Design Of Ultra-high Performance Concrete Research Articles

Multi-level micromechanical analysis of elastic properties of ultra-high performance concrete at high temperatures: Effects of imperfect interface and inclusion size

Ultra-high performance concrete (UHPC), as a typical multi-phase composite material, is vulnerable to mechanical degradation and explosive spalling at high temperatures. In this work, the temperature-dependent elastic modulus of UHPC is estimated through the step-by-step homogenization from gel matrix level to ultra-high performance fiber-reinforced concrete level. Totally three forms of C-S-H gels are considered to determine the elastic properties of the initial matrix, and the phase transformation and volume fractions of individual phases at high temperatures are calculated based on the hydration and dehydration kinetics models. Moreover, the effects of imperfect interface and inclusion size are taken into account by introducing the spring-layer interface model and log-normal size distribution function to the homogenization scheme. The predicted elastic moduli are validated by analytical and experimental results, showing that accurate predictions can be obtained at up to 800°C. Interestingly, the imperfect interface matters when high-modulus inclusions are embedded in the low-modulus matrix, and the effect of imperfect interface on effective elastic modulus varies a lot at different microscopic levels. The findings highlight the synergistic effects of imperfect interface and inclusion size on the elastic properties, which is helpful to the mechanical design of UHPC at high temperatures.

Optimized design of steel fibres reinforced ultra-high performance concrete (UHPC) composites: Towards to dense structure and efficient fibre application

This paper presents a method for designing steel fibre reinforced Ultra-High Performance Concrete (UHPC) composites with dense micro/meso structure and efficient fibre application. Based on the modified Andreasen & Andersen packing model, the UHPC skeleton is first designed. Then, the effects of steel fibre shape and content on particle packing of UHPC are evaluated, which depicts that the hooked-end fibres have the most obvious negative effects on this system compared to other shaped fibres. Hence, to minimize the negative effects of fibres, the D-Optimal Design (DOD) method is utilized for the theoretical optimized design of UHPC via modelling of wet packing density with hybrid fibres. Meanwhile, an aided design model called Artificial Neural Network (ANN) is built for the prediction of wet packing density against different fibres, which shows very satisfactory accuracy, convenience and practicability. Finally, the basic properties of the developed UHPC are evaluated in detail to prove the feasibility and advancement of the proposed design method. In general, based on the approach proposed herein, an optimized UHPC composite with dense structure, high fibre efficiency and admirable mechanical properties could be produced.

A novel development of green ultra-high performance concrete (UHPC) based on appropriate application of recycled cementitious material

Recycled construction waste cementitious material (RCWCM) can be considered as a kind of waste solid with potential cementitious characteristics. In the past, landfill and stacking methods were generally used for treating this type of wastes, which is harmful for the sustainable development of construction industry. Hence, based on this premise, an effective method for developing a green Ultra-High Performance Concrete (UHPC) by incorporating RCWCM is addressed in this study. Specifically, dehydrated cementitious powder (DCP) is obtained from heating treatment of RCWCM. Then, the DCP is used to gradually replace the cement and included in the design of UHPC based on Modified Andreasen & Andersen (MAA) particle packing model. Afterwards, the physical and chemical characteristics of the newly produced UHPC are evaluated. The results show that when DCP is used to replace up to 25% cement, the variation of UHPC compressive strength is difficult to be noticed. Corresponding to that, the microstructure and durability of the developed green UHPC are also advanced when the added DCP content is less than 25%. Additionally, the ecological assessment shows that the UHPC incorporating DCP has a relatively small impact on the environment, which provides a broad prospect for the future sustainable development of UHPC.

