An automated method for the measurement of surface water absorption into permeable materials

Lbnsfruction and Building Materials, Vol. 9, No. I, pp. 3-I I, 1995 Elsevm Science Ltd Printed in Great Britain 09504618/95 $lO.oO+O.OO

An automated method for the measurement of surface water absorption into permeable materials A. E. Noble*,

E. R. MillertS

and H. Derbyshiret

*Department of Electronic Engineering and Applied Physics, Aston University, Birmingham, UK tTimber Division, Building Research Establishment, Garston, Watford WD2 7JR, UK Received

16 May 1994; accepted 29 August

1994

The initial surface absorption test procedure specified in BS 1881 Part 5 for the determination of the surface permeability of concrete has been successfully adapted to continuous operation using a line-scan camera and computer control. Results are presented for a range of permeable surfaces including concrete, treated concrete and calcium silicate bricks. The apparatus will reliably measure absorption rates over the range 0.5-50 000 mL m-2 h-l. A modification of the procedure to permit the measurement of surface displacement during the absorption test enables the method to be applied to swelling substrates such as wood. Keywords:

automated

measurement;

surface water absorption;

The movement of water into and out of building materials is an important factor in their performance, influencing for example the chemical and frost resistance of stone, clay tiles and concrete, and the risk of corrosion of concrete reinforcement. With wood-based materials, the pore structure can affect the interaction of coatings and adhesives, as well as durability under conditions of outdoor exposure. The porosity of building materials may vary over a wide range; a collection of published results has been compiled by Whiteley et uf’. Numerous methods exist for the determination of surface porosity. One which has been used over many years as a successful guide to the weathering durability of architectural products and cast stone is the initial surface absorption test (ISAT), which is described in BS 1881 Part 5*. This test, which has been the subject of rigorous evaluation by Levitt’, has the advantages of relative simplicity and low apparatus cost, and of yielding results which correlate with field experience. However, the test procedure relatively lacks accuracy when rates of absorption are very high or very low and is labour intensive, so a programme of work has been carried out in BRE in association with Aston University to adapt and develop the method for continuous monitoring and to improve resolution and accuracy. The basic ISAT apparatus is shown in Figure l. it consists typically (for laboratory work) of a domed cap, preferably in clear plastics, which is clamped tightly onto the test surface. A rubber gasket ensures a watertight seal. Water is supplied from a reservoir at a head XZorrespondence to Dr E. R. Miiler 0 Crown copyright (I 995)

materials

of 200 mm relative to the test surface via the inlet tube. The outlet tube is connected to a horizontally mounted capillary tube at the same height. At the start of a test, water is supplied from the reservoir to fill completely the cap and connected capillary tube. The connection to the water supply is then closed and any water subsequently absorbed by the sample is translated into travel of the meniscus along the capillary. The range of absorption rates which can be accommodated is influenced by the sizes of capillary bore and test area. The capillary tubes used in the present work were of 1 mm2 and 6.2 mm2 bore cross-sectional area. Two cap sizes were used of external diameter 40 mm (test area 1257 mm2) and 85 mm (test area 5675 mm2). BS 1881 specifies the use of a graduated capillary tube

rJ\

Reservoir ._.-. .-

r9

-__________.-_

-_-_-_-_ -_-_-_-_ -_-_-_-_ __-_

200 mm

,...__:...

.-.-.-.

1 .-

Based clampingframe

Figure 1

Construction

permeable

ISAT

water absorption

and Building Materials

cell

1995 Volume 9 Number

1

3

Measurement of surface water absorption: A. E. Noble et al. and a stop-watch to monitor the movement of the meniscus, and requires that the rate of absorption be determined at specified intervals from the time that water is first introduced into the cap. In the apparatus described below the meniscus is monitored continuously by a linescan camera controlled by a BBC Master computer.

