GRP composite materials in construction: properties, applications and durability

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N 2 i J

National Research

Conseil national

'

8

23

I

Council Canada

de recherche Canada

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GRP COMPOSITE MATERIALS IN CONSTRUCTION:

PROPERTIES, APPLICATIONS AND DURABILITY

-nn,

by

Aurel Blaga

3

Reprinted from

Industrialization Forum

VoL

9,

No.

1,

1978

p. 27 32

DBR Paper No. 823

Division of Building Research

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SOMMAIRE

L'article porte sur la nature et les caractsristiques des materiaux en polyester renforcE 21 la fibre de verre (PRV) et traite brisvement de leurs principales applications dans le domaine du bztiment. L'influence de la composition, du type de renforcement, des techniques de fabrication et de la nature du fini de surface sur les propri6tEs et la durabilits des matEriaux en polyester renforcs 5 la fibre de verre est analysEe. La nature et le mscanisme de di5tErioration des composEs en polyester renforcg 5 la fibre de verre dans divers milieux sont illustrEs par des micro photographies obtenues 5 partir d'un microscope Electronique

B balayage, en mettant l'accent sur la dGtGrioration causEe par les intempsries. Le r3le de leur composition et du milieu sur la d6thrioration des matgriaux, qui commence d'ordinaire 5 la surface extcrieure, est discut6 5 la lumiere d'studes faites 1 la DRB/CNR.

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Application. Building Products. Cracks(Fisurrs). Deterioration. Labomtory Tests. Manufacturing Processes, Physical Properties, Weatherability,

Articles., Blaga., Industrialization Forum., Year 1978..

GRP

Composite Materials in Construction

:

Properties,

Applications and Durability.

Aurble Blaga.

The nature and properties of glass-fiber reinforced polyester (GRP) materials are described, and their prin- cipal applications in construction outlined. The influence of formulation, type of reinforcement and fabrication technique, and the nature of the surface finish, on the properties and durability of the GRP composite material, is analyzed.

GRP composites are common in construction; the main components are the matrix and the reinforcing glass, which cooperate to provide the special properties of the composite. Production techniques include hand lay-up, continuous production and matched-die molding. Properties depend on composition and fabrication, e.g. resin formulation, fillers, curing, reinforcement, coupling agent, surface finish and work- manship. GRP composites are used as light-transmitting panels, sandwich panels, modular units and for miscellaneous applications (storage tanks, formwork, windows etc.) Breakdown occurs at the glass- resin interface and by surface micro-cracking.

The text is accompanied by microphotographs and by an extensive bibliography.

Introduction.

Glass-fiber reinforced polyester (GRP) composites are the most common of all reinforced plastics used in the construction industry. Glass-fiber reinforced polyester is also referred to as FRP (fiber-glass reinforced polyester). Sheet-like materials are often called "GRP laminates".

GRP composites are a range of materials ; depending on the formulation and use requirements, they may be fabricated into products (structures and articles) that are light in weight, transparent, translucent or opaque, colorless or colored, flat sheet or shaped. Further- more, there is no limit as to the size of object that can be made from GRP.

General Nature and Fabrication of

GRP

com- posites.

The two main components of a GRP composite are the matrix and the reinforcing glass. The matrix is the continuous phase. In itself, the matrix does not provide strength; its role is essential, however, since it serves to bond the reinforcing glass-fiber together and to transfer the load to the reinforcing phase. The matrix is based on cured thermosetting polyester resinl.2. The raw material is supplied in the form of a viscous, syrupy liquid, and comprises the following basic ingredients : a linear unsaturated polyester ; a crosslinking monomer (curing agent), usually styrene; and an inhibitor to retard crosslinking until the resin is to be used by the fabricator. Other ingredients,

which can be added either by the resin manufacturer or by the fabricator of the GRP product, are fillers, pigments, fire retardants, ultraviolet (u.v.) light stab- ilizers and thixotropic agents.

A catalyst (initiator) and glass reinforcement are added by the fabricator and the resulting mixture is then ready for the production of the GRP item. During fabrication, the monomer reacts with the polyester through free radical initiation, resulting in crosslinking of the polyester chain and final cure. The operation can be carried out with or without application of external heat, depending on the catalytic system2.3. The ultimate result is a rigid solid material in which the matrix has joined chemically and mechanically with the reinforcing glass-fibers to provide a synergistic composite structure whose properties are very different froh, and significantly superior to, those of either material alone.

The glass reinforcement provides the strength for the GRP composite. It is used in bundles of fibers or filaments (diameter ranging from 0.005 to 0.01 mm or more) combined to form a strand. Glass-fiber reinforcement can be of several types, the most important being chopped strand, chopped strand mat, roving or cloths. Chopped strand mat is the most widely used form of glass reinforcement, especially in sheet- like materials. The strands (2 to 5 cm long) are distributed randomly. A coupling agent, usually a silane derivative, is added and serves to bond the glass reinforcement to the resin. Use of an appropriate coupling agent with a particular formulation and for a Vol. 9, (1978), No. 1.

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Type of Reinforcement

Propew* Chopped-Strand Parallel 143 Fabric Mat or Premix Roving Parallel Laminated Glass Content,

(by weight, O h ) 25

-

45 50 - 70 62 - 67

Specific Gravity 1.4

-

1 8 1.7

-

1.9 1.7 - 1.9

Tensile Strength, MPa 76

-

160 550

-

900 540

-

600

(103 psi) 1 1 -23 80 - 130 78 -87

Tensile Modulus, MPa 5.6

-

12

-

31

( lo6 psi) 0.82

-

1.8

-

4.5

Flexural Strength, MPa 140

-

260 690 - 1400 590

-

720

(103 psi) 20 - 38 100

-

200 85

-

105

Flexural Modulus, GPa 6.9

-

14 34

-

49 31

-

38

(lo6 psi) 1.0 -2.0 5.0

-

7.0 4.5 - 5.5

Compressive Strength, 120

-

180 340

-

480 280

-

340

MPa (lo3 psi) 18-26 50

-

70 40

-

50

The matrix is based on general purpose unsaturated thermosetting polyester resin. " Resistance to continuous heat (150-205°C; 300-400°F)9.

Table 1. Physical properties of glass-fiber reinforced polyester* sheet (reinforced with various glass

fiber constructions) 12.13.

specific application is very important. Good bonding results in good mechanical and electrical properties, improved resistance to moisture and to thermal effects of the service environment2.4. This is particularly important for fibers close to the surface, which are subjected to greater stresses induced by the environment 596.

The performance of a GRP material in a given application will depend to a large extent on the method of man~facture2,~.8. GRP components for construction applications may be made by any of the conventional techniques, including hand lay-up, continuous process, spray-up process, cold press or hot press molding, and filament winding2.3J. The actual method of manufacture will be determined by the number of components which are needed, their di- mensions, the properties required and cost. A designer must, therefore, determine the method of manufacture of a component at an early stage in the design.

The hand lay-up, the oldest method and one that

requires a minimum of equipment, is at present used to make large, unconventional components and a relatively small number of small parts whose size and/or quantity would not justify the expense of production tooling. This method consists of impregnating the glass-fiber reinforcement with the liquid, thermo- settable resin in an open mold (of wood, plaster, concