Even if mix design (cement content, W/C ratio) has an effect on durability of concrete mixed with blended cement, curing conditions, particularly curing time, also ...
Construction and Building Materials, Vol. 9, No. 2, pp. 91-95, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights resewed 095tM618/95 $lO.OO+O.OO
0950-0618(95)00001-1
of curing upon carbonation of concrete
Effects
J. P. Balayssac, Ch. H. D&rich6
and J. Grandet
Laboratoire Matkiaux Complexe Scientifique
et Durabilith des Constructions, de Rangueil, 31077, Toulouse,
Received
1994; revised
9 September
30 November
INSA-UPS GEnie Civil, Cedex, France
1994; accepted
2 December
1994
Even if mix design (cement content, W/C ratio) has an effect on durability of concrete mixed with blended cement, curing conditions, particularly curing time, also affect concrete durability against carbonation. The choice of curing time depends on cement content, and must be adjusted to concrete design (28 day strength, W/C ratio). Concretes with a low cement content (or a high W/C ratio) must be cured longer than those having higher cement content. Finally, some correlations showthat strength at the end of curing, which takes into account both mix design and curing conditions, is able to assess durability of concrete against carbonation. Keywords: durability; carbonation;
curing
To produce a durable concrete in all storage conditions, it might appear to be sufficient by limiting its porosity, to have a minimum 28 day strength. This is easily achieved for current concretes, either by increasing cement content (currently used) or by providing mixing factors (aggregate quality and addition of microparticles). The standard test which makes it possible to assess this strength is easy and reliable. However, this strength must be generally taken as a function of core concrete. By having a minimal 28 day strength, the durability of core concrete will be assured’. Curing conditions can change this core strength and also have an important effect on cover concrete properties. The cover concrete role is of prime importance for reinforced concrete durability as it protects reinforcing bars from penetration by harmful agents (carbonation, chloride ingress etc.)2. Hence, in this paper, durability is expressed in terms of carbonated depths. Four different concretes are cast. After three different curing times (1, 3 and 28 days), they are stored at 20°C and 60% relative humidity (with a CO, content of 0.03%), for up to 18 months. Curing effects (especially curing time) on durability of the cover concrete to natural carbonation are investigated. The influence of cement content and cement type is analysed. Finally, it is shown that the influence of both cement (quantity and type) and curing time can be expressed by a single parameter which makes it possible to estimate durability.
sition of two other cements, an ordinary Portland cement (CPA 55) and a slag cement (CLK 45) with 65% of slag. The sand and gravel are composed of river aggregate from the Garonne. The maximum grain size is 6 mm for sand and 12.5 mm for gravel. Concrete mixing
The concretes mixed cover a range after 28 days of storage in water, They are mixed to provide the Cement content, W/C ratios and given in Table 2.
of strength measured from 25 to 40 MPa. same slump (8 cm). 28 day strength are
Test samples curing and storage
After demoulding at one day, the test samples are stored in a controlled environment at 20°C and 60% relative humidity, or kept in water up to 3 or 28 days, before exposure to the environment described above. The effects of carbonation are evaluated on 11 cm diameter and 10 cm high cylinders whose ends are sawn after demoulding. After curing the side surfaces are covered with adhesive aluminium sheet. Table la Oxide composition of cements Cement
SiO, W)
A&O, W)
Fe,O, (%)
CaO (%)
MgO (%)
SO, (%)
K,O (x)
Na,O (%)
CPJ45 CPA55 CLK45
26.2 20.1 30.3
3.1 5.4 11.5
3.2 3.2 -
58.9 63.2 45.2
0.60 2.20 4.15
2.60 2.30 2.90
1.22 0.76
0.60
Table lb Physical properties of cements
Experimental details Density
Cement
Cements and aggregate
(mS:kg)
A cement with fillers (CPJ 45) is used, containing 75% of clinker and 25% of limestone fillers. Oxides are given in Table 1. This table also presents the chemical compoConstruction
CPJ45 CPA55 CLK45 and Building Materials
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f~ects
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Table 2 Properties
of concrete:
