instruction for preparing a paper for a symposium in dubrovnik

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instruction for preparing a paper for a symposium in dubrovnik
CONCRETE EXPOSURE CLASSES CONCERNING
CORROSION INDUCED BY CHLORIDES – CRITERIONS
– EXSPERIMENTAL RESULTS AND POSSIBLE
MISLEADS
Elica Marušić*, Nedjeljko Akrap*, Tin Dragović*, Inga Čujić* & Marija
Jurašin*
*Institut IGH, d.d., Laboratory IGH
Regional unit in Split, Matice hrvatske 15, 21000 Split, Croatia
e-mail: [email protected], web page:www.igh.hr
Keywords: concrete, chlorides, exposure classes, criterions
Abstract: Chloride migration coefficient is one of the main parameters for describing the
durability of concrete constructions concerning the risk of reinforcement corrosion caused
by chlorides. Exposure classes related to corrosion induced by chlorides from sea water
are defined in Croatian regulation, but criterions for satisfying exposure classes aren't. In
Croatian practice these criterions are usually defined by values of chloride migration
coefficient tested on concrete according to standard NT BUILD 492 from non-steady-state
migration experiment There are also some values of mentioned coefficient that are
accepted in practice for defining exposure classes, but they aren't officially approved.
During performance of laboratory tests it was noticed that results obtained by test method
NT BUILD 492 on same concrete were significantly different depending on moisture
content of concrete although preconditioning was prescribed in the test method. So, results
obtained by tests could mislead and put the same concrete in different exposure classes.
In order to find the improvement for test method, interlaboratory test was organized.
Results are shown in this article together with suggestion for improvement of standard NT
BUILD 492 and suggestion for defining state of saturation in project requirements together
with required value of chloride migration coefficient.
Besides, as permeability properties, especially of concrete with mineral additives, depend
on age of concrete, it is necessary to prescribe age in which chloride migration coefficient
should have required value.
1. INTRODUCTION
In this study suggestions for defining criterions for exposure classes of concrete will be
presented. Before defining criterion values it is necessary to choose test method. Criterion
value will depend on it.
2. CHLORIDE MIGRATION COEFFICIENT –TEST METHOD
Since 2005 in IGH laboratory for testing concrete in Split chloride migration coefficient has
been tested according to NT BUILD 4928 on more then 300 concrete specimens. For part of
these specimens reliable data exist about concrete composition because mixes were
prepared in laboratory. In some cases tests were performed on specimens which were dried
before preconditioning in Ca(OH)2 according to NT BUILD 492. On such specimens it was
noticed that results had surprisingly high values.
The conclusion was that it had been happening because preconditioning according to NT
BUILD 492 was insufficient for obtaining such specimen saturation which wouldn't cause
significant differences in test results on different specimens made from same concrete but
cured in different conditions.
2.1 Interlaboratory test results
In 2007 we organized interlaboratory test7 to exclude the possibility of mistake in
performing test method. Three laboratories took participation in interlaboratory test: IGH
laboratory in Split, IGH laboratory in Zagreb and Laboratory of Faculty of Civil
Engineering in Zagreb. In Croatia they are the only laboratories which were performing that
method. All three laboratories have similar equipment produced by Germann Instruments,
which is one modification of equipment prescribed in NT BUILD 492. One mix of high
strength concrete (68,1 MPa after 28 days) was prepared, ten samples were made: six cubes
15x15x15 cm and four cylinders with diameter of 100 mm and height 200 mm. Finally,
after drilling and sawing there were twenty four specimens in total (cylinders with diameter
of 100 mm and height of 50 mm) for testing. Each laboratory had eight specimens to test.
Specimens were distributed in laboratories so that each laboratory had similar combination
of specimens and positions. After curing of more then forty days in water, half of total
number of specimens in each laboratory were dried to constant mass in oven on
temperature of 105±5°C and then cooled in dessicator, so that before start of testing in each
laboratory there were four specimens dried to constant mass and four specimens saturated
in water to constant mass. After that, each specimen was tested according to test method
NT BUILD 492. Preconditioning for surface dried specimens in saturated Ca(OH) 2 with
vacuum treatment were performed for all specimens, no matter they were dried or water
saturated. Preconditioning takes place for twenty to twenty four hours.
Test results confirmed presumptions that preconditioning defined in test method was
insufficient and that state of saturation before preconditioning had significant influence on
test results.
