CEMENT and CONCRETE RESEARCH. Vol. 23, pp. 683-692, 1993. Printed in the USA. 0008-8846/93. $6.00+130. Copyright © 1993 Pergamon Press Ltd.

THE INFLUENCE OF CURING CONDITIONS ON THE PERMEABILITY AND DURABILITY OF CONCRETE. RESULTS FROM A FIELD EXPOSURE TEST.

C. Ewertson and P.E. Petersson Swedish National Testing and Research Institute

Bor~s Sweden

(Communicated by G. Fagerlund) (Received July 22, 1992)

ABSTRACT Two concrete qualities were subjected to different curing conditions and then exposed to three different climates; outdoors exposed to rainfall, outdoors pro- tected from rainfall and indoors. After one or two years of exposure the water impermeability and the carbonation depth were determined.The test results show that the differences between different curing conditions becomes more pronounced the drier the climate. This means that laboratory tests cannot always be used for predicting the concrete behaviour of a real structure. Field exposure tests are to be preferred. According to the results from the carbonation tests, it seems that wet curing and covering with plastic foil are equally efficient. This is relevant for Swedish field conditions, and other climatic conditions probably give other results.

Introduction

This field investigation was carried out in order to study the influence of curing conditions on permeability and carbonation of concrete. Two concrete qualities have been subjected to different curing conditions and then placed in three different climates for 2 years; outdoors exposed to rain, outdoors sheltered from rainfall and indoors. The carbonation depth and water impermeability were then determined.

Results from tests dealing with the influence of the curing on the permeability and the carbonation rate have been reported, for example in (1,2,3,4,5,6). Most of the investigations have, however, been performed under well controlled laboratory conditions on water-cured specimens and very few results have been reported for field exposure tests on specimens subjected to different curing conditions.

Concrete oualitt'es

Two concrete qualities were used, with water-cement ratios of 0.35 and 0.5. Ordinary

683

684 C. Eweztson ~md P.E. Pet~rsson Vol. 23, No. 3

Portland cement (the Swedish cement Degerhamn Std P) was used. This cement has a low alkali content (eq 0.5% N%O), low C3A (about 2%) and a low value of the heat of hydration

(7). Natural gravel was used with a maximum aggregate particle size of 16 mm, the slump was i00 + 10 m m and the air content was 5.5 + 0.5 % for both mixes.

Manufacturing of s_ovchnens

The aggregate and the cement were first mixed for one minute in a paddle mixer with a ca- pacity of 350 liters. The air-entraining agent was mixed with some of the water and then added to the mixer together with the first mixing water. Then the concrete was mixed for 2 minutes, after which the slump was determined. The mixing then continued for another minute and the slump was checked again.

The specimens were cast in steel moulds. The moulds were t-died in two layers and each layer was compacted on a vibrating table for 15 seconds. Finally the top surface was smoothed by use of a steel rod.

For some specimens a surface cast against the mould was used as the test surface. In this case plastic-coated plywood moulds were used and then the bottom surface of the specimen was used as the test surface.

Curint,

Directly after casting, the specimens were subjected to different types of curing according to table 1. The curing always took place at a temperature of 20 + I°C. During the first day the specimens were kept in the moulds and sealed with plastic foil so that evaporation was prevented. For the water-cured specimens wet sponges were then placed under the plastic foil during the first day in order to obtain a high humidity after which the specimens were placed in water.

TABLE 1

The different curing conditions used in the test (W--water, PF---plastic foil, A=air and 0, 1 and 3 represent the number of days before form removal).

Type of Time until Water curing Covered with In the air Curing time at curing form removal I) plastic foil (RH--50%) 20°C, t2o

(days) (days) (days) (days) (hours) W0 0 5 0 23 120 PF0 0 0 5 23 120 A0 0 0 0 28 0 Wl 1 5 0 22 144 PF1 1 0 5 22 144 A1 1 0 0 27 24 W3 3 5 0 20 192 PF3 3 0 5 20 192 A3 3 0 0 25 72

i) 0 day before form removal is relevant only for the horizontal top surfaces of the specimens while and 3 days before form removal are relevant for surfaces cast against the mould.

Vol. 23, No. 3 CURING, PERMEABILITY, DURABILITY, FIELD TEST 685

F~¢ld ex_oosure conditions

Specimens were subjected to two different outdoor field exposure conditions - exposed to or protected from rainfall. The test surface was turned upwards for the exposed and vertically for the protected specimens. Parallel with the field test, reference specimens were stored in a climate chamber at 65% RH and a temperature of +20°C. Two specimens were used for each combination of concrete quality, curing condition and exposure condition. The specimens were placed in the different exposure conditions at the age of 28 days.

