Efeito da Umidade Relativa e Permeabilidade do Ar na Carbonatação do Concreto

Prévia do material em texto

Effect of relative humidity and air permeability on prediction of
the rate of carbonation of concrete
D. Russell, P. A. M. Basheer, G. I. B. Rankin and A. E. Long
The effect of relative humidity on both air permeability
and rate of carbonation is investigated.Descriptivemodels,
based on data from an accelerated carbonation test, were
developed to quantify the effects of the relative humidity
and the air permeability on the rate of carbonation.
The relationships between the rate of carbonation and
physical properties, such as the permeation properties and
compressive strength were established. It was found that
the permeation measurements should only be used to
estimate the likely rate of carbonation when the relative
humidity of the concrete specimen is known.
1. INTRODUCTION
The moisture content of concrete in service can be decisive in
determining the life span of a concrete structure. This is due,
not only to its effect on the permeation properties of the con-
crete, but also to its effect on various deterioration processes
acting in concrete. Carbonation is one of the mechanisms that
is influenced by both the permeation properties and the
moisture content of the concrete.
The moisture conditions of the cover concrete are most often
expressed in terms of relative humidity (RH). This varies with
variations in the ambient conditions. Therefore, a cyclic wetting
and drying exposure could lead to fluctuations in RH in the
cover concrete.1 In this situation, carbonation effectively stops
when the concrete is saturated and only proceeds when the
concrete dries out enough to allow the ingress of carbon
dioxide. Another possibility is when the concrete is dry enough
to allow the ingress of CO2, but still contains sufficient amounts
of moisture to allow the chemical reactions, necessary for
carbonation, to occur. In this situation carbonation reaches a
maximum level at relative humidities between 50 and 70%.2–4
At relative humidity values above 70% the carbonation slows
down due to the slower rate of diffusion of CO2 through water
filled pores. At relative humidity values below 50% there is
insufficient moisture to allow carbonation reactions to take
place.
The influence of moisture on permeation properties is well
documented.5–9 Nagataki et al.10 proposed that the air
permeability is related to the water evaporated from the
concrete. Parrott11 has related the internal relative humidity to
the air permeability; it has been shown that an increase in
relative humidity caused a decrease in the air permeability of
the concrete. Parrott5 also presented the effect of relative
humidity on air permeability in concrete from different
researchers. The influence of the different concretes on
this relationship is most pronounced at high relative
humidities.
1.1. The effect of air permeability on carbonation of
concrete
Dhir et al.12 have related permeability to carbonation and it has
been shown that concrete with low permeability carbonates at a
lower rate in comparison to one with high permeability. They
have also found that there is a close relationship between
intrinsic permeability of cover concrete and its resistance to
carbonation, irrespective of mix proportions and curing.
The relationship between permeability and carbonation is very
much an interactive one, with the permeation controlling the
ingress of carbon dioxide and the carbonation altering the
permeation properties. Dewaele et al.13 report permeability
reductions of between three and five orders of magnitude in
fully carbonated cement paste compared to uncarbonated
cement paste. This is not the only interactive process involved
in carbonation, but it is significant. It is further complicated by
the fact that both the permeability and the process of
carbonation are affected by the relative humidity, as discussed
previously. This paper discusses the quantitative influence of
the moisture content on both the air permeability and the rate
of carbonation, in an accelerated carbonation test. Furthermore,
the combined effect of air permeability and relative humidity
on carbonation is presented.
2. EXPERIMENTAL PROGRAMME
2.1. Test variables
The test specimens consisted of eight different concrete mixes,
with variables of four water/cement ratios, (w/c) and two
cement contents (CC). Three batches of these eight mixes were
cast for testing at three RH conditions: 55, 65 and 75% RH. The
curing, conditioning and exposure to the CO2 environment
were held constant for all mixes and RH conditions that are
discussed in subsequent sections.