2.2 Chloride migration coefficient – influence of moisture content
Short analyze of interlaboratory test results is presented on figure 1 and in table 1. Diagram
on figure 1 shows chloride migration coefficient test results grouped according to state of
saturation before preconditioning, in one group there are specimens which were dried and
in the other group there are specimens which were water saturated. In each group there are
two subgroups according to specimen origin: specimens drilled from cubes and then sawn
and specimens sawn from cylinders.
40
dried specimens - from cube
dried specimens - from cylinder
saturated specimens - from cube
saturated specimens - from cylinder
dried specimens - from cube - average
dried specimens - from cylinder - average
saturated specimens - from cube - average
saturated specimens - from cylinder - average
-12
2
chloride migration coefficien (x10 m /s)
DRIED SPECIMENS
35
30
specimens taken from
cube 15x15x15 cm
specimens cut from
cylinder 10 x 20 cm
25
20
15
WATER SATURATED SPECIMENS
10
specimens taken from
cube 15x15x15 cm
5
specimens cut from
cylinder 10 x 20 cm
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
specimen
Figure 1: Results of chloride migration coefficient grouped according to state of saturation
before preconditioning7
Chloride migration coefficient
Origin of
specimen
(diameter 100
mm and 50 mm)
Average
value
10-12 m2/s
Standard deviation
Repeata- Reproducerepeata- reproduce- repeata- reproduce- bility
bility
bility
bility
bility
bility
sr
sR
sr
sR
r
R
10-12 m2/s
%
%
Specimens water saturated before preconditioning
Specimens taken
from cube
Specimens taken
from cylinder
3,08
0,40
0,46
13,0
14,9
36,8
42,1
2,39
0,11
0,24
4,6
10,0
13,0
28,3
Specimens dried before preconditioning
Specimens taken
from cube
Specimens taken
from cylinder
24,19
1,06
4,32
4,4
17,9
12,4
50,6
23,35
4,60
8,43
19,7
36,1
55,7
102,1
Table 1: Summary of test results
Average value of chloride migration coefficient for all water saturated specimens is 2,73x
10-12 m2/s.
Average value of chloride migration coefficient for all dried specimens is 23,77x 10-12 m2/s.
As in test method there are no data about precision, obtained precision could be an
indication. Experimental results show that precision of test method for specimens which
were water saturated before preconditioning according to the method is better then that for
dried specimens. Test results of dried specimens sometimes show total penetration of
chlorides through specimen. In such cases test result is questionable. Real coefficient is
probably greater.
Average value of chloride migration coefficient for samples which were dried before
preconditioning according to the method (23,77x10-12 m2/s) is 8,7 times greater then that for
samples which were water saturated before preconditioning (2,73x10-12 m2/s). Such
significant difference in values of chloride migration coefficient leads to conclusion of
insufficiency of prescribed preconditioning in the method. It is especially important in case
when criterions which concrete should satisfy are defined.
Specimens which were taken from cylinder show slightly better results then that taken from
cube.
Presently, new program is in progress where different moisture contents were taken into
account, not only extreme states of saturation as totally water saturated and oven dried
concrete specimens. Those tests haven’t yet been completed.
2.3 Chloride migration coefficient – influence of age of concrete
It isn’t recommended to test early age concrete because chloride migration coefficient
decreases with age of concrete due to filling of cement stone structure. It is especially
noticeable in concrete with addition of micro silica. As chloride attack is type of aggression
which develops in time, only thin layer of concrete will be attacked in early age of concrete.
Early age chloride migration coefficient should be used as indication of coefficient in later
ages.
According to experimental results, chloride migration coefficient tested in age of 7 days is
two times greater then that tested in age of 30 to 40 days. With further aging decrease of
chloride migration coefficient is lower. After three months it decreases for further 10 to 20
% in average.
2.4 Results of Round Robin test1
Precision of method NT BUILD 492 obtained in Round Robin Test between 27 laboratories
is represented through repeatability of 18 % and reproducibility of 36 %. Results of
interlaboratory test presented in this study shows that precision for water saturated
specimens taken from cylinder is better then that obtained by Round Robin test.
3. CRITERIONS, EXPOSURE
COMPOSITION
CLASSES
AND
CONCRETE
Requirements for concrete exposed to chloride ingress in Croatian regulation are considered
only in HRN EN 2062 where concrete classes of exposure for concrete are defined.
Recommended limiting values for composition and properties of concrete are defined, such
as minimum cement content, maximum water-cement ratio and minimum strength class.
Any technologist knows that it isn’t enough and that concrete which only satisfies such
minimum requirements should hardly satisfy requirements for increased resistance to
chloride attack. Concrete designed for significant constructions exposed to chloride ingress
has to satisfy requirements for durability. So, it is necessary to recommend test method and
criterions in order to protect reinforcement from chloride attack and corrosion as its result.
