Reactive Powder Concrete (RPC) is a type of ultra-high strength concrete. Due to its dense microstructure, is vulnerable to explosive spalling at elevated temperatures. Remarkable application of RPC in special structures throughout the world has drawn the attention to understand the performance of RPC at elevated temperatures, which has not been investigated extensively yet. The main objective of this work was to evaluate the performance of RPC at elevated temperatures from 200 C to 800 C, by obtaining residual mechanical properties after exposure. The study aims to find an optimum fiber dosage for spalling protection of RPC. To improve the mechanical properties, RPC incorporating fiber dosage from 0.1% to 0.9% is studied. The thermal deterioration of RPC is assessed using ultrasonic pulse velocity, water absorption and sorptivity. Results shows that 0.1% fiber dosage is enough to control spalling of RPC up to 800 C. To enhance the residual properties of RPC exposed to elevated temperatures, it is recommended to use fiber dosage of 0.5%. The study also includes microstructural analysis of RPC subjected to elevated temperatures, to assess and evaluate the formation of pores and cracks.
Trang 1Performance evaluation of reactive powder concrete with polypropylene
fibers at elevated temperatures
Department of Civil Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore 575025, India
h i g h l i g h t s
Performance of RPC with different fiber dosages at elevated temperatures is investigated
RPC with at least 0.1% polypropylene fiber dosage, checks spalling at elevated temperatures
0.5% fiber dosage, has shown superior fire endurance characteristics
a r t i c l e i n f o
Article history:
Received 31 July 2017
Received in revised form 18 February 2018
Accepted 1 March 2018
Keywords:
Reactive powder concrete
Polypropylene fibers
Elevated temperature
Mechanical properties
Microstructure
a b s t r a c t Reactive Powder Concrete (RPC) is a type of ultra-high strength concrete Due to its dense microstructure,
is vulnerable to explosive spalling at elevated temperatures Remarkable application of RPC in special structures throughout the world has drawn the attention to understand the performance of RPC at ele-vated temperatures, which has not been investigated extensively yet The main objective of this work was to evaluate the performance of RPC at elevated temperatures from 200°C to 800 °C, by obtaining residual mechanical properties after exposure The study aims to find an optimum fiber dosage for spal-ling protection of RPC To improve the mechanical properties, RPC incorporating fiber dosage from 0.1% to 0.9% is studied The thermal deterioration of RPC is assessed using ultrasonic pulse velocity, water absorption and sorptivity Results shows that 0.1% fiber dosage is enough to control spalling of RPC up
to 800°C To enhance the residual properties of RPC exposed to elevated temperatures, it is recom-mended to use fiber dosage of 0.5% The study also includes microstructural analysis of RPC subjected
to elevated temperatures, to assess and evaluate the formation of pores and cracks
Ó 2018 Elsevier Ltd All rights reserved
1 Introduction
Concrete is a composite material consisting of various
ingredi-ents, that are entirely different in their properties from each other
It is very difficult to assess the fire resistance of concrete, due to
different thermal characteristics of each ingredient The most
vencher part is presence of moisture and porosity of the concrete
The utilization of High Strength Concrete (HSC) for last few
dec-ades, throughout the world, has proved itself to be promising
con-struction material [1] But in case of fire performance, some
research studies have shown that HSC has disadvantage to resist
fire, i.e., it is more prone to explosive spalling, due to low
perme-ability and high brittleness when compared to normal strength
concrete[2] The same observation was made in case of High
Per-formance Concrete (HPC) due to dense microstructure and very
low permeability seems to be a disadvantage, in the situations where HPC is exposed to fire[3] From earlier studies, it is proven that HPC, is susceptible to spalling or even explosive spalling when subjected to rapid rise in temperature during fire exposure Spal-ling of concrete depends on many parameters such as ingredients
of mix, type of aggregate, rate of temperature exposure, thermally induced mechanical stress, density of concrete, moisture content etc The main two reasons for explosive spalling of HPC are, ther-mal stress induced by rapid temperature rise and water vapour which may cause high pore vapour pressure To overcome spalling
of concrete, addition of fibers, especially polypropylene fibers to concrete is well known fact in the field of construction Addition
of polypropylene fiber has been proved to be very efficient in reducing spalling of concrete at elevated temperatures Polypropy-lene fibers melt at temperature of 170°C, whereas spalling occurs between 190°C and 250 °C [4] Presence of polypropylene fibers reduces the internal vapour pressure and eliminates the chances
of spalling under fire The length of fibers has significant effect in
https://doi.org/10.1016/j.conbuildmat.2018.03.020
0950-0618/Ó 2018 Elsevier Ltd All rights reserved.
⇑ Corresponding author.
E-mail address: subhashyaragal@yahoo.com (S.C Yaragal).