4

The automatic

Construction

ISAT apparatus

and Building

Materials

..!_P

--I light

Figure 2

-- --_.* water_ -

._-__--

The automatic ISAT apparatus The optical arrangement developed for monitoring the meniscus in the present studies is shown in Figure 2. The tube is illuminated by a short fluorescent lamp and observed by the line-scan camera (LSC) focused on the capillary bore. The LSC is fitted with a standard Nikon 55 mm micro lens and a solid-state sensor which consists of a linear array of photodiode sensors onto which the image of the capillary bore is projected. The capillary bore viewed by eye from the position of the camera appears as shown in Figure 3. The variation in light intensity along the axis of the tube (also depicted in Figure 3) shows how the meniscus is detected by the sensor. The photodiode array contains 1024 photodiodes spaced 25 km apart and the information on the array is obtained by scanning through its entire length to determine the amount of light falling on each diode in turn. This operation. which takes 7 milliseconds, is carried out by a dedicated microprocessor unit which in turn communicates with the BBC Master computer via a serial data link. Each scan of the array generates a voltage ‘video’ which is analogue in amplitude, according to the light intensity. and digital in position being stepped from 1 to 1024.

: : : :

f

_--

_-_------

+ ;; j:

air

1 _--

--

+

pixel positm

Figure 3

Detection

of meniscus

In order to interpret the voltage video the instrument must first be calibrated to establish the current levels associated with light and dark and to determine the image magnification. Calibration is accomplished using two black calibration marks set 80 mm apart on the capillary tube and with the microprocessor operating in calibrate mode. In this mode the microprocessor scans the array. searching for the two dark lines on the image. These are then related to individual photodiodes (pixel positions) on the array and stored by the microprocessor together with the light and dark current levels. Once this has been done the microprocessor automatically switches from calibrate mode into run mode and the capillary tube is rotated by hand to remove the calibration marks from the field of view. Run mode is the normal operating mode of the microprocessor. In this mode the microprocessor repeatedly scans the photodiode array searching for a single dark line, interpreted as the position of the meniscus, and communicating this information on demand to the BBC Master computer. With the optical arrangement adopted, the camera monitors a length of approximately 100 mm of the capillary tube. This gives a resolution of approximately 0.1 mm, which corresponds to water volumes of 0.1 FL and 0.62 p,L for the two tubes used. One sweep of the meniscus across the field of view accordingly corresponds to quantities of 100 (*L and 620 pL. Continuous operation of the apparatus requires that the tube be periodically refilled to ensure that the meniscus constantly remains in the camera’s field of view. This is achieved under software control so that as the meniscus approaches the lower limit an electromechanical valve in the water feed to the cell is opened momentarily in order to restore the position of the meniscus to the upper limit. Results are then recorded repeatedly and automatically, as shown in Figure 4, which illustrates 14 sweeps taken over a period of 2 hours. As detailed in BS 1881 the test procedure involves the determination of the initial surface absorption rate in mL m ? s ’ after intervals of 10 min, 30 min, 1 h and 2 h, from the start of the test. The LSC provides a continuous measurement of the absorption process and permits the expression of absorption rate at any desired interval.

1995 Volume 9 Number 1

Measurement of surface water absorption: A. E. Noble et al. 0

concrete treated concrete cement-bonded p-rticleboard medium-density fi reboard calcium silicate bri :k wood

l l l g

l

60

l

E g 50 e a a 40 30

0' 10

20

30

40

50 Elapsed

Figure 4

60

70

60

90

100

110

time (min)

Typical line-scan output

Measurement of surface swelling When conducting tests on wood it was found that the total absorption calculated from the ISAT data was markedly lower than total uptake measured gravimetritally. This could only be explained by the inevitable swelling of the wood during the test, a consequence of which would be to expel water from the cap. The cap was accordingly modified as shown in Figure 5 to permit surface swelling to be monitored during testing, by creating a hole in its top through which could pass the stem of a linear variable differential transformer (LVDT) fitted with a circular foot approximately 100 mm2 in area resting on the wood surface. The gap between the stem and the walls of the hole was made watertight by injecting grease. During testing the output from the LVDT probe is sent to the analogue port of the BBC Master computer, which records meniscus position and surface movement simultaneously. The movement of the meniscus is then corrected to take account of swelling of the surface. It is assumed for the purposes of calculation that surface swelling is uniform over the test area, though this is a simplification. The resolution of the probe is 1 km, giving a resolution in volume of 5.7 PL for the large cap and 1.2 PL for the small cap. An improvement in this resolution would be desirable, but it would require a resolution better than 0.1 km in the probe in order to match the resolution of the camera system. This is not possible with the present LVDT apparatus.

