SELF-DESICCATION AND ITS IMPORTANCE IN CONCRETE TECHNOLOGY Bertil Persson, M.Sc., Lic. Tech. Lund Institute of Technology, Div. Building Materials, Lund University, P O Box 118, 221 00 Lund, Sweden. ABSTRACT Self-desiccation occurs in all types of concrete due to the chemical shrinkage that takes place when water is attached to the cement. In High Performance Concrete the low porosity makes the effect of self-desiccation much more pronounced. This paper presents a summary of a Nordic Seminar on Self-Desiccation in Concrete held in Lund. For this purpose aspects of mix design, curing conditions, desiccation, micro-cracking, self-stresses, shrinkage and frost resistance related to self-desiccation are detailed. Initially a background is given on the principal effects ofself-desiccation. This Nordic Seminar took place in Lund on 10 June 1997. Keywords: Autogenous shrinkage, Chemical Shrinkage, Frost resistance, High Performance Concrete, Hydration, Internal relative humidity, Moisture, Self-desiccation, Selfstresses. 1. INTRODUCTION 1.1High Performance Concrete Prestressed High Performance Concrete (HPC) with water-cement ratio, w/c < 0.38, has been used in Sweden since 1948 for the fabrication of water pressure pipes. The Swedish invention was also licensed abroad in more than 100 factories. The pipes exhibit superior durability compared with pipes made of normal strength concrete. Besides pipes, columns for electrical power lines and self-desiccating concrete slabs (more than 1 million square metres used to date) became the first large applications of HPC in Sweden. Self-desiccation is studied during moisture-insulated conditions (constant weight) and at constant temperature conditions. No exchange of moisture takes place to or from the concrete specimen during the test period. 1.2Self-desiccation The fundamental cause of self-desiccation is the chemical shrinkage that takes place during hydration of water to cement. As compared with the specific volume of water in the capillary pores the specific volume of the hydrated water in the gel of concrete is reduced by about 25%. Due to the decreased size of the capillary pores especially at low w/c < 0.38, the effect
SELF-DESICCATION AND ITS IMPORTANCE IN CONCRETE TECHNOLOGY
Bertil Persson, M.Sc., Lic. Tech.Lund Institute of Technology, Div. Building Materials, LundUniversity, P O Box 118, 221 00 Lund, Sweden.
ABSTRACT
Self-desiccation occurs in all types of concrete due to thechemical shrinkage that takes place when water is attached to thecement. In High Performance Concrete the low porosity makesthe effect of self-desiccation much more pronounced. This paper
presents a summary of a Nordic Seminar on Self-Desiccation inConcrete held in Lund. For this purpose aspects of mix design,curing conditions, desiccation, micro-cracking, self-stresses,shrinkage and frost resistance related to self-desiccation aredetailed. Initially a background is given on the principal effects of self-desiccation. This Nordic Seminar took place in Lund on 10June 1997.
Keywords: Autogenous shrinkage, Chemical Shrinkage, Frostresistance, High Performance Concrete, Hydration, Internal
Prestressed High Performance Concrete (HPC) with water-cement ratio, w/c < 0.38, has beenused in Sweden since 1948 for the fabrication of water pressure pipes. The Swedish inventionwas also licensed abroad in more than 100 factories. The pipes exhibit superior durabilitycompared with pipes made of normal strength concrete. Besides pipes, columns for electrical
power lines and self-desiccating concrete slabs (more than 1 million square metres used todate) became the first large applications of HPC in Sweden. Self-desiccation is studied duringmoisture-insulated conditions (constant weight) and at constant temperature conditions. Noexchange of moisture takes place to or from the concrete specimen during the test period.