The field exposure station is located close to the National Testing and Research Institute in Bor~s. The mean temperature (1991) in the area was about +6°C with the highest value in July (+17°C) and the lowest in February (-3°C). The total amount of annual rainfall is about 800 mm. The relative humidity varies between about 90% in January and 65% in June. The specimens exposed to rainfall are shown in figure 1.

Test methods

At an age of one year the water impermeability was determined. For each measurement two specimens with the dimensions 250x250x125 m m were tested. The test surface was then subjected to a water pressure of 0.8 + 0.03 MPa within a circular area with a diameter of 170 mm. After 24 hours the specimen was split and the penetration depth of the water front was measured. The testing procedure and the equipment is similar to the one used in SS 13 72 14 but the test age and the curing method is different.

FIG. 1

The field exposure station with the specimens exposed to rainfall (and snow).

686 C. Ewertson znd P.E. Petersson Vol. 23, No. 3

In order to determine the water content, the specimens were weighed directly before the start of the water impermeability test. After the test and the splitting, the two pieces of the speci- men were dried at 105°C and then they were weighed again. The moisture ratio was calcu- lated as the difference between the weight before and after drying divided by the dry weight. Possible weight loss during the splitting of the specimen was compensated for.

For the carbonation testing, two specimens with the dimensions 100xl00x400 mm were pro- duced for each combination of concrete quality, curing condition and exposure condition. At the test, the specimens were split about 50 mm from one end and the carbonation depth was determined by spraying 3 % phenolphtalein-solution on the surface directly after splitting. Here the results after two years of exposure are presented.

Test results Water imuermeabilitv

The results after 1 year for the water impermeability tests are presented in table 2. Each value represents the mean value for two specimens.

For a W/C-ratio of 0.35 the penetration depths are small, especially for the specimens expo- sed to rain. The difference between the results for water curing and protection with plastic foil is small while the concrete is more permeable for the specimens which are "cured" in air, i.e. no protection against evaporation.

For the concrete with a W/C-ratio of 0.5, all results indicate that water curing is superior to

TABLE 2

The results from the water impermeability test. The figures represent the penetration depth (ram) of the water front after 24 hours (EXP=exposed to rain, PROT--protected from rain, LAB=in the laboratory and the curing methods are W=water, PF---plastic foil and A=air).

Exposure and TOP SURFACE SURFACE AGAINST MOULD

curing conditions (not against mould) Time until form removal 1) Time until form removal

[ W EXP ~ PF

lA r w

PROT ~ PF LA [ W

LAB t PF LA

0 day W/C--0.35

2 2 2 6 4 31 19 30 55

0 day W/C=0.50

26 40 73 46

>125 >125

67 115

>125

1 day w/c---0.50

16 33 27 26

>125 >125

4 8

125 125

3 days W/C=0.50

24 35 35 56 82

>125 64 93

>125

i) 0 day before form removal is relevant only for the horizontal top surfaces of the specimens while and 3 days before form removal are relevant for surfaces cast against the mould.

Vol. 23, No. 3 CURING, PERMEABILITY, DURABILITY, FIELD TEST 687

other curing methods and the difference between curing by protection with plastic foil and no curing at all seems to be small. This is in good agreement with the results presented in (1).

According to the test results, the penetration depth seems to be lowest for the specimens ex- posed to rain and highest for the specimens stored in the climate chamber, i.e. the drier the environment, the more permeable the concrete becomes. In order to study the influence of the moisture content on the test results the moisture ratio in the specimens directly before the start of the test was determined. The results are presented in table 3.

TABLE 3

The moisture ratio (percent of the dry weight) of the specimens directly before the start of the water impermeability test (EXP=exposed to rain, PROT=protected from rain, LAB=in the laboratory and the curing methods are W--water, PF---'plastic foil and A=air).

