INFLUENCE OF CONCRETE’S MINERALOGICAL COMPONENTS ON
FRACTURE COMPRESSIVE AND TRACTIVE
M.P.Morales Alfaro and F.A.I.Darwish
Rua Passo da Pátria 156-3 andar – Sala 365 - bloco “D”
São Domingos – Niterói - RJ Brasil – CEP 24210-020
ABSTRACT
The present work represents an approach to study the fracture behavior in concretes of standard, medium and high
compressive strength, tested under compressive and tensile loads. Tensile loading was carried out by diametrical compression
and also in a direct way using notched short rod cylindrical specimens. The study was focused on analyzing the problem of
crack initiation and propagation in light of the water-cement ratio as well as the physical characteristics of the aggregates used
in producing the concrete. It was conducted by classification for concrete according to its characteristic compressive strength
at 45 days, for standard and medium level of compressive strength’s concrete were molded specimens by ratio water-cement
0,5 and 0,6. For high strength concrete was used 0,36. Each group of concretes molded had three different geological origin
coarse aggregate so its physical and mineralogical influence on the concrete’s microstructure could be analyzed by scanning
electronic microscopy of the surfaces failure by compressive and tensile tests. The conclusions obtained for standard
compressive strength concretes did not show an important influence of the mineralogical aggregate’s composition in the
increment of 0,5 to 0,6 ratio’s water-cement on the compressive, tensile strengths and fracture toughness. On medium
compressive strength’s concretes an increase on fracture toughness to increase the ratio water-cement 0,5 to 0,6 and a
significant influence of the mineralogical composition of the aggregates in compressive and tensile strengths, were observed.
About the high strength concrete the strong matrix composed by additive and silica fume resulted as the principals influenced
in highest results of compressive, tensile and fracture toughness proprieties. The influence of the aggregates was not
significantly noted.
Introduction
Diverse specialties as Geology, Chemistry, and Civil Engineering, are responsible for the selection of materials utilized to
produce and prepare construction materials. The isolated knowledge between who manufactures and who constructs, leads, in
general, to control its quality for its mechanical behavior without knowledge of external properties of the mineralogical
components of the matrix that conforms them. There are few researches about the mineralogical influence of concrete
components on the mechanical properties, could not explain many mechanical behaviors. Said researches have generated a
controversy since 1964 in relation to the reason for the effect of water/cement in the concrete fracture toughness.
According to Dos Santos (1998, p.16) [1]:
"As well as Peterson (1980), one concludes that the value of the relation water-cement inversely influences of proportional
form the tenacity to the fracture of the concrete, or either, when the relation is increased water/cement the tenacity to the
fracture diminishes, this conclusion opposes the position of LOTT and KESLER (1964)".
The objective of this research is to explain the apparent two-way of the water-cement ratio, which influences the fracture
toughness. The procedure for the problem analysis was involved using two water-cement ratios in, standard and medium
strength concrete, one of each. Also for each group produced were used three different coarse aggregates to discover if it
variably changes the water-cement ratio’s actuation. High strength concrete had just one water-cement ratio. The tests were
done by direct and diametrical compressive tensile loads, and simple compressive load. To reinforce the conclusions obtained
by the mechanical tests, some of the surfaces of failure by the three tests done were observed by electronic microscopy.
Fracture Mechanics of Concretes
The concrete is an almost-fragile material due to formation of micro fissures in its microstructure its hardening process as
consequence of thermal changes that happen when the respective chemical reactions take place. These micro fissures are not
visible externally. The theory of Fracture Mechanics was formulated by Griffith (1920) and applied to concretes since 1961 by
Kaplan. The concrete fracture toughness is calculated by application the theory of Linear Elastic Fracture Mechanics (LEFM)
based on the application of the theory of elasticity to bodies containing cracks or defects. There are three modes of loading,
which involve different crack surface displacements. Mode I, or tensile mode is opening the crack faces are pulled apart. Mode
II, is sliding or in-plane shear (the crack surfaces slide over each other) and mode III is tearing or anti-plane shear (the crack
surfaces move parallel to the leading edge of the crack and relative to each other).