1.2 Self-desiccation
The fundamental cause of self-desiccation is the chemical shrinkage that takes place duringhydration of water to cement. As compared with the specific volume of water in the capillarypores the specific volume of the hydrated water in the gel of concrete is reduced by about
25%. Due to the decreased size of the capillary pores especially at low w/c < 0.38, the effect
of self-desiccation becomes more pronounced for HPC. Self-desiccation influences theproperties of the young concrete as well as the long-term behaviour of the concrete, i.e.deformations caused by self-generated stresses, stability and durability (frost resistance andcorrosion). Concrete with low w/c deforms even with sealed curing owing to self-desiccation,
free of imposed stresses (autogenous shrinkage). Silica fume concrete exhibits a very lowlong-term increase of the compressive strength due to self-desiccation, which may influencethe long-term stability and durability. Low internal relative humidity close to thereinforcement bars is a favourable parameter related to self-desiccation and perhaps decreasesthe rate of corrosion. The air-filled volume due to self-desiccation created by the chemicalshrinkage clearly improves the frost resistance of materials and structures. There is nosignificant effect of self-desiccation in normal strength concrete (NSC), and it has thus little orno effect on the properties of NSC or on NSC design. Low-w/c concrete has a shortage of water already from casting compared with the amount required for the chemical reactions tocome to an end. This means that a pronounced self-desiccation takes place, which is anadvantage for solving problems with moisture in the concrete during the time of construction,i.e. the amount of built-in moisture will be reduced since most of the moisture is consumedduring the hydration process. The mechanisms behind self-desiccation and the design criteriaof the stresses that occur in the concrete owing to the self-desiccation are largely unknown.One purpose of this Nordic Seminar was to compile the available knowledge on self-desiccation and to edit a state-of-the-art report in the field related to self-desiccation inconcrete. Another purpose was to initiate new applications of HPC and new ways to estimatethe effect of self-desiccation.
2. MIX DESIGN – EFFECT OF W/C AND AIR CONTENT
Since the effect of self-desiccation was more generally applied in Finland and Sweden in orderto solve problems related to built-in moisture in the concrete during the time of construction,intensive development of the mix design took place. Too high an amount of built-in moisturecaused additional costs as high as SEK 3 billion yearly in Sweden. A good part of this cost isrelated to built-in moisture from concrete. The high moisture content in the pores of theconcrete affected adhesives or wood placed in direct contact with the concrete. One solutionto avoid built-in moisture problems for NSC is to increase the drying time, often as much as 1year. However, owing to economic requirements this solution is not feasible. Another solution
is the use of HPC with very low content of built-in moisture. The initial development of themix design led to the conclusion that it was favourable to use air-entrainment and silica fumein the concrete to make the self-desiccating HPC more workable /2/. The governingparameter in mix design was the water-cement ratio, w/c, which should not exceed 0.38 if aninternal relative humidity, RH < 0.85, was required even under wet outer conditions such asrain or snow /3-5/. If w/c was larger than 0.38 the time of desiccation of the concrete becamevery long when the concrete was wetted at early ages, which was recently confirmed during alarge-scale field test, Figure 1 /6/. In Finland the effect of extreme air-entrainment has recentlybeen studied, up to 11% air content /7/. However, w/c still had the largest effect on the self-desiccation, Figure 2 /7/. If the strength was held constant, the air-entrainment became
favourable since lower w/c was then required, Figure 3 /7/. Another demand was to lower theamount of superplasticiser in the HPC, which was possible after use of an ideal grading curve
of HPC in the fresh state /8,9/. The high cost of the superplasticiser was a reason for therequired reduction. An ideal semi-linear logarithmic grading curve also improved the stabilityof the air-entrainment /10/.
60
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75
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100
0 5 10 15 20 25 30 35 40
Age (weeks)
I n t e r n a l r e l a t i v e h u m i d i t y ,
R H ( % )
w/c=0.4 A w/c=0.4 W w/c=0.5 A w/c=0.5 W w/c=0.7 A w/c=0.7 W
0.4 A
0.4 W
0.5 A
0.5 W
0.7 A
0.7 W
Figure 1. Development of RH in field tests of 125 mm floating concrete slabs /6/. Depth of measurement: 50 mm. A= air curing; W= air curing after water curing for 1 months.
