Study of the Fracture Behavior of Mortar and Concretes with Crushed Rock or Pebble Aggregates

Abstract

The objective of this work was to compare the fracture energy of mortar and concretes produced with crushed rock and pebble aggregates using zero, 10, 20, 30 and 40% of aggregates mixed with standard mortar and applying the wedge splitting method to achieve stable crack propagation. The samples were cast in a special mold and cured for 28 days, after which they were subjected to crack propagation tests by the wedge splitting method to determine the fracture energies of the mortar and concrete. The concretes showed higher fracture energy than the mortar, and the concretes containing crushed rock showed higher resistance to crack propagation than all the compositions containing pebbles. The fracture energy varied from 38 to 55 J.m-2. A comparison of the number of aggregates that separated from the two concrete matrices with the highest fracture energies indicated that the concrete containing pebbles crumbled more easily and was therefore less resistant to crack propagation.

Full text

*e-mail: sebastiao@demar.eel.usp.br
Study of the Fracture Behavior of Mortar and Concretes with
Crushed Rock or Pebble Aggregates
Sebastião Ribeiro*, Diego de Campos Ribeiro, Mateus Botani de Souza Dias,
Giseli Cristina Ribeiro Garcia, Ésoly Madeleine Bento dos Santos
Department of Materials Engineering – DEMAR, Lorena School of Engineering – EEL,
University of São Paulo – USP, Estrada Santa Lucrecia s/n, Bairro Mondezir,
CP 116, CEP 12600-970, Lorena, SP, Brazil
Received: September 8, 2010; Revised: December 23, 2010
The objective of this work was to compare the fracture energy of mortar and concretes produced with crushed
rock and pebble aggregates using zero, 10, 20, 30 and 40% of aggregates mixed with standard mortar and applying
the wedge splitting method to achieve stable crack propagation. The samples were cast in a special mold and
cured for 28 days, after which they were subjected to crack propagation tests by the wedge splitting method to
determine the fracture energies of the mortar and concrete. The concretes showed higher fracture energy than
the mortar, and the concretes containing crushed rock showed higher resistance to crack propagation than all the
compositions containing pebbles. The fracture energy varied from 38 to 55 J.m
–2
. A comparison of the number
of aggregates that separated from the two concrete matrices with the highest fracture energies indicated that the
concrete containing pebbles crumbled more easily and was therefore less resistant to crack propagation.
Keywords: portland cement, mortar, concrete, crushed rock, pebbles
1. Introduction
Concrete, the second most widely used material worldwide,
can be defined as a macroscopically heterogeneous material whose
properties depend on its phases, on their relationship to each other,
and on its constituents
1-8
.
The materials commonly used for the production of concretes
are Portland cement, sand, aggregates, additives and water. Each of
these components has a specific function
3,6,7,9
.
Hardening of concretes take place in the curing stage, which is
very important from the structural standpoint since this is the stage
when the interactions among the initial and final constituents of the
concrete are defined. Concrete curing time is variable, but at seven
days of age, normal concretes usually reach 80% of the final strength
they will have achieved after 28 days
10
.
Concretes are materials with complex structures composed of
several starting materials, which are also complex, which makes
them difficult to understand, e.g., the problem of interactions between
matrix/aggregates, the region between the aggregate and the matrix,
the size and quantity of aggregates, and so forth
1,2,8,9
. For example,
the size and amount of aggregates have been shown to influence the
fracture energy and fracture toughness of concretes
8
.
The matrix can be constituted of different materials in variable
quantities which can modify their properties, e.g., the cement/water
ratio, cement/sand ratio, among others. The same applies to the
aggregate, which may consist of pebbles, which is basically SiO
2
,
with smooth rounded surfaces, or of crushed rock with irregular
shapes and with varied chemical and mineralogical compositions and
highly rough surfaces. The shape and texture of aggregates have an
important effect on the interlocking between mortar and aggregate
9-14
.
Depending on the properties of their raw materials, concretes
may present a strong matrix and aggregate, a strong matrix and weak
aggregate, or a weak matrix and strong aggregate. Lastly, in any
of the aforementioned situations, there is the interfacial transition
zone, ITZ – the region between the aggregate and the matrix, which
is normally weaker where the probability of a crack surrounding an
aggregate is high
9,12,15,16
.
