Ultra-high Performance Concrete Matrix Research Articles

Numerical simulation and visualization of motion and orientation of steel fibers in UHPC under controlling flow condition

To improve fiber orientation in ultra-high performance concrete (UHPC), one L-shape device was developed to control the flow of fresh mixture. A smoothed particle hydrodynamics (SPH) method was used to analyze and simulate the motion of steel fibers in UHPC with flow through this L-shape apparatus. SPH simulation indicated that steel fibers tended to rotate to be parallel with local streamlines and most fibers were aligned in the horizontal channel of this L-shape device, inducing an improvement of fiber orientation in hardened specimens. This modification effect of fiber orientation can be a little strengthened with the decreasing height of the horizontal channel. On the other hand, a colorless, transparent and viscous concentrated suspension was obtained by adding super absorbent polymer (SAP) into water for representing fresh UHPC matrix and the visualization of fiber motion was achieved. This visualization approach, coupled with image analysis of steel fibers in hardened UHPC specimens, well verified the SPH numerical simulation results. Therefore, this type of L-shape apparatus provides a potential method to manually improve the steel fiber orientation in UHPC for better mechanical properties.

Pullout behavior of straight and hooked-end steel fibers in UHPC matrix with various embedded angles

The study provided a systematic evaluation of the pullout behavior of straight and hooked-end steel fibers embedded in ultra-high performance concrete (UHPC) with various angles using a single fiber pullout test. Three steel fibers, including one straight fiber and two hooked-end fibers, were aligned at 0°, 30°, and 45° with respect to the loading direction. The fibers were pulled out from non-fiberous UHPC matrix with a compressive strength of 151.5 MPa. Experimental results showed that when the fiber embedded angle increased from 0° to 30° and 45°, the average bond strengths of straight fibers were increased by 19.2% and 52.9%, while the average bond strengths of hooked-end fibers were increased by 10.3–13.6% and 16.2–26.1%, respectively. The percentage of end hook contribution to total pullout energy ranged from 50% to 56.8%, indicating significant mechanical anchorage contribution. Two types of fiber failure modes, including fiber pullout failure and fracture failure, were found, depending on fiber angles. From Scanning Electronic Microscopy (SEM) observation, interfacial transition zone failure, fiber surface scratching, and matrix failure were identified. Two new indices, namely energy dissipation index and bond strength index, were proposed to evaluate the fiber-matrix bond behavior. It was found that hooked-end fibers with a smaller diameter could be a better choice for structural application according to the proposed evaluation indices. The proposed indices offered an alternative and practical method to assess the fiber-matrix bond behavior.

Experimental investigations of internal energy dissipation during fracture of fiber-reinforced ultra-high-performance concrete

Split-cylinder fracture of fiber-reinforced ultra-high-performance concrete (UHPC) was examined using two complementary techniques: X-ray computed tomography (CT) and acoustic emission (AE). Fifty-mm-diameter specimens of two different fiber types were scanned both before and after load testing. From the CT images, fiber orientation was evaluated to establish optimum and pessimum specimen orientations, at which fibers would have maximum and minimum effect, respectively. As expected, fiber orientation affected both the peak load and the toughness of the specimen, with the optimum toughness being between 20% and 30% higher than the pessimum. Cumulative AE energy was also affected commensurately. Posttest CT scans of the specimens were used to measure internal damage. Damage was quantified in terms of internal energy dissipation due to both matrix cracking and fiber pullout by using calibration measurements for each. The results showed that fiber pullout was the dominant energy dissipation mechanism; however, the sum of the internal energy dissipation measured amounted to only 60% of the total energy dissipated by the specimens as measured by the net work of load. It is postulated that localized compaction of the UHPC matrix as well as internal friction between fractured fragments makes up the balance of internal energy dissipation.

Effect of inclination angle on hooked end steel fiber pullout behavior in ultra-high performance concrete

The bond relationship between the concrete matrix and steel fiber is a significant factor that affects the performance of ultra-high performance fiber reinforced concrete (UHPFRC). In the present research, pullout performances of hooked end fibers embedded in ultra-high performance concrete matrix under various inclination angles are systematically investigated, with special attention on fiber dimension and embedded length. Pullout load-slip curves are obtained and experimental observations including complete fiber pull-out, fiber rupture and matrix failure are analyzed in detail. The effects of the pullout angle are then studied quantitatively by parameter calculations and mechanism analysis. A new analytical model for evaluating the snubbing and spalling effects of the hooked end steel fiber is proposed and validated. It is shown that the influences of the inclination angle on the peak pullout load vary with different fiber types, embedded lengths and fiber diameters, which are also associated with the occurrences of the fiber rupture and the matrix failure. In addition, optical microscope and scanning electron microscopy observations at mesoscale are performed to further analyze the effects of orientation angle.

