Ultra-Durable Concretes: Structure at the Micro- and Nanoscale
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Ultra-Durable
Concretes: Structure at the Micro- and Nanoscale Christian P.Vernet
Abstract Ultrahigh-performance concretes (UHPCs) are obtained by optimizing several technologies: minimizing the amount of water added, using superplasticizers and a wide particle size distribution, and packing the particles to improve fluidity with minimized water additions and to optimize load-carrying capacity. Fibers can be incorporated to increase ductility, leading to ultrahigh-performance fiber-reinforced concretes (UHPFRCs). Such enhanced concretes can approach the compressive strength of steel, with a remarkable improvement in durability. UHPCs offer new solutions for innovative construction, especially in aggressive environments. Keywords: durability, microstructure, ultrahigh-performance concrete.
crete. Reinforcing this material with short (12 mm long) steel fibers yields a concrete with a flexural strength of as high as 140 MPa, close to that of aluminum. Since then, other types of ultrahighperformance concrete (UHPC) and fiberreinforced UHPC (UHPFRC) have been developed3,4 and used in applications such as the construction of footbridges, aircooling towers in power stations, decorative panels, and oil well cementing. One UHPFRC (Ductal) is available as a premix and has been licensed in Japan, France, the United States, and Canada.4 Its mechanical properties far surpass those of conventional fiber-reinforced concrete, as shown in Figure 1. Due to the higher strength, structures built from this material are typically three times less massive2 than the traditional ones, as can be seen in a recently completed footbridge in Seoul, South Korea (Figure 2). Along with their ultrahigh mechanical performance and ductility,2 these materials also have remarkable durability, thanks to their very dense microstructure,5–7 with porosity reduced by a factor of 10, compared with ordinary concrete, and disconnected pores—an important factor in resistance to aggressive environments. Nine years of accelerated tests and natural weathering have shown that UHPCs are ultrahigh-durability concretes. This article explains the origin of this high durability in the structure at the micro- and nanometer scales.
Physical Principles Introduction Strong and long-lasting—those are two attributes that one tends to associate spontaneously with concrete. Well, in fact, not always. Concrete is in effect an artificial sedimentary rock, and as such, it has the intrinsic drawbacks of sedimentary rock. It is basically a brittle material, especially in tensile or bending conditions, due to its porosity. This porosity also makes it sensitive to various kinds of environmental attack from the atmosphere or from human practice (e.g., the use of de-icing salts). This is why traditional concretes—even those with the highest performance—are always reinforced with steel bars and why 80% of bridges and roads have to undergo some repairs during their service life. Can this intrinsic weakness be overcome? Sintering at high temperature is a possible way, but it makes no s
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