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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 2preventing explosive spalling of concrete Using 12 mm length of
polypropylene fibers efficiently mitigates the explosive spalling
when compared to 6 mm length[5] This is in agreement with
find-ings of[6]who reports that as length of fiber increases such as 12,
19 and 30 mm, it proves to be more effective in preventing spalling
when compared to shorter polypropylene fibers of length 3 mm
and 6 mm From literature, it is learnt that adding polypropylene
fiber in HSC was good to reduce chances of spalling, but however
some of researchers have reported that this has an adverse effect
on strength
RPC is a new emerging construction material in the modern era
RPC with its high strength and durability properties, has been
grad-ually replacing HSC and HPC, especially in special structures like
long span bridges, tall structures and nuclear power plants As it
is becoming commonly used, the chances of being exposed to high
temperature also increases in the event of accidental fires
How-ever, so far only a few researchers have reported on its
perfor-mance at elevated temperatures RPC has dense microstructure
this seems to be a disadvantage in the situation where the fire
endurance is a necessity The absence of voids does not relieve
the internal stress that creates a major problem This problem
can be solved by addition of polypropylene fibers to the mix
How-ever only a few studies have been carried out on RPC subjected to
elevated temperatures and they also revealed contrary results,
necessitating further research
An experimental investigation made in 2015[7,8]found that,
plain RPC spalled under high temperature and spalling starts at
360°C, whereas RPC with polypropylene fibers shows no spalling
The reason behind spalling and performance of RPC prepared with
different fiber dosages under elevated temperatures remains to be
investigated Furthermore, mechanical properties including
com-pressive strength, split tensile strength and weight loss of RPC,
exposed to elevated temperatures are also of great concern from
the serviceability requirements Experiments indicate that, despite
the positive effects of polypropylene fibers in enhancing the
resid-ual strength of the heated RPC, an overdose of fiber could have an
adverse influence on the RPCs thermomechanical properties A
proper dosage of fibers to improve the spalling resistance of RPC
depends on the mix proportion and the geometry of the fibers In
case of elevated temperatures resistance performance, of RPC
remains a concern, more so in relation to explosive spalling Earlier
researchers have investigated that RPC is vulnerable to explosive
spalling under elevated temperatures, which seriously jeopardizes
the safety of RPC applications Yang et al.[9]studied performance
of RPC under elevated temperature in the range of 400°C–800 °C
Results show considerable reduction in strength and elastic
modu-lus values due to elevated temperatures
Liu and Huang[10], have reported that the residual strength of
RPC at elevated temperatures decreases significantly at
tempera-ture beyond 300°C when compared that of RPC at room
tempera-ture The reduction in strength is mainly because of the pore
pressure mechanism in RPC that prevents water vapour from free
transport within and its escape from the matrix, when exposed
to elevated temperatures Pore pressure mechanism is caused
due to dense microstructure and mainly due to disconnected pores
Explosive spalling occurs when the pore pressure in the matrix
accumulates to a threshold, exceeding the tensile strength of
con-crete Kalifa et al.[11]suggested that mixing polypropylene fibers
could reduce the pore pressure of concrete and decreases the risk
of spalling and also as fiber content increases pore pressure
decreases Essential problem associated in understanding, spalling
of RPC including pore characteristics, pore size distribution, pore
pressure and factors related to explosive spalling are yet, not
exhaustively studied However, dense microstructure of RPC
pre-vents evaporation and escape of free water from the interior
por-tion of RPC specimen at elevated temperatures Due to its low
permeability and discontinuous pore network, the risk of explosive spalling has jeopardized the safety of RPC structure This hinders the commercial development and application of RPC in the field
of modern constructions Therefore, the physical parameters like weight loss, colour change, crack development, mechanical proper-ties such as compressive strength, split tensile strength and water absorption of RPC at elevated temperatures is required to be investigated
The effect of different fiber dosage on spalling has not been reported for RPC Since, the RPC is more likely to spall than HSC,
it is necessary to investigate and understand the spalling behavior
of RPC and recommend optimum fiber dosage to prevent spalling without compromise on fresh and hardened properties The effect
of elevated temperatures up to 800°C on fiber reinforced RPC has been the scope of this study Mechanical properties such as com-pressive strength, split tensile strength and physical parameters like weight loss, crack development at different temperatures were determined Durability properties like water absorption and sorp-tivity have also been studied This study also investigates the degradation of microstructure and its effect on residual mechanical properties of RPC after exposure to elevated temperatures To iden-tify the deuteration portion at microstructural level, RPC speci-mens were subjected to Scanning Electron Microscope (SEM) analysis The SEM results strengthen and reinforce the reason behind reduced mechanical properties after exposure The previ-ous researcher’s results on mechanical properties of RPC contain-ing polypropylene fibers are not in agreement with each other This is due to differences in curing condition of specimen, material used for RPC production and the way of experimentation The opti-mum fiber dosage to prevent explosive spalling and simultane-ously maintaining the residual strength to the expected range for RPC is yet to be investigated in detail Therefore, the focus of pre-sent investigation is to determine minimum dosage of polypropy-lene fibers required to mitigate spalling and to possess acceptable residual strength levels