Depending on the pathway of the crack, aggregates may display
two basic behaviors: one, in which the aggregates are pulled out of the
matrix, and the other, in which the aggregates become fractured. This
behavior can be evaluated on the two fracture surfaces of a sample
subjected to the stable crack propagation test
12
.
When the crack propagates in the ITZ, which is considered
weak in relation to the matrix, and the aggregates are fairly strong,
they are dislodged from the matrix rather than fractured, with little
consumption of energy. This interfacial region may even display
points where the aggregate/matrix interface is completely detached.
The opposite may also occur, since the aggregates may be strongly
bonded to the mortar, becoming fractured during the crack opening
process
1,2,12
.
To study the fracture behavior of the mortar and concretes, the
wedge splitting method was used to produce stable crack propagation
in the sample, which is a necessary situation for calculating fracture
energy
17-25
. The wedge splitting method was patented in 1986 by
Tschegg
26
and since then numerous studies have focused on improving
this test, which is currently performed with well designed and accurate
devices, as well as highly accurate testing machines. This favors
the stable propagation of the crack, a thermodynamic condition
of transformation of elastic energy stored in the testing machine
and the sample into surface energy per unit of newly formed crack
surface
5,27-32
. Fracture energy, which represents the energy per unit
of the fracture area of mortars and concretes, can be calculated by
the following equation
5,19,21,23,24,27,31-33
:
DOI: 10.1590/S1516-14392011005000004
Materials Research. 2011; 14(1): 46-52
© 2011

Study of the Fracture Behavior of Mortar and Concretes with Crushed Rock or Pebble Aggregates
1
2
wof
Pds
A
γ=
∫
(1)
where A is the projected area of the fracture surface, P is the vertical
load applied by the testing machine, and s is the displacement of the
machine’s actuator. The value of the integral Pds, which is determined
by integrating the area under the load-displacement curve, indicates
the total work of fracture.
The fracture energy results reported in the literature vary
significantly because the authors of these studies used different
compositions, raw materials, preparation conditions and methodologies
to measure the materials’ properties
3,5,8,10,12,13,14,16
. The results may vary,
for example, from 10 to 200 J.m
-2
.
2. Materials and Methods
2.1. Materials
The materials used here were crushed rock with a maximum
size of 9.5 mm and minimum size of 4.75 mm, pebbles in the same
granulometric range, washed medium-grained sand with a mean size
of 500 µm, maximum size not exceeding 2800 µm and minimum
53 µm, Portland CPII-E-32 cement manufactured according to the
Brazilian NBR 11578/1991 standard, and potable water. These
aggregates have different shapes and surfaces. Crushed rock is
irregular with rough surfaces while pebbles are rounded and have
smooth surfaces. Crushed rock aggregate consists of a mineral
complex whose main minerals are quartz, feldspars, cordierite
and anortite. Pebbles are composed only of SiO
2
in the form of
quartz mineral. The mineral compositions of these aggregates were
determined by X-ray diffraction, applying 2θ from 10 to 90°.
2.2. Molding of mortar and concrete samples
Initially, a mortar was produced with a composition of 1:2
in weight of cement:sand and water, with a quantity of water of
0.46 relative to the weight of cement. This composition was chosen
(1:2 rather than 1:3) to work with a mortar more resistant to crack
propagation. The mortar was cast in a special mold to produce notch
and grooves on the samples. The samples were demolded 24 hours
after casting.
The above described mortar was used for the production of the
concrete samples by adding crushed rock or pebbles as aggregates, in
proportions of 10, 20, 30 and 40% in weight. Before the aggregates
were added to the mortar, they were wetted and drained to prevent
them from consuming water or contributing water to the mortar. For
each material (mortar, concrete with crushed rock and concrete with
pebbles), six samples with dimensions of 100 × 100 × 75 mm were
used to measure the fracture energy, see Figure 1.
2.3. Curing of the samples
All the samples (mortars and concretes) were cured for 28 days at
25 °C in a chamber with a moisture-saturated atmosphere. After two
days of curing, the samples were subjected to stable crack propagation
tests to study the fracture of the three materials: mortar, concrete with
crushed rock, and concrete with pebbles.
