aged concrete, Alkali Silica Reaction, ASME, ASR, ASR damage, chloride damage, concrete containment, concrete structures, cracked concrete, earthquake hazards, material testing, nuclear power, Nuclear Power Stations, nuclear reactors, potassium chloride, radiation damage, radiolysis, Safety, salt, salt damage, sodium chloride, USNRC
[The problem of concrete degradation at nuclear power stations, including spent fuel pools and dry cask storage remains, even though the comment period for which this was written is past.]
The comment period deadline was on March 30th, for whether the US NRC should use improved identification techniques to better detect concrete degradation at Nuclear Power Stations: “The petitioner requests that the NRC amend its regulations to improve identification techniques against ASR concrete degradation at U.S. nuclear power plants. The petitioner suggests that the reliance on a visual inspection does not ‘adequately identify Alkali-Silica Reaction (ASR), does not confirm ASR, or provide the current state of ASR damage (if present) without petrographic analysis under current existing code.” http://www.regulations.gov/#!docketDetail;D=NRC-2014-0257
The average age of US commercial nuclear reactors is 34 years with the oldest being over 45 years old (EIA). Even in the best of circumstances, concrete suffers age related damage. But, nuclear power stations suffer from extreme conditions. According to William et. al., for the NRC (2013):
“All commercial NPPs in the United States contain concrete structures whose performance and function are necessary for protection of the safety of plant operating personnel and the general public as well as the environment. A myriad of concrete-based structures are contained as a part of a light-water reactor (LWR) plant to provide foundation, support, shielding, and containment functions. Typical safety-related concrete structures contained in LWR plants may be grouped into four general categories: primary containments, containment internal structures, secondary containments/reactor buildings, and other structures… there may be a coupling effect between radiation and ASR that can potentially accelerate ASR activity or cause ASR to occur with aggregates that are not normally reactive. As plants age, the potential of ASR to occur in structures forming the biological shield or support for the reactor pressure vessel may increase as these structures are located in areas in which they are subjected to moderate elevated temperature in combination with radiation… ASR capable of being detected visually, however, is probably in a fairly advanced stage of development“. (William et. al. NRC, 2013, p. 83, pp. 88-89)
Thus, a big problem is that concrete can be damaged and weakened even before cracks become visible. Here visible cracks are being examined. However, concrete strength can be lost before damage becomes visible.
“NRC Chairman Allison Macfarlane uses a flashlight to closely examine cracks in a concrete wall at the Seabrook nuclear plant in New Hampshire. The cracks are caused by a chemical reaction in the components of the concrete. The NRC is monitoring how the plant owner is dealing with the issue.” (USNRC-Flickr)
Even more frightening, visual inspections are also inadequate for detecting damage to the nuclear reactor pressure vessels, so it is important to request ultrasonic testing for the pressure vessels, while you are commenting: https://miningawareness.wordpress.com/2015/03/03/nuclear-reactor-cracks-widespread-disease-scourge-warns-nobel-in-chemistry-nominee/
The request for better standards comes from an organization which is particularly concerned about the safety status of Seabrook nuclear plower station just 40 miles up the coast from Boston. However, your comment can support better testing for safety in nuclear power stations all over America.
ASR Seabrook Location of ASR, USNRC
The chloride in sea salt speeds up concrete Alkali Silica Reaction degradation, as well as corrosion of rebar. For nuclear reactors and waste, radiolysis of water and materials also speeds up degradation. As can be seen on this map, much of the United States is exposed to sea salt (NaCl), which speeds up degradation of concrete, as well as corrosion of metals (which itself can further damage concrete).
Figure 36: NADP Deposition Maps of NaCl
“Concrete degradation may have various causes. Concrete can be damaged by fire, aggregate expansion, sea water effects, bacterial corrosion, calcium leaching, physical damage and chemical damage (from carbonatation, chlorides, sulfates and distilled water). This process adversely affects concrete exposed to these damaging stimuli.” Additional problems for nuclear reactors include radiation damage: “Exposure of concrete structures to neutrons and gamma radiations in nuclear power plants and high-flux material testing reactor can induce radiation damages in their concrete structures,” and possibly thermal damage “Up to about 300 °C, the concrete undergoes normal thermal expansion. Above that temperature, shrinkage occurs due to water loss; however, the aggregate continues expanding, which causes internal stresses.” http://en.wikipedia.org/wiki/Concrete_degradation
The Alkali in the Alkali Silica Reaction refers to alkali metals, such as sodium in sea salt (NaCl). The silica is in the concrete itself: “Alkali metals are the chemical elements found in Group 1 of the periodic table. The alkali metals include: Lithium (Li), Sodium (Na), Potassium (K), Rubidium (RB), Cesium (Cs), and Francium (Fr)“. http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/s-Block_Elements/Group__1%3A_The_Alkali_Metals
“Sodium chloride and potassium chloride accelerate ASR in concrete“, according to Desai, Purvi, 2010. Chloride, as a strong oxidizer, speeds up the formation of hydroxyl radicals, as does ionizing radiation.