2 Experimental program 2.1 Materials
RPC is composed of cement, silica fume, quartz powder and sil-ica sand with very low w/b ratio to achieve required workability High range water reducing admixtures are used RPC is cement based concrete mixture In the present study, Ordinary Portland Cement of 53 grade was used which complies with
IS:12269-2013 The chemical and physical properties of cement are shown
in Tables 1 and 2 respectively Silica fume is the second basic important ingredient of RPC which fills the voids of micro particu-lates in the cement It also produces secondary hydrates products
by pozzolanic reaction from the results of primary hydration Undensified silica fume was used in the present study, which com-plies with ASTM C1240-03 a and IS:15388-2003 Chemical compo-sition of undensified silica fume is presented in Table 1 The particle size of silica fume is extremely very fine of size 0.1mm The physical properties of silica fume are tabulated in Table 2 Quartz powder is the finest material compared to cement The par-ticle size of quartz powder ranges from 10mm to 45 mm It acts as a filler material in the mix proportion of RPC The chemical and physical properties of quartz powder are tabulated in Tables 1 and 2respectively Silica sand is largest particle size material in mix proportion of RPC In the present study silica sand was used with particle size ranging from 150mm to 600 mm The sand con-firms to zone IV grading requirement as per IS: 383-2016 To main-tain the required workability, a second generation polycarboxylic ether polymer, high range water reducing superplasticizer Master
Trang 3Glanium Sky 8276 was used in the present study which meets the
requirements of IS: 9013-1999 Polypropylene fibers of length 12
mm were used in the present study
2.2 Mix proportion and specimen preparation
The mixing method adopted in producing RPC is quite involved
when compared to conventional concrete production[12,13] The
only change is the addition of polypropylene fibers and increment
of superplasticizer dosage to maintain required flowability Mix
proportion of RPC adopted is presented inTable 3
The mixing sequencing of RPC as followed is, dry mixing of all
ingredients in first stage, later in the second stage addition of half
volume of water containing half the amount of superplasticizer In
the present study, same mixing sequence was adopted[12]with
one more value-added ingredient that is polypropylene fibers
These fibers were added at the end of third mixing stage with
increased mixing time by two minutes in total mixing duration
The entire mixing process is completed within 14 min Normal
pan mixer with mixing speed of 80 RPM was used After
comple-tion of mixing, fresh mix of RPC was poured in 100 mm cube
moulds and compacted on vibration table to remove air voids
After one day of setting, cubes were removed from moulds and
kept for curing under water for 28 days
2.3 Parameters studied
Effect of different dosages of polypropylene fibers in preventing
explosive spalling of RPC has been focused in the present study
The polypropylene fibers content is varied from 0.1% to 0.9% The
performance of RPC prepared with different fiber dosages at
ele-vated temperatures of 200°C, 400 °C, 600 °C and 800 °C, are
inves-tigated using digitally programable electric muffle furnace as
shown inFig 1 The rate of heating was 5°C/min The retention
period at elevated temperature, is taken as 30 min for all
temper-ature levels The microstructural investigation was carried out on
RPC samples exposed to different temperature levels Physical
parameters, such as colour change and crack development at
dif-ferent elevated temperatures were recorded by physical
observation
2.4 Parameter evaluated
In this study an attempt was made to study the performance of plain RPC at elevated temperature RPC after exposure to elevated temperatures, under goes changes in physical and chemical prop-erties Generally assessment of fire damaged structure, usually starts with visual observation of colour change, crack development and spalling of concrete [14] Changes in physical properties including colour change and crack development have been consid-ered here for assessment after exposure The variation in colour change and initiation of crack for RPC mixes prepared with differ-ent fiber dosages have been evaluated by careful inspection The weight loss at a given temperature was measured from three spec-imens Average percentage of weight loss were determined for RPC specimen prepared with various fibers dosages The weights of samples were taken before and after exposure to elevated temper-atures for weight loss evaluation Portable Ultrasonic Nondestruc-tive Digital Indicating Tester (PUNDIT) measurement is one of the quick methods that indicates, the qualitative degree of damage Deterioration of RPC after exposure to elevated temperatures, is assessed by using PUNDIT The UPV test is conducted on 100 mm cubes as per IS:13311 (Part 1):1992 Compressive strength test was conducted on cubes of 100 mm as per IS: 516-1959 The
resid-Table 1
Chemical composition of RPC ingredients.
Constituents Chemical compositions (%)
Table 2
Physical properties of RPC ingredients.
Mix Constituents Physical properties
Specific surface area/Particle size Specific Gravity Density (kg/m 3
Silica Fume 20,000 (m 2
Table 3
Mix proportion of RPC.
Fig 1 Electric Muffle Furnace.
Trang 4ual compressive strength is evaluated for each temperature level.