Figure 1 shows a mortar specimen representative of all the
samples used in this work, ready to be subjected to the stable crack
propagation test by the wedge splitting method.
2.4. Crack propagation tests by the wedge splitting method
The mortar and concrete samples were subjected to stable crack
propagation tests using an MTS model 810M universal testing
machine operating at an actuator speed of 30 µm/min. Load vs.
displacement curves were built based on the load and displacement
data of the load cell. Loads were measured using a 5 kN load cell
with 5 N resolution, model MTS 661–19F-01. The software programs
used to control the test were TestStar-790.00, version 4.0E and
TestWare-SX, version 4.0D. The MTS actuator was operated with
a displacement of 0.03 mm/min. The work of fracture was then
determined based on these curves, using Origin Pro 7.5 software,
while the fracture energies were calculated from the projected fracture
area of the specimens, using Equation 1. Total, dislodged, fractured
and mixed aggregates were counted on the surfaces of the specimens
using a stereoscopic microscope.
2.5. Structures of the samples
The structures of the three materials were examined using a
LEIKA DM IRM optical microscope equipped with LEIKA QWIN
Stander imaging software, a LEO model 1450 VP scanning electron
microscope, and conventional photographs of polished surfaces and
fracture surfaces. In addition to the microstructural analyses, chemical
analyses were carried out by energy dispersive spectrometry (EDS).
The behavior of the aggregate and matrix (detached, fractured and
mixed) was evaluated using a QUIMIS model Q740Z-TR stereoscopic
microscope coupled to a KODO KC-512DN Color Camera.
3. Results and Discussion
Figure 2 shows the general structure of two concrete samples,
one with pebbles and the other with crushed rock (a and b). These
micrographs show coarse pebble aggregates (a) and crushed rock
aggregate (b), and fine sand grains, as well as the cement matrix
with finer particles. Both cases reveal a heterogeneous and complex
microstructure in which the coarse aggregates are embedded in the
mortar matrix.
Figures 3 and 4 depict samples after the stable crack propagation
test by the wedge splitting method. The fractured parts of the sample
represent the behavior of the matrix and the aggregates when the
crack runs through the entire section, indicating when the aggregate is
completely detached from one of the sides, and showing the respective
Figure 1. Photograph of the mortar sample prior to the stable crack propagation
test by the wedge splitting method.
2011; 14(1)
47

Ribeiro et al.
Figure 2. Backscatter SEM micrograph of a concrete containing: a) pebbles; and b) crushed rock.
Figure 3. Photograph of a sample of concrete containing pebble aggregate after the fracture test by the wedge splitting method.
hole on the other side. Also clearly visible are the two parts of the
fractured aggregate, with its respective fractions held in the matrix.
This mixed behavior occurs when part of the aggregate is partially
fractured and part of it is detached from one side, remaining trapped
at the other side.
Table 1 illustrates the behavior of the pebble and crushed rock
aggregates in the concretes under study. At least five fractured samples
of each composition were used to count the aggregates, based on
the fractured surfaces illustrated in Figures 3 and 4. The detached,
fractured and mixed aggregates were counted using a stereoscopic
microscope.
An evaluation of the results listed in Table 1 indicates that,
in all the compositions, the number of aggregates detached from
the concrete containing pebbles is much larger than from the one
containing crushed rock, and that the opposite applies to the fractured
Table 1. Behavior of the aggregates in concrete containing crushed rock
aggregate and in concrete containing pebble aggregates.
Concrete
(identification)
Behavior of the aggregates
Fractured (%) Detached (%) Mixed (%)
Crushed
rock
10 60.92 23.12 15.96
20 63.19 22.49 14.41
30 55.78 27.64 16.58
40 63.48 27.42 9.10
Pebble
10 21.35 56.17 22.48
20 20.67 54.47 24.86
30 21.52 55.63 23.18
40 26.47 57.94 15.59
48
Materials Research

Study of the Fracture Behavior of Mortar and Concretes with Crushed Rock or Pebble Aggregates
Figure 4. Photograph of a sample of concrete containing crushed rock aggregate after the fracture test by the wedge splitting method.
