A STUDY FOR PREVENTING THE RISK OF ALKALI-SILICA REACTION DUE TO THE AGGREGATE PLANNED TO BE USED IN MASS CONCRETE OF DERINER DAM AND HEPP PROJECT

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1 A STUDY FOR PREVENTING THE RISK OF ALKALI-SILICA REACTION DUE TO THE AGGREGATE PLANNED TO BE USED IN MASS CONCRETE OF DERINER DAM AND HEPP PROJECT Aydin SAGLIK, M. Fatih KOCABEYLER General Directorate of State Hydraulic Works Technical Research & Quality Control Department Concrete-Materials Laboratory 06100, Yucetepe/Ankara/TURKEY Abstract Deriner Dam and HEPP located in the northestern part of Turkey is being built on the river Coruh and when completed, will be the highest of Turkey and the fifth highest of the world with 252 m height from the base. Being built as a double curvature thin arched concrete dam, for about 4,000,000 m 3 concrete casting will be accomplished in the dam s body. The petrographical examination of the crushed aggregate intended to be used in mass concrete design have shown that the aggregate consisted of about 20% of reactive silica. Another source of the aggregate investigated was obtained from the intersection of Coruh and Berta rivers so called Givit. The latter is more reactive than crushed fine aggregate and actually not planned to be used in mass concrete but other concrete structures of dam construction. Moreover, the Portland cement, which could be used in mass concrete construction, contains alkalies over limit specified in the standard but mineral admixture (Catalagzi Thermal Power Plant, Class F Fly Ash) intented to be used for mass concrete has alkalies below the limits given in the standards. Particularly from the aggregate point of view, there is a serious potential risk of alkali-silica reaction (ASR) to occur in mass concrete. In this study, an experimental research programme has been carried out to reduce or even prevent the alkali-silica reaction. Two types of aggregate samples, one having for about 20% and the other having more than 30% reactive silica, were prepared for the test conforming to the grading requirements of ASTM C 1260 standard. As a result of the trials using fly ash with different percentages by replacement of cement, it was determined that the alkali-silica reaction was prevented significantly by fly ash usage of at least 25% with crushed fine aggregate and at least 35% with naturally shaped fine aggregate. 1. Introduction Alkali silica reaction (ASR) is the reaction between the alkali hydroxide in Portland cement and certain siliceous rocks and minerals present in the aggregates, such as opal, chert, chalcedony, tridymite, cristobalite, strained quartz, etc. The products of this detrimental Aydin SAGLIK, MSc. ChE., Page 1 of 2

2 reaction often result in significant concrete expansion and cracking, and ultimately failure of the concrete structure. The alkali silica reaction occurring in concrete is a chemical phenomenon taking place between the alkalis in the cement and the aggregates including some reactive siliceous minerals. The product created as the result of the reaction alkali silica gel accumulates in the pores in concrete or on surface of the aggregate particles. Thereafter the water penetrating the concrete inside continues the reaction and the gel expands, as a result of this the concrete begins to crack being subject to internal tensions. The formation of cracks result in a more permeable concrete and makes it subject to harmful environmental effects [1]. Among the reactive formations of silica mineral found in the aggregates, opal, chalcedony, zeolites and tridymite can be mentioned. These reactive materials may occur in opaline or chalcedonic chert, silicious limestones, riolite and riolitic tuffs, dacites and dacite tuffs, andesite and andesite tuffs and phyllites. [2]. Alkalis are found in cement and are given as sodium oxide equivalent (%Na 2 O e = %Na 2 O x%K 2 O). Alkali content is expressed as soluble with acid in proportion with the cement mass in TS EN general cement standard. When the concrete is thought as a whole, it is expressed as Na 2 O e in kg in 1m 3 of concrete. Alkalis are found in cement in the form of neutral sulfates in general such as Na 2 SO 4, K 2 SO 4 veya (Na,K) 2 SO 4. These compounds have a very high solubility in water. When they are dissolved, they