Compression testing machine of capacity 2000 kN was used in
the present study Split tensile test was conducted on exposed
100 mm RPC cube specimen The testing procedure as per IS:
5816-1970, was fallowed The most important essential
parame-ters governing the concrete durability is penetration of water,
gas and ions which mainly depends upon micro structure and
porosity of concrete It is well known that RPC consists of dense
microstructure The development of pores and micro cracks under
elevated temperatures has a major impact on durability properties
like water absorption and sorptivity The water absorption test was
carried out on exposed 100 mm RPC cubes as per BS 1881-122 RPC
cubes after exposure to elevated temperature were cooled down to
room temperature Later weight of the cubes were taken and
immersed under water for 24 h After 24 h, the cubes were
removed and kept outside till it reaches to surface saturated
condi-tion then weight of cubes were taken The same procedure was
fol-lowed for all RPC cubes with different fiber dosages, which were
subjected to elevated temperature exposure
Sorptivity test determines the rate of capillary rise in absorption
of water as a function of time when only one surface of the
speci-men is exposed to water ingress by capillary suction, during initial
contact with water Before conducting sorptivity test, four side
sur-faces of exposed RPC cubes were sealed with paraffin wax to
ensure free water movement only through the bottom surface
Then RPC specimen were kept on plastic strip in a tray such that
the free water level was about 5 mm above the bottom surface of
specimen in contact with water The mass of water absorbed per
unit area before immersion and subsequently after intervals of 5
min, 10 min, 20 min, 30 min, 60 min, 180 min, 360 min and 1440
min was determined Test setup of sorptivity is as shown in
Fig 2 Three cubes were used for each test
2.5 Microstructure analysis
In the present study SEM was used to understand the
morphol-ogy of RPC after exposure to different elevated temperatures The
presence of polypropylene fiber in RPC at low temperature
expo-sure, that created channels through melting process of
polypropy-lene fiber was confirmed by secondary electron images at high
magnification using SEM The SEM was aided with Energy
Disper-sive X-ray Spectroscopy (EDS) which facilitates understanding the
elemental composition and atomic weight of chemical compounds
developed in RPC, when exposed to elevated temperatures Based
on the presence of chemical compounds such as Ca, Si, Al and S
and their atomic weight, the ratio of Si to Ca was determined A
detailed microstructural investigation was carried out to
under-stand the structural arrangements of hydrated and unhydrated
products when RPC specimen were exposed to different elevated
temperature levels
3 Results and discussion
3.1 Performance of plain RPC at elevated temperatures Current study includes performance of plain RPC at elevated temperatures The results are as shown inFig 3andTable 4 From results, it can be observed that as temperature increases strength also is observed to increase The increase in strength observed from 100 to 350°C is around 20% The increase in strength may be due to rapid hydration of unreacted cement and silica fume which produce large amounts of hydrated products Thermoactive nature of quartz powder also participates in the hydration process when RPC is exposed to elevated temperatures, that leads to formation of dense hydrated products When plain RPC is exposed to 400°C, explosive blasting of RPC was observed
as shown inFig 4 Therefore, the results of residual strengths of RPC, have been reported in the above Table up to 350°C
3.2 Physical observation The damage to the fiber reinforced RPC, after being exposed to elevated temperature can be roughly detected by observing the RPC surface.Fig 5(a)–(c) shows RPC surface with different fiber dosage exposed to various elevated temperature levels (200°C,
400°C, 600 °C, 800 °C), along with the one at ambient temperature FromFig 5(a)–(c) it can be observed that, at 200°C, there is no colour change and visible crack development on RPC surface At
400°C, formation of light micro cracks was observed on RPC sur-face composed of 0.1% fiber dosage The RPC cubes composed of 0.5% and 0.9% have not shown any crack development at 400°C
of exposure RPC exposed to 600°C has shown visible cracks on surface of all RPC cubes composed of different fiber dosages Among three different fiber contents, the RPC prepared with low fiber dosage (i.e 0.1%) has shown considerable surface cracks, compared to other RPC prepared with high fiber dosages From
Fig 5it can be observed that as fiber content increases number
of cracks appears to get reduced The presence of small pores were observed on the surface of RPC specimen composed of 0.1% fiber content at 600°C The presence of these pores were less in number
on surface of RPC specimen composed of 0.5 and 0.9% fiber dosages The slight colour change was observed in this tempera-ture range Colour of cubes turns to slight pink reddish and the intensity of colour decreases as fiber dosage increases
When RPC cubes were exposed to 800°C, the development of crack was more pronounced, especially for RPC cubes produced with 0.1% fiber as shown in Fig 5(a) The RPC specimen have
Trang 5shown, major colour change when exposed to 800°C FromFig 5
(a), it can be observed that colour of cubes turns to grey reddish
The presence of more surface voids was observed on RPC
com-posed of low fiber dosages Spalling was not observed in entire
exposure condition at elevated temperatures This indicates that
0.1% of polypropylene fiber is sufficient to prevent explosive
spal-ling of RPC up to 800°C
From the above observation, it can be concluded that, as fiber
content increases, leading to decreases in surface cracks and pores
The thick cracks were observed on RPC samples composed of low
fiber dosage at elevated temperatures This is may be due to low
fiber dosage inefficient to reduce spalling of concrete The vapour pressure created inside the concrete at elevated temperature leads
to formation of micro cracks These small width cracks grew to large widths at higher elevated temperatures
3.3 Weight loss
Fig 6shows the variation of average percentage loss in weight with elevated temperature for RPC specimen of different fiber dosages From Fig 6, it can be observed that as temperature increases, there is an increase in percentage weight loss in all RPC specimen prepared with different dosages In case of RPC pre-pared with 0.1% fiber dosage show, lower percentage of weight loss
at elevated temperatures compared to other RPC mixes prepared with 0.5% and 0.9% This is because at temperatures above 170
°C, polypropylene fibers melt and create channels inside the con-crete RPC with high dosage of fiber, suffer more weight loss due
to fiber evaporation at high temperatures
At temperatures, above 600°C, these fibers turn to molten state and evaporates As fiber content increase, the rate of evaporation also increases Therefore, there is an increase in percentage weight loss with increase in fiber dosage at elevated temperatures The weight loss predominantly occurs, due to loss of water in all three forms, namely free water, adsorbed water and chemically bounded water From results, it can be observed that there is no significant difference observed between RPC mixes prepared with different fiber dosages at elevated temperature of 200°C
Table 4
Compressive strength results at elevated temperatures.
