Evaluation and Rehabilitation of Concrete Dams

by Kalpesh Parikh

Abstract: The Evaluation and Rehabilitation process used for concrete dams by way of past technical papers and case study. Dams damage by Alakali Silica Reaction (ASR). To evaluate ASR using core test, Petrographic Analysis and Uranyl Acetate to identify gel formation. Non destructive test to locate the concrete deterioration. Summa software to predict the ASR and moisture transfer. New concrete with Supplementary cement materials reduces ASR. Lithium compounds as an add mixture to reduce ASR in existing structure. Roller Compact Concrete used for rehabilitation of Jordan dam. Geocomposite membrane has been applied to dam to stop seepage in concrete.

Keywords: Alkali Silica Reaction, Petrographic Analysis, Uranyl Acetate, Summa software, Supplementary Cement Materials, Lithium Compounds, Roller Compact Concrete, Geocomposite membrane, Jordan dam.

1. Introduction. The paper main emphasis is on evaluation and repair of dams damaged by alkali silica reaction in concrete structure. Alkali-silica reaction (ASR) was first identified as a form of concrete deterioration in the late 1930s (Stanton 1940).As dam is surrounded by water, to assess and repairs concrete structure which is sitting under water is an engineering challenge. The concrete structure damage by alkali silica reaction (ASR) can cause serious expansion and cracking in concrete, resulting in major structural problems. As the structure is sitting under water it is difficult to evaluate and rehabilitate, to stop the ASR in the concrete, to do so would require stopping of moisture penetration or the application of chemical treatment on surface of structure. It is hard to resolve the concrete structure damaged by ASR, because most of dam are made years ago at that time concrete which had high percentage of alkali in the cement. The concrete deterioration by ASR is more susceptible because of high percentage of alkali content. This paper discusses concrete dams and procedures taken to evaluate and rehabilitate dam damage by alkali-silica reaction. The discussion includes the ASR in concrete, method to identify ASR in concrete. Rehabilitating concrete dam damage by ASR and case study explaining the dam assessment and rehabilitation carried out.

2. Alkali-Silica Reaction (ASR) Chemical reaction in either concrete or mortar between hydroxyl ions (OH−) of the alkalies (sodium and potassium) from hydraulic cement or other sources and certain siliceous rocks and minerals, such as opal, chert, microcrystalline quartz, and acidic volcanic glass, present in some aggregates. This reaction and the development of the alkali-silica gel reaction product can, under certain circumstances, lead to abnormal expansion and cracking of the concrete.

1

2.1 Mechanism of ASR Concrete is a porous material (typically about 10 percent of the volume of concrete is occupied by pores) , in saturated concrete, the pores contain a solution composed of alkali hydroxides (NaOH and KOH).The origin of the sodium (Na) and potassium (K) is, from the portland cement. Three requirements must be met for expansive ASR to occur: 1) reactive forms of silica or silicate in the aggregate, 2) sufficient alkali (Na and K ) primarily from the cement, 3) and sufficiently available moisture in the concrete. If any one of the three requirements is not met, expansion due to ASR cannot occur. In its simplest form, ASR can be visualized as a two-step process.

Alkali + Silica Gel Reaction Products Gel Reaction Products + Moisture Expansion

Figure 1 Sequence of Alkali-Silica Reaction (Courtesy of the U.S. Department of Transportation- Federal Highway Administrator)

ASR depends on the crystalline structure of the silica. Quartz has a well ordered crystal and is very stable in concrete at normal temperature on the other hand opal has an internal structure consisting of more-or-less densely packed and is highly reactive in concrete. This is found in number of rocks such as andesite, arenite, agillite, arkose, chert, gneiss, greywacke, hornfels, quartz-arenite, rhyolite, silicified carbonate, tuff, basalt, flint, granite, quartzite, sandstone, shale, and siltone and its reactive mineral and glass are cristobalite, quartz, opal, strained quartz, tridymite, volcanic glass.

2

Figure2 crystal structure of opal and quartz Figure3 Requirement of Deleterious ASR

2.3 Formation of cracks The formation of the gel per se is not deleterious. The deterioration of the concreteStructure is due to the water absorption by the gel and its expansion. It is reportedthat the RH must be higher than 80% for the gel to swell although it can be formed atlower relative humidity. According to Hobbs [3] the progression of the swelling of ASRgel follows the general patterns as shown in Figure 4. As the tensile strength of the system is exceeded, cracks will form and propagate. As there is not a preferential direction for cracks to propagate and also the sites of crack initiation are randomly distributed in the specimen, map cracking will be characteristic of ASR deterioration.

