Today, the advancement of technology and the achievement of increasing innovations in the field of building materials have increased high-strength concrete (HSC) production. The use of this material has been increased due to economic and technical reasons in the construction of concrete sections. However, the more compressive strength of the concrete is, the more concrete becomes brittle and its tensile strength does not increase with increasing compressive strength. HSC is also more vulnerable to high temperatures due to its high density and low porosity compared to conventional concrete. Researchers have proposed different methods including the use of polypropylene fibres in concrete mix designs in order to overcome these defects of HSC. In this study, a new type of polypropylene fibres, called high performance synthetic macro polypropylene fibres (HPP), have been used in dosages of 1, 2 and 3 kg/m3 . Tests on hardened concrete include compressive strength, tensile strength and flexural strength at temperatures of 25, 100, 200 and 300 C. By adding 1 kg of fibres to HSC, its compressive strength, tensile strength and flexural strength increased up to 14, 17 and 8.5%, respectively. Furthermore, the greatest improvement in the mechanical properties of concrete exposed to high temperatures was obtained when 1 kg/m3 of fibres was added to HSC.
Trang 1Effects of high temperatures on mechanical behavior of high strength
concrete reinforced with high performance synthetic macro
polypropylene (HPP) fibres
Department of Civil Engineering, Isfahan (Khorasgan) Branch, Islamic Azad University, Isfahan, Iran
h i g h l i g h t s
HPP Fibers did not have significant effect on the compressive strength of concrete
Mechanical properties of HSC were enhanced when HPP fibers were added
Addition of HPP fibers postponed the spalling of HSC when exposed to high temperatures
Discussion about the optimum dosages of HPP fibers was made
a r t i c l e i n f o
Article history:
Received 29 September 2017
Received in revised form 8 January 2018
Accepted 9 January 2018
Keywords:
High strength concrete
High performance synthetic macro
polypropylene fibres (HPP)
Fibre reinforced concrete
High temperatures
a b s t r a c t
Today, the advancement of technology and the achievement of increasing innovations in the field of building materials have increased high-strength concrete (HSC) production The use of this material has been increased due to economic and technical reasons in the construction of concrete sections However, the more compressive strength of the concrete is, the more concrete becomes brittle and its tensile strength does not increase with increasing compressive strength HSC is also more vulnerable
to high temperatures due to its high density and low porosity compared to conventional concrete Researchers have proposed different methods including the use of polypropylene fibres in concrete mix designs in order to overcome these defects of HSC In this study, a new type of polypropylene fibres, called high performance synthetic macro polypropylene fibres (HPP), have been used in dosages of 1, 2 and 3 kg/m3 Tests on hardened concrete include compressive strength, tensile strength and flexural strength at temperatures of 25, 100, 200 and 300°C By adding 1 kg of fibres to HSC, its compressive strength, tensile strength and flexural strength increased up to 14, 17 and 8.5%, respectively Furthermore, the greatest improvement in the mechanical properties of concrete exposed to high tem-peratures was obtained when 1 kg/m3of fibres was added to HSC
Ó 2018 Elsevier Ltd All rights reserved
1 Introduction
Concrete is one of the most important and most popular
building materials, featuring advantages such as plasticity prior
to hardening, good compressive strength and the availability of
its constituent materials Due to advances in technology, the use
of high-strength concrete (HSC) has been increasing in recent
years In parallel, many studies have been done to improve the
weaknesses of this type of concrete, including its low tensile
strength and ductility compared to its compressive strength[1,2] and its greater vulnerability at high temperatures among various types of concrete Studies show low resistance of concrete in high temperatures, so that exposure of concrete to high temperatures leads to cracking and explosive spalling Accordingly, the strength and modulus of elasticity of HSC drops significantly[3–9] Concrete may be exposed to high temperatures in cases such as the occurrence of fire in concrete structures, in the explosion of jet engines, in factories in the extraction and melting of metals, in some chemical plants where concrete is close to the furnace, and related-nuclear activities Adding fibres is the most widely known method to prevent spalling of HSC[10–17] Among fibres, adding polypropy-lene (PP) into HSC shows better performance in order to increase
https://doi.org/10.1016/j.conbuildmat.2018.01.064
0950-0618/Ó 2018 Elsevier Ltd All rights reserved.
⇑ Corresponding author.
E-mail addresses: hamidbehbahani@gmail.com , hbehbahani@khuisf.ac.ir
(H.P Behbahani).