Figure 5. Optical microscopy images of the fracture surface of the concrete with pebbles: a) detail of the aggregate dislodged from the matrix; and b) hole left
behind by the dislodged aggregate.
aggregates, where the concrete with crushed rock shows higher
values. The reason for this behavior is quite obvious when one
evaluates the aggregate-matrix interactions, especially considering
roughness. This property provides good anchorage, favoring a better
interaction between the crushed rock and the matrix when compared
with the pebble-matrix interaction, as can be seen by comparing the
interfacial regions in Figure 2a and b. The path of the principal crack
deflects from the surface of the pebble, thus requiring less energy to
propagate than cracks in crushed rock. Therefore, this justifies the
higher resistance to crack propagation in the concretes with crushed
rock than in those with pebbles, as will be discussed below.
Figure 5 illustrates the behavior of the pebble aggregate when it
is dislodged from the matrix, showing a smooth surface in (a). In (b),
note the hole left by the dislodged pebble. This is a clear indication
that the aggregate/matrix interface was weaker than the aggregate,
allowing the aggregate to become dislodged rather than fractured.
Figure 6 shows the crushed rock aggregate detached from the
matrix (a), and the region from which the respective aggregate was
detached, called the “hole” (b). In this figure, note that the detached
aggregate does not have a smooth surface like that of a pebble and
that it appears encrusted by the material of the matrix, Figure 6a,
indicating that the interaction between aggregate and matrix is
stronger in concretes with crushed rock than in concretes with pebbles.
Note, also, that the surface of the hole from which the aggregate
was detached is not as smooth as the one shown in Figure 5b, which
represents a hole produced by a dislodged pebble in the concrete
containing pebble aggregate.
Figure 7 shows three representative load-displacement curves
chosen randomly among the tested samples: a) mortar 1:2; b) concrete
with pebbles; and c) concrete with crushed rock.
The curves show stable crack propagation behavior, indicating
that elastic deformation occurs as the load increases until the
2011; 14(1)
49

Ribeiro et al.
Figure 6. Optical microscopy images of the fracture surface of the concrete with crushed rock: a) detail of the aggregate detached from the matrix; and b) hole
left behind by the detached aggregate.
Figure 7. Representative load-displacement curves of the samples of a) mortar; b) concrete with pebbles; and c) concrete with crushed rock.
Figure 8. Fracture energy of the 1:2 mortar and of the concretes containing
10, 20, 30 and 40 wt. (%) of pebbles or crushed rock.
maximum load is reached, whereupon this deformation begins to
diminish, indicating a damping behavior. Also note the three distinct
regions in these curves, one of them indicating the elastic deformation
of the samples (region 0-a), another characterized by the onset and
growth of the crack (region a-b), and lastly the region of crack
propagation (region b-c), characterized by damping of the curve. The
behavior and characteristics of these curves depend on each material,
and provide inputs for a perfect evaluation of the fracture behavior of
concretes and mortars. It is important to note the differences between
the two concretes. The maximum load of the concrete with pebbles is
lower than that of the concrete with crushed rock. However, the two
concretes show opposite displacement behavior, indicating that the
concrete containing crushed rock is stiffer than that containing pebble,
but less resistant to crack propagation. This behavior is illustrated
in Figure 7b and c, which shows a displacement of approximately
4.5 mm for concrete with pebbles and of 2.8 mm for concrete with
crushed rock.
Figure 8 shows the fracture energy of the three materials of this
study. The mortar was used for purposes of comparison, since it was
employed to produce the concretes with added aggregates.
Upon examining Figure 8 one can see that the fracture energy
of the mortar is lower than that of the concretes. The addition of
50
Materials Research

Study of the Fracture Behavior of Mortar and Concretes with Crushed Rock or Pebble Aggregates
aggregates increases the fracture energy, causing the concretes to
present a certain degree of reinforcement due to the binding effect
of the aggregates in the matrix.
In both concretes, the fracture energy increased in proportion to
the quantity of aggregates added to them. These results are consistent
with other findings reported in the literature, which indicate that the
fracture energy increases with the quantity of aggregate
8,14
.
