bs & rilem

Materials and Structures/Mat6riaux et Constructions, Vol. 32, October 1999, pp 571o578

British Standard and RILEM water absorption tests: A critical evaluation

M. A. Wilson, M. A. Carter and W. D. Hoff Department of Building Engineering, UMIST, P 0 Box 88, Manchester, M60 1QD, UK

Paper received: February 15, 1999; Paper accepted: May 10, 1999

A B S T R A C T R I~ S U M I':-

British Standard and RILEM capillary suction rate and water absorption tests used for clay bricks, stones and pre-cast concrete products are critically examined. Experimental data are reported comparing the initial rate of suction with the sorptivity, an analytically based method of measuring capillary suction rate. Experimental work is also reported comparing water contents attained as a result of vacuum saturation absorption and the British Standard 5 h boiling test. Results of 24 h and 30 min immersion tests are also reported.

It is concluded that the initial rate of suction test is fundamentally flawed and may produce misleading results because of its use of only a single point measurement. It is further concluded that vacuum saturation provides the most accurate measurement of water absorption, and therefore porosity. The 5 h boiling test generally produces results significantly below those obtained by vacuum saturation with samples attaining approximately 90% of vacuum saturation. Immersion tests, used to provide comparative data on the rates of absorption of different materials, can only be valid if the specimens have identical dimensions.

Les essais de succion par capillaritd et d'absorption d'eau bas& sur la norme anglaise et sur la recommandation RILEM, utilis& pour les briques d'argiles, les pierres et les produits pr~fabriqu& en b~ton sont examin&. Des donn&s exp&imentales sont pr&ent&s, en comparant le taux de succion initial et la sorptivitd, une mdthode analytique de mesure de la sucabn par capillaritY. On rapporte dgalement, quelques donn&s exp&imentales comparant les teneurs d'eau atteintes par absorption sous vide selon la norme anglaise apr~s 5 heures d'~bullition. Les rOsultats des essais d'immersion de 24 heures et 30 minutes sont aussi prOsent&.

On conclut que l'essai de taux de succion initial est fondamentalement d~fectueux et peut produire des r&ultats trompeurs dus h l'utilisation d'un seul point de mesure. II est aussi conclu que la plus pr&ise des m~thodes de mesure d'absorption d'eau, et par consequent de la porositd, est celle de saturation sous vide. Le test de 5 heures d'~bullition donne des r&ultats proches de ceux obtenus par saturation sous vide pour des &hantillons atteignant environ 90% de saturation de vide. Les essais d'immersion, utili- s& pour obtenir des donn&s comparatives sur les taux d'absorption de diff&ent mat&iaux, ne sont valides que si les sp&imens sont de dimensions identiques.

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1. I N T R O D U C T I O N

Two aspects of the water absorption characteristics of porous construction materials are of particular interest and practical significance. These are the total (water absorption) porosities and the capillary suction properties of the materials.

The total porosity of any masonry material has a determining influence on the compressive strength and also on the permeability of the material to water or liquid flow. Thus in BS 3921 [1] clay bricks of low porosity and high strength are classified as engineering bricks, whilst mea-

surement of the total water absorption porosity is also used in this standard in the definition of those types of clay brick which are suitable for use as damp proof courses.

In the use and specification of clay bricks water absorption porosity is often taken as a guide for the pre- diction of frost resistance, in that bricks of high strength and low porosity have generally proved to be frost resistant in practice. However, it has been shown [2] that frost damage is a result of complex patterns of freezing of pore water, and porosity alone is not the determining parameter of frost resistance. Thus several types of relatively porous clay bricks (notably some hand-made bricks)

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Materials and Structures/Mat6riaux et Constructions, Vol, 32, October 1999

prove, in testing and in practice, to be as resistant to damage on freeze-thaw cycling as many engineering bricks.

The capillary suction properties of masonry materials are relevant in respect of both construction practice and weathering performance. During construction wet trade practices rely upon the suction of masonry materials to encourage bonding between mortar or plaster and brickwork or blockwork. In brick or block laying excessive suction causes rapid removal of water from the soft mortar and can, in principle, result in incomplete hydra- tion and a weak or porous hardened mortar. Similar considerations apply in the application of renders or plasters to brick or block substrates. The use of additives or admixtures in mortars and plasters to improve the water retaining properties of the soft solid can help to balance the effects of excessive suction. Pre-wetting of the masonry material with the aim of reducing its suction is widely adopted although the effectiveness of this tech- nique has been questioned [3].