Fig 4 Explosive blasting of plain RPC at elevated temperatures.
(a)
(b)
(c)
Trang 6At 400°C the percentage weight loss, is more for RPC mix
pre-pared with 0.9% fiber dosage As fiber content increases the
per-centage weight loss is also increasing with temperature[15] The
same trend can be observed at 600 and 800°C At 800 °C,
maxi-mum variation in weight loss was observed between RPC mixes
prepared with different fiber dosages There seems to be
approxi-mately 5% and 7% variation in weight loss with 0.5% and 0.9% fiber
dosage when compared to 0.1% fiber dosage
There is a sudden increase in weight loss beyond 400°C The
rate of weight loss increase drastically as fiber dosage increase
beyond 400°C From the above observations, it can be stated that
the apparent bulk mass loss occurs between 400°C and 800 °C
This loss may be due to evaporation of water and in addition
beyond 400°C there may be decomposition of hydrated and
unhy-drated compounds The process of dehydration starts beyond 400
°C, which release considerable amount of chemically bound water
creating the interior pores At higher temperature levels,
polypropylene fibers have been completely vaporized Hence
cumulative effects of these parameters make RPC specimen to
suf-fer considerable amount of weight under elevated temperatures
3.4 Ultrasonic pulse velocity
This test is a qualitative one, used to evaluate the quality of
con-crete and this technique is sensitive to degradation phenomena
including internal cracking and other deuteration due to thermal
treatment UPV test was carried out to determine the severity of
damage, when RPC specimen were exposed to elevated
temperatures
Fig 7shows results of RPC specimen prepared with various
fiber dosages subjected to elevated temperatures From Fig 7it
can be observed that, as fiber dosage increase, the UPV also
increases at room temperature In case of 0.1% fiber dosage, the
UPV results show sudden drop in velocity beyond 200°C
For the case of 0.1% fiber dosage, the value of UPV decreases
continuously with increase in temperature up to 400°C The
smal-ler the relative UPV value, more is the severity of the damage The
reduction in UPV value in 0.1% fiber dosage, is due to insufficient
fiber content which is unable to release vapour pressure from core
of concrete, that leads to micro-cracks As number of cracks
increases there will be chances of discontinuous matrix and
forma-tion of voids This leads to reducforma-tion in UPV values
The RPC mixes, prepared with 0.5 and 0.9% fiber dosages have
shown higher UPV values compared to RPC mix prepared with
0.1% fiber dosage for 200°C and 400 °C Values for the case of 0.1% fiber dosage, shows reduction in velocity from 4.78 to 2.00 km/s with increase, in temperature from 27°C to 800 °C and con-sequently gradual deterioration in the quality of concrete It is observed that, beyond 200°C the UPV values decrease rapidly, due to sharp deterioration in the physical state of exposed RPC Beyond 400°C the UPV values of RPC mixes prepared with 0.5% and 0.9% fiber dosages have shown a large decrease This is may
be due to evaporation of fibers that create channels which leads
to increase of internal micro cracks As number of microcracks increase, the quality of concrete decreases directly This leads to higher reduction in UPV values for cases of 0.5% and 0.9% fiber dosage when compared to the case with 0.1% fiber dosage for
600°C and 800 °C
The lowest UPV value, was observed for RPC specimen prepared with 0.9% fiber dosage, and at 800°C Formation of pores and cracks through melting of fibers in case of higher dosage of fibers, under elevated temperatures leads to physicochemical changes in cement paste and thermal incompatibility between cement paste and aggregate which is, believed to be responsible for the deterio-ration in mechanical properties
At 800°C, the UPV values for 0.1% dosage, is quite higher than for the specimen prepared with 0.9% This is due to less number
of cracks developed inside the concrete after evaporation but in case of 0.5% and 0.9% fiber dosage, the number of microcracks in specimen seems to be more, hence there is sharp decrease in UPV values at 800°C From the above discussion, it was observed that as fiber dosage increases, UPV value also increases at room temperature as well as at 200°C Beyond 200 °C there is a sharp decrease in UPV values for 0.1% and 0.9% fiber induced RPC speci-men After 400°C, also 0.5% fiber embedded RPC specimen have shown better UPV values, that fall in the range of good quality of concrete as perTable 5
3.5 Compressive strength RPC belongs to HSC category, the dense microstructure of RPC does not allow release of pressure, created by vapour under ele-vated temperatures This may lead to development of internal cracks in the cement matrix zone of RPC Which in turn may cause spalling of concrete, so to reduce this event from happening, differ-ent polypropylene fiber dosages were added to RPC mixes to study their performance at elevated temperatures Compressive strength
at room temperature and residual compressive strengths at
differ-Fig 6 Average percentage weight loss at different elevated temperatures.
