1、ACI 364.11T-15TechNote1 Managing alkali-aggregate reaction expansion in Mass concreteKeywords: alkali-aggregate reaction; maintenance; mass concrete; repair.IntroductionWhen the alkalis in cement react with susceptible aggregate particles, a reaction rim of alkali-silica or alkali-carbonate gel is f
2、ormed around the aggregate. If this gel is exposed to moisture, it expands, causing an increase in volume of the concrete mass. This expansion will develop if the relative humidity (RH) in concrete is higher than 80 to 85% and temperature is 50 to 104F (10 to 40C). Alkali-aggregate reaction (AAR) in
3、 mass concrete sections typically results in extensive cracking with crack widths that can be much wider compared with cracking caused by other types of deterioration (Fig. 1). The crack width can range from 0.004 in. (0.1 mm) to as much as 0.4 in. (10 mm) in extreme cases. The severity of cracking
4、appears to be a function of the rate of internal expansion, ambient environmental conditions, and the degree of restraint present in a given concrete section. Cracking in large mass concrete struc-tures can be unsightly, although structural integrity may or may not be impaired. Expansion of the conc
5、rete can cause major operation and maintenance problems, such as inoperable gates in locks and dams and misalignment of hydropower generators in dams.While the causes of AAR and maintenance issues may be applicable to other types of concrete structures, the discussion is often directed toward mass c
6、oncrete struc-tures. Mass concrete is the volume of concrete with dimensions large enough to require that measures be taken to cope with the generation of heat and tempera-ture gradients from hydration of the cementitious mate-rials and attendant volume change due to internal or external restraint.Q
7、uestionWhat are the maintenance and repair techniques for mass concrete structures damaged by AAR?AnswerThere is no universal approach to maintenance or repair of concrete structures affected by AAR, and prob-lems usually are addressed on a case-by-case basis. Maintenance and repair ranges from moni
8、toring the AAR-affected concrete (Fig. 2) to complete removal and replacement.Fig. 1Wide cracks in concrete as a result of AAR (width of the order of 1 in. 25 mm in certain areas).Fig. 2Monitoring changes in crack width.American Concrete Institute Copyrighted Material www.concrete.org2 MANAGING ALKA
9、LI-AGGREGATE REACTION EXPANSION IN MASS CONCRETE (ACI 364.11T-15)DiscussionMost research is directed at investigating mechanisms of deterioration, identifying materials susceptible to reac-tion, and preventing AAR (201.2R; 221.1R) in new construc-tion. In theory, drying the concrete is the most judi
10、cious way to mitigate the expansion process in an existing struc-ture; however, in practice, the size of mass concrete struc-tures makes it nearly impossible to completely dry the concrete sufficiently to mitigate the AAR reaction (201.2R; 221.1R). If the concrete cannot be dried, then applications
11、of surface treatments, such as penetrating sealers and thin coatings, can be used to minimize the ingress of moisture. This will, at best, maintain the moisture content present in the concrete at the time of treatment. The moisture content in the interior of mass concrete will almost always be above
12、 the threshold value required for expansive activity. For example, results of tests on five dams in the desert southwestern United States show that atmospheric drying has little or no effect on the internal RH and moisture content of concrete. Even after 50 years of exposure, values for RH exceeded
13、the 85% threshold for expansion as little as 0.5 to 1 in. (12 to 25 mm) from exposed surfaces (Stark and DePuy 1987). Therefore, appli-cation of surface treatments should not be expected to mitigate expansive activity in mass concrete and, in some cases, it actually may increase the moisture content
14、 of the concrete by preventing migration of moisture through the concrete surface.Because AAR in mass concrete sections typically results in extensive wide cracking, sealing the cracks by epoxy injection may be the most obvious repair action (Fig. 3). While crack injection may increase structural st
15、ability and minimize ingress of external contaminants, it will also block egress of reaction products, poten-tially increasing the pressure from internal gel expansion and enhancing the formation of new cracks. Injection with flexible materials may provide some improvement in repair performance. Con