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0 2 4 6 8 10 12 14 16
Age (weeks)
I n t e r n a l r e l a t i v e h u m i d i t y ,
R H
( % )
w/c=0.34 +12 °C w/c=0.34 +20 °C w/c=0.36 +12 °C w/c=0.36 +20 °C
Figure 2. Internal relative humidity, RH, in 100 mm cubes versus time at varying water-cement ratio, w/c, and air-entrainment. Raw data from /7/.
3. EARLY CURING
Early drying desiccation from the surface of fresh concrete results in so-called plasticshrinkage /11/, which can be controlled by sealing or addition of moisture to the surface /12/.HPC is more vulnerable owing to the lack of bleeding water and to a more pronounced earlychemical shrinkage compared with NSC /13/. Lately also water-based plastic solutions havebeen used in order to control the plastic cracking due to early shrinkage /10/. If required, thesurface with the sealing solution may be ground away 1 or 2 days after the casting. As analternative additional water may be supplied to the surface by the channels of a special type of plastic fleece /12/. Also the sides and the bottom of the formwork may be covered with fleecein order to avoid possible surface drying through the formwork. Studies have shown that this
method of controlling the plastic shrinkage also improved the durability of the surface sincethe amount of microcracking and the diffusivity decreased.
4. AUTOGENOUS SHRINKAGE
Autogenous shrinkage occurs when RH in a sealed concrete decreases due to the chemicalshrinkage that takes place when the water is attached to the cement during hydration /14/.The relationship between RH and shrinkage was nearly linear, Figure 4 /15/. The aggregate inthe concrete restrained the shrinkage substantially compared with the free shrinkage thatoccurred in cement paste. Since the autogenous shrinkage in low-w/c HPC is of the sameorder as the drying shrinkage, the risk of cracking even of sealed structures must be preventedby suitable working joints or by a sufficient amount of reinforcement. Use of prestressing maybe an alternative crack-controlling measure. However, most of the autogenous crackingoccurs within 3 months, which makes it possible for the contractor to repair any cracks withinthe construction time. The magnitude of the autogenous shrinkage also was dependent on thetype of the cement /16/. Low-alkali cement causes less autogenous shrinkage than normalalkali cement; a slowly hardening cement has less shrinkage than a normal cement /16/. Testsshow that the autogenous shrinkage in concrete with lightweight aggregate, LWA, perhaps iseliminated /13/. LWA seemed to perform like a reservoir for water supply in the concrete.
I n t e r n a l r e l a t i v e h u m i d i t y , R
H ( % )
K20-4% K30-4% K30-8% K35-1% K50-1% K50-4%
K20-4%
K35-1%
K30-4% och 8%
K50
Figure 3. RH in 100 mm cubes versus time at varying strength, and air-entrainment. K = cubestrength (MPa). 1% = 1% of air-entrainment. Raw data from /7/.
Figure 4. Shrinkage versus RH in concrete and cement paste. c = cement content; s= contentof silica fume; w = water content. Raw data from /15/.
0
1
2
3
4
0 2 0 4 0 6 0 8 0 1 0 0
I n t e r n al r e l a t i v e h u m i d i t y ( % )
S h r i n k a g e ( p e r m i l )
Co nc r e te w /c =0 .2 6 ;s / c =0 .1 Co n c r e te w / c =0 . 48
C e me n t p a st e w / c= 0 .1 9 ;s /c = 0. 1 C e me n t p a st e w / c= 0 .3 4
A large number of studies have been performed concerning the effect of autogenous shrinkage
on the self-generated stresses in concrete /13-18/. During the measurements of self-generatedstresses in HPC a stress rig with controlled length is most often used /13/. Owing to externalrestraint, self-generated compressive stresses in reinforcement reaching as much as 40 MPahave been observed in HPC /17/. In a most interesting method to measure the internal self-generated stresses in the cement paste, a porcelain ball with manganin wire around it is used
/19/. The manganin wire displayed changes in the pressure of the cement paste surrounding it.Alternatively a mercury thermometer may be used cast in the concrete. Small volume changesin the concrete are transferred to the mercury in the thermometer and then read as a change intemperature /19/. Both types of detectors were first calibrated in oil subjected to hydraulicpressure. Figure 5 shows self-generated stresses in cement pastes with w/c= 0.30 at differentamounts of silica fume /19/. The amount of silica fume is significant with regard to thedevelopment of self-generated stresses in the concrete. This effect is in turn explained by thepore distribution in the concrete. The pore distribution may be described as the degree of saturation, S0, related to RH, Figure 6 /14/. For example S0= 0.83 gives RH = 0.90 inPortland cement concrete but RH = 0.80 in concrete with 10% silica fume. According to thewell-known Kelvin equation, lower RH causes greater underpressure in the pore water andthus also larger autogenous shrinkage.
