Nondestructive Testing of Moisture in Cellulose Fibre Cement Boards

Tomasz GORZELAŃCZYK, Krzysztof SCHABOWICZ Faculty of Civil Engineering, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27,

50-370 Wrocław, Poland, e-mail: tomasz.gorzelanczyk@pwr.edu.pl, k.schabowicz@pwr.edu.pl

Abstract

This paper presents nondestructive tests of moisture in cellulose fibre cement boards. Cellulose fibre cement

boards are commonly used in architectural engineering in many exterior and interior applications, such as façade,

wall or roof cladding. The moisture content of such boards after final curing is one of their vital parameters,

determining their further treatment. The moisture content of cellulose fibre cement boards was tested in the

course of their production process. Maps of board moisture distribution were plotted on the basis of the NDT

results. The nondestructive testing method has been found useful for determining the moisture content in

cellulose fibre cement boards and detecting defective areas in them and so it seems to be a promising factory

production control tool.

Key words: nondestructive testing, cellulose fibre cement boards, moisture

1. Introduction

Fibre cement elements, also called cellulose fibre cement elements, have been used in

construction for over 100 years. They were invented by the Czech engineer Ludwik Hatschek.

In 1900 he developed and patented a technology for manufacturing light, tough, durable and

nonflammable asbestos cement sheeting which he called “eternit”[1, 2].

Eternit sheeting has become one of the most popular roofing materials in the 20th century

until it was conclusively proven that asbestos is harmful to health. As a result of the action of

many international and national medical associations and administrative bodies and

institutions in the world, eternit production has been discontinued, eternit products have been

phased out of use and the dangerous to health component has been replaced with safe

cellulose fibres [1-3].

Today cellulose fibre cement boards are manufactured from clean and harmless raw materials.

They are made up of 50-70% cement. The other components are different kinds of mineral

fibres (usually cellulose) and fillers (e.g. limestone powder, kaolin, perlite, quartz sand). Such

boards do not contain asbestos or other harmful components. Besides being durable, they are

characterized by high bending strength, moisture resistance and biological corrosion

resistance [1-3].

Cellulose fibre cement boards are typically used as ventilated façade cladding for both new

built and renovated buildings, interior wall covering, balcony balustrade filling, base course

and chimney cladding, enclosure soffit lining and so on. Cellulose fibre cement boards can be

applied to unfinished, painted or merely impregnated surfaces [1-3].

This paper describes the manufacture of cellulose fibre cement boards, with a special focus on

changes in the moisture content of the board in the course of this process, and presents the

results of the determination of the moisture distribution in finished boards after all the

production stages, by the drying-weighing method and the nondestructive dielectric method as

well as an analysis of the results.

11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech RepublicM

ore

Info

at O

pen

Acc

ess

Dat

abas

e w

ww

.ndt

.net

/?id

=16

337

2. Process of producing cellulose fibre cement boards

Figure 1 shows a sample flowchart of the technological process of producing cellulose fibre

cement boards with distinguished seven zones occupied by the particular production stages. In

each of the stages the board has a different moisture content, which is investigated by means

of nondestructive and destructive test methods [4-6].

Fig. 1. Flowchart of technological process of producing cellulose fibre cement boards.

The production of cellulose fibre cement boards starts in the preparation zone (zone 1) where

cellulose fibres are mixed with water in a mixer (pulper) until they are uniformly dispersed.

Loose components (cement with additions) in specified proportions are introduced into the

batched water and mixed until a homogenous plastic compound is obtained. Then the

cellulose cement mixture passes to a board forming machine. The Hatschek or flow-on

machine is used (zone 2) to form boards with a set thickness of 4-14 mm. The next (optional)

production stage is the pressing of stacked boards (zone 3). A pressing force proper for the

type of board (exterior or interior cladding board) is applied. Directly after pressing or

forming the warm (due to cement hydration heat) cellulose fibre cement boards are

transported to a pre-curing tunnel (zone 4) where they remain for about 14 hours. After this

stage the boards can be taken off the stack and placed on a pallet. But this should be done as

quickly as possible since the boards are still quite warm and moist and they should not cool

and dry too much or too quickly. After they are placed on pallets the boards mature and are

cured in steady thermal-moisture conditions in, e.g., special airtight tarpaulin tents (zone 5)

where they remain for about 14 days. During this time the boards acquire the proper bending

strength and get rid of some of the moisture in a natural way. After the maturation period the

boards pass to a final drying oven (zone 6) where they are subjected to three-stage drying at

the temperature of 180°C, 160°C and 120°C in respectively stage 1, 2 and 3. Then the boards

are naturally cooled, which takes about 20-30 minutes depending on board thickness. This is a

critical stage in the production process. Boards with a too high moisture content are not fit for

further treatment, such as impregnation or painting. In the last stage of their manufacture the

boards are trimmed and, if necessary, their surface is ground at the edges (zone 7).

