Performance of polypropylene-reinforced cement corrugated sheeting

Performance of polypropylenereinforced cement corrugated sheeting E.G. KEER and AM. THORNE (University of Surrey, UK) Cement sheeting reinforced by continuous, opened networks of fibrillated polypropylene film has been developed as an alternative to asbestos-cement. Comparative tests on the behaviour of full-size polypropylene-reinforced cement corrugated sheet and asbestoscement sheet under simulated uniformly distributed load and under concentrated load are reported. The polypropylene-reinforced cement sheeting can sustain the loads recommended by International Standards and remain serviceable. The behaviour of the sheeting is quasi-ductile and the consequent high impact resistance is a considerable advantage over the brittle behaviour of asbestos-cement.

Keywords: composite materials; reinforced cement; polypropylene films; asbestos cement; corrugated sheet materials; load capacity

The growing awareness in recent years of the health hazard associated with the handling and working of asbestos fibres has led to considerable activity in the development of alternative cement-based composites) The size of the asbestos-cement market world-wide has been considerable, with an annual production probably in excess of 20 million tonnes. An alternative material in which the cement matrix is reinforced by layers of continuous, opened networks of fibrillated polypropylene film was developed by H a n n a n t and Zonsveld at the University of Surrey. 2'3 Commercial development of networks and composite has followed. 4,~ The use of continuous opened networks allows a sufficient fibre volume to be incorporated in thin sheets to ensure that, under increasing tensile load, fine multiple cracking of the matrix occurs (with consequent very fine crack widths) prior to composite failure due to fibre fracture, rather than fibre pull-out, at strains in excess of 5%. The resulting polypropylene-reinforced cement composite is, therefore, quasi-ductile in nature with a high impact resistance. The behaviour of strip and coupon specimens cut from flat sheets has been reported elsewhere in the hterature: -~° The UK and European market for asbestos-cement is dominated by its use as corrugated sheeting of various profiles. The main application of the sheeting is for the roofing and cladding of agricultural and industrial buddings. This paper reports tests conducted to establish the behaviour under load of full-size corrugated sheeting made from polypropylene-reinforced cement. Comparisons are drawn with the behaviour of asbestos-cement sheeting.

SERVICE LOADS ON CORRUGATED SHEETING A roofing or cladding sheet will have to be designed to sustain the following loads in service, in addition to its own weight: 1)

uniformly distributed (UD) roof loading to take account of snow loads and loads imposed during access for maintenance purposes (corrugated cement sheeting is unlikely to be used in circumstances where general access to a roof is permitted). In the UK this loading requirement is generally taken as 0.75 kPa. H In Western Europe, snow loading requirements vary with geographical location and may be up to and, exceptionally, in excess of 2 kPa) 2 Snow loads may be long-term in certain locations;

2)

wind loading short-term and essentially uniformly distributed. Pressure/suction values depend upon a number of factors but can be 2-3 kPa locally on sheets:

3)

concentrated loads arising from foot traffic, for example during erection or maintenance. In U K practice, the concentrated load is generally taken as 0.9 kN. 1~ Additionally, resistance to body impact is desirable, but little guidance is given in existing standards on the impact resistance deemed satisfactory. In acceptance tests specified in existing and proposed standards, UD loading is generally simulated by a simple loading arrangement (eg a single-line load at midspan ~3 or two-line loads at quarter span positions ~4) rather than the more realistic~ yet relatively complex

0010-4361/85/010028-05 $03.00 © 1985 Butterworth 8- Co (Publishers) Ltd 28

COMPOSITES. VOLUME 16. NO. 1. JANUARY 1985

system of airbags or similar devices. The two-line load system is considered preferable in that it simulates the bending moment and shear force distributions of the UD load case more accurately than the single-line load. Concentrated loading is simulated by loading through square rigid plates. Sizes specified vary between 100 × 100 m m and 300 x 300 mm.

Table 2. Dimensions and section properties of corrugated sheets

A major but generally unquantified stress, which may vary daily to give a significant fatigue effect, is induced by restrained moisture or temperature movements and gradients in the sheet. Rapid changes from dry to wet conditions have been known to crack asbestos-cement sheets in use. A material which is tolerant to such induced tensile strains may be useful in this respect.