6. SURFACE MOISTURE AND VOLATILE EMISSIONS
Large volatile emissions from the adhesive between the concrete and a plastic carpet mayoccur when the RH = 0.90 both at the surface of the concrete and at the critical depthprovided that the concrete did not carbonate /20-23/, i.e. the concrete was cured withaluminium foil. Due to the very dense structures of silica fume concretes, the available volumeis less in silica fume concretes than in Portland cement concretes with RH held constant,Figure 6 /14/. The required degree of saturation to absorb the glue is more or less constant insilica fume concretes and Portland cement concretes. For example, at a degree of saturation,S0= 0.83, RH = 0.90 is obtained in Figure 6. In concrete silica fume with 6% silica fume RH =0.84 according to Figure 6. If adhesive and a plastic carpet are placed on a concrete with 6%
silica fume (at RH = 0.84 in the surface) the critical degree of saturation most certainly will beexceeded, which explained the large volatile emissions /20-23/.
The solution to the problem is perhaps to create a free pore volume in the surface by drying,with a volume that is large enough for the glue to be absorbed /24/. The required surfacedrying time is about 1 month. During the surface drying time carbonation will also take place,which substantially lowers the pH in the surface of the concrete, which is beneficial in order todecrease the reactions with the adhesives /25/. In low-w/c HPC carbonation takes placeprovided that no silica fume is used /26/. After two months of carbonation the volatileemissions from a plastic placed with adhesives on concrete were reduced to one sixth
compared with emissions from a surface that did not carbonate before the adhesive wasapplied /27/.
Even after several years water-cured HPC with low w/c maintains a very low internal relative
humidity, RH, due to self-desiccation, at least in the inner part of the concrete a couple of centimetres from the surface /28/. The self-desiccation thus creates a free pore volume in HPCwhere the ice may expand during freezing. (The total amount of water is also less in HPC thanin NSC, which also in turn increases the frost resistance.) Young HPC tolerates freezing atearlier age than a NSC, Figure 7 /28/. Furthermore, the scaling from HPC with low w/cseemed to be very small provided that no silica fume was used /28,27/. The scaling of silicafume concrete shows the opposite after 30 cycles of testing. Perhaps the porosity of silicafume concretes was altered, decreasing the available volume for ice to expand, cp. Figure 6
/14/.
0
10
20
30
40
50
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Water-cement ratio, w/c
D e g r
e e o f h y d r a t i o n ( % ) a n d
c u r i n g t i m e ( h ) r e s p e c t i v e l y
Degree of hydration (%) Curing t ime (h)
Figure 7. Required degree of hydration (%) and curing time (h) in order to avoid frostdamage. Raw data from /28/.
8. SUMMARY AND CONCLUSIONS
A Nordic Seminar on Self-Desiccation and Its Importance in Concrete Technology was heldin Lund on 10 June 1997. This article summarised the Seminar and gave some practicalaspects on the effect of self-desiccation. The following conclusions were drawn:
Figure 8. Scaling from Portland cement and silica fume concrete respectively. Raw data from /28,27/.
• The fundamental cause of self-desiccation was the chemical shrinkage that takes placeduring hydration of water to cement.
• The effect of self-desiccation was clearly observed in low-w/c concretes, i.e. in concretewith w/c < 0.38.
• Autogenous shrinkage, one effect of self-desiccation, was nearly linearly dependent on theinternal relative humidity, RH, of HPC.
• Autogenous shrinkage also caused self-generated compressive stresses up to 40 MPa inthe reinforcement in HPC.
