Nanomechanical Properties of Interfacial
Transition Zone in Concrete
P. Mondal, S.P. Shah, and L.D. Marks1
Abstract. This research provides better understanding of the nanostructure and the
nanoscale local mechanical properties of the interfacial transition zone (ITZ) in
concrete. Nanoindentation with in-situ scanning probe microscopy imaging was
used to compare the properties of the bulk paste with the properties of the ITZ between paste and two different types of aggregates. ITZ was found to be extremely
heterogeneous with some areas as strong as the bulk matrix. Higher concentration
of large voids and cracks along the interface was observed due to poor bonding.
Nanoindentation results on relatively intact areas of the interface disagreed with
the notion of increasing elastic modulus with distance from the interface. Depending on the aggregate type, average modulus of the ITZ was 70% to 85% of the average modulus of the paste matrix. The main problem the ITZ poses on the overall
mechanical properties of concrete was concluded to be due to extreme heterogeneity within the interface and poor bonding between aggregate and paste. It was
noted that the connectivity of the weaker areas such as large voids and cracks
along the interface governs failure.
1 Introduction
In concrete, paste matrix works as the glue that holds aggregates together to behave as a whole. However, this composite action depends on a thin interface layer
(ITZ) that exists between the aggregates and the paste matrix. This is a region of
gradual transition of properties, where the effective thickness of the region varies
with the microstructural feature being studied, and with degree of hydration [1]. In
many studies, it was concluded that in ordinary Portland cement concrete, the ITZ
P. Mondal
University of Illinois at Urbana Champaign, IL, USA
S.P. Shah
ACBM Center, Northwestern University, IL, USA
L.D. Marks
Northwestern University, IL, USA
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P. Mondal et al.
consists of a region up to 50 μm around each aggregate with fewer unhydrated
particles, less calcium-silicate-hydrate, higher porosity, and greater concentration
of calcium hydroxide and ettringite.
Simeonov et al. [2] reported the influence of the ITZ on the overall elastic
properties of mortar and concrete. In normal strength concrete, it is considered to
be the weakest link in the mechanical system [1]. Although it is widely accepted
that the properties of the ITZ have to be taken into account in modeling the overall
mechanical properties of concrete [2-5], it is difficult to determine its local mechanical properties because of the complexity of the structure and the constraints
of the existing measurement techniques [6]. Most of the time, the modulus of the
ITZ is assumed to be uniform and less than that of the paste matrix by a constant
factor. This factor is assumed to have a value between 0.2 and 0.8, although there
is not enough theoretical or experimental evidence to support this assumption [5].
In some recent studies, attempts were made to determine the local mechanical
properties of the ITZ using microindentation or microhardness testing [7-9]. Asbridge et al. reported about 20 percent reduction in Knoop microhardness in the
ITZ than the bulk matrix at w/c of 0.4 and 0.5 [9], however similar Knoop microhardness at w/c 0.6. In their study, the width of indentation was reported as 10-15
μm, which is comparable with the width of the ITZ itself. Therefore, though microindentation revealed some information, effects of adjacent phases on hardness
results make microhardness test not suitable for determining the local properties of
the ITZ. Furthermore, there is still very little information available about the nanoscale mechanical properties of the ITZ. Nanoindentation has been used successfully in the recent past to investigate the local mechanical properties of cement
paste and concrete [10-13]. This study is one of the first efforts which strive to
characterize the nanomechanical properties of the ITZ using nanoindentation.
