ACI Committee 209 –CREEP AND SHRINKAGE IN CONCRETE MINUTES ACI 2006 FALL CONVENTION Denver CO, November 6, 2006 Attendees VOTING MEMBERS Carlos Videla (Chair) Domingo J Carreira (Secretary) Akthem Al-Manaseer Zdenek Bazant Marwan Daye Walter Dilger John Gardner Will Hansen David McDonald Hani Nassif Ian Robertson Jason Weiss Klaus Alexander Rieder ASSOCIATE MEMBERS Mario Sassone Mahmoud Reda Taha Keiichi Imamoto Nakin Suksawang VISITORS Paul Brooks Will Lindquist Jason Draper Adam Ihab Ken Leland Opening of the Meeting and Welcome The meeting was called to order by Videla at 15:35 and all in attendance were welcomed to the meeting The following voting members offered apologies for not being able to attend the meeting: Mario Chiorino, Lawrence Novak and Kenji Sakata Also associate members Adam Neville and Mauricio Lopez sent apologies for not attending the meeting Meeting attendees introduced themselves Copies of the following documents were distributed: • Agenda for this meeting • Attendance list • Minutes of the Charlotte meeting • TAC Review of ACI 209.XX-XX “Modeling and Calculation of Shrinkage and Creep in Hardened Concrete” (former Chapter 5) • Draft of responses to “P” comments of TAC Review (Carlos Videla) • Guide of Testing Methods for Creep and Shrinkage of Concrete (Hani Nassif) • Draft changes to ACI 209.1R document (David McDonald) Announcements Next meeting will be on Monday 23rd April, 2007 in Atlanta, GA Changes in membership: The chair informed that: • Professor Harald Muller has been appointed as Dean and Member of the Presidium at Karlsruhe University and has been elected as member of the German Research Foundation, representing the area of Civil Engineering, and member of the Presidium of Fib Because these new and important academic commitments are very time consuming he has resigned to the voting member status and his membership was changed to Associate Members, effective October 16, 2006 He expects to join again ACI 209 committee in the future • ACI sent official notification to the Chairman for the appointment of the following three new associate members: Nakin Suksawang (Assistant Professor, Florida International University), Mauricio Lopez (Assistant Professor, Pontificia Universidad Católica de Chile) and Patrick Earney (Student, University of Missouri) The committee roster was circulated No changes were made Other announcements: • The chair announced that the committee has 18 voting members, 19 associate members and consulting members • Gardner stated that the committee has preliminary approval for two sessions at the Fall 2007 Convention in Puerto Rico titled “Structural Effects due to Creep and Shrinkage.” Six abstracts have been received but more are needed An SP is proposed to be available by the time of the session Abstracts are required before the end of November Approval of Minutes from Spring 2006 Meeting in Charlotte, NC Videla asked for review and approval of the minutes Robertson moved, seconded by McDonald, to approve the minutes of the Charlotte meeting All members present voted in favor of approving the Charlotte meeting minutes with the exception of Bazant who abstained Current action updates Status of reports: ACI 209.XX-YY “Modeling and Calculation of Shrinkage and Creep of Hardened Concrete” The chair informed the committee that he and the secretary met with TAC on Sunday morning to discuss the TAC comments which had been received on Friday November rd This did not give time to distribute the TAC comments to the members before the meeting with TAC One hundred and one comments had to be addressed, 22 primaries, 58 editorial and 20 secondary – some comments overlapped All primary, (P) comments were discussed with TAC and reached agreement on the changes to satisfy the P comments TAC requires that only the responses to the comments be addressed and balloted by the committee The document itself is not for changes or balloting According to TAC recommendation Primary, Editorial and Secondary comments could be balloted separately P and E comments identifies a primary technical and a specific or general editorial issue, respectively The committee must consider these comments and take action before publication of the document S comment identifies a secondary technical or editorial issue and the committee is expected to consider and take action, but need not document the response The committee reviewed all the P comments and made proposals on how to resolve them Some proposals were accepted and others were tabled The agreed proposals are listed below by comment number Response to Comment Nº 1: Models ACI 209 and CEB MC90-99 were not included because are already published However the committee has no problem to include both models For ACI 209 it is suggested to use the summary included in Neville & Brooks book entitled “Concrete Technology” With respect to CEB MC90-99 there are copyright problems that should be solved by ACI Headquarters or to point out in the corresponding appendix the source where it can be found Ballot result: in favor, against and abstention Response to Comment Nº 2: Numeric examples should be added using the MathCAD program facilitating engineers to develop their own programs Ballot result: in favor, against and abstention Response to