Irwing (1940-1950), postulated that the energy due to plastic deformation must be added to the surface energy associated with
the creation of new crack surfaces. He studied on ductile materials; the surface energy term is often negligible compared to the
energy associated with plastic deformation. Further, he defined a quantity, G, the strain energy release rate or "crack driving
force," which is the total energy absorbed during cracking per unit increase in crack length and per unit thickness. Then
showed that the local stresses near the crack tip are of the general form [1]
 ij 
KI
2 r
f i j   +…
(1)
where r are cylindrical coordinates of a point with respect to the crack tip,  = cylindrical coordinates of a point with respect to
the crack tip and Ki = stress intensity factor.
It showed that the energy approach (the "G" approach above) is equivalent to the stress intensity approach and that crack
propagation occurs when a critical strain energy release rate, G, (or in terms of a critical stress intensity, Kc) is achieved.
In this research when applied, the mode I of the fracture mechanics of concrete theory and the stress intensity factor KIQ,
resulted in reaching the value of critical stress intensity factor Kc. The equation to use is determinate by ISRM (1988) [2]
K IQ 
24 Fc
B
3
(2)
2
where KI Q is an apparent stress intensity factor, Fc is a maximum load and B is a specimen’s diameter.
Experimental
Compressive and tensile by diametric compression strengths were tested using cylindrical specimens with 150 mm of diameter
and 300 mm of high. To determinate the concrete fracture toughness was used mode I (Figure 1), it carried tension-tension
loads by displacement-controlled tests on the specimens by Instron 5500 R machine; the velocity of transversal displacement
was 1mm/min registered the maximum load in this condition. The procedure was based on the methodology developed by
Santos (1998) [1] to held the specimens at the machine (Figure 2) and as use the recommendations of the ISRM (1988) [2],
which defined the geometrics dimensions of specimens in rocks but was applied to concrete materials using notched short rod
cylindrical specimens. The geometrics dimension (Figure 3); were tested with mode II by deformation control. Some
specimens were prepared and tested by a methodology developed by Hanson  Ingrafea (1999) [3] to determinate concrete
fracture toughness based in a flexion tests at three points by specimens of diameter 150 mm and high 300 mm (Figure 4).
Due to high loads of the Hanson  Ingrafea’s methodology, it was not possible to apply it. However, it presented better way to
prepare the specimens and obtain the maximum load. Santos’ methodology to prepare specimens needs some evolution
because it did not lose many specimens at time of preparation and when tested, it reached the maximum load with low
machine’s efforts. The conclusions about these two methodologies to determinate concrete fracture toughness agree with a
research done by a Brazilian Cooperation Group [4] who’s compared three different methods in respect to practice preparing
specimens and the precision of results, but they did the final tests by Hanson  Ingrafea’s method.
H
Mode1
Mode 2
Mode 3
Santos method.
Figure1: Loading’s Mode for test
Figure2: Position
pppp pppFracture Toughnesspppp pfor held the machine
B150mm. H (high)  1,45 B (diameter)
H. I. methodology
Figure3: Geometric dimensionppppp
Pp
short rod specimen
Figure4: Flexion test
at three points
Results and Analysis of the Results
This Study used Cement Portland V-ARI-RS for high initial resistance, course aggregate had been triturated and were from
three geologic origins with size nominal maximum ½ inch, fineness modulus 7.6 and the same granulometry. Fine aggregate
had fineness modulus 2.43. The tests were done in cylindrical concretes, diameter of 150 mm and high of 300 mm except the
high strength concrete, which was done by cylindrical specimens, diameter 100 mm and high of 200 mm and for it was used
silica fume and additive superplasticier. The test in concrete fracture toughness used notched “short rod” cylindrical specimen.
Table I displays Physic characteristics of aggregates.
TABLE 1 - Granulometria dos Agregados
Physic’s characteristics
Coarse Aggregates
Color
3
Specific mass (g/cm )
Absortion (%)
2
Specific surface area (cm /g)
Fine Aggregate
white
black
gray
dark gray
2.64
2.79
2.99
2.43
0.79
0.71
0.13
0.86
1,63
1,55
1,44
22,41
The present work was conducted by classification by the characteristic compressive strength concrete. To standard and
mediums strengths at 45 days and to high strength at 100 days:
TABLE 2: Characteristic Compressive Strength’s Concretes and slump
Water - cement
Standard (MPa) Medium (MPa)
High (MPa)
ratio
0,5
24
40
0,6
20
30
0,36
100
Slump (mm)
125 – 150
125 – 150
75
STANDARD CONCRETE
Results of compressive, tensile by diametric compression and fracture toughness are summarized on table 3.
TABLE 3 – Concrete’s Mechanics Properties to water-cement’s ratio 0,50 and 0,6
Coarse Aggregate
water-cement ratio
Mechanics
Propierties
0,5
0,6
0,5
0,6
0,5
0,6
Compressive Strength (MPa)
26,0
23,7
26,6
24,1
26,2
23,9
Tensile by diametrical compression (MPa)
3,43
3,32
3,40
3,05
3,27
3,03
Fracture Toughness (MPa/m)
0,98
0,97
0,96
0,97
0,96
0,94
white
black
Gray
The behavior of the standard concrete in the compressive strength indicate that the influence of mineralogy and physical
characteristics of aggregates to increase water-cement ratio 0,5 to 0,6 had not been significant. This fact can have it that the
compressive strength of the mortar is superior to the concrete ones until approximately 30%, accord mortar’s tests done. It is
proved by observation that the failure surfaces, approximately 30% - 40% of aggregates unglued from the mortar.
Tensile by diametrical compression, got a percentage of the order of 12% in relation the resistance of the compression in
concrete with black and gray coarse aggregates. For the concrete with white coarse aggregate was 14%. These two percents
are high; probably the main influence is the fineness of the cement but in the case of the white coarse aggregate also
influences the mineralogical composition of this.
Concrete fracture toughness, results are apparently invariable for the increase water-cement ratio 0,5 to 0,6, can be attributed
to the fact that the standard concrete contains a "high" percent of pores and humidity. The tensile strength, becomes
essentially, insensitive to the water contain as well as the type of aggregate used. It is proved by observation on the failure
surfaces, approximately 25% - 30% aggregates unglued from the mortar.
MEDIUM STRENGTH CONCRETE
Results of compressive, tensile by diametric compression and fracture toughness are summarized on table 4.
TABLE 4 – Concrete’s Mechanics Properties to water-cement’s ratio 0,50 and 0,6
Coarse Aggregate
water-cement ratio
Mechanics
Propierties
0,5
0,6
0,5
0,6
0,5
0,6
Compressive Strength (MPa)
46,4
31,0
41,9
34,2
40,0
33,6
Tensile by diametrical compression (MPa)
5,49
5,21
5,31
4,40
5,03
4,39
Fracture Toughness (MPa/m)
1,08
1,60
1,09
1,51
1,02
1,43
white
black
Gray
The behavior of the medium strength concrete in the compressive strength indicates that the influence of mineralogy and
physical characteristics of aggregates to increase water-cement ratio 0,5 and 0,6 have been significant. For water-cement ratio
0,5, the failure surfaces indicate that most of coarse aggregates broke, it could mean that the mortar’s compressive strength is
significantly stronger than the aggregates resistance and it last represents the critical stage in this way of fracture. Thus being,
a reduction in the size of coarse aggregates, must result in an improvement of the resistance of the concrete, fact that is
proven by the level of this situated resistance of 46 MPa for the concrete with white coarse aggregate. It is 10% stronger than
concretes with black and gray coarse aggregates with 41,9 MPa and 40 MPa respectively. For the water-cement ratio 0,6, the
mortar lost resistance and the failure surfaces showed aggregates unglued from the mortar like standard concrete and also
repeated some independence of the coarse aggregate’s types that are situated at 31 MPa for the white and 34 MPa for the
two others.
Tensile by diametrical compression, got a percentage of the order of 12% in relation the resistance of the compression in
concrete with black and gray coarse aggregates, the percent is the same obtained by standard concrete. The concrete with
white coarse aggregate had an important increment of 12% to 16%. It discloses influences of the mineralogical composition,
specific surface area, specific mass, density and shape of the aggregate. This fact can have it that the value of density of white
coarse (2,64 tn/cm3 ) and the fine aggregates ones (2,64 tn/cm3) are nears. It produces a homogeneous distribution of strains
and a soft transference of the strengths between the concrete’s components that retard the cracks propagation. The other two
aggregates, black and gray are 2,8 tn/cm3 and 2,9 tn/cm3 , working with the fine aggregate, the same for all the concretes,
with 2,64 tn/cm3, cracked propagation occurs more quickly, and to increase the water-cement ratio does not increase its 12%
of the compression strength.
