scholarly journals Fluorescence Microscopic Investigations of the Retarding Effect of Superplasticizers in Cementitious Systems of UHPC

Materials ◽  
2020 ◽  
Vol 13 (5) ◽  
pp. 1057 ◽  
Author(s):  
Johannes Arend ◽  
Alexander Wetzel ◽  
Bernhard Middendorf
Keyword(s):  
Molecular Structure ◽  
High Performance ◽  
High Performance Concrete ◽  
Ultra High Performance Concrete ◽  
Adsorption Characteristic ◽  
Beneficial Effects ◽  
Retarding Effect ◽  
Cementitious Systems ◽  
Mixing Water

The adsorption of superplasticizer molecules to particle surfaces in cementitious systems is a very important aspect for the desired liquefaction of pastes and concretes. This way, the comb shaped polymers shield attractive forces between the particles and induce a well-dispersed, homogeneous suspension. These admixtures allow the usage of fine fillers even in combination with low amounts of mixing water, and thus, are the basis for modern high performance concretes. However, the adsorption does not cause beneficial effects only: The polymer covered particle surfaces, especially clinker, are hindered to interact with water, thus hydration is retarded. This is the reason for lower early strength and is very disadvantageous for certain applications. Today it is known that the molecular structure of the polymers, for instance the chain length and charge density, affects the retardation strongly. The complexity and diversity of cementitious systems is the main reason why research in this field is quite empiric and time as well as cost intensive. To investigate the adsorption of superplasticizers in various systems in-situ, a fluorescence microscopic approach was applied: By staining the polymers with fluorescent dye they become localizable and the adsorption quantifiable. This work shows the influence of molecular structure to adsorption characteristic of different polymers and the correlation to the retarding effect of superplasticizers, especially concerning the presence of silica fume, which is indispensable for ultra-high performance concrete (UHPC).

Numerical analyses on flexural performance of prefabricated UHPC link slab

2021 ◽  
Author(s):  
Bruno Briseghella ◽  
Wei Xu ◽  
Jun-Qing Xue ◽  
Jian-hui Lin ◽  
Camillo Nuti
Keyword(s):  
Crack Resistance ◽  
Bearing Capacity ◽  
High Performance ◽  
High Performance Concrete ◽  
Labor Cost ◽  
Ultimate Bearing Capacity ◽  
Ultra High Performance Concrete ◽  
Steam Curing ◽  
Expansion Joints

<p><br clear="none"/></p><p>The expansion joints in the multi-span simply supported bridge can be eliminated by using the link slab. The ultra-high performance concrete (UHPC) with high tensile strength and crack resistance is an effective material for the link slab. However, the cast-in-situ UHPC link slab need to be cured with steam curing. Therefore, the construction processes are complicated and the construction quality is difficult to guarantee. In this paper, the prefabricated UHPC link slab which can be assembled on site to simply the construction process, accelerate the construction speed and reduce the labor cost was proposed. Finite element models of the prefabricated and cast-in-situ UHPC link slabs under bending were built by using ABAQUS. The ultimate bearing capacity of the prefabricated link slab was nearly the same of the cast in situ and the crack resistance slightly lower. Finally, the influence of the bolt (used to connect the prefabricated link slab) number and the distance from the bolt to the edge of the link slab on the crack resistance and ultimate bearing capacity of the prefabricated link slab were obtained.</p>

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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