• Self-desiccation may have a beneficial effect on the frost resistance of Portland cement
based HPC (primarily given that no silica fume is used in the mix design).• Self-desiccation in concrete with low w/c decreased the risk of built-in moisture in
structures since a great part of the capillary water was hydrated to the cement.
ACKNOWLEDGEMENT
The Swedish Council of Building Research financed part of the Seminar, which is herebygratefully acknowledged. I am also most grateful to Professor Göran Fagerlund for hisreview.
/1/ B. Persson, G. Fagerlund. Self-desiccation and Its Importance in Concrete Technology.
Report TVBM-3075. Div. Building Materials. Lund Institute of Technology. (1997).
/2/ M. Karlsson. Rapid-Desiccating Concrete. Cementa 2.92. Cementa. Danderyd (1992).
/3/ G Hedenblad. Desiccation of Moisture during the Construction Period - Drying Timeand Measurement of Moisture. Report T12:1995. Swedish Council of BuildingResearch. Stockholm (1995).
/4/ B. Persson. DRY for Choice of Water-Binder Ratio in Concrete Free of Moistureduring the Construction Period. Report TVBM-3075. Div. Building Materials. LundInstitute of Technology. Lund (1997).
/5/ M. Gerlam. SBUF Guide for Drying of Moisture in Concrete. Sabema. Kållered(1996).
/6/ B. Persson. Effect of Grinding on Surface Alkalis of Concrete. Report 97.07. Div.Building Materials. Lund Institute of Technology. Lund (1997).
/7/ V. Penttala, L Wirtanen. Drying of Concrete with Low Water Binder Ratio and HighAir Content. Report TVBM-3075. Div. Building Materials. Lund Institute of Technology. Lund, 209-226 (1997).
/8/ B. Persson. Ideal Grading Curve in Fresh Concrete. BETONG 3/95. Stockholm(1995).
/9/ B. Persson. Concrete in Efficient Construction. Bygg & Teknik 7/96. Stockholm(1996)
/10/ J. Persson. Private communication. Sydsten LTD. Malmö (1996).
/11/ A. Radocea. Autogenous Volume Change of Concrete at Very Early Age - Model andExperimental Data. Report TVBM-3075. Div. Building Materials. Lund Institute of Technology. Lund, 56-71 (1997).
/12/ U. Guse, H.K. Hilsdorf. Surface Cracking of High Strength Concrete - Reduction byOptimisation of Curing Regimes. Report TVBM-3075. Div. Building Materials. LundInstitute of Technology. Lund, 239-249 (1997).
/13/ Ø. Bjøntegaard , T.A. Hammer, E. Sellevold. High Performance Concrete at EarlyAges: Self-Generated stresses due to Autogenous Shrinkage and Temperature. ReportTVBM-3075. Div. Building Materials. Lund Institute of Technology. Lund, 1-7(1997).
/14/ B. Persson. Experimental Studies of the Effect of Silica Fume on the ChemicalShrinkage and Self-Desiccation in Portland Cement Mortars. Report TVBM-3075.Div. Building Materials. Lund Institute of Technology. Lund, 116-131 (1997).
/15/ V. Baroghel-Bouny. Experimental Investigation of Self-Desiccation in High-Performance Materials - Comparison with Drying Behaviour. Report TVBM-3075.Div. Building Materials. Lund Institute of Technology. Lund, 72-87 (1997).
/16/ E-I. Tazawa, S Miyazawa. Effect of Self-Desiccation on Volume Change and FlexuralStrength of Cement Paste and Mortar. Report TVBM-3075. Div. Building Materials.Lund Institute of Technology. Lund, 8-14 (1997).
/17/ F. Tomozawa, T Noguchi, K.B. Park. Experimental Determination and Analysis of Stress and Strain Distribution of Reinforced High-Strength Concrete Column Causedby Self-Desiccation and Heat of Hydration. Report TVBM-3075. Div. BuildingMaterials. Lund Institute of Technology. Lund, 99-115 (1997).
