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Solid State Phenomena
ISSN: 1662-9779, Vol. 259, pp 46-51
doi:10.4028/www.scientific.net/SSP.259.46
© 2017 Trans Tech Publications, Switzerland
Submitted: 2016-10-17
Revised: 2016-12-21
Accepted: 2017-02-07
Online: 2017-05-19
Drying Shrinkage of Concrete: Experiments and Numerical Models
VINKLER Marek1, a *and VÍTEK Jan L.2,b
1
Faculty of Civil Engineering, CTU in Prague, Czech Republic
2
Faculty of Civil Engineering, CTU in Prague and Metrostav, a.s., Czech Republic
a
b
marek.vinkler.1@fsv.cvut.cz, jan.vitek@metrostav.cz
Keywords: Drying Of Concrete, Shrinkage Of Concrete, Strain Gauges, Shrinkage Models
Abstract. The paper presents some results of experimental program focused on drying and
shrinkage of large concrete specimens. Segments of walls with thicknesses 200, 400 and 800 mm
and standard cylinders 150x300 mm were used as specimens. Each segment has embedded 4
vibrating wire strain gauges in axis plane for measurements of shrinkage strain and plastic tubes of
various lengths for measurements of pore relative humidity in different depths. Relative humidity
and temperature of ambient environment were not controlled, however they were recorded very
closely. Measure shrinkage strains are compared with prediction based on shrinkage models. The
most important predictive models are used for comparison: Model Code 2010, Eurokód 2, Model
ACI 209-R92, Model B4 a Model B4s.
Introduction
Phenomenon drying of concrete has direct consequence in shrinkage of concrete and drying creep
of concrete. These two phenomena belong to major factors influencing cracking of concrete
elements Due to uneven drying of concrete cross-section, uneven tendency to shrink arise. As a
consequence concrete member shrinks and internal stresses are generated – they are tensile in
surface layers and they might overcome tensile strength of concrete.
In order to obtain more detailed information on the mutual relations between drying and shrinkage
of concrete, the experimental program has been carried out since January 2015. The progressive
drying of concrete is measured in large concrete elements and laboratory specimens. Such
experiment is able to provide direct measurements of drying and shrinkage. Measured values are
compared with predictive shrinkage models.
Layout of Experiment
Three wall segments of the thicknesses of 200 mm, 400 mm and 800 mm were cast into wooden
formworks. Individual segments had the same longitudinal and vertical size 800 x 800 mm. A
relative humidity of concrete in different depths under the surface exposed to environment was
measured. Four sides of the wall segments were sealed, so that one dimensional drying was
simulated. Four vibrating wire strain gauges were embedded into each segment for measurements of
shrinkage of concrete. Measurements of strains in concrete started 3 hours after concreting.
Measurements of pore relative humidity of concrete started 10 days after concreting. The values of
pore relative humidity were measured in the corresponding depths for each drying surface of each
segment (15, 30, 50, 70, 100, 150, 200, 300, 400 mm).
A typical concrete mix was provided by the professional concrete producer. The concrete
strength class C30/37 was selected because of the most frequent production. Basic mix parameters
were: water cement ratio 0,50; cement content 360 kg/m3 (Portland cement 42,5R). The measured
average cube strength in compression at 28 days was about 55 MPa. More information about the
experiment can be found in literature [5]. Layout of the experiment is displayed in Fig. 1.
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
Tech Publications, www.scientific.net. (#103349884, University of Auckland, Auckland, New Zealand-12/11/17,07:07:56)
Solid State Phenomena Vol. 259
47
Fig. 1 Layout of the experiment
Results
Chosen results of measured relative humidity and shrinkage to the age of 552 days are presented
below. Time development of pore relative humidity of concrete in different depth are shown in
Fig. 3 for 200 mm thick wall and in Fig. 4 for 800 mm thick wall.
The environmental conditions in the laboratory were carefully recorded with 5 minutes time step.
The average relative humidity was 39,1 %, while the average temperature was 21,5 °C for the
period of 552 days (see Fig. 2).