penetrate into to the concrete pores as Na + - -, K and OH ions and the ph of the pore solution is approximately in accordance with the alkali content. Alkalis may come from other sources such as mixture water, salty aggegate sources and / or ligno-sulfonate based chemical admixtures. The cements containing Na 2 O e more than 0.6% in accordance with the American standards are included in high alkali cements class [3]. In general, possibility of occuring alkali aggregate reaction in concrete and its degree of detrimental affect to the concrete depends on the alkali content in the cement and therefore the pore water alkalinity and the amount of cement to be used in concrete. For this reason, a limit value must be determined taking into account not the alkali amount in the cement but the alkalinity of the pore water. In concrete with high cement factor (>300 kg/m 3 ) the expansion caused by the alkali aggregate reaction may damage the concrete more. Not all of the alkalis in the cement dissolve and enter into reaction during the hydratation of the cement, the dissolved part is known as the active alkalis. The ability of the alkali amount in the cement to cause alkali aggregate raction at a harmful level depends on the reactivity of the aggregate and different limit values can be determined for different aggregates. For example, if the amount of Na 2 O e in the cement is known, the highest cement factor can be determined depending on the amount of use of aggregate in the concrete. In a similar way, maximum permissible Na 2 O e amount can exist in the specific cement factor. [4]. The reaction starts with the dissolution of the alkaline hydroxides in pore water derived from the alkalis of cement (Na 2 O and K 2 O) and after they are converted into hydroxide, with their effect on the siliceous minerals in the aggregate. As a result of this reaction, an alkali silica gel is formed, either in planes of weakness or pores in the aggregate (where reactive silica is present) or on the surface of the aggregate particles. In the latter case, a characteristic altered surface zone is formed. This may destroy the bond between the aggregate and the surrounding hydrated cement paste. Since the water absorption capacity of alkali silica gel is quite high, very big volume expansions occur in unstrained case. When the expansions are restricted, tensile stresses occur in concrete and thereafter the cement paste deterioriates and causes the formation of cracks in concrete. The expansion event can be explained with the hydraulic Aydin SAGLIK, MSc. ChE., Page 2 of 2

3 pressure that occurs as the result of osmosis, but expansion can also be caused by the swelling pressure of the still solid products of the alkali-silica reaction. For this reason, the swelling of aggregate particles is considered to have a harmful effect on the concrete. The size of the siliceous particles containing reactive silica affects the speed of reaction. While expansion occurs in 1 or 2 months after the reaction of small particles, the expansion occurring as the result of larger particles is seen many years later. In accordance with the information in the literature, gel formation occurs due to the existence of Ca ++ ions in the medium [5]. When puzzolonic material is used in concrete, the ions are removed from the medium since they will react with Ca(OH) 2. Alkali-silica reaction can take place only in places where moisture exists. The reaction begins and continues when the relative humidity ratio in the concrete is minimum 75 85% at a temperature of 20 C. At higher temperatures, it has been seen that the reaction begins at a lower relative humidity ratio. The temperature rise increases the speed of reaction, but it does not affect the total expansion that will occur as the result of this reaction. With the increase in the temperature Ca(OH) 2 solubility decreases but the solubility of silica increases. Existence of moisture in the medium is important for the occurrence of alkali silica reaction. When moisture is removed from the medium or when the penetration of humidity into the concrete is prevented, the reaction does not continue. On the other hand, in concrete that is continuously subjected to wetting and drying the diffusion of the alkali ions moves from the wet regions to the dry regions, therefore