Concrete Figure 6 shows the results f. 3rn tests on concrete in the form of a plot of time again t absorption rates. Values of absorption rate were calcu lted for each of the time intervals between successive r adings and shown as a series of dots. Plots are sho vn for two concretes. Sample A relates to a sample of site concrete, and sample B relates to a similar gra 1e of concrete cast in the laboratory and cured for 28 dh ‘s under water. Both curves have been derived from dati of the form shown in Figure 4, after elimination of thr points associated with refilling the capillary. Table I lists the absorption rates for the two samples calculated for several specific intervals from the start of test. Levitt3 used the Poiseuille formula to show that if the absorption process is controlled by capillary flow into the substrate, the rate of absorption after elapsed time will be given by Rate = cP where c is a constant and the power (m) is theoretically equal to 0.5. Levitt also gives reasons why the value of m obtained in practice may differ from this value. In general m is found to lie between 0.3 and 0.7 although values outside this range are occasionally encountered. The units of the constant (c) depend upon the units in which the rate and the elapsed time are measured and

2500. ‘r “E 2000

-

; z1500.

B IOOOSample

500 Sample

Results and discussion

A

6

=I

Lb. IO

The operation of the line-scan camera was evaluated using the following substrates:

20

30

40

50

60

Elapsed

Figure 6

tmw

70

60

90

100

110

120

(mfn)

Absorption rates for concrete

Table 1 Absorption rates (mL mm2h-l) for two different concrete samples at different elapsed times seal Foot of probe

Penpex cap

Elapsed time (min) Gasket Sample

Figure 5

LSAT apparatus modified to monitor surface swelling

Construction

Sample A (site) B (laboratory)

and Building

I

3

5

10

30

60

120

3113 2054

1152 815

1154 425

759 289

473 123

364 66

263 37

Materials

1995 Volume 9 Number 1

5

Measurement

of surface water absorption:

A. E. Noble et al.

also upon the value of the power. Throughout this paper absorption rate is expressed in mL m z h ’ and elapsed time in minutes. Such a power-law relation between absorption rate and elapsed time was found to be valid for the concrete samples tested and the solid lines in Figure 6 show the calculated power curves for the two samples. The results of regression analysis for the concrete samples are given in Table 2. The regression coefficients for both samples are in excess of 0.99 showing that the data fit the theoretical curve extremely well. Indeed, the majority of the data points in Figure 6 lies exactly under the regression curve. The value of power for the ‘laboratory’ sample was unusually high, reflecting the rapid decline in absorption rate for this sample and its low overall absorption.

3000

I

A before treatment C rnmedlatelyafiertreatment D afiertreatmentand weathering

10

40

Figure 7 The effect of treatment rates of concrete

90

60

100

110

120

and weathering

on water absorption

1200 .: r ; 1000 --J ii. 600 t =

600 400 200

20

different

50

40

concrete

(b)

10000 9000 -8000 z

J

; 7000

-

; 6000

-

j$ 5000 iI 4000 -

samples

10

-0.841

7417 2054

using absorption

rate measured

0.992 0.99x

2h ’ and time in

‘Table 3 Absorption rates (mL m : h ‘) for treated concrete at different elapsed times

and

untreated

20

30

40

60

Figure board:

70

3

IO

30

60

120

A (untreated) 3113 C (newly treated) n/a D (treated and 1698 weathered)

1552 43 1518

759 17 458

473 8 51

364 x 20

263 x 23

60

100

90

8 Absorption rates for (a) cement-bonded (b) cement-bonded wood particleboard

mineral

Table 4 Absorption rates (mL m ’ h ‘) for cement/bonded boards at different elapsed times

I

Board A (mineral particle) B (wood particle) *Test terminated

1995 Volume 9 Number 1

1508 10 871

at 60 min

3 829 3046

110

120

(mln)

Elapsed

I

Materials

50

Elapsed Me

Time from start (min)

and Building

30 Elapsedtlme (min)

for

Construction

70

1400

10

The results for two types of cement-bonded particleboard with very different absorption characteristics are shown in Figures S(a) and 8(b). Details of the absorption rates are given in Table 4. Some boards, for example the mineral particleboard (board A) used as a replacement for asbestos cement shown in Figure S(a). followed the power-law behaviour. Other boards did not show this type of behaviour. an example being the cement-bonded wood particleboard B shown in Figure X(b).

6

60

1600

Cement-bonded purticleboard

Sample

50

(4

For the evaluation of concrete treatments. samples of concrete mix A were vacuum impregnated with an oligomeric siloxaneesilane fluid. Water absorption tests were carried out a few weeks after treatment (sample C) and after 18 months of natural weathering (sample D). The results are shown in Table 3 and Figure 7. Neither of the treated samples conformed to the power-law behaviour. The results show clearly that the treatment was very effective initially in preventing water penetration but that resistance to water uptake was reduced by exposure to outdoor weathering.

“Calculated minutes

30

ElapsedtIme (mln)

Treated concrete

B (laboratory)

20

particle-

particle-

time (min) 5

638 1493

IO 471 638

30

60

120

297 224

240 145

* 118

Measurement of surface water absorption: A. E. Noble et al. Table 5 shows the total water uptake for these boards. The total water uptake is calculated firstly from the gain in weight during the test and secondly from the total distance travelled by the meniscus. Ideally the two values should agree and provide a useful check on the validity of the test. For the particleboards tested, the difference between these two estimates was rarely greater than 10%. Usually the total water uptake calculated from the total meniscus movement was slightly lower than the value calculated by weighing. This is to be expected since the absorption which takes place at the start of the test while the system is filled is not accounted for in the total distance travelled by the meniscus. For this reason samples with high initial absorption rates showed a larger discrepancy between the two estimates of total absorption.

The water absorption properties of three different grades of medium-density fibreboard were investigated using duplicate samples of each board. The boards were of similar density; the mean value of density for the set of six samples was 755.3 kg mm3and the standard deviation 13.5 kg m-3. The agreement between duplicate samples of each type was good and the mean value of absorption rate for each pair is presented in Table 6. Figure 9 and Table 7 show the water absorption for the three boards and it will be seen that despite the consistency of density, there were differences in the water absorption characteristics of the different types of board. Boards D2 and D8 showed a similar rate of absorption which remained constant throughout the 2 hour test; board D4 showed less moisture penetration and a decreasing rate of absorption. Differences between the three types of board were more pronounced in the second test carried out on the samples. Boards D2 and D8 showed higher rates of absorption than in the first tests and the increase was Table 5 Comparison two particleboards

of gravimetric

uptake

with LSC absorption

Board

Test time (min)

Weight

increase

Total travel of meniscus (mL mm2)

422 750

398 632

60 120

Elapsed Board

Test

O-15

15-30

D2

First Second First Second First Second

100.9 138.7 65.6 57.0 84.3 314.3

77.1 102.1 42.4 33.0 87.2 198.9

D8

for

(g m 2,

Table 6 Absorption rates (mL m 2 h-‘) for medium-density boards of different elapsed times

D4

10

20

30

40

50

60

70

60

90

100

110

120

130

Elapsed time (min)

Figure 9

Water

absorption

of medium-density

fibreboard

300

Fibre building board

A (mineral particle) B (wood particle)

200

fibre-

time (min) 30-60 82.3 78.9 34.1 24.4 83.4 131.0

60-120 84.8 84.4 28.4 23.0 88.3 99.6

Construction

250

_I 10

20

30

40

50

60

70

60

90

100

110

120

130

Elapsed time (min)

Figure 10

Change

in water absorption

of fibreboard

D8

particularly marked for board D8 (Figure IO). Board D4 showed a slight reduction in absorption rates during the second test. The reason for the differences in moisture uptake of the different boards is not known but could possibly be linked to differences in resin content or type. The results illustrate the capability of the ISAT test and indicate differences in the water absorption characteristics of fibreboards. Calcium silicate brick The capability of the line-scan camera system to measure very high rates of absorption over prolonged periods of time was demonstrated by the tests carried out on samples of calcium silicate brick. Two grades of brick, sandlime and flintlime, conforming to BS 187: 1978, were tested for 24 hours. The bricks were found to be quite variable in porosity, and absorption rates for a typical sample of each type are shown in Figure 11. Table 8 shows the mean values of absorption rate for all the samples tested. It will be evident that initial rates of absorption were extremely high especially for the more permeable sandlime bricks. The absorption rate decreased with time (although not conforming to the power-law behaviour) but rates remained high even after 24 hours under test. and Building