A comparison of the values of fracture energy for the two types
of aggregate reveals that concrete containing crushed rock showed
higher fracture energy. This finding is consistent with the literature
8,14
.
The higher fracture energies of concretes with crushed rock can be
attributed to the greater interaction between the rough aggregates
and the matrix, which did not allow for the formation of regions of
weaker binding with the mortar. Other mechanisms that spend energy
may also occur during the fracture of the crushed rock aggregate.
However, this did not occur with the concrete containing pebbles,
in which the aggregate-matrix interaction differed considerably
because the smoother surface of the aggregates did not provide
effective anchoring. This produces a zone of low adherence and hence
lower resistance to crack propagation, translating into lower fracture
energies. These statements are supported by the results shown in
Figures 3 and 4, in Table 1, and in the literature
8,12,13,14
.
4. Conclusions
Concretes with crushed rock as aggregate are more resistant to
crack initiation but less resistant to crack propagation than concrete
produced with pebbles as aggregate, considering aggregates in same
size range. Less energy is consumed in elastic deformation than in
crack propagation in concrete. This behavior is more enhanced in
concrete containing pebble aggregates.
Due to their irregular and rougher surface and other possible
mechanisms that consume the energy of crushed rock aggregate,
concretes containing crushed rock have higher fracture energy than
those containing pebble aggregates.
Due to their smoother surface and therefore weaker anchorage,
pebble aggregates present areas dislodged from the matrix, leading to
lower values of fracture energy, since the crack follows these regions
without consuming energy.
The physical structure of aggregates is also important, since
crushed rock aggregates show internal cracks due to the compressive
forces applied during the crushing process, while pebbles are bodies
that shift back and forth by the action of water, causing them to collide
with one another and form cracks. This characteristic of aggregates
can influence the properties of concrete.
Although the concretes containing pebbles exhibited lower values
of fracture energy in the compositions studied here than in those
containing crushed rock, they are applicable in civil construction.
The important point is to know the resistance values of concretes
with pebble aggregates in order to employ them in specific cases.
Acknowledgements
The authors gratefully acknowledge the financial support of this
work by FAPESP (Fundação de Amparo à Pesquisa do Estado de
São Paulo) through process 2007/55964-3, and by CNPq (Conselho
Nacional de Desenvolvimento Científico e Tecnológico) through a
Research Productivity Grant, process 302387/2007-2.
References
1. Stroeven P. A stereological approach to roughness of fracture surfaces and
tortuosity of transport paths in concrete. Cement and Concrete Composite.
2000; 22:331-341.
2. Sadowski T and Golewski G. Effect of aggregate kind and graining on
modeling of plain concrete under compression. Computational Materials
Science. 2008; 43:119-126.
3. Xiao J, Schneider H, Dönnecke C and König G. Wedge splitting test
on fracture behaviour of ultra high strength concrete. Construction and
Building Materials. 2004; 18:359-365.
4. Rosselló C and Elices M. Fracture of model concrete: 1. Types of fracture
and crack path. Cement and Concrete Research. 2004; 34:1441-1450.
5. Ribeiro S, Santos EMB, Garcia GCR and Rodrigues JA. Elastic work
and fracture energy of concretes made with crushed stones and pebbles
aggregates. Materials Science Forum. 2010; 636-637:1215-1221.
6. Wu KR, Chen B, Yao W and Zhang D. Effect of coarse aggregate type
on mechanical properties of high-performance concrete. Cement and
Concrete Research. 2001; 31:1421-1425.
7. Guinea GV, El-Sayed K, Rocco, CG, Elices M and Planas J. The effect
of the bond between the matrix and the aggregates on the cracking
mechanism and fracture parameters of concrete. Cement and Concrete
Research. 2002. 12:1961-1970.
8. Chen B and Liu J. Effect of aggregate on the fracture behaviour of high
strength concrete. Construction and Building Materials. 2004; 18:585-
590.
9. Prokopski G and Halbiniak J. Interfacial transition zone in cementitious
materials. Cement and Concrete Research. 2000; 30:579-583.
10. Nilica R and Harmuth H. Mechanical and fracture mechanical
characterization of building materials used for external thermal insulation
composite systems. Cement and Concrete Research. 2005; 35:1641-1645.