the condition gets worse. The humidity gradients also have the same effects [6]. The development of alkali-silica reaction takes place very slowly in fact and therefore its effects may not be seen for years. Since its reason can not be determined completely and since the reaction has a very complex structure, the researches about this issue is continuing today. Even if the alkali aggregate reaction risk can be shown as the result of the experiments carried out on the materials, it is not possible to calculate when and to what extent it will damage the concrete only by using the amount of the reactive materials. The actual reactivity of the aggregate depends on the particle size and porous structure since alkali silica reaction occurs on the specific surface of the aggregate. When the only source of the alkalis is the cement, the concentrations of the aggregate on the reactive surface will be affected by the size of this surface area. The aggregate that is planned to be used in Deriner Dam and HEP construction, mass concrete design, has been determined to to be containing reactive silica at a rate of 20% as the result of the petrographic inspections carried out on the aggregate. More detailed information about the aggregate, which has altered granodiotrite characteristics as a rock class, is given in Table 1. Utilization of fly ash in concrete has been acknowledged by the researchers for years. Many advantages provided with the utilization of fly ash in concrete are known today. Its use is widespread especially the improvement of the characteristics of fresh and hardened concrete and increasing the durability of the concrete and its economy provides a further advantage. As the result of the researches carried out in recent years, it is highlighted that utilization of fly ash at places where the use of aggregates containing reactive silica is unavoidable, decreases the ASR to a great extent and even prevents it. In mortar bar experiments carried out in compliance with ASTM C 1260 using both aggregates it has been proven that alkali silica reaction is reduced to innocuous levels with the use of minimum 25% fly ash with crushed fine aggregate and 35% fly ash with naturally shaped fine aggregate. Aydin SAGLIK, MSc. ChE., Page 3 of 3

4 2. Materials 2.1. Aggregate The aggregate that must be used in the mass concrete of Deriner Dam ve HEP construction is obtained from altered granodiorite rocks and has been found out that it includes 20% reactive silica as the result of the petrographic inspections carried out. The result of the petrographic inspection is given hereunder in Table 1. The inspection is carried out by taking a thin section samples from the rock specimen. Crushed fine aggregate is produced from this material also. A different fine aggregate that is considered for use in other concrete structures other than mass concrete is obtained from the intersection of Coruh river and Berta Branch. The experiments are carried out also on the naturally shaped fine aggregate obtained from Givit source. This second sand includes a higher ratio of reactive silica in comparison with the crushed sand. The petrographic analysis results related with this aggregate are given in Table 2. In the mortar bar experiments performed using ASTM C 1260 standard, the gradation of fine aggregate has been adjusted as given in Table 3 hereunder. Table 1 Results of petrographic inspection experiment for crushed fine aggregate (1) Type of Rock ( ) mm Number of particles, % (2) ( ) ( ) mm mm (63-125) mm Altered Granodiorite (I) Altered Granodiorite (II) Altered Granodiorite (III) (3) Altered Granodiorite (IV) Diabase Chlorite-schist (1) ASTM C 295, (2) 300 particles have been counted for each sieve fraction. The number of particles of each component in each fraction is given in %. (3) Quartz and feldspat particles are also included in the rock components in this sieve fraction. Table 2 Thin section petrographic inspection experiment results for naturally shaped fine aggregate Type of Rock Number of particles, % 2.36 mm 1.18 mm 0.60 mm Limestone (Micritic) Siliceous or cherty limestone (**) Sandstone (**) Chert (*) Riolite (**) Basalt (Moderately altered) Basalt (With volcanic glassy matrix) (**) Diabase (Moderately altered) Quartzite (*) Muscovite schist (**) Quartz (*) Feldspat (Clayed) (*) Alkali reactive rock or mineral (**) Contains alkali reactive mineral Aydin SAGLIK, MSc. ChE., Page 4 of 4