Materials

1995 Volume 9 Number 1

7

Measurement of surface water absorption: A. E. Noble et al. absorption due to evaporative losses serves to show that the ISAT test is a more reliable method of determining water absorption rates in very permeable materials. Wood

Figure 11

Absorption

rates for calcium silicate brick

Table 7 Comparison of gravimetric uptake with LX’ uptake for medium-density fibreboard

Board D?

Weight increase (g m 3

Test

167 213 X6 Xh I97 330

First Second First Second First Second

D4 D8

Total travel of (mL m >)

meniscus

162 IX8 72 61 175 305

Table 8 Absorption rates (mL m 2 h ‘) for calcium silicate brick ‘I( different elapsed times

Time from start (min) Grade Sandlime Flintlime

1.5

3

5

15

9429 7516

6701 4644

560X 3399

3300 2016

30

60

770

1340

2580 2106 1524 1123

1331 571

267 ‘16

Despite the large sample size (sample weight was typically 1.2 kg) it was possible to determine the total uptake by gravimetric means because of the large volume of water absorbed during the test. However. it was evident from the comparison of the two estimates of total uptake that water was being lost by evaporation from the brick as the test proceeded. This was confirmed by sealing the bricks with wax. leaving unsealed only the test area and a small area at the base of the brick to allow the escape of air as water penetrated the sample. The effect of the seal on the difference between the two estimates of total uptake can be seen from the data in Tuhle 9. The error in the gravimetric determinations of water

Water absorption measurements on wood require that results be corrected for surface swelling and as described earlier this was achieved by making measurements of meniscus movement simultaneously with measurements of surface displacement (using the LVDT probe). The tests were carried out on carefully orientated discs or cylinders of wood (see below) measuring 50 or 95 mm in diameter and varying in depth from 40 to 60 mm. They were sealed around the periphery for the upper 5 mm in depth with two coats of aluminium sealer. Wood is anisotropic and its absorption properties differ substantially for transverse (end grain) surfaces which are cut perpendicularly to the stem of the tree and for surfaces which intersect the growth rings tangentially or radially. Accordingly wood was selected and converted with care so that test surfaces provided examples of radial, tangential and transverse grain orientation. Figure 12 shows the result of an LsCiLVDT test on a tangential surface of Scats Pine sapwood. In this case, it will be seen that, when the movement of the meniscus is corrected for the volume of water displaced by swelling of the surface, the agreement between the gain in weight of the sample during the test and the corrected distance travelled by the meniscus is very good. Table 10 gives details of LSC~LVDT tests on the radial, tangential and transverse faces of wood species. The results are the mean values of duplicate samples, except for Norway Spruce where four replicate samples were tested. Tests on radial and tangential faces were carried out over a 2 hour period but tests on the transverse face were terminated as soon as it was evident that water had penetrated the full depth of the test sample. The results show the significant degree of swelling which takes place as water penetrates the wood test samples in all directions. In absolute terms. the surface movement is broadly similar for all three test faces, but

6006

9 Comparison of gra\imetrlc uptake wth LSC absorption during 24 hour absorption tests on calcium silicate brick Table

Grade

Sandlime - not waxed Sandlime - waxed Flintlime not waxed Flintlime - waxed

8

Construction

Weight increase (g 111-)

Total travel of meniscuh (mL m ‘)

34 916 26 492 35 951 I’ 601 IO 493 21 631

40 331 29 509 36 “9 I5 6x0 14 765 ‘I 021

and Building

Materials

10

20

30

40

50 Elapsed

Figure I2

1995 Volume 9 Number 1

Correction

60

70

80

90

100

110

t!me(m(n)

of ISAT measurements for surface swelling

120

Measurement of surface water absorption: A. E. Noble et al. Table 10

Water

absorption

characteristics

of softwood

species

Observed meniscus movement (mL mm2)

Surface displacement (mL m ‘)

