Assoc. Dr. Hasan YILDIRIM / Istanbul Technical University Faculty of Civil Engineering Construction Materials Group Faculty Member
A) Alkali-Aggregate Reaction
In the years when it was first encountered alkali-aggregate reaction (AAR) was dubbed “concrete cancer”, since it was considered to be the decomposition of concrete from the inside. This study will discuss the history of the reaction, its occurrence, effects, and measures that should be taken to prevent it.
1. The History Of AAR
Thomas E. Stanton showed that concrete cracks observed in structures in California in the 1930s were due to the reaction between alkalis and siliceous aggregates in Portland cement. From then on, the alkali content in cement was limited in the USA. Nine international conferences were held to address the problem that had quickly become widespread throughout the world.
Table 1. International conferences on alkali-aggregate reaction.
2. Formation Of The Reaction
Cracks are formed in concrete primarily due to external factors or at locations with low strength. Subsequently, the quantity of water that penetrates into the cracks increases particularly during winter, due to freezing-thawing. Salt that melts into the concrete via water reacts with portlandite and alkali solution is formed. If the concrete contains reactive-silica aggregate this will absorb more water with the alkali solution and form alkali-silica gel, which causes expansion. Once the gel is formed, internal pressure causes swelling and cracks shown in Figure 1 are observed.
Figure 1. Cracks occurring in concrete due to ASR
The reaction occurs in three different ways:
• Alkali-silica reaction
• Alkali-silicate reaction
• Alkali-carbonate reaction
2.1. Alkali-silica reaction (ASR)
In such type of reactions, silica participation from aggregate and alkali participation from cement occurs. The chemical product that is formed is alkali-silica gel, which has the characteristic of expanding by absorbing water as shown in the following equation. Expansion causes cracks in the concrete. An important point to note is that for the reaction to occur, water must be present, in addition to alkali and silica.
ASR is the most frequently observed reaction type and it causes map-like cracks in the concrete typically called “map cracking”. In addition, white colored alkali-silica gel leaking from the cracks can occasionally be observed. The gel has a white powder appearance in dry weather.
SiO2 + 2NaOH + H2O ==> Na 2SiO3 . 2H2O
Silica Alkali Water Alkali-Silica Gel
2.2. Alkali-silicate reaction
This reaction is the same as ASR, however, the silica is not free in this reaction and is in the form of other silicates (vermiculite, mica, etc.).
2.3. Alkali-carbonate reaction (ACR)
ACR is the reaction between alkali and dolomite of dolomite limestone containing clay in the aggregate. As the result of the reaction, rock that contains dolomite transforms into another rock containing calcide. Due to this, the rock opens and water penetrates inside causing the swelling of clay. Swelling clay produces cracks in concrete.
CaMg(CO3)2 + 2NaOH ==> Mg(OH)2+CaCO3 + Na2CO3
Dolomite Alkali Brucite Calcide
2.4. Reactive silica types
Reactive silica contained in the aggregate can be found primarily in the following forms:
• Amorphous silica
• Opal
• Quartz
• Chalcedony
Tatematsu & Sasaki found that there was a negative correlation between the regularity of the crystal structure and reactivity when they analyzed micro crystal quartz, crypto crystal quartz, and chalcedony minerals in 1989 using x-ray and compared the results. Accordingly, chalcedony, which has the least regular chemical structure, has the highest reactivity, and micro crystal quarts mineral, which has the most regular structure, has the lowest reactivity.
2.5. Alkali varieties
Among principle elements in the periodic table, sodium (Na) and potassium (K) react with water and form alkali hydroxides, referred to in brief as alkalis, that can be dissolved in water (NaOH and KOH). The total alkali oxide quantity is specified with the definition named equivalent alkalinity. The calculation is carried out simply by indicating the total molecular weights of Na2O and K2O alkali oxides (62 and 94.2 respectively) in terms of Na2O.
Equivalent alkalinity= Na2O (%) + (62 /94,2) K2O (%
3. Effects Of The Reaction
In addition to the actual magnitude of expansion, the effect of ASR on the other engineering characteristics of the concrete is also important. However, since expansion speed and total expansion depend mainly on aggregate reactivity, cement type, cement dosing, and environmental factors, the effects of ASR on concrete engineering characteristics cannot be generalized. Additionally, ASR’s effects on the strength and elasticity characteristics of concrete are shown in Table 2. The table shows the effect of aggregate with two different reactivities. One of these is (opal) aggregate, which has rapid and high reactivity, and the other is (fused silica) aggregate, which has slow and medium level reactivity (Swamy, 1988). The results clearly show that significant drops (losses) occur in the strength and elasticity module due to ASR. It is an important aspect that such losses are not at the same rate in all parameters and do not occur at the same rate (parallelism) as expansion (elongation) as the effect of ASR. It can be seen from Table 2 that an increase reaching the rates of 40%-60% in compressive strength and a decrease varying between the rates of 65%-80% in tensile strength is demonstrated depending on the type of reactive aggregate (Figure 2).
