Effect of Fiber Hybridization on Durability Related Properties of Ultra-High Performance Concrete
International Journal of Concrete Structures and Materials
Effect of Fiber Hybridization on Durability Related Properties of Ultra-High Performance Concrete
The purpose of the paper is to determine the influence of two widely used steel fibers and polypropylene fibers on the sulphate crystallization resistance, freeze-thaw resistance and surface wettability of ultra-high performance concrete (UHPC). Tests were carried out on cubes and cylinders of plain UHPC and fiber reinforced UHPC with varying contents ranging from 0.25 to 1% steel fibers and/or polypropylene fibers. Extensive data from the salt resistance test, frost resistance test, dynamic modulus of elasticity test before and after freezing-thawing, as well as the contact angle test were recorded and analyzed. Fiber hybridization relatively increased the resistance to salt crystallization and freeze-thaw resistance of UHPC in comparison with a single type of fiber in UHPC at the same fiber volume fraction. The experimental results indicate that hybrid fibers can significantly improve the adhesion properties and reduce the wettability of the UHPC surface.
ultra-high performance concrete; steel fibers; polypropylene fibers; salt resistance; frost resistance; contact angle; surface free energy
Ultra-high performance concrete is often used in
underground waterproof projects, roads, bridges and seismic
structures in harsh environmental conditions. The material is
characterized by high strength, low absorbability, low water
permeability and high freeze resistance, which results in high
durability (Smarzewski and Barnat-Hunek 2013; Li and Liu
2016; Kang et al. 2016). This material has a low porosity and
might be filled with a gaseous phase or liquid phase. The
liquid phase contains water and various types of pollutants
including salts, which penetrate the concrete through the
interconnected pores (Koniorczyk et al. 2013). Therefore,
knowledge about the wettability, absorptivity, frost resistance
and salt resistance of a given UHPC formulation is extremely
important when considering its durability. The frost
resistance of fiber reinforced concrete is affected by its porosity,
type of aggregate, fiber characteristics and environmental
conditions; hence, the frost resistance of ultra-high
performance concrete mixtures has been investigated by some
researchers (Yun and Wu 2011). Chemrouk and Hamrat have
shown that a water/cement ratio below 0.4 reduces the
penetration of chlorides, decreases the risks of concrete spalling
and rebar corrosion in reinforced concrete structures
(Chemrouk and Hamrat 2002). UHPC is characterised by a
maximum aggregate size of 8 mm. The water/binder ratio is
frequently below 0.25 and highly reactive silica fume must be
added to the mix. Workability can be ensured by applying
large amounts of superplasticizer (2009). Fibers are added to
the matrix as reinforcement to control cracking and to
increase material ductility (Dinh et al. 2016; Abdallah et al.
2016; Bencardino et al. 2010; Abou El-Mal et al. 2015;
Sorensen et al. 2014). The cracks in concrete generally occur
over time due to a number of reasons. Cracks weaken the
waterproofing capabilities and expose the microstructure to
moisture, bromine, chloride and sulphates (K o¨ksal et al.
2008; Song et al. 2005; Sivakumar and Santhanam 2007;
Toutanji 1999). Thus, improving concrete properties is an
important aim in concrete science (Nili and Afroughsabet
2012). Cracks occur in different sizes and at different stages
of concrete exploitation; consequently, the use of different
fibers with various lengths and varied characteristics are a
good way to solve this problem. The interaction between the
fibers and the concrete is more advantageous when two types
of fibers are used (Yao et al. 2003; Dawood and Ramli 2010;
Yang 2011). Concrete deterioration is associated with
destructive chemical attacks and the impact of freezing and
thawing (Chemrouk 2015; Bondar et al. 2015). Another
factor that affects concrete durability is resistance to the
ingress of aggressive ions (Afroughsabet and Ozbakkaloglu
2015). The most frequent sulphate salts are calcium sulphate
(Ca2SO4), sodium sulphate (Na2SO4), potassium sulphate
(K2SO4) and magnesium sulphate (MgSO4), the most
aggressive salts of which are sodium sulphate and potassium sulphate
(Chemrouk 2015). Not only does the growth of salt crystals in
concrete pores generate stress leading to damage, but the
growth of ice is another reason for concrete quality
deterioration. Scherer (Scherer 1999) proved that the crystallization
pressure of salt is low in large pores; on the other hand, the
impact of freeze–thaw cycles is more evident in the small pores
of UHPC. Nevertheless, few investigations of freezing-thawing
resistance can be found for fiber reinforced concrete (Miao et al.
2002; Colombo et al. 2015). In practice, concrete degradation
mechanisms are the combined results of mechanical stress,
physical and chemical attacks (Miao et al. 2002). The resistance
of concrete to freezing and thawing depends on the degree of
saturation, system of pores, permeability and the water/binder
ratio. High freezing-thawing resistance depends on low
permeability and a low water/binder ratio (Colombo et al. 2015).
For concrete with a high freezing-thawing resistance, the
distance from the hydrated cement paste to an air void cannot
exceed 0.2 mm. In addition, very small pores with diameters
less than 0.3 mm are desirable (Yang et al. 2015). High strength
concrete with a very low water/binder ratio generally has a low
capillary porosity. Hence, the amount of water able to freeze is
low enough and concrete can be damaged by frost action after
long and continuous exposure to water (Guse and Hilsdorf
1998). However, the doubts regarding the frost resistance of
high performance concretes have not been entirely removed
(2009). The expansive reactions in concrete are related to the
sulphates contained in ground water, sea water, soils and
sewage. Sulphate attack is associated with the groundwater
environment and the discharge of industrial wastewater ( C¸avdar
2014). Sulphate ions during penetration of the concrete may
react with calcium hydroxide or with calcium aluminate
hydrates. Severe damage, and in the end complete
disintegration occur if sulphate solutions penetrate the hardened concrete.
UHPC are often exposed to aggressive impacts from the
environment, thus they must have high resistance to chemical
corrosion, frost corrosion, weathering or the influence of
aggressive water. In paper ( C¸avdar 2014), the flexural strength,
compressive strength and modulus of dynamic elasticity of
fiber reinforced mortar samples after 100 freeze–thaw cycles
were reduced. Frost and salt resistance are considered to be
significant features in evaluating concrete durability.
The purpose of this article is to determine the influence of
steel and polypropylene fibers on the freeze–thaw resistance,
sulphate crystallization resistance and surface free energy of
ultra-high performance concrete.
2. Experimental Methods
In the mixes Portland cement CEM I 52.5 N-HSR/NA,
condensed silica fume, two types of coarse aggregates:
granodiorite and granite, quartz sand, water, superplasticizer
and two types of fibers: steel (SF) and polypropylene (PF),
were used. Tests on the cements were carried out according
to PN-EN 197-1:2012. The properties of the cement are
shown in Table 1.
Based on PN-EN 933-1:2000, the particle size distribution
for coarse and fine aggregate was performed. In order to attain
the same workability, a highly effective superplasticizer with
a density of 1.065 g/cm3 at 20 C, based on polycarboxylate
ethers intended to create a self-compacting concrete, was used.
The SF properties are the following: hooked-end with density
7.8 g/cm3, length 50 mm, diameter 1 mm, modulus of
elasticity 200 GPa, tensile strength 1100 MPa and the PF
characteristics are: density 0.9 g/cm3, length 12 mm, diameter
25 lm, modulus of elasticity 3.5 GPa, tensile strength
350 MPa. The mixes were prepared using: cement—
670.5 kg/m3, granodiorite aggregate 2/8 mm (in C1, SC,
SPC1 mixes) or granite aggregate 2/8 mm—990 kg/m3 (in
SPC2, SPC3, PC, C2 mixes), quartz sand 0.1/2 mm—500 kg/
m3, water—178 l/m3, silica fume—74.5 kg/m3, and
superplasticizer—20 l/m3. The quantities of SF and PF were varied.
The purpose of using two types of aggregates was to determine
their effect on the fracture parameters of UHPC (Smarzewski
and Barnat-Hunek 2015). The abbreviations of the concrete
mixes, quantities of steel and polypropylene fibers and types
of coarse aggregate are shown in Table 2.
The mixtures were prepared using a concrete mixer with a
capacity of 100 l. At the beginning of mixing the coarse
aggregate and sand were homogenized with a half of the
amount of water. Subsequently, cement, silica fume, the
remaining water were added and finally the superplasticizer.
After the concrete components had been thoroughly mixed,
steel and polypropylene fibers were added by hand to obtain
a homogeneous and workable consistency.
Samples were formed directly after all the components had
been mixed. Moulds coated with anti-adhesive oil were filled
with a concrete batch and compacted on a vibrating table. The
cubical samples were compacted in one layer, while the
cylindrical samples were done in two layers. After
compacting, the samples were covered with foil to minimize the loss of
moisture. All the samples were stored at a temperature of about
23 C until the time to remove them from the moulds after
24 h, then they were placed in a water tank for 7 days to cure.
Over the next few days, until completion of the test—after
28 days, the samples remained in air-dry conditions.
2.2 Test Methods
Tests were carried out on cube samples and cylindrical
samples. The physical properties of the concrete were
described in paper (Barnat-Hunek and Smarzewski 2016).
The dynamic modulus of elasticity before and after the
cyclic freezing-thawing test and the salt crystallization
resistance test was measured.
2.2.1 Resistance to salt crystallization
The resistance to salt crystallization was tested according
to standard EN 12370:2001. Six cubes from each mix were
used for the test. The dimensions of the samples were the
following: 100 9 100 9 100 mm. The samples were
immersed in a 14% solution of sodium sulphate dehydrate
for the period of 2 h after drying and weighing, Fig. 1.
Specific surface area
Commencement of bonding
End of bonding
Compressive strength at 2 days
Tensile strength at 2 days
Then they were dried in conditions of a progressive
temperature increase until 105 C for about 10 h, maintaining a
high relative moisture at the initial stage of drying.
Afterwards, the samples were saturated in sodium sulphate again.
The cycle of saturation and drying was repeated 15 times.
Then the samples were stored in water for 24 h. After
saturation the specimens were washed, dried and weighed. The
obtained results are presented in percent as the relative
difference in mass in relation to the initial mass of the sample
and the number of cycles till concrete surface degradation,
which meant a lack of resistance to salt crystallization.
2.2.2 Frost Resistance Test
Frost resistance was determined using the direct method
according to EN 12012:2007 and EN 13581:2004. Six
cylindrical samples 150 mm in diameter and of a height of
300 mm from each mix were subjected to 180 cycles of
Fig. 1 Experimental set-up of UHPC resistance to sodium
sulphate crystallization test.
Table 1 Cement properties.
Table 2 Variable components of concrete mixtures.
Percentage of fibers (%)/mass (g/cm3)
Steel fibers SF Polypropylene fibers PF
Type of coarse aggregate 2/8 mm
freezing and thawing (F-T). These samples were placed in a
freezing chamber in which the temperature control range is
-30 to 60 C (Fig. 2). The cyclic freeze–thaw scheme is
illustrated in Fig. 2. At the beginning the temperature was
kept constant at 20 C for 1 h to eliminate an initial
temperature difference between the samples and the chamber.
The rate of freezing in water ranged from 20 to -20 C for
6 h. In turn, the rate of thawing in water ranged from -20 to
20 C for 6 h. One F–T cycle duration was 12 h. After
90 days all the samples were dried to a constant mass, then
the mass loss, dynamic modulus of elasticity and contact
angle were determined. During the measurements the
samples were stored under similar conditions.
2.2.3 Dynamic Modulus of Elasticity
Dynamic modulus of elasticity testing was performed on
six cylinders 150 mm in diameter and of a height of 300
mm, from each mix before and after the frost resistance test.
Determination of the dynamic modulus of elasticity was
performed based on the resonant frequency of vibration of a
sample generated by an impact and sensed by an
accelerometer. The test was conducted based on ASTM
C666 and ASTM C215. In the experiment, an accelerometer
was installed on a cylindrical sample and was attached to the
battery operated instrument, Fig. 3. A hardened steel ball
was used as the impactor. The steel ball struck the top
surface of the cylindrical specimen, then the apparatus
Fig. 2 Experimental set-up, schematic diagram, and cyclic temperature scheme for freeze–thaw test.
computed the maximum amplitude. The transverse (flexural)
resonance is valid to determine the durability of UHPC by
testing the degradation of concrete after freezing-thawing
cycles and aggressive environments on concrete samples.