Total uptake Corrected meniscus movement

(mL m 2,

Species

Botanical

Scats Pine sapwood

Pinus sylvesrris

Radial Tangential Transverse

120 120 40

426 411 5923

229 149 214

655 560 6137

624 576 6518

Scats Pine heartwood

Pinus sylvestris

Radial Tangential Transverse

120 120 120

53 38 978

108 68 111

161 105 1090

181 175 1084

Norway Spruce

Picea abies

Radial Tangential Transverse

120 I20 30

76 107 1064

121 75 20

197 182 1085

158 153 1180

Douglas Fir

Pseudorsuga

Cypress

Cupressus sp.

Radial Tangential Transverse Radial Tangential Transverse

120 120 30 120 120 120

62 87 I068 106 401 2170

74 56 28 71 108 113

136 143 1096 176 509 2283

91 142 1313 326 549 2333

Western Red Cedar

Thuja plicaja

Radial Tangential Transverse

120 120 120

155 0 2201

78 48 97

233 48 2298

365 168 3439

name

menziesii

Test face

Test duration (min)

for penetration into transverse surfaces it represents only a small part of the total volume of water absorbed. For penetration through the radial or tangential surfaces, however, the volume of water displaced by surface swelling is a significant proportion of the total uptake. The results in Table 10 show that the application of a simple correction for surface movement to the observed movement of the meniscus results in a value of total uptake (corrected movement of the meniscus) which is very much closer to the gravimetric determination of total water absorbed. At its best this simple approach gave extremely good results (e.g. Douglas Fir tangential samples), with the two estimates of total uptake showing less than 1% difference. However, there were

Table 11

Absorption

rates (mL m ? h i) for softwood

Weight increase

instances where the applied correction appeared to be either excessive or insufficient, suggesting that the assumption of uniform surface swelling may not be valid. The two estimates of uptake differed by as much as 70% in the case of tangential samples of Western Red Cedar. A more accurate method of determining surface displacement and profile during the absorption test would be required to give a reliable measurement of the volume of water displaced by swelling. However, despite the limitations of the measurements of surface displacement, the LSC/LVDT apparatus has proved useful in determining rates of water absorption for different wood species. Table 11 gives the values of absorption rates after different times for several wood

species at different

elapsed

times

Time from start (mm) Species

Botanical

name

Test face

I

3

5

15

30

60

120

Pinus sv/vesrris

Radial Tangential Transverse

3210 2098 21 283

1475 1417 17 985

1186 786 12 800

538 429 8306

309 258 645 1

189 182 *

131 133 *

Scats Pine heartwood

Pinus sylvesiris

Radial Tangential Transverse

199 233 4723

196 123 I969

171 116 1311

196 74 678

82 66 468

52 43 343

38 33 237

Norway Spruce

Picea rrbies

Radial Tangential Transverse

816 1177 8954

402 427 2232

276 179 I505

144 100 726

93 75 666

72 56 *

51 39 *

Douglas Fir

Pseudotsuga menziesii

196 178 3430 221 766 2583

196 196 2257 167 585 2098

75 69 1027 120 404 1437

33 65 *

28 33 *

Cupressus sp.

300 672 9085 295 1033 3960

63 89 *

Cypress

Radial Tangential Transverse Radial Tangential Transverse

89 278 1082

59 190 787

49 128 570

Western Red Cedar

Thuja plicafa

Radial Tangential Transverse

1078 163 5746

716 150 2985

408 127 2234

134 37 1432

84 17 1083

51 12 852

32 9 504

Scats Pine sapwood

*Test terminated

when water had penetrated

the full depth

Construction

of the sample

and Building

Materials

1995 Volume 9 Number 1

9

Measurement

of surface water absorption:

A. E. Noble et al.

species through the three test faces. The rates of transverse penetration were many times greater than the rates of radial and tangential penetration for all the species tested. Differences in penetration rates through radial and tangential faces were relatively small, although absorption rates through the tangential face were generally greater than through the radial face during the early part of the absorption test. For Scats Pine and Norway Spruce this difference was reversed as the test proceeded resulting in higher total uptake through the radial face for these species. All the wood species showed a reduction in absorption rate with time and analysis of the corrected values of the position of the meniscus showed that for wood too the absorption rate decreased with time according to a power law. Figure 13 shows the regression curve for a tangential Scats Pine sapwood sample. There was rather more scatter in these results as the measurements of surface displacement introduce an additional error but the data follow the regression curve well. Banks’ described the flow of preservatives into wood under applied pressure using Darcy’s law modified to take account of wetting effects. The observed power-law dependence of absorption rate on time followed from this approach.

