Absorption Tests

Absorption tests involve the intake of a fluid due to capillary suction in the pores.9 _{Under perfect}

elapsed, and the constant of proportionality is called the sorptivity. Ideally, this is the property that should

be measured in an absorption test.9 _{There are, however, practical limitations that hinder this, for example,}

difficulty in achieving a unidirectional penetration of water and problems of determining the water penetration depth without actually splitting open the concrete specimen. Because of these difficulties, the absorption characteristics of concrete are usually measured indirectly by one of the following types of test.

6.4.1.1 Standpipe Tests

One of the simplest techniques available for measuring the water absorption characteristics of concrete is the standpipe test. This consists essentially of a vertical tube of suitable diameter glued onto the horizontal concrete surface to be tested. Water is filled into the tube up to a certain level and then allowed to be absorbed by the concrete. The amount of water absorbed per unit of time is reported as an index of water absorptivity.

There are different versions of this type of test reported in the technical literature; however, in this discussion only a brief description of three different methods is given. The first version is commonly

referred to as the “chimney method.”10 _{Its main features are that it has a constant head of 100 mm, a}

standpipe diameter of 67 mm, and the water absorptivity index is calculated from the amount of water absorbed over a 6-day period (measurements are taken every 24 h for the entire duration of the 6 days).

The second method is known as “Karsten’s pipe test.”11 _{This has a falling head of 100 mm, a 60-mm-}

diameter standpipe, and the volume of water absorbed between minute 5 and minute 15 from the start

of the test is used as a measure of the absorptivity. The third approach is called the “Australian test.”12

This has a 200-mm falling head, a 100-mm-diameter standpipe, and the water absorptivity index is determined from a graphical relationship developed between the head and time. This graphical relation- ship can be developed either by observing the head for 2 h or by measuring the time until the head has fallen by 100 mm. Although all of these tests are simple to carry out on site, they each have a low level of sensitivity and thus are not that successful at detecting different levels of concrete performance. Therefore, they are not used extensively.

6.4.1.2 Initial Surface Absorption Test (ISAT)

The initial surface absorption is defined as the rate of flow of water into the concrete surface per unit

area at a stated interval from the start of the test at a constant applied head and temperature.13 _{The first}

version of a test to measure this property was proposed by Glanville in 1931.14 _{This was further developed}

into a commercial test by Levitt15 _{and was incorporated into the British Standards in 1970.}16 _{A brief}

description of the British Standards version of this test is presented here.

The main components and general arrangement of the ISAT are shown in Figure 6.3. A circular cap

with a surface area of at least 5000 mm2 _{is clamped tightly onto the concrete surface and filled with water}

from a reservoir. To ensure that all the air is removed from the cap during this operation, the flexible tube (connected to the capillary) should not be connected to the outlet. Another requirement to ensure that no air is trapped is that the cap should be made from a transparent material to allow the detection of air bubbles. After the cap has been filled, the flexible tube is connected to the outlet. The other end of the tube is connected to a horizontal, calibrated glass capillary tube. To commence a test the reservoir is filled with water to a level that is 200 mm above the concrete surface (the same height as the horizontal capillary tube). Once this has been achieved and there are no leaks or air bubbles in the system, the inlet tap from the reservoir is closed. The movement of the water in the capillary tube is then monitored. This monitoring should take place over a 2-min period at intervals of 10 min, 30 min, 1 h, and 2 h after the start of the test. From the movement of the water in the calibrated capillary tube, the initial surface

absorption value can be calculated in units of mL/m2_{/s. The reason for measuring absorption at specified}

intervals after the start of the test is that water absorption of a dry surface is initially high but decreases

as the water-filled length of the capillaries increases. A recommendation in the British Standard17 _suggests

that it is not necessary for the test be conducted at 2 h after the start of the test because there will probably be only a small amount of movement in the capillary tube.

The main advantage of the ISAT is that it is a quick and simple nondestructive in situ test method that can be used to measure water penetration into a concrete surface. It can also be used on exposed aggregate and profiled surfaces provided a watertight seal is achieved. The difficulty of ensuring a watertight seal is probably one of the greatest limitations of this test because of the problems achieving this in practice. Another limitation is that the measured property is greatly affected by the moisture condition of the concrete. This, however, applies to nearly all near-surface absorption and air permeability

tests and is best summarized by a quote from Neville:7 _{“A low value of initial surface absorption may be}

due either to the inherent low absorption characteristics of the concrete tested or else to the fact that the pores in poor-quality concrete are already full of water.” Another disadvantage is that the 200-mm head of water is considered quite low, and although the results may be related to surface weather exposure they are of little relevance to behavior under high water pressures. In view of these advantages and disadvantages, the main application of the ISAT is as a quality control test for precast concrete units that can be tested whenever they are “dry.” The technique can also be used as a comparative test on in situ concrete for assessment of potential durability.

6.4.1.3 Autoclam Sorptivity Test

The idea for the “Clam” test was first reported by Montgomery and Adams18 _{in the early 1980s. Initially,}

it was only a water permeability test. It was then modified and the “Universal Clam” was produced. This could measure both water and air permeability. Although the test was quite advanced at the time, it still was essentially a manual test with piston movement (which controlled the pressure) performed by a micrometer screw.

In the early 1990s, Basheer19 _{completed further development work that not only standardized the tests}

but also made the whole process fully automatic. This version of the Clam, the “Autoclam,” is controlled by a microprocessor and has a complete data acquisition and transfer facility to enable computer analysis of the results. Three types of test are now possible: water absorption, air permeability, and water perme- ability. The Autoclam is commercially available. In this section, the water absorption test is described, and in a later section the air and water permeability tests are presented.

