CORRELATION BETWEEN PARAMETERS OF DURABILITY
AND AERATION OF AERATED CONCRETE
Adam Hubáček[1]–
Rudolf Hela[2]
ABSTRACT
This paper deals with determination of level of dependence of different
parameters of aeration of fresh and hardened concrete (determined in accordance
with standardized procedures in common use) and durability of the concrete
based on observation of properties of aerated concrete. Fluctuation of values of air content in
hardened and fresh concrete are observed as well as influence of this fluctuation
on durability of concrete.
1 AERATED
CONCRETE
There is a boom of aerated
concrete in particular in highway and road construction. These concretes are
manufactured with aerated and plasticizing admixtures and their effect enhances
properties of concrete, in particular workability, water-tightness, frost
resistance and surface resistance to action of water and chemical de-icers, and
thus the life-time of concrete structures exposed to corrosive environment,
acid rain and frost is longer.
Air-entraining admixtures
create a system of pores of different sizes in concrete. These pores, called
also “effective air” serve as space for expansion to eliminate pressures
developed during crystallization of ice or de-icing salts. In general,
desirable size of pores is considered to be between 25 – 300 mm; larger pores cause unproportional
deterioration of mechanical properties of concrete.
Method most often used to
determine measure of entrained aeration is so-called pressure method, which is
carried out with a sample of fresh concrete. Nevertheless, this method gives no
information of space distribution of pores in concrete or their sizes. Spacing
factor is another method of determination of air content in concrete. This
method tests hardened concrete and, using microscopic measuring and special
software, it is capable of determining detailed and complex information about
size and distribution of different air pores in hardened concrete.
Once concrete hardens, air
pores stay in the form of voids. This is usually called a “system of air
bubbles – pores” in hardened concrete. Main parameters of air pores are total
air content, average spacing of pores and specific surface of pores. The
effective air consists of small enclosed pores of microscopic size with diameter
10 to 300 mm. These micro-pores are evenly
distributed throughout the mass of concrete. Relative distance between pores is
called the Spacing factor. It is the distance of travel of water before it can
enter an air pore – this decreases the pressure. Specific surface indicates
relative number and size of air micro-pores for given content of air. Larger
value of specific surface is better since it indicates higher number of smaller
pores. Spacing factor value < 0.2mm is necessary to reach high resistance to
frost and defrost. Total air content is usually measured with fresh concrete.
Required air content decreases with growing gradation of aggregate.
As for aerated concrete,
problems of conformity of repeated testing of aerated concrete still remains
unresolved. Namely the dependence of air content of fresh concrete before
placing and after placing and compaction is not determined by pressure method
used with fresh concrete. There is also no correlation between air content in
fresh concrete (pressure method), characteristics of air pores in hardened
concrete (Spacing factor) and resistance to action of frost, water and chemical
de-icers.
2 EXPERIMENTAL
PART
The topic of this paper is
unification and testing of different mix-designs of aerated concrete and observation
of statistical dependence between air content in fresh concrete and
characteristics of space distribution or air pores in hardened concrete
determined after 28 days. Air-entrainers were dosed so that the air content in
fresh concrete determined by pressure test was between 4.0 and 7.0%. Air
content of different mix-designs determined by pressure test was compared to
air content of hardened concrete determined microscopically. Method of this test is described in the
following chapter. At the same time, resistance to chemical de-icers and
resistance to frost of concrete were tested in automated freezing device. After
evaluation of all tests, individual curves of dependencies of tested aerated
concrete were drawn and correlation relationships were described, in particular
those between air content in hardened concrete determined microscopically and
resistance of concrete to chemical de-icers determined in accordance with CSN
73 1326.
15 mix-designs of concrete
of the same composition of strength class C 30/37, degree of corrosive attack
XF2 – XF4 were made for the purpose of laboratory tests. Cement used was
Portland cement class 42.5 R, Mokrá, mined aggregate of fraction 0-4mm,
site Tovačov, coarse mined aggregate fraction 4-8 mm site Náklo and
coarse crushed aggregate fraction 8-16 mm and 11-22 mm, site Želešice.
Plasticizer used was based on sulphonated condensates of naphthalene Stachement
NN combined with air-entrainer Microporan by the same manufacturer.
Each of the mix-designs was
tested to determine properties of fresh concrete and evaluation of
physico-mechanical properties of hardened concrete. Based on these properties,
the curves of dependency were drawn.
Table 1: Resulting parameters of mix-designs of
air-entrained concrete.
lLbl.
|
n
|
Slump
|
Air
content
in FC
|
Volume weight FC
|
Volume weight HC
|
Compressive strength
28 days
|
Resistance to water and chemical de-icers
|
|
50 cycles
|
100 cycles
|
|||||||
|
[-] |
[mm] |
[%] |
[kg/m3] |
[kg/m3] |
[N/mm2] |
[g/m2] |
[g/m2] |
|
|
PB |
1 |
170 |
6,7 |
2400 |
2350 |
40,5 |
73,3 |
185,6 |
|
2 |
170 |
7,5 |
2380 |
2330 |
39,0 |
60,5 |
95,3 |
|
|
3 |
130 |
4,6 |
2410 |
2360 |
42,0 |
125,5 |
215,7 |
|
|
4 |
140 |
5,7 |
2380 |
2330 |
42,5 |
108,6 |
198,3 |
|
|
5 |
190 |
7,8 |
2360 |
2310 |
42,5 |
32,6 |
116,9 |
|
|
6 |
180 |
8,5 |
2330 |
2290 |
39,0 |
53,9 |
89,8 |
|
|
7 |
150 |
6,5 |
2360 |
2330 |
42,5 |
95,3 |
189,3 |
|
|
8 |
140 |
7,0 |
2420 |
2390 |
44,0 |
25,3 |
52,5 |
|
|
9 |
200 |
7,7 |
2380 |
2340 |
46,5 |
31,5 |
66,5 |
|
|
10 |
150 |
6,0 |
2420 |
2390 |
50,5 |
24,3 |
54,7 |
|
|
11 |
150 |
6,5 |
2420 |
2380 |
48,5 |
37,2 |
69,4 |
|
|
12 |
140 |
4,2 |
2410 |
2380 |
41,0 |
65,3 |
185,6 |
|
|
13 |
180 |
7,5 |
2380 |
2340 |
42,0 |
37,2 |
89,7 |
|
|
14 |
160 |
5,6 |
2400 |
2360 |
45,0 |
69,3 |
189,6 |
|
|
15 |
150 |
6,1 |
2380 |
2330 |
43,5 |
128,3 |
223,9 |
|
|
Average |
160 |
6,5 |
2390 |
2350 |
43,5 |
64,5 |
134,9 |
|
Table 2: Parameters of
aeration of fresh and hardened concrete
Lbl.