The dynamic modulus, EDM (GPa), was calculated from
2.2.4 Contact Angle and Surface Free Energy
Contact angle measurements were conducted to calculate
the surface free energy (SFE). Measurement of the contact
angle of a distilled water drop was carried out on a research
stand consisting of a goniometer integrated with a camera for
taking photos of a drop put onto the sample surface, Fig. 4.
Distilled water drops of 2 mm3 were deposited by means
of a syringe. Due to the heterogeneity of the material surface,
six drops were put on each sample. The measurements were
carried out at the time of drop application and after 35 min.
Measurements were performed before and after the frost
resistance test of 180 cycles of freezing-thawing in order to
verify change in the adhesive properties and wettability of
UHPC. Measurement of the contact angle of a distilled
water drop is shown in Fig. 5.
The Neumann model was applied to calculate SFE. The
total SFE for the distilled water, cw = 72.8 mJ/m2, was
adopted. In the Neumann model the following equation for
calculating SFE was used (Barnat-Hunek and Smarzewski
Fig. 3 Set-up of dynamic modulus of elasticity test for
3. Results and Discussion
3.1 Physical and Mechanical Properties
A higher air content has a detrimental effect on the
durability related properties of UHPC. Depending on the UHPC
components, the air content ranges from 0.3 to 6% by
volume of mixture (Wille et al. 2011; Wang and Gao 2016). The
air content in UHPC was 1% owing to using superplasticizer
in the amount of 20 kg/m3, and a water/binder (w/b) ratio
equal to 0.17 (Pierard and Cauberg 2009). A higher w/b ratio
and higher superplasticizer dosage led to an increased air
content (Abbas et al. 2016). On the other hand, special
mixing technology under lowered air pressure reduces the air
content (Dils and De Schutter 2015). Moreover, the addition
of steel fiber decreased the entrapped air content of fresh
UHPC mixtures when a high dosage of superplasticizer was
used (Wang and Gao 2016). Less workable concrete contains
more entrapped air which cannot escape from the mass
because of the high viscosity of the mixture. Poor
workability leads to a higher content of air in the cavities and pores
of the capillaries which have an important impact on the
mechanical properties and the durability properties of the
concrete (Dils et al. 2013). When the capillary porosity
decreases, a small amount of water is absorbed and
saturation does not occur so easily. This gives a higher resistance
to freeze–thaw cycles, and penetration of chlorides and
sulphates (Dils et al. 2013; A¨ıtcin 2003). The mean values of
UHPC properties shown in Table 3 are based on papers
(Smarzewski and Barnat-Hunek 2015; Barnat-Hunek and
A greater amount of polypropylene fibers (PF) causes a
decrease in density. The pore volume increased the number
of micro-cracks which are formed in the cement mortar due
to a poor transition zone and weaker adhesion between PF
and the cement matrix. This affected an increase in concrete
absorptivity. The test results for polypropylene fiber
reinforced UHPC indicate the negative impact of PF on
decreasing the water absorption of concrete. An inverse
relationship for UHPC with a high content of steel fibers
Fig. 4 Equipment used in contact angle measurement.
Table 3 UHPC properties.
(SF) is observed. A larger amount of SF increases the
strength of UHPC. The compressive strength of PF
reinforced UHPC is significantly lower than concrete without
fibers, and with granite aggregate. The reason for the
decrease in the compressive strength is probably that
dispersion at 1% PF volume was difficult, caused poor
workability, and incomplete consolidation of UHPC. A decrease
in compressive strength by adding polypropylene fibers was
also observed by other researchers (Khitab et al. 2013;
Afroughsabet et al. 2016).
3.2 Resistance to Salt Crystallization
The examined cube samples were visually monitored after
15 cycles of sulphate exposure. No deterioration was
observed on the cube surfaces. A loss in mass was observed
in the samples with a high SF content. On the other hand,
there was an increase in weight in the samples without fibers,
and with a PF content at least 0.5% (Fig. 6).