The first step for all three Autoclam tests is to clamp or glue a steel base ring onto the concrete surface (Figure 6.4A). If clamping is being used, a rubber ring is placed between the base ring and the concrete to ensure an air- and water-tight seal. The base ring usually has an internal diameter of 50 mm. The actual Autoclam test apparatus is then clamped onto the base ring. This overall arrangement is shown in Figure 6.4B. For a water absorption test, both the priming and bleed valves are opened and the piston is raised to the top of the cylinder using the controller. Using a syringe, the user introduces water into the test area through the priming valve with air escaping through the bleed valve. After a steady flow of water out of the bleed valve has been achieved, the bleed valve is closed and, using the syringe, the water pressure is increased to slightly below 0.02 bar. The priming valve is then closed and the test started by means of the control panel. At this point, a stepper motor increases the water pressure to exactly 0.02 FIGURE 6.3 Schematic of initial surface absorption test (ISAT).

Reservoir Tap is opened to fill chamber and capillary tube with water Flexible tube

Calibrated glass capillary Specimen

Measurement

Time taken for the water meniscus to move by a certain distance at a water head of 200 mm after closing the tap connected to the reservoir Watertight cap

Chamber In Out

bar and the test commences. An applied pressure of 0.02 bar is used because this corresponds to a water head of approximately 200 mm, which is the same as in the ISAT. As the test commences, water is absorbed immediately into the concrete and thus the pressure tends to decrease; however, the piston moves downward to maintain the pressure. This piston travel is monitored every minute for 15 min and, knowing the cross-sectional area of the piston, the volume of water absorbed can be determined. If the cumulative volume of water absorbed is plotted against the square root of time, a linear relationship is achieved and

the gradient of this relationship is reported as the sorptivity index, in units of m3_/÷_{min. Start-up errors}

can be avoided if only the portion of the straight-line relationship between minute 5 and minute 15 is used. The main advantages of this test are that it is relatively simple and easy to perform, the recorded results have a high level of accuracy, and the apparatus is completely portable allowing easy use on-site and in the laboratory. The disadvantages are generally the same as the ISAT; the main problem is the practical difficulties in clamping the base ring onto the concrete surface.

FIGURE 6.4 Schematic of Autoclam tests. (From Basheer, P.A.M., Ph.D. thesis, The Queen’s University of Belfast,

1991.)

Steel ring Epoxy adhesive

A Bonding steel ring to test surface

Display of pressure Priming Bleed valve Piston Hydraulic cylinder Pressure transducer Mounting screw Measurement

Piston is pushed down to keep pressure constant; piston travel is monitored to determine flow into concrete

Test pressure

Sorptivity test: 0.02 bar Water permeability test: 0.5 bar

Test duration

15 minutes for both tests Base ring Water

Priming valve

B Operation for water flow test

Cylinder

Measurement

Air pressure is increased to 1.5 bar and pressure decay is monitored every minute for 15 minutes

C Operation for air permeability test

Display of pressure Bleed valve Piston Pressure transducer Mounting screw Base ring

6.4.1.4 Figg Water-Absorption Test

The development of this test, known as a “drill-hole” absorptivity test, was first reported by Figg in 1973.20

The commercial trade name for the apparatus is the “Poroscope.” A schematic showing the apparatus and test setup is presented in Figure 6.5. The first step in performing a test is to drill a hole, 10 mm in diameter and 40 mm deep, normal to the concrete surface with a masonry bit. After thorough cleaning of the hole, a silicone plug is placed into the hole to leave a 20-mm-long cavity, 20 mm below the concrete surface. A hypodermic needle with a very fine canula passing through it is inserted through the silicone plug until the canula touches the bottom of the cavity. A two-way connector and various flexible tubes are used to connect the hypodermic needle to a syringe and a horizontal calibrated capillary tube set 100 mm above the bottom of the cavity. The syringe is used to inject water through the canula into the cavity. As water enters through the canula, air in the cavity escapes through the flexible tubes. When the system is full of water and a check has been made to ensure no air is trapped, the syringe is isolated. At this point the concrete being tested is subjected to a water head of approximately 100 mm. As time elapses, water is absorbed into the concrete and thus the meniscus in the capillary tube moves. The time required for this meniscus to move 50 mm is taken as a measure of the water absorption of the concrete. This obtained value is called the absorption index and is measured in seconds. Obviously, a higher absorption index value corresponds to a better-quality concrete.

FIGURE 6.5 Schematic of Figg tests.

Silicone plug 20 mm 40 mm 10 mm diameter Hypodermic needle To vacuum pump To pressure gauge Valve Measurement

Time in seconds for pressure to change from 55 kPa to 50 kPa below atmospheric pressure

B Air permeability test

20 mm 40 mm 10 mm diameter Silicone plug Hypodermic needle

Calibrated capillary tube

Valve

Measurement

Time in seconds for the meniscus in capillary tube to travel 50 mm under water head of 100 mm

100 mm

The main advantages of the Figg water absorption test are that it is a simple and easy test to complete and the cost of the equipment is relatively low. One of the main limitations is that during drilling of the hole microcracks may be formed in the surrounding concrete, which may defeat the purpose of the test by altering the absorption mechanism. Another possible limitation is that, because this test involves concrete that is approximately 20 mm below the surface, this test cannot be used to assess the performance

of surface treatments, for example, surface penetrants or controlled permeability formwork. Bungey,13

however, has also described this limitation as an advantage because the recorded results are not influenced by localized surface effects such as carbonation of the outer few millimeters of concrete.