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Accelerated Carbonation Test on Concrete Made with Micro-Silica

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The addition of supplementary cementitious materials such as fly ash, ground granulated blast furnace slag, silica fume, microsilica etc. is known to enhance the durability of reinforced concrete structures. In addition of being durability enhancer, inclusion of those materials in concrete also reduces problems of storage. These waste materials (i.e. Fly ash, GGBS, silica fume etc.) can be added to OPC at the time of preparation of concrete or can be blended with OPC clinker and inter-grinded at the time of manufacturing. The effect of such blending on durability properties of concrete structure may or may not be similar to that of addition at the time of casting. However, where relative control at site is less especially in remote areas, the use of blended cements is generally preferred. In this paper an accelerated carbonation test has been carried out to study carbonation of concrete made with addition of microsilica by weight of cement. Concrete cubes were prepared with PPC cement with w/c cement ratio 0.5 and different percentage of addition of micro-silica by different percentage of PPC cement. All concrete cube specimen were exposed to accelerated carbonation environment (CO 2 concentration = 10±0.5%, RH = 60% and temperature 30 o C) after 28 days of moist curing followed by laboratory conditioning. Carbonation depth was measured by spraying phenolphthalein solution on the exposed surface made with spit test at exposure age of 30 days. From the carbonation test results it is observed that carbonation depth is decreases with increase in microsilica content. A small increase in compressive strength also observed with increase in microsilica content.
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International Conference on Recent Trends and Challenges in Civil Engineering
December 12-14, 2014, MNNIT Allahabad, India
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Accelerated Carbonation Test on Concrete Made with Micro-Silica
Manish Kumara and Jitu Kujurb
aM.E. Student, Civil & Environmental Engineering, BIT, Mesra, Ranchi, Jharkhand, 835215
bAssistant Professor, Civil & Environmental Engineering, BIT, Mesra, Ranchi,Jharkhand, 835215
ABSTRACT
The addition of supplementary cementitious materials such as fly ash, ground granulated blast furnace slag, silica
fume, microsilica etc. is known to enhance the durability of reinforced concrete structures. In addition of being
durability enhancer, inclusion of those materials in concrete also reduces problems of storage. These waste materials
(i.e. Fly ash, GGBS, silica fume etc.) can be added to OPC at the time of preparation of concrete or can be blended
with OPC clinker and inter-grinded at the time of manufacturing. The effect of such blending on durability
properties of concrete structure may or may not be similar to that of addition at the time of casting. However, where
relative control at site is less especially in remote areas, the use of blended cements is generally preferred. In this
paper an accelerated carbonation test has been carried out to study carbonation of concrete made with addition of
microsilica by weight of cement. Concrete cubes were prepared with PPC cement with w/c cement ratio 0.5 and
different percentage of addition of micro-silica by different percentage of PPC cement. All concrete cube specimen
were exposed to accelerated carbonation environment (CO2 concentration = 10±0.5%, RH = 60% and temperature
30oC) after 28 days of moist curing followed by laboratory conditioning. Carbonation depth was measured by
spraying phenolphthalein solution on the exposed surface made with spit test at exposure age of 30 days. From the
carbonation test results it is observed that carbonation depth is decreases with increase in microsilica content. A
small increase in compressive strength also observed with increase in microsilica content.
1. INTRODUCTION & OBJECTIVE
Concrete is widely construction material in the world. The Concrete, when used in reinforced concrete structures
should be free from degradation problem. But it is well known that concrete is not free from degradation problem in
aggressive environment. Corrosion of steel in concrete is a major deterioration problem mainly due to carbonation
and presence of chloride ions at reinforcement level. Due to carbonation, the pH of the concrete decreases from pH
12.6 13 to 11 at which protective layer on the reinforcement bar destroyed and corrosion of reinforcement takes
place [1-2].