Study on Optimization of Working Performance of Ultra High Performance Concrete

The water binder ratio is a key parameter in the mix design of ultra-high performance concrete. Aiming at the high sensitivity of ultra-high performance concrete to water consumption, the influence of water consumption on the performance of ultra-high performance concrete was studied in a narrow range. The compatibility ratio of raw materials of ultra-high performance concrete can be adjusted, but the space is small, so we try to improve the fluidity of concrete by physical and chemical means. The experimental results show that the fluidity of concrete increases slightly with the addition of glass beads, but the flexural properties of the concrete are adversely affected. With the addition of viscosity reducer, the workbility of concrete increases, but the compressive strength decreases.

Optimized design of ultra-high performance concrete (UHPC) with a high wet packing density

This study presents an optimized design method in developing ultra-high performance concrete (UHPC) with high wet packing density, in which the multiply effects of solid and liquid phases on UHPC packing mode are considered. Specifically, a model based on D-optimal method is firstly established to assess the influence of solid granular particles and superplasticizer (SP) on the UHPC packing density. Based on the theoretical analysis, the mixture proportions can be optimized, and UHPC with high wet packing density could be produced. Then, to evaluate the reliability of the math-physical method in developing UHPC, its compressive strength and pore structure are tested. The obtained experimental results show that the designed UHPC contributes high packing density, leading to optimized pore structure and extraordinary compressive strength. The experimental verification further highlights that D-optimal method is a promising and effective approach to design UHPC, and eventually for the development of sustainable UHPC.

Chloride-induced corrosion of steel fiber near the surface of ultra-high performance concrete and its effect on flexural behavior with various thickness

The results of experimental investigation of ultra-high performance concrete (UHPC) by evaluating the flexural response are presented in order to qualitatively evaluate the effects of chloride-induced corrosion. The third-point bending test is carried out using the hydraulic servo-controlled testing machine. The experimental variables are thickness of specimen (10, 25 and 50 mm), immersion duration, and mixture design of UHPC. The specimens are immersed in 10 wt% NaCl solution up to a year to induce corrosion. The effect of chloride-induced corrosion of steel fiber of UHPC is evaluated in terms of compressive strength, flexural strength and flexural toughness. The experimental results indicate that there is no significant loss in flexural strength and toughness for the specimens which were thicker than 25 mm over a period up to 365 days of immersion in chloride solution; whereas, the specimen with thickness of 10 mm showed about 10% decrease in maximum stress and corresponding toughness after 180 days of the immersion. Furthermore, it is revealed by comparing the compressive strength with and without fibers that the presence of steel fibers in UHPC under corrosive environments do not lead to substantial strength decrease.

Environmental Assessment of Ultra-High-Performance Concrete Using Carbon, Material, and Water Footprint.

There is a common understanding that the environmental impacts of construction materials should be significantly reduced. This article provides a comprehensive environmental assessment within Life Cycle Assessment (LCA) boundaries for Ultra-High-Performance Concrete (UHPC) in comparison with Conventional Concrete (CC), in terms of carbon, material, and water footprint. Environmental impacts are determined for the cradle-to-grave life cycle of the UHPC, considering precast and ready-mix concrete. The LCA shows that UHPC has higher environmental impacts per m3. When the functionality of UHPC is considered, at case study level, two design options of a bridge are tested, which use either totally CC (CC design) or CC enhanced with UHPC (UHPC design). The results show that the UHPC design could provide a reduction of 14%, 27%, and 43% of carbon, material, and water footprint, respectively.

Autogenous Shrinkage, Microstructure, and Strength of Ultra-High Performance Concrete Incorporating Carbon Nanofibers.

The mix design of ultra-high performance concrete (UHPC) is complicated by the presence of many “ingredients.” The fundamental packing density allows a simpler mix design with fewer ingredients to achieve optimum packing density and dense microstructure. The optimum particle grading increases the flowability of UHPC and eliminates entrapped air. This study presents a simplified particle grading design approach that positively influences the strength, autogenous shrinkage, and microstructure characteristics of UHPC. Carbon nanofibers (CNFs) of superior mechanical properties were added to enhance the strength of UHPC and to reduce its autogenous shrinkage. In addition, ground granulated blast-furnace slag (GGBS) was used as a cement replacement material to reduce the amount of cement in UHPC mixes. Test results showed that the presence of homogeneously dispersed CNF increased the compressive strength and compensated the autogenous shrinkage of UHPC. The findings indicated that an ideal particle distribution, which is close to the modified Andreasen and Andersen grading model, contributed to achieving high compressive strength and CNFs were capable of providing nano-bridges to compensate the shrinkage caused by GGBS.

Properties of reactive powder concrete incorporating silica fume and rice husk ash

Reactive Powder Concrete (RPC) is composed of very fine powders (cement, sand, and pozzolanic materials), and superplasticizers. A very dense matrix is found, and this tightness provides RPC with ultra-high strength and durability. Recently, using supplementary cementing materials associates greatly with ultra-high strength and the mix design of ultra-high performance concrete (UHPC). These materials could be natural, by-products or industrial wastes. They could be also less energy consuming and little time produced materials. Silica fume (SF), rice husk ash (RHA) and granulated blast furnace slag (GBFS) etc. are among the major supplementary cementing materials utilized. The detailed experimental investigation done to study the impact of partial alteration of cement with SF, RHA, and GBFS on concrete properties. This study aims to a minor replacement of Portland cement by SF, RHA and GBFS to reach UHPC. Twenty-five different concrete mixes (fc =150.1 to 188.2 MPa) with and without SF, RHA and GBFS were prepared with local materials in Egypt. Concrete mixes were cast with 0, 10, 15, 20, and 25% cement replaced by either SF or RHA, and another proportions taken combination between SF and RHA or SF and GBFS or RHA and GBFS about percentages from 10 to 15%. The mixes were tested for slump flow, air content, mechanical properties and water permeability. The findings of hardened properties indicate that optimum level for partial changing of cement by SF and RHA was 20% and it is observed that though the strengths of SF or RHA concrete goes on decreasing after the 20% addition of SF or RHA. Test results have indicated that RHA exhibits lower pozzolanic activity than SF.

Interface Shear Strength at Joints of Ultra-High Performance Concrete Structures

When ultra-high performance concrete (UHPC) is fabricated as precast members such as in a UHPC segmental bridge, the joint design between the precast members can significantly affect the overall integrity and safety of the structure. Therefore, the structural behavior of the UHPC joint was experimentally investigated in this study with test variables including joint type, number and height of shear keys, type of filler, curing temperature, and lateral compressive stress. The UHPC considered in this study is the K-UHPC developed in Korea with a specified compressive strength as high as 180 MPa and high flowability. The joint shear strengths affected by the test variables were investigated in detail. The test results were also compared with two representative predictive equations for interface shear strength to determine an appropriate equation for the joint design of UHPC. These equations did not match well with the test data because they were originally proposed for normal strength concrete. However, the JSCE equation could be improved by modifying a coefficient to show good agreement with the test especially in the case of the dry joint with epoxy application.

Performance assessment and design of ultra-high performance concrete (UHPC) structures incorporating life-cycle cost and environmental impacts

Ultra-high performance concrete (UHPC) as a novel concrete material is associated with very high strength and low permeability to aggressive environment. There have been many studies focusing on the development of UHPC materials. More studies are needed to implement the knowledge obtained from material level into the structural design and construction level. This paper emphasizes on the structural modeling and performance assessment of bridge girders made of UHPC considering the major improvements in terms of structural performance, durability, environmental impacts, and cost-effectiveness in a long-time interval. Additionally, the effect of the concrete strength increase on the life-cycle environmental impact and cost is assessed on a structural scale. An illustrative example is established to demonstrate the use of UHPC within precast-prestressed girder bridge. It is found that the use of UHPC can result in a significant reduction of concrete volume and CO2 emissions compared with conventional bridge with the same span length. Additionally, the life-cycle cost and equivalent annual cost associated with these two bridges are compared. This study aims to aid the development and adaptation of novel materials within civil engineering to make optimal use of the favorable material properties.

