus and hardness of the indented material were extracted from nanoindentation load-depth curves. The statistical deconvolution analysis was then utilized to analyze the modulus and hardness spectra. The experimental and analysis results revealed that the modulus and hardness data obtained from nanoindentation tests can be used in the accurate and reliable identification of the microstructure phases and their properties in UHPC. For the present UHPC, the microstructure can be characterized into 6 phases with distinguishable mechanical properties, including micro porosity, Low Density Calcium Silicate Hydrates (LD CSH), High Density Calcium Silicate Hydrates (HD CSH), silica powder and sand, and residual cement clinker. The obtained modulus and hardness values of these phases were in the range of various reported ones for cement-based materials and UHPC. Keywords: ultra-high performance concrete; microstructures; micromechanical properties; nanoindentation; statistical analysis.

A Local Compression Tests of UHPC Anchor Blocks for Post-Tensioning Tendons

2012 ◽  
Vol 525-526 ◽  
pp. 561-564
Author(s):  
Eun Suk Choi ◽  
Jung Woo Lee ◽  
Chang Joh ◽  
Jong Won Kwark ◽  
Jee Sang Kim ◽  
...  
Keyword(s):  
High Performance ◽  
High Performance Concrete ◽  
High Strength ◽  
Construction Technology ◽  
Compression Tests ◽  
Ultra High Performance Concrete ◽  
European Organization ◽  
The Matrix ◽  
Post Tensioning ◽  
Anchor Block

In the application of Ultra High Performance Concrete (UHPC) to PSC girders by using the post-tensioning system, the high strength and ductility of UHPC in tension can be exploited to substitute the confined reinforcing bars which control the rupture around the anchorage device. The exploitation of such properties is expected to simplify the reinforcing details around the anchorage zone. Taking advantage of UHPC can downsize a cross section with the attributes of high compression and tensile strength. This paper reports the local behavior of UHPC anchor block under compression. Test specimens were made based on mix proportion of K-UHPC (KICT-Ultra High Performance Concrete) developed by the Korea Institute of Construction Technology (KICT). The performance of the anchor block was evaluated according to ETAG-013 (European Technical Agreement Guide No.13) of EOTA (European Organization for Technical Approvals). As the results of the experiment, it is found that the details and reinforcement of UHPC anchorage zone can be simplified with the interconnection effect and the high intensity of the matrix itself.

Flexural design of precast, prestressed ultra-high-performance concrete members

PCI Journal ◽  
2020 ◽  
Vol 65 (6) ◽  
pp. 35-61
Author(s):  
Chungwook Sim ◽  
Maher Tadros ◽  
David Gee ◽  
Micheal Asaad
Keyword(s):  
Prestressed Concrete ◽  
High Performance ◽  
High Performance Concrete ◽  
Limit State ◽  
Design Guidelines ◽  
Design Tool ◽  
Supplementary Cementitious Materials ◽  
Ultra High Performance Concrete ◽  
Concrete Members ◽  
Cylinder Strength

Ultra-high-performance concrete (UHPC) is a special concrete mixture with outstanding mechanical and durability characteristics. It is a mixture of portland cement, supplementary cementitious materials, sand, and high-strength, high-aspect-ratio microfibers. In this paper, the authors propose flexural design guidelines for precast, prestressed concrete members made with concrete mixtures developed by precasters to meet minimum specific characteristics qualifying it to be called PCI-UHPC. Minimum specified cylinder strength is 10 ksi (69 MPa) at prestress release and 18 ksi (124 MPa) at the time the member is placed in service, typically 28 days. Minimum flexural cracking and tensile strengths of 1.5 and 2 ksi (10 and 14 MPa), respectively, according to ASTM C1609 testing specifications are required. In addition, strain-hardening and ductility requirements are specified. Tensile properties are shown to be more important for structural optimization than cylinder strength. Both building and bridge products are considered because the paper is focused on capacity rather than demand. Both service limit state and strength limit state are covered. When the contribution of fibers to capacity should be included and when they may be ignored is shown. It is further shown that the traditional equivalent rectangular stress block in compression can still be used to produce satisfactory results in prestressed concrete members. A spreadsheet workbook is offered online as a design tool. It is valid for multilayers of concrete of different strengths, rows of reinforcing bars of different grades, and prestressing strands. It produces moment-curvature diagrams and flexural capacity at ultimate strain. A fully worked-out example of a 250 ft (76.2 m) span decked I-beam of optimized shape is given.

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