By Frank Bowen
As I scrolled through the few articles and reports I have collected over the past decade regarding alkali-silica reactions, I began to realize I had developed an ignorance to this elusive subject. Determined to understand more, I have spent the past several weeks diving into the mysteries of ASR using recent reports from the Federal Highway Administration and various other scholarly professionals in an effort to fill in the blanks. During the process, I discovered I’m not the only one undereducated on the subject and there’s a lot more we need to learn. What we do know, however, is important for precasters to understand.
A brief history
More than a century ago, it was noticed that concrete could deteriorate from seawater and freezing weather cycles. At least, this was the blame at the time. However, some cases of concrete failure could not be attributed to either of these causes, prompting a deeper investigation. A young civil engineer with the California State Division of Highways, Thomas E. Stanton, began his research on a few of these structures and found an expansive reaction between cement and aggregate taking place. Though Stanton might not have been the first to visually recognize ASR, he is credited with the first serious research into why it occurred. In addition to discovering ASR, he was the first to explain that expansion was negligible when the alkali content of the cement was below 0.6% and that expansion could be reduced by supplementary cementitious materials, thereby setting the groundwork for mitigation procedures.
If you look at your cement mill certifications, you will see the above finding in the lower left corner (look for limit < 0.6% next to potassium or sodium equivalent). Look back through your historic mill certifications and log the range of your cement provider’s results. If you compare this with the composition of your aggregates, you should give yourself a pat on the back. You have just begun the journey into understanding your own materials as they relate to ASR.
ASR from the chemistry perspective
While the fundamental physical and chemical reactions remain poorly understood, it is well known that ASR destroys the durability of concrete and can cause serious maintenance and reconstruction costs. This is a result of a number of sequential reactions. For ASR to occur, four things must be present:
- A source of reactive silica (commonly contained in aggregates).
- Alkalis, or more correctly, hydroxide ions, in concrete pore solution to attack silica.
- A source of soluble calcium (e.g., portlandite) to react with dissolved silica, which forms a deleterious gel.
- Access to moisture to allow gel expansion.
When siliceous aggregate is attacked by an alkali solution, it is converted to a viscous alkali silicate gel. This prompts the dissolution of calcium ions (Ca²+) in the pore structure, which then reacts with the viscous gel to convert it to a hardened calcium-silicate hydrate. Eventually, the tensile stresses caused by the increase in volume exceed the tensile strength of the concrete, causing cracks. The cracks radiate from the interior of the aggregate out into the surrounding paste. These empty voids, when created, allow the gel to migrate into the cracks.
That, honestly, is about as simple as I can explain this reaction. If you can read like a chemist, it looks like this:
Ca(OH)2 + H4SiO4 • Ca+²⁺ + H2SiO4–²⁻+ 2 H2O • CaH2SiO4 · 2 H2O
Still confused? Don’t worry, the first time I read about this process, I got lost at ions. If you research Ca²+, you will find more information on biology and how it plays a role in cell structure than how it reacts with ASR in concrete.
Ions are electrically charged particles formed when atoms lose or gain electrons. Alkali metals, for example, which are found in the far left column of the periodic table, have a single electron in their outermost band. This makes them highly reactive. It’s very easy for them to find a partner atom to share that electron with.
Atoms are comfortable with themselves when they are fully loaded on that outer band. When lithium, sodium or potassium lose their single outermost electron, they become positively charged. The same goes for calcium, but since it is in the second column of the periodic table, it must lose two electrons for this to happen.
Metal atoms lose the electron, or electrons, in their highest energy level and become positively charged ions. Non-metal atoms gain an electron, or electrons, from another atom to become negatively charged ions.
This is all critical to ASR to show where the reactivity is taking place. Knowing this is essential to understand what needs to be done to mitigate this destructive process.
Signs of ASR are typically random map-cracking and spalled concrete. Cracking usually appears in areas with a frequent supply of moisture. Map cracking, also known as three-point cracking, can cause other issues as well, such as shrinkage. Petrographic examination can conclusively identify ASR. Overall, ASR is characterized primarily by four main features:
- Presence of ASR aggregates.
- Map crack pattern.
- Presence of alkali silica gel in cracks and/or voids.
- Calcium hydroxide depleted paste.
If ASR has been identified by the specifying authority within the region, it is recommended that precast producers perform long-term testing on concrete samples. That is, if you have questions about the durability of your concrete, save some samples and leave them exposed to the elements. Then simply check on them periodically and record your results.
When dealing with ASR, a process that takes place over decades, accelerated tests should be conducted with bias. It is important to understand the benefits and deficiencies offered by various testing procedures. Below is a listing of common ASTM test methods for evaluating ASR potential. A more detailed listing can be found in the appendix of ASTM C33, “Standard Specification for Concrete Aggregates.”