J. P. Balayssac et al.
of concretes
Concrete
Cement
c (kgim’)
Bl B2 B3 B4
CPJ45 CPJ45 CPJ45 CPJ45 CPA55 CPA55 CPA55 CPA55 CLK45 CLK45
300 340 380 420 250 280 300 350 340 400
C: cement content G/S: gravel on sand ratio R28: 28 day strength (after storage tested)
wit
G/S
0.65 0.61 0.53 0.48 0.73 0.65 0.59 0.54 0.61 0.50
I .oo
R28 (MPa) 25.1 32.6 37.8 43.5 26.4 30.0 35.0 41.8 24.9 31.9
1.13 1.13 I.15 0.92 0.94 0.96 I .04 1.17 1.15
&e (W
in water at 2O”C, three samples
After an appointed period (90, 180, 360 and 540 days) three test samples are split to obtain a break without wet sawing. The carbonated depth is then measured - referring to RILEM Recommendations” after application of phenolphthalein to the free surface obtained by splitting. The measurement uncertainty is 0.5 mm, and the dispersion is around 5%. Experimental results and discussion Table 3 presents carbonated depths according to curing time and testing ages. Carbonation ingress up to 18 months is shown for each curing time in Figure I. Whatever curing time, carbonated depth decreases with increasing cement content (also 28 day strength). Concrete Bl (cement content: 300 kg/m3) shows more significant carbonated depths than others, for 3 and 28 day curing. Figure 2 presents carbonated depths according to cement content and curing, for all concretes. The beneficial effect of curing time on durability can be seen immediately. So, after 18 months, for a concrete with a cement content of 350 kg/m3, increasing the period of curing from 1 to 28 days halves the carbonation depth. Carbonated depth versus curing time is plotted on Figure 3, for all concretes. Curing effects do not increase in the same proportion as curing time. Indeed, carbonated depth decreases rapidly when the curing period increases from 1 to 3 days, followed by a slower Table 3 Carbonated
depths
(mm)
Concrete
Properties
Bl
C=300 kg/m’ W/C=O.65 R,,=25.1 MPa C=340 kg/m’ W/C=O.61 R2,=32.6 MPa C=380 kg/m’ w/c=o.53 R,,=37.8 MPa C=420 kg/m’ W/C=O.48 R,,=43.5 MPa
B2
B3
B4
92
Construction
Curing Id id 2nd Id 3d 28 d Id 3d 28d Id 3d 28 d
and Building
90 6.5 4 3 5.5 3.5 2.5 4.5 3 2 4 2.5 1.5
Age (day) 180 360 11 6 5 10 5 4 9 5 3.5 7.5 4 3
Materials
13 9.5 6 12 6 4.5 IO 5.5 4 8.5 3.5 3
540 15 13 9 13 8 6 11.5 7 5 9.5 4 3.5
0
Figure I
30
Carbonation
160
270
ingress (cement
3643
used
450
540
: CPJ45)
variation, This result should be specified in terms of cement content. Thus, Figure 4 presents the PJP,, ratio between carbonated depth for x day curing (P,,s), with .Y = 3 or 28 days and carbonated depth for one day curing (PC,) - versus cement content. For a concrete stored for 18 months, increasing the curing period from 1 to 3 days increases durability performance by a value of 10% for a concrete with a cement content of 300 kg/m3 and 50% for a concrete with a cement content of 420 kg/m3; increasing the curing period from 3 to 28 days still improves by a value of 300/;, the concrete with the lowest cement content, but only by a value of 10% the concrete with the highest cement content. Considering one day curing, two additional days halves carbonated depth for a concrete with a cement content of 420 kg/m3 and 27 additional days are necessary for a concrete with a cement content of 300 kg/m3. In fact, a concrete with a low cement content, even if
1995 Volume 9 Number 2
Effects of curing upon carbonation CmboiWon
16
depth (mm) Curing (day):
14
16
Cerboneuon
dqth
of concrete:
JR
Balayssac et al.
(mm)
14 I[ Ags:SOdeye
,6
Cubonatlon
depth (mm) Curing (day)
:
16 14
12
12
10
i
10
“,’
0
4
6 4
I
2 I O---
--t_---,
250
300
i
350 cement content
Age:S4Odeye
2
Age:MOdaye 400
0
450
350
300
250
(kg/m3)
450
460
camantcontent(kg/m3)
Figure 2 Effect of cement content on carbonation (different curing periods). (Cement used: CPJ45)
16 14 ..
Concrete Bl
12 -. 10 '.
14
-540
12
6
- --•---
6 ~4 .-
L-
---
i&O
6
L
4
-90
3
2
2
O-
0 5
0
10 Curing
14
Concrete 82
10
--3&J
8
16 T
16
ABEWV) : ~-
fhrbonllon
15
20
25
30
c 0
5
10
time (dry)
Curing
depth(mm)
Carbonrtlon
Age&v):
15
20
25
30
tlme(dey)
depth (mm)
Age (day):
Concrete B3
.
..-540 *
360
--.-
180
--+
90 -.~-_+
oi--d+ 0
/ 5
10
15
---_--_+.--_~~
-Y
20
25
30
0
5
Curingtime(day)
Figure 3
---------v-10 Curing
15
20
25
Urn0 (day)
Effect of curing on carbonation (cement used: CPJ45)
improved by adequate curing, will still have a largely porous cover concrete, encouraging ingress of harmful agents. Furthermore a too high cement content can prevent additional hydration by stopping hydrate
Construction
formation into a much denser structure because of the presence of a large number of anhydrous grains4. Therefore, the curing period must be optimized according to concrete strength and cost restraints, These
and Building
Materials
1995 Volume 9 Number 2
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Effects of curing upon carbonation of concrete: J. P. Balayssac et al.