Figure4 Progression of the swelling of ASR gel

2.4 Microscopic Appearance Alkali silica reaction is diagnosed primarily by four main features

Presence of alkali silica reactive aggregates Crack pattern Presence of alkali silica gel in cracks and/or

voids Ca(OH)2 depleted paste Figure5 Microscopic Appearance of Concrete

“Fluorescent light mode, 2x3 mm, exposed surface is towards the top the photograph shows a concrete which is cracked from alkali silica reaction. The reactive aggregate (right) is porous flint (appears light green due to high porosity). The wide crack running from the porous aggregate into the cement paste is filled with alkali silica gel (dark green and cracked due to shrinkage).Courtesy Concrete Expert International”

3

2.5 Symptoms of ASR observed in Concrete Dams

Field symptoms of ASR in concrete dam are being illustrated below:

Cracking which may be random in direction (map or pattern cracking)

Discoloration around cracks. Gel exudation from cracks. Misalignment of adjacent sections. Closing of joints, extrusion of joint

sealant and crushing/spalling of concrete around joints.

Figure6 Crack Pattern in Concrete

3 Evaluation of ASR for Concrete Dam3.1 Laboratory and Field method A wide variety of standard test methods is available for identifying potentially reactive aggregates. A list of tests that have been standardized by ASTM. Petrographic examination of aggregates (ASTM C295) is often seen as the essential first stepof a ASR testing program. Here a concrete specimen from a coring is usually submitted for petrographic analysis where the mineral content and structure are analyzed. A microscopic test is performed to find areas of gel formation on fluorescent-impregnated samples. ASTM C 1567 is used to test the efficacy of pozzolans and slags for controlling concrete expansion due to ASR. AASHTO T 299-93, standard method of test for rapid identification of Alkali-Silica Reaction in concrete by use of uranyl acetate, it is applied on the concrete surface as it is a rapid method after 5 minute when sample dries use of ultraviolet lamp is made to notice the ASR gel formation. Visual inspection method, the inspection is difficult to carry out because of underwater structure use of advance appliance by inspector use of Camera where the photograph can be taken. Objective of the test is to determine the concrete has undergone ASR and as symptoms discussed above will be an indicator of deterioration of concrete by ASR.

3.2 Destructive Test The process of core drilling will occur when visual inspection has revealed signs of degradation such as cracks on the outside of the concrete dam. Precaution should be taken when core sample is taken from dam, as it may have contamination due to chemicals in reservoir water, the sample should be washed with distilled water. After that core sample are taken into the laboratory to undergo property tests and experiment to determine the cause of deterioration. ASTM provides several standard testing methods related to alkali aggregate reaction as stated by the U.S. Department of Transportation. Among the entire test, the concrete prism test – length change measurements (ASTM C1293) is the most recommended test method. The test measures the expansion of the specimen over a time in a controlled environment.

4

3.3 Nondestructive Test There are several methods to evaluate the concrete deterioration by nondestructive test methods.

Hammer method, Ultrasonic Pulse Velocity (UPV), Side Scan Sonor & echo sounder, Spectral analysis of surface waves

Underwater acoustic profiler is good for accurate mapping for underwater structures.

Finite Element Analysis- An extensive coring sample only represents 0.1% of a common concrete dam’s volume. A finite element analysis will give assume average material values.There are many more nondestructive evaluation methods for the testing of concrete (see Concrete Repair Manual, 2002)

3.4 Evaluation of concrete Structure using software To evaluate existing structure Material Service Life, LLC is developing a comprehensive service life modeling software for existing structure and for new construction. MSL solution was successful in developing SUMMA and STADIUM models can predict ASR, corrosion, sulfate attack damage, carbonation and various concrete contamination damage. Stadium service life prediction of saturated or unsaturated concrete structures exposed to aggressive environment such as water, deicing salts, chloride or sulphate contamination of soil. Hydraulic structure can be analyzed by Stadium Software. Use of Stadium Moisture software can predict moisture transfer (Material moisture content and moisture flux) with in concrete structure. Stadium Heat Prediction of heat transfer (temperature field and heat flux) with in concrete structure. Roller Compacted Concrete mixture compaction software, MSL engineer use a theoretical model recently developed in laboratory of Paris, RCC mixtures can design mixture with optimum packing density and adequate workability. Software can also used for precast industry to design dry concrete mixtures. (Material Service Life)

4 Measures to Prevent ASR One of the most efficient means of controlling ASR in concrete containing reactive aggregates is the appropriate use of supplementary cementing materials (SCM). Such materials include pozzolans (e.g. flyash, silica fume, calcined clay, or shale) and ground-granulated blast furnace slag).The amount of SCM required generally increases as:

Reactivity of the aggregate increases. Alkali content of the concrete increases. Available alkali content of the SCM increases. Calcium-to-silica ratio (CaO/SiO2) of the SCM

increases.

Figure 7 Effect of SCM on the expansion of concrete (Prism Test)

5

Pozzolans, such as silica fume, which have a high content of reactive silica and low levels of calcium and alkali tend to be very efficient in controlling ASR and can be used at relatively low levels of replacement (typically 7 to 15 percent2) for this purpose. On the other hand, SCM with relatively high levels of calcium and lower amounts of silica, such as Class C fly ash or slag, generally need to be used at replacement levels of 35 percent or more. Low-calcium Class F fly ash is relatively more efficient (i.e.can be used at lower replacement levels) than Class C fly ash or slag, but has to be used at higher replacement levels than silica fume. Figure7 shows typical expansion behavior of concrete containing high-alkali cement, reactive aggregate and different types of SCM.

5. Rehabilitation of Concrete Dams 5.1 Rehabilitation of Concrete Dams damage by ASR ASR could be easily reduce if the structure is not sitting under the water, since the difficulty is that dam structure is always under water. The only two practical means for addressing the cause of damage (i.e., to retard or prevent further reaction), are to either dry the concrete to eliminate the moisture required to sustain ASR or to change the nature of the reaction by introducing lithium compounds. Silane sealers have been used successfully to reduce the relative humidity in ASR-affected concrete piers (Kojima et al., 1992), railway sleepers (Oberholster et al., 1992) and median barriers (Bérubé et al., 1998). Silanes applied to concrete render the surface of the concrete hydrophobic and prevent the ingress of liquid water into the concrete. However, water vapor can still exit through the layer, reducing the moisture content, and hence reducing the relative humidity, with time. Figure8 shows a photograph of Silane-treated and untreated sections of a barrier wall in Quebec.

Figure8 Silane Sealer Providing restraint in the form or rock anchors or post-tensioned tendons also has been used in hydraulic structures to prevent unwanted expansion and distortion of the structure. Fiber-reinforced polymers (FRPs) have been used to wrap elements such as columns. Use of new of a low-lime ASTM Class F fly ash in new concrete containing recycled concrete as coarse aggregate greatly reduced expansion due to ASR in the new concrete However to bring expansions to less than the test criterion without exception, also required the use of low alkali cement with the fly ash. (Portland Cement Association).Methods for mitigating the symptoms include filling cracks; cutting joints to allow further expansion to take place, thereby relieving internal stresses within the concrete or pressures on adjacent members or structures; and providing restraint to further expansion. Caulking cracks with an epoxy grout (or similar compound) can help protect embedded reinforcement and reinstate the integrity of the cracked concrete. However, it will not significantly retard the rate of reaction and expansion, and new cracks will inevitably form with time if the reaction is allowed to proceed. Cutting joints to allow for expansion to take place has been used in a number of hydraulic structures, with the principal aim in these cases being to relieve stresses on embedded mechanical equipment such as sluice gates or turbines. Joints can also be cut

6

to isolate expanding structures from adjacent structures or to relieve internal stresses in pavements. Providing space for expansion does not deal with the reaction, and it is likely that the expansion and cracking will continue.

5.2 Rehabilitation of Concrete Dam Structure Adding rock fill, concrete, or precast slabs to either the downstream or upstream

face is used to increase the strength of the dam. Use of concrete technology, use of preplaced aggregate concrete where the coarse

aggregate is placed first in the form. Cement grout is injected to fill the voids. This method is used for spillway and piers that is damage from abrasion or freezing/thawing.

Tremie concrete that is fed by a pipe where concrete simply flows from its own weight. It is used to place concrete under water and generally it is used for large volume repairs that reach deep into the structure.