2.2. Mix constituents
Details of the eight mixes used in the test programme are given
in Table 1. Class 42.5N Portland cement (OPC) conforming to
Proceedings of the Institution of
Civil Engineers
Structures & Buildings 146
August 2001 Issue 3
Pages 319^326
Paper 11814
Received 09/09/1998
Accepted 24/01/2001
Keywords:
code of practice & standards/
concrete technology & manu-
facture/field testing & monitoring
Derek Russell
Structural Engineer, Kirk
McClure Morton Consulting
Engineers, Belfast, Northern
Ireland
P. A. Muhammed Basheer
Professor of Structural Materials,
School of Civil Engineering, The
Queen’s University of Belfast,
Northern Ireland
G. I. Barry Rankin
ManagerofConstructionDivision,
Northern Ireland Technology
Centre, TheQueen’s University
of Belfast, Northern Ireland
Adrian E. Long
Dean, Faculty of
Engineering, The Queen’s
University of Belfast,
Northern Ireland
Structures & Buildings 146 Issue 3 Effect of relative humidity and air permeability Russell et al. 319
BS12: 199114 was used in the manufacture of all mixes. The
coarse aggregate used was 10 mm maximum size basalt and
the fine aggregate was medium size natural sand.15 All the
aggregates were oven dried at 408C for at least 24 h and cooled
to 208C prior to casting in order to remove moisture from
aggregates so that it did not affect the effective water/cement
ratio of the mixes. However, in order to remove any effect of
water absorption by aggregates during the mixing, a pre-
determined quantity of water, based on the one-hour water
absorption of the aggregates, was added to the mixing water.
The concrete mixes were designed in accordance with a
procedure given in ‘Design of Concrete Mixes’ by the Building
Research Establishment.16
2.3. Preparation of test specimens
Six 2506 2506 80 mm slabs were cast for each mix for each
of the relative humidity sets. Three slabs from these were used
for the measurements of both the air permeability and the
relative humidity. From the remaining three slabs, 27 cores of
50 mm diameter were removed, which were used for the
determination of the rate of carbonation. Along with the slabs,
100 mm cubes were cast to determine the compressive strength
of the concrete.
The concrete was manufactured in accordance with BS 1881:
Part 12517 using a 0?085 m3 capacity pan mixer. The moulds
were filled in two equal layers and each layer was compacted
using a vibrating table until no air bubbles appeared at the
surface. The concrete in the moulds was then covered with a
polythene sheet and left for 24 h in the laboratory at 208C
(+28C).
After 24 h the specimens were removed from the moulds and
placed in a curing tank at 208C (+18C). After three days the
specimens were removed from the tank and wrapped and sealed
in polythene sheets. The specimens were then placed in a
conditioning room at 208C (+28C) and 55% (+2%) RH for
25 days. At 28 days the cubes were crushed and cores taken
from the three slabs. The curved sides of the cores and the four
sides of the slabs received three coats of Sikaguard, 680 ICOSIT
concrete cosmetic acrylic paint that had in previous tests been
shown to provide good resistance to both CO2 and moisture
penetration. Initial air permeability and relative humidity
readings were taken at this stage before the specimens were
subjected to drying.
2.4. Drying
After initial testing for air permeability and relativehumidity at
the age of 28 days, both the slabs and the cores were placed in
a drying cabinet at 408C (+28C). The drying period depended
on the target moisture conditions required and was selected
based on previous experience with the selected mixes. The
target uniform relative humidities of 55, 65 and 75% after
conditioning resulted in drying periods of 4, 2 and 1 week
respectively.
2.5. Conditioning
After drying, the specimens were conditioned at 408C (+28C)
for 6 weeks to establish a uniform moisture profile in the cover
concrete in which carbonation could occur. In order to achieve
this the specimens were wrapped in 300-gauge polythene sheets
and the joints sealed with heavy-duty parcel tape. This was
based on a method used by Parrott16 wherein it was shown
that the increased temperature allowed the redistribution of
moisture within the concrete to proceed more freely than at
lower temperatures. After the conditioning period, the trowel-
finished face of both the cores and the slabs was treated with
three coats of paint in order to leave only the face cast against
the mould exposed to the environment.
2.6. Accelerated carbonation test
The carbonation test was carried out in a specially built
carbonation chamber, manufactured by LEEC, England. This
chamber was capable of maintaining a constant environment
with respect to temperature, relative humidity and carbon
dioxide concentration. The chamber was set to maintain a 5%
CO2 (+0?5%) concentration at 208C (+18C) during the testing.
The relative humidity was programmed to each of the three
target RH levels when each set of specimens were tested, the
accuracy of the relative humidity control was +1%. The
specimens were exposed to this environment for 6 weeks.
2.7. Measurement of carbonation
The depth of carbonation was measured after each week of the
6-week exposure to carbon dioxide in the chamber. At these
times three cores from each mix were removed from the
chamber, split open longitudinally and the freshly fractured
Mix no. w/c ratio Cement content:
kg/m3
Water content:
kg/m3
Fine aggregate
content:
kg/m3
Course aggregate:
content:
kg/m3
Achieved a/c
ratio
1 0?50 375 192 685 1243 5?14
2 0?57 375 215 664 1207 4?99
3 0?63 375 234 647 1174 4?86
4 0?70 375 256 625 1140 4?71
5 0?50 315 164 732 1329 6?54
6 0?57 315 184 711 1296 6?37
7 0?63 315 201 696 1296 6?23
8 0?70 315 220 679 1233 6?07
Note: Fine aggregae/course aggregate ratio held constant at 0?55.
Table 1. Details of mixes used in investigation
320 Structures & Buildings 146 Issue 3 Effect of relative humidity and air permeability Russell et al.
surfaces were sprayed with a 1% phenolphthalein solution to
determine the depth of carbonation. After 24 h the depth of
carbonation was measured at three locations at right angles to
the exposed surface to an accuracy of 0?5 mm in accordance
with the RILEM recommendations CPC-18.18 An average of
three readings from the three cores was reported as the depth of
carbonation after each week’s exposure in the chamber.
2.8. Measurement of air permeability
The air permeability was measured before the specimens were
exposed to CO2 and at weekly intervals to coincide with
measurement of the carbonation depth. The air permeability
was measured using the Autoclam permeation system7 and the
result reported as an air permeability index (API). The test was
carried out on a total of three 2506 2506 80 mm slabs for
each mix and relative humidity condition. An average air
permeability index was determined from the three results.
2.9. Measurement of relative humidity
The internal relative humidity of the concrete was monitored at
the same time as the air permeability and the depth of
carbonation. It was measured at 0, 10, 20, 30 and 40 mm depths
from the exposed surface. A chilled mirror dew point probe
purchased from Protimeter plc, England was used to measure
the relative humidity in preformed cavities of 22 mm diameter.
The probe was used in conjunction with a shield device to
ensure a constant and sealed volume of air was used in each
test.18
3. EXPERIMENTAL RESULTS ANDDISCUSSION
The 28-day compressive strength values for the eight mixes are
presented in Table 2. The results show that mixes with an
adequate range of quality control were produced. In addition,
there was sufficient repeatability between the three sets of
mixes which were cast several months apart.
3.1. Effect of relative humidity on air permeability of
concrete
The relative humidity profiles were determined to a depth of
40 mm from the concrete surface and the variation over the
depth was found to be within a narrow band of 5% deviation
from the target RH. The relative humidity at 10 mm (RH10) was,
chosen to determine the effect of RH on air permeability. It was
considered that the RH at 10 mm was less likely to be affected
by short term fluctuations in the ambient RH, whilst still
reflecting the RH of the near surface concrete, as the RH
profiles in the specimens were stable. The API values obtained
at both prior to and after the 6 weeks of carbonation are
reported in Table 3.
