Very High Performance Concrete

In very high performance concretes with low water/cement ratio the air-entraining agents may be considered unnecessary (cf. Section 13.4.2). In many countries, application of air-entrainers for such structures is imposed by standards. 

For practical calculations, the permeability of concrete is considered as equal to that of hardened paste. In that assumption, the existence of microcracks and aggregate grains in concrete compensate each other for water flow. For higher concrete strength and low wic ratio the coefficient and permeability itself decrease considerably. A further decrease may be expected for high and very high performance concretes with low values of wIc ratio and with extensive use of microfillers. 

The examples of composition of high performance concrete and very high performance concrete presented in Table 13.4 show the features listed above. 

Other admixtures are used for high performance concrete as for ordinary concretes. However, their necessity should be verified in terms of improved impermeability and early strength. It has been observed that air-entraining agents are not needed for high performance concrete in most cases and certainly not for very high performance concrete, because pores are already well distributed and total porosity is low. Furthermore, it can be difficult to produce an additional pore system in a dense and coherent fresh mix. However, this question is still subject to tests and discussions. 

Several authors propose the value of 0.22 as an optimum wic ratio (ACI 1998). In many published compositions of very high performance concrete (VHPC), wic ratio remains between 0.25 and 0.30 and between 0.30 and 0.35 for high performance concrete (HPC), probably for higher percentage of hydration of Portland cement. Also, lower dosages of snperplasticizer might be possible for required workability, and consequently, lower cost. 

Figure 13.9 Grading of all components of a very high performance concrete, after Lecomte and Thomas (1992).

The brittleness of high performance concrete (HPC) and very high performance concrete (VHPC) deserves more attention. Experimental results obtained by de Larrard and Malier (1992) on specimens made with three different concretes are given below as an example ... 

The carbonation process in high performance concrete is basically reduced because CO2 cannot penetrate its dense structure. Accelerated tests executed by Levy (1992) showed practically no traces of carbonation. When SF is added as admixture, e.g. for very high performance concrete, the pH of concrete may decrease, thus creating conditions more favorable for corrosion of steel reinforcement. However, in that case the electrical resistance of the concrete is enhanced and consequently it will slow down the corrosion process. 

Special kinds of very high performance concrete and ultra high performance concrete... 

There are several kinds of high performance concrete and very high performance concrete proposed by different authors and companies. Some of them are covered by patents and others are presented openly in published papers. A few of them are briefly described below in order to present a large variety of cement-based composites of the highest quality. Not all are further developed and used after successful laboratory investigations, but all have some particularities that deserve attention. These achievements perhaps indicate directions of future development of cement-based composites. 

Considerable effort in goal-oriented research is still needed in relation to applications of advanced cement-based materials. Notwithstanding recent achievements, the use of very high performance concrete and ultra high performance concrete is still limited to single special structures or so-called hot points in the structures. 

In concrete technology an important field of interest is a self-compacting concrete, which flows and compacts itself due to gravity (20). Therefore, no external vibration or other compaction is needed. It is possible to produce a very high performance concrete as self-compacting concrete. Because compacting work is not needed, the noise level during construction is lowered remarkably. 

This equation applies for pore capillary partially filled, when the balance is achieved in interfaces solid-liquid-gas [6], Assuming this mechanism as the main cause of shrinkage for pores with high relative humidity, say, 80 %, SRA are very useful to control shrinkage in high-performance concrete (HPC). 

Durability of concrete can be attained first of all by producing material of low permeability, which causes the migration of aggressive solution into this composite very difficult. The tightness of concrete will be the result of low porosity, whieh can be assured by low w/c ratio it should be generally lower than 0.4. That is why the high performance concretes are produced in majority with w/c of about 0.35. In case... 

The texture of cement pastes in high-performance concretes is characterized by high density and very low porosity. Owing to the pozzolanic reaction between the added microsilica and calcium hydroxide, the amount of ciystalline portlandite in the hardened paste is reduced, and the formed C-S-H phase has a lower C/S ratio (Cheong et al, 1997). The mature cement paste contains a significant fraction of non-hydrated cement. 

Because of its very dense microstractrrre, high-performance concrete may be effectively used in applications where it is exposed to chemical attack or abrasion. Very often additional protective meastrres, which are necessary if normal concrete is used, may be avoided. 

Mechanical Behavior of High Performance Concretes, Vol. 5 Very High Strength Concrete. Documents laboratory investigations of the mechanical behaviour of high performance concrete for highway... 

The influence of the alkali content of the SF on its pozzolanic properties has been studied by de Larrard et al. (1990) using 20 different SF compositions with various kinds of cements and plasticisers. The authors have proposed certain empirical relations to forecast the compressive strength of very high performance mortars and concretes. 

Two main groups of admixtures are considered to be very important components of high performance concrete (i) superplasticizers, which improve the workability of the fresh mix with low values of wic ratio and (ii) microfillers, which increase the density of the hardened material. 

Furthermore, the specific cost of the material, calculated after the cost of components and execution, is only a small part of the total cost, which also comprises maintenance cost of various kinds, including repairs, possible replacement, breaks in normal exploitation, etc. Using high performance materials, smaller dimensions of structures are necessary, and lower maintenance cost may be expected. There are also other advantages already Eiger (1933) has observed that high performance concrete elements may be subjected very early to partial service loads. All these factors decide upon the total cost of the entire structure. As a result, apparently expensive composite materials appear very interesting economically (Malier 1992 CEB 1989). 

The development of various kinds of high performance and ultra high performance concretes, also reinforced with dispersed fibres, has resulted in the creation of a group of very important building materials. At present, for many outstanding structures or for construction in special conditions, application of these materials is considered a necessity, and this situation will be extended in the future (Brandt 2008). 

---