Only in concretes with the high contents of steel fiber (1
and 0.75%) was a slight decrease in the mass of the concrete
(0.14 and 0.05% respectively) observed. In this study, the
water absorptivity corresponds to the increase in mass in the
sample after the salt crystallization test (Fig. 7).
The polynomial trend was characterized by correlation
coefficient R2 = 0.7822 and relatively low errors in the
intercept. A higher increase in mass after the salt
crystallization test was observed in the concretes with 1 and 0.75%
PF, with the granite aggregate and with the highest
absorptivity. This is due to the greater amount of free pores which
were filled by salt.
3.3 Frost Resistance Test
There are many tests that can be used to determine the
frost resistance of UHPC. How many cycles of freezing and
thawing should be performed to consider it resistant to
freezing and thawing remains an open question.
ASTM C 666 recommends procedure A (freezing and
Fig. 6 Relative increase/decrease in sample weight after salt
Fig. 8 Failure mode of concrete sample with 1% SF content
after freeze–thaw resistance test.
Fig. 7 Relationship between water absorptivity and increase
in mass after salt crystallization test.
thawing in water) or procedure B (freezing in air and
thawing in water) to determine freeze–thaw resistance. In
both procedures, the number of F–T cycles is equal to 300
and mostly adopts a cycle of 2 h of freezing and 2 h of
thawing. The ASTM C 666 sequence is very high and it does
not represent well natural freeze–thaw exposure,
nevertheless, this method is commonly used. In this research the
number of cycles was reduced to 180 and the duration of a
freezing-thawing cycle was extended to 12 h.
The addition of steel fibers significantly increased UHPC
degradation after 180 freezing-thawing cycles. The steel
fibers can change the failure mode of samples subjected to
cyclic freezing and thawing. After the experiment, we
observed cracks on the concrete surface and corrosion of the
steel fibres in the samples with the 1% SF content, see
More cracks and voids can be observed in the concrete
with SF in the amounts of 1% (SC) and 0.75% (SPC1) with
the granodiorite aggregate which resulted in a higher mass
loss. In other cases, the swelling pressure of ice was
sufficient to cause disruptive micro-cracks that during the process
became macro-cracks which extended throughout the
affected concrete. The average mass loss of the samples with
error bars is shown in Fig. 9.
Fig. 9 Relative decrease in mass after freeze–thaw
The greatest mass loss was demonstrated by the concrete
containing 1% steel fibers; it was 22.3 times greater than the
one without fibers, with granodiorite aggregate. The concrete
with 1% PF had an 11% higher mass loss than the one
without PF. In the SC concrete, the increased amount and
volume of capillary pores are the main causes of expansive
internal pressure during the freezing of water. For the seven
series of UHPC, the correlation between the percentage of
mass decrease after the frost resistance test and the decrease/
increase in mass after the salt crystallization resistance test is
determined (Fig. 10).
This relationship can be described by the equation:
y = 0.00304x2 - 0.0535x ? 0.0832. The polynomial trend
was characterized by a good correlation coefficient
R2 = 0.8322 and relatively low errors in the intercept. It was
observed that the results for UHPC with the highest SF
content significantly differ from the other results as they
have the highest mass loss in both the salt and frost
resistance tests. It was observed that the addition of steel fibers
appeared to decrease the internal material degradation due to
the freeze–thaw cycles (Cwirzen et al. 2008). The
experimental results showed that the weight loss of UHPC
subjected to cycles of freezing and thawing in water was higher
than the weight loss of UHPC after the salt crystallization
resistance test. Steel fibers can have a major influence on
Fig. 10 Relationship between percentage of mass loss after
frost and salt resistance tests.
workability; one special concern is their orientation. In steel
fiber reinforced concrete a well-finished sample surface
helps encapsulate the fibers within the mortar. Material
degradation due to freeze–thaw exposure can be caused by
an imprecise finish of the sample surface and dense
distribution of steel fibers near the outer edge of the samples.