The capillary suction properties of masonry materials also define their behaviour in driving rain. Walls of masonry units having high suction tend to absorb incident rainwater readily and the time to surface saturation is relatively long [4, 5]. In low suction walls - typically built of engineering bricks of low porosity- surface saturation occurs quickly and incident rain will then flow across the wall surface and tend to penetrate at shrinkage microcracks in perpend joints and at other defects.

That it is necessary to be able to measure, in routine testing, the water absorption porosities and the capillary suction properties of masonry materials is recognised in a number of British Standard [1, 6] and 1LILEM [7-12] tests. In this paper we report the results of laboratory measurements of these properties and critically review the appropriateness of the present British Standard and RILEM procedures. Recommendations are made for possible changes to current testing practice.

2. WATER A B S O R P T I O N TEST PROCEDURES

2.1 Capillary suction tests

Sorptivi~ The capillary absorption of water into a porous solid

is described by the non- linear diffusion equation [13]:

0~-= V. DV0 (1)

where 0 is the volume fraction water content of the material and D(0) its unsaturated hydraulic diffusivity.

Solution of the one-dimensional form of equation (1) gives a partial differential equation describing absorption through one end of a semi-infinite bar:

00 _ ~ D 00 (2) 0t Ox ax

such that: 0 = 0 satx = 0, t_> 0 and 0=00forx > 0, t=0.

0 0 and 0 s are the volume fraction water contents of the solid in the dry and saturated states respectively. Equation (2) has a solution of the form [13]:

x(O,t) : r 2 (3)

so that the advancing water content versus distance pro- file maintains constant shape ~(0).

The cumulative absorbed volume of water per unit area of supply surface, i, is given by:

1/2 0s (4) i = t Ioo ~adO = S t 1/2

where S is the sorptivity [14, 15], a parameter defining the ability of a material to absorb and transmit water by capillarity. Clearly, from equation (4), the sorptivity depends on the initial water content of the material [16] and is highest when the material is dry and zero when it is saturated. Sorptivity to water scales with surface ten- sion, o, and viscosity, ~, as (o/q)1/z [14]. Thus the sorptivity to water increases with temperature.

A useful application of the sorptivity in respect of masonry structures is in the calculation of the time to surface saturation, ts, of a wall exposed to driving rain [4, 5] using:

t s = 3'$2. (5)

where 7 is a constant (0.64 for most building materials) and v 0 is the velocity of the driving rain. At times less than t S all incident rainwater is absorbed. At times greater than ts, some water continues to be absorbed and the remainder runs off.

A precise procedure for the measurement of sorptivity has been described by Hall and Tse [17]. The sample, typically rectangular in section, is dried to constant mass in an air oven at 105~ and its dry mass noted. After cooling to room temperature the sample is immersed to a depth of 3-5ram in a tray of water. The sample is removed at intervals (times of 1, 4, 9, 16 and 25 min are convenient) and weighed. The sorptivity is determined from the gradient of the straight line (equation (4)) obtained by plotting the cumulative volume of water absorbed per unit area, i, against tl/2. It is convenient to express the sorptivity in units mm min-lS 2. Typical i (# z) graphs are shown in Fig. 1. Neither of these lines passes through the origin, one line having a very significant positive intercept. One dimensional water absorption data usually produce an intercept at t = 0 [18]. Thus the general equat ion def ining one-d imens iona l water absorption is more correctly written

i = St1~ 2 + B (6)

Most bricks produce a positive intercept (B > 0). One cause of this is the depth to which the sample is immersed in the water allowing some water to be absorbed through the sides of the sample as well as the base. Some bricks however produce a negative intercept (B < 0). This has been attributed to a dense surface layer formed during firing [18]. A minor factor in either a positive or negative intercept is the accuracy of the tim-

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Wilson, Carter, Hoff

90

60

�9 40

30

20

10

0

t V2/minl~ Fig. 1 - i(t 1/2) graphs for a clay brick, D, and an aerated autoclaved concrete (aac) block, O, [3]. The slopes of the dotted lines in the insert define the initial rate o f suction values for these materials.

ing procedure. Since the absorption rate is very high at short times (absorption rate = di/dt = 1/2 St -1/2) a small delay in starting the clock will contribute to a negative intercept. If timing is started slightly early the intercept will be increased in the positive direction.

