Kay Wille

Ultra-High Performance Concrete with Compressive Strength Exceeding 150 MPa (22 ksi): A Simpler Way

Although intensive research related to ultra high-performance concrete (UHPC) and its composition has been conducted over the past 2 decades, attaining compressive strengths of over 150 MPa (22 ksi) without special treatment, such as heat curing, pressure, and/or extensive vibration, has been nearly out of reach. This paper describes the development of a UHPC with a compressive strength exceeding 200 MPa (30 ksi), obtained using materials commercially available in the U.S. market and without the use of any heat treatment, pressure, or special mixer. The influence of different variables such as type of cement, silica fume, sand, and high-range water reducer on compressive strength is evaluated. The test results show that the spread value, measured through a slump cone test on a flow table, is a good and quick indicator to optimize the mixture packing density and thus its compressive strength.

Nanoengineering Ultra-High-Performance Concrete with Multiwalled Carbon Nanotubes

Ultra-high-performance concretes (UHPCs) are characterized by extremely high packing densities. These densities can be achieved with optimization of grain size distribution by incorporating a homogeneous gradient of fine and coarse particles during mixing. Addition of steel fibers can increase the ductility under tensile loading tremendously. The packing density is a key parameter for the high bond strength between steel fibers and the UHPC matrix. The objective of this study is twofold: to enhance the behavior of the bond between the steel fibers and the matrix by increasing packing density with the inclusion of nanometer-sized particles while preserving the workability of the concrete mix. Multiwalled carbon nanotubes (MWNTs) were chosen for their unique physical and impressive mechanical properties (i.e., ultimate strength, stiffness, and ductility). However, because of the nanotubes’ inherent tendency to agglomerate, an extensive study was conducted to optimize their dispersion in solutions suitable for concrete mixing. An experimental validation study was then conducted to explore the effects of incorporating MWNTs in mixes with only 0.022% by cement weight and how they affect the bond behavior of two different high-strength steel fibers. Mechanical characterization studies revealed that low concentrations of MWNTs can significantly improve the bonding behavior of single pulled-out high-strength steel fibers. No conclusive evidence can be drawn with regard to MWNTs’ influence on the UHPCs’ compressive and bending strengths [i.e., 200 and 13 MPa (30 and 1.9 ksi), respectively].

Effect of Ultra-High-Performance Concrete on Pullout Behavior of High-Strength Brass-Coated Straight Steel Fibers

The objective of this research was to investigate the pullout behavior of straight high-strength steel fibers embedded in different ultra-high- performance concretes (UHPCs) with a compressive strength ranging from 190 to 240 MPa (28 to 35 ksi). Particular attention was placed on obtaining matrixes with high packing density to enhance the physicochemical bond with the embedded fiber. The parameters investigated included the use of different sand ratios, silica fume (SF) and glass powder with different mean particle sizes, different superplasticizers, and the addition of hydrophilic or hydrophobic nanosilica particles. Thus, by tailoring the matrix composition, significantly different bond stress versus slip-hardening behaviors were achieved. This is atypical for straight smooth steel fibers, which are normally characterized by a bond-slip softening behavior. Microscopical studies revealed that scratching and delaminating of the brass-coated fiber surface by fine sand and by abrading matrix particles is one reason for this phenomenon, and help explain the maximum equivalent bond strength observed of up to 20 MPa (2.9 ksi).

Strain Rate Dependent Tensile Behavior of Ultra-High Performance Fiber Reinforced Concrete

Ultra High Performance Fiber Reinforced Concretes (UHP-FRC) can be designed to resist increasing tensile loading after first matrix cracking, which results from strain hardening tensile characteristics accompanied by multiple cracking. Previous investigations carried out under static loading conditions have clearly shown that matrix composition, fiber material and geometry as well as fiber volume fraction and fiber orientation influence the strain hardening tensile behavior. This paper describes research that was conducted to study the direct tensile behavior of UHP-FRC loaded at various speeds. A hydraulic test machine was used to apply load up to 103 times faster than static loading, i.e. up to a strain rate of \(\dot{\varepsilon}\) = 0.1 s− 1. The test setup was designed to permit reliable measurement of direct tension test results at the different loading speeds considered, taking into consideration a reasonable gage length for multiple crack development while minimizing the inertial effects associated with the specimen and attached measurement equipment. The strain rate dependent tensile behavior is analyzed in terms of peak strength, strain at peak strength, hardening modulus and energy absorption capacity prior to softening. The results show the strain rate sensitivity of each of these parameters at fiber volume fractions of 2, 2.5 and 3%.