Overall length (mm)

LOAD TESTS ON CORRUGATED SHEETS

Average thickness (mm)

Test specimens The asbestos-cement corrugated sheet tested was a typical, commercially available sheet of overall depth 57 mm, cross-sectional thickness 6-9 m m and pitch of corrugations 146 mm. It had been produced by the Magnani process. 15 Two polypropylene-reinforced cement sheets were manufactured by hand in the laboratory. The mix proportions of the matrix used are given in Table 1. The fibrillated polypropylene film used was supplied in opened packs of 4 layers, the modulus of elasticity and ultimate strength of the film networks, measured from tensile tests on the composite, being 2-3 G P a and 300-400 MPa, respectively. The corrugated sheets were manufactured in the laboratory as follows. Layers of flat sheet about 1 m m thick were first built by handworking the fresh matrix into layers of film networks laid on a flat surface. Each flat sheet was then laid into a corrugated mould (made of concrete using an asbestos-cement former similar to that tested) and pressed into the corrugations. The final thickness of the corrugated sheet was built up from a n u m b e r of flat sheets pressed successively onto the mould one on top of the other. The arrangement of the layers of film was such that a corrugated sheet reinforced with a film volume fraction (If) of about 5% in the longitudinal direction (ie parallel to the corrugations) and 3% transversely resulted. The film volume fractions were achieved by placing about eight layers of network longitudinally and five layers transversely per m m thickness of sheet. After 24 h under polythene sheets, water was ponded over the corrugated sheet and it was left to cure for 7 days, after which it was carefully removed from the mould and stored in air prior to testing.

Property

Asbestoscement

Polypropylenereinforced cement Sheet A

Sheet B

1820

1660

1810

Overall breadth (mm) 1080

920

1010

56

55

Overall depth (mm)

57

crest

9.8

9.1

7.2

side

6.1

8.0

6.3

trough

9.0

8.0

6.2

2885

2182

99

77

Second moment of area (mm4/m width) x 103 2643 Section modulus (mm3/m width) x 103

90

polypropylene-reinforced corrugated sheet B was deliberately made thinner than sheet A, and the smaller section properties are evident in Table 2. In addition, two flat sheets of polypropylene-reinforced cement composite approximately 600 × 600 m m were made from the same matrix proportions and arrangement of film layers as the corrugated sheets and cured in the same manner. Specimens 300 m m long by 25 m m wide were cut both longitudinally and laterally for testing under direct tension.

Test procedure The corrugated sheets were loaded in the test rig shown in Fig. 1. Deflections were measured at midspan and quarter span under each corrugation by displacement transducers linked to a data logger and micro-computer. Strain gauge readings were similarly monitored

The overall sheet dimensions as-tested and the calculated section properties are given in Table 2. The

Table 1. Composition of cement matrix, proportions by weight Cement

1.00

Pulverized fuel ash

0.25

Sand

O. 19

Water

0.34

Superplasticizer (Melment L10, Hoescht Chemicals)

0.01 7

COMPOSITES. JANUARY 1985

Fig. 1 Loading rig for simulated UD load; polypropylene-reinforced cement sheet under equivalent UD load of 6.1 kPa

29

at various positions over the sheets. Preliminary work was carried out to investigate the effect of different support conditions and loading configurations. A consequence of this preliminary work was the adoption in the test rig of individual, adjustable end supports to each corrugation. This overcame the necessity to pack beneath corrugations at supports in cases where sheets were warped and rested on only a few corrugations. Asbestos-cement sheets bought ex-stock were particularly bad in this respect.