11. Donza H, Cabrera O and Irassar EF. High-strength concrete with different
fine aggregate. Cement and Concrete Research. 2002; 32:1755-1761.
12. Eleces M and Rocco CG. Effect of aggregate size on the fracture and
mechanical properties. Engineering Fracture Mechanics. 2008; 75:3839-
3851.
13. Rocco CG and Elices M. Effect of aggregate shape on the mechanical
properties of a simple concrete. Engineering Fracture Mechanics. 2009;
76:286-298.
14. Amparano FE, Xi, Y and Roh, Y-S. Experimental study on the effect of
aggregate content on fracture behavior of concrete. Engineering Fracture
Mechanics. 2000; 67:65-84.
15. Lothenbach B, Le Saout G, Gallucci E and Scrivener K. Influence of
limestone on the hydration of Portland cements. Cement and Concrete
Research. 2008; 38:848-860.
16. Yan A, Wu K-R, Zhang D and Yao W. Effect of fracture path on the fracture
energy of high-strength concrete. Cement and Concrete Research. 2001;
31:1601-1606.
17. Barinov SM. The work-of-fracture of brittle materials: Principle,
determination, and applications. Journal Material Research; 1994;
6(9):1412-1424.
18. Hillerborg A, Modéer M and Peterson P-E. Analysis of crack formation
and crack growth in concrete by means of fracture mechanics and finite
elements. Cement and Concrete Research. 1976; 6:773-781.
19. Harmuth H. Stability of crack propagation associated with fracture
energy determined by wedge splitting specimen. Theoretical and Applied
Fracture Mechanics. 1995; 23:103-108.
20. Sakay M and Bradt RC. Graphical methods for determining the nonlinear
fracture parameter of silica and graphite refractory composites. In:
Fracture mechanical of ceramics. New York: Publishing Corporation;
1986. p. 127-142.
21. Sakai M and Bradt RC. Fracture toughness testing of brittle materials.
International Materials Reviews. 1993; 2(38):53-78.
22. Nakayama J. A bending method for direct measurement of fracture energy
of brittle material. Journal Applied Physics. 1964; 3:422-423.
23. Nakayama J. Direct measurement of fracture energies of brittle
heterogeneous materials. Journal of the American Ceramic Society. 1965;
18(11):583-587.
2011; 14(1)
51

Ribeiro et al.
29. Tschegg EK, Fendt KT, Manhart C and Harmuth H. Uniaxial and biaxial
behaviour of refractory materials. Engineering Fracture Mechanics. 2009;
76:2249-2259.
30. Auer T, Manhart C and Harmuth H. Contributions to refractory fracture
mechanical and fractographic investigations. RHI Bulletin. 2006; 1:38-41.
31. Larson DR, Coppola JA and Hasselman DPH. Fracture toughness and
spalling behavior of high-Al
2
O
3
refractories. Journal of the American
Ceramic Society. 1974; 57(10):417-421.
32. Ribeiro S and Rodrigues JA. The influence of microstructure on the
maximum load and fracture energy of refractory castables. Ceramics
International. 2010; 36:263-274.
33. Chou C-C, Cheng I-L, Chen K-J and Ko Y-C. Fracture energy and thermal
stress resistance parameter of high alumina brick. Ceramic Bulletin. 1986;
65(7):1042-1046.
24. Nakayama J, Abe H and Bradt RC. Crack stability in the work-of-fracture
test: Refractory application. Journal of the American Ceramic Society.
1981; 64(11):671-675.
25. Harmuth H, Rieder K, Krobath M and Tschegg E. Investigation of the
nonlinear fracture behaviour of ordinary ceramic refractory materials.
Materials Science & Engineering A. 1996; 214:53-61.
26. Tschegg E. Republik Österreich. Patent number 390328B; 1986.
27. Harmuth H and Tschegg EK. Fracture mechanical characteristic of
ordinary ceramic refractory materials. Veitsch-Radex Rundschau. 1994;
2(1):465-479.
28. Trunk B, Schober G, Helbling AK and Wittmann FH. Fracture mechanics
parameters of autoclaved aerated concrete. Cement and Concrete
Research. 1999; 29:855-859.
52
Materials Research