Table 2. Effect of ASR on concrete characteristics
Figure 2. Decrease in single axial compressive strength and tensile strength under effect of ASR (Swamy, 1992)
Figure 3. Decrease in Dynamic Elasticity module and Ultrasonic Pulse speed under effect of ASR (Swamy, 1992)
Dynamic modulus loss was realized at high rates at the level of 60%-80% (Figure 3). Therefore, two important characteristics of concrete influenced by ASR are the modulus of rupture and elasticity. Both of these are parameters that influence the bending rigidity of structural components.
In the experimental results provided in Table 2 and Figure 3, a decrease is observed in pulse speed with the increase of elongation. When the results are analyzed carefully, it can be observed that dynamic modulus and pulse speed are also very sensitive to the change in the internal structure of concrete due to ASR, as is the case with bending strength (Swamy, 1988). All of these characteristics are parameters that can be measured in low expansion while no visible cracks are formed in the new concrete. Therefore, these characteristics can be used for monitoring the structural degradation influenced by ASR.
Another characteristic shown in Table 2 is that the loss in concrete engineering characteristics does not occur at the same level or in proportion to expansion. This result indicates the risk of determining identical expansion limits for all structure types. The necessity for determining critical results according to the type of concrete structure and the surrounding environment arises for harmful expansion ratio. Therefore, limit values indicated in ASTM C 227 may be required to be changed according to application conditions (harmful if lengthwise elongation is more than 0.20%).
Another important result obtained from the data in Table 2 and Figure 3 is the possibility of successfully using non-destructive test methods, such as pulse speed and dynamic modulus measurement, in the detection and observation of the damage outset and progress in concrete influenced by alkali-silica reaction (Mullick, 1988).
4. Measures Against The Reaction
Applications from two different perspectives are used in order to take alkali-aggregate reaction under control. First of all, a non-reactive aggregate must be selected. Secondly, silica quantity in cement mixture must be limited. Binder and admixtures are used for the latter.
4.1. Binders
Binders can be premixed in cement and added into concrete together, or can be added to the mixture while mixing the concrete. There are three types of binders; pulverized fuel ash (volatile ash), microsilica (silica fume), and coarse ground blast furnace slag.
4.1.2. Pulverized fuel ash (volatile ash)
While some of the ash produced by the incineration of pulverized coal in thermal power stations accumulates on the bottom of the furnace, a greater part (nearly 75-80%) is driven out of the chimney along with the gases. Such ash is called “volatile ash”. Volatile ash contains high quantities of silica and alumina. With its very fine grains, this material has an amorphous structure. Therefore, volatile ash demonstrates pozzolanic characteristics. Pozzolanic reaction occurs between free calcium and hydroxide, which are formed as the result of the hydration of ash and cement when water is added.
4.1.3. Microsilica (silica fume)
These are minerals with pozzolanic characteristic consisting of amorphous and transparent silicium dioxide (Si20) spheres produced during silicon or ferrous silicium manufacture. Silica fume is obtained from exhaust gases of electrical arc furnaces. The average particle size is less than 0.1 micron.
Figure 4. Electrical arc furnace
4.1.4. Blast furnace slag
In the creation of granulated blast furnace slag, foreign matters, such as silica and clay other than ferrous oxide in iron ore, are accumulated on the top layer while obtaining iron in the blast furnace. If the slag solution at a temperature of 1500-1600˚C discharged from the furnace is put in water and cooled instantly, it becomes granulated and gains an amorphous structure.
Figure 5. Blast furnace
4.1.5. Impacts of binders on ASR
The expansion rate changes depending on binder varieties (Figure 6). Binder quantity also impacts expansion size (Figure 7).
Figure 6. Miscellaneous binder materials in cement and expansion of concrete containing no binder
ASR control were subjected to testing with the addition of cement exclusively; slag, with 50% binder slag in cement; PFA, with binder slag; PFA, with 50% binder ash in cement; microsilica, with addition of 10% microsilica.