Determining the Environmental Benefits of Ultra High Performance Concrete as a Bridge Construction Material

Ultra High Performance Concrete (UHPC) is a material that is attracting attention in the construction industry due to the high mechanical strength and durability, leading to structures having low maintenance requirements. The production of UHPC, however, has generally higher environmental impact than normal strength concrete due to the increased demand of cement required in the concrete mix. What is still not sufficiently investigated, is if the longer lifetime, slimmer construction and lower maintenance requirements lead to a net environmental benefit compared to standard concrete bridge design. This study utilizes life cycle assessment (LCA) to determine the lifetime impacts of two comparable highway crossing footbridges spanning 40 meters, designed respectively with UHPC and normal strength concrete. The results of the study show that UHPC is an effective material for reducing lifetime emissions from construction and maintenance of long lasting infrastructure, as the UHPC design outperforms the normal strength concrete bridge in most impact categories.

Material efficiency in the design of ultra-high performance concrete

This research investigates the material efficiency in the design of ultra-high performance concrete. The material efficiency is influenced by the flowability, mechanical performance, durability and cost. Suitable material constituents have been pre-selected based on their properties and availability in the United States of America. The efficiency of the constituents is progressively investigated using the following three step approach emphasizing: (1) ultra-high performance paste, (2) ultra-high performance matrix, and (3) ultra-high performance fiber composite. Mix design recommendations for performance and cost effective ultra-high performance concretes are made and conclusions for further enhancements are drawn.

Effect of nano-silica on the hydration and microstructure development of Ultra-High Performance Concrete (UHPC) with a low binder amount

This paper presents the effect of nano-silica on the hydration and microstructure development of Ultra-High Performance Concrete (UHPC) with a low binder amount. The design of UHPC is based on the modified Andreasen and Andersen particle packing model. The results reveal that by utilizing this packing model, a dense and homogeneous skeleton of UHPC can be obtained with a relatively low binder amount (about 440kg/m3). Moreover, due to the high amount of superplasticizer utilized to produce UHPC in this study, the dormant period of the cement hydration is extended. However, due to the nucleation effect of nano-silica, the retardation effect from superplasticizer can be significantly compensated. Additionally, with the addition of nano-silica, the viscosity of UHPC significantly increases, which causes that more air is entrapped in the fresh mixtures and the porosity of the hardened concrete correspondingly increases. In contrary, due to the nucleation effect of nano-silica, the hydration of cement can be promoted and more C–S–H gel can be generated. Hence, it can be concluded that there is an optimal nano-silica amount for the production of UHPC with the lowest porosity, at which the positive effect of the nucleation and the negative influence of the entrapped air can be well balanced.

Innovative design approach of precast–prestressed girder bridges using ultra high performance concrete

The construction of new bridges and the maintenance and renewal of aging highway bridge network using ultra high performance concrete can lead to the construction of long life bridges that will require minimum maintenance resulting in low life cycle costs. Ultra high performance concrete (UHPC) is a newly developed concrete material that provides very high strength and very low permeability to aggressive agents such as chlorides from de-icing salts or seawater. Ultra high performance concrete could enable major improvements over conventional high performance concrete (HPC) bridges in terms of structural efficiency, durability, and cost-effectiveness over the long term. A simplified design approach of concrete slab on UHPC girders bridge using the Canadian Highway Bridge Design code and the current recommendations for UHPC design is proposed. An illustrative example demonstrates that the use of UHPC in precast–prestressed concrete girders yields a more efficient design of the superstructure where considerable reduction in the number of girders and girder size when compared to conventional HPC girders bridge with the same span length. Hence, UHPC results in a significant reduction in concrete volume and then weight of the superstructure, which in turn leads to significant reduction in the dead load on the substructure, especially for the case of aging bridges, thus improving their performance.