- Compressive behavior
- Shrinkage and Creep
- Bending behavior of Ductal®
- Fatigue behavior
- Behaviour under fire
Ductal® exhibits excellent performance in compression: It is 4 to 8 times higher than conventional concretes. Compression behaviour is almost linear elastic up to the maximum stress and exhibits no damage to the material during this phase.
Stress curve - deformation for a sample of Ductal® with metallic fibers
(The shape of the curve is identical for Ductal® with organic fibers although the peak value is lower.)
Creep tests have been carried out in France at the Ecole Centrale de Nantes and at the Laboratoire Central des Ponts et Chaussées (LCPC) and in the United States at the Federal Highway Administration (FHWA) Research Center in McLean, Virginia, USA.
For ordinary concrete, the creep coefficient can reach 3-4; for high-performance concrete, this is reduced but the recorded deformation remains higher than the elastic deformation. The creep coefficient of Ductal® is less than 0.8, and if a heat treatment is applied, the creep factor is less than 0.2, as shown in the figure below. As a rule, a value of 0.3 is considered for calculations.
Since the water to cement ratio is very low, Ductal® does not exhibit drying shrinkage. An endogenous shrinkage is observed (300 to 400 µm/m), but when heat treatment is applied, shrinkage is complete by the end of the treatment and there is no subsequent residual shrinkage, as shown in the figure below.
The fibers give the material a ductile behaviour during bending (i.e., when loaded in flexure beyond the elastic limit, micro-cracks occur and the fibers hold the cracks tightly closed, providing a ductile performance rather than a sudden or brittle failure) as shown in the following graph.
The ductility behaviour observed during bending is characterised by a multiple cracks before the stress peaks, without localization and without the presence of any major cracks.
The second figure above shows an image obtained by X-ray scanning, where it can be observed the high density of fibres (2 % volume) of a 40x40x40 mm cube sawed out off a Ductal® beam (courtesy of TOMO-ADOUR).
Fatigue tests on pre-loaded test samples were carried out at the CSTB. The loading applied was between 10 and 90% of the elastic limit. The figure below shows a crack opening displacement curve in relation to the number of cycles. Note: There is no increase in the crack opening, i.e. no crack propagation, at 1.2 million cycles.
Analysis of the rate of increase of the deflection in relation to the number of cycles shows that the loading applied is below the material's threshold of endurance. When calculating the design of structures subjected to fatigue action-effects, the service stress is limited to the material's resistance to direct tension. The results presented above verify that the application of the UHPC rule is particularly reliable in the case of Ductal® products with metallic fibers.
Ductal® materials have French classification M0 (non-combustible). Ductal®-AF was specially formulated for fire resistance. Its composition uses metal and organic fibers. ISO 834 fire resistance tests using loaded and unloaded posts and beams were conducted at the Centre Scientifique et Technique du Bâtiment (CSTB, in Marne-la-Vallée, France) and at VTT in Finland. These tests demonstrated the excellent fire performance of the material and an almost complete lack of flaking. The photo below shows a column before and after the ISO fire test.
The characterization of Ductal® at high temperatures was carried out at CSTB in Grenoble, at SFC in France, at the University of Braunschweig in Germany, at the Politecnico di Milano in Italy, and at Imperial College London in the UK. A summary of the results is given below.
Evolution of the resistance under compression according to temperature Compressive tests at high temperatures were carried out on test samples of Ductal®. Some of the tests were carried out on samples after cooling having been maintained at a given temperature T: so-called ‘residual’ tests. Some of the test samples were tested at the constant temperature T: so-called ‘hot' tests. The figure below shows all the results. We can see that the results obtained are almost all higher than the "DTU Feu" (French fire safety standard) curve specified for HPCs (extension of the "DTU Feu" for HPCs between 60 and 80 MPa).