Trang 7ent elevated temperatures, for RPC with different fiber dosages, are
presented inTable 6and in Fig 8 Normalized residual strength
variation of RPC at elevated temperatures is also shown inFig 9
Increase in fiber dosage has shown increase in compressive
strength at room temperature At 200°C, polypropylene fiber
dosage of 0.1, 0.5 and 0.9% has shown 3, 4 and 4% higher
compres-sive strength compared to RPC at room temperature respectively
The increased compressive strength is due to strengthened
hydrated cement paste after evaporation of free water at 200°C,
which leads to greater Van der waals forces that cause cement
gel layer to come closer to each other[16]
At 400°C exposure, the compressive strengths have increased
drastically for all RPC mixes as seen fromFig 9 RPC mix prepared
with 0.1, 0.5 and 0.9% fiber dosage have shown 6, 13 and 13%
increase in compressive strengths respectively when compared
to RPC at room temperature Increase in strength is mainly
attrib-uted to further hydration process, with catalyzed hydration
through non-reacted cement products, in the presence of steam
upshot under autoclaving effect formed in pastes, which is
consid-ered as advancement in chemical bonding process [17] Among
three different fiber dosages, 0.5% and 0.9% have shown consider-able higher compressive strengths up to elevated temperature of
400°C FromFig 8it can be observed that up to 400°C as fiber tent increases strength is also observed to increase Porosity of con-crete has a significant impact on pore vapour pressure Polypropylene fibers melt at temperature less than 300°C, which results in an increase in concrete porosity and creation of more escape routes leading to reduction in bond water vapour pressure However, melting of polypropylene fibers causes thermal incom-patibility between the aggregate and cement paste which leads
to increase in free space and creates a thermal shock absorber The melting of polypropylene fibers is beneficial for evaporation
of water vapour and improves the compressive strength of RPC,
up to 0.5% Fiber dosage of 0.5% in RPC has shown best performance
at elevated temperatures
Beyond 600°C the residual compressive strength decreases sig-nificantly for all RPC mixes prepared with different fiber dosages This decrease is due to the transformation of calcium hydroxide
to calcium oxide in the range of 400°C to 500 °C and reduction and disintegration of Calcium Silicate Hydrated between 400°C and 600°C[17] Among three fiber dosage, RPC with 0.5% fiber con-tent has shown highest residual compressive strength This may be due to right fiber dosage addition leads to proper and proportion-ate channels for release of vapour pressure there by reducing the chances of spalling However, at higher fiber dosage than optimum, due to more pores, the strength suffers more
The residual strength of RPC with 0.1% fiber dosage after expo-sure to 600°C was 77.3% of its original strength at 27 °C, that is 22.7% reduction in compressive strength For RPC with 0.5% fiber
Fig 7 UPV results of RPC exposed to different elevated temperatures.
Table 5
Velocity criterion for concrete quality grading [IS:13311 (Part 1):1992].
Sl No Pulse velocity (km/s) Concrete quality grading
Table 6
Compressive strength results of RPC at elevated temperatures.
