Concrete durability is often governed not only by its compressive strength but also by the quality of its surface zone. The outer layer of concrete is the first line of defence against rainwater, moisture and other environmental actions. A concrete surface that absorbs water rapidly may indicate poor finishing, inadequate curing, a porous surface layer or other durability-related concerns.
The Initial Surface Absorption Test (ISAT) provides a method for assessing how quickly water is absorbed into the surface of hardened concrete.
Reference:
IS 516 (Part 2/Sec 2): 2020 – Hardened Concrete: Methods of Test, Section 2: Initial Surface Absorption.
The standard describes the test as a method for determining the initial surface absorption of
- Oven-dried concrete,
- Non-oven-dried laboratory specimens and
- Site concrete.
What is Initial Surface Absorption?
Initial surface absorption is defined as the rate of flow of water into concrete per unit area, measured from the start of the test up to a specified time interval, under a constant applied water head.
In simple terms, the test answers the question:
How quickly does water enter the surface layer of concrete?
The test primarily evaluates the surface zone, which is the region immediately behind the exposed concrete surface. Depending upon the nature and condition of the concrete, the depth of concrete influencing the result may range from a few millimetres to several centimetres.
Why is the ISAT Important?
The ISAT provides information about the uniaxial water penetration characteristics of a concrete surface. The results can be related to:
- Quality of concrete finishing;
- Uniformity of the surface zone;
- Likely durability of the exposed surface under natural weathering; and
- Comparative performance of different concrete mixes or construction practices.
The test can be particularly useful for comparing different batches of concrete, different structural members, or concretes made with different compositions or cementitious materials.
However, an important limitation must be understood.
ISAT is not a permeability test
The test results are of little relevance to concrete behaviour under high water pressure. Therefore, ISAT should not be used to determine the permeability of concrete.
Principle of the Test
A watertight cap is sealed onto the concrete surface. The cap is connected to:
- A water reservoir;
- A control tap; and
- A calibrated capillary tube.
Water is allowed to enter the cap under a controlled head. As the concrete absorbs water, the movement of water in the capillary tube is measured.
The apparatus is calibrated such that the movement of water along the capillary scale can be directly related to the initial surface absorption rate in ml/(m²·s).
The standard requires the test assembly to maintain a water head of 180 mm to 220 mm above the concrete surface.
Main Components of the ISAT Apparatus
The test assembly consists of the following major components:
1. Watertight Cap
The cap is fixed tightly onto the concrete surface and should provide a minimum water contact area of 5,000 mm².
For smooth horizontal laboratory specimens, a cap with a soft elastomeric gasket may be clamped onto the surface. For rough surfaces, vertical surfaces, sloping surfaces or soffits, a knife-edged cap with suitable sealing arrangements may be used.
The outlet is positioned at the highest point of the cap so that trapped air can escape. A transparent cap is useful because it allows the operator to observe the filling of water and removal of air. The typical apparatus and cap arrangements are illustrated in following figures.
2. Water Reservoir
A reservoir, typically of approximately 100 mm diameter, supplies water to the cap.
The reservoir must be positioned so that the specified water head of 180–220 mm is maintained throughout the test.
3. Capillary Tube and Scale
The capillary tube should:
- Be at least 200 mm long; and
- Have a bore radius between 0.4 mm and 1.0 mm.
The capillary size and length should be selected considering the expected absorption characteristics of the concrete. Highly absorptive concrete may require a larger bore or a longer tube.
4. Other Equipment
The apparatus also includes:
- Flexible connecting tubes;
- Stands and clamps;
- Stopwatch or clock accurate to 0.5 s;
- 10 ml measuring cylinder;
- Thermometers accurate to 0.2°C;
- Drying oven capable of maintaining 105 ± 5°C;
- Cooling cabinet; and
- Balance for determining specimen mass where required.
Calibration of the Apparatus
Proper calibration is essential because the movement of water in the capillary tube is used to determine the absorption rate.