Assessment of the automated

ISAT

apparatus

The absorption tests described in the preceding section have clearly demonstrated that the LSC provides a reliable method of determining the water absorption properties of permeable materials. Its ability to operate continuously over long periods without attention is a considerable advantage and would be of particular benefit when testing relatively impermeable materials. During the absorption tests a wide range of rates of meniscus travel was encountered and the apparatus coped efficiently with both very high and very low rates of absorption. When measuring low rates of absorption, the rate of meniscus travel can be enhanced by use of

the large cap and a narrow-bore capillary tube. The factors which limited the detection of very low rates were firstly the resolution of the LSC (0.1 mm on the capillary tube), secondly the effects of any relaxation of the gasket seal and thirdly slight changes in ambient temperature. In practice it was found that the LSC output associated with rates of absorption less than 0.5 mL m-l h-l was subject to a great deal of noise and the results accordingly were uncertain. When dealing with high rates of absorption, the speed of the meniscus can be reduced by using the small cap and a wide-bore tube. The greatest meniscus speed which can be measured at the maximum resolution of the camera is limited by the time taken for the LSC to scan the pixel array and send the data to the computer (14 ms). For maximum resolution, the distance travelled by the meniscus between scans should not exceed the distance along the capillary tube corresponding to the pixel spacing. This implies an upper limit of meniscus speed of 7 mm ss’, corresponding to an absorption rate of approximately 124 000 mL mm2h-’ when using the small cap and wide capillary tube. The instrument is capable of measuring even higher absorption rates but at the price of some loss of resolution. However, in practice, factors other than the scan time are more likely to limit the measurement of high rates of absorption. For example, a rapidly moving meniscus requires frequent refilling of the capillary tube. Each refill operation involves some loss of data and as the frequency of refilling the tube increases, the proportion of the total data lost in this way will eventually become unacceptably high. A further consideration at high rates of meniscus travel is the time taken for the supervising computer to write the received data to disk; this can interrupt regular data sampling if readings are taken at closely spaced time intervals. The highest rate of absorption measured during these tests was 44 000 mL m-2 h-’ and it is considered unlikely that the present optical and hardware configuration could measure absorption rates much in excess of 50000 mL mm2h-i.

Conclusions

3000

The initial surface absorption test procedure has been automated by the application of a line-scan camera and computer control to the determination of meniscus position. The apparatus permits continuous measurement over long periods and has shown great accuracy of measurement over a wide range of rates of absorption. The test can be applied to materials such as wood, which may swell during absorption, using the means developed for measuring the surface displacement and correcting the observed movement of the meniscus. 10

20

30

40

50

60

70

80

so

100

110

120

Elapsedtime(min) Figure 13 Variation

of absorption sapwood tangential sample

10

rate with time - Scats Pine

Acknowledgements The authors would like to thank Dr Maurice Levitt for helpful discussions during the course of this work.

Construction and Building Materials 1995 Volume 9 Number 1

Measurement

References 1 Whiteley, P., Russman, H. D. and Bishop, J. D. Porosity of building materials - a collection of published results. J. Oil Col. Chem. Assoc. 1977, 60, 142-150 2 BS 1881. Methods of Testing Concrete Part 5: Methods of testing hardened concrete for other than strength. British Standards

Construction

of surface water absorption:

A. E. Noble et al.

3

Institution. London, 1970 Levitt, M. Non-destructive testing of concrete by the initial surface absorption method. Proc. Symp. on Non-destructive

4

Institution of Civil Engineers, London, 1970, pp. 23-28 Banks, W. B. Factors affecting the introduction of preservatives into wood. Pestic. Sci. 1972, 3, 219-227

Testing of Concrete

and Building

and Timber, London, 11-12 June 1969,

Materials

1995 Volume 9 Number 1

11