Atmospheric carbon dioxide diffuses through the pore of the concrete and reacts with alkaline hydration product in
presence of moisture at the surface of pore forming calcium carbonate [5-10]. As the outermost portion of the
concrete becomes carbonated the carbon dioxide will ingress further and as soon as the pH of the concrete at
reinforcement level is nearly neutrality the passivation of the steel will be lost. The penetration depth of this pH is
generally called carbonation depth. The formation of H2CO3 continues so long as the moisture and CO2 are available
in concrete [1-4]. All concrete will end up carbonated with a pH lower than 9 [2]. Because concrete is a micro
porous material the penetration of carbon dioxide will be determined by the form of the pore structure whether the
pores of the concrete are water filled or not. Carbonation also depends on binding capacity the concrete I.e.
composition and cement content in the concrete [1-3]. Further diffusion rate of CO2 depends on permeability o the
concrete when governed by composition, compaction and curing condition and age of concrete. From the Literature
it is observed that most of the studies have been carried out is on carbonation of concrete made with fly ash, GGBFS
[10-16]. Work on carbonation of concrete made with micro-silica is very less. Therefore it planned to perform study
on carbonation of PPC concrete made with addition of micro-silica. The addition of supplementary cementitious
materials such as fly ash, ground granulated blast furnace slag, silica fume, micro-silica etc. is known to enhance the
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durability of reinforced concrete structures. In addition of being durability enhancer, inclusion of those materials in
concrete also reduces problems of storage.
2. EXPERIMENTAL WORK
2.1 Material used
Portland pozzolanic cement (PPC) having 25% pozzolana (fly ash) content and satisfying ASTM type IP and
IS:1489-1991 (Part I ) and having The specific surface area of 354 m2/kg has been used. W/c ratio 0.5 is kept
constant thought. Coarse aggregates of MSA 20 mm of quartzite origin were usedsatisfy the grading requirement of
coarse aggregate as perASTM C 33-92a and IS: 383-1970. Sand from local river conforming to zone III
classificationas per IS: 383 was used as fine aggregate. The specific gravity of coarse and fine aggregates are 2.69
and 2.5. Tap water from laboratory of deep groundwater source was used for mixing. The w/c ratio, cement content,
fine aggregate and coarse aggregate contents of the concrete mixes are presented in Table 1. All the concrete mixes
have been designed as per the guideline given in IS.10262:2009 method for similar workability using super
plasticizer of 0.35 percent by weight of cement with slump range, 60100 mm keeping water content constant to 190
kg/m3.
Table 1.Concrete mix proportion
Cement
w/c
Cement
content (kg/m3)
Fine aggregate
Coarse aggregate
type
ratio
Content (kg/m3)
Content (kg/m3)
PPC
0.5
380
560
1210
2.2 Preparation of Concrete cube and exposure condition
Concrete cubes of size 150mmx150mmx150mm are prepared using PPC cement, w/c ratio= 0.5 demoulded after 24
hours. These cubes were then moist cured for 7 days and 27 days. 7 days of cured cubes were crushed in
compression testing machine after taking out from moist curing tank to determine the compressive strength. Rest of
the cubes were taken out from curing tank after 27 days and kept in an accelerated carbonation chamber for 12
weeks where CO2 concentration, temperature and relative humidity maintained were 10± 0.5%, 33oC and 69± 2%
respectively. The interval of relative humidity most critical for carbonation is from 60 to 70% [2, 11]. The concrete
cubes were kept in laboratory condition for 14 days at temperature 28-33oC and RH 60-65% before exposing to
accelerated carbonation environment.
2.3 Accelerated carbonation:
In order to provide accelerated carbonation to the concrete specimens, an experimental set up was planned based on
the specification given in European draft prCEN/TS 12390- XX:2008 (E) [118] and fabricated as shown in Fig.1.
Experimental set up consists of carbonation chamber, CO2 gas cylinder and CO2 meter. Carbonation chamber was
connected to CO2 gas cylinder through the pipe. An industrial gas cylinder was used with CO2 gas of 99% purity.
Carbonation chamber was fabricated in such a way that it was air tight with a top cover that was removable for
placing and taking out the concrete specimens. An electric fan was also attached at the center of top cover of
carbonation chamber to create aggressive environment through the movement of gas inside the carbonation
chamber.