The calibration is arranged so that the movement of water along the capillary scale during one minute corresponds directly to the initial surface absorption rate in ml/(m²·s).
For calibration:
- The capillary tube is measured;
- It is flushed with soap solution followed by distilled or de-ionized water;
- A water head of 200 ± 5 mm is maintained;
- Water temperature is maintained at 27 ± 1°C;
- The time required to collect 10 ml of water is measured three times; and
- The mean time is used to determine the bore radius.
The scale is then prepared based on the contact area of the cap and the bore area of the capillary tube. Each division of the scale represents 0.01 ml/(m²·s).
Selection and Preparation of Test Specimens
At least three separate specimens or locations should be tested, and they should represent the concrete under examination.
Other important requirements include:
- Minimum concrete section thickness: 75 mm;
- Areas with visible surface cracking should normally be avoided;
- Surface contamination such as mould oil or curing membranes may influence the results.
Oven-Dried Specimens
For standardized dry conditions, specimens are dried at:
105 ± 5°C
Drying continues until constant mass is achieved, defined as not more than 0.1% change in mass over a 24-hour drying period.
The specimen is then placed in a cooling cabinet until its temperature is within 2°C of room temperature.
Concrete made using high alumina cement should not be conditioned by oven drying.
Non-Oven-Dried Laboratory Specimens
For laboratory testing, the concrete specimen should remain in the laboratory for at least 48 hours at 27 ± 2°C before testing.
Site Concrete
For site testing:
- Protect the test surface from water for at least 48 hours before testing;
- The protective material should not come into contact with the concrete surface;
- Protect the surface from direct sunlight and rainfall for at least 12 hours before and during the test.
These precautions are important because the moisture condition of concrete significantly affects the measured absorption.
Step-by-Step Test Procedure
Step 1: Fix the Cap
The cap is sealed tightly to the concrete surface.
For gasket-type caps, slight greasing may be required. For knife-edged caps, a suitable sealing material is applied around the cap to prevent water leakage.
Before starting the test, the sealing arrangement should be checked for leakage. If leakage occurs during the test, the test should be discontinued.
Step 2: Assemble the Apparatus
The reservoir is positioned to provide a water head of 180–220 mm above the concrete surface.
The capillary tube is supported horizontally at approximately the same level as the water surface in the reservoir. For non-horizontal surfaces, the water head is measured from the mid-height of the concrete under the cap.
Step 3: Control the Water Temperature
For laboratory testing, water temperature should be maintained at:
27 ± 2°C
For site testing, significant temperature fluctuations during the test should be avoided.
Step 4: Start the Test
Measure the concrete surface temperature adjacent to the cap.
Fill the reservoir, open the tap and allow water to enter the cap. The moment the tap is opened is considered the start of the test.
All trapped air must be flushed out through the capillary tube. The water level in the reservoir must be maintained so that the specified water head remains constant.
When are Readings Taken?
Readings are normally taken at:
- 10 minutes
- 30 minutes
- 1 hour
The absorption rate normally decreases as the test progresses because the capillary pores near the test surface gradually become filled with water.
How is the Reading Measured?
Before each specified reading interval, the capillary tube is filled with water and positioned horizontally.
When the specified test interval is reached:
- Close the tap;
- Allow water to move back along the capillary tube;
- Start the stopwatch when the meniscus reaches the scale;
- Observe the movement after 5 seconds; and
- Select the measuring period based on the initial rate of movement.
The standard specifies the following:
| Movement in First 5 Seconds | Measurement Period |
| Less than 3 divisions | 2 minutes |
| 3 to 9 divisions | 1 minute |
| 10 to 30 divisions | 30 seconds |
| More than 30 divisions | Record as > 3.60 ml/(m²·s) |
One scale division represents 0.01 ml/(m²·s).
If the measurement is taken for two minutes, the number of divisions is multiplied by 0.5 to obtain the equivalent one-minute movement. If measured for 30 seconds, the number of divisions is multiplied by 2.
When Should the Test be Stopped?
