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IS 6925:1996 is the Indian Standard (BIS) for methods of test for concrete admixtures. This standard specifies the methods for testing chemical admixtures to assess their effect on concrete properties. It details procedures for evaluating workability, water reduction, stiffening time, air entrainment in fresh concrete, and compressive strength of hardened concrete, alongside uniformity tests for the admixture itself.
Prescribes methods for testing the performance and compliance of chemical admixtures for concrete.
Overview
Status
Current
Usage level
Frequently Used
Domain
Materials Science — Admixtures and Chemical Products for Concrete
! Crucially, this standard has been superseded and replaced by IS 9103:2023, which combines both specifications and test methods into a single document.
! This code provides the 'how-to' for testing, while the pass/fail acceptance criteria were listed in the now-superseded IS 9103:1999. The two were designed to be used together.
! When performing comparative tests, it is essential that the control concrete mix and the test concrete mix are identical in every respect (materials, proportions, temperature) except for the admixture being tested.
EN 480 SeriesCEN (European Committee for Standardization), Europe
HighCurrent
Admixtures for concrete, mortar and grout — Test methods
Direct equivalent series dedicated to testing methods for concrete admixtures.
ASTM C494 / C494M - 23ASTM International, USA
HighCurrent
Standard Specification for Chemical Admixtures for Concrete
A specification standard that prescribes the required test methods for admixture classification.
BS EN 480 SeriesBSI (British Standards Institution), UK
HighCurrent
Admixtures for concrete, mortar and grout. Test methods
The British adoption of the European EN 480 series of testing standards.
AS 1478.1 - 2000Standards Australia, Australia
MediumCurrent
Chemical admixtures for concrete, mortar and grout - Part 1: Admixtures for concrete
Australian specification standard that defines the test methods for evaluating admixtures.
Key Differences
≠The methodology for determining setting time is fundamentally different. IS 6925 uses the Vicat apparatus on cement paste, whereas ASTM C494 (referencing C403) and EN 480-2 use a penetration resistance method on mortar sieved from the concrete.
≠The standards specify different reference concrete mix designs. For example, the target slump for the control mix in IS 6925 is 50 ± 10 mm, while in ASTM C494 it is significantly higher at 90 ± 15 mm (3.5 ± 0.5 in).
≠Standard compressive strength test specimens vary. IS 6925 and EN standards use 150 mm cubes, whereas ASTM standards mandate the use of 150 x 300 mm (6 x 12 in) cylinders.
≠The structure of the standards differs. IS 6925 is a single, comprehensive document for various tests. The European equivalent is a multi-part series (EN 480-1, EN 480-2, etc.), and ASTM C494 is a specification that references numerous separate ASTM test method standards (e.g., C143, C403).
Key Similarities
≈All standards are based on the core principle of comparing a 'test' concrete mix containing the admixture against a 'control' or 'reference' concrete mix without it.
≈The primary performance characteristics evaluated are consistent across all standards, including water content, workability (slump), setting time, compressive strength, and air content.
≈Compressive strength is a key performance indicator in all standards, with tests typically conducted at standardized ages such as 3, 7, and 28 days to assess strength development.
≈The methods for determining the air content of fresh concrete are very similar, with IS, ASTM, and EN standards all prescribing the use of the pressure method (as detailed in IS 1199, ASTM C231, and EN 12350-7 respectively).
Parameter Comparison
Parameter
IS Value
International
Source
Reference Mix - Target Slump (Control)
50 ± 10 mm
90 ± 15 mm (3.5 ± 0.5 in)
ASTM C494 / C494M
Compressive Strength Specimen
150 mm Cube
150 x 300 mm Cylinder
ASTM C494 / C494M
Setting Time Test Apparatus
Vicat Apparatus (on paste)
Penetration Resistance Needles (on mortar)
EN 480-2
Reference Mix - Cement Content
350 ± 5 kg/m³
Typically 300 ± 5 kg/m³ (for reference concrete C1)
EN 480-1
Compressive Strength Specimen
150 mm Cube
150 mm Cube
EN 480 Series
Bleeding Test - Concrete Sample Volume
Approx. 14 Litres (0.014 m³)
14 ± 0.6 Litres (0.5 ft³)
ASTM C232 (referenced by C494)
Length Change (Shrinkage) Test Duration
28 days
28 days (plus additional readings)
ASTM C494 / C494M
⚠ Verify details from original standards before use
Key Values6
Quick Reference Values
Standard laboratory temperature for tests27 ± 2 °C
Standard laboratory relative humidity for tests65 ± 5 %
Penetration resistance for initial set time0.5 N/mm² (MPa)
Penetration resistance for final set time3.5 N/mm² (MPa)
Standard size of cube for compressive strength150 mm
Minimum concrete mixing time after adding water2 minutes
Key Formulas
Water Reduction % = [(Wc - Wt) / Wc] x 100 — Where Wc is water in control mix, Wt is water in test mix.
Air Content % (Pressure Method) = A1 - G — Where A1 is apparent air content and G is aggregate correction factor.
Compressive Strength (MPa) = P / A — Where P is the maximum load in Newtons and A is the cross-sectional area in mm².
Tables & Referenced Sections
Key Tables
No tables data
Key Clauses
Clause 4.1 - Workability
Clause 4.2 - Water Content
Clause 4.3 - Stiffening Time
Clause 4.4 - Air Content of Fresh Concrete
Clause 5 - Compressive Strength
Clause 6 - Tests for Uniformity (pH, Dry material content, etc.)
To provide uniform procedures for testing and evaluating the performance of concrete admixtures, ensuring consistent assessment across different labs and products.
How is the setting time of concrete with an admixture measured?+
By testing the penetration resistance of mortar sieved from the concrete over time. Initial set is the time to reach 0.5 MPa resistance, and final set is the time to reach 3.5 MPa resistance (Clause 4.3).
Does this standard provide the required performance limits for admixtures?+
No, this standard only defines the test methods. The performance requirements and compliance criteria were provided in IS 9103:1999 (which is also now superseded by IS 9103:2023).
What is the 'control mix' mentioned in the tests?+
It is a reference concrete mix made with the same materials and proportions as the 'test mix', but without the admixture, used as a baseline for comparison.