16、sider crack sealing and injec-tion only after a careful evaluation of its compatibility with future expansive reactivity.The chances for successful repair of a mass concrete structure where the concrete exhibits continuing AAR expansion are very low. Such structures can often continue in operation f
17、or extended periods, provided appro-priate maintenance programs are implemented. The most common remedial action to keep mass concrete struc-tures in service (Fig. 4) is opening joints or cutting slots by sawing to relieve internal stresses. In this process, a continuous loop of wire embedded with c
18、utting diamonds makes the desired cut. Some dams have been post-tensioned before slot-cutting to prevent excessive shear at the slots as the stress was relieved and to improve earthquake resistance (Newell and Wagner 1995).One of the first applications of diamond wire technology to cut slots in a ma
19、ss concrete structure was in the intake structure at Mactaquac Generating Station (Thompson 1990). Waterstops were installed in the open slots to control leakage. Diamond-wire cutting has also been used successfully to reestablish or widen contraction joints in several mass concrete structures. The
20、first application of diamond wire technology for cutting slots in a powerhouse began in 1993 at R. H. Saunders Generating Station (Kee et al. 1998). The cuts were made in critical areas of monolith joints (80 ft 24.3 m wide and 80 ft high 24.3 m) between generators. The closed joints that were origi
21、nally 0.2 in. (5 mm) wide were enlarged to 0.6 in. (15 mm). The concrete expanded by 0.2 in. (5 mm) almost immediately after the slots were cut, and the concrete expanded another 0.2 in. (5 mm) during the next year. Subsequent expansion was at a rate of approximately 0.04 in. (1 mm) per year. New sl
22、ots will be cut as necessary to relieve stress and accommodate future growth in the concrete, a process that will be easier in the future because of the availability of existing access holes.Expansion of the concrete at Center Hill Dam resulted in binding of the spillway gates and closing of the exp
23、an-sion joints in bridge spans over the spillway. AAR expansion occurred for approximately 40 years before it was necessary to alter the structure to maintain operational capability. The bridge spans were shortened in 1985 by cutting the concrete deck and steel girders. The supports and expansion jo
24、ints were also reset (Hugenberg and Hull 1995). Two gates were shortened, and the gate seal plates embedded in two monoliths were built out to vertical so that the gates were again functional. Binding of one of the modified gates necessitated additional shortening of the gate and modification of the
25、 seal plates in 1991. Modifications of the bridge spans and spillway gates are recognized as nonpermanent solutions to operational deficiencies in a structure that is exhibiting Fig. 3Crack repair by epoxy pressure injection.American Concrete Institute Copyrighted Material www.concrete.orgMANAGING A
26、LKALI-AGGREGATE REACTION EXPANSION IN MASS CONCRETE (ACI 364.11T-15) 3continued expansion, their life expectancy depending on the concrete residual expansion rate, the dimensions of the affected elements, and the extent of the geometrical correc-tions being performed. It should be stressed that in m
27、any instances, a permanent solution may not be available.All concrete affected by AAR does not continue to expand or deteriorate throughout the useful life of the structure. Parker, Coolidge, and Stewart Mountain dams, located in the Arizona desert, and Tuscaloosa Lock, located in Alabama, are examp
28、les of structures where the mass concrete exhib-ited early-age expansion and cracking that proceeded fairly rapidly and then essentially ceased after approximately a decade in service. The dams are currently in service more than 70 years after construction. The reaction appears to have stopped prima
29、rily because of the depletion of alkalis rather than from loss of moisture in the concrete (Stark and DePuy 1987). Tuscaloosa Lock was ultimately replaced after more than 40 years in service because of the need for a larger, more modern structure. Additional information on investigation and manageme
30、nt of AAR is available in literature published by The United States Committee on Large Dams (USCOLD 1995).SummaryBecause of their typically massive size, difficult access, and complex geometry, it is almost impossible to mitigate expansive AAR reactivity in existing mass concrete structures such as