|
n
|
Air content
in FC
|
Air content in HC
|
Microscopical air content A300
|
Sparing
factor L
|
|
[-] |
[%] |
[%] |
[%] |
[mm] |
|
|
PB |
1 |
6,7 |
5,72 |
3,02 |
0,13 |
|
2 |
7,5 |
8,31 |
3,34 |
0,16 |
|
|
3 |
4,6 |
4,21 |
2,86 |
0,11 |
|
|
4 |
5,7 |
5,78 |
2,40 |
0,13 |
|
|
5 |
7,8 |
9,48 |
2,96 |
0,11 |
|
|
6 |
8,5 |
8,41 |
1,44 |
0,19 |
|
|
7 |
6,5 |
6,98 |
2,53 |
0,13 |
|
|
8 |
7,0 |
6,16 |
4,35 |
0,09 |
|
|
9 |
7,7 |
6,31 |
2,86 |
0,14 |
|
|
10 |
6,0 |
7,10 |
3,22 |
0,11 |
|
|
11 |
6,5 |
6,81 |
2,01 |
0,15 |
|
|
12 |
4,2 |
4,39 |
1,82 |
0,12 |
|
|
13 |
7,5 |
5,83 |
2,16 |
0,13 |
|
|
14 |
5,6 |
5,31 |
1,66 |
0,20 |
|
|
15 |
6,1 |
5,79 |
2,31 |
0,14 |
|
|
Average |
6,5 |
6,4 |
2,60 |
0,14 |
|
Diagram 1: Comparison of
air content in fresh concrete and resistance to chemical de-icers
Diagram 2: Comparison of
air content in fresh and hardened concrete
Diagram 3: Comparison of
air content in hardened concrete and resistance to chemical de-icers
Diagram 4: Comparison of
air content A300 and resistance to chemical de-icers
Diagram 5: Comparison or
air content A300 and Spacing factor L
3 CONCLUSION
Individual parameters of
partial correlations of aerated concrete were evaluated one another.
Relationship between air content in fresh concrete and air content in hardened
concrete showed value of correlation between observed parameters 61.0%, which
indicates emergent dependency between air content in fresh and hardened
concrete of observed mix-designs. Relationship between air content in fresh
concrete and resistance to chemical de-icers showed correlation 39%, which
means that no important interdependency can be expected between observed
parameters. As for comparison of air content in hardened concrete and
resistance to chemical de-icers, the value of correlation 27.5% does not
indicate any interdependency of these two properties. No dependency was found
between content of microscopic air and resistance to chemical de-icers; the
value of determination index was below 12%. This fact was not caused by low
resistance of concrete or low content of microscopic air, but by lower
compatibility of plasticizing and air-entraining admixtures. The total content
of air in hardened concrete varied considerably as well as air content of
microscopical air. This is in accord with other indexes, where measure of
correlation between air content in hardened concrete and air content A300
was as low as 2%. Comparison of Spacing factor L and air content of hardened
concrete and air content A300 showed values lower than 3%. As for
the second set of properties, the level of correlation between content of
microscopical air A300 and Spacing factor was 45.8%, which shows
slight relationship. In general it is true that interdependence between two
observed values can be considered if correlation is 60% or more. It means that
no interdependence was found between the measure of aeration of fresh and
hardened concrete of observed mix-designs and its durability.
Stated values imply there is no solid
correlation between assessed values. It becomes clear that the currently used
system cement – water – admixtures – additives is very complex and sensitive to
accuracy and sequence of dosage of individual components, time and way of
mixing and temperature of its constituents. Moreover the test methods used are
not always exact – e.g. there is no definition and prescript for the procedure
of placing concrete into pressure vessel for the purpose of measuring air
content of fresh concrete or for the procedure of compacting. Treatment of
surface of test samples before the test of resistance to chemical de-icers is
also more or less left to a laboratory analyst. These and other factors may
contribute to considerable differences of results even if the mix-designs are
identical.
This result
was achieved through financial contribution of MSMT CR, project No. VVCEZ
002163051 and grant GAČR GA 103/07/1662 –Modeling of the process of
disintegration of degraded layer of construction materials during treatment
before rehabilitation.
Literature
[1]
HUBÁČEK,
A. Study on Issue of aerated concrete– Doctoral Thesis, VUT Brno, FAST 2008.
[2]
HELA,
R; HUBÁČEK, A. Technické listy CIDEAS
[3]
ČSN
73 1326/Z1 - Stanovení odolnosti povrchu cementového betonu proti
působení vody a chemických rozmrazovacích
látek
[4]
ČSN
EN 480-11 - Přísady do betonu, malty a
injektážní malty - Zkušební metody -
Část 11: Stanovení charakteristik vzduchových
pórů ve ztvrdlém betonu