Pumping the most common method for underwater repair is pumping the concrete or grout. It uses a flexible hose that can reach difficult location

5.3 Rehabilitation using Roller Compacted Concrete (RCC) Roller compacted concrete (RCC) is a dry material, which has been consolidated by external vibration utilizing vibratory rollers. RCC needs to be consistency that is dry enough to support the vibratory equipment used on it wet enough to permit adequate paste binder distribution throughout the mass. RCC is a no slump concrete that contains coarse aggregates and develops properties similar to conventional concrete. It is most commonly used in concrete gravity and earth-fill dams, but can also be used in heavily loaded pavements. In some cases, failed gravity dams were completely replaced by gravity dams composed entirely of roller compacted concrete. (Abdel-halim, 1998)

5.4 Rehabilitation using Rockfill Buttressing Rockfill buttressing, often placed on the downstream face of concrete gravity dams, is a rehabilitation method used to improve the stability of existing dams against hydrostatic and seismic loading. This stabilization occurs due to horizontal earth pressure caused by this rockfill buttress opposing the forces of the hydrostatic loading on the upstream face of dam. Despite varying earth pressures during an earthquake, the horizontal forces created by the buttresses are enough to deter a concrete dam from downstream sliding. For this reason, rockfill buttresses may be a viable option in situations where hydrostatic and seismic loading are concern. (Pierre Leger, 2006)

5.5 Rehabilitation using Geocomposite Membrane Geocomposite membrane to control water seepage in a concrete structure.Use of geocomposite consisting of a PVC (polyvinyl-chloride) geomembrane backed with geotextile reinforcement made of polyster. The successful underwater installation of the membrane repair system demonstrated the feasibility of the system. Although results of the demonstration were more qualitative than quantitative, it is evident that the system is constructible and will perform acceptably when designed and installed correct. Compared to dewatering of a structure for repair, a geomembrane system that can be installed

7

underwater minimizes the impact of the repair on project operations such as hydropower generation, and recreation. Also, the underwater repair system eliminates the potentially adverse environmental impacts associated with dewatering of many structures Mc donald

5.6 Rehabilitation using Lithium as an Admixture to control ASR for new concrete Lithium compound can control ASR expansion by using lithium monohydrate (LiOH.H2O) and lithium nitrate (LiNO3) are used as an admixture to concrete to reduce ASR in new concrete. For the new concrete mixes ASR expansion is reduce if the ratio of Li/(Na+K) is greater than 0.74.Pavement structure is being tested by LiNO3 solution to reduce ASR in existing concrete structure. Result showed that penetration was about 1-2 inch in three years. As shown in figure 9 the depth of concentration of lithium.

Figure 9 Lithium Concentration Profile Figure10 Electrochemical Lithium Test Electrochemical lithium impregnation experiment is conducted where D.C. Voltage is applied between the concrete surface anode coated with a lithium borate electrolyte and the reinforcement cathode. The Penetration is higher is noted after several week was satisfactory to affect ASR expansion. The experiment is being treated on the columns.Vaccum impregnated lithium technique which is in development. The procedure is its uses pressure injection of the lithium compound treated to the cracks on the concrete structure. Application is being done to many structure but process is yet to be proven.It would be difficult to interpret that the above process can be used underwater because Research and Development is going on in lithium treatment.

6 Case Study: Assessing the Sama El-Serhan Dam (Abdel-Halim, 1999) The Sama EI-Serhan dam is located approximately ten miles north of Mafraq, Jordan. It is concrete face rock fill dam 10m in height and 120m in crest width. The Concrete spillway has experienced physical attack of erosion due to flood. Rehabilitation of spillway is to be carried out. Roller Compacted Concrete rehabilitation method was adopted. Rehabilitation was to provide support on down stream side of spillway to increase its strength. Determining the material used for RCC, here local materials used for the mixture of the RCC

8

.Figure 11 RCC section used to protect Foundation Spillway

Constituent of the RCC are regular concrete which compose of aggregates, cement and admixtures. The aggregate used for this concrete was crushed limestone; size is approximately 38mm and 12% fines. The cement used was Pozzolanic Portland Cement. Chemical consist of Na, Silica, Al, Fe, use of plasticizer was also included in RCC mix to improve the concrete workability and reduce water content. Evaluation of RCC properties is necessary to determine the section performance. Core sample were taken 90 days after placement of the RCC section. The test undergone included field densities, compressive strength, tensile strength, modulus of elasticity, temperature rise, and permeability. The test result attained from the RCC section was satisfactory. The project revealed the feasibility of using RCC for dam rehabilitation. Since this report only mentioned the properties of the RCC section after 90 days , future analysis may reveal different results. Since pozzolan, which comprised of silica and sodium was used in concrete, there is a chance Alkali silica reaction could take place. It is recommended that periodic check of core test is to be done in order to determine ASR expansion in concrete.

