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 email@example.com, firstname.lastname@example.org 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 . 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 , Eurokód 2 , Model B4 , Model B4s  a Model ACI 209R-92 . 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  ACI Committee 209, (2008): Guide for Modeling and Calculating Shrinkage and Creep in Hardened Concrete. ACI Report 209.2R-08, Farmington Hills.  ČSN EN 1992-1-1: Eurokód 2: Navrhování betonových konstrukcí – Část 1-1: Obecná pravidla a pravidla pro pozemní stavby, (2005). Český normalizační institut.  fib Model Code for Concrete Structures 2010 – Final Draft, (2013). fib – International Federation for Structural Concrete.  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.  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.