In HRN 11283 which is national addition to HRN EN 206 there isn't any movement
forward. In practice, small number of designers in cooperation with technologists prescribes
in most cases two coefficients9: 6x10-12 m2/s for XS3 exposure class and 9x10-12 m2/s for
XS2 exposure class. Recently, requirement for chloride migration coefficient of 3x10 -12
m2/s appears in some projects.
Some designers define only class of exposure without criterions. They can't say anything
about predicted lifetime of designed construction. Situation is much better when some
permeability parameter is used as criterion.
Designers who don’t know much about technology of concrete aren’t aware of the fact that
recommended limiting values in HRN EN 206-1 can hardly enable resistance to chloride
ingress. On the other side, requiring too small value of chloride migration coefficient causes
another kind of problems. Such requirement will result in significant increase of expenses
because in that case high performance concrete is designed. Companies which make the
offers for execution works usually forget to include all expenses.
As in four years in our laboratory more then 300 concrete specimens were tested according
to NT BUILD 492, and for more then 50 specimens there are reliable data about
composition, we analyzed part of these results regarding composition and classed them. In
table 2 recommended limiting values for composition and properties of concrete according
to HRN EN 206-1 are presented.
Exposure classes for chloride induced
corrosion (sea water)
Max. water-cement ratio (w/c)
Minimum strength class
Minimum cement content (kg)
XS1
XS2
XS3
0,50
C30/37
300
0,45
C35/45
320
0,45
C35/45
340
Table 2: Recommended limiting values for composition and properties of concrete
according to HRN EN 206-1
Cement content (kg)
Water-cement ratio
Compressive strength (on cube) (MPa)
Chloride migration coefficient (x10-12m2/s)
Exposure class according to table 2
Suggested class
Note
365
0,58
40-46
18-15
Normal
concrete
330
0,50
60
12
XS1
XS1
360
0,45
63
12-10
XS3
XS1
410
0,40
70
10-9
XS3
XS1
High strength concrete
Table 3: Few examples6 from practise with parameters close to those recommended in HRN
EN 206-1
In table 3 few concrete compositions are classified according to criterions shown in table 2.
Changing the type of cement from CEM I 42,5 to cement with addition of slag (CEM II/AS 42,5N) decreases chloride migration coefficient for more then 20 %6.
Further increase of cement content and decrease of water-cement ratio doesn’t result in
significant decrease of chloride migration coefficient. In the most number of cases it will be
lowered to value from 6 to 9 x10-12m2/s.
Cement content (kg)
Water-cement ratio
Compressive strength (MPa)
Difference in constituents
Chloride migration coefficient (x10-12m2/s)
Exposure class according to table 2
Suggested class
Note
430-480
420-440
430-480
0,33-0,36
0,34-0,36
0,33-0,34
74-88
69-83
77-81
Addition of
Different
micro silica
aggregate
<3
7-9
9 – 10,5
XS3
XS3
XS3
XS3
XS2
XS1
High strength concrete
Table 4: Results of experimental program with ten mixes4
It was confirmed in another test program with ten mixes4 which have water-cement ratio
from 0,33 to 0,36, cement content from 420 to 480 kg per cubic meter of concrete and
compressive strength from 69 to 88 MPa that only three mixes which were prepared with
addition of micro-silica have chloride migration coefficient lower then 3x10-12 m2/s. Value
of chloride migration coefficient for five mixes ranges from 7 to 9x10-12 m2/s and for two
mixes ranges from 9 to 10,5 x10-12 m2/s. All these concrete compositions can be declared as
high strength concrete.
Cement content (kg)
Water-cement ratio
Compressive strength (MPa)
490
0,33
85
Difference in constituents
-
Chloride migration coefficient (x10-12m2/s)
Exposure class according to table 2
Suggested class
Note
8
XS3
XS2
-
430
036
72
Addition of micro
silica
<3
XS3
XS3
-
Table 5: Results of one more experimental program5
These are typical examples which illustrate different classes of concrete concerning
resistance to chloride ingress. Classification made in HRN EN 206-1 is insufficient. As type
of cement and addition of mineral additions significantly decreases value of chloride
migration coefficient it is clear that classes of exposure should be defined through one of
chloride transport parameters in concrete, directly, not through concrete composition.
From tables 3, 4 and 5 it can be seen that increasing of cement content and decreasing of
water-cement ratio can improve resistance of concrete to chloride ingress only to certain
limit. Suggested classes of exposure are given.