Confinement effects on fiber pullout forces for ultra-high-performance concrete

Fiber reinforcement of ultra-high-performance concrete (UHPC) is necessary for adding toughness and ductility to an otherwise very brittle matrix. The action of fibers pulling out of the concrete matrix is one of the primary mechanisms of energy dissipation. Of interest in this work is the effect of a compressive stress in the concrete matrix on the fiber pullout behavior. An experimental fixture was developed to apply confining forces to the UHPC matrix while a fiber is pulled out of the matrix. Approximately 90 pullout tests were conducted with specimens subjected to confinements of 2, 38, and 76 MPa. Complete force-pullout length data were recorded and analyzed. The results showed that confinement stresses had a positive correlation with both peak force and work of the pullout force, but the latter was only valid for the first few millimeters of pullout. When the entire response was considered, the correlation with work of pullout disappeared. An analytical pullout model was employed to isolate the effects of bond fracture energy and friction. Fitting the model to the data showed that bond fracture energy was not affected, but friction stress increased up to 60% for high confining stresses.

How do fiber shape and matrix composition affect fiber pullout behavior and flexural properties of UHPC?

The use of steel fiber is essential to secure high strength and ductility in producing ultra-high performance concrete (UHPC). In this study, the interfacial bond properties between embedded steel fibers with different shapes (straight, hooked, and corrugated fibers) and UHPC matrices proportioned with either 15% or 20% silica fume, by mass of binder, under different curing times were investigated. Flexural properties of UHPC reinforced with 2% different shaped fibers were also evaluated. Test results showed that corrugated and hooked fibers significantly improved the bond properties by three to seven times when compared to those with straight fibers. The flexural strength of UHPC with corrugated and hooked fibers were enhanced by 8%–28% and 17%–50%, respectively. Microstructural results from MIP, BSEM, and TG confirmed the change in bond properties. The bond strength of straight fibers exponentially increased with the decrease of calcium hydroxide content. Based on the composite theory, the flexural strengths of UHPC made with different shaped fibers can be efficiently predicted using the fiber-matrix bond strength, the flexural strength of the UHPC matrix (non-fibrous matrix), and the parameters of fibers. The ratios of predicted to measured flexural strengths ranged between 0.8 and 1.1, in which straight fibers showed a larger discreteness due to higher sensitivity of flexural strength associated with the orientation of fibers.

CHARACTERIZATION OF ULTRA-HIGH-PERFORMANCE CEMENTITIOUS COMPOSITE INCORPORATING CARBON NANOTUBES

Ultra-High-Performance Concrete (UHPC) is a type of concrete with unique mechanical and durability characteristics, developed to meet the global demand for extreme construction. Typically, UHPC is produced using customized mix design and subjected to special curing condition. Concrete is a brittle material in nature, where UHPC has its drawbacks in terms of lower tensile strength ratio and brittleness. Carbon nanotubes (CNT) is a potential candidate to act as nano-reinforcement in UHPC matrix, to create a denser and a more ductile Ultra-High-Performance Cementitious Composite (UHPCC) system. The consistent dispersion of CNT in the cementititious matrix is a challenge due to their high aspect ratio and its agglomerating behavior. This paper presents on the fundamental UHPCC mix design which optimizes on its packing density with fewer constituent materials. The mechanical strength and microstructure characteristics of three types of UHPCC developed with CNT, which were produced with different dispersion methods are reported. It was found that stable CNT dispersion enhanced the microstructure characteristics of the UHPCC matrix, and achieved higher compressive and flexural strengths compared to control specimens without CNT.

