Understanding Water Absorption in Concrete

Water absorption in hydraulic cement concrete is a crucial aspect that influences its durability and performance. This process involves measuring the sorptivity, or water absorption rate, of both the outer and inner surfaces of the concrete. The test focuses on observing how the mass of concrete samples increases over time when only one face is exposed to water.

Factors Affecting Water Absorption

Several factors impact the water absorption of concrete:

• Concrete Mix Proportions
• Chemical Admixtures and Supplementary Materials
• Cement and Aggregate Properties
• Entrained Air Content
• Curing Type and Duration
• Hydration Level
• Presence of Microcracks
• Surface Treatments
• Placement Method
• Moisture Condition during Testing

Purpose of the Water Absorption Test

The primary goal of the water absorption test is to determine how quickly hydraulic cement concrete absorbs water. This information is crucial for assessing the concrete’s performance and ensuring its long-term durability.

Tools and Materials Required

To conduct the water absorption test, you will need:

Equipment	Purpose
Pan	Container for the test
Support Devices	Hold the concrete specimens
Top-pan Balance	Accurate measurement of mass
Timing Device	Track the time during the test
Paper Towel or Cloth	Wipe out surface water
Environmental Chamber	Control temperature and humidity during storage
Polyethylene Storage Containers	Store test specimens
Caliper	Measure specimen diameters
Sealing Materials	Seal specimen surfaces
Plastic Bag or Sheeting	Additional sealing for storage

Concrete Sample Preparation

1. Specimen Dimensions: Concrete specimens should be 100 ± 6 mm in diameter and 50 ± 3 mm in length.
2. Test Result Average: The test result is the average of a minimum of two samples.
3. Vacuum Saturation: Saturate drilled core specimens in a vacuum desiccator.
4. Initial Mass Measurement: Measure the mass of each specimen to the nearest 0.01 g.
5. Environmental Chamber: Place specimens in an environmental chamber for three days.
6. Storage: Store specimens in containers for a minimum of 15 days before the absorption test.

Vacuum-Saturation Procedure

1. Desiccator Placement: Place the specimen in a vacuum desiccator with both end faces exposed.
2. Vacuum Pump Operation: Start the vacuum pump and maintain for three hours.
3. Water Filling: Fill a separatory funnel with de-aerated water and drain it into the container.
4. Additional Vacuum: Allow the vacuum pump to run for an additional hour.
5. Specimen Soaking: Soak the specimen underwater for 18 ± 2 hours.

Water Absorption Test Procedure

1. Initial Weighing: Weigh specimens to the nearest 0.01 g.
2. Diameter Measurement: Measure at least four diameters and calculate the average.
3. Sealing: Seal side surfaces and unexposed end with suitable materials.
4. Initial Mass: Record the initial mass of the sealed specimen.
5. Test Setup: Place the specimen on supports in a pan with water.
6. Timing: Start the timing device and note the initial contact time.
7. Mass Recording: Record the mass at specified time intervals (see Table-1).
8. Specimen Handling: Remove the specimen for weighing, wipe off surface water, and repeat the process.

Calculations

The absorption rate (I) can be calculated using the formula:

I=mta⋅dI=a⋅dmt​

Where:

• $mt$: Change in specimen mass in grams at time $t$,
• $a$: Exposed area of the specimen in mm²,
• $d$: Density of water in g/mm³.

The initial and secondary rates of water absorption are determined by plotting $I$ against the square root of time within specific intervals.

Conclusion

Understanding and measuring the water absorption rate of hydraulic cement concrete is essential for ensuring its durability and performance over time. By following a systematic testing procedure and considering various influencing factors, engineers and construction professionals can make informed decisions about the suitability of concrete for specific applications.