Surface Morphology of Porous Cementitious Materials Subjected to Fast Dynamic Fractures

This paper presents a study of the surface height irregularities of cement pastes subjected to fast dynamic fractures. The height irregularities are quantified by the values of the three-dimensional profile parameters. The studied dynamical irregularities show a similar analytical behavior to those obtained by static fractures.


Introduction
We have recently published several studies [1][2][3] on the surface morphology of fractured specimens made from hydrated cement pastes.This material shows a high value of porosity, which has been proved to be an influential factor governing the irregularities of fracture surfaces.
For practical surface analyses of porous materials, it would be valuable to know whether the surface height irregularities are also influenced by the method of fracture.For this purpose we performed a large series of experiments with cement pastes.This material was chosen because its porosity can easily be controlled within a broad interval by means of the water-to-cement ratio r = w/c.Correct knowledge of the dependence of surface roughness on the fracturing method may be useful for further surface studies of fractured porous materials.

Experimental arrangement
Ordinary Portland cement CEM 42,5 I R-sc of domestic provenance was used to create 144 specimens of hydrated pastes with six different water-to-cement ratios r (0.3, 0.4, 0.5, 0.6, 0.7, 0.8).The specimens were rotated during hydration to achieve better homogeneity.All specimens were stored for the whole time of hydration at 100 % RH and 20 • C.After 90 days of hydration the specimens were fractured both in the static regime (three-point bending tests) and also in the dynamic regime (impulse fractures caused by a chisel and a heavy hammer).The fracture surfaces were then immediately used for microscopic analysis.
The three-dimensional profile parameter H a was used to characterize the roughness of the fracture surfaces of the hydrated cement pastes.In fact, H a represents the averaged 'absolute' height of the fracture relief z = f (x, y)

Results and discussion
It is well known that hydrated cement is a composite material consisting of several solid hydrated products and pore spaces (see the photo in Figure 2).Porosity (P ) influences most of the mechanical properties of this material, and it is therefore not surprising that the surface irregularity (H a ) was also found [1][2][3] to be among the dependent properties.
Since porosity to a large extent determines compressive strength, a strong functional relation between compressive strength σ c and surface irregularity of the fracture surfaces was also revealed where σ o , H o , h o and ρ are fitting parameters.In the present study, relation ( 2) is tested with specimens fractured dynamically using a sharp chisel and a heavy hammer that simulate an impulsive load.Figure 3 shows the resulting graph σ c (H a ).The dy-namical strength values in Figure 3 were evaluated on the basis of the static values by adding corrections corresponding to dynamic processes [4].As shown in Figure 3, the experimental points are fitted well by function (2), which means that this function may describe a universal behavior of the compressive strength and the height irregularities of the surfaces formed by both the static fracture processes [3] and the dynamic fracture processes.

Conclusion
The experiments have proved similar behavior of the height surface irregularities formed by static and dynamic fractures.The fast fracture process accomplished with the wedge-shaped chisel generates very similar graphs σ c (H a ) to those in the case of slow fracture processes.These results have been achieved using three-dimensional profile parameters H a evaluated on the basis of a reconstruction of the confocal surface.The graphs σ c (H a ) seem to be convenient candidates for calibration curves.
The properties of the surface irregularities of fractured cement pastes presented here may also be useful for morphological and structural studies of other porous materials.

Fig. 2 :Fig. 3 :
Fig. 2: A photograph of the surface of a hydrated cement paste