The standard specifies two important limits at the 10-minute reading:
Very Low Absorption
If the 10-minute reading is less than:
0.05 ml/(m²·s)
the test may be stopped and reported as:
Concrete too impermeable to be sensitive to a longer-term test.
Very High Absorption
If the 10-minute reading is greater than:
3.60 ml/(m²·s)
the test should be stopped and reported as concrete being too permeable to fall within the sensitivity range of the method.
Factors Affecting ISAT Results
The initial surface absorption of concrete can be influenced by several factors:
- Moisture condition of concrete;
- Concrete mix composition;
- Aggregate characteristics;
- Surface finish and surface type;
- Curing;
- Age of concrete;
- Cracking; and
- Temperature.
Therefore, ISAT results should not be interpreted in isolation. Two concretes with similar mix proportions may produce different results if their curing, finishing or moisture conditions are different.
Effect of Moisture Condition
Non-oven-dried concrete may contain residual moisture, which affects the initial absorption rate. The influence of this moisture tends to reduce as the duration of the test increases.
This is one reason why the moisture conditioning of specimens must be properly recorded.
Effect of Surface Variability
ISAT is sensitive to the actual condition of the concrete surface. Results may vary significantly because of differences in:
- Surface finishing;
- Local concrete quality;
- Surface pores; and
- Near-surface material characteristics.
Oven drying can also alter the cement paste structure and may therefore produce results different from those obtained on non-oven-dried concrete.
Effect of Temperature and Correction to 27°C
The viscosity of water changes with temperature. Therefore, the measured absorption can be affected significantly when the concrete surface temperature differs substantially from 27°C, which is the reference temperature used for calibration.
For site testing, the standard provides correction factors to convert the measured result to an equivalent value at 27°C.
| Concrete Surface Temperature | Multiply Result By |
| 5°C | 1.7 |
| 10°C | 1.5 |
| 15°C | 1.3 |
| 20°C | 1.2 |
| 25°C | 1.1 |
| 27°C | 1.0 |
| 30°C | 0.9 |
| 35°C | 0.8 |
| 40°C | 0.7 |
Where Can ISAT be Used?
The test is useful for:
Quality Control of Precast Concrete
Precast units can be tested under relatively standardized drying conditions, making comparison of results more reliable.
Assessment of In-Situ Concrete
Although achieving uniform moisture conditions on site is difficult, ISAT can be used to assess surface characteristics and compliance with specifications related to weathering performance.
Comparative Studies
The test is particularly useful for comparing:
- Different concrete batches;
- Different structural members;
- Different concrete mixes;
- Different cementitious materials; and
- Different finishing or curing practices.
Limitations of the Test
ISAT should not be used blindly. The test is not suitable for:
- Concrete containing obvious honeycombing;
- Highly porous surfaces;
- Cracked areas;
- Thin concrete sections through which water may penetrate during the test; or
- Locations that have already been subjected to the test.
The test may be performed on exposed aggregate or profiled surfaces only when a reliable watertight seal can be achieved.
What Should be Included in the Test Report?
The test report should include, as applicable:
- Date, time and place of testing;
- Age of concrete;
- Identification of specimen or structural element;
- Test location and positions tested;
- Description of the concrete surface;
- Orientation of the surface;
- Conditioning before the test;
- Any surface heating;
- Method used for sealing the cap;
- Water contact area and cap dimensions;
- Capillary tube length;
- Concrete surface temperature;
- All ISAT results in ml/(m²·s); and
- Results corrected to an equivalent temperature of 27°C.
Conclusion
The Initial Surface Absorption Test is a useful method for evaluating the quality and water absorption behaviour of the near-surface zone of concrete. Unlike a conventional compressive strength test, it focuses on the part of concrete that is directly exposed to the environment.
The test can help engineers identify differences caused by poor surface finishing, inadequate curing, variation in concrete quality or differences between concrete mixes. However, the result must always be interpreted with an understanding of the concrete’s moisture condition, temperature, surface characteristics and test preparation.