The Study of UHPC Precast Concrete Containing Incinerator Fly Ash

Abstract The objective of this study was to evaluate ultra high performance concrete (UHPC) as a precast product material containing portland type II cement, quartz powder, silica fume, silica sand, steel fiber, superplasticizer, water, and a specific amount of incinerator fly ash (IFA). The UHPC mixes containing different volumes of IFA were evaluated to find the optimum quality of precast production, while 2.5 % of steel fiber by volume was blended at every mix. The results indicate that the properties of UHPC exhibit high strength, ductility, and bonding stress. The compressive strength of UHPC decreases greatly as IFA for part replacement of cement or quartz powder increases. However, the flexural strength of UHPC decreases a little as IFA for part replacement of quartz powder increases. The results demonstrate that the UHPC products with 10 % or 15 % IFA were able to withstand a 98-kN proof load to the cover and frame without cracks or permanent deformation. It has to be noted that heavy metals that originated in the IFA are confined in the UHPC matrix and yield rather low leachability results.

Investigation of Microstructural Damage in Ultrahigh‐Performance Concrete under Freezing‐Thawing Action

This work aims to investigate the damage in ultrahigh‐performance concrete (UHPC) caused by freezing‐thawing action. Freezing‐thawing tests were carried out on UHPCs with and without steel fibers. Mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and X‐ray computed tomography (X‐ray CT) were applied to detect the microstructure of the UHPC matrix before and after the freezing‐thawing tests. The results showed that UHPC possessed very excellent freezing‐thawing resistance due to its dense microstructure. After the freezing‐thawing action, cracks occurred and were prone to initiate at the sand‐paste interface in the UHPC matrix. MIP results also indicated that cracks appeared in the UHPC matrix after the freezing‐thawing action. The number of defects that can be seen by X‐ray CT increased in UHPC after the freezing‐thawing action as well. The mismatch of the thermal expansion coefficients of the aggregate and the paste is considered to be the reason for the cracking at the sand‐paste interface. The steel fibers in UHPC inhibited the propagation of cracks in the matrix and improved the freezing‐thawing performance of UHPC.

Monitoring hydration of UHPC and conventional paste by quantitative analysis on Raman patterns

Cement hydration governs physical, chemical and mechanical properties of cement pastes. Therefore, tracing hydration progress in different types of cement pastes can help to understand different hydration mechanisms and paste behaviours. In the present research, a quantitative analysis on Raman spectroscopy (RS) patterns for conventional cement paste and ultra-high performance concrete (UHPC) matrix at different hydration ages has been applied. It has been found that quantitative analysis on peak area of Raman patterns is fully capable to differentiate between conventional cement hydration and pozzolanic reaction in UHPC. The result also showed that silica fume included in the UHPC matrix was more reactive at early ages, which was completely in agreement with reduction of calcium hydroxide (CH) content at the same hydration period.

Size effect in ultra-high-performance concrete beams

In order to investigate the size effect on the flexural performance of ultra-high-performance concrete (UHPC), numbers of UHPC beams with three different sizes, including two with smooth steel fibers with aspect ratios of 65 and 100 and one with twisted steel fibers with an aspect ratio of 100, were fabricated and tested. The beams made of UHPC matrix without fiber were also considered for evaluating the effect of fibers on the size effect. In addition, image analyses at crack surfaces were performed to investigate the size effect on the fiber distribution characteristics and their implication on the size-dependent flexural performance. Test results indicated that the flexural performance of UHPC noticeably decreased with an increase in the specimen size, caused by a decrease in the number of fibers and poor fiber orientation. The use of longer steel fibers with a higher aspect ratio was effective in reducing the sensitivity to the size effect of UHPC beams compared with shorter steel fibers. Finally, by applying similar fiber distribution characteristics for all test specimens of different sizes, an insignificant size effect on the flexural strength was obtained for UHPC with 2% by volume of steel fibers.

Structural performance of ultra-high-performance concrete beams with different steel fibers

In this study, ten large ultra-high-performance concrete (UHPC) beams reinforced with steel rebars were fabricated and tested. The experimental parameters included reinforcement ratio and steel fiber type. Two different reinforcement ratios (ρ=0.94% and 1.50%) and steel fiber types (smooth and twisted steel fibers) were adopted. In addition, three different fiber lengths (Lf=13, 19.5, and 30mm) for the smooth steel fibers and one fiber length (Lf=30mm) for the twisted steel fiber were considered. For a control specimen, a UHPC matrix without fiber was also considered. Test results indicated that the addition of steel fibers significantly improved the load carrying capacity, post-cracking stiffness, and cracking response, but it decreased the ductility. Specifically, with the inclusion of 2% by volume of steel fibers, approximately 27–54% higher load carrying capacity and 13–73% lower ductility were obtained. In addition, an increase in the length of smooth steel fibers and the use of twisted steel fibers led to the improvements of post-peak response and ductility, whereas no noticeable difference in the load carrying capacity, post-cracking stiffness, and cracking response were obtained according to the fiber length and type. Sectional analysis incorporating the suggested material models was also performed based on AFGC/SETRA recommendations, and the ratios of flexural capacities obtained from experiments and numerical analyses ranged from 0.91 to 1.19.