From the US Dept. of Transportation “Publication Number: FHWA-HRT-06-133 Date: March 2007:
Alkali-aggregate reaction (AAR) is a reaction in concrete between the alkali hydroxides, which originate mainly from the portland cement, and certain types of aggregate. Two types of AAR are currently recognized; these are alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR). As the names imply, these types of reaction differ in that they involve reactions with either siliceous or carbonate phases in the aggregates (table 3). ASR is far more widespread than ACR and is the focus of this document. ACR will not be discussed further, however, it should be noted that the measures used to control ASR have generally shown limited effectiveness in controlling ACR.
Table 3. Terminology for alkali-aggregate reactions (CSA A23.1-04).
Alkali-Aggregate Reaction AAR
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 constituents of some aggregates; under certain conditions deleterious expansion of concrete or mortar may result.
Alkali-Carbonate Reaction (ACR)
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 carbonate rocks, particularly calcitic dolostone and dolomitic limestones, present in some aggregates; the reaction causes dedolomitization and expansion of the affected aggregate particles, leading to abnormal expansion and cracking of concrete in service.
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.
3.2 Mechanisms of ASR
Concrete is a porous material (typically about 10 percent of the volume of concrete is occupied (by pores) and, 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, principally, the portland cement. Table 4 shows an oxide analysis of a typical portland cement.
Table 4. Typical chemical analysis for portland cement.
Oxide SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 LOI
Percent 20.55 5.07 3.10 64.51 1.53 0.15 0.73 2.53 1.58
It is convenient to convert the potassium oxide to an equivalent amount of sodium oxide using a molar ratio and to express the alkalies in terms of the equivalent alkali content of the cement; which is defined as:
Equivalent Alkalies, Na2Oe = Na2O + 0.658 x K2O (1)
For the analysis given in table 4, the equivalent alkali content is calculated as follows:
Na2Oe = 0.15 + 0.658 x 0.73 = 0.63% (2)
The equivalent alkali content of portland cements produced in or imported into North America typically ranges from 0.20 to 1.20 percent Na2Oe.
Although the alkalies represent a small fraction of the portland cement, they dominate the pore solution of the concrete, which, as a result, is highly alkaline with a pH in the range of 13.2 to 13.8. Diamond (1989) indeed found a direct correlation between the cement alkali content and the pH of the pore solution.
Figure 6 summarizes the sequence of ASR in concrete. Some forms of silica (SiO2) found in some aggregates are unstable at high pH and react with the alkali hydroxides to form an alkali-silica gel. This gel has the propensity to absorb large quantities of water and swell. Under certain conditions, the swelling pressures can cause expansion and, eventually, cracking of the concrete.
Figure 6 demonstrates the sequence of alkali-silica reaction (ASR) in concrete.
Not all forms of silica are reactive in concrete. The reactivity depends on the crystal structure of the silica rather than its chemical composition. For example both quartz and opal are silica minerals and are predominantly composed of silica (SiO2); i.e., they are of similar composition (although opal has varying proportion of water, usually 3 to 9 percent). Quartz has a well-ordered crystal structure (figure 7) and is very stable in concrete at normal temperatures. Opal, on the other hand, has an internal structure consisting of more-or-less densely packed aggregate of spheres of silica (cristobalite and/or tridymite) and is highly reactive in concrete.
3.3 Symptoms of ASR
Common symptoms of ASR in affected structures include (figure 9)” [see toward top of this blog post]:
Cracking which may be random in direction (i.e., map or pattern cracking) or may show preferred orientation if expansion is restrained in one direction.
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.
Pop-outs over reactive aggregate particles.
Operation difficulties (e.g., jamming of sluice gates in dams).
Field symptoms of ASR in concrete structures have been described and illustrated in a number of documents, including SHRP 315 (1995), CSA A864 (1992), and BCA 1992.
3.4 Methods of Evaluating Potential Reactivity of Aggregates
3.4.1 Field Performance
Field performance survey of concrete structures can be used to determine the potential alkali-reactivity of concrete aggregates (CSA A23.1). When field performance is used for that purpose (CSA A23.1):
The structure examined should have similar or higher cement/alkali contents compared to the new structure to be built.