The scanning microscopy electronic images of failure surfaces under tensile by diametrical compression of concretes with
white and gray coarse aggregate to water-cement ratio 0,6, prove the influence of its mineralogical composition mainly at the
zone of the cement paste-aggregate interfacial transition zone (ITZ).
Picture 1, presents the failure surface that reach 16% of tensile by diametrical compression in respect to the compression
strengths. Observing the crystals at cement paste-aggregate interfacial transition zone, it can be seen that they are very little
as the microscopy’s ampliation (5000x) did not discriminate. The size of its particles is the possible answer for the high
resistance. Picture 2, with ampliation (3000x) presents the failure surface that reach 12% of the same test on concrete
produced by same characteristics, just change the white aggregate coarse to gray. Comparing with Picture 1, shows bigger
size of the crystals at the cement paste-aggregate interfacial zone, and presents disorder and more humidity.
Interfacial transition zone of failure surface under tensile by diametric compressive strength on concretes
Picture 1: concrete by white coarse aggregate, 5000X
Picture 2: concrete by gray coarse aggregate, 3000X.
Concrete fracture toughness, increases its values when the water-cement ratio increase 0,5 to 0,6. It is probably due to the
water-cement ratio 0,6, which had an optimum quantity of pores to dissipate the energy, in this way the crack propagation was
retarded and had more toughness. For the concretes with white coarse aggregates was obtained some vantage about another
two concretes with black and gray coarse aggregate.
High Strength Concrete
Results of compressive, tensile by diametric compression and fracture toughness are summarized on table 5.
TABLE 5 – Concrete’s Mechanics Properties to water-cement’s ratio 0,50 and 0,6
Coarse Aggregate
water-cement ratio
Mechanics
Propierties
white
black
0,36
Gray
Compressive Strength (MPa)
90,8
91,5
90,3
Tensile by diametrical compression (MPa)
11,54
9,33
9,42
Fracture Toughness (MPa/m)
>2,02
>1,97
>1,94
The behaviour of the high strength concrete in the compressive loads indicates that the influence of mineralogy and physical
characteristics of aggregates had been very important. For water-cement ratio 0,36 the failure surfaces indicate the coarse
aggregates broken, The mortar’s compressive strength is stronger than the aggregates resistance and it last represents the
critical stage in this way of fracture. Thus being, the specific mass of black (2,80 tn/cm3) and gray (2,99 tn/cm3) coarse
aggregates, result in a positive factor. The white coarse aggregate (2,64 tn/cm3) compressed its factor with its mineralogical
composition, rich in quartz, and with its major specific surface to obtain similar results as seen on table 5.
Tensile by diametrical compression, got a percentage of the order of 10% in relation to the resistance of the compression in
the concretes, it is an acceptable result. The percent decrease with respect to the standard and medium strength concrete it is
due to the capacity for deformation diminishes at increment of the compressive strength.
Concrete fracture toughness increase for the water-cement ratio used 0,36; but it does not mean that there was an inverse
relation between this variables. The matrix of the high strength concrete is different as the standard and medium resistance
concretes because it has silica fume and a superplasticier additive in its composition. The effect of it is in the micro structure
and in the interfacial transition zone paste-aggregate so is not possible to do comparisons between the standard, medium or
high strength concretes. An evidence of the difference is for example the different way as the failures in the surfaces occurred
by compression test. The increase on the value of fracture concrete is explained with the increase of all its mechanics
properties characterized by a strong matrix, strong interfacial transition zone paste-aggregate, low quantity of pores mainly.
Fractography of Failure Surfaces
The failure surfaces of the different types of tests, compressive, tensile by diametric compressive, fracture toughness presents
different fractographies. All of them are of concretes of medium resistance, with water-cement ratio 0,6 and white coarse
aggregate and on the interfacial transition zone.
Failure surface broke by compressive strength shown on Picture 3. On it was observed the interfacial transition zone pasteaggregate with predominance of humidity. Probably the way of the primary crack propagation happens with the humidity zone.
It is a possible explanation to the known Abraham’s law, which is based on experimental methods, that affirm, the compressive
strength concrete decrease with the water-cement ratio increase.
Failure surface broke of tensile by diametric compressive strength is shown on Picture 1. Failure surface broke by direct
tension as shown on Picture 4, on it was observed a possible fracture of topography, probably own to the characteristics of the
aggregates.
Picture 3: Failure surface
by compression strength.
Picture 4: Failure surface by
direct tension.