Furthermore, this may also be related to the differences in
the physical properties of the frozen water and the salt
solution such as freezing point, deformability or ductility
(Miao et al. 2002). In this study, the steel fibers did not delay
the onset or spread of micro-cracks, and thus do not protect
against concrete degradation during cycles of freezing and
thawing. The steel fibres are protected against corrosion in
the alkaline environment of concrete, nevertheless, single
fibres may corrode in the presence of moisture in the edge
zone of the concrete samples. This corrosion caused
significant failure and visual imperfections in the form of rust
stains on the surface of UHPC with a content of at least
0.75% steel fibers (see Fig. 8). On the other hand, due to the
extremely low porosity of UHPC compared to ordinary
concrete and even high strength concrete, the rate of
degradation is much slower and leads to significantly greater
durability. The pores of UHPC are very fine and
discontinuous and reduce the flow of reactive agents within the
material, hence leading to limited material deterioration
3.4 Dynamic Modulus of Elasticity After Frost Resistance Test
Dynamic modulus is important to evaluate the load
bearing capacity of fiber reinforced UHPC, whose value is also a
measure of frost resistance. Based on the theory of
resonance, the frequencies of the concrete specimens before and
after freezing-thawing cycles were measured. The results of
the dynamic modulus of elasticity before and after the
freeze–thaw cycles are presented in Fig. 11.
The highest dynamic modulus was displayed by the
concrete with 1% SF, and is 27% higher than that of the concrete
with 1% PF. After 180 freezing-thawing cycles, decreases in
the dynamic modulus values were observed: 0.1% for C1,
Fig. 11 Dynamic modulus of elasticity before and after
Fig. 12 Relationship between mass loss and dynamic
modulus ratio after/before F–T cycles.
4% for SC, 1% for SPC1, 0.3% for SPC2, 0.2% for SPC3
and 0.6% for C2. For all the mixtures the relative values of
the dynamic elastic modulus do not drop below 95% of the
baseline. Based on the above observation, it was found that
all the examined UHPC are frost resistant. It was noted that
the decreases in the dynamic modulus of elasticity are in
close relationship with the mass losses for all types of
UHPC. The correlations between the mass loss and the ratio
of dynamic modulus before and after freezing and thawing
(F–T) cycles were determined, (see Fig. 12).
The relationship between the loss in mass after the F–T
cycles and the dynamic modulus of elasticity ratio is
presented in the form of the polynomial ax2 ? bx ? c. The
high correlation coefficient value of more than 0.9636
indicates that the loss in mass has a strong relation to the
ratio of the dynamic modulus of elasticity before and after
F–T cycles. There is a clear grouping of the results
depending on the type and quantity of fibers in UHPC,
which was also observed in the relationship between the
percentage of mass loss after the frost resistance test and
after the salt resistance test (Fig. 10). The lowest results were
noticed for the UHPC without fibers and with 1% PF.
Conversely, the highest values were observed for the UHPC
with SF (Fig. 12). The middle part of the curve are the data
obtained for the hybrid fiber reinforced UHPC.
3.5 Contact Angle and Surface Free Energy
Measuring the contact angle is one of the methods of
monitoring changes in the wettability of the material surface.
Surface free energy is often used as a measure of adhesive
properties. The contact angle and SFE enable forecasting of
material surface durability.
The measured contact angles of distilled water and the
calculated SFE of UHPC after 0 and 35 min are included in
Table 4. The contact angle before the frost resistance test
after 0 and 35 min, and the contact angle before and after
180 F–T cycles at the beginning of the drop test are shown in
Figs. 13 and 14 respectively.
It can be noticed that the distilled water contact angles (hw)
decreased in the course of time and they are different before
and after 180 F-T cycles. Decreases in the contact angle
between 0 and 35 min after the frost resistance test were
observed: 40.8 (C1), 34.8 (SC), 16.5 (SPC1), 28.4
(SPC2), 31.3 (SPC3), 31.9 (PC), 45.5 (C2). The contact
angles after the frost resistance test after 0 min were high for
the concrete without fibers and with steel fibers. Therefore,
these UHPC were characterized by the lowest initial surface
wettability. The hybrid fiber reinforced concrete and the
polypropylene fiber reinforced concrete were characterized
by initial wettability by up to 100% higher, except for the
concrete with 0.25% SF and 0.75% PF (but in this case a
smooth surface can lead to a higher value of contact angle).