The concrete examined should be at least 10 years old.
The exposure conditions of the structure examined should be at least as severe as those likely to affect the new structure to be built.
In the absence of documentation, a petrographic report should confirm that the aggregates used (existing structure) and for use (new structure) are identical.
The possibility that supplementary cementitious materials (SCMs) have been used should be considered / investigated so that if present SCMs for use (new structure) are as close to identical as possible in the structure under investigation.
Evaluating potential alkali-reactivity from field performance requires the involvement of an engineer and/or scientist with experience of assessing ASR in concrete structures.
3.4.2 ASR Testing in the Laboratory
A wide variety of standard test methods is available for identifying potentially reactive aggregates. Table 6 lists the tests that have been standardized by ASTM. Only the concrete prism test (ASTM C1293) and the accelerated mortar bar test (ASTM C1260) are currently recommended for use in identifying reactive aggregates. Petrographic examination of aggregates (ASTM C295) is often seen as the essential first step of an ASR testing program; however, it should not be used to accept an aggregate source without expansion testing in concrete or mortar. (ASTM C1105 is recommended for evaluating alkali-carbonate reactive rocks). ASTM C 1567 is used to test the efficacy of pozzolans and slags for controlling concrete expansion due to ASR rather than to identify reactive aggregates.
Table 6. ASTM test methods related to alkali-aggregate reaction.
C1293-05 Standard Test Method for Concrete Aggregates by Determination of Length Change of Concrete Due to Alkali-Silica Reaction1,3
C1260-05a Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar Bar Method)1
C227-03 Standard Test Method for Potential Alkali Reactivity of Cement Aggregate Combinations (Mortar Bar Method)
C289-03 Standard Test Method for Potential Alkali-Silica Reactivity of Aggregates (Chemical Method)
C1105-05 Standard Test Method for Length Change of Concrete Due to Alkali-Carbonate Rock Reaction
C295-03 Standard Guide for Petrographic Examination of Aggregates for Concrete2
C1567-04 Standard Test Method for Determining the Potential Alkali-Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated Mortar-Bar Method)3
C441-05 Standard Test Method for Effectiveness of Mineral Admixtures or Ground Blast Furnace Slag in Preventing Excessive Expansion of Concrete Due to the Alkali-Silica Reaction
1Only the concrete prism test (ASTM C1293) and the accelerated mortar bar test (ASTM C1260) are currently recommended for use in identifying reactive aggregates.
2Often seen as the essential first step of an ASR testing program; however, it should not be used to accept an aggregate source without expansion testing in concrete or mortar.
3Only the modified version of the concrete prism test (ASTM C 1293) and the ASTM C 1567 are recommended for evaluating the efficacy of pozzolans and slag for controlling expansion due to ASR.
The concrete prism test is generally considered to be the most reliable laboratory test in its ability to predict field performance. In this test, concrete prisms are stored in sealed containers over water at 38 °C (100 °F). Figure 10 shows the test setup, and figure 11 provides the length change measurements. The test uses the following parameters:
Aggregate is tested in a concrete mixture containing 420 kilograms per cubic meter (kg/m3) (705 pounds per cubic yard (lb/yd3)) of cement.
Cement alkalies are raised to 1.25 percent Na2Oe from a cement with an alkali content of 0.9 ± 0.1 percent Na2Oe.
Concrete is molded into prisms measuring 75 millimeters (mm) by 75 mm by minimum 250 mm gauge length (3 inches by 3 inches by minimum 10 inches).
Concrete prisms are stored over water in sealed containers at 38 °C (100 °F).
Length change is measured for 12 months to determine aggregate reactivity and 24 months to determine efficacy of preventive measures (CSA A23.2-27A).
The appendices to ASTM C1293 (the standard test method) and ASTM C33 (the specification for aggregates) consider aggregates that produce expansions greater than or equal to 0.04 percent at 1 year to be potentially deleteriously reactive. In Canada, in a test almost identical to ASTM C1293
(CSA A23.2-27A), the expansion after 12 months is used to classify the aggregate as (i) nonreactive (expansion 0.12 percent). This test method can also be used to evaluate preventive measures (see section 3. 5).
Read more here: http://www.fhwa.dot.gov/publications/research/infrastructure/pavements/concrete/06133/002.cfm Also see three US NRC documents in References below, specific to concrete degradation in nuclear power stations.