31、dams. Effective maintenance can accommodate continuing expansion, thus allowing most structures to achieve a long service life. Ongoing maintenance to manage AAR expansion in mass concrete has progressed to a relatively routine operation at several projects. Consequently, fewer than 5% of the dams a
32、nd spillways affected by AAR have been abandoned or replaced. Experience in modifying structures to accommodate continuing AAR expansion is used to develop and implement a growth management plan for AAR-affected structures.ReferencesACI Committee 201, 2008, “Guide to Durable Concrete (ACI 201.2R-08)
33、,” American Concrete Institute, Farm-ington Hills, MI, 49 pp. ACI Committee 221, 1998, “Report on Alkali-Aggregate Reactivity (ACI 221.1R-98) (Reapproved 2008),” Amer-ican Concrete Institute, Farmington Hills, MI, 31 pp.Hugenberg, T. L., and Hull, K., 1995, “Alkali-Aggregate Rock Reaction at Center
34、Hill Dam, Tennessee,” Second International Conference on Alkali-Aggregate Reactions in Hydroelectric Plants and Dams, The United States Committee on Large Dams, Denver, CO, pp. 173-192.Kee, D. C.; Liscio, L. L.; Ho, M. S.; and Eastman, K. T., 1998, “Rehabilitating R. H. Saunders: Enhancing Value,” H
35、ydro Review, V. 17, No. 7, July, pp. SR2-SR9.Newell, V. A., and Wagner, C.D., 1995, “Modifications to Hiwasse Dam and Planned Modification to Fontana and Chickamauga Dams by the Tennessee Valley Authority to Manage Alkali-Aggregate Reaction,” Second Interna-tional Conference on Alkali-Aggregate Reac
36、tions in Hydroelectric Plants and Dams, The United States Committee on Large Dams, Denver, CO, pp. 83-100.Stark, D., and DePuy, G., 1987, “Alkali-Silica Reaction in Five Dams in Southwestern United States,” Katharine and Bryant Mather International Conference on Concrete Durability, SP-100, American
37、 Concrete Institute, Farm-ington Hills, MI, pp. 1759-1786.Thompson, G. A., 1990, “Alkali-Aggregate Reactivity Remedial Measures, Mactaquac Intake Structure” Inter-national Workshop on Alkali-Aggregate Reactions in Concrete: Occurrences, Testing and Control, CANMET, Ottawa, ON.USCOLD, 1995, Second In
38、ternational Conference on Alkali-Aggregate Reactions in Hydroelectric Plants and Dams, The United States Committee on Large Dams, Denver, CO.Fig. 4Slot cutting with diamond wire saw.American Concrete Institute Copyrighted Material www.concrete.org4 MANAGING ALKALI-AGGREGATE REACTION EXPANSION IN MAS
39、S CONCRETE (ACI 364.11T-15)ACI TechNotes are intended for reference for the design and construction of concrete structures. This document is intended for the use of individuals who are competent to evaluate the significance and limitations of its content and who will accept responsibility for the ap
40、pli-cation of the information it contains. The American Concrete Institute disclaims any and all responsibility for the accuracy of the content and shall not be liable for any loss or damage arising therefrom. Reference to this document shall not be made in contract documents.ACI 364.11T-15 was adop
41、ted and published August 2015.Copyright 2015, American Concrete Institute.All rights reserved including the rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by electronic or mechanical device, printed, written, or oral, or recording
42、for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors.For additional copies, please contact: American Concrete Institute, 38800 Country Club Drive, Farmington Hills, MI 48331 Phone: +1.248.8
43、48.3700, Fax: +1.248.848.3701 www.concrete.orgReported by ACI Committee 364Fred R. Goodwin, Chair Marjorie M. Lynch, SecretaryRandal M. Beard Kal R. Hindo David A. VanOckerBenoit Bissonnette Charles J. Hookham Alexander M. VaysburdChristopher D. Brown Ashok M. Kakade Kurt F. von FayDouglas Burke Jam
44、es M. Kasper James WarnerRyan Alexander Carris Keith E. Kesner David W. WhitmoreBruce A. Collins Erick N. LarsonBrian Lee Cope John S. Lund Consulting MembersBoris Dragunsky Pritpal S. Mangat Robert V. GeveckerPeter Emmons Surendra K. Manjrekar Stephen A. JohansonPaul E. Gaudette James E. McDonald Emory L. KempTimothy R. W. Gillespie William R. Nash Howard H. Newlon, Jr.Zareth B. Gregorian Jay H. Paul Weilan SongPawan R. Gupta K. Nam Shiu Dela TharmabalaJohn L. Hausfeld Thomas E. Spencer Robert TracyRon Heffron John A. Tanner William F. WescottRobert L. Henry Valery Tokar