5 Table 3 Aggregate gradation used in compliance with ASTM C 1260 Sieve size openings, mm Passing the sieve Retained on the sieve Mass, % 4.75 (No.4) 2.36 (No.8) (No.8) 1.18 (No.16) (No.16) 0.60 (No.30) (No.30) 0.30 (No.50) (No.50) 0.15 (No.100) Cement and Mineral Additives It is unavoidable to use one of the natural puzzolan and / or fly ash mineral additives together with PC 42.5 Portland cement that is considered for use in Deriner Dam mass concrete. In terms of both decreasing temperature due to heat of hydration and preventing the alkali silica reaction, utilization of such additives is necessary also due to economic reasons. Especially as the result of the recent researches carried out, it is indicated that fly ash is very effective in terms of preventing alkali silica reaction [7]. The mechanism of fly ash to prevent ASR is included in the general remarks part. In the research, it has been decided to use Unye Cement Factory product PC 42.5 (TS EN 197-1, CEM I) Portland cement, which was chosen because of the logistical reasons, together with fly ash (Catalagzi Power Plant, located in the northwestern coast of Turkey) and it has been tried to determine the optimum mineral additive utilization ratio at different ratios (20, 30, 40, 50%). The chemical and some physical characteristics of the cement and mineral additives used in the research are given in Table 4 hereunder. The alkali content of Portland cement (Na 2 O e ) is higher than the limit given in the standards. Table 4. Chemical characteristics of the cement and fly ash Chemical Characteristics Portland cement CEM I, PC 42.5 Catalagzi fly ash (Class F) SiO SiO 2 +Al 2 O 3 +Fe 2 O Insoluble residue Loss on Ignition CaO SO MgO Alkalis (Na 2 O e ) 1.23* 0.20 *The high alkalinity value given here shows total alkalinity. The soluble amount is below this value. Aydin SAGLIK, MSc. ChE., Page 5 of 5

6 Table 4. Physical characteristics of the cement and fly ash Portland cement Physical Characteristics CEM I, PC 42.5 Catalagzi Fly Ash (Class F) Specific gravity Blaine Fineness, cm 2 /g Compressive strength, ASTM C 109 MPa, 7, 28, 90 Days 36.3, 51.3, Puzzolanic activity, %, 7, 28, 90 days in accordance with ASTM C , 85, Experimental Study In the experiments, the alkali silica reaction potential of cement fly ash aggregate combinations has been determined using ASTM C 1260 mortar bar method. The changes in length of the mortar bars has been measured at the end of 14 th and 28 th days as the result of mineral additive addition at different %s through substitution to Portland cement and assessment has been carried out in accordance with the criteria determined in ASTM C 1260 standard. As the result of this study, the ratios of mineral additives with the aggregates including a certain amount of reactive silica to minimize or completely prevent the effects of this harmful reaction have been determined. The data for correlating the results of the experiment performed by using the method defined in the standard with different cement types aggregate combinations and the petrographic analysis results of the aggregate must be assessed in accordance with the criteria given in the attachment interpretation of the test results given in ASTM C The criteria given in the standard are indicated in Table 4 hereunder. Table 4. Assessment of experiment results in accordance with ASTM C1260 Size change, % < > 0.2 Alkali silica reaction potential with the method of length change measurement 16 days later 4. Results innocuous Reading must be repeated at the end of the 28 th day At a level that is potentially deleterious The measurement results obtained using the mortar bars in accordance with ASTM C1260 using both aggregate materials are given in Table 5 hereunder. Since the naturally shaped fine aggregate obtained from Givit source at the intersection of dam downsteam of Coruh river and Berta branch includes a high ratio of silica ASR can be prevented through the use of fly ash at a minimum ratio of 35%. But the harm that would be caused by ASR can be prevented with a minimum utilization ratio of 25% with crushed sand. In the production of mass concrete, crushed stone fine aggregate shall be used. It is estimated that the fly ash utilization ratio will be around 30-35% in the mass concrete production, taking into account the other parameters. But in order to examine the long term effects, 180 day results are also included in the Aydin SAGLIK, MSc. ChE., Page 6 of 6

