Other material properties of UHPFRC

The material properties referred to in the following sections, although important in providing correct structural performance, are not the subject of the present research. For this reason only the principal points and relevant references are summarised.

3.4.1 Long term behaviour: shrinkage, creep, self-healing

According to some authors, creep and shrinkage are “the most outstanding characteristics” of UHPC [Acker, Behloul 2004], since the effect of both phenomena is significantly decreased.

Shrinkage is a process primarily caused by the self-dessication, and due to the low water-cement ratio in UHPFRC, it is reported that the major part of shrinkage occurs immediately after settling, while practically no shrinkage occurs after early age. A difference in behaviour is reported between thermally treated and non-treated UHPC. According to French recommendations [SETRA, AFGC 2002], autogenious shrinkage is defined as a function of the water-cement ratio, and for thermally non–treated material with a water-cement ratio of 0.17 - 0.20, for time t , the shrinkage value is 550 μm/m. For thermally treated material no shrinkage develops according to these recommendations. Certain manufacturers of UHPFRC give similar values: zero to 10 μ^m/m post-cure shrinkage and 550 μm/m for thermally non-treated material characterises Ductal, the UHPFRC manufactured by Lafarge [Graybeal 2006].

Creep is closely related to shrinkage behaviour from the microstructural point of view, [Acker 2004a ], and it is observed that thermal treatment significantly reduces creep [SETRA, AFGC 2002]. The evolution of creep function depends on the age of the material at loading. For example, a creep coefficient of 0.2-0.8 is suggested for Ductal, with the lower values corresponding to thermally treated material and the higher to non-treated material. In normal concrete, the creep coefficient can reach the value of 3 to 4. Low creep values are of particular interest for the application of prestressing (reduction of prestress losses).

More details concerning the creep and shrinkage of UHPC, HPC, and ordinary concrete, based on the description of microstructural changes, can be found in [Acker 2004], [Acker 2004a] and [Kamen 2007].

Another positive consequence of a low water-cement ratio and the fact that a conciderable percentage of the cement paste remains unhydrated, is that, in the presence of humidity, further hydration can take place in microcracks. This is known as the self-healing effect. The results of an experimental programme conducted in order to quantify the self-healing of UHPC materials are reported in [Granger et al. 2007], [Hearn, Morley 1997]. Both the recovery of global stiffness and the improvement in strength of initially cracked specimens are reported, as a function of the time of healing. The stiffness recovery is due to the formation of new crystals with a stiffness close to that of the C-S-H crystals that were initially formed.

3.4.2 Porosity, durability

As already mentioned in relation to the conception principles of UHPC, the matrix is tailored in order to decrease porosity on different scales, resulting in hardly any capillary porosity in comparison with other concrete materials [Vernet 2003], [ Schmidt, Fehling 2005], Figure 3.27.

Figure 3.27: Distribution of pores of different size in UHPC, HPC and ordinary concrete, from [Schmidt, Fehling 2005]

The durability of UHPFRC is related to this impressively decreased porosity, and to generally improved material homogeneity. This results in highly improved resistance to the penetration of chlorides, frost and freezing attack, etc. Various laboratories are investigating these issues worldwide. More detailed information on these properties and characteristic durability values can be found in [Vernet 2003], [Schmidt, Fehling 2005].

Microcracking, characterised by small crack openings, has been proved to have only a small impact on permeability, and consequently durability [Aldea et al. 1999].

3.4.3 Energy-dissipation capacity, impact resistance

UHPFRC has a high energy-dissipation capacity, which ensures cracking stability even in the case of relatively strong impact. For typical rates of impact loading on civil engineering structures, it is demonstrated that tensile strength increases up to two times, and compressive strengths up to 1.5 times according to [SETRA, AFGC 2002] and based on research on RPC, e.g. [Toutlemonde et al.

1998].