The sorptivity is not included in either the British Standard or the RILEM water absorption test procedures although it is being increasingly adopted in concrete technology [19].

The Initial Rate of Suction (IRS) The initial rate of suction tests described in BS 3921

[1] and LUM A5: Initial rate of suction (IRS) of masonry units [8] are identical.

The IRS is measured by immersing a dry brick of known mass in water to a depth of 3 + 1ram. After one minute the brick is removed from the water and, after the removal of any excess water from its surface, is weighed. The initial rate of suction, I, is calculated from [1]:

I (m2 - ml ) (7) 1000A

where m 1 is the mass of the dry sample (in g), m e the mass of the wet sample (in g) and A the area of the absorbing surface (in mm2). From this definition the units of I are given kg mm -2 min -1 as recommended in BS 3921.

In order to be consistent with our definition of S, it is convenient to express 1in units o fmm min <. Thus:

I=(m2-ml) (8) 1 0 0 0 p A

where p is the density of water in g cm -3. Because "41 = 1 the initial rate of suction I as defined by

equation (8) will be numerically equal to the sorptivity, S, if the straight line graph of/versus tl/2 passes through the origin (i.e. B = 0 in equation (6)). The units of / (nun rain <)

are different from the units ofS (mm rain-I/2), but if/is to be a practically useful measure of suction the numerical values o f /and S must be equal or in simple proportion to each other. Unfortunately this is rarely the case because B in equation (6) is usually not zero. An example of the con- sequence of a non-zero value of B is shown by the experimental data in Fig. 1 [3]. The values of I for brick and autoclaved aerated concrete (aac) block from these data are numerically equal to the gradient of a line from t = 1 to the origin for each material. The values of I suggest that the aac material has the higher suction which is clearly not the case when the data are examined as a whole. The value of I for brick also overestimates the true suction of the brick.

A further shortcoming of the IRS test is the lack of any means of correction for variations in temperature. As discussed, the sorptivity scales with temperature as (o/11)1/2 [14]. Sorptivity data may therefore be normalised to a standard temperature of, say, 20~ However, the temperature effect is not large: the sorptivity increases by about 1% for every 1~ rise in temperature.

Variations on the IRS test are found in PAN 1: Testing methods for natural stones [7] and C P C l l . 2 : Absorption of water by concrete by capillarity [12]. In PAN 1 a dry stone specimen is immersed to a depth of 2 mm and allowed to absorb water through one face. Measurements of change in mass are taken at intervals until the water reaches the upper surface of the specimen. However it is only the last of these measurements which is used in the determination of the capillarity coefficient, Cap, which is defined by:

rn c Cap = A~/t (9)

where m c is the total mass of water absorbed since the start of the test, A the absorbing area and t the elapsed time. This test is a more accurate measure of suction than the IRS test because it is taken over a longer time and recognises that absorption is linear with tl/2. However it is a single point measurement and does not correct for intercepts at t = 0 or for possible departures from linearity of the i(tl/2) graph as the water reaches the top of the specimen.

In CPC 11.2 the dry concrete specimen is immersed to a depth of 5 + lmm and changes in mass at 3, 6, 24 and 72 h are measured. Again only the change in mass measured since the start of the test is used to calculate the "absorption of water by capillarity" which is expressed in g/mm 2. This test does not take into account the tl/2 relationship defining the volume of water absorbed; nor does it take account of the complex issues associated with long-term water absorption into concretes which are affected by long-term reactivity and swelling of the gel [20, 21].

2.2 Water absorption tests

vacuum saturation porosity Vacuum saturation is a method of assessing the total

water absorption porosity of a material. To measure the

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Materials and Structures/Mat4riaux et Constructions, Vol. 32, October 1999

vacuum saturation porosity the sample is dried to constant mass at 105~ before being allowed to cool. In a typical procedure the sample is then placed in a vacuum chamber which is connected by a hose to a rotary vacuum pump capable of producing a vacuum of~ 0.1 mm of mercury. The chamber (and therefore the sample) is evacuated and pumping is continued for some time. The hose is then clamped, disconnected from the vacuum pump and immersed in a second tank containing water. When the clamp is released water is drawn into the vacuum chamber. When the sample has become fully immersed, the hose is removed from the water tank and air allowed to enter the vacuum tank so that atmospheric pressure forces water into the evacuated pores of the material. After an appropriate period of soaking the sample is removed from the water and weighed. The water absorption porosity is often quoted as a percentage of dry mass. The volume fraction porosity, f is calculated from:

f = volume of water absorbed (10) volume of sample

There are no vacuum absorption tests in the British Standards, although an earlier version of BS 3921 did contain such a test. There are a number of variations on vacuum absorption in the IKILEM tests in which various reduced pressures and soaking times are recommended. In LUM A4: Water absorption and water porosity of masonry units [9] an unspecified pumping time is followed by a 1 h soak. PAN 1: Testing methods for natural stones [7] specifies 3 h pumping followed by 3 h immersion under vacuum which is then followed by a 21 h soak at atmospheric pressure. CP 11.3: Absorption of water by concrete by immersion under vacuum [10] specifies a 24 h period of evacuation followed by a 2 h immersion under vacuum followed by 24 h immersion under atmospheric pressure before the first weighing. The material remains immersed and weighings are continued until the material reaches constant mass.

Variation in the amount of water absorbed with both pumping and immersion times has been investigated by Prout [22] for clay bricks using a 20 litre vacuum tank and an appropriate rotary pump. Taking the water content after 5 h pumping followed by a 5 day immersion period as a nominal value of complete saturation, it was found that 5 h pumping followed by 10 min soaking or 6 min pumping followed by 15 min soaking gave essentially complete saturation. Prout also noted that very long soaking times (~ 2 months) gave a small increase in mass for some bricks. These increases were less than 0.25% for a high porosity brick and less than 1% for a low porosity brick. In an early paper Washburn and Footitt [23] discuss the relationship between pore size and soaking time when vacuum saturation is used. Peake and Ford [24] found that following evacuation dense bricks may require immersion times as long as 23 days to reach constant mass.

Water absorption by boiling BS 3921 [1] and LUM A4: Water absorption and water

porosity of masonry units [9] both contain an identical water absorption (boiling) test. For this test dry bricks of

known mass are placed in a tank of water at room temperature. The temperature of the water is increased to boiling point over a period of approximately 1 h, main- tained at 100~ for 5 h and then allowed to cool to room temperature for between 16 and 19 h. The wet bricks are then removed and weighed and the water absorption porosity, A, calculated from:

100 (wet mass-dry mass) Z = (11)

dry mass

with the results expressed as a percentage of the dry mass. The tests referred to in LUM A4 include an option to calculate the water absorption by volume which is referred to as the "water porosity".

In a second water absorption test described in PAN 1: Testing methods for natural stones [7], air-dried specimens of stone are boiled for 3 h followed by a 24 h soak. After being weighed the specimens are then oven dried to enable their water contents to be calculated. The water content is expressed as a percentage of the dry mass.

The water absorption tests discussed here have two essential components: the soaking time during which the water is being heated or cooled and the boiling time. By the time the brick reaches 100~ it will have a high water content. During the boiling period the water within the pores will generally turn to vapour and expand. This will create considerable pressure within the pores and both water vapour and air will be expelled from the brick. During the 5 h boiling period the air within the pore space will be replaced by steam which on cooling will undergo the phase change back to water with an associated large volume contraction. The consequent lowering of pressure in the pore space of the brick will result in water being forced into these pores by atmospheric pressure acting on the bulk water.

These boiling tests are empirical test procedures which are believed to give the true total water absorption porosities of the materials. Washburn and Footitt [23] express concern about the use of boiling at atmospheric pressure as a method of measuring porosity and conclude that it is not sufficiently reliable as a reference method. They also discuss the damage to the brick ceramic material which may result from boiling.

Although the porosity (by whatever means obtained) may be a useful measurement, it must be borne in mind that there are only small differences in porosity across a range of materials. For example, over a range of brick materials the volume fraction porosity may vary from 0.t to 0.5 (a factor of 5) whereas the sorptivity over the same range of materials may vary from 0.02 to 3 mm min -1/2, a factor of 150 [25]. The porosity, even when measured by vacuum saturation, is an inherently insensitive parameter.

Water absorption by soaking There are a number of tests which determine water

absorption by soaking. One use of such tests is in the determination of the saturation coefficient which is defined as the percentage of pore volume filled in a 24 h soak. The saturation coefficient is used as an empirical guide to the durability of building stones [26, 27].