7. Summary and Conclusions As from our overall study while reviewing several technical papers the common problems in concrete dams where, deterioration of concrete due to Alkali silica reaction, which is an material property. So the deterioration is true for old concrete, because of deterioration the service life of the dam decreases. Research is needed to develop methods to stop ASR in existing underwater concrete structure. Use of software should be promoted as we have discussed that the Summa software which evaluates the sulphate attack, ASR etc. would be helpful way to evaluate the deterioration in concrete dam and appropriate measure could be taken before, deterioration beings in concrete. As current research and development is going on to enhance software feature. In order to get a success a group of company like Sika, Holicm has invested money. Silane Sealer and Geocomposite membrane are developing method use on concrete surface. The main concern is these methods are workable under water structure. Study should be conducted how the application would be taken place under water. Roller Compacted Concrete used as a rehabilitate method for Jordan dam. But concern would be that concrete consist of silica and sodium there are very good chances of ASR expansion in presence of moisture will take place. With the use of new concrete with supplementary cement material like flyash, silica fume, calcined clay, or shale and ground-granulated blast furnace slag, the alkali content is less so ASR expansion will be less.

9

Concrete dam, the main emphasis should be on the material property. If right material is fixed considering the environment. The deterioration of concrete due to ASR, AAR etc will not be a concern.

References1 Alkali Silica Reaction “Cracking in concrete”http://www.understanding-cement.com/alkali-silica.html (accessed October 04, 2009)2 Handbook for the Identification of Alkali-Silica Reactivity in Highway Structures, Revised Ed.“Gel formation in concrete”http://leadstates.transportation.org/asr/library/c315/ (accessed October 04, 2009)3 Use of Lithium to Prevent or Mitigate Alkali Silica Reaction in Concrete Pavements and Structure, Publication No. FHWA-HRT-06-133, March 2007http://www.fhwa.dot.gov/pavement/concrete/pubs/06133/ (accessed October 04, 2009)4 Concrete Experts International “Deterioration mechanism in concrete”http://www.concrete-experts.com/ (accessed October 04, 2009)5 Matco Service Inc- Petrographic Analysishttp://www.matcoinc.com/concrete_testing.html (accessed October 04, 2009)6 Emmon, P.H. Concrete Repair and maintenance Illustrated, Kingston: RS Means, 19937 Schmick, B.L.,ed. Concrete Repair Manual, 2nd edition. ACI International, et.al 20038 Material Service Life, Inc. SUMMA Software Development – U. S. Navy NFESC – service life prediction software- (accessed October 04, 2009)http://www.mslexperts.com/case_studies/files/Summa_USN_NFESC.pdf9 Olson Engineering, Inc. Larry D. Olson, P.E. (Presenter); Dennis A. Sack, “Nondestructive evaluation of concrete dams and other structures”.10 Virginia Transportation Research Council, Gerardo G. Clemeña, .Evaluation of “Nondestructive evaluation methods for application in early detection of deterioration in concrete pavement”.11 Water Based Silane and Siloxane Penetrating Sealer- “Silane sealer and epoxy grouting”http://www.epoxysystems.com/39.htm (accessed October 04, 2009)12 David Stark “ The Use of Recycle-Concrete Aggregate from Concrete Exhibiting Alkali Silica Reactivity” Portland Cement Assosciation Bulletin RD11413 Leger, P., Javanmardi, F. “ Structural Stability of concrete gravity dams strengthened by rockfill buttressing: Hydrostatic load” . Journal of Geotechnical and Geoenvironmental Engineering, 2006: 1592-1599.14 Mc Donald, J.E. “Development of a Geomembrane System for Underwater Repair of Concrete Structure” (accessed October 04, 2009)http://www.wes.army.mil/REMR/bulls/vol14/no1/text/geomem.html15 Shayan Ahmad and Jack Grimstad “Deterioration of concrete in hydroelectric concrete gravity dam and its characterization.” Cement Concrete Research 36 (2006): 371-8316 Mohammad A. H. Abdel-Halim, Mohammad A. Al-Omari and Mohammad M. Iskender “Rehabilitation of the Spillway of Sama EL-Serhan dam in Jordan, using roller compacted concrete” Engineering Structures Volume 21, Issue 6, June 1999.

10