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The water absorption test determines the water absorption rate (sorptivity) of both the outer and inner concrete surfaces. The test involves the measurement of the increase of mass of concrete samples resulting from the absorption of water as a function of time when only one face of the specimen is exposed to water.

The concrete specimens are either taken from drilled cores or molded in cylinders. The samples should be saturated and weighted before the test. The absorption can be estimated at different distances from the exposed surface.

Factors Influencing Water Absorption of Concrete

  1. Concrete mix proportions.
  2. Chemical admixtures and supplementary cementitious materials in the concrete.
  3. Composition and physical properties of the cementitious component and the aggregates
  4. Entrained air content
  5. Type and duration of curing
  6. Degree of hydration
  7. Presence of microcracks
  8. Concrete surface treatments such as sealers or form oil
  9. Placement method, including consolidation and finishing.
  10. Concrete moisture condition at the time of testing.

Purpose

The water absorption test aims to determine the rate of water absorption by hydraulic cement concrete.

Tools and Materials

  1. Pan
  2. Support device, rods, pins, or other devices
  3. Top-pan balance, accurate to at least ±0.01 g
  4. Timing device
  5. Paper towel or cloth
  6. Environmental chamber
  7. Polyethylene storage containers
  8. Caliper
  9. Sealing materials like duct tape or aluminum tape
  10. Plastic bag or sheeting

Concrete Sample Preparation

  1. Concrete specimens are either drilled cores or molded cylinders. They should be 100 ±6 mm in diameter and with a length of 50 ±3 mm.
  2. The test result is equal to the average test result of a minimum of two samples. The test surfaces should be at the same distance from the original exposed surface of the concrete.
  3. Vacuum saturate the drilled core specimens obtained from the field.
  4. After that, measure the mass of each test specimen to the nearest 0.01 g.
  5. Put test samples in the environmental chamber at a temperature of 50 ± 2 degrees and RH of 80 ±3 % for three days.
  6. After that, put each specimen in a sealable container, leave a small space between the sample and container wall to allow free airflow around the specimen.
  7. Store the container at 23 ±2 degrees for a minimum of 15 days at the start of the absorption test.
  8. Conduct the absorption procedure at 23 ±2 degrees with tap water conditioned to the same temperature.

Vacuum-saturation Procedure

  1. Place specimen directly in vacuum desiccator. Both end faces of the sample must be exposed.
  2. Seal desiccator and start the vacuum pump and maintain it for three hours.
  3. Fill separatory funnel with the de-aerated water.
  4. With vacuum pump still running, open water stopcock and drain sufficient water into the container to cover the specimen.
  5. Close water stopcock and allow the vacuum pump to run for one additional hour.
  6. Close vacuum line stopcock, and then turn off the pump.
  7. Turn vacuum line stopcock to allow air to re-enter desiccator.
  8. Soak specimen underwater in the beaker for 18 ± 2 hours.

Water Absorption Test Procedure

  1. Take out specimens from the storage container and weigh them to the nearest of 0.01 g.
  2. Measure a minimum of four diameters of the specimen at the surface to be exposed to water and calculate the specimen’s average diameter.
  3. Seal the side surface of specimens with suitable seal material, and seal one end that is not exposed to water with a plastic sheet that can be secured using an elastic band or other equivalent systems.
  4. Weigh the sealed specimen and record it as the initial mass.
  5. Place specimen support at the pan’s bottom and pour tap water into the pan until it rises nearly 3 mm above the specimen supports, see Figure-1. This level of water in the pan needs to be maintained during the test.
  6. Start the timing device and put the unsealed surface of the specimen on the supports in the pan. Write the time and date of the initial contact of the sample with the water.
  7. Record the mass of the specimen after first contact according to the time interval provided in Table-1.
  8. For each record of a mass, remove the concrete specimen from the pan. Then, stop the timing device if the contact time is less than 10 minutes, and wipe out any surface water with dampened paper or cloth. Invert the specimen so that the wet surface does not contact the balance pan. Measure the mass within 15 seconds of specimen removal from the pan. Finally, place the specimen on the support in the pan and restart the timing device.
Schematic Representation of Absorption Test
Figure-1: Schematic Representation of Absorption Test

Table-1: Time interval for Recording the Mass of Concrete Specimen  

Time intervalTolerances
60 second2 second
5 minutes10 second
10 minutes2 minutes
20 minutes2 minutes
30 minutes2 minutes
60 minutes2 minutes
Every hour up to 6 hours5 minutes
Once a day up to 3 days2 hours
Day 4 to 7 3 measurements 24 hours apart2 hours
Day 7 to 9 One measurement2 hours

 Calculations

The following expression can be used to compute the absorption rate of concrete:

Absorption (I)= mt/ (a*d) Equation 1

Where:

mt: the change in specimen mass in grams, at the time t,

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a: exposed area of the specimen, mm2

d: density of the water, g/mm3

As shown in Figure-2, the initial rate of water absorption is the slope of the line that is the best fit to I plotted against the square root of time using all the points from 1 minute to 6 hours.

The secondary rate of water absorption is the slope of the line that is the best fit to I plotted against the square root of time using all the points from
1 day to 7 days.

Initial Rate Absorption and Secondary Absorption of Concrete
Figure-2: Initial Absorption rate and Secondary Absorption of Concrete

FAQs

What is water absorption rate test (sorptivity test) of hydraulic-cement concrete?

It is a test method by which the rate of absorption (sorptivity) of water by hydraulic cement concrete is estimated by measuring the increase of mass of the specimen resulting from absorption of water as a function of time when only one surface of the specimen is exposed to water.

How do you calculate the water absorption rate of concrete?

The water absorption rate of concrete is equal to the change in mass of concrete specimen divided by area of the sample exposed to water times the density of water.

What are the factors that influence water absorption of concrete?

1. Concrete mixture proportions
2. Entrained air
3. Moisture condition of the concrete
4. Type and duration of curing
5. Composition and physical properties of cement materials and aggregate
6. Chemical admixtures and supplementary cementitious materials in conrete
7. Concrete surface treatment like sealers
8. Presence of microcracks
9. Concrete placement method, including compaction and finishing
10. Degree of cement hydration

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