Production Of Ultra High Performance Concrete Research Articles

Novel ultra high performance concrete mixing technology with preliminary dry forced packing

It is a universally recognized fact that the production of ultra high performance concrete (UHPC) requires mixing technologies that go beyond what is typically used for ordinary concrete. A generally accepted approach is to adapt and repurpose unique mixers from other industries to mix UHPC. However, in some cases these tools have intrinsic limitations that preclude the production of UHPC that exhibits uniform dispersal of all constituents. This study discusses a method of mixing that utilizes preliminary forced packing of dry multi-component mixtures, and studies the effect of such forced packing on the physicochemical properties of the composition, microstructure, homogeneity, and mechanical properties of the resultant UHPC. The methodology achieved complete de-agglomeration of ultra-fine particles in the mixture and facilitated their uniform distribution within the cementitious matrix, leading to increased homogeneity, density and strength of the concrete. The study discusses methods of direct measurement of uniformity of the hardened cementitious matrix.

Bond behaviour of high-strength steel rebars in normal (NSC) and ultra-high performance concrete (UHPC)

Bond is the interaction and force transfer between rebar and concrete, which directly affects the performance of concrete structures. On the other hand, innovations in the production of ultra-high performance concrete (UHPC) and its unique advantages, such as high strength, low permeability, and high resistance to chemical attacks, have increased the interest in using it in the building industry. While most of the recommended relationships in the standards are based on normal strength concrete (NSC), it is necessary to know more about UHPC behaviour. In this study, 60 cubic concrete specimens reinforced with a rebar in its centre and three bond lengths (2, 4, and 6 times the diameter) were prepared using UHPC and NSC and two types of rebars (high-strength and normal strength). A pull-out test was performed on all specimens. The results show that the concrete strength, the ratio of concrete cover to the diameter (c/d), bond length, rebar yield strength and geometry of the rebar play an important role in determining the failure mode. The use of a high-strength rebar (AIV) in comparison with normal strength (AIII) rebars, by increasing the possibility of applying a higher bond tension to concrete, provides the opportunity to use more concrete capacity. UHPC reduces the embedded length of rebars by increasing bond strength 5-fold relative to NSC. In NSC, increasing the bond length increases the maximum bond stress, while in UHPC, for increasing bond length, the maximum bond stress decreases. New relationships have been proposed to predict bond-slip behaviour.

A Review on Raw Materials and Curing Methods Applied in Production of Ultra High Performance Concrete

Ultra high performance concrete (UHPC) refers to cement based materials exhibiting high compressive strength, tensile strength and excellent durability. This paper reviews the raw materials and curing methods currently applied in UHPC manufacturing process. Raw materials, preparation techniques and curing methods have significant influence on properties of UHPC. The use of very high cement content, high strength cement and silica fume may improve the strength and compactness but increase the production cost. The use of high temperature curing leads to denser microstructure and better performance than room temperature curing and water curing do, but obviously limits its application of UHPC. Thus, preparation of UHPC using widely and locally available raw materials such as ordinary Portland cement CEM Type 1 42.5N and common technology such as room temperature curing are trends in production of UHPC.

Low carbon design of an Ultra-High Performance Concrete (UHPC) incorporating phosphorous slag

Abstract In this study, a low carbon design of an Ultra-High Performance Concrete (UHPC) is performed by incorporating high-volume phosphorous slag (PS). The modified Andreasen and Andersen (MAA) particle packing model is employed to obtain a densely compacted particle skeleton, while the PS is employed to replace cement by 10–50% (weight). Then, the effect of the added PS on fresh and hardened properties of the designed UHPC are investigated. The results indicate that the addition of PS significantly improves the flowability and reduce its autogenous shrinkage during the early stage. Compared to the reference sample, the UHPC with PS has lower compressive strength at 3 days, while presents comparable strength at 28 days. Microstructure analyses verify that the added PS obviously prolong the dormant period of cement hydration in UHPC, which actually is beneficial for obtaining a stable and microstructure optimized UHPC. Additionally, the carbon foot print calculation demonstrates that appropriate application of PS in the production of UHPC is a promising approach to develop a cleaner building material.

