Alkali-Silica Reactivity

Report on Determining the Reactivity of Concrete Aggregates and Selecting Appropriate Measures for Preventing Deleterious Expansion in New Concrete Construction

4.0 Preventing Damaging Expansion in Concrete Containing Alkali-Silica Reactive Aggregates

This report provides two approaches for selecting preventive measures. In the first approach, the performance of the preventive measure is tested in combination with the reactive aggregate using either the CPT or the AMBT. This approach is suitable for selecting the appropriate level of SCM's or lithium nitrate admixtures. The second is a prescriptive approach where the preventive measure is selected on the basis of the reactivity of the aggregate, the nature of structure and its exposure, the required service life, and the availability of alkalis in the system. This approach is suitable for selecting the appropriate level of SCM and/or the maximum alkali content of the concrete.

The preventive measures determined by either performance testing or prescription using this report will generally reduce the risk of expansion due to ASR to an acceptable level for most highway applications. However, it should be noted that the level of prevention arrived at by following this report may not be sufficient for certain critical structures such as hydraulic dams or power plants where ASR expansion of any level cannot be tolerated.

4.1. Performance Testing Using the Concrete Prism Test (CPT)

The ability of SCM's such as fly ash, slag, silica fume and natural pozzolans, or of chemical admixtures, such as lithium compounds, to control deleterious expansion with a reactive aggregate can be evaluated using a modified version of the concrete prism test, ASTM C 1293. When testing SCM's, the total cementitious content is maintained at 708 lb/yd³ (420 kg/m³), but the portland cement is partially replaced with the desired amount of the SCM (or combination of SCM's) under investigation. The alkali content of the portland cement component of the mix only is raised to 1.25% Na2Oe.⁶ It is prudent to conduct a number of tests using varying levels of SCM(s) to optimize the proportions in terms of meeting the expansion criteria. The test duration for evaluating preventive measures is two years and the expansion criterion used to demonstrate that the combination of SCM and reactive aggregate is suitable for use in concrete construction is expansion ≤ 0.04% at 2 years.

The only lithium compound included in this report is an aqueous solution of lithium nitrate.⁷ When testing lithium nitrate solution the procedure in ASTM C 1293 is followed with the following exceptions:

• The alkali content of the portland cement is raised to 1.25% Na₂Oe.
• The desired quantity of lithium nitrate solution is added to the mix water prior to mixing. It is prudent to conduct a number of tests using varying amounts of lithium to determine the minimum "safe" level required to sufficiently suppress expansion.⁸
• The amount of water contained in the lithium nitrate solution should be included in the calculation of W/CM. In other words, this amount of water should be subtracted from the mix water content required for the same mix without lithium.
• The test is extended to two years and the expansion criterion used to demonstrate that the combination of lithium and reactive aggregate is suitable for use in concrete construction is expansion ≤ 0.04% at 2 years.

4.2. Performance Testing Using the Accelerated Mortar Bar Test (AMBT)

Before the accelerated mortar bar test (AMBT) can be used to determine the performance of a specific SCM–aggregate or lithium–aggregate combination, it must first be demonstrated that the aggregate being evaluated responds well to the accelerated test. This requires a comparison of the results from the AMBT and the CPT test for the aggregate being used (without preventive measures). After subjecting the aggregate to both tests, the results are plotted on Figure 4. Provided the data fall within the region indicated in Figure 4, the AMBT can be used to determine the efficacy of both SCM's and lithium nitrate. The AMBT and CPT should be compared every two years unless the results of petrography or other tests indicate a significant change in the composition of the material in the quarry, in which case new tests should be commenced immediately.

When testing the SCM in the AMBT, the modified version of the test, ASTM C 1567, should be used; this test was developed specifically for "determining the potential alkali–silica reactivity of combinations of cementitious materials and aggregates." Combinations of cementitious materials and aggregates will be deemed acceptable for use if the expansion ≤ 0.10% after 14 days immersion in 1 M NaOH.⁹ Note: this test method is not suitable for evaluating SCM's with high levels of alkalis (fly ash with > 4.5% Na₂Oe, and slag and silica fume with > 1.0% Na₂Oe) and such materials should be evaluated using the concrete prism test.

Figure 4. Comparison of AMBT and CPT Data for the Purpose of Determining Whether the AMBT is Suitable for Evaluating Preventive Measures with a Specific Aggregate

When using the AMBT to determine the lithium dose required with a specific aggregate the approach proposed by Tremblay et al. (2008) will be used; the procedure is as follows:

1. Test the aggregate using the standard AMBT (ASTM C 1260). Extend the duration of the test such that the mortar bars are exposed to 1M NaOH at 80°C (178°F) for a period of 28 days. Let E1 = expansion of bars without lithium at 28 days.