Effects of nano-silica and nano-limestone on flowability and mechanical properties of ultra-high-performance concrete matrix

The effects of nano-silica/SiO2 (NS) and nano-limestone/CaCO3 (NC) on the flowability, strengths and microstructure of ultra-high-performance concrete (UHPC) matrix under different curing conditions were investigated in this study. The NS and NC were incorporated at different ratios as partial mass replacements for cement. On the microstructure aspect, the results verify that the NS acts as an effective filling material, which reduced porous areas and accelerated the cement hydration process by pozzolanic effect. On the other hand, the NC acts mainly as an inert filler material that created a denser microstructure, but accelerated the cement hydration process through boundary nucleation growth effect. On the mechanical properties aspect, a threshold value of the NS and NC contents were found so that the compressive, flexural strengths and flexural to compressive strength ratio of the UHPC matrix were found to increase as the NS and NC contents increased towards the threshold content, and then to decrease with the increase of NS and NC contents when the threshold was surpassed. Corresponding to the highest measured mechanical strengths of UHPC matrix, the optimal contents of NS and NC are around 1.0% and 3.0%, respectively. The research concluded that the NS and NC contents are critical to the performance of UHPC matrix.

The Determination of Frost Resistance on Ultra High Performance Concrete

The ultra high performance concrete (UHPC) has very special properties that are expressively different of normal concrete. Due to its high compression strength greater than 150 MPa, tensile strength greater than 20 MPa and improved durability, these represent significant advances in concrete technology. These materials include Portland cement, silica fume, quartz flour, fine silica sand, high-range water-reducer, water and either steel or organic fibres. Depending on the type of fibres used can influence the compressive strength. The article describes the tests of frost resistance on UHPC plates with different types of textiles armatures. The aim of the testing is describe influence of textiles armatures in UHPC matrix in extreme conditions.

Effect of shrinkage reducing agent on pullout resistance of high-strength steel fibers embedded in ultra-high-performance concrete

The interfacial bond strength of long high-strength steel fibers embedded in ultra-high-performance concrete (UHPC) reinforced with short steel microfibers was investigated by conducting single-fiber pullout tests. In particular, the influence of the addition of a shrinkage-reducing to a UHPC matrix on the pullout resistance of high-strength steel fibers was investigated. The addition of a shrinkage-reducing agent produced a noticeable reduction in the fiber pullout resistance owing to the lower matrix shrinkage, although the reduction of pullout resistance differed according to the type of fiber. Long smooth and twisted steel fibers were highly sensitive to the addition of the shrinkage-reducing agent whereas hooked fibers were not. Among the various high-strength steel fibers tested, twisted steel macrofibers showed the highest interfacial bond resistance, although twisted fibers embedded in UHPC showed slip softening pullout behavior rather than the typical slip hardening behavior observed in mortar.

Effect of Natural Wollastonite Microfibers on Early-Age Behavior of UHPC

In this study, the effect of incorporating wollastonite microfibers in ultrahigh performance concrete (UHPC) on its early-age properties was investigated. Wollastonite microfibers were added at 0, 4, 8, and 12% as partial volume replacement for cement. Results show that the early-age properties of UHPC mixtures incorporating wollastonite microfibers are highly affected by the microfiber content and aspect ratio. Increasing the wollastonite microfiber content resulted in a compressive strength comparable to or higher than that of the control mixture without microfibers. Wollastonite microfibers reduced shrinkage strains and increased cracking resistance compared to that of the control mixture. However, no significant improvement in the flexural behavior was achieved with the addition of wollastonite microfibers because of a sudden rupture of microfibers within the matrix. Wollastonite microfibers can act as an internal restraint for shrinkage, reinforcing the microstructure at the microcrack level, and leading to an enhancement of the early-age engineering properties of UHPC matrix. Partially replacing cement with natural wollastonite microfibers leads to a reduction in the cement factor, which represents economic and environmental benefits.