Contents lists available atScienceDirect Construction and Building Materials
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c o n b u i l d m a t
Trang 2resistance of HSC at elevated temperatures[18–20] In addition, it
caused improvement in the mechanical properties of HSC and its
shrinkage control[21–27] Investigating the spalling phenomena
for concrete by incorporating polypropylene fibres, Lura and Terrasi
[18]found that spalling was substantially decreased by adding to the
concrete small quantities (almost 0.1% by volume) of fibres made
from a low melting-point polymer Noumowe[28]and Sahmaran
et al.[29]studied the mechanical and microstructure properties of
HSC in face of high temperatures It was found that the pore
struc-ture at high temperastruc-ture may have a considerable influence on the
spalling behavior of the high strength polypropylene fibre concrete
Polypropylene fibres are melted when exposed to high
tempera-tures, and creating channels in concrete mass prevents the
forma-tion of high vapor pressure in concrete pores, which reduces the
spalling of concrete In addition, the fibrous concrete cool slower
than normal concrete, resulting in fewer cracks in cooling phase
Other researches have studied properties of HSC with combination
of Polypropylene and other fibres, e.g steel fibres in order to improve
the mechanical properties of HSC[30,31]
This study investigates the effects of adding a new type of
polypropylene fibres, called high performance synthetic macro
polypropylene fibres (HPP), on the mechanical properties of
con-crete at elevated temperatures up to 300°C These fibres are made
of polymer materials that have an especial sine-shape The physical shape of these fibres makes them superior for concrete mixture when compared with the common type of fibres In addition, com-pared to typical fibres, they also have a higher modulus of elasticity and tensile strength[32,33] Among advantages of this type of fibre are enhancing the concrete resistance to stress, fatigue, heat, and increase tensile, shear and flexural strength in concrete.Table 1 shows the differences between properties of HPP fibers and a common type of polypropylene fibers Their physical shapes are displayed inFig 1 The objectives of this research are (i) to obtain the effect of high temperatures on the mechanical properties of conventional normal concrete (NC) and HSC; (ii) to study the effect
of adding polypropylene fibres with different dosages on mechan-ical properties of HSC; (iii) to examine mechanmechan-ical behavior of high strength HPP fibre concrete at elevated temperatures up to 300°C
2 Test program and procedures 2.1 Concrete mix design and testing Two different types of concrete including normal concrete and HSC are used in this research with strength of 25 and 69 (MPa)
Table 1
Technical specifications of studied HPP fibres compared with a typical PP fibre.
Type of Fibre Physical Shape Density (gr/cm 3
) Tensile Strength (MPa) Modulus of Elasticity (MPa) Melting Point (°C) Diameter (mm) Length (mm)
Fig 1 Physical shape of fibres (a) studied HPP fibres (b) typical type of PP fibres.
Table 2
Concrete mix design of samples.
Type of Concrete Sample W/C D max
(mm)
Coarse Aggregate (kg/m 3 )
Fine Aggregate (kg/m 3
(kg/m 3 ) Fibre Content (kg/m 3 )
Super Plasticizers (%)
Trang 3Effects of adding HPP fibres on properties of HSC were studied at
three different dosages of 1, 2 and 3 kg per cubic meter of concrete
The mix design to achieve HSC was in accordance with DOE
method [34] and is presented in Table 2 Naming the samples
was done in such a way as to represent the type of sample HSC
represents high strength concrete and the number after that
indicates the fibre content in a cubic meter of HSC NC represents
the normal concrete
Concrete samples were tested at day 7 and day 28, and three
samples were made for each design The experiments carried out
in this study are compressive strength test in accordance with B
S 1881: Part 116 standard, splitting tensile strength test according
to ASTM C496/C496 M-04 regulation, flexural strength test
follow-ing ASTM C293 The samples were exposed to three different
tem-peratures of 100, 200 and 300°C according to ISO-834 standard
using oven The electric oven used in this study is shown in
Fig 2 The number and type of studied specimens and testing
pro-cedure are presented inTable 3
Different methods are used to make fibre reinforced concrete
Type of work, facilities and equipment are among the factors that
are important in choosing production methods It should be noted
that in any method of production of fibre reinforced concrete, the
distribution of fibres in a mixture should be homogeneous to
pre-vent fibres from balling The balling or conglobation of fibres
dur-ing the mixdur-ing process depends on several factors, with the most
important parameter being the aspect ratio Other factors that
affect the fibre distribution include fibre percentages, grain size,
aggregate size and quantity, the ratio of water to cement, mixing
method Higher amount of aspect ratio, volume percentage of
fibres, and size of the aggregates increased the tendency for
bal-ling The fibre concrete mixing design is similar to other concrete
and for a specified mixture, the slump decreases with increasing
fibre content For a uniform distribution of fibres in the mixture,
the proper performance of the concrete is important Additives
materials may be used to create air bubbles, water reduction and
shrinkage control InFig 3, an illustration of the studied fresh fibre
reinforced HSC is presented
2.2 Materials The cement used in this research was the production of Ardestan cement plant with the shown properties in Table 4 In order to make experimental samples, a washed sand from Isfahan flood plain was used with a fineness modulus of 1/3 in accordance with ASTM C-125 standard The sand used in this study was moun-tainous materials in Isfahan with a maximum size of 19.5 mm, specific gravity of 2.68 gr/cm3and bulk density of 1500 kg/m3 In this research, super plasticizer Dynamon SP 5600, without any chlorine based on formulations containing advanced poly-carboxylate molecular chains, is used to provide the required
Fig 2 A view of used electric oven.