Conclusions
The apparent two-way of the water-cement ratio, which influences the fracture toughness, is possible after the present work, to
explain what occurred. Must be comparing fracture toughness and the water-cement ratio’s influence into each band of each
type of concrete: standard, medium or high strength concrete, because each of one had different cements contents and it
influenced the behavior of the mechanic properties. Other way will show unreal conclusions, for example, If comparing a
fracture toughness of medium concrete’s strength (1,08 MPa/Öm) of water-cement ratio 0,5 with high strength concrete
fracture toughness (2 MPa/Öm) with water-cement ratio 0,36; the apparently conclusion is the ratio decrease 0,5 to 0,36, it
influences an increase in the fracture toughness of 1,08 MPa/Öm to 2 MPa/Öm, but it is an unreal conclusion because the
matrix are conformed by different variables as quantity of cement, aggregates coarse or fine, additives all of them influencing
in the interfacial transition zone and the concrete’s matrix.
The present work determinates for standard concrete. The fracture toughness is independent to the type of coarse aggregate,
for water-cement ratios 0,5 to 0,6, because the quantity of pores of the zone interface paste-aggregate has the mainly
responsibility to the formation of the failure surface, it occurred when the aggregates unglued.
For medium strength concrete, the mineralogical composition of the coarse aggregate were very important for fracture
toughness to water-cement ratio 0,6. The white coarse aggregate had a chemistry reaction with the paste and obtained a
strong interfacial transition zone paste-aggregate. For the other two aggregates, black and gray, observed is the transition
zone were the firsts failures occurred. The water-cement 0,5 to 0,6 influenced increasing the fracture toughness, probably was
conformed and optimum mechanism to dissipate the strains and the crack propagation was retarded and obtained more value
of fracture toughness for ratio 0,6 than 0,5. According to it, the quantity of pores is another important variable to influence the
value of fracture toughness. For high strength concrete, all the properties increase with use of the silica fume and the
superplasticier additive.
The fracture toughness of standard concrete increase in generally respect to a medium concrete, probably for the major
cement contents, it affects the results in a better interfacial transition zone. The same effect occurred with respect to the high
strength concrete. According to it, is very probable that the cement’s quantity is another important variable to influence the
value of fracture toughness.
The fractographies scanning by electronic microscopy showed there is a characteristic failure surface for the way of load is
carrying the specimen, compression, tensile by diametric compression or direct tension.
Acknowledgments
This research is dedicate to GOD, without his blessing could not have been done, to Sara, my mother; Michelle, my sister;
Ricardo, my baby and Ricardo y husband. I will be eternally grateful to Ph.D Fathi Aref Ibrahim Darwish, PhD Vicente
Custodio, PhD Marcus Vinicius, Lafarge S.A, Structural Tests Laboratory - UFF, Mechanicals Tests Laboratory ITUC-PUC,
Ph.D Jorge Augusto Sales Pereira, MsC Katia Allende, Paulinho, Bira, Luciano, Wellington, Casia. My friends, Pilar Alva, Mery
Gomez, Fernanda Lima, Helinette, Marinette by the care given to me and my son. To all the persons who helped me, I wish
God blesses you.
References
1.
2.
3.
4.
SANTOS, A.C; SOUSA, J.L.A.O.; “Determination of concretes fracture toughness by “short rod” specimens”, MsC.
Dissertation, Universidade Estadual de Campinas, Campinas, S. P, Brazil (1998).
ISRM: “Suggested methods for determining the fracture toughness of rock”, F. Ouchterlony, Working Group Coordinator,
Int. J. Rock Mech Min. Sciences, vol. 25, pp 71-96, (1988).
Hanson, J. H. and Ingraffea, A.R.; “An experimental – computational evaluation of the accuracy of fracture toughness
tests on concrete – Volume II”, PhD. Dissertation, Cornell University, Ithaca, N.Y, (August 2000).
Bittencourt, T. N., Santos, A. C., Borges, A. J. U., Prado, E .P., Guimarães, A. E. P., Ferreira, L. E. T., “Experimental
Study of structural concrete fracture toughness by cilíndricals specimens”, Boletim Técnico da Escola Politécnica da
Universidade da São Paulo, (2000).
http://www.lmc.ep.usp.br/people/tbitten/gmec/Boletins_T%E9cnicos/BT_PEF_0001.pdf
Apendix
author’s mail: [email protected] or [email protected]