The moisture content of the boards changes very significantly in the course of the production

process. Moisture content tests and studies of moisture content change in the course of

production, carried out using destructive and nondestructive methods are presented below.

3. Tests of moisture content in cellulose fibre cement boards

As already mentioned, cellulose fibre board moisture content is a key parameter supplying

information about the quality of the board and its treatment during the production process.

Thus it is essential to measure and control board moisture content at each of the production

stages. As part of this research, the authors tested the moisture content of the exterior cladding

(façade) and interior cladding boards in mainly the final production stage, i.e. after they left

the final drying oven. The approximate compositions of the tested boards are shown in table

1.

Table 1. Approximate compositions of cellulose fibre cement boards.

No.

Kind of raw material

Content in %

Type

of board Exterior cladding board Interior cladding board

1. Filler 27-32 33-43

2. Cellulose 6-12 6-10

3. Reinforcing fibres 0.25-1.25 -

4. Cement 50 -70 44-64

5. Hydrophobic agent 0.30-0.90 -

The tests were carried out using the drying-weighing method and the nondestructive dielectric

method (by means of a Trotec T650 meter). Nondestructive tests would be conducted on the

top surface of stacked 1200 × 3050 mm boards. Measuring points located at least 100 mm

from the board edges and about 200-255 mm from one another, as shown in fig. 2, were

selected. There were 72 measuring points on each board. In total about 300 cellulose fibre

cement boards were tested.

Fig. 2. Location of moisture content measuring points on cellulose fibre cement board.

The authors developed their own graduation curve, shown in figure 2, for determining the

moisture content of the cellulose fibre cement boards. Sample moisture content test results for

one batch of cellulose fibre cement boards are presented in table 2.

y = 4E-07x4 - 0,0001x3 + 0,0117x2 - 0,3902x + 4,1875

R² = 0,9656

0

5

10

15

20

25

30

0 20 40 60 80 100 120 140 160 180

Mo

istu

re c

on

ten

t in

[%

]

Measured value [digits]

Fig. 2. Graduation curve for determining moisture content of cellulose fibre cement boards.

Table 2. Sample test results for moisture content in fibre cement boards.

Board symbol Measuring point Moisture content in %

Exterior cladding board Interior cladding board

PZ

1/07.08.2013/ 1 7.23 -

2 7.14 -

3 6.90 -

4 6.54 -

5 6.13 -

6 5.57 -

7 5.19 -

9

.

.

4.88

.

.

-

72 3.37 -

PW

47/16.09.2013/ 1 - 8.50

2 - 8.39

3 - 8.12

4

.

.

-

7.69

.

.

72 - 3.97

The drying-weighing method was used, in accordance with [5, 7, 8-10], to determine the mass

moisture content (wm) in the earlier production stages. This method was also used to verify the

results obtained by the dielectric method. For this purpose small samples were cut out in the

places where the dielectric measurements had been carried out. The samples would be

weighed and subsequently dried at a temperature of 105°C to a constant mass. Then the mass

moisture content in per cent would be calculated from the following relation:

[ ]%

s

swm

m

mmw

−= …………………………………(1)

where:

mw – the mass of the sample with the actual moisture content [g],

ms – the mass of the sample dried at 105°C [g].

4. Results of tests of moisture content in cellulose fibre cement boards and

their analysis

The experimentally determined mass moisture content (wm) of the cellulose fibre cement

boards at all the production stages is shown in table 3.

Table 3. Results of tests of moisture content in cellulose fibre cement boards at

particular production stages.

State of board Production stage

(zone)

Test method Determined moisture content wm [%]

Liquid fibre cement

mixture 1 - 100

Freshly formed board 2 drying-weighing 35 – 50

Board after pressing 3 drying-weighing 25 – 35

Board after passing

through pre-curing tunnel 4 drying-weighing 20 – 25

Board after 14 days of

maturing in tents 5

dielectric / drying-

weighing

12 - 16

(exterior-cladding board)

13 - 18

(interior-cladding board)

Board after passing

through final drying oven 6

dielectric / drying-

weighing

3 - 7,5

(exterior-cladding board)

4 - 8,5

(interior-cladding board)

Figures 4 and 5 show exemplary maps of the distribution of mean moisture content in the

tested exterior and interior cladding fibre cement boards.

6-8 4-6 2-4 0-2 Moisture content in board [%]

Direction of travel of board in oven

Fig. 4.Map of distribution of mean moisture content in exterior cladding (façade) board (no. 32).