5 7 4


It is well established [3] that a simple immersion test does not result in saturation due to air becoming entrapped within the porous solid. This air is held under the curved menisci of the water and is therefore under pressure. The entrapped air will eventually escape from the brick by dissolving in the water and hence diffusing out through the pore system. However this will take some time. It has been shown that a brick placed on end in a tray of water under conditions of free evaporation from the exposed surfaces will reach full (vacuum) saturation after 2 years [3].

The water absorption which occurs during soaking will essentially be suction-driven. In this paper we focus on the 24 h immersion test as described in LUM A4 and the 30 min immersion test described in BS 7263.

LUM A4: Water absorption and water porosity of masonry units [9] measures the percentage water content either by mass or by volume of initially dry specimens which have been soaked in water at room temperature for 24 h. It does however state that soaking times longer than 24 h may be required for some materials but makes no men- tion of achieving constant mass. The dimensions of the specimens are not specified

PAN1: Testing methods for natural stones [7] measures the percentage water content by volume of initially dry specimens which have been soaked for several days. The specimen size is not defined, but must be of greater volume than 20 dm 3. Measurements of change in mass are taken every 24 h until constant mass has been attained. However it is clearly unlikely that saturation will occur in a matter of a few days and the results reported in [3] suggest that the timescale for true constant mass may be of the order of a year or more. Variations on this test include progressive immersion and absorption under reduced or increased pressures.

CP 11.1: Absorption of water by concrete by immersion [11] measures the increase in mass of hardened concrete which has been soaked until 2 weighings, taken 24 h apart, result in a change in mass of less than 1%. The specimen size is not defined, but must be of volume not less than 0.001 m 3. The water content of the specimen is calculated after oven drying and is expressed as a percentage of the mass of the wet material. The major dis- advantage of this test is that since the material is not dry

before the test commences, the absorption rate will be greatly reduced (equation (4)) leading to a large increase in the time taken to reach saturation.

An absorption test of much shorter duration is described in BS 7263 [6]. Although not applicable to bricks or stones we include a brief discussion of this test together with experimental results for brick and stone materials to illustrate the results of absorption tests carried out over short times.

The test described in BS 7263 is applied to pre-cast concrete products such as flags, kerbs, channels and edgings for which maximum water absorption values are specified. Dry samples of specified dimensions and known dry mass are immersed in a tank of water at 20 + 1~ to a cov- ered depth of 25 + 5 mm. After 30 + 0.5 min the samples are removed from the water and weighed and the water absorption, W, determined from:

w=(Wet mass-dry mass.) (12) dry mass

with the results expressed as a percentage of the dry mass.

3. EXPERIMENTAL W O R K

Brick and limestone materials of widely varying hydraulic properties were selected for the experimental work. M1 the samples were approximately the same size. The materials used are summarised in Table 1.

The sorptivity of each material was measured as described with an immersion depth of approximately 4 mm. In the case of the bricks the sorptivities were measured through the bed faces. Values of initial rate of suction were not measured in a separate set of experiments but were obtained from the same data used to determine the sorptivities. This ensured that both sorptivity and initial rate of suction values for each material were determined from identical immersion depths and experimental conditions.

To illustrate the effect of immersion depth on both sorptivity and initial rate of suction, three absorption experiments were carried on the same brick (a clay facing brick of the same type as sample 5) at depths of 1, 3 and 6 mm.

Table 1 - Summary of materials used in the experimental work together with experimental results obtained from the test methods described

Sample Description

1 TELFORD BLUE (Wire cut engineering brick)

2 ANSTRUDE DE JAUNE (Limestone)

3 BRICK PAVER

4 St. MAXlMIN FINE (Limestone)

5 LIGHT BUFF (Wire cut facing brick)

6 STAFFORDSHIRE BLUE (Engineering brick)

7 ACCRINGTON NORI (Engineering brick)

Sorptivity

(ram mm -1/2)

0.05

0.27

0.19

3.98

1.94

0.20

0.48

Initial rate of suction

(mm mm "t)

0.096

0.41

0.22

2.41

2.3

0.28

0.34

Vacuum saturation porosity

% by mass

6.72

9.87

7.96

22.03

22.45

6.48

6.85

Water absorption

(5 h boiling) % by mass

6.15

9.03

7.2

22.07

20.07

5.2

6.83

Water absorption (24 h soak) % by mass

4.06

4.67

5.88

15.83

16.18

3.18

5.23

Water absorption

(30 rain soak) % by mass

0.6

3.4

2.4

13.4

15.3

2.13

3.80

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Materials and Structures/MaMriaux et Constructions, Vol. 32, October 1999

The water absorption (boiling) tests were carried out in accordance with BS 3921 and LUM A4. The samples were left to soak for 18 h after boiling for 5 h giving a total immersion time of 24 h (including lh taken to reach boiling point). After drying again to constant weight the materials were vacuum saturated. Pumping was continued for 2 h and, after flooding the chamber, the samples were left to soak for 24 h giving equal immersion times for both the boiling and the vacuum tests.