Table 3
Number and type of concrete samples for testing.
10 50
59
50
0 20 40 60 80
Temperature (°C )
HSC NC
Fig 4 Effects of high temperatures on NC and HSC samples.
Fig 3 Studied fresh concrete containing HPP fibres.
Table 4 Properties of used cement.
Chemical analysis Blaine (cm 2
/gr) 3200 ± 100
Al 2 O 3 (%) 5.2 ± 0.2
Fe 2 O 3 (%) 4.6 ± 0.2
Trang 4performance for high-strength concrete This product is
manufac-tured in accordance with type F, G ASTM C-494 and EN 934-2
standards
3 Results and discussion
This part is categorized into four sections Section3.1presents
results of compressive strength test on NC and HSC specimens
exposed to high temperatures In Section3.2, the results of adding fibres on workability of HSC are discussed Section3.3presents the effects of HPP fibres on mechanical properties of HSC, including compressive strength, tensile strength and flexural strength Finally, mechanical behavior of HSC with addition of HPP fibres
at designated high temperatures are described in Section3.4 3.1 Compressive strength of ordinary concrete and HSC against heat The results of the compressive strength test on ordinary con-crete and HSC samples are demonstrated inFig 4at temperatures
of 100°C, 200 °C, and 300 °C It should be noted that the heated samples were tested after removal from the oven, followed by cooling to ambient temperature of about 25 ± 2°C
It was seen that the strength of both NC and HSC reduced when exposed to high temperatures The compressive strength of normal concrete in the face of heat was reduced with a slight slope and reduced by 4%, 8%, and 14% when exposed to temperatures of
100, 200 and 300°C, respectively Compressive strength of HSC decreased significantly such that its strength reduced up to 7.2%, 14.5%, and 27.5% after exposure to 100°C, 200 °C and 300 °C, respectively This proves that the HSC is more susceptible to spalling than normal concrete at high temperatures
3.2 Effects of adding HPP fibres on workability of fresh HSC The slump test was used in order to measure workability of concrete samples.Table 5indicates the slump value of different types of samples
Table 5
Results of the slump test on fresh concrete.
66.12
60.2
54
56
58
60
62
64
66
68
70
Type of Sample Fig 5 The effect of HPP fibres on compressive strength of HSC.
12.2
12.5
11.5
11.7
11.9
12.1
12.3
12.5
12.7
Type of Sample Fig 6 The effect of HPP fibres on splitting tensile strength of HSC.
6.5
6.71
6.8
7.09
6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7 7.1 7.2
Type of Sample Fig 7 The effect of HPP fibres on the flexural strength of HSC.
Trang 5According to mentioned table, the amount of concrete
worka-bility reduced by adding HPP fibres and it continued to reduce by
increasing amount of fibres However, addition of 1 kg HPP fibres
had no significant effect, reducing workability by only 5%
3.3 Effect of adding HPP fibres on mechanical properties of HSC
HSC specimens with different dosages of HPP fibres The results
showed that the compressive strength of the HSC had negligible
change by adding 1 kg fibres The compressive strength decreased
with increasing fibers content; however, this reduction is not
sig-nificant Addition of fibres into HSC led to decrease by 4.3%, and
12.7% in compressive strength of mixtures with fibre content of
2 kg and 3 kg, respectively Similarly, reduction in compressive
strength of concrete due to addition of different types of
polypropylene fibres was obtained in other studies, e.g.[27,35]
This reduction could be attributed to the presence of voids due
to the addition of HPP fibre and the existence of weak interfacial
bonds between the HPP fibers and cement particles[36]
tensile strength of HSC As can be seen, addition of 1 kg fibres did
not affect the tensile strength of HSC The tensile strengths of HSC
specimens containing higher content of fibres were higher than those of the HSC specimens without fibres When the splitting occurred and was sustained, the HPP fibres bridging the split parts of the specimens acted over the stress transfer from the matrix to the fibres, and gradually supported the full tensile stress The transferred stress enhanced the tensile strain capacity
of the concrete matrix, and thus improved the tensile strength of
Table 6
Testing results of HSC specimens with fibres in different temperatures.
Type of Sample Temperatures (°C) Compressive strength (MPa) Tensile strength (MPa) Flexural Strength (MPa)
7-day 28-day change (%) 7-day 28-day change (%) 7-day 28-day change (%)
45
50
55
60
65
70
75
Temperature (° C)
Fig 9 28-day compressive strength of HSC with different dosages of fibres in
different temperatures.