By using sand, which is harmful in terms of alkali, and miscellaneous binders, Aydın and Yıldırım observed that pozzolanic matters decrease alkali aggregate reaction, as shown in the below table.
Table 16. Expansion ratios with miscellaneous pozzolans
Figure 7. Expansion in mortar with/without slag at two different levels in cement.
OPC, Portland cement; GGBFS, granulated blast furnace slag
4.2. Admixtures
Lithium compound, super plasticizers and air entrainers are the admixtures used.
4.2.1. Lithium compounds
Lithium compounds added to cement can prevent alkali-aggregate reaction in mortar bar expansion tests (Stark, 1992). As shown in Figure 8, lithium fluoride (LiF) and highly reactive aggregate were mixed and it was seen that for 0.5% of the resulting cement weight for LiF dosing, the expansion decreased significantly. A similar test was also conducted for lithium carbonate (Li2CO3) and it was required to increase the dose to 1% to decrease the expansion (Figure 9). The reason for this was the rate of Li in the first compound being higher than that in the second.
Figure 8. Expansion in mortar with/without LiF addition
Figure 9. Expansion in mortar with/without Li2CO3 addition
References:
1. E.G. Aydın, H.Yıldırım, “The effect of mineral admixtures and micronized calcite on alkali silica reaction expansions in the usage of aggregates from different origin”, International Journal of Physical Sciences , 5996-6011 pp., 2012.
2. Swamy, R.N., 1992, Alkali-Aggregate Reactions in Concrete: Material and Structural Implications, Sciences in Concrete Technology, Energy,Mines and Resources, Ottawa, Canada,533-581
3. Swamy, R.N., 1988, Expansion of Concrete due to Alkali-Silica Reaction, ACI Materials Journal,V.85,No.1,33-40.
4. Mullick,A.K.,1988,Distress in a Concrete Gravity Dam due to Alkali Silica Reaciton, Int.J.of Cement Composites and Lightweight Concrete,V.10,No.4,225-232.
5. West, G., 1996, Alkali-aggregate reaction in concrete roads and bridges
6. Taşdemir M.A., Bayramov F., Kocatürk N.A, Yerlikaya M., “Betonun Performansa Göre Tasarımında Yeni Gelişmeler”, Beton 2004 Kongresi, 2004
7. Baradan B., Yazıcı H., Ün H., “Betonarme Yapılarda Kalıcılık”, Nisan 2002.
8. Gerwick B.C., “International Experience In The Performance of Marine Concrete”, Concrete International, May 1990.
9. TS EN 206-1 “Beton- Bölüm 1: Özellik, Performans, İmalat ve Uygunluk”
10. ACI 357.R-84 “Guide for the Design and Construction of Fixed Offshore Concrete Structures”
11. ACI 201.2R-01 “Guide to Durable Concrete”
12. Zhang M.H., Bremner W.T., Malhotra M.V. “The Effect of Portland Cement Type on Performance”, Concrete International, January 2003.
13. Lafave M., Pfeifer W.D., Sund D.J., Lovett D., Cıvjan S.A. “Using Mineral and Chemical Durability Enhancing Admixtures in Structural Concrete”, Concrete International, August 2002.
14. Ilıca, T., Yıldırım, H. and Sengul, O. “Effect of Cement Type on the Resistance of Concrete against Rapid Chloride Permeability”, 11th International Conference on Durability of Building Materials and Components, Istanbul, Turkey, 11 – 14 May 2008, pp. 481 – 488.
15. E.G. Aydın, H.Yıldırım, “The effect of mineral admixtures and micronized calcite on alkali silica reaction expansions in the usage of aggregates from different origin”, International Journal of Physical Sciences , 5996-6011 pp., 2012.
16. Swamy, R.N., 1992, Alkali-Aggregate Reactions in Concrete: Material and Structural Implications, Sciences in Concrete Technology, Energy,Mines and Resources, Ottawa, Canada,533-581
17. Swamy, R.N., 1988, Expansion of Concrete due to Alkali-Silica Reaction, ACI Materials Journal,V.85,No.1,33-40.
18. Mullick,A.K.,1988,Distress in a Concrete Gravity Dam due to Alkali Silica Reaciton, Int.J.of Cement Composites and Lightweight Concrete,V.10,No.4,225-232.
19. West, G., 1996, Alkali-aggregate reaction in concrete roads and bridges