Dr. Digby MacDonald explains that chloride, a major oxidizer, disrupts the protective passivity coating of metals. (Video short version, explains from 1 min to 1 min 50 sec): http://youtu.be/ah_Y3Z5rRvc
According to Purvi Desai (2010): “Free chloride levels may be affected by type of cations and chloride dosage . Several research studies have shown that alkali chlorides seem to trigger the alkali silica reaction in concrete. Mortar specimens have been subjected to alkali chlorides by introducing them as additives in the mix or by using external alkali salt solution…. Sodium chloride tends to increase the hydroxyl ion concentration in pore solution [22,23]. This may be due to the formation of chloride intruded ASR products… Sodium chloride and potassium chloride accelerate ASR in concrete. Calcium chloride and magnesium chloride seem to cause minimal ASR effect. Typically alkali silica gel is rich in alkalis and silica. Increase in calcium concentration in the gel leads to the formation of a non-swelling gel. This is supported by findings from Standard ASTM C 1260, Mortar bar test and SEM/EDX analysis results. Potassium ions accelerate the ASR reaction in comparison to sodium ions at lower temperatures.” “Higher temperature accelerated the ASR” “Sodium chloride and potassium chloride accelerate ASR in concrete… SEM/EDX results show that calcium concentration in ASR gel increases with distance from aggregate. During the ASR reaction, some of the gel migrates to the matrix surrounding the aggregate. This gel on coming in contact with cement hydration products becomes rich in calcium. This gel further propagates the ASR reaction by increasing the alkali availability to more reaction sites. Expansion results imply that chlorides play an important role in accelerating the ASR reaction. Chloride ions by itself do not contribute to the expansive effect. This can be concluded based on the fact that sodium chloride causes more expansion than calcium chloride solution. Higher temperature accelerated the ASR distress, regardless of the source of alkali.” Desai, Purvi, “Alkali Silica reaction under the influence of chloride based deicers” (2010). All Theses. Paper 944. (Clemson U).
References and Additional Reading
The first three documents look like they may be useful. Unfortunately, we just found them so have not had time to read them. They were written by contractors for the NRC. So, if they say something is bad you know it probably is. If they say something is ok, skepticism may be in order. The quotes cited from the first one suggest that it is on the up and up.
“A Review of the Effects of Radiation on Microstructure and Properties of Concretes Used in Nuclear Power Plants“, Manuscript Completed: August 2013 Date Published: November 2013, Prepared by: Kaspar William University of Houston Department of Civil and Environmental Engineering Houston, TX 77204-4003, Yunping Xi University of Colorado Department of Civil, Environmentaland Architectural Engineering Boulder, CO 80309-0428, Dan Naus Oak Ridge National Laboratory Materials Science and Technology Division Oak Ridge, TN 37831-6069, Herman L. Graves, III, NRC Technical Monitor, NRC Job Code N6978, Prepared for: Division of Engineering Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission Washington, DC 20555-0001
NUREG/CR-7171 ORNL/TM-2013/263 http://permanent.access.gpo.gov/gpo42084/ML13325B077.pdf
“Primer on Durability of Nuclear Power Plant Reinforced Concrete Structures – A Review of Pertinent Factors“, Manuscript Completed: November 2006 Date Published: February 2007 Prepared by D.J. Naus, Oak Ridge National Laboratory Managed by UT-Battelle, LLC P.O. Box 2008 Oak Ridge, TN 37831-6283, H.L. Graves, III, NRC Project Manager
Prepared for Division of Fuel, Engineering and Radiological Research Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 NRC Job Code N6002 http://permanent.access.gpo.gov/LPS106396/LPS106396/www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr6927/cr-6927.pdf
“A Compilation of Elevated Temperature Concrete Material Property Data and Information for Use in Assessments of Nuclear Power Plant Reinforced Concrete Structures” Manuscript Completed: October 2010 Date Published: December 2010, Prepared by D.J. Naus Oak Ridge National Laboratory Managed by UT-Battelle, LLC Oak Ridge, TN 37831-6283 H.L. Graves, NRC Project Manager NRC Job Code N6511
NUREG/CR-7031 ORNL/TM-2009/175 http://permanent.access.gpo.gov/gpo42084/ML13325B077.pdf
NRC Powerpoint presentation images from : “Seabrook Station Seabrook Station Safety Performance in 2011 & & Seabrook Station Safety in light of the Alkali-Silica Reaction Occurring in Plant Structures Plant Structures, 2011 Reactor Oversight Process Nuclear Regulatory Commission – Region I”
“The average age of U.S. commercial reactors is about 34 years. The oldest operating reactors are Oyster Creek in New Jersey, and Nine Mile Point 1 in New York. Both entered commercial service on December 1, 1969“. http://www.eia.gov/tools/faqs/faq.cfm?id=228&t=21