Exposure and TOP SURFACE SURFACE AGAINST MOULD curing conditions (not against mould)

Time until form removal 1) Time until form removal

[ W PF

[A [ W

PROT ~ PF [ A

0day W/C---0.35

3.6 3.6 3.1 3.1 3.1 3.0

0 day W/C--0.50

4.5 3.6 3.4 3.9 3.5 2.6

1 day W/C=0.50

4.2 4.2 4.2 3.7 3.5 2.4

3 days W/C=0.50

4.2 4.1 4.3 3.6 3.5 3.3

[ W 2.9 3.1 2.9 3.1 LAB ~ PF 2.9 2.8 2.7 2.9

[ A 2.6 2.2 2.5 3.0 l) 0 day before form removal is relevant only for the horizontal top surfaces of the specimens

and 3 days before form removal are relevant for surfaces cast against the mould. while 1

From the results in table 3 it is clear that the moisture content decreases as the environment becomes drier and this indicates that the differences between the test results may be dependent on the actual moisture content. On the other hand, the differences between the spe- cimens cured in water and under plastic foil seem to be very small (except for top surface, W/C=0.S) while the two curing methods give rise to completely different values for per- meability, cf table 2. This implies that other factors than water content influence the test results.

h can be observed that the curing method affects the moisture content also after a year of ex- posure to different climatic conditions. The difference between water curing and protection with plastic foil is normally small, while the water content is often lower for specimens "cu- red" in air. I f the moisture content after a defined exposure history is dependent on the material structure, and therefore indirectly dependent on the curing conditions, then water cu- ring and protection with plastic foil seem to be about equivalent, while no curing seems to produce a different material structure.

688 C. Ewertson mad P.E. Petersson Vol. 23, No. 3

The values for the carbonation depth after two years of exposure are presented in table 4. The results show that the carbonation rate increases as the environment becomes drier, i.e. the value of the carbonation depth is highest for the specimens stored in the laboratory and lowest for the specimens exposed to rainfall. It can also be observed that the carbonation depth de- creases when the time until form stripping increases. This effect is most pronounced where no further curing is performed after the removal of the formwork.

For the specimens stored in the laboratory, a significant difference between the different curing methods was found. Water curing gives the smallest carbonation depths, protection with plastic foil gives higher values and no curing gives the poorest results. This is in agreement with the results in (1). The results are relevant for top surfaces as well as for surfaces cast against the mould.

For the field exposed specimens protected from rainfall there does not seem to be any dif- ference between water curing and protection with plastic foil. This is contradictory to the re- suits in (1) and also to the results regarding water impermeability presented above. The results for the air-"cured" specimens are, however, much poorer than the results for the other curing methods.

For the specimens exposed to rain, the difference between the three curing methods seems to be very small. For the water-cement ratio of 0.35 as well as for the case where the form is kept in place for three days there is not any difference at all. This means, according to these

TABLE 4

The carbonation depth (ram) after two years of exposure in different climates (EXP=exposed to rain, PROT=protected from rain, LAB=in the laboratory and the curing methods are W--water, PF--plastic foil and A=air).

Exposure and TOP SURFACE SURFACE AGAINST MOULD

curing conditions (not against mould)

Time until form removal l) Time until form removal

r W EXP ~ PF

[A r W

PROT ~ PF [ A r W

LAB t PF [ A

0 day w/c--0.35

1.0 1.0 1.0 1.5 1.5 3.0 1.0 3.5 5.0

0day W/C--0.50

1.5 2.0 2.5 3.0 4.0 7.5 5.0 6.0 12.0

1 day W/C---0.50

1.0 0.5 1.5 2.5 1.5 6.0 3.5 5.0 9.5

3 days W/C--0.50

1.0 1.0 1.0 2.5 2.0 3.5 3.5 5.0 6.0

I) 0 day before form removal is relevant only for the horizontal top surfaces of the specimens while and 3 days before form removal are relevant for surfaces cast against the mould.

Vol. 23, No. 3 CURING, PERMEABILITY, DURABILITY. FIELD TEST 689

test results, that rain water is often able to cure the defects caused by insufficient curing directly after casting.

In figure 2, the carbonation depth is plotted against the curing time at 20°C (wet curing or protection against evaporation). As found above, the influence of the duration of curing is small for specimens exposed to rain. For the other two exposure conditions the carbonation depth decreases rapidly with increased curing time up to about 100-120 hours. Further curing seems to have a less pronounced effect on the carbonation rate. According to these results a good curing for this concrete quality (W/C---'0.50) should therefore last for about 100-120 hours.

The carbonation rate is normally assumed to follow (8):

X = k ~ eq I

where X is the carbonation depth, t is the time and k is a constant depending, among other things on environmental conditions and material properties.

Using eq 1 and the results in table 4 it is possible to make a rough estimate of the carbonation depth at an arbitrary age. Such estimations are presented in figure 3 where the carbonation depth after 50 years of exposure is shown as a function of the curing time at 20°C. Results are given for three different exposure conditions and are relevant for an OPC concrete with a water-cement ratio of 0.50 and the curing method is protection against evaporation.