14 12

~ ~o A

~

tMultiple erockin(Ji r region ~

~

_/

m ~tudl~o~

| / I-'l

I 2

I i 6 8 Strain (%) Stress/strain curves in direct tension for strip specimens

0

Fig. 2

~4

Tensile specimens

~d 7,3 o

Typical stress/strain curves for longitudinal and lateral polypropylene-reinforced cement specimens are shown in Fig. 2. The stiff, uncracked region is followed by a region of increased composite strain at approximately constant stress whilst multiple cracking of the matrix occurs. The post-cracking slope is essentially a function of the product of the film modulus and film volume fraction. The high strain capacity to failure is apparent. Of particular relevance to the load/deflection behaviour of corrugated sheet are the initial and mean composite cracking stresses (Crco and ~co, respectively) and the uncracked composite modulus, Ec (see Fig. 2). These values, summarized in Table 3 as average values of at least eight specimens for each direction from each sheet, can be used to obtain the matrix cracking strain for comparison with strains measured on the corrugated sheet.

I 4

A

5

no

RESULTS

t~

~.s S''

'~'

I I0

///

1 I.... ,, ...... ........ ,"",:" f / /.,~. / t ..... .". .""'" .,. ,"" / / i ... ..... .'" / I

..'"

..'"

..j

ii/

..."

I'/_ / I

/

,, / /

1 I

I.-"i

I.,9

5

I

I

~-reinfwc~acement

---

..."" / / ....." / /

0 Fig. 3

~i~n

- L.ateral specimen

/I

The flat sheet specimens were tested in direct tension in an Instron 1122 machine at a strain rate of about 10% per min. Strains were recorded over a 100 m m gauge-length by a clip-on extensometer ~6 linked to X-Y-Y chart recorders.

.~..-" ""

.--"'-

6

The tests to 'failure' reported here employed two loading beams at quarter-span positions (see Fig. 1). The test span for each sheet was 1.38 m, the recommended purlin spacing for the asbestos-cement sheet. Deflections and surface strains were monitored at increments of load.

Polypropylene-reinforced cement sheet A

........ Polypropylene-remforced' cement sheet B I

I

I

I

IO

I

I

I

I

15

1 I

I

I

20

Deflection (ram) Equivalent UD load vs deflection for corrugated sheets

are shown in Fig, 3. The loading has been expressed as an equivalent UD load per unit area of sheet between supports. The deflection is the average of the deflections recorded at midspan on the underside of each corrugation. 1. Asbestos-cement The asbestos-cement sheet failed at an equivalent UD load of 5.5 kPa at a deflection of span/200 or 7 mm. Failure was sudden and the sheet broke into two

Corrugated sheets under simulated UD load The load/deflection curves for the three sheets tested

Table 3.

I

Direct tensile test results Flat Sheet A

Flat Sheet B

Property Longitudinal

Lateral

Longitudinal

Lateral

%0 (MPa)

6.1

5.1

5.8

4.1

~co (MPa)

6.8

5.6

6.3

4.8

24.5

22.4

*

*

Ec (GPa) ~'mu (microstrain)

279

250

258

212

*Results from Sheet A used. Sheet B results unreliable due to warped specimens
30

~co

Ec

mean matrix cracking strain

COMPOSITES. JANUARY 1985

sections as a transverse crack propagated rapidly across the sheet beneath a loading beam. The extreme bending stress at failure was calculated, using simple beam theory, as 14.5 MPa, surprisingly less than the recommended minimum value of 15.7 MPa, ~7 although it must be noted that the latter value relates to sheets tested under midspan line loading, The modulus of elasticity calculated from the load/deflection curve was about 17 GPa, within the range of published values, TM and the calculated maximum strain at failure was 840 microstrain, which compares reasonably with the average measured value close to failure of 798 microstrain.

2. Polypropylene-reinforced cement The quasi-ductile behaviour and great toughness of polypropylene-reinforced cement sheeting is apparent from the shape of the load/deflection curve compared with that for asbestos-cement. The difference in performance between sheets A and B is to be expected from their relative thicknesses and section properties (see Table 2). It proved impossible to break the sheets and loading was halted when deflections increased rapidly for very small increments of load. Both sheets exhibited good recovery upon removal of load. Fig. 1 shows sheet A under the maximum equivalent UD load of 6.1 kPa at a deflection of about 20 mm or span/70.