/18/ H Hedlund, G Westman. Measurement and Modelling of Volume Change andReactions in Hardening Concrete. Report TVBM-3075. Div. Building Materials, 174-192 (1997).
/19/ B. Dela, H. Stang. Eigenstresses in Concrete due to Autogenous Shrinkage. ReportTVBM-3075. Div. Building Materials. Lund Institute of Technology, 46-51 (1997).
/20/ H.W. Johnsson. Chemical Emissions from Flooring - Effect of Concrete Quality and
Moisture. Publication P95:4. Chalmers University of Technology. Gothenburg (1995).
/21/ M. Fritsche, A. Sjöberg, H.W. Johnsson. Chemical Emissions from Combined GluedFlooring on Self-Desiccating Concrete - Effect of Method of Gluing, Type of Drying,Type of Cement, Glue and Plastic Carpet. Publication P97:1. Chalmers University of Technology. Gothenburg (1997).
/22/ A. Sjöberg. Ongoing Research concerning Flooring System, Moisture and Alkali.AMA-News - Ground - Housing 1/97. Swedish Building Service. Solna, 15-17 (1997).
/23/ A. Sjöberg. Ongoing Research. Floor System, Moisture and Alkali. Contribution atHB/IAQ’97. Washington (1997).
/24/ B. Persson. Drying of Surface in Building-Moisture Free Concrete. AMA-News -Ground - Housing 1/96. Swedish Building Service. Solna, 20-23 (1997).
/25/ O. Peterson. Priv. com. Div. Building Materials. Lund Institute of Technology. (1997).
/26/ B. Persson. Long-term Shrinkage in High Performance Concrete. Contribution 2i073.10th International Congress on the Chemistry of Cement. Gothenburg (1997).
/27/ B. Persson. Carbonation and Emissions. BETONG 2/98. Stockholm (1998).
/28/ G. Fagerlund. Effect of Self-Desiccation on the Internal Frost Resistance of Concrete.TVBM-3075. Div. Building Materials. Lund Institute of Technology, 227-238 (1997).
/29/ P.E. Petersson. Salt-frost Resistance of Concrete - Field Tests. Report SP 1995:73.The Swedish Testing and Research Institute. Borås (1995).
APPENDIX: LIST OF PARTICIPANTS AT THE NORDIC SEMINAR IN LUND
Baroghel-Bouny, Veronique , Laboratoire Central des Ponts et Chaussées, LCPC, Paris.
Bentz, Dale; National Institute of Standards of Technology, NIST, Gaithersburg.
Bjøntegaard, Øyvind; The Norwegian University of Science and Technology, Dept. Structural
Engineering, Trondheim.
Dela, Birgitte Fries, Dept. Structural Engineering and Materials, DTU, Lyngby.
Fagerlund, Göran, Div. Building Materials, Lund Institute of Technology, Lund.
Hammer, Tor Arne, The Research Institution SINTEF, Civil and Environmental Engineering,Trondheim.
Hansen, Kurt Kielsgaard , Jensen, Ole Mejlhede, Dept. Structural Engineering and Materials,
Technical University of Denmark, DTU, Lyngby.
Hedenblad, Göran, Div. Building Materials, Lund Institute of Technology, Lund.
Hedlund, Hans; Div. Structural Engineering, Luleå University of Technology, Luleå.
Juvas, Klaus; Partek Concrete Development Ltd, Pargas.
Mejlhede Jensen, Ole, Dept. Structural Engineering and Materials, DTU, Lyngby.
Koenders, Eddie A B; van Breugel, Klaas, Faculty of Civil Engineering, Delft University of
Technology, Delft.
Leivo, Markku, Building Technology, Technical Research Centre of Finland, Espoo.
Miyazawa, Shingo, Ashikaga Institute of Technology, Dept. Civil Engineering, Ashikaga.
Mjörnell, Kristina, Dept. Building Materials, Chalmers University of Technology, Gothenburg.
Persson, Bertil, Div. Building Materials, Lund Institute of Technology, Lund.
Radocea, Adrian, Dept. Building Materials, Chalmers University of Technology, Gothenburg.