Development of a novel cleaner construction product: Ultra-high performance concrete incorporating lead-zinc tailings

This paper addresses a development for a novel sustainable Ultra-High Performance Concrete (UHPC) incorporating lead-zinc tailings. The modified Andreasen & Andersen particle packing model is utilized to achieve a densely compacted cementitious matrix and the lead-zinc tailings are utilized to replace cement by 10%, 20%, 30% and 40% (wt.), while the packing densities of all the designed UHPC mixtures are almost keep the same. Then, the impact of added lead-zinc tailings on the fresh and hardened behaviors of the developed UHPC is investigated. The obtained experimental results reveal that the workability, compressive strength and chlorine anion penetration resistance of the UHPC decrease with the addition of lead-zinc tailings. However, it is possible to develop self-compacting UHPC with up to 30% replacement of cement by lead-zinc. Moreover, it is noteworthy that the addition of lead-zinc tailings could significantly decrease the early autogenous shrinkage of UHPC, which is beneficial for its microstructure development and application. Additionally, the leaching toxicity of heavy metals for this developed sustainable UHPC is far lower than that of the National Standard of China. The ecological evaluation demonstrates that replacing cement by lead-zinc tailings in the production of UHPC can significantly reduce its environmental impact, which provides a promising view for developing and applying a novel cleaner construction product with advanced 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.

Microstructural Investigation of Heat-Treated Ultra-High Performance Concrete for Optimum Production.

For optimum production of ultra-high performance concrete (UHPC), the material and microstructural properties of UHPC cured under various heat treatment (HT) conditions are studied. The effects of HT temperature and duration on the hydration reaction, microstructure, and mechanical properties of UHPC are investigated. Increasing HT temperature accelerates both cement hydration and pozzolanic reaction, but the latter is more significantly affected. This accelerated pozzolanic reaction in UHPC clearly enhances compressive strength. However, strength after the HT becomes stable as most of the hydration finishes during the HT period. Particularly, it was concluded that the mechanical benefit of the increased temperature and duration on the 28 day-strength is not noticeable when the HT temperature is above 60 °C (with a 48 h duration) or the HT duration is longer than 12 h (with 90 °C temperature). On the other hand, even with a minimal HT condition such as 1 day at 60 °C or 12 h at 90 °C, outstanding compressive strength of 179 MPa and flexural tensile strength of 49 MPa are achieved at 28 days. Microstructural investigation conducted herein suggests that portlandite content can be a good indicator for the mechanical performance of UHPC regardless of its HT curing conditions. These findings can contribute to reducing manufacturing energy consumption, cost, and environmental impact in the production of UHPC and be helpful for practitioners to better understand the effect of HT on UHPC and optimize its production.

Comparative study on flexural properties of ultra-high performance concrete with supplementary cementitious materials under different curing regimes

This study investigated the effects of curing regimes (standard, hot water, and steam curing) on mechanical properties of ultra-high performance concrete (UHPC) with different supplementary cementitious materials (SCMs). The flowability, compressive strength, flexural load-deflection relationship, ultimate flexural strength, and toughness of UHPC mixtures with 0, 20%, 40%, and 60% of either ground granulated blast-furnace slag (GGBS) or fly ash, by the mass of cement, were evaluated. Test results indicated that the increase in GGBS or fly ash content had limited or negative influence on the compressive strength of UHPC in terms of curing regime type . For the flexural strength, there existed an optimal SCM content, which was 40% for GGBS and 20% for fly ash. Excessed this dosage could result in a drop in flexural strength and toughness. The hot water and steam curing significantly improved the flexural properties. However, prolonging the standard curing over 28d led to comparable flexural properties as those under hot water and steam curing showed. The production of UHPC with appropriate GGBS or fly ash content with adequate standard curing, therefore, offers an effective way to obtain satisfactory flexural properties.