The changes in API over the carbonation period were erratic,
with the API of some mixes increasing whilst some others
decreased due to the carbonation. However, as can be seen in
Table 3, there was a general trend of slightly increasing API
due to carbonation and this was most pronounced for the
0?7 w/c ratio mixes, particularly the 375 kg/m3 mixes. The API
of the mixes varied differently at the three RH levels. However,
no clear patterns were established.
Parrott16 reported that air permeability remained constant
between 40–70% RH, with API decreasing above 70%. These
results were supported by Nolan.19 The results presented in
Table 3 support these findings, with the exception of the high
w/c ratio mixes, whose API decreased marginally from 55 to
65% RH, particularly in uncarbonated concrete. In addition, the
results show that the lower the RH of the concrete at the time of
carrying out the API test the easier it becomes to distinguish
between the various mixes. This finding is in agreement with
that reported by Nolan. The results also demonstrate the
importance of using the API measured at the same RH for
comparing the relative performance of concretes.
3.2. Effect of relative humidity on carbonation of
concrete
The rates of carbonation (RoC) of all mixes given in Table 4 are
calculated from the results of depth of carbonation over the
6-week period plotted against the square root of the exposure
period. The overall trend in Table 4 is that of decreasing RoC
with increasing RH. However, the RoC does not appear to
change at a constant rate over the range of RH studied. For the
lower w/c ratio mixes the greatest change in RoC occurred
between 55 and 65% target RH. This is the range for which the
change in API was not constant, as highlighted in the previous
section.
Maximum carbonation occurred at 55–65% RH. However, the
API, while not consistent, did not alter significantly within this
range. This would mean other factors might have influenced
the RoC between these RH values. From 65 to 75% RH there
was a decrease in both the carbonation and API. Hence, the
lack of carbon dioxide may be the factor controlling the RoC in
this range of RH values.
The results show that while the RH affects the RoC, the effect is
not the same for all the mixes investigated. The effect of w/c
ratio on API and RoC is the largest at 55% RH and is the least
at 75% RH. This can be attributed to the fact that the pores are
free of moisture at low RH, i.e. the open porosity effect is
obtained at the low RH.
Overall the trend of decreasing RoC with increasing RH within
this range is similar to that which could be expected. However,
what the results show is that there is a wide range of variability
depending on the mix proportions and the quality of the
concrete.
Target relative humidity
Cement 55% 65% 75%
content:
w/c ratiokg/m3 Compressive strength: N/mm2
0?50 375 58?8 61?5 61?7
0?57 375 48?4 51?8 50?4
0?63 375 41?1 42?2 41?1
0?70 375 26?5 33?7 35?5
0?50 315 55?7 63?0 59?7
0?57 315 42?9 50?0 57?8
0?63 315 35?7 44?6 49?5
0?70 315 29?3 35?3 39?5
Table 2. The 28-day compressive strength of mixes used in
investigation
Structures & Buildings 146 Issue 3 Effect of relative humidity and air permeability Russell et al. 321
3.3. Combined influence of API and RH on rate of
carbonation
The previous sections discussed the effect of the RH on both
API and RoC. This section firstly tries to relate the API to the
RoC first and then discusses the combined effect of API and RH
on RoC. The RoC was plotted against the initial and the final
API in Figs 1 and 2 respectively. In these figures the effect of
changing the RH is clear (55% at top to 75% target RH at
bottom). The graphs show that a good relationship can be
determined for individual sets of mixes.
The trend lines in both Figs 1 and 2 for the two cement
contents (CC) are separated. A close examination of Fig. 1, by
comparing the RoC of mixes with the same w/c ratio but
different CC, would reveal
that the RoC did not differ
much between the two
cement contents. That is, the
separation of the two trend
lines is primarily due to the
effect of the CC on the API. A
similar observation can be
made in Fig. 2 as well. The
two figures also demonstrate
the necessity of carrying out
RH measurements when
measuring the API.