Carbonation chamber was fabricated with 10 mm thick polymethyle methacrylate (Perspex sheet). Size of the
carbonation chamber was kept 1.3m x 0.92m x 0.35m. The carbonation chamber was fabricated and was connected
to the CO2 gas cylinder through 5mm diameter nylon pipe in series with appropriate control valves. A perforated
platform was fabricated at height of 0.12 m from bottom of the chamber to avoid direct contact between the concrete
specimen and salt solution. A saturated laboratory reagent grade sodium bromide salt solution was kept below this
perforated platform in the chamber to maintain the required relative humidity. Saturated sodium bromide solution is
used to maintain relative humidity of 50-60% as specified in the ASTM E 104. Fig. 3.2 shows the line diagram for
accelerated carbonation set up. Carbon-dioxide concentration (10 ± 0.5%) was maintained inside the carbonation
chamber with above mentioned relative humidity and temperature. Carbon dioxide was injected by means of 5mm
diameter nylon pipes to chamber from CO2 gas cylinder. The pressure gauge was fitted to the carbon dioxide gas
cylinder through which the supply pipes are connected to the chamber. CO2 concentration inside the chamber was
monitored by using CO2 meter regularly at an interval of six hour to check the CO2 concentration inside the chamber
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if less then CO2 gas was injected and brought to required level. A portable thermo-hygrometer was used to measure
the relative humidity and temperature inside the chamber.
Plate 3.1 Experimental set up for accelerated carbonation
Fig. 3.2Line diagram for accelerated carbonation
2.4 Compressive strength and Carbonation depth
The 28 days cube compressive strengths of the concrete mixes were determined in a compression testing machine as
per the guideline given in ASTM C39. Carbonation test was measured by spraying the phenolphthalein solution on
the exposed surface of the cube. Carbonation depth is measure using slide calipers. In this paper an accelerated
carbonation test has been carried out to study carbonation of concrete made with addition of micro silica in PPC
concrete. Concrete cubes were prepared with PPC cement with w/c cement ratio 0.5 and different percentage of
replacement of cement by different percentage of micro silica. All concrete cube specimen were exposed to
accelerated carbonation environment (CO2 concentration = 10±0.5%, RH = 60% and temperature 30oC) after 28
days of moist curing followed by laboratory conditioning. Carbonation depth was measured by spraying
phenolphthalein solution on the exposed surface made with spit test at exposure age of 30 days. From the
carbonation test results it is observed that carbonation depth is decreases with increase in micro silica content.
Compressive strength also increases with increase in micro silica content.
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3. RESULTS & HIGHLIGHTS OF IMPORTANT POINTS
In this paper an accelerated carbonation test has been carried out to study carbonation of concrete made with partial
replacement of cement by microsillica. Concrete cubes were prepared with PPC cement with w/c cement ratio 0.5
and different percentage of replacement of cement by different percentage of microsillica. All concrete cube
specimen were exposed to accelerated carbonation environment (CO2 concentration = 10±0.5%, RH = 60% and
temperature 30oC) after 28 days of moist curing followed by laboratory conditioning. Carbonation depth was
measured by spraying phenolphthalein solution on the exposed surface made with spit test at exposure age of 30
days. From the carbonation test results it is observed that carbonation depth is decreases with increase in microsillica
content. Compressive strength also increases with increase in micro silica content. Results of compressive strength
and carbonation test are tabulated in Table 2.
Table 2. Compressive strength and carbonation test results
Particular of test
Percentage of Microsilica
0%
5%
10%
Compressive strength (7 days) N/mm2
18.4
19.25
20.33
Carbonation depth (mm)
11
9
8
The 7 days cube compressive strengths of the concrete mixes were determined in a compression testing machine.
Concrete cubes made with PPC and w/c=0.5 with addition of micro silica 0%, 5% 10% and 15% are 18.4 n/mm2 ,
19.25 n/mm2, 20.33 n/mm2 respectively. From the result it is observe that compressive strength of the concrete
made with PPC marginally increases with micro silica addition. Although it very early to comment on this nature
because we have only 7 days compressive strength data. some cube are still to be tested for 90 days and 180 days
compressive strength for all addition of micro silica and results will be published in some journal in future with
more results.
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Chapter
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