Comment Nº 8: The models allow estimating short-term and long-term creep and shrinkage, i.e since the end of the curing period The following changes are suggested to page 4, lines to 7: … including the prediction of the time-dependant behavior of the hardened concrete The prediction of short-term shrinkage and creep is also important to assess the risk of concrete cracking and stripping-reshoring and unshoring times It was agreed to a search of the words “short” and “long” term and be changed to “time-dependant” Ballot result: 12 in favor, against and abstentions It was agreed that from this comment on, the committee would not ballot the remaining items, and a consensus agreement was reached for the remaining items Response to Comment Nº 15: Accepted to delete the following sentence: If drying creep requires loss of water from the concrete to the environment, however, superposition would require ingress of water into the concrete on unloading, which is generally inconceivable Response to Comment Nº 21: the creep models were developed from specimens tested on compression, however it is assumed that the creep in tension is the same as the creep in compression The following change is suggested to page 5, lines 18 and 19: This document was developed is limited to address the issues related to the prediction of creep under compression and shrinkage-induced strains in hardened concrete Similar change should be considered on page 1, line 21 adding “in compression” after the word creep Response to Comment Nº 22: See response to TAC Comments Nº and 21 Response to Comment Nº Nº 23: See response to TAC Comment Nº 21 Response to Comment Nº 34: … overestimation of the tensile stresses and, therefore, it is may be an appropriate a safe basis for design For restrained autogenous shrinkage, however, the opposite occurs Limited information exists for tensile creep Response to Comment Nº 37: The notation will be changed to: f’c = concrete specified characteristic strength at 28 days And change in the document Also it was agree to include a note that compressive strength refers to cylinder strength Response to Comment Nº 47: The committee could not agree on a method for comparison of test data with predictions from models for shrinkage and creep Reducing the comparison between a large number of experimental results and a prediction method to a single number is fraught with uncertainty The following sentence is suggested to be added at the end of line 12, page 13: Therefore the committee strongly recommend designers to perform sensitivity analysis of the response of the structure using the analyzed models or/and to carry out short term testing to calibrate the models and improve the prediction The committee summarized the statistical indicators in Tables 4.1 and 4.2 to provide the user with basis of comparison without endorsing any method Response to Comment Nº 48: Figures 4.1 to 4.8 clearly show that as time increases, the spread of data increases Paragraph re-written to eliminate misunderstanding: One of the problems with many of the standard statistical techniques is that shrinkage and creep data exhibit heteroscedacity of the comparison of shrinkage and creep data with a model’s prediction is the increasing divergence with time as shown in Figures 4.1 to 4.8 That is, as time increases, the spread of data increases Thus, when techniques such as linear regression are used, the weighting of the later data is greater than that of the earlier data (Bazant 1987; Bazant et al 1987) On the contrary, the comparison of the percent deviation of the model from the data tends to weight earlyage data more than later-age data The spread is a measure of the limitation of the model’s capabilities and variability in the experimental data Response to Comment Nº 54: The term corresponds to the one used by RILEM Committee TC 107 In order to clarify the meaning it is suggested that page 17, line 19 should read: Drying shrinkage and drying creep should be bounded (do not increase indefinitely with time); Response to Comment Nº 59: It is unfair because the comparison of ACI 209 model was done with data outside the range of applicability of the model and the model was developed before the Rilem Databank was established It is suggested that page 20, lines to 13 should read: Figure 4.1 through 4.8 (Gardner 2004) compare the predicted values for two sets of input information for RILEM data sets extending longer than 500 days, concrete 28day mean strengths fcm28 between 16 and 82 MPa (2320 and 11,890 psi), water-cement ratios between 0.4 and 0.6, duration of moist curing longer than day (possibly unfair biased against to ACI 209R-92 because this model was developed assuming moist curing of at least days and age of loading beyond days), and age of loading greater than the duration of moist curing and volume-surface ratios V/S greater than 19 mm (3/4 inches) Response to Comment Nº 75: The Committee has converted the units of the model Units will be deleted from model’s description in Chapter Response to Comment Nº 76: The scatter of the data is due to the heteroscedacity (see response to TAC comment Nº 48), not to the size effect Therefore the following change is suggested: The comparison of shrinkage data in Fig 4.1 clearly shows the