The results presented in this
section show that the API
can be related to the RoC.
However, it depends on both
the internal RH and the
cement content. That is, the
many factors influencing the
RoC of concrete make assess-
ment of concrete’s carbonation
resistance difficult to deter-
mine on the basis of API
measurements alone. Then it
should be borne in mind that
API measurements should be
taken in conjunction with RH
measurements. Furthermore,
any additional information
about a concrete, for example, its w/c ratio or CC, or another
material property such as strength, can improve the accuracy
of the prediction.
3.4. Prediction of the rate of carbonation based on
relative humidity and air permeability
In this section, the development of prediction models to
describe the relationship between the rate of carbonation,
the relative humidity and the air permeability of concrete is
discussed. In order to do this multiple linear regression (MLR)
models were employed. The first stage in developing the
MLR models was to ensure that the data was normally
distributed and to transform any data that was not normally
distributed. All of the data used in developing the models
were normally distributed, with the exception of the API data,
which required a logarithmic transformation to provide the
normality.
The next stage was to determine the coefficient of correlation
between the various factors under consideration, namely, the
air permeability and the relative humidity and the rate of
carbonation, Table 5. MLR models were developed with the
variable with the highest coefficient included first. Altogether
three models were developed from this study and these are
presented in Table 6. Two of the models relate the air
permeability to the rate of carbonation whilst the final model
also includes the compressive strength. All of the models in this
section should be considered as descriptive as they relate to
accelerated carbonation tests, not real time exposure. The
intercepts and multipliers for the models are presented in
Table 7. The results obtained from the three models within this
Target relative humidity
Cement 55% 65% 75%
content:
w/c ratio kg/m3 API at start of carbonation: (ln mbar)/min>100
0?50 375 7?28 8?95 2?81
0?57 375 20?90 16?25 7?61
0?63 375 15?64 13?65 5?01
0?70 375 54?63 24?68 13?59
0?50 315 11?35 7?76 4?87
0?57 315 13?81 13?51 5?05
0?63 315 12?40 12?57 4?04
0?70 315 18?53 21?07 13?48
Target relative humidity
Cement 55% 65% 75%
content:
w/c ratio kg/m3 API at end of carbonation: (ln mbar)/min>100
0?50 375 7?71 12?01 2?52
0?57 375 13?79 21?21 7?60
0?63 375 18?62 18?57 9?58
0?70 375 75?52 46?33 22?46
0?50 315 11?16 7?57 4?61
0?57 315 13?15 12?82 4?65
0?63 315 15?75 16?62 5?20
0?70 315 23?69 31?39 14?36
Table 3. Measured API for all mixes before and after 6 weeks of accelerated carbonation
Target relative humidity
Cement 55% 65% 75%
content:
w/c ratio kg/m3 Carbonation rate: mm/(week0?5)
0?50 375 3?32 1?43 1?97
0?57 375 5?56 2?90 2?39
0?63 375 5?49 6?08 3?06
0?70 375 9?84 7?94 2?74
0?50 315 4?40 1?71 1?32
0?57 315 4?48 4?20 2?42
0?63 315 8?72 6?34 2?82
0?70 315 10?96 7?62 4?18
Table 4. Rate of carbonation for all mixes
322 Structures & Buildings 146 Issue 3 Effect of relative humidity and air permeability Russell et al.
section are compared to the actual data in graphical form in
Figs 3, 4 and 5.
3.5. Model based on log (initial air permeability) and
relative humidity
This model combines the logarithm of the initial measurement
of the air permeability (APIi) with the relative humidity at
10 mm depth from the surface
(RH10). The APIi was used to
represent the property of the
concrete’s structure at an
early age and the RH to
indicate the service condi-
tions. The model is presented
below:
RoC = b0 + b1[ln(APIi)]
+ b2(RH10) + eI
1
This model produced a coeffi-
cient of multiple deter-
mination (R2) of 0?595. This
result was disappointing given
the high initial correlation
coefficients. However, it must
be remembered that there was
a wide scatter of API values
for the mixes investigated.