8-10 6-8 4-6 2-4 0-2 Moisture content in board [%]

Direction of travel of board in oven

Fig. 5. Map of distribution of mean moisture content in interior cladding fibre cement board (no. 176).

An analysis of the results presented in table 3 shows that the moisture content of the board

very significantly changes in the course of the production process. With regard to the quality

of the finished board, the moisture content in the board after it passes through the final stage

(the example of the final drying oven shown in fig. 6) is critical.

It clearly emerges from the maps of the distribution of mean moisture content in the boards

that the distribution is uneven along the board length. This applies to both the exterior

cladding (façade) board, in which the final moisture content ranges from 3 to almost 8%, and

the interior cladding board in which the moisture content is similar, ranging from 4 to almost

9%. This a rather large, but unfortunately typical, scatter of moisture content values in the

board after it passes through the final drying oven. This is probably due to the oven design

and the location of drying agents in the oven. On the basis of the test results a board moisture

content control system was introduced and improvements in the oven structure were made

whereby the distribution of heat during the drying of the boards and consequently the

distribution of moisture content in both the exterior cladding board and the interior cladding

board became uniform.

Fig. 6. View of the example of final drying oven used in production of cellulose fibre cement boards.

5. Conclusions

As part of this research, maps of the distribution of mean moisture content in the finished

(after passing through the final drying oven) exterior cladding (façade) and interior cladding

boards were plotted. Moisture content measurements were also carried out for the boards at

the earlier stages in the production process in order to determine the changes in board

moisture content in the course of the whole production process, whereby a proper board

quality control system could be proposed.

The test results indicated that the distribution of moisture content was uneven. In the authors’

opinion it was due to the atypical design of the final drying oven. It should be noted that the

moisture content in the cellulose fibre cement boards is a key parameter indicating their

fitness for use and durability. The nonuniform distribution of moisture content adversely

affects not only the strength parameters of the board, but also its further treatment,

particularly its impregnation, varnishing or covering with some other decorative structure.

Such boards are characterized by poor surface adhesion, which shortens their service life

under variable weather conditions.

It emerges from the tests that the dielectric method of measuring moisture content is not

highly precise due to the scatter of results and the rather low accuracy of the nondestructive

measuring equipment. In the authors’ opinion, other nondestructive testing methods, such as

the noncontact ultrasonic method exploiting Lamb waves, could be used to measure the

distribution of moisture content in cellulose fibre cement boards and to detect material

imperfections in them [11, 12]. Pilot tests of cellulose fibre cement boards by means of an

ultrasonic scanner designed specifically for this purpose has been successfully carried out and

their results are reported in [12]. Changes in moisture content are expected to have an

influence on the amplitude of the Lamb wave. The explanation of this phenomenon will be

the subject of further investigations and studies by the authors.

References

1. Information acquired from the webpage: http://www.euronit.de/.

2. Information acquired from the webpage: http://www.cembrit.com/.

3. EN 12467 – ‘Fibre-cement flat sheets. Product specification and test methods’, 2013.

4. American Concrete Institute Report ACI 228.2R-98, ‘Nondestructive Test Methods for

Evaluation of Concrete in Structures’, ACI, Farmington Hills, Michigan, 1998.

5. J Hola and K Schabowicz, ‘State-of-the-art nondestructive methods for diagnostic testing

of building structures – anticipated development trends’, Archives of Civil and

Mechanical Engineering, 11, pp. 5-11, 2010.

6. V M Malhort and J N Carino, ‘Handbook on nondestructive testing of concrete’, CRC

Press, 2004.

7. A M Neville, ‘Properties of concrete’, 2000.

8. J Hoła and Z Matkowski, ‘Selected problems relating to damp-proof protections of walls

in existing masonry building structures’ (in Polish), 24th Scientific-Technical Conference,

Szczecin-Międzyzdroje, 26-29 May 2009 Szczecin, Wydawnictwo Uczelniane

Zachodniopomorskiego Uniwersytetu Technologicznego, pp. 73-92, 2009.

9. J Karyś and K Zwierzyński, ‘Measurement of moisture content in building space dividers’

(in Polish), 5th Workshop for Mycological Building Surveyors, PSMB, Wrocław, 2006.

10. Z Matkowski and A Pawlonka, ‘Analysis of nondestructive methods of concrete moisture

content testing’ (in Polish), 1982.

11. R Drelich, T Gorzelańczyk, M Pakula, K Schabowicz, ‘NDT testing of cellulose fibre

cement boards using non-contact ultrasound’, e-Journal of Nondestructive Testing &

Ultrasonics, No 6, pp. 21-28, 2013.

12. S Yashiro, J Takatsubo, N Toyama, ‘An NDT technique for composite structures using

visualized Lamb-wave propagation’, Composites Science and Technology, 67, pp. 3202-

3208, 2007.