8 9 10 11 12 13
Temperature (° C)
Fig 10 28-day tensile strength of HSC with different dosages of fibres in different temperatures.
4 5 6 7 8
Temperature (° C)
Fig 11 28-day flexural strength of HSC with different dosages of fibres in different temperatures.
Trang 6the fibrous mixtures over the non- fibrous concrete mixture
counterpart
HSC specimens with different contents of fibres The results
show that using fibres increases the flexural strength of HSC
Adding different contents of 1 kg, 2 kg and 3 kg fibres into HSC
caused increase in flexural strength up to 3.1%, 4.6% and 9%,
respectively It could be due to bridging mechanism of fibres
which prevent the growth of cracks and reduce crack width
under flexural test
Overall, through observing the mechanical properties of HSC
equipped with HPP fibres, it can be obtained that the addition of
fibres caused reduction in compressive strength of HSC; however,
this reduction is negligible when 1 kg fibres were added Tensile
strength and flexural strength of concrete increased by addition
of fibres
3.4 Effects of adding HPP fibres on properties of HSC exposed to high
temperatures
HSC specimens without fibre and with fibres were put inside
the furnace and heated to temperatures of 100, 200 and 300°C
The samples were naturally cooled to reach ambient temperature
The results of measurement of compressive strength, tensile
strength and flexural strength of HSC with different dosages of
fibres in different temperatures are recorded in Table 6 and
depicted inFigs 9–11 All of experimentally obtained results are
provided in detail and presented inAppendix A
Generally, it can be seen that the concrete samples lost their
strength when exposed to high temperatures Non-fibrous HSC
were damaged more than fibrous concrete With respect to
Table 6, the plain HSC experienced drops in compressive
strength up to 7.2%, 14.5% and 27.5% when exposed to
temper-atures of 100, 200 and 300°C, respectively However,
compres-sive strength of HSC.1 reduced 5.2, 8.12 and 14% when
exposed to temperatures of 100, 200 and 300°C, respectively
In addition, Table 6 reveals that the specimens with content
of 1 kg fibres showed better performance in the face of high
temperatures compared to other specimens containing higher
amount of fibre Fig 12 shows an exterior of heated HSC
spec-imen reinforced by fibres and the channels formed on the sam-ple are observed
The results are in agreement with other studies Porosity of HSC increases with addition of fibres, resulted in reducing vapor pres-sure in the pores in the deeper concrete areas and control cracking
In addition, the melted polypropylene fibres due to high tempera-tures cause the creation of channels in the concrete mass that allows water vapor to evacuate, releasing pore pressure, gradually reducing the temperature, and decreasing the cracks in the cooling phase[8,22,37–39]
Finally, it should be mentioned that results of previous section showed addition of 1 kg/m3HPP fibres did not sacrifice workability and compressive strength of the concrete while increasing the flex-ural strength of the HSC Thus, it could be concluded that the opti-mum dosage of addition of HPP fibres into concrete is 1 kg/m3, which not only improves the mechanical properties of the HSC, but also caused higher resistance of the HSC at elevated tempera-tures up to 300°C
4 Conclusion This paper investigated effects of different dosages of HPP fibres on mechanical behavior of HSC The effects of high temper-atures on properties of non-fibre and fibrous HSC was also stud-ied Normal concrete was found to be less damaged than HSC when exposed to high temperatures The addition of HPP fibres
to HSC improved the tensile strength and flexural strength of HSC which could be due to distribution of tensile stresses and the prevention of growth of cracks in concrete Addition of HPP fibres reduced compressive strength of HSC which could be due
to fibre compression and reduction in concrete condensation However, this reduction was negligible when 1 kg of fibres were added Adding more than 1 kg/m3 of fibres caused significant reduction in workability of fresh concrete as well It could be con-cluded that the best improvement in properties of HSC was achieved by adding content of 1 kg/m3 fibres The addition of HPP fibres to HSC improved the behavior of concrete when exposed to high temperatures HSC exposed to heat at a temper-ature of 300°C experienced compressive strength reduction of about 28% while a 14% reduction was found in concrete contain-ing 1 kg/m3HPP fibres
Fig 12 View of the channels created by the melting of the fibres in the cube sample.
Trang 7Appendix A
Following tables display the experimentally obtained results for concrete specimens in detail
Table A.1 The 7-day test results of HSC specimens with fibres in different temperatures
Type of Sample Temperatures (°C) Compressive strength
(MPa)
Tensile strength (MPa) Flexural strength (MPa)
⁄ N.A.: Not Available.
Table A.2 The 28-day test results of HSC specimens with fibres in different temperatures
Type of Sample Temperatures (°C) Compressive strength
(MPa)
Tensile strength (MPa) Flexural strength (MPa)
⁄ N.A.: Not Available.
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