2 Experimental Details
Since different factors such as w/c ratio, age of sample, aggregate type and size affect properties of the ITZ, it was decided to keep most of them constant and vary
one or two factors at a time: w/c ratio and age of the sample were respectively 0.5,
and 1 month. Two types of model concrete samples were made, one with round
gravel and the other one with limestone aggregates. Samples were cured under
water at 25° C. For nanoindentation, samples were prepared following the method
described in a different paper by the authors [12]. Figure 1 shows scanning
electron microscopy image of polished model concrete sample with gravel. In
most of the areas, there were large voids adjacent to an aggregate or a crack running along the interface. This proves weak bonding between aggregates and cement paste. Even in areas with no obvious interfacial zone (no voids or cracks),
fewer unhydrated cement particles were found. Using a Hysitron Triboindenter,
nanoindentation was done away from large voids and cracks. This is due to the
practical constraints that nanoindentation can not be done on such a large void or a
Nanomechanical Properties of Interfacial Transition Zone in Concrete
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Fig. 1 Scanning Electron Microscopy Images of Different Areas of the Interfacial Transition Zone in the Model Concrete Sample
crack. In addition, these features are associated with the bonding issues between
aggregates and paste rather than nanoscale properties of materials present in ITZ.
AFM like imaging capability of the Triboindenter was indispensable to find the
narrow area of the interface and position the indenter.
3 Results and Discussion
Figure 2 shows the SEM image of cement paste adjacent to gravel and results of
nanoindentation on the same area. From the SEM image (Figure 2 (a)), no obvious
differences in properties were found at the interface. To determine nanomechanical properties, exactly the same area was identified using the AFM imaging mode
of the indenter. Indentation was performed on the selected locations as shown in
Figure 2 (b). Figure 2 (c) shows the modulus in GPa obtained from each indent.
No difference in the modulus was observed in this area adjacent to the gravel.
In a different area of the same sample, cement paste close to the gravel showed
higher porosity in SEM and AFM image. Local elastic modulus determined from
nanoindentation on this area reflected the effect of higher porosity. The indentation modulus obtained was lower than the previous case. The average modulus in
the interfacial zone was found to be 85% of that of the paste matrix. This is close
to the value assumed by a few resent researchers for modeling purposes [5]. The
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Fig. 2 (a) Scanning Electron Microscopy Image of ITZ, (b) 60 μ × 60 μ Image of the ITZ
between Paste and Gravel Showing Indent Locations and Indentation Modulus in GPa
Written on each Indent Locations
modulus obtained from indentation all around one gravel plotted against distance
from the gravel showed no trend of increasing modulus of cement paste with increasing distance from the gravel. This is in contrast to what has been reported by
Zhu et al. [8], although one has to keep in mind that Zhu et al. used microindentation to study the ITZ between paste and steel bar.
To eliminate any uncertainty that might be present due to the mixed chemical
composition of round gravel, limestone aggregates were used for further study. To
determine the variation of modulus with distance from limestone, the experiment
was repeated on eight different locations around the same limestone aggregate.
Fig. 3 Comparison between the Modulus of the ITZ and the Bulk Paste
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Analyzing results of over 200 indents, no trend of increasing modulus with distance from aggregate was found. Figure 3 shows a comparison between the distributions of modulus obtained from nanoindentation on the interfacial zone with
modulus of bulk cement paste. Comparisons showed that the average modulus of
the ITZ is 30% lower than the average modulus of the bulk cement paste. Furthermore, the percent of data within the modulus range of 5 to 15 GPa is much
higher in the case of the ITZ compared to that of the bulk paste. This implies
higher porosity in the interfacial zone.
4 Conclusions
Nanoindentation with imaging was proved to be indispensable to identify the narrow area around aggregate and position the indenter in the same area. Interfacial
transition zone was found to be extremely heterogeneous with some areas as
strong as the bulk matrix. A higher concentration of large voids and cracks along
the interface was observed due to poor bonding. Nanoindentation was performed
on relatively intact areas of interface. Results from this study disagree with the notion of increasing elastic modulus with distance from the interface. However, results show higher porosity in the ITZ and the average modulus of the ITZ is 70%
to 85% of the average modulus of the bulk paste depending on the aggregate type.
Still, extreme heterogeneity within the interface and poor bonding between the aggregate and paste remain as the main problem that affects the overall mechanical
properties of concrete. It is important to note that the connectivity of the weaker
areas such as large voids and cracks along the interface will govern failure. Thus,
modeling concrete as three phase material considering the ITZ as a weak zone
around aggregate with average property some percentage less than the paste matrix may not be sufficient to predict overall strength.
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