0.6 t 0.4 t
,,~
_:
c
l
r, x=3
/!
_
.
0.6
+ x=26
1
0.4 *
0.2 1
.
.
*
x=3 x=26
0.2
Age : 90 dye
Age:lBodaye
OL
0
260
300
360
400
450
250
3co
Cameal content (kg/m3)
0.6
.
0.4
/, x=3
c
’
400
450
I
0.2
* Age:srOdeye
Age:36Odeye ~.~
0
400
360
450
047
x=26
0.2
300
400
06.
..
350 Cement content (kg/m3)
0.8
250
460
350
300
Cement content (kg/m3)
Cement content (kg/m3) Figure 4 Effect of increasing curing time beyond one day (cement (PC,, with x=3 or 28) and carbonated depth for one day curing
used: (‘PJ45).
PJP, :I: ratio
between
carbonated
depth
for .Y days’ curing
Bier’s” results. On the other hand concretes with a Portland or blended cement content of 340 kg/m3 have higher strengths at end of curing and lower carbonated depths. Such a relation can provide quite a good correlation between durability factor and strength whatever cement content and type. Using a simple mechanical test, it will be possible to assess longer term durability of a concrete against carbonation, whatever cement type and content, simply by knowledge of its curing time.
16
Figure
5 Relationship between carbonation depth and end of curing (concretes: 3 day curing; age: 18 months)
strength
at
results are in agreement with others which showed that curing must depend on W/C ratio5. The lower the latter the shorter may be the curing period. Curing effects depend on cement type. Increasing curing period does not have the same effect, for the same cement content, on a concrete of Portland cement as on a concrete of slag cement. Curing is more appreciable for a concrete with slag cement, as has been demonstrated by other&‘. Then, in Figure 5 carbonated depths after 18 months of concretes mixed with three different cements (ordinary cement, cement with limestone fillers and slag cement) are plotted versus strength at end of curing. For all these concretes the curing period is 3 days. Cement content is within the range of 250 to 420 kg/m”. The two concretes, with a slag cement content of 340 and 420 kglm3, show low strength at end of curing and high carbonated depth according to Parrott’s and 94
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l
Construction
and Building
Materials
Conclusions Test results for carbonation have shown that curing conditions, and especially curing time, have a large effect on the durability of commonly used concretes. This effect depends on cement content, and curing must be optimized in relation to the latter. Regarding our results, one day curing is not sufficient for all the concretes whatever the cement content (in a range between 300 and 420 kg/m3); indeed at 18 months for one day curing, carbonated depths vary between 10 and 15 mm, relating to cement content, which is close to the usual underlying reinforcing bars in non-aggressive environments. Increasing the curing period to 3 days is sufficient for concretes with a cement content higher than 380 kg/m’; for others, curing must be longer. The curing effect also depends on cement type. For example, increasing the curing period much improves durability of concretes mixed with a slag cement.
1995 Volume 9 Number 2
Effects of curing upon carbonation of concrete: J.P. Balayssac et al. RILEM
By correlating the carbonated depths after 18 months exposure of concretes cured over 3 days (using several cements), and strength at the end of curing, it can be seen that the latter makes it possible to assess longer term durability versus carbonation; this can be realized whatever the cement content and cement type.
Acknowledgments
6
The authors acknowledge financial support from the Association Technique de 1’Industrie des Liants Hydrauliques (ATILH).
7
References 1 2
8 9
Baron, J. Introduction a la durabilite des betons. La Durabilite des Betons, Presses de l’ENPC, 1993 Francois, R. and Arliguie, G. Reinforced concrete : correlation between cracking and corrosion. CANMET-ACI 1991, MontrPal
Construction
Recommendations : TC56-MHM
Hydrocarbon materials.
CPC-18 Measurement of hardened concrete carbonated depth Granju, J.L. and Maso, J.C. Hardened cement pastes, modelisation of the microstructure and evolution of mechanical properties. I. basic results (pp. 249-256). II. Compressive strength law (pp. 303-310). III. Elastic modulus (pp. 539-545). Cem. Concr. Rex 1984, 14 Durable Concrete Structures, ComitC Euro-International du B&on (C.E.B.), 1st edn, 1984 Gowripalan, N., Cabrera, J.G., Cursens, A.R. and Wainwright, P.J. Effect of curing on durability. Concr. Intern. : Des. Constr. 1990 12(2) 47-54 Haque, M.N. Some concretes need 7 days initial curing. Concr. Intern. Des. Constr. 1990, 12(2) 42-46
Parrott, L.J. Effects of curing and cement type upon the performance of cover concrete and the durability of reinforced concrete, B. C.A. Interim report, January 1992 Bier, T.A., Kropp, J. and Hilsdorf, H.K. The formation of silica gel during carbonation of cementitious systems containing slag cements. Proc. 3rd International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete. Trondheim 1989,
AC1 SP-114, Vo1.2, 1413-1428
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