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Acta Polytechnica Vol. 51 No. 5/2011

Surface Morphology of Porous Cementitious Materials Subjected to
Fast Dynamic Fractures

T. Ficker

Abstract

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.

Keywords: 3D profile analysis, fracture surfaces, cement-based materials, confocal microscopy.

1 Introduction
We have recently published several studies [1–3] 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 materi-

als, it would be valuable to know whether the sur-
face height irregularities are also influenced by the
method of fracture. For this purposewe performed a
large series of experiments with cement pastes. This
material was chosen because its porosity can easily
be controlledwithin a broad interval bymeans of the
water-to-cement ratio r = w/c. Correctknowledgeof
the dependence of surface roughness on the fractur-
ing method may be useful for further surface studies
of fractured porous materials.

2 Experimental arrangement

Ordinary Portland cement CEM 42,5 I R-sc of do-
mestic provenance was used to create 144 specimens
of hydrated pasteswith six differentwater-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 ho-
mogeneity. 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 frac-
ture surfaces were then immediately used for micro-
scopic analysis.
The three-dimensional profile parameter Ha was

used to characterize the roughnessof the fracture sur-
faces of the hydrated cement pastes. In fact, Ha rep-
resents the averaged ‘absolute’ height of the fracture
relief z = f(x, y)

Fig. 1: 3D confocal relief of fractured cement paste

Ha =
1

L · M

∫ ∫
(LM)

|f(x, y)|dxdy (1)

where L × M is the area of the vertical projection
of the three-dimensional fracture profile f(x, y) into
the plane xy. The parameter Ha has great statistical
relevancy, since it is a global averaged characteris-
tic covering the entire tested surface L × M. The
three-dimensional profiles f(x, y) were created using
an Olympus Lext 3100 confocal microscope. One of
these profiles is shown in Figure 1. The profiles are
formed by software that processed a series of optical
sections created by the confocal microscope at vari-
ous heights of the fracture surfaces. Approximately
100 image sectionswere taken for eachmeasured sur-
face site, starting from the very bottom of the sur-
face depressions (valleys) and proceeding to the very
top of the surface protrusions (peaks). The investi-
gated area L × M =1280 μm×1280 μm (1024 pix-
els×1024 pixels) was chosen in five different places
of each fracture surface (in the center, and in fourpo-
sitions near the corners of the rectangular area), i.e.
each plotted point on the graphs of the profile pa-
rameters corresponds to an average value composed

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Acta Polytechnica Vol. 51 No. 5/2011

of 60measurements (12 samples×5 surfacemeasure-
ments). Each measurement was performed for mag-
nification 20×.

3 Results and discussion
It is well known that hydrated cement is a composite
material consisting of several solid hydrated prod-
ucts and pore spaces (see the photo in Figure 2).
Porosity (P) influencesmost of the mechanical prop-
erties of this material, and it is therefore not sur-
prising that the surface irregularity (Ha) was also
found [1–3] to be among the dependent properties.
Since porosity to a large extent determines compres-
sive strength, a strong functional relation between
compressive strength σc and surface irregularity of
the fracture surfaces was also revealed

σc(Ha)= σo

(
Ho

Ha − ho

)ρ
(2)

where σo, Ho, ho and ρ are fitting parameters.

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

Fig. 3: Compressive strength independence on theheight
irregularities of a surface formed in a dynamic fracture

In the present study, relation (2) is tested with
specimens fractured dynamically using a sharp chisel
andaheavyhammer that simulate an impulsive load.
Figure 3 shows the resulting graph σc(Ha). The dy-

namical strength values in Figure 3 were evaluated
on the basis of the static values by adding correc-
tions 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.

4 Conclusion
The experiments have proved similar behavior of the
height surface irregularities formed by static and dy-
namic fractures. The fast fracture process accom-
plished with the wedge-shaped chisel generates very
similar graphs σc(Ha) to those in the case of slow
fracture processes. These results have been achieved
using three-dimensional profile parameters Ha eval-
uated on the basis of a reconstruction of the confocal
surface. The graphs σc(Ha) seem to be convenient
candidates for calibration curves.
Theproperties of the surface irregularities of frac-

tured cement pastes presented here may also be use-
ful for morphological and structural studies of other
porous materials.

Acknowledgement

This work was supported by the Ministry of the
Czech Republic under Contract no. ME09046 (Kon-
takt).

References

[1] Ficker,T.,Martǐsek,D., Jennings,H.M.: Rough-
ness of fracture surfaces andcompressive strength
of hydrated cement pastes, Cem. Concr. Res. 40
(2010) 947–955.

[2] Ficker, T., Martǐsek, D., Jennings, H. M.: Sur-
face roughness and porosity of hydrated cement
pastes, Acta Polytechnica 51(2011), no. 3, 7–20.

[3] Ficker, T.: Fracture surfaces of porous materials,
Acta Polytechnica 51(2011), no. 3, 21–24.

[4] Ficker, T.: Quasi-static compressive strength of
cement-based materials, Cem. Concr. Res. 41
(2011) 129–132.

Prof. RNDr. Tomáš Ficker, DrSc.
Phone: +420 541 147 661
E-mail: ficker.t@fce.vutbr.cz
Department of Physics
Faculty of Civil Engineering
Brno University of Technology
Veveř́ı 95, 662 37 Brno, Czech Republic

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