Ultra-High Performance Concrete: Mechanical Performance, Durability, Sustainability and Implementation Challenges

In this study, an extensive literature review has been conducted on the material characterization of UHPC and its potential for large-scale field applicability. The successful production of ultra-high performance concrete (UHPC) depends on its material ingredients and mixture proportioning, which leads to denser and relatively more homogenous particle packing. A database was compiled from various research and field studies around the world on the mechanical and durability performance of UHPC. It is shown that UHPC provides a viable and long-term solution for improved sustainable construction owing to its ultra-high strength properties, improved fatigue behavior and very low porosity, leading to excellent resistance against aggressive environments. The literature review revealed that the curing regimes and fiber dosage are the main factors that control the mechanical and durability properties of UHPC. Currently, the applications of UHPC in construction are very limited due to its higher initial cost, lack of contractor experience and the absence of widely accepted design provisions. However, sustained research progress in producing UHPC using locally available materials under normal curing conditions should reduce its material cost. Current challenges regarding the implementation of UHPC in full-scale structures are highlighted. This study strives to assist engineers, consultants, contractors and other construction industry stakeholders to better understand the unique characteristics and capabilities of UHPC, which should demystify this resilient and sustainable construction material.

Early-Age Strength of Ultra-High Performance Concrete in Various Curing Conditions.

The strength of Ultra-High Performance Concrete (UHPC) can be sensitively affected by the curing method used. However, in contrast to the precast plant production of UHPC where a standard high-temperature steam curing is available, an optimum curing condition is rarely realized with cast-in-place UHPC. Therefore, the trend of the compressive strength development of UHPC was experimentally investigated in this study, with a focus on early-age strength by assuming the various curing conditions anticipated on site. Concrete specimens were cured under different conditions with variables including curing temperature, delay time before the initiation of curing, duration of curing, and moisture condition. Several conditions for curing are proposed that are required when the cast-in-place UHPC should gain a specified strength at an early age. It is expected that the practical use of UHPC on construction sites can be expedited through this study.

Modeling of temperature history in the hardening of ultra-high-performance concrete

Ultra-high-performance concrete (UHPC) consists of cement, silica fume (SF), sand, fibers, water and superplasticizer. Typical water/binder ratios are 0.15 to 0.20 with 20 to 30% silica fume. In the production of ultra-high performance concrete, a significant temperature rise at an early age can be observed because of the higher cement content per unit mass of concrete. In this paper, by considering the production of calcium hydroxide in cement hydration and its consumption in the pozzolanic reaction, a numerical model is proposed to simulate the hydration of ultra-high performance concrete. The heat evolution rate of UHPC is determined from the contributions of cement hydration and the pozzolanic reaction. Furthermore, by combining a blended-cement hydration model with the finite-element method, the temperature history in the hardening of UHPC is evaluated using the degree of hydration of the cement and the silica fume. The predicted temperature-history curves were compared with experimental data, and a good correlation was found.

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.

Development of UHPC mixtures utilizing natural and industrial waste materials as partial replacements of silica fume and sand.

In the exploratory study presented in this paper, an attempt was made to develop different mixtures of ultrahigh performance concrete (UHPC) using various locally available natural and industrial waste materials as partial replacements of silica fume and sand. Materials such as natural pozzolana (NP), fly ash (FA), limestone powder (LSP), cement kiln dust (CKD), and pulverized steel slag (PSS), all of which are abundantly available in Saudi Arabia at little or no cost, were employed in the development of the UHPC mixtures. A base mixture of UHPC without replacement of silica fume or sand was selected and a total of 24 trial mixtures of UHPC were prepared using different percentages of NP, FA, LSP, CKD, and PSS, partially replacing the silica fume and sand. Flow and 28-d compressive strength of each UHPC mixture were determined to finally select those mixtures, which satisfied the minimum flow and strength criteria of UHPC. The test results showed that the utilization of NP, FA, LSP, CKD, and PSS in production of UHPC is possible with acceptable flow and strength. A total of 10 UHPC mixtures were identified with flow and strength equal to or more than the minimum required.