7 experimental study schedule as indicated in ASTM C 227. By the time the paper is written, 180 day results have not been obtained yet. Table 5. Length changes in % of the mortar bars prepared in compliance with ASTM C1260. Length Change % s Cement and fly Crushed fine aggregate Naturally shaped fine aggregate ash at different %s 14 th day 28 th day 14 th day 28 th day PÇ PÇ + 20% FA PÇ + 30% FA PÇ + 40% FA PÇ + 50% FA When the assessment in accordance with Table 5 herein above is performed, it is observed that utilization of fly ash decreases the alkali silica reaction and when used at high ratios, it prevents this reaction to a great extent or reduces it to harmless levels. Altough the reactive silica content of naturally shaped sand is higher, fly ash reduces the alkali silica reaction so as not to damage the concrete. In order to provide this condition to be seen more clearly, the results given in Table 5 are shown in Figure 1 and Figure 2. Mortar Bar Length Change, % 0,18 0,16 0,14 0,12 0,10 0,08 0,06 0,04 Test Results Obtained From Naturally Shaped Fine Aggregate 14 Day 28 Day ASTM C 1260 Limit : 0,1% 0,02 0,00 PÇ 42,5 PÇ+%20 FA PÇ+%30 FA PÇ+%40 FA PÇ+%50 FA Cement and Fly Ash Combinations, % Figure 1. Comparison of length changes as measured from the mortar bars prepared with naturally shaped fine aggregate. Aydin SAGLIK, MSc. ChE., Page 7 of 7

8 0,18 Test Results Obtained From Crushed Fine Aggregate Mortar Bar Length Change, % 0,16 0,14 0,12 0,10 0,08 0,06 0,04 0,02 ASTM C 1260 Limit : 0,1% 14 Day 28 Day 0,00 PÇ 42,5 PÇ+%20 FA PÇ+%30 FA PÇ+%40 FA PÇ+%50 FA Cement and Fly Ash Combinations, % Figure 2. Comparison of length changes as measured from the mortar bars prepared with crushed fine aggregate. 5. General Remarks The alkali silica reaction and the expansion that occurs as the result of this reaction must be distinguished. Expansion may occur as the result of alkali silica reaction but this expansion does not always cause tensile stresses that may damage the concrete. Alkali silica reaction and then the expansion occurring in the concrete occurs when some conditions take place at the same time; 1) In case the concrete is sufficiently saturated with water (at 20 C temperature, approximately 75-85% relative humidity), 2) If there is a sufficient amount of silica in the aggregate used in the concrete, and 3) In case there is a high ratio of alkali (sodium, potassium) in the cement or mineral addtive used in the concrete the ph value of the pore water increases and in this case, the concentration of hydroxyle ions in the solution increases. With the increase in the ph value of the solution, the silica solubility in the aggregate containing reactive silica increases and alkali silica gel is created. For this reason, the following conditions must be complied with in order to take the alkali silica reaction under control; 1. Taking the relative humidity in the medium under control, 2. Taking the type or amount of the concrete or aggregate containing reactive silica under control, 3. Keeping the ph value of the pore water at a low level so that it will not be able to dissolve the silica in the aggragate (by reducing the amount of alkali that will come from the cement and mineral admixture). 4. In case the conditions herein above can not be met, using a mineral additive containing low alkali (puzzolan) in addition to the cement in the concrete. Aydin SAGLIK, MSc. ChE., Page 8 of 8