Table 4 summarizes important values from the load/ deflection curves for the two polypropylene-reinforced cement sheets. The load at the limit of proportionality (LOP) was determined in accordance with the procedure (reproduced in Fig. 4) outlined in the Danish Standard for non-asbestos fibre-reinforced cement sheeting. 19 This is one of the first standards to recognize that a new generation of quasi-ductile cement sheeting material will require definitions of loads at the limits of serviceability (in this case close to the onset of cracking), in addition to failure or maximum loads. No cracks were visible with the naked eye at the LOP. The average values of longitudinal strain measured at midspan at loads close to the LOP on sheets A and B (267 and 291 microstrain, respectively) compared reasonably with the matrix cracking strain (279 and 258 microstrain) measured from direct tensile tests on flat specimens. Values of modulus of elasticity calculated from load/deflection curves were 21.0 GPa and 20.3 GPa for sheets A and B respectively, slightly lower than the value of 24.5 G P a from tensile tests.

Equivalent maximum UD load (kPa)

6.1

4.3

Equivalent UD load at LOP (kPa)

3.5

2.8

Water was ponded on some corrugations whilst the sheets were under a load of about 5 kPa, considerably in excess of the load required to be sustained and sufficient to crack the sheet. Although the underside of the sheet at each crack became damp, no water droplets formed even while cracks were 'open' under load, evidence that very fine crack widths had been achieved. Having been left for 12 h or more in the unloaded condition, the cracks had healed themselves, probably due to the continued hydration of previously unhydrated cemenL 20 No more water permeated through the sheets, and the underside was dry although water was still ponded in corrugations on the top surface.

Deflection at LOP (mm)

3.0

3.8

Concentrated edge loading

Equivalent UD load at a deflection of span/250 (kPa)

4.1

3.2

20.2

14.8

7.4

5.1

Table 4. Important values from load/deflection curves for polypropylene-reinforced corrugated cement sheeting Property

Sheet A Sheet B

Approximate maximum average deflection recorded (mm) Average residual deflection at zero load (mm)

/

I

Initial tangent

/

Pbr

The tests to 'failure' of the polypropylene-reinforced cement sheets were followed by loading the sheets at midspan with a concentrated load on a 125 X 125 mm plate positioned on an edge corrugation. Both sheets easily sustained loads in excess of 1 kN. The transverse

Maximum load,

l / I f

~

Pb,

. / ,,f

P~ A

.~I~

Load at the limitof

J

proportionality, Ppr

- ~ o.i a,,,

-5

v

-to

%.

-e.

0"4Pbr

"%, -15 _ --e- --e_ _ ..e,.....k.

0.I ~r UA

Deflection, U

Fig. 4 Load at the limitof proportionality according to Danish Standard for non-asbestos fibre-reinforced cement sheeting.19 (The initialtangent is drawn through points on graph at 0.1 Pbr and 0.4 Pbr)

COMPOSITES . JANUARY 1985

At zero load following UD loading Under concentrated load of 1.5 kN at midspon on on edge corrugation At zero load following concentrated load test

\ \

-20 Fig. 5 Transverse deflection profiles at midspan for polypropylenereinforced cement Sheet A

31

Hannant, for his helpful advice and Dr D.C. Hughes for his assistance with the development of the test facility.

REFERENCES 1 2

3 4

5

6

7 Fig. 6 Polypropylene-reinforced cement sheet supporting three people after conclusion of simulated UD and concentrated load tests

deflection profile at midspan for sheet a under an edge concentrated load of 1.5 kN is shown in Fig. 5. Additionally, the residual deflection profile after the quarter span line loading test and the final residual deflection profile after the concentrated load test are shown in Fig. 5. The m a x i m u m residual deflection recorded in either sheet upon completion of the tests was only span/120, and both sheets were subsequently able to support three people safely (see Fig. 6).

8 9

l0 ll 12

CONCLUSIONS 1)

2)

3)

4)

Polypropylene-reinforced cement corrugated sheets of similar size to a typical asbestos-cement profile can sustain the uniformly distributed loads recommended in International Standards without reaching a limit state of serviceability. It proved impossible to break the polypropylenereinforced sheets in the test rig and m a x i m u m loads were well above the loads required to be sustained. The sheets exhibited good recovery upon removal of loads. After tests to 'failure' under equivalent UD loads, polypropylene-reinforced cement sheets were still able to support the recommended concentrated loading on the edge of the sheet. The great toughness of the material is an important asset in its use as a roofing or cladding element.