lack of a size effect in the average thickness function that ACI 209 model overestimate measured shrinkage at low shrinkage values (equivalent to short drying times) and underestimate at high shrinkage values (typical of long drying times) This result indicates the limitation of the model’s equation used to predict shrinkage The simplest creep-time function does not contain a size effect term, but the effect is masked by the contribution of basic creep to the total creep The ACI 209R-92 compliance comparison is rather insensitive to using all of the available data, including mix proportions, compared with just using the measured concrete strength Response to Comment Nº 78: Delete “Kristek et al (2001) developed a user-friendly website to calculate shrinkage and compliance” Although the sentence could be replaced by: Chiorino et al (2006) developed a userfriendly website to calculate shrinkage and compliance (www.polito.it/creepanalysis), is it suggested to delete the sentence because the committee can not assure the permanence of this web site in time Response to Comment Nº 82: Accepted to provide in.-lb unit equivalents for equations Response to Comment Nº 83: The following change is suggested to page 25 line 11: … sensitive structures have accepted this model as preferable to the ACI 209 model (based on 1971 Branson and Christiason model) Response to Comment Nº 86: Ideal or theoretical optimum values will be added The information will be provided by Al-Manaseer Response to Comment Nº 87: See response to TAC Comment Nº 86 Response to Comment Nº 91: Although the models are not sensitive to minor changes in input values, the effect of air conditioning in moist climates and exposure to pool in dry climates can be very significant Response to Comment Nº 96: The models can not make up considerations for data that does not exist or for concretes yet to be developed However the committee agrees to state the limitations of the models up front of the document in the first page, as follows: This guide describes the prediction of shrinkage and creep in hardened concrete It outlines the problems and limitations in developing prediction equations for shrinkage and compressive creep of hardened concrete It also presents and compares the prediction capabilities of four different numerical methods This document is aimed at designers who wish to predict shrinkage and creep in concrete; however, it does not include information on structural design issues associated with shrinkage and creep The prediction models are for concretes of typical composition not including very highstrength concrete nor concrete with supplementary cementing materials as silica fume, high fly ash content and natural pozzolan or high range admixtures, The models can be used in such circumstances if are calibrated with test data Response to Comment Nº 100: The committee can not change published work ACI 209.XX-YY “Time Dependent Effects in Concrete Structures” (former Chapter 6) Dilger informed that he has sent to committee members a revised Section 6.6 Composite Members of the draft document ACI 209.XX-YY “Time Dependent Effects in Concrete Structures” (March 20, 2006) However, Chiorino is behind schedule in the revision of his part of the report due to other important assignments Chairman Videla states that we are far behind the original schedule Also, the members of the sub-committee still have the mission of agreeing and submitting a draft document prior to be balloted by ACI 209 committee Therefore suggests that a final dead line should be the Spring 2007 Atlanta Convention for having the results of a balloted document and an approved printed version of the report for the session at next year's Fall Convention in Puerto Rico This means Chiorino and the sub-committee should agree and commit to finalize the revision of the report by early January 2006 and a new commonly agreed draft of the document by the end of January 2006, respectively If these target dates cannot be accomplished, the sub-committee compromise should be to propose the best alternative among the two documents New Business – future action “Early Age Shrinkage and Cracking of Concrete” Jason Weiss No action “Test Methods for Creep and Shrinkage” Hani Nassif A document was distributed with a proposed content of the “Guide on Testing Methods for Creep and Shrinkage of Concrete” A sub-committee was formed with the following members: Nassif (designated subcommittee chair), Burg, McDonald, Dilger, Carreira, Imamoto, Gardner and Videla ACI 209.1R-05 Updating document “Report on Factors Affecting Shrinkage and Creep of Hardened Concrete” David McDonald submitted preliminary suggested revisions Chairman Videla pointed out that some inconsistencies with respect to the two new documents under the process of TAC approval and final drafting should be resolved, particularly with respect to notation Presentations Keiichi Imamoto presented the paper "The Influence of Specific Surface Area of Coarse Aggregate on Drying Shrinkage of Concrete" from 18:30 to 18:50 Videla thanked Dr Imamoto for his presentation Other Business No other business Adjourn A motion to adjourn was submitted by Robertson and seconded by Carreira All members present voted to adjourn The meeting adjourned at 18:53