Strength dependent tensile behavior of strain hardening fiber reinforced concrete

The influence of matrix strength on the tensile behavior of Fiber Reinforced Cement Composites [FRCC] is investigated. The test parameters included four cementitious matrices with compressive strength of 28 MPa (4 ksi) [M1], 56 MPa (8 ksi) [M2], 84 MPa (12 ksi) [M3] and 190 MPa (28 ksi) [M4], respectively, two types of high strength deformed steel fibers, Hooked [H-] and Twisted [T-] fibers, and two volume fractions of fibers, 1% and 2%. It is observed that while the first cracking strength, post cracking strength and energy absorption capacity of FRCC are strongly influenced by the compressive strength of the matrix their strain capacity at peak stress and cracking behavior are not as much affected. While both H- and T- fibers led to improved performance when the matrix strength was increased, T- fibers take better advantage of higher strength matrices. A post-cracking tensile strength exceeding 15 MPa at a peak strain of 0.5% was achieved by using 2% T- fibers with an ultra-high strength matrix (M4).

Equal Arc Segment Method for Averaging Data Plots Exemplified for Averaging Stress versus Strain Curves of Pervious Concrete

AbstractIn common engineering practice, data plots, such as the stress versus strain relationship of materials, are averaged among a series of samples using the mean ordinate (MO) method. This method is reliable as long as the sample performance does not vary significantly. If the stress versus strain curve of individual samples differs significantly in terms of shape, peak stress, and corresponding strain, the MO method may result in misleading average curves that do not adequately represent the material behavior. In this paper, an equal arc segment (EAS) method for averaging data plots is presented. Based on experimentally obtained compressive stress versus strain curves for pervious concrete, a comparison has been conducted between the proposed EAS method and the MO method. The results show that the EAS method outperforms the MO method, especially where there are high variations in the data pool. The EAS method is not limited to pervious concrete as showcased here, but can be applied to other data plot...

Experimental Study of UHPC Repair for Corrosion-Damaged Steel Girder Ends

AbstractCorrosion of girder ends is a prevalent problem that significantly reduces the bearing capacity of bridges. Current repair methods are expensive and difficult to implement. A novel repair m...

Rehabilitation of Steel Bridge Girders with Corroded Ends Using Ultra-High Performance Concrete

The end corrosion in steel girders at the bearings due to joint leakage is a significant problem in many of the old bridges around the nation. This critical damage impairs the shear and bearing capacities of girders. This paper discusses a novel method for retrofitting the corroded ends of steel bridge girders using ultra-high performance concrete (UHPC) encasings. This repair method involves casting thin UHPC panels on each side of girder web. Shear studs welded to undamaged portion of the web and flange engage the UHPC panels and provide an alternate load path. This repair method is expected to be superior to the current practice of attaching steel cover plates. It can be easier to design and install, reduce obstruction to traffic during the repair, prevent future corrosion to the girder end, and reduce the total cost of repair. To investigate the effectiveness of the repair in recovering the capacity of the corrosion damaged girders, three large-scale experiments were performed on the undamaged, damaged and repaired rolled girders. It was found that a 1 3/4-in. thick UHPC panel cast two-third of the height of the girder effectively restores the bearing capacity. A high fidelity finite element model was created from the results of the large-scale experiments. This model was then used to design eight repair techniques for full size plate and rolled girders with heavy corrosion damage. This innovative repair method may offer the bridge design community a superior alternative retrofit method for large scale application on aging bridges.

Understanding the Dispersion Mechanisms of Nanosilica in Ultra High Performance Concrete

One of the current challenges to nanoengineering cementitious composite materials is obtaining proper dispersion of nano-sized particles in the cementitious composite matrix. Proper dispersion of particles can lead to improved particle packing density, a key parameter in improving the mechanical, chemical, and sustainable properties of the cementitious composite. Current advances in optimizing particle packing density have led to the development of higher strength, higher durable cementitious composite materials, such as ultra-high performance concrete (UHPC). Further advancement of UHPC can be achieved by the addition of properly dispersed nano-sized particles which will assist in broadening the particle size distribution for further optimization of the particle packing density. Among the vast variety of potential nano-particles, nanosilica is the most studied to date. Nanosilica has been shown to improve macroscopic properties such as mechanical strength, durability, and chemical resistivity of cementitious materials. However, as the average particle diameter size decreases the water demand increases and challenges a balanced mix design. More fundamental work needs to be done in order to understand how nanosilica disperses inside the hydrating matrix. Our research investigations will focus on the interfacial interactions between nanosilica and the hydration products, the ions of the pore solution, and the polymers of admixtures.

Enhancing Printable Concrete Thixotropy by High Shear Mixing

Our results show that the storage elastic modulus as a function of time increases at a higher rate for cement paste mixed at higher vesus lower mixing intensity. Hence, higher mixing appears to be enhancing thixotropy. Using calorimetry analysis we find that higher mixing decreases the setting time and enhances the peak of the heat flow. By analyzing the nanoparticles present in the suspending fluid of the cement paste, we show, in accordance with literature, that an appropriate combination of mixing energy and super-plasticizer dosage promotes hydration by scratching hydrates from the surface of cement particles, stabilizing them in the suspending fluid and hence generating additional nucleation surfaces. These results open the door for the design of printing heads including high-shear micro mixers allowing for a faster liquid-to-solid transition of the printable material.