13 14 15 16 17 18 19

20

Wells, IL "Future developments in fibre-reinforced cement, mortar and concrete" Composites 13 No 2 (April 1982) pp 169-172 Hannant, D.J. and Zonsveid, J.J. 'Polyolefin fibrous networks in cement matrices for low cost sheeting' Phil Trans Roy Soc. London A294 (1980) pp 591-597 Hannant, D.J., Zonsveld, J.J. and Hughes, D.C. 'Polypropylene film in cement-based materials' Composites 9 No 2 (April 1978) pp 83-88 Bijen, J. and Geurts, E. 'Sheets and pipes incorporating polymer film material in a cement matrix" Concrete International 1980." Fibrous Concrete Proc (The Construction Press Ltd, Lancaster, UK) pp 194-202 Vittone~ A. 'Net like products in polypropylene fibrillated films - - production and applications' 3rd lnt Confon Polypropylene Fibres and Textiles (Plastics and Rubber Institute, 1983) pp 40.1-40.10 Galloway, J.W., Williams, R.I.T. and Raithby, K.D. 'Mechanical properties of polyolefin-reinforced cement sheet for crack control in reinforced concrete' TRRL Supplementary Report 658 (Transport and Road Research Laboratory, Department of the Environment, Crowthorne, UK, 1981) Hughes, D.C. and Hannant, D.J. 'Brittle matrices reinforced with polyalkene films of varying elastic moduli' J of Mater Sci 17 (1982) pp 508-516 Keer, J.G. 'Behaviour of cracked fibre composites under limited cyclic loading' Int J of Cement Composites and Lightweight Structures 3 No 3 (August 1981) pp 179-186 Hibbert, A.P. and Hannant, D.J. 'Toughness of cement composites containing polypropylene fibres compared with other fibre cements' Composites 13 No 4 (October 1982) pp 393-399 Hannant, D.J. 'Durability of cement sheets reinforced with fibrillated polypropylene networks' Magazine of Concrete Research 35 No 125 (December 1983) pp 197-204 'British Standard Code of Practice for P,erformance and Loading Criteria for Profiled Sheeting in Building' BS 5427." 1976 'Design Loads for Buildings, Snow Load and Ice Load' DIN 1055: Part 5: 1975: German Standard 'Asbestos-cement products - - Corrugated Sheets and Fittings for Roofing and Cladding" ISO/DIS 393. International Standard 'Standard Methods of Conducting Strength Tests of Panels for Building Construction' ASTM E72 American National Standard Ryder, J.F. 'Applications of fibre cement" Fibre-Reinforced Cement and Concrete, RILEM Symposium 1 (Construction Press Ltd, 1975) pp 23-35 de Vekey, R.C. 'An LVDT extensometer for tensile studies of composite materials' J Mater Sci Letters 9 (1974) pp 1898-1900 'Corrugated Sheets' BS 690." Part 3: 1973: Specification for Asbestos-Cement Slates and Sheets Allen, H.G. 'Tensile properties of seven asbestos cements" Composites 2 No 3 (June 1971) pp 98-103 'Corrugated Sheets of Fibre-reinforced Cement for Roofing' DS/R 1112-1114 (Danish Standards Institution, Dansk Standardiseringsrad, 1979) Hannant, D.J. and Keer, J.G. 'Autogenous healing of thin cement-based sheets" Cement and Concrete Research 13 (1983) pp 357-365

AUTHORS ACKNOWLEDGEMENTS The authors would like to thank the Head of the Construction Materials Research Group, Dr D.J.

32

The authors are with the Department of Civil Engineering, University of Surrey, Guildford, Surrey, GU2 5XH, UK. Inquiries should be addressed to Dr Keer in the first instance.

COMPOSITES. JANUARY 1985