After drying again to constant weight the 30rain absorption test (by soaking) was carried out in accordance with BS 7263. After weighing the samples were returned to the water to complete a 24 h soak in accordance with LUM A4.

4. EXPERIMENTAL RESULTS

4.1 Comparison between sorptivity and initial rate of suction

Fig. 2 shows experimentally determined i(tl/2) data for a selection of the materials tested�9 Both positive and negative intercepts at t = 0 are apparent�9 Clearly these intercepts will affect the calculation of initial rate of suction.

Sorptivity values obtained from the gradients of the lines in Fig. 2 are summarised in column 3 of Table 1. Values of the initial rate of suction, expressed in mm min-1 rather than kg mm -2 min -1 to aid comparison, are given in column 4 of Table 1. Fig. 3 compares sorptivity with initial rate of suction for each material,

5 -

0!

f 54

i 0- O ~ ~ ~ : : : : l ~ ~ m

1 2 3 4 5

- 5 -

t v2 / min v2

Fig. 2 - Experimentally determined i(t 1/2) data for a selection of the materials tested: &, sample 3; � 9 sample 4; x sample 5; t , sample 7. The solid lines are least squares fits through the data points.

Fig. 3 - Comparison between sorptivity values (black) and values of the BS 3921 initial rate of suction (grey) for all the samples tested.

arranged in order of increasing sorptivity. Fig. 3 shows that although both sets of data follow broadly the same trend, there are some inconsistencies. The initial rate of suction test shows samples 4 and 5 to have similar suction properties whereas the sorptivity of sample 4 is over twice that of sample 5; this test also shows 2 to have a higher suction' than 7 when the reverse is the case. The initial rate of suction test further shows 6 to have a higher suction than 3 when both these materials have the same sorptivity. These anomalies are undoubtedly caused by the factors discussed earlier.

The effect of immersion depth on both the sorptivity and initial rate of suction are summarised in Table 2 and

- 1 2 . . . . . . . . . -

'2

-2 t 1/2 / rainY2

Fig. 4 - i(t 1/2) data measured at immersion depths o f l mm, � 9 3mm, A; and 6mm, , . The solid lines are least squares fits through the data points.

576


Table 2 - The effect of different immersion depths on the " sorptivity and the BS 3921 initial rate of suction for an

ordinary quality clay facing brick of the same type as sample 5

Immersion depth Sorptivity Initial rate of suction (mm) (mm min -1/2) (mm min "1 )

1 2.54 2.19

3 2.51 2.85

6 2.53 3.9

illustrated in Fig. 4. The i(tl/2) data in Fig. 4 clearly show the intercept increasing with immersion depth in the positive y direction. Table 2 shows that immersion depths of 1, 3 and 6 mm produce different initial rate of suction values which increase significantly with immersion depth. The sorptivity however is not affected by these small changes in immersion depth.

4.2 Comparison between vacuum saturation, water absorption (boiling) and water absorption (soaking) tests

The results of all the water absorption tests - vacuum saturation porosity (2 h vacuum followed by 24 h immersion); the BS 3921 and LUM A4 water absorption test (5 h boiling followed by 18 h soak); the LUM A4 24 h soak and the BS 7263 30 min soak are summarised in columns 5 to 8 of Table 1. All the results are expressed in terms of percentage mass fraction to aid comparison. Fig. 5 compares the volume of water absorbed by the samples during each test. The water contents attained by the samples during the boiling test, the 24 h soak and the 30 rain soak are shown in Table 3 as a percentage of those attained by vacuum saturation.