- ASTM C1260, “Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method)”
This test exposes specimens to a solution of sodium hydroxide (NaOH). The alkali content of cement is not a significant factor in affecting the expansion. This method is one of the best ways to see ASR happen if you intentionally use aggregates with a high silica content (like glass) in concrete. However, because this test uses NaOH, rather than potassium hydroxide (KOH) when raising alkali loadings, there may be significant differences not accounted for between Na and K regarding ASR. Keep in mind that some cements contain a higher proportion of potassium sulfate (K2SO4) than sodium sulfate (Na2SO4).
- ASTM C1293, “Standard Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction” (also known as the concrete prism test or CPT)
Many consider C1293 to be the best test method for evaluating deleterious ASR potential because it provides the strongest correlation to field performance. However, C1293 takes one year to perform when evaluating aggregate reactivity, or two years when evaluating the efficacy of SCMs to mitigate deleterious expansion.
- ASTM C1567, “Standard Test Method for Determining the Potential Alkali-Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated Mortar-Bar Method)”
This test is similar to C1260 with a testing period of 14 days. C1260 is used to test the reactivity potential of aggregates, while C1567 includes the use of secondary cementitious materials within the mortar bar samples to determine the effective reduction of ASR. Consequently, the two tests are often run together. C1260 is used as an initial screening test for aggregates and C1567 determines the effectiveness of the SCM being considered to moderate the aggregate activity.
- ASTM C295, “Standard Guide for Petrographic Examination of Aggregates for Concrete”
A petrographic examination of aggregate will give a comprehensive description of the aggregate source and quantify the various rock and mineral constituents present. Ask your aggregate provider first for test results before contacting a laboratory.
- ASTM C1778, “Standard Guide for Reducing the Risk of Delterious Alkali-Aggregate Reaction in Concrete,” and AASHTO PP-65, “Selecting Measures to Prevent Deleterious Alkali-Silica Reaction in Concrete”
This new ASTM standard and related AASHTO provisional standard provides detailed protocols and guidelines using previous ASR testing data, petrography and historical performance to provide a prescriptive decision-making process for mitigation.
Mitigation methods can be split into those methods suitable for new concrete and those that can be used for existing structures. For precasters in identified ASR areas, the following methods for new concrete may be the most critical:
- Use of non-reactive aggregates (aggregates which expand less than 0.04%) in the ASTM C1293 test or less than 0.1% in the ASTM C1260 test. Precasters are limited to regional aggregate production, and consequently this option may not be practical. If ASR is a concern in the precaster’s marketing area by departments of transportation or other primary specifying agencies, likely local aggregate suppliers have conducted or are in the process of conducting reactivity testing.
- Use of SCMs (particularly Type F fly ash and/or silica flume) reduces the alkalinity (pH) of pore solution by OH-consumption and reduces permeability.
- Calculation of ASR concrete potential using both cement and aggregate alkali values. The old rule that simply using cement with the alkali content of < 0.6% would alone mitigate ASR has shown to be unreliable. The current concern with this method is that it only accounts for the alkali content in cement and not for alkalis in aggregates, SCMs and other admixtures.
Lithium compounds have also been used, but they may be cost-prohibitive and limited in availability. However, this chemical approach, which alters the reaction gel to a less expansive nature, has us in what I feel to be the right direction. Development of a more economical and more abundant admixture may be the future for ASR mitigation.
Need to know ASR facts
Throughout my research I could not help but see the repeated phrases, “needs further investigation,” “more research should be conducted,” “insufficient data exists,” and so forth. Regardless, here is a list of key ASR points for precasters to know:
- Internal alkali release from non-portland cement sources in concrete can also contribute to ASR in structures with intended long service lives.
- Calcium is required for the formation of ASR gel. It can replace alkalis in ASR gel, thereby keeping a high pH in the pore solution.
- Aggregate reactivity depends on more than the type of silica mineral it contains. The size and particle distribution of the aggregates are also a factor.
- ASR does not always occur at the interface transition zone, where the aggregate and cement paste meet. Sometimes it may only occur in the interior of some aggregates, depending on the aggregates’ porosity.
As stated earlier, many precasters will not have issues with ASR. However, we must take the time to review the current ASTM standards and check our mix designs to assure material confidence.
Frank Bowen, a 2013 Master Precaster graduate, received his M.B.A. from Middle Tennessee State University through the Concrete Industry Management graduate program in 2014 and is the director of quality assurance at Piedmont Precast in Atlanta, Ga.
Federal Highway Administration, fhwa.dot.gov
“Alkali-Silica Reaction: Current Understanding of the Reaction Mechanisms and the Knowledge Gaps,” Farshad Rajabipour, Eric Giannini, Cyrille Dunant, Jason H Ideker, and Michael D.A. Thomas, published 2015
“The Alkali-Silica Reaction in Concrete” by R. N. Swamy, published 1992
Design and Control of Concrete Mixtures, 16th edition, 2016, Portland Cement Association
“Evaluation of Alkali Silica Reactivity (ASR) Mortar Bar Testing (ASTM C1260 and C1567) at 14 days and 28 days,” Portland Cement Durability Subcommittee, PCA R&D SN2892b