For concrete structures exposed to rain, the carbonation depth after 50 years is also small for poor curing conditions. When the structure is protected from rain, the carbonation rate in- creases considerably and in order to limit the carbonation to for example 25 mm, the curing time at 20°C must exceed about 45 hours. The corresponding value for the specimens stored in the laboratory (+20°C, 65% RH) is about 200 hours. The latter case may represent the situation in a wanner, drier climate than Sweden's.

Conclusions

The results presented in this paper are relevant primarily to Swedish climatic conditions, and the conclusions may be completely different for field exposure tests carried out in other countries. The results of this investigation implies that a drier, warmer climate than Sweden's probably produces a higher carbonation rate and also a higher sensitivity to different curing conditions.

On the basis of the results presented in this report, the following conclusions can be drawn:

. The results from the water impermeability test are, at least to some extent, contradictory to the results from the carbonation test. This means that the water impermeability test seems unsuitable for predicting the carbonation rate in concrete structures.

. Both the water penetration and the carbonation rate increases as the exposure conditions become drier. Storage in the laboratory (+20°C, 65% RH) gave the

690 C. Eweztson md P.E. Petersson Vol. 23, No. 3

-r" I,.-

E3 z o I- .<

ca or .< o

10-

LAB

\ r:l /,4k J CURING

,. W/C=0.50

." °U

W/C=0.35

\ A

! o lo1) 200

CURING TIME AT 20 °C (HOURS)

10

D Z O

Z

o., <

PROT

[o} ~,P. PROT [.} w,,,E, CURING

5 ,- ~ O 0 W/C=0.50

w/c=o.35 I"

0 100 200

CURING TIME AT 20 °C (HOURS)

EXP

"r" I-- ,,=,

z __Q

o~ o-

o

1 0 -

5 -

PROT

WATER CURING

WlC=0.50

CURING TIME AT 20 "C (HOURS)

FIG. 2

The carbonation depth after two years of exposure in different climates as function of curing time at 20°C (EXP=exposed to rainfall, PROT---protected from rainfall, LABf in the laboratory).

Vol. 23, No. 3 CURING, PERMEABILITY, DURABILITY, FIELD TEST 691

L~ C3

Z _o

Z o

o

20- "-..

1 0 - ' ...... " " " . . . . . . . . . . . . . . . . . . . . . . . . .

! 0 1 O0 200

CURING TIME AT 20 "C (HOURS)

FIG. 3

Calculated values of the carbonation depth after 50 years of exposure as function of curing time at 20°C for different exposure conditions. The curves are relevant for an OPC concrete with a water-cement ratio of 0.50, and the curing method is protection against evaporation.

highest values, field exposed specimens protected from rainfall gave lower values and specimens exposed to rain produced the best results.

. The drier the climate, the more pronounced the difference between different curing conditions. For the specimens exposed to rain the differences are very small, while they are significant for the specimens stored in dry laboratory conditions.This means that laboratory tests cannot always can be used for predicting the concrete behaviour in a real structure. Field exposure tests are preferred.

. According to the water impermeability test, wet curing is superior to the other curing methods while the difference between protection with plastic foil and no protection at all seems to be small.

. According to the results from the carbonation tests it seems that wet curing and covering with plastic foil are equally efficient. "Curing" in air gives poorer results, also when the form not is removed until three days after casting.

Referenc¢~

1. C. Ewertson and P.E. Petersson.The influence of cu~_ng conditions on the permeability and durability of concrete. SP-report 1987:7, Swedish National Testing and Research Institute, Building Technology, Bor~s, Sweden (1987).

692 C. Ewertson and P.E. Petersson Vol. 23, No. 3

2. A. Meyer. Investigations on the carbonation of concrete. Proc Chem Cement, p.394, Tokyo(1968). 3. M. Kurz. The influence of extremely short-term curing on carbonation in concrete. Proc RILEM seminar, p.250,Hannover (1984) 4. N. Fattuhi. Materials and Structures. 19, 131 (1986). 5. T. Bier. Influence of type of cement and curing on carbonation and pore structure of HCP. MRS Symp proc., Boston (1986). 6. K. SchiSnlin. Permeabilit/it als Kennwert der Dauerhaftigkeit von Beton. Heft 8, Instituts fiir Massivban und Baustofftechnologie, Universitiit Karlsruhe (1989). 7. K. MalmstriSm. The importance of cement composition on the salt-frost resistance of con- crete. SP-report 1990:07, Swedish National Testing and Research Institute, Building Techno- logy, Bor~s, Sweden (1987). 8. K. Tuutti.Corrosion of steel in concrete. Report fo 4.82, Cement and Concrete Research Institute, Stockholm (1982).