Temperature (°C) Fiber dosage (%)
Strength MPa Normalized Strength Strength MPa Normalized Strength Strength MPa Normalized Strength
Trang 8dosage, the residual strength was 90% of its original strength at 27
°C, which was much more than residual strength obtained for RPC
with 0.1% fiber dosage The reduction in compressive strength was
about 10% for RPC composed of 0.5% dosage at 600°C RPC with
0.9% fiber dosage at 600°C has shown 22% reduction in
compres-sive strength which is almost same as that of 0.1% fiber dosage at
600°C This shows that minimum fiber dosage 0.1% is not efficient
to maintain residual strength within acceptable range as well as
0.9% fiber dosage seems to be over dosage for RPC under elevated
temperatures Results indicate that, 0.5% fiber dosage effectively
serves the purpose of reducing explosive spalling as well as
main-tain reasonable residual strength The same observation was made
in the study of polypropylene reinforced concrete at elevated
tem-perature[18]
RPC mixes exposed to 800°C have shown considerable strength
loss The residual compressive strength obtained for RPC with 0.1%
fiber dosage at 800°C was around 68% which indicates 32% loss in
strength However, for 0.5% fiber dosage, the residual strength
being 76% of its original strength at room temperature For 0.5%
fiber dosage, there was only 24% reduction in strength which is
comparatively better than 0.1% fiber dosage
The drastic reduction in residual compressive strength (40%)
was observed for RPC with 0.9% fiber content This may be due to
adverse effect of over dosage of fibers that leads to creation of large
number of channels due to evaporation of fibers at high tempera-ture These channels propagate and enlarge in size causing early failure of concrete The reduced strength at 800°C is due to trans-formation of quartz fromatob form that cause the volumetric expansion of the RPC at approximately 571°C, which results in reduction of bonding between the aggregate and cement paste
[19] The other strong reason for reduced strength at 800°C is the decomposition of calcium hydrate gel that causes severe dete-rioration of RPC[20]
3.6 Split tensile strength The split tensile strength results of RPC with fiber dosage of 0.1, 0.5 and 0.9% at room temperature and at elevated temperatures are presented inTable 7 The variation of split tensile strength is also shown inFigs 10 and 11 At 200°C the split tensile strength of RPC increases considerably when compared to RPC at 27°C From
Fig 10it can be observed that, as fiber content increases strength
is also increasing up to 400°C This shows positive impact of polypropylene fibers in strength enhancement of RPC under tensile loading All three-different fiber dosage i.e 0.1, 0.5 and 0.9% have shown 26, 28 and 18% increase in split tensile strength at 200°C respectively
Fig 8 Compressive strength results of RPC at elevated temperature.
Fig 9 Normalized compressive strength of RPC at elevated temperature.
Trang 9The increased split tensile strength was also observed when RPC
specimen exposed to 400°C There was about 50, 56 and 45%
increase in tensile strength observed for fiber dosage of 0.1, 0.5
and 0.9% respectively The drastic increase of strength of RPC for
all fiber dosages is due to development of secondary hydrated
and conversion of remaining unhydrated cement grains that
rapidly hydrated producing secondary hydrated gel with active
participation of silica fume at this temperature
The increase in strengths is may be due to bonding effect
between fiber and matrix as we can observe from Fig 10 with
increase in fiber dosage strength is also observed to increase This
indicates positive influence of fiber effect on tensile strength
enhancement of RPC up to 400°C The tensile strength of RPC at
400°C increases due to autoclave effect and the creation of shorter and stronger siloxone elements that causes an increase in strength
in temperature range of 250–350°C[19] The drastic decrease in tensile strength of RPC after 600°C was observed for RPC mixes with various fiber dosages The RPC with 0.5% fiber dosage has shown less deterioration in tensile strength The residual tensile strength is around 91% of its original strength
at 27°C[21] For RPC with 0.9% fiber dosage the residual strength
is less when compared to 0.5% fiber dosage and it is around 78% of its original strength at 27°C The lowest tensile strength of RPC with high dosage of polypropylene fibers is possibly be due to dense network of melted channels created through evaporation
of fibers under elevated temperature that accumulate at a single
Table 7
Split tensile strength results of RPC at elevated temperatures.
Temperature (°C) Fiber dosage (%)
Strength MPa Normalized Strength Strength MPa Normalized Strength Strength MPa Normalized Strength
Fig 10 Split tensile strength of RPC after exposure to elevated temperature.
Fig 11 Normalized split tensile strength at elevated temperature.