2. Test the aggregate in a modified version of the AMBT. In this test add sufficient lithium nitrate to the mortar bar mixture and the soak solution to achieve lithium-to-alkali molar ratios of [Li]/ [Na+K] = 0.74 in the mortar and [Li]/ [Na+K] = 0.148 in the soak solution. Conduct the rest of the test in accordance with ASTM C 1260 extending the period in 1 M NaOH to 28 days. Let E2 = expansion of bars with lithium at 28 days.

3. If (E₂ - E₁)/E₁ < 0.1 then use the following lithium-to-alkali molar ratio:
   (Eq. 1)

   LA=[LI]/[Na + K] = 1.0 + 0.7X [(E₂ - E₁)/E₁]

4. If (E₂ - E₁)/E₁ ≥ 0.1 then use the concrete prism test to determine the lithium content required (see Section 5.2).

4.3. Prescriptive Approach for Selecting Preventive Measures

The level of prevention is determined by considering the class, size and exposure condition of the structure, the degree of aggregate reactivity, and the level of alkalis from the portland cement (when SCM's are used as preventive measures). This approach is similar to that developed in Canada (CSA A23.2–27A) and in Europe (RILEM TC191–ARP: AAR–7).

4.3.1. Degree of Aggregate Reactivity

The degree of alkali–silica reactivity of an aggregate is determined by testing the aggregate in the CPT and using the expansion value at one-year. Aggregate–reactivity classes are given in Table 1.

If data from the CPT are not available, the aggregate may be considered as very highly reactive (R3).

Where the coarse and fine aggregates are of different reactivity, the level of prevention should be selected for the most reactive aggregate.

4.3.2. Level of ASR Risk

The risk of ASR occurring in a structure is determined by considering the aggregate reactivity and the exposure conditions using Table 2.

• ^†A massive element has a least dimension > 3 ft (0.9 m)
• ^††A dry environment corresponds to an average ambient relative humidity lower than 60%, normally only found in buildings
• ^†††Examples of structures exposed to alkalis in service include marine structures exposed to seawater and highway structures exposed to deicing salts (e.g. NaCl) or anti-icing salts (e.g. potassium acetate, sodium formate, etc.)

4.3.3. Determination of the Level of Prevention

The level of prevention required is determined from Table 3 by considering the risk of ASR from Table 2 together with the class of structure from Table 4.

• ^††It is not permitted to construct a Class S4 structure (see Table 4) when the risk of ASR is Level 6. Measures must be taken to reduce the level of risk in these circumstances.

• ^†Note: this table does not consider the consequences of damage due to ACR. This protocol does not permit the use of alkali–carbonate aggregates
• ^††The types of structures listed under each Class are meant to serve as examples. Some owners may decide to use their own classification system. For example, sidewalks and culverts may be placed in the S3 Class is some jurisdictions.

4.3.4. Identification of Preventive Measures

Option 1 – Limiting the Alkali Content of the Concrete

Damaging alkali–silica reaction can be prevented by limiting the alkali content of the concrete. Maximum permissible alkali contents are given in Table 5. The alkali content of concrete is calculated on the basis of the alkali contributed by the portland cement alone.

• ^†SCM's must be used in Prevention levels Z and ZZ.

Option 2 – Using Supplementary Cementing Materials (SCM)

Damaging alkali–silica reaction can be prevented by using a sufficient quantity of a suitable supplementary cementing material (SCM) such as fly ash, slag, silica fume or natural pozzolan. Table 6 provides minimum replacement levels for Class F fly ash with less than 18% CaO and meeting the requirements of ASTM C 618, silica fume with more than 85% SiO₂ and meeting the requirements of ASTM C 1240, and slag meeting the requirements of ASTM C 989. Class C fly ashes or Class F fly ashes with more than 18% CaO are not covered by these prescriptive measures; the ability of these materials to control ASR with a particular reactive aggregate should be determined by performance testing (see sections 4.1 and 4.2).

Many natural pozzolans such as metakaolin, calcined clays and shales, and volcanic ash have been shown to be effective in controlling expansion due to ASR. However, no prescriptive measures are provided for natural pozzolans in Table 6 as this class of materials covers a wide variety of pozzolan types with a broad range of properties. When natural pozzolans are to be used to control ASR, the efficacy of a particular aggregate–pozzolan combination should be determined by performance testing (see sections 4.1 and 4.2). Information on natural pozzolans can be found in ACI 232.1R Use of Raw or Processed Natural Pozzolans in Concrete.

• ^†The minimum level of silica fume (as a percentage of cementing material) is calculated on the basis of the alkali (Na₂Oe) content of the concrete contributed by the portland cement and expressed in either units of lb/yd³ (LBA in Table 6) or kg/m³ (KGA in Table 6). LBA is calculated by multiplying the cement content of the concrete in lb/yd³ by the alkali content of the cement divided by 100. For example, for a concrete containing 500 lb/yd³ of cement with an alkali content of 0.81% Na₂Oe the value of LBA = 500 x 0.81/100 = 4.05 lb/yd³. For this concrete, the minimum replacement level of silica fume for Level Y is 1.8 x 4.05 = 8.1%. KGA is calculated by multiplying the cement content of the concrete in kg/m³ by the alkali content of the cement divided by 100. For example, for a concrete containing 300 kg/m³ of cement with an alkali content of 0.91% Na2Oe the value of KGA = 300 x 0.91/100 = 2.73 kg/m³. For this concrete, the minimum replacement level of silica fume for Level X is 2.5 x 2.73 = 6.8%. Regardless of the calculated value, the minimum level of silica fume shall not be less than 7% when it is the only method of prevention.
• ^††Note: the use of high levels of SCM in concrete may increase the risk of problems due to deicer salt scaling if the concrete is not properly proportioned, finished and cured.

When two or more SCM's are used together to control ASR, the minimum replacement levels given in Table 6 for the individual SCM's may be reduced provided that the sum of the parts of each SCM is greater than or equal to one. For example, when silica fume and slag are used together, the silica fume level may be reduced to one–third of the minimum silica fume level given in the table provided that the slag level is at least two–thirds of the minimum slag level.

The minimum replacement levels in Table 6 are appropriate for use with portland cements of moderate to high alkali contents (0.7 to 1.0 % Na₂Oe). Table 7 provided recommendations for adjusting the level of SCM when the alkali content of the portland cement is above or below this range. Where SCM's are combined with lower alkali cements (< 0.7% Na₂Oe) it is probably safe to adopt the value of the minimum replacement level for the next prevention level down. For example, if slag is to be used in prevention level Y with a low–alkali cement, the level of slag can be reduced to the level specified for prevention level X (35%). The replacement levels should not be below those given in Table 6 for prevention level W, regardless of the alkali content of the portland cement. Similarly, if higher alkali cements (> 1.0% Na₂Oe) are used together with SCMs, the replacement level of SCM should be increased to that required for the next prevention level up. For example, if slag is to be used in prevention level Y with a high–alkali cement, the level of slag should be increased to the level specified for prevention level Z (65%). This report does not provide guidance for using preventive measures with reactive aggregates when the alkali content of the portland cement exceeds 1.25% Na₂Oe.

• ^†The replacement levels should not be below those given in Table 6 for prevention level W, regardless of the alkali content of the portland cement.

Option 3 – Controlling the Alkali Level of Concrete and using SCMs when Exceptional Levels of Prevention are Required (Levels Z and ZZ)

Where extreme levels of prevention are required, a combination of Options 1 and 2 may be required. This approach requires that a minimum level of SCM is used and that a maximum limit is placed on the alkali content of the concrete contributed by the portland cement. Options for Prevention Levels Z and ZZ are given in Table 8.

⁶ The expansion of concrete prisms produced with cement alkalis raised to 1.25% Na₂Oe provides a reliable prediction of the field expansion of concrete produced with cement with alkalis up to 1.0% Na₂Oe. If the cement to be used in the field has an alkali content above 1.00% Na₂Oe, this same cement should be used for the concrete prism test and the alkalis should be raised by 0.25% Na₂Oe by the addition of NaOH to the mix water.

⁷ At the time of writing, the only commercially available products were all solutions containing 30% LiNO₃.

⁸ The published literature indicates that the level of lithium required increases as the concentration of the other alkalis (Na+K) in the system increases. For many aggregates, deleterious expansion appears to be prevented when the lithium–to–sodium–plus–potassium–molar ratio, [Li]/ [Na+K] ≥ 0.74. For a 30%–solution of LiNO₃, the molar ratio of 0.74 is achieved when the dose of lithium is equal to 0.55 gal LiNO3 solution per lb Na2Oe (4.6 L LiNO₃ solution per kg Na2Oe). The alkalis added to the mix water as NaOH should be included in the calculation of the lithium–to–sodium–plus–potassium–molar ratio.

⁹ If it has been determined that an extended test duration of 28 days in 1M NaOH and a lower expansion limit of 0.08% is required to correctly identify the aggregate as deleteriously reactive then the same requirements should be used to evaluate the preventive measures.