This is evident in Fig. 3,
which presents the model’s
effectiveness graphically. A
close examination of Fig. 3
would indicate that another
possible reason for the low R2
value is due to the nature of
the data itself. A curve rather
than a straight line better
represents the trend in Fig. 3.
3.6. Model based on log
(final air permeability)
and relative humidity
In this model the final air permeability (APIf) data is used to
provide a more mature representation of the concrete’s
condition, while the RH indicates the service conditions. The
model is presented below:
RoC = b0 + b1[log(APIf)] + b2(RH10) + eI2
This model proved slightly better than the previous model
producing a coefficient of multiple determination of 0?644. As
can be seen in Fig. 4 there is less scatter of the data points with
this model. Once again, a curvilinear relationship could better
express the relationship in Fig. 4.
3.7. Model based on log (initial air permeability),
compressive strength and relative humidity
The final model combined the first model with the fcu data for
the mixes. In this model it is considered that both the strength
and permeation measurements would be used in the assessment
of a structure’s carbonation resistance. The model is shown
below:
RoC = b0 +b1[log(APIi)] + b2( fcu) + b3(RH10) + eI3
The fcu is well established as an indicator of concrete quality and
is related to the rate of carbonation of concrete. The addition of
the fcu to the model dramatically improves the coefficient of
12
10
8
6
4
2
0
12
10
8
6
4
2
0
12
10
8
6
4
2
0
0 20 40 60 80 100
Initial air permeability index, API: Ln mBar/min × 100
(c)
(b)
(a)
R
at
e 
of
 c
ar
bo
na
tio
n,
 R
oC
:
m
m
/w
ee
k0
·5
R
at
e 
of
 c
ar
bo
na
tio
n,
 R
oC
:
m
m
/w
ee
k0
·5
R
at
e 
of
 c
ar
bo
na
tio
n,
 R
oC
:
m
m
/w
ee
k0
·5
0·50/375
0·57/375
0·63/375
0·70/375
0·50/315
0·57/315
0·63/315
0·70/315
(w/c)/CC
315 kg/m3 CC, R 2 = 0·51
375 kg/m3 CC, R 2 = 0·96
315 kg/m3 CC, R 2 = 0·83
375 kg/m3 CC, R 2 = 0·66
315 kg/m3 CC, R 2 = 0·60
375 kg/m3 CC, R 2 = 0·71
Fig. 1. Rate of carbonation versus initial permeability index
Correlation coefficient: R
Response variable Rate of carbonation: RoC
Measured properties
Compressive strength ”0.694
Relative humidity, at 10 mm depth ”0?583
Air permeability index
Initial 0?708
Final 0.708
log (air permeabilityindex)
Initial 0?754
Final 0.753
Table 5. Correlation between rate of carbonation and
experimental variables
Structures & Buildings 146 Issue 3 Effect of relative humidity and air permeability Russell et al. 323
correlation to 0?781. This is
presented graphically in
Fig. 5. The results from this
model indicate that the addi-
tion of fcu to the model in
equation (1) provides a better
representation of the likely
rate of carbonation. This is
presumably due to fcu’s non-
dependence on RH while the
API is dependent on the RH.
3.8. Discussion on the
models for describing the
rate of carbonation
All of the models developed
in this study are of the
descriptive type. The models
incorporate both the relative
humidity at 10 mm depth from
the surface and a measure
of the air permeability of
the concrete. The relative
humidity is meant to give
some indication of the service
conditions and the API an
indication of the concrete
quality.