3.4.4 Fire resistance

UHPFRC reinforced with steel fibres only exhibits relatively unfavourable behaviour. Due to the low content of connected pores, and higher enclosed porosity, steam cannot escape, resulting in an increase in internal stresses in the presence of high temperatures. Fire resistance problems can be avoided by the use of organic fibres, [Acker, Behloul 2004], which, in melting, create communication between the pores. The addition of approximately 0.7 % in volume of polypropylene fibres provides sufficient fire resistance [Heinz et al. 2004].

3.5 Conclusions

UHPFRC is a new, advanced concrete material of elaborated composition, with remarkably superior mechanical strengths and durability in comparison to other concretes.

As a result of material optimization on different scales, the obtained UHPC microstructure is very homogeneous and compact, with almost no capillary porosity. The developed microstructure explains the significantly improved mechanical properties. Ductility is achieved by the addition of short fibres, in an optimal quantity that also maintains the workability of the fresh mixture.

The compressive strength of UHPFRC is higher than 150 MPa; tensile strength, in the range of 10 MPa, is characterised by significant ultimate strain (approx. 2-3 ‰), and the cracking behaviour of the composite is characterised by a fracture energy higher than 10 kJ/m².

From a qualitative point of view, the compressive behaviour of UHPFRC does not differ substantially form the behaviour of ordinary concrete, and it is possible to model it with a slight adaptation of material laws used for ordinary concretes. More detailed conclusions on the behaviour in compression and modelling are given in § 3.3.1.6.

The tensile behaviour of UHPFCR is characterised by the strain-hardening phase, often with a small slope, that distinguishes it from other concretes and FRCs. The potential of the material to develop strain-hardening phase depends on the quantity of fibres and their orientation, the latter being strongly influenced by the casting procedure. Details of this behaviour and modelling possibilities for further design needs are discussed in § 3.3.2, and conclusions are given in § 3.3.2.9.

In addition to mechanical strengths, the properties of interest for structural application such as creep and shrinkage, durability, and impact strengths are also impressively improved due to the material microstructure.

The combination of these properties postulates more advanced structural application:

- high strengths and ductility, combined with material durability, suggest that applications without passive reinforcement, and with a decrease in element size, may be possible;

- the plasticity of the fresh material allows easy placement in a variety of formworks, without additional vibrating; combined with the fact that the ordinary reinforcement can be excluded, a gain in production time and a greater freedom of form are possible.

in bending

The objective in this chapter is to study bending behaviour of UHPFRC elements and develop a practical procedure for their analysis and design. Elements without conventional reinforcement are considered.

The specific mechanical properties of the material lead to an element bending response with a pronounced non-linear part, governed by multi-microcracking in the tensile region and the propagation of a localised macrocrack. For this reason fracture mechanics theories are required for understanding UHPFRC element response in bending. An analytical model describing the behaviour of elements in the presence of pseudo-plastic tensile strain (representing multi-microcracking) and the localised crack is developed in this study. The results are compared with the experimental results obtained in a test programme on thin UHPFRC elements in bending (Appendix T1). The results of the analytical model are also compared with the results of a developed numerical model for finite element analysis. The plausibility of analytical results is also demonstrated for elements made of other quasi-brittle materials, with and without the multi-microcracking phase. A simplified version of the model is proposed for design purposes.

The influence of the specific tensile properties of UHPFRC on bending strengths and ductility are studied by means of parametric analysis, with the major parameters for the analysis being the maximal deformation achieved during the microcracking phase and the slope of tensile softening law. Based on these results, the size effect in relation to strength and to the deformational capacity of elements made of UHPFRC and other quasi-brittle materials is discussed. Conclusions regarding practical design procedures for UHPFRC thin elements without ordinary reinforcement are drawn.

a)

microcracking

localised macrocrack

b)

microcracking localised macrocrack

Figure 4.1: UHPFRC structural elements failing in bending, without the possibility of redistributing internal force; thus the resistance of the element corresponds to the resistance of a critical section; a) thin elements without ordinary reinforcement;

b) deep elements, with prestressing reinforcement