Simplified Guide to Testing Alkali Aggregate Reactivity (IS 2386 Part VII - 1963)

 IS.2386.7.1963

Simplified Guide to Testing Alkali Aggregate Reactivity (IS 2386 Part VII - 1963)

The standard covers two methods to evaluate the potential alkali-aggregate reactivity of fine and coarse aggregates used in concrete:

1. Mortar-Bar Method: Measures expansion in mortar bars to assess physical reactivity.
2. Chemical Method: Analyzes chemical interactions between aggregates and sodium hydroxide to determine potential reactivity.

These tests are critical to prevent alkali-silica reaction (ASR) or alkali-carbonate reaction (ACR), which can cause concrete deterioration due to expansive gel formation.

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Why Test Alkali Aggregate Reactivity?

Alkali-aggregate reactions occur when alkalis (e.g., sodium or potassium oxides) from cement react with certain minerals in aggregates, forming gels that expand in the presence of moisture. This can lead to cracking, reduced durability, and structural failure in concrete. The tests ensure aggregates are safe for use in concrete structures like dams, bridges, and buildings.

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1. Mortar-Bar Method

Purpose

To measure the expansion of mortar bars made with the test aggregate to determine if it causes deleterious alkali-aggregate reactions.

Apparatus

• Scales and Weights: Scales with a permissible variation of ±2.0 g at 2000 g load (new scales: ±1.0 g). Weights as per Table 1 (e.g., ±1 g for 100 g weight).
• Sieves: Square-hole, woven wire cloth sieves per IS 460:1962 (e.g., 850-micron, 300-micron, 150-micron).
• Glass Graduates: 200 ml capacity, calibrated at 20°C with ±0.5 ml deviation.
• Moulds: Single or double moulds for 25 x 25 mm specimens, with gauge studs (6.5 mm diameter) penetrating 15.0–18.5 mm. Moulds must be rigid, made of steel or brass.
• Mixing Bowl: As shown in Fig. 3.
• Comparator: For precise length measurements (Fig. 4), calibrated with a reference bar.
• Moist Closet/Room: Maintains 27 ± 2°C and ≥90% relative humidity.
• Storage Room: Maintains 30 ± 2°C for specimen storage.
• Measuring Device: Dial gauge or micrometer, calibrated regularly, with a range of ≥7.5 mm.

Materials

• Cement: Low-alkali Portland cement (≤0.1% sodium oxide by weight). Pass through an 850-micron sieve to remove lumps.
• Aggregate:
  • Fine aggregate: Processed with minimal crushing.
  • Coarse aggregate: Crushed to produce a graded product (per clause 2.4.2).
  • Both must be representative of the material proposed for use.
• Water: Distilled or equivalent purity.

Procedure

1. Aggregate Preparation:
   • Fine aggregate: Process with minimal crushing.
   • Coarse aggregate: Crush to achieve grading as per clause 2.4.2.
   • Sieve to ensure proper particle size (e.g., 300–150 micron for chemical method).
2. Mortar Mixing (Clause 2.5.4):
   • Sequence: Add water to the bowl, mix cement for 30 seconds, add half the aggregate and mix for 30 seconds, then add the remaining aggregate and mix for 14 minutes.
3. Moulding Test Specimens (Clause 2.5.5):
   • Fill moulds immediately after mixing, using compressed air to level the top.
   • Compact mortar around gauge studs and mould surfaces.
   • Smooth the surface with a trowel after compaction.
   • Place moulds in a moist closet (27 ± 2°C, ≥90% humidity) for 24 ± 2 hours.
4. Storage and Measurement (Clause 2.6):
   • After 24 ± 2 hours, demould specimens, measure initial length, and store in a moist room (23 ± 2°C) for 28 days.
   • Measure length again at 28 days and calculate the change.
   • For longer-term tests, measure at 1, 2, 3, 6, 9, and 12 months, and every 6 months thereafter.
   • Store specimens in a container over (but not in contact with) water at 35 ± 2°C. Before measuring, cool to 21 ± 2°C for at least 1 hour.
5. Examination (Clause 2.8.2):
   • Inspect for cracking patterns, surface mottling, and other notable conditions.
   • Conduct petrographic analysis if gel-filled pores or reactive aggregate particles are suspected.
6. Calculation (Clause 2.7):
   • Calculate length change as a percentage to the nearest 0.001%: $$\text{Length Change (\%)} = \frac{\text{Length at time t} - \text{Initial Length}}{\text{Initial Length}} \times 100$$
   • Measure curvature by placing the specimen on a flat surface and measuring the gap to the nearest 0.2 mm.