Fig. 2 Day average environment conditions
48
23rd Concrete Days 2016
Fig. 3 Time development of pore relative humidity for 200 mm thick wall
Fig. 4 Time development of pore relative humidity for 800 mm thick wall
Solid State Phenomena Vol. 259
49
Fig. 5 Measured shrinkage strain on wall segments and cylinders
Comparison with Shrinkage Models
Shrinkage strains measured on individual specimens are presented in Fig. 5. Comparison of
measured shrinkage and shrinkage models are shown in Fig. 6 for 200 mm thick wall and in Fig. 7
for 800 mm thick wall. Specifically, following shrinkage models were used: Model Code 2010 [3],
Eurokód 2 [2], Model B4 [4], Model B4s [4] a Model ACI 209R-92 [1]. Measured data are
displayed twice in each figure: by its measured value (denoted as EXP) and by modified value with
subtracted initial swelling (denoted as EXP 2).
Several observations can be made from the comparison of measured and predicted total
shrinkage strain until the age of 371 days: 1) If no subtraction of initial swelling strain had been
carried out, comparison would have been almost futile. Measured shrinkage strains (EXP) lay below
all model predictions. This difference might be smaller after longer period. It seems that shrinkage
models overestimate measured shrinkage strains; 2) Except for the wall segment ST3, Model ACI
209R-92 provides too high prediction of strains. It also overestimates early-age shrinkage rate; 3)
The best overall prediction (compared with values EXP2) for all wall segments is provided by
Eurocode 2.
50
23rd Concrete Days 2016
Fig. 6 Measured shrinkage strain vs shrinkage models for 200 mm thick wall
Fig. 7 Measured shrinkage strain vs shrinkage models for 800 mm thick wall
Summary
(i)
Progressive drying and shrinkage of concrete were measured on large concrete specimens.
Drying of concrete was recorded as very slow phenomenon and it will take years until
approximate equilibrium between moisture in concrete and ambient relative humidity With
respect to the fact that relative humidity of ambient environment is variable, true equilibrium
Solid State Phenomena Vol. 259
51
will never occur and moisture in concrete will change according to environment conditions..
Obtained results are no surprise, nevertheless presented measurements, which are still in
progress, represents important results for typical concrete mixture used in concrete industry.
(ii) The shrinkage measurements show that the shrinkage strains are highly dependent on the size
of the specimen. It is also a well-known fact, but the experiments provide exact relations
between the size and shrinkage strain also for the thickness up to 800 mm. The results clearly
show that strains measured on cylinders are multiples of the strain in realistic concrete
elements.
(iii) A comparison of the measured shrinkage strain and predicted shrinkage strain using the
models shows the necessity to measure the strains immediately after casting of elements. The
initial swelling was observed in all specimens which is not usually recognized if shrinkage
measurements start later. Many shrinkage values in databases probably involve values which
do not include early measurements. Prediction models which are built on these databases are
not able to capture the swelling effect properly.
Acknowledgements
Shrinkage measurement is a long term research activity. Results presented in this paper were
obtained with financial support of project TAČR No. TE01020168, project GAČR No. 16-04454S
and student grant SGS16/039/OHK1/1T/11. All the supports are gratefully acknowledged.
References
[1] ACI Committee 209, (2008): Guide for Modeling and Calculating Shrinkage and Creep in
Hardened Concrete. ACI Report 209.2R-08, Farmington Hills.
[2] ČSN EN 1992-1-1: Eurokód 2: Navrhování betonových konstrukcí – Část 1-1: Obecná pravidla
a pravidla pro pozemní stavby, (2005). Český normalizační institut.
[3] fib Model Code for Concrete Structures 2010 – Final Draft, (2013). fib – International
Federation for Structural Concrete.
[4] RILEM Technical Committee TC-242-MDC (chair: Bažant, Z. P.): RILEM Draft
Recommendation: TC-242-MDC Multi-decade Creep and Shrinkage of Concrete, (2015): Material
Model and Structural Analysis. Model B4 for Creep, Drying Shrinkage and Autogenous Shrinkage
of Normal and High-strength Concretes with Multi-decade Applicability. Materials and Structures,
vol. 48, pgs. 753-770.
[5] Vinkler, M., Vítek, J. L., (2015): Progressive Drying and Shrinkage of Concrete. Proceedings
of the 10th International Conference on Mechanics and Physics of Creep, Shrinkage, and Durability
of Concrete and Concrete Structures. Concreep 10, Vienna.
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