9 ASTM C 150 has limited Na 2 O e alkali content for Portland cements with 0.6% and ASTM C 618 has limited the same parameter for mineral additives (fly ash and puzzolan) with 1.5%. Alkali content is determined with the raw material from which they are obtained for cement and mineral additives. The effectiveness of the mineral additive depends on its chemical and physical characteristics, reactivity of the aggragate and the alkali content of Portland cement. In general use of higher ratios of puzzolan is necessary with aggragates containing active silica at a high amount. As the result of the study carried out, data having the same parallelity with the literature has been obtained. Use of minimum 25% of fly ash in mixtures where crushed sand is used is sufficient for reducing and stopping the alkali silica reaction. For naturally shaped sand, minimum 35% fly ash use is necessary. Reduction and even the prevention of alkali silica reaction through the use of fly ash (class F) can be explained through the mechanisms indicated hereunder; 1. The cement fly ash reaction has a lower CaO:SiO 2 ratio than the calcium silicate product that is created as the result of the reaction of only Portland Cement. Besides this, the alkali bonding capacity of calcium silicate hydrate (C-S-H) gel is higher and as a result, since the ph value of the pore water will be reduced, alkali silica reaction is prevented. 2. The puzzolanic reaction of the fly ash will reduce the calcium hydroxide in the medium (Ca(OH) 2 ) a smaller amount of calcium hydroxide will be used for alkali silica gel formation and expansion shall not be that big to cause damage. 3. Since as the result of the fly ash s puzzolanic reaction, the Ca(OH) 2 which is in a great amount in the medium will enter into reaction and create a bigger amount of C-S-H in the cement paste, a tighter micro structure will occur and therefore the diffusion and hence the reaction of alkali ions will get more difficult. Another benefit of this is the prevention of entry of moisture in the medium since the concrete is less permeable. When the design is made taking into consideration the mechanisms given herein above, it has been observed that the expansion effect that may occur as the result of alkali silica reaction can be reduced to a level that will not damage the concrete. While the ratio of fly ash to be used for crushed sand in the mass concrete is determined to be around 25 35%, minimum 35-40% fly ash utilization with naturally shaped sand will be able to prevent the risk that will occur as the result of alkali silica reaction. As the reactive silica content of the aggregate increases (as is the case for naturally shaped fine aggregate), the ratio of fly ash that must be used in the concrete also increases. The findings regarding the required fly ash ratios are also validated with the literature information. Another finding is that because the specific surface area of the crushed aggregate is much larger than that of the naturally shaped aggregate, there is bigger surface for the alkali-silica reaction to occur and the expansion due to the reaction is distributed equally in cement paste or on the aggregate surface and may not create detrimental effects to the concrete. Aydin SAGLIK, MSc. ChE., Page 9 of 9

10 6. Acknowledgements The authors would like to appreciate General Directorate of State Hydraulic Works, Technical Research and Quality Control Department Presidency for its support for the realization of this research. Furthermore, the authors would like to extend their gratitude to Chemistry Lab. Manager Mr. Metin HALICI and Rock Mechanics and Petrography Lab. Chief Mr. Yalcin ORKUN for their assistance for the experimental studies during the performance of this study. 7. References 1. Canadian Strategic Highway Research Program, C-SHRP Transportation Association of Canada, Technical Brief # 3, Alkali-Aggregate Reactions in Concrete: An Annotated Bibliography, Washington DC, A.J. Goldbeck, Needed research, ASTM Sp. Tech. Publ. No.169, pp.26-34, Oberholster, R.E., Allkali reactivity of siliceous rock aggregates: Diagnosis of the reaction, testing of cement and aggregate and prescription of preventative measures, Alkalis in concrete, research and practices, proceedings of the 6th international conference, Copenhagen, Denmark, June 1983, pp Addis B., Owens G., Fulton s Concrete Technology, Cement and Concrete Institute, Midrand, South Africa, Chatterji S., The role of Ca(OH) 2 in the breakdown of portland cement concrete due to alkali-silica reaction, Cement and concrete research, 9, No.2, pp , Neville A.M., Properties of concrete, Fourth Edition, John Wiley & Sons, Inc., 605 Third Avenue, New York, NY, USA, Detwiller, R.J., Substitution of Fly Ash for Cement or Aggregate in Concrete : Strength Development and Suppression of ASR, RD 127, Portland Cement Association, Skokie, Illinois, 2002, 28 pages. Aydin SAGLIK, MSc. ChE., Page 10 of 10