Reporting Results (Clause 2.9)

• Aggregate type and source.
• Cement type, source, and alkali content (% Na₂O, K₂O, and Na₂O equivalent).
• Average length change (%) at each measurement.
• Grading details if different from clause 2.4.2.
• Mixing water percentage by cement weight.
• Observations from specimen examination (e.g., cracking, gel presence).
• Repeatability: Expansion should not differ by more than 0.05% from the batch average.

Amendments

• Amendment No. 1 (June 1999) (Clause 2.11):
  • For aggregates with >30% strained quartz (undulatory extinction angle >15°), conduct additional tests in a metal container with sodium hydroxide solution to assess microcrystalline quartz reactivity.
• Amendment No. 2 (June 2009):
  • Table 1: Corrects permissible variation for 1000 g weight to ±1.8 g (from ±1.0 g).
  • Clause 2.5.3: Specifies “ten 12.5-mm drops” for the flow test.

Example

A river sand is tested:

• Mix: Cement with 0.1% Na₂O, sand graded per clause 2.4.2, water.
• Moulding: Prepare 25 x 25 mm bars, cure for 24 hours at 27°C, then store at 23°C for 28 days.
• Measurement: Initial length = 250.00 mm; after 28 days = 250.25 mm.
• Calculation: Length change = $(250.25 - 250.00) / 250.00 \times 100 = 0.1$.
• Interpretation: If expansion >0.1% at 6 months (per typical limits), the aggregate may be reactive, requiring further petrographic analysis.

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2. Chemical Method

Purpose

To assess the potential reactivity of aggregates by measuring dissolved silica and reduction in alkalinity when reacted with sodium hydroxide solution.

Apparatus

• Scales and Balances:
  • Scales: ±2.0 g variation at 2000 g (new: ±1.0 g).
  • Analytical balance: ≥100 g capacity, ±0.0002 g reproducibility, ±0.0005 g stability.
• Sieves: 300-micron and 150-micron IS sieves.
• Containers: 250 ml polyethylene containers.
• Reagents:
  • Distilled water.
  • Ammonium molybdate solution (10 g in 100 ml water).
  • Hydrochloric acid (concentrated, sp gr 1.19; 0.05 N; 1:1 dilution).
  • Hydrofluoric acid.
  • Oxalic acid solution (10 g dihydrate in 100 ml water).
  • Phenolphthalein indicator (1 g in 100 ml ethanol).
  • Standard silica solution (10 millimoles SiO₂/litre).
  • Standard sodium hydroxide solution (1.000–1.010 N).
  • Concentrated sulphuric acid (sp gr 1.84).

Procedure

1. Sample Preparation (Clause 3.5.3):
   • Crush aggregate and sieve to obtain material passing 300-micron and retained on 150-micron IS sieve.
   • Wash over 150-micron sieve to remove fines, dry at 100–105°C for 20–24 hours, and re-sieve.
   • Repeat washing if silty or clayey coatings persist.
2. Reaction Procedure (Clause 3.6):
   • Weigh three 25.00 ± 0.05 g portions of the prepared aggregate.
   • Place each in a 250 ml polyethylene container with 25 ml of 1.000 N NaOH solution.
   • Add 50 ml NaOH to a fourth container (blank).
   • Cure at 80°C for 24 hours, then filter the liquid.
   • Dilute filtrate to 200 ml in a volumetric flask for analysis.
3. Dissolved Silica (Gravimetric Method) (Clause 3.7):
   • Take 100 ml of diluted solution, evaporate with 5–10 ml hydrochloric acid (sp gr 1.19) on a steam bath.
   • Repeat evaporation to remove residual acid.
   • Heat residue, dissolve in hydrochloric acid, and filter to collect silica.
   • Calculate silica concentration: $$S_1 = (W_1 - W_2) \times 3.330$$ where $S_1$ is silica concentration (millimoles/litre), $W_1$ is silica weight in 100 ml sample, and $W_2$ is silica in the blank.
4. Dissolved Silica (Photometric Method) (Clause 3.8):
   • Use for rapid analysis of crystalloidal silica, avoiding interference from tannins.
   • Follow specific photometric procedures (not detailed in the provided excerpt).
5. Reduction in Alkalinity (Clause 3.9):
   • Titrate filtrate with 0.05 N hydrochloric acid to phenolphthalein endpoint ($P$).
   • Optionally, titrate to methyl orange endpoint ($T$) and calculate: $$R_1 = 2P - T$$ where $R_1$ is the reduction in alkalinity.