Mechanical properties of concrete-reinforced fibres and powders with crushed thermoset composites: The influence of fibre/matrix interaction

This paper focuses firstly on the formulation and mechanical behaviour of concrete reinforced fibre and powder (CRFP). Secondly, we present the effect of silane treatment on the bond characteristics of the fibre–matrix of ultra-high-performance concrete (UHPC). The initial objective of the study was to optimize the incorporation of fibres and powders in high performance concrete (HPC). Two materials were investigated. The first involved replacing a volume of sand by an equivalent volume of fibres and powders. The second type was obtained by replacing an equivalent volume of silica fume with powders. Testing of the first material showed that the incorporation of fibres and powders by sand substitution increased the bending and tensile strength by 32%. In the second material, the substitution of silica fume by the powder led to an increase of 37% in flexural and tensile strengths.The second section of this paper focuses on silane-treated fibres. The experimental results on fibre pullout tests under different conditions are reported. Various silane treatment contents ranging from 0% to 1% were studied. Fibre pullout tests were conducted to measure the bond characteristics of fibres in the UHPC matrix. The treatment by vinyl trimethoxy silane (VTMO) can effectively enhance the fibre–matrix interfacial properties, especially the fibre pullout energy. It was concluded that in terms of the bond characteristics, the optimal silane content is 0.5% under these conditions.

Ultra-high performance concrete and fiber reinforced concrete: achieving strength and ductility without heat curing

Ultra-high performance concrete (UHPC) and ultra-high performance fiber reinforced concrete (UHP-FRC) were introduced in the mid 1990s. Special treatment, such as heat curing, pressure and/or extensive vibration, is often required in order to achieve compressive strengths in excess of 150 MPa (22 ksi). This study focuses on the development of UHP-FRCs without any special treatment and utilizing materials that are commercially available on the US market. Enhanced performance was accomplished by optimizing the packing density of the cementitious matrix, using very high strength steel fibers, tailoring the geometry of the fibers and optimizing the matrix-fiber interface properties. It is shown that addition of 1.5% deformed fibers by volume results in a direct tensile strength of 13 MPa, which is 60% higher than comparable UHP-FRC with smooth steel fibers, and a tensile strain at peak stress of 0.6%, which is about three times that for UHP-FRC with smooth fibers. Compressive strength up to 292 MPa (42 ksi), tensile strength up to 37 MPa (5.4 ksi) and strain at peak stress up to 1.1% were also attained 28 days after casting by using up to 8% volume fraction of high strength steel fibers and infiltrating them with the UHPC matrix.

Comment on ‘penetration into semi-infinite reinforced-concrete targets with spherical and ogival nose projectiles’ ( Int. J. Impact Engng6, 2913̄07, 1987)

The unparalleled attributes of ultra-high-performance concrete (UHPC) reported over the last three decades sum up the development of this material in two phases: UHPC and ultra-high-performance fiber-reinforced concrete (UHP-FRC)/ultra-high-performance hybrid fiber-reinforced concrete (UHP-HFRC). The integration of scientific knowledge pertaining to material interactions and technical observations of developed UHPC has laid the foundation for further research and development. Microstructural research has revealed the contribution of finer sizes mineral admixtures along with lower water/binder ratios in the range of 0.16–0.24 to the development of high-density calcium silicate hydrate with a very low possibility of ettringite formation and alkali silica reactions in UHPC matrix. The addition of fibers is challenging when seeking to maintain the rheology of UHPC; therefore, technically sound concrete experts are required for the field applications. The challenges preventing the widespread use of UHPC are the higher initial cost, the requirement of special skilled labor for execution, and the lack of open discussions about various UHPC standards available worldwide to reach an agreement on minimum strength achievements and test standards. The hurdles hindering the use of UHPC in recent emerging 3D printing technology are also discussed in the present study. The vast literature compilation also reveals that less research has concentrated on UHP-HFRC as compared to UHPC and UHP-FRC. Therefore, a comprehensive review regarding the effects of mineral admixtures and fibers on the microstructural characteristics, fresh properties, and mechanical properties overall to address the upcoming challenges preventing the widespread use of UHPC and UHP-FRC/UHP-HFRC is essential.