4. SUMMARY
In case of defining criterion for chloride migration coefficient in concrete design it is
necessary to define: 1.test method, 2.age of concrete specimen in the time of testing, 3.state
of saturation before testing.
We suggest using of NT BUILD 492 test method because test equipment is easy to get and
test is short and reliable if test specimen is water saturated before start of preconditioning
according to test method. Test specimens in age of more then 28 days should be tested.
To make decision about value of chloride migration coefficient as requirement in project it
is necessary to know: class of exposure, projected lifetime of construction, kind of
reinforcement, thickness of concrete protective layer and concrete surface protection.
Considering all these parameters together should lead to define optimal chloride migration
coefficient.
Taking all this into account it can be concluded that we are at very beginning of setting up
the solution for this problem in regulations. Results of Round Robin tests show that
situation isn’t better in other parts of the world.
In this study it is tried to facilitate designers to decide about value of chloride migration
coefficient as criterion for exposure classes of concrete. It was done by analyzing different
concrete compositions together with chloride migration coefficient test results and their
grouping.
Designers have to be aware of the fact that each level of coefficient of migration requires
new type of concrete.
5. CONCLUSIONS
Experimental results obtained by testing different concrete specimens in laboratory and
confirmed by interlaboratory tests lead to conclusion that conditions of preconditioning in
test method NT BUILT 492 are insufficient. Previous state of saturation has significant
influence on test results so that specimens from same concrete mix have significantly
different test results, depending on previous state of saturation. In this study it is proposed
to test completely saturated specimens, because test results are more reliable, more precise
and problems with total penetration of chlorides through specimen don’t exist.
Analyse of experimental results leads to conclusion that compositions of concrete could be
grouped in following classes concerning chloride migration coefficient: <3x10 -12 m2/s, 3-6
x10-12 m2/s, 6-9x10-12 m2/s, 9-12x10-12 m2/s and >12x10-12 m2/s. First two groups could
satisfy exposure class XS3, third group could satisfy XS2 and forth group could satisfy
class XS1.
REFERENCES
[1] Castellote, M. & Andrade, C. 2006. Round-Robin Test on methods for determining
chloride transport parameters in concrete. Materials and Structures, vol.39, 955-900.
RILEM .
[2] HRN EN 206-1. 2006, Beton – 1.dio: Specifikacije, svojstva, proizvodnja I sukladnost
(uključuje amandmane A1:2004 I A2:2005), (EN 206-1:2000+A1:2004+A2:2005,
Concrete – Part 1: Specification, performance, production and conformity (includes
amendments A1:2004 and A2:2005).
[3] HRN 1128, 2007. Beton – Smjernice za primjenu norme HRN EN 206-1 (Concrete –
Guidelines for the implementation of HRN EN 206-1).
[4] Institut IGH, Regional Unit in Split, 2005. Ispitivanje tehničkih značajki svježeg i
očvrslog betona klase C 40/50 visoke trajnosti za sastav betona kupole i temeljnog
prstena sportske dvorane Višnjik u Zadru za Lavčević Inženjering, Split (Testing of
properties of fresh and hardened concrete C40/50 of high durability for compositions of
concrete for dome and foundation ring of sports hall Višnjik in Zadar for Lavčević
Inženjering, Split), Test report 3611-B-635/08, 2005-06-10.
[5] Institut IGH, Regional Unit in Split, 2007. Istraživanje sastava betona razreda najmanje
C 25/30, C 40/50 i C 50/60, različitih razreda izloženosti za Mucić&Co. (Exploring the
composition of concrete, class at least C 25/30, C 40/50 i C 50/60, of different exposure
classes for Mucić&Co.), Test report 3610-0081/07, 2007-03-01.
[6] Institut IGH, Regional Unit in Split, 2008. Ispitivanje primjene raznih vrsta cementa u
betonu različitih sastava za Dalmacijacement (Exploring the effect of usage of different
kinds of cement (in different compositions of concrete) for Dalmacijacement), Test
report 3610-B-0035/08, 2008-01-23.
[7] Institut IGH, Regional Unit in Split, 2008. Međulaboratorijsko ispitivanje koeficijenta
migracije klor iona u betonu (Interlaboratory test of chloride migration coefficient in
concrete), Test report 3610-0053/08, 2008-09-24.
[8] NT BUILD 492, Nordtest Method 1999. Cncrete, mortar and cement-based repair
materials: Chloride migration coefficient from non-steady-state migration experiments.
Nordtest.
[9] Port Authority of Dubrovnik 2005. Passenger Port Dubrovnik, Rehabilitation and
Construction of Berth 10-16, Tender Documents, Vol.I: The Tender.
.

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