Fig. 5 shows that the water absorption (5 h boiling) test consistently produces lower water contents than the vacuum saturation test with the exception of samples 4 and 7 which have, within the bounds of experimental error, absorbed the same amount of water in each test. In the other samples the shortfall in water content for the 5 h boiling test varies from 15 cm 3 for sample 1 to nearly 60 cm 3 in the case of sample 5. However, there is a correlation between the amount of water absorbed during this test and that absorbed during vacuum saturation. Table 3 shows that the boiling test, again with the exception of samples 4 and 7, results in the samples consistently attaining 80 to 90% of vacuum saturation. Samples 4 and 7 have very different hydraulic properties: 4 has over 8 times the sorptivity of 7; in terms of volume fraction, 4 has nearly 3 times the water porosity of 7. It seems reasonable to assume that 4, being a very high suction and highly porous material, may well attain the same level of saturation in both the vacuum saturation and boil tests. However in the case of 7, a relatively impervious material of low sorptivity and porosity, the same level of saturation in each test implies that the post- evacuation soak in the vacuum test may not have been

600

500

"~ 400

"" 300 4

"6 200

1 3 7 6 2

Sample Fig. 5 - Comparison between the volume of water absorbed during each of the 4 water absorption tests described. First column in each case: 30 min immersion; second column: 24 h immersion; third column: 5 h boil; fourth column: vacuum saturation.

long enough. This is consistent with the results reported in [24] where it was found that a dense brick only reached approximately 85% of full saturation after a 24 h soak following evacuation.

Fig. 5 also shows that all the materials tested have absorbed proportionally different amounts of water in the 30 rain and 24 h immersion tests compared to vacuum saturation. In the case of the 30 min soak the degree of vacuum saturation attained varies from 8% for sample 1 to nearly 70% for sample 5. For the 24 h soak water contents vary from 47 to 74% of vacuum saturation (i.e. saturation coefficients range from 0.47 to 0.74).

The results of the soaking tests depend on the sizes of the samples under test. This is not recognised in the specifications of the various tests. For example in the 30 min soak test the concrete materials for which this test is designed are of varying specified size. Consider a cube

Table 3 - Water contents attained during the 5 h boiling, 30 rain immersion and 24 h immersion water

absorption tests expressed as percentages of the vacuum saturation water contents

Sample 5 h boil 30 min 24 h immersion immersion

91.5

91.0

90.5

100.0

89.0

80.8

99.7

8.5

34.4

30.0

60.8

68.2

32.98

55.84

60.5

47.3

73.9

71.84

72.0

49.0

76.45

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Materials and Structures/Mat6riaux et Constructions, Vol. 32, October 1999

of side x mm into which water has penetrated a distance a mm through all sides. For samples of different sizes the volume depends on x 3 but the volume of absorbed water depends onf [x 3 - (x - 2a)3], wherefis the volume fraction porosity. A large sample will therefore absorb proportionally less water than a small sample of the same material. In contrast, the vacuum saturation porosity would be constant irrespective of sample size.

6. CONCLUSIONS

The experimental results show an inherent inaccuracy in the measurement of initial rate of suction. The use of only a single data point means that the results are influ- enced by initial experimental error arising fi'om the immersion depth or the timing. In the sorptivity test, since the cumulative increase in mass is measured, all the data points are similarly affected resulting in an upward or downward displacement of the entire data set to produce an intercept on the y axis; the slope of the line, which indicates the sorptivity, is unchanged. The intercept is therefore a measure of the initial experimental error. The initial rate of suction test, using only a single data point, cannot compensate for this error. The sorptivity is therefore a more satisfactory measurement of capillary suction. Measurement of the sorptivity only differs from that of the initial suction rate in the number of data points required. Thus the initial rate of suction test could be improved upon with only minor changes to the experimental procedure. Other suction tests (PAN 1, CPll.2) in which several data points are collected could easily be used to determine sorptivity values with no change to the experimental procedure. However the data would have to be checked to ensure the i(tl/2) plots were linear over the time scale of these tests. Departure from linearity, as is often seen in cementitious materials, would provide evidence of other long-term effects.

The experimental results also show that the water absorption (5 h boiling) test consistently results in samples attaining approximately 80 to 90% of vacuum saturation. However there is some correlation between the amount of water absorbed during both vacuum saturation and boiling. Vacuum saturation can give a more accurate measurement of a material's water porosity with no increase in experimental time over the 5 h boiling test.