In summary, on the basis of
the results obtained, the API
and RH do not provide a good
indication of the likely rate of
carbonation. However, the
12
10
8
6
4
2
0
12
10
8
6
4
2
0
12
10
8
6
4
2
0
0 20 40 60 80 100
Initial air permeability index, API: Ln mBar/min × 100
(c)
(b)
(a)
R
at
e 
of
 c
ar
bo
na
tio
n,
 R
oC
:
m
m
/w
ee
k0
·5
R
at
e 
of
 c
ar
bo
na
tio
n,
 R
oC
:
m
m
/w
ee
k0
·5
R
at
e 
of
 c
ar
bo
na
tio
n,
 R
oC
:
m
m
/w
ee
k0
·5
0·50/375
0·57/375
0·63/375
0·70/375
0·50/315
0·57/315
0·63/315
0·70/315
(w/c)/CC
CC = 315 kg/m3, R 2 = 1·00
CC = 375 kg/m3, R 2 = 0·95
CC = 315 kg/m3, R 2 = 0·98
CC = 375 kg/m3, R 2 = 0·74
CC = 315 kg/m3, R 2 = 0·77
CC = 375 kg/m3, R 2 = 0·88
Fig. 2. Rate of carbonation versus final air permeability index
Variablity: % Coefficient
Response Variables included of multiple
variable in model Explained Improvement Unexplained determination, R2
Rate of carbonation (RoC) log APIi
log APIi+RH10
56?85
59?52 +2?67
43?15
40?48 0?595
Rate of carbonation (RoC) log APIf
log APIf+RH10
56?70
64?43 +7?73
43?30
35?57 0?644
Rate of carbonation (RoC) log APIi
log APIf+ fcu
log APIf+ fcu+RH10
56?85
63?58
78?11
+6?73
+14?53
43?15
36?42
21?89 0?781
Table 6. Development of MLR descriptive models for the rate of carbonation
Response variable Explanatory variable Intercept Multiplier R2
Rate of carbonation (RoC) log APIi
RH10
2?377 6?041
70?062 0?592
Rate of carbonation (RoC) log APIf
RH10
4?264 4?864
70?080 0?644
Rate of carbonation (RoC) log APIi
fcu
RH10
26?735 70?916
70?183
70?194 0?781
Table 7. Intercepts and multipliers for MLR models developed
324 Structures & Buildings 146 Issue 3 Effect of relative humidity and air permeability Russell et al.
inclusion of the API and RH parameters into a model based
on the compressive strength improves the accuracy of the
model.
4. CONCLUSIONS
On the basis of the results obtained from this investigation the
following conclusions were reached:
(a) The air permeability of the specimens fluctuated during
the course of exposure to carbon dioxide. This contradicts
the generally held belief that carbonation reduces the
permeation properties of the concrete. Further investigation
to assess the effects of pore refinement and shrinkage
cracking during carbonation is required in order to
establish the reasons for the fluctuation in air permeability.
(b) The dependence of both air permeability index and rate
of carbonation on relative humidity was as expected based
on published work. That is, an increase in API with a
decrease in RH and RoC reaches a peak between 55 and
65% RH.
(c) The difficulty in using API in combination with RH
measurements in order to estimate the rate of carbonation
was attributed to the susceptibility of API measurements to
changes in RH. A test that describes the pore structure or
gives an indication of its quality (strength) in combination
with RH measurements may be of more use. Furthermore,
the results may have been influenced by the fact that an
accelerated test was used.
(d ) The models based on permeation properties were found to
be inadequate to describe the rate of carbonation. On the
basis of the results from this investigation air permeability
could not be used to represent accurately the rate of
carbonation. Age and exposure related inputs might be
required to improve these models.
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0
2
4
6
8
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12
0 2 4 6 8 10 12
M
ea
su
re
d 
R
oC
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Please email, fax or post your discussion contributions to the secretary: email: kathleen.hollow@ice.org.uk; fax: +44 (0)20 7799 1325;
or post to Kathleen Hollow, Journals Department, Institution of Civil Engineers, 1^7 Great George Street, London SW1P 3AA.
326 Structures & Buildings 146 Issue 3 Effect of relative humidity and air permeability Russell et al.
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