Trang 10place, initiating cracks That leads to sudden failure of RPC
speci-men under tensile loading
The similar strength deterioration trend was observed when
RPC specimen were exposed to 800°C The reduction in strength
is mainly because of chemical degradation and microcracking
due to excessive pore pressure and thermal incompatibilities
between aggregate and cement paste The other reason for large
strength deterioration is phase changes of quartz aggregates At
high temperature more than 600°C transformation of quartz from
atob form causes the volumetric expansion of RPC at
approxi-mately 571°C, which results in reduction of bonding between
the aggregate and cement paste[22]
RPC with 0.5% fiber dosage has shown good residual strength
when compared to RPC with 0.1% and 0.9% fiber dosages The mean
high strength is due to proper bonding between aggregate and
cement paste without major cracks developed due to channels
cre-ated through melting of polypropylene fibers However, after
expo-sure to 800°C, still the residual strength is around 45% of its
original strength at 27°C RPC with 0.9% fiber dosage has shown
reduced split tensile strength (30%) compared to RPC with 0.5%
fiber dosage The addition of higher content of fibers creates
con-tinuous and dense channel that leads to propagation of micro
cracks to large scale This phenomenon causes deterioration of
con-crete under low tensile loading condition
Th reduction of tensile strength at 600°C and 800 °C due to
pores and channels created due to evaporation of bond water
and melting of fibers, which increase the internal defects of RPC
matrix and also weaken the bonding between cement paste and
aggregate This is considered as adverse effect of over dosage of
polypropylene fibers On the contrary polypropylene fiber has
pos-itive effect, if and only when optimum fiber content is embedded
in RPC The maximum reduction of tensile strength at 800°C is
due to phase change of quarzitic material As temperature
increases, the tetrahedral chains of quartz molecules gets
elon-gated and reorient, leading to significant volume increase, that
causes radial cracking around the perimeter of the particle in
heated specimen[23]
3.7 Water absorption
The most important essential parameters governing the
con-crete durability is penetration of water, gas and ions which mainly
depends upon micro structure and porosity of concrete It is well
known that RPC consists of dense microstructure The
develop-ment of pores and micro cracks under elevated temperatures has
a major impact on durability properties like water absorption
and sorptivity Hence water absorption studies on exposed RPC
specimen were carried out, to assess the properties like intrinsic
porosity and permeability of concrete RPC cubes after exposure
to elevated temperatures were cooled down to room temperature
Later weight of the cubes were taken and immersed under water
for 24 h After 24 h, the cubes were removed and kept outside till
it reaches to surface saturated condition then weight of cubes were
taken
The quantity or volume of moisture, that enters concrete,
depends on the concrete permeability and interconnectivity
between pores Results of water absorption of RPC specimen with
different fiber dosages, and exposed to elevated temperatures are
presented inFig 12
It is observed that, as temperature of exposure increases water
absorption is also increasing At 200°C, there is no much difference
between water absorption values obtained for RPC with different
fiber dosages At 400°C percentage of water absorption for RPC
specimen with 0.9% fiber dosage has shown a little higher value
compared to RPC specimen with 0.5% and 0.1% The increase in
water absorption, is may be due to penetration of more water
through surface voids and channels created by melted polypropy-lene fibers Strength enhancing chemical compounds starts to decay from 450°C The decomposition of these compounds creates porosity in internal matrix RPC specimen after exposure to 600°C has shown considerable amount of water absorption The results also show that as fiber content increases, the percentage of water absorption is also observed to increase This is obvious as fiber con-tent increases, the rate of fiber evaporation also increases at ele-vated temperatures, which leaves more number of channels and enhances interconnected voids through which water easily pene-trates into the body of concrete, thus increasing the water absorp-tion value The presence of molten fiber and channels created by melting of fibers are confirmed through SEM images at different magnifications which is discussed in Section3.9
Among the three fiber dosages, RPC with 0.9% fiber content has shown higher percentage of water absorption after exposure to elevated temperatures Also, the rate of water absorption increased rapidly beyond 400°C for all fiber contents The percentage of water absorption till 400°C was lower than 4%, this may be due
to the presence of molten polypropylene fibers blocking the chan-nels and not allowing water to enter the concrete core However, at higher temperatures, due to melting of fibers more channels are created through which water has easy access into the body of con-crete At 800°C, the RPC with 0.9% fiber dosage has 9.7% of water absorption, which is comparatively higher than RPC specimen pre-pared with 0.5 and 0.1% fiber dosages The bunches of inter con-nected channels created through melting of fibers, makes the concrete much more porous, which is the reason for higher water absorption values
3.8 Water sorptivity After RPC exposure to elevated temperatures, the water sorptiv-ity of the specimen were assessed to determine the inner concrete properties, since the test is directly related to the formation of pores and cracks in the heated RPC specimen.Fig 13, shows results
of water absorption per unit area for RPC with 0.1% fiber dosage at different elevated temperatures The rate of water sorptivity increased sharply with increase in temperature From Fig 13it can be observed that, at 200°C the rate of sorptivity increases up
to 20 min and then on it gradually decreases In case of 400°C the rate of sorptivity increases till 30 min
This is likely due to more surface damage of RPC at 400°C that leads to propagation of surface cracks These cracks allow water to penetrate inside the body of concrete increasing sorptivity When RPC specimen were exposed to 600°C there is colour change and formation of visible hair cracks on surface of RPC specimen The sorptivity results at 600°C, show decrease in rate of sorp-tivity after 20 min This is because the heated RPC specimen absorb more water to fill the voids and pores, and also shrinkage cracks created due to thermal effect within short period of time Later the rate of water absorption reduces due to saturation condition attained inside the body of concrete From theFig 13it is further observed that, the high rate of sorptivity was obtained for the RPC exposed for 800°C It reaches to 29.11 10 4
mm/min0.5 within 10 min duration, after which sorptivity sharply drops From the overall sorptivity results it can be concluded that, the rate and total sorptivity of RPC specimen after exposure can be attributed to the effect of moisture loss and crack development which in turn is due to thermal incompatibility between cement paste and aggre-gate under elevated temperatures
Fig 14shows sorptivity results of RPC with 0.5% fiber dosage, after exposure to elevated temperatures At 200°C, the sorptivity value is gradually increasing till 30 min where it attains maximum value of sorptivity 1.22 10 4
mm/min0.5, which is comparatively less than RPC with 0.1% fiber dosage at 200°C This is due to the