T h e soaking tests are o f l imited value, but the satura- t ion coeff icient may be useful as an indicator o f stone durabil i ty [26]. It mus t be emphasised that i f soaking measurements are to have any value for comparat ive pur - poses the spec imen d imens ions mus t be def ined p re - cisely, and comparisons only made be tween specimens o f identical dimensions.

REFERENCES

[1] British Standards Institution. British Standard specification for clay bricks. BS 3921, (1985).

[2] Pront, W. and Hoff, W. D., 'Durability of Building Materials and Components', Proceedings of the 5th International Conference, Brighton, (1990) 39-51.

[3] Gummerson, R. J., Hall, C. and Hoff, W. D., 'Capillary water transport in masonry structures; building construction applica- tions of Darcy's Law', Construction Papers I (1980) 17-27.

[4] Hall. C. and Kalimeris A. N., 'Water movement in porous building materials - V. Absorption and shedding of rain by building surfaces', Bldg. Envir. 19 (1982) 13-20.

[5] Hall. C. and Kalimeris A. N., 'Rain absorption and runoff on porous building surfaces', Canadian Journal of Civil Engineering 11 (1984) 108-111.

[6] British Standards Institution. Pre-cast concrete flags, kerbs, channels, edgings and quadrants. Part 1: Specification. BS 7263, (1994).

[7] PAN 1: 'Testing methods for natural stones', in 'RILEM Technical Recommendations for the Testing and Use of Construction Materials', (E&FN Spon, London,1994).

[8] LUM A5: Initial rate of suction (IRS) of masonry units. Ibid. [9] LUM A4: Water absorption and water porosity of masonry units.

Ibid. [10] CPC 11.3. Absorption of water by immersion. Ibid. [ 11] CPC 1 i. 1: Absorption of water by concrete by immersion. Ibid. [12] CPC 11.2: Absorption of water by concrete by capillarity. Ibid. [13] Philip, J. R., 'Theory of infiltration', Advances in Hydroscience 5

(1969) 215-296. [14] Gummerson, R.J., Hall, C. and Hoff, W. D., 'Water movement

in porous building materials - II. Hydraulic suction and sorptivity of brick and other masonry materials', Bldg. Envir. 15 (1980) 101-108.

[15] Hall, C., 'The water sorptivity of mortars and concretes: a review', Mag. Concrete Res. 41 (1989) 51-61.

[16] Hall, C., Hoff, W. D. and Skeldon, M., 'The sorptivity of brick: dependence on initial water content', Journal of Physics D, Applied Physics 16 (1983) 1875-1880.

[17] Hall, C. and Kam Ming Tse, T., 'Water movement in porous building materials - VII. The sorptivity of mortars', Bldg. Envir. 21 (1986) 101-108.

[18] Gummerson, R.J., Hall, C. and Hoff, W. D., 'The suction rate and sorptivity of brick', Transactions and Journal of the British Ceramic Society 80 (1981) 150-152.

[19] Reinhardt, H. W. (Ed), 'Penetration and permeability of concrete. Barriers to organic and contaminating liquids', RILEM Report 16, (E&FN Spon, London, 1998).

[20] Taylor, S. C., 'A study of the liquid transport properties of cement-based materials', PhD Thesis, UMIST, (1998).

[21] Hall, C., Hoff, W. D., Taylor, S. C., Wilson, M. A., Beom-Gi Yoon, Reinhardt, H. W., Sosoro, M., Meredith, P. and Donald, A. M., 'Water anomaly in capillary absorption by cement-based materials',Journal of Materials &ience Letters 14 (1995) 1178-1181.

[221 Prout, W. 'Studies of Frost Damage in Masonry', PhD Thesis, UMIST, (1989).

[23] Washburn, E. W. and Footitt F. F., 'Porosity: III. Water as an absorption liquid',Journal of the American Ceramic Society 4 (1921) 527-537.

[24] Peake, F. and Ford, R. W., 'A comparison of the vacuum and boiling methods for measuring the water absorption of bricks', Trans.J. Br. Ceram. Soc. 81 (1982) 160-162.

[25] Hall, C., Hoff, W. D. and Prout, W., 'Sorptivity - porosity rela- tions in clay brick ceramic', American Ceramic Society Bulletin 71 (1992) 1112-1116.

[26] Building Research Establishment, 'Selecting natural building stones', BRE Digest 420 (1997).

[27J Ross, K. D. and Butlin, R. N., 'Durability tests for building stone', BILE Report BR141, Garston, (1989).

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