The highest contact angle was noted for the concrete with
1% SF both at the beginning of the test and after 35 min.
The smallest contact angle was obtained by the concrete with
the addition of 0.75% SF and 0.25% PF, which was about
38% lower than in the case of the concrete without fibers
with the same granodiorite aggregate at the beginning of the
experiment. These samples exhibited the greatest mass loss.
Due to the corrosion of steel fibers on the concrete surface,
the wettability and adhesion properties increased. The
contact angle decreased by about 18% for the concrete with 1%
SF and 13% for the concrete with 1% PF, whereas for the
concretes without fibers, the contact angle decreased by
about 5–6%. The contact angles after the frost resistance test
after 35 min were high for the hybrid fiber reinforced
concrete and polypropylene fiber reinforced concrete. The
concretes with hybrid fibers had the least wettable surface of
fiber reinforced concretes. The lowest contact angle was
observed for the concrete without fibers with granite
aggregate. This contact angle was about twice lower than
that of the concrete without fibers with granodiorite
aggregate. The absorptivity of coarse aggregate had an impact on
the contact angle.
Fig. 13 Contact angle before F–T cycles after 0 and 35 min.
Fig. 14 Contact angle before and after 180 F–T cycles at
beginning of drop test.
Table 4 UHPC contact angle and SFE.
Fig. 15 Correlations between mass loss and SFE ratio
before and after F–T cycles.
The total SFE values for all the UHPC were calculated
(see Table 4). The highest SFE value cS = 59.7 mJ/m2,
which shows the finest adhesion properties, was obtained by
the hybrid fiber reinforced UHPC with 0.75% SF and 0.25%
PF. After the frost resistance test, all the SFE values were
higher than before the test. The results were interpreted on
the basis of the SFE ratio before and after F–T cycles. For
the all the types of UHPC, the correlation between the mass
loss and SFE ratio before and after the frost resistance test
was determined (Fig. 15).
This relationship can be described by the equation
y = 28.095x2 - 60.94x ? 30.084. The polynomial trend
was characterized by an excellent coefficient R2 = 0.98 and
quite low errors in the intercept. The results for the concrete
with the highest SF content were significantly different from
the other results. They have the highest mass loss in the frost
resistance test and the highest differences between SFE
before and after the test.
Studies were performed to determine the impact of fiber
hybridization on the salt crystallization resistance, the
freeze–thaw resistance and the surface wettability of
ultrahigh performance concrete. Through careful analysis of the
test results, the following conclusions can be drawn:
The degree of ice pore saturation is sufficiently low. The
matrices of the UHPC with steel fibers in the amounts of
1% (SC), and 0.75% (SPC1) are slightly damaged. The
increased volume of free pores in UHPC with
polypropylene fibers affected the greatest rise in mass
after the salt crystallization test. All the UHPC exhibited
good resistance to salt crystallization.
Freezing-thawing cycles cause cracking and degradation
of the UHPC with the high content of steel fibers, affecting
deterioration of the dynamic modulus and significant loss
in mass, which influences the durability. However, for all
the UHPC the relative values of the dynamic elastic
modulus do not drop below 95% of the baseline.
The contact angles after the frost resistance test were
high for hybrid fiber reinforced UHPC and
polypropylene fiber reinforced UHPC. The lowest contact angle
was observed for the concrete without fibers with granite
aggregate. The highest SFE value was obtained by the
hybrid fiber reinforced UHPC with 0.75% SF and 0.25%
Ultra-high performance concrete demonstrated good
correlations between: the mass loss and the dynamic
modulus ratio before and after F–T cycles, the water
absorptivity and increase in mass after the salt
crystallization test, the percentage of mass loss after the frost
resistance test and salt resistance test. The adhesive
properties and wettability were determined by the
correlation between the mass loss and SFE ratio before
and after the F–T cycles.
Fiber hybridization increases the resistance to salt
crystallization and freeze–thaw resistance, improves the
adhesion properties and reduce the wettability of the
UHPC surface in comparison with one type of fiber at
the same fiber volume fraction.
This work was financially supported by Ministry of Science
and Higher Education—Poland, within the statutory research
number S/15/B/1/2016, S/14/2016.
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