Reporting Results

• Aggregate type and source.
• Dissolved silica concentration (millimoles/litre).
• Reduction in alkalinity.
• Any deviations from standard grading or procedure.
• Chemical analysis of reagents used.

Interpretation (Clause 3.10)

• Compare results with established data on aggregate performance in structures or mortar-bar tests.
• High silica dissolution (>100 millimoles/litre) or significant alkalinity reduction may indicate potential reactivity.
• Confirm with petrographic examination or mortar-bar test results.

Example

A quarry aggregate is tested:

• Sample: 25 g (300–150 micron) reacted with 25 ml 1 N NaOH at 80°C.
• Silica Analysis: 100 ml filtrate yields 0.03 g silica; blank yields 0.001 g.
• Calculation: $S_1 = (0.03 - 0.001) \times 3.330 = 96.57$ millimoles/litre.
• Alkalinity: Titration shows significant reduction.
• Interpretation: High silica dissolution suggests potential reactivity, requiring mortar-bar testing for confirmation.

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Key Tips for Students and Engineers

• Mortar-Bar Method:
  • Ensure precise length measurements (±0.001%) using a calibrated comparator.
  • Maintain strict temperature (27 ± 2°C) and humidity (≥90%) conditions to avoid variability.
  • Use low-alkali cement to isolate aggregate reactivity.
• Chemical Method:
  • Handle reagents (e.g., hydrofluoric acid) with care due to their hazardous nature.
  • Ensure thorough washing to remove fines, as they can skew silica measurements.
  • Standardize NaOH and HCl solutions daily for accurate titration.
• General:
  • Cross-check results with petrographic analysis for aggregates with suspected reactive minerals (e.g., opal, chalcedony).
  • Refer to Amendments No. 1 and 2 for updated procedures and tolerances.

Common Mistakes to Avoid

• Mortar-Bar:
  • Inconsistent mould filling or compaction, leading to variable expansion.
  • Incorrect storage conditions (e.g., temperature deviations).
  • Neglecting regular comparator calibration.
• Chemical Method:
  • Incomplete removal of fines, affecting silica dissolution results.
  • Using outdated or contaminated reagents.
  • Improper filtration, leading to inaccurate silica or alkalinity measurements.
• Document Errors:
  • The standard is inconsistently labeled (e.g., “2366” or “2396” instead of “2386”).
  • “AGREGATES” is a typo for “AGGREGATES” (Page 7, 29).
  • Truncated text limits procedural details; consult the full standard for clarity.

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Notes on Document Issues

• Typographical Errors: References to “2366”, “2396”, or “2.96” are typos for “2386”. “AGREGATES” should be “AGGREGATES”.
• Truncation: Large sections are truncated, omitting critical details (e.g., full procedures for photometric analysis, grading specifics in clause 2.4.2).
• Amendments:
  • Amendment No. 1 (1999): Adds testing for aggregates with strained quartz.
  • Amendment No. 2 (2009): Corrects weight tolerances and flow test details.
• Reference Standards:
  • IS 460:1962 (Test Sieves).
  • IS 1727:1960 (Pozzolanic Materials Testing).
  • IS 2:1960 (Rounding Off Numerical Values).
• Year Discrepancy: The standard is from 1963, reaffirmed in 2002, with amendments in 1999 and 2009.

For complete details, refer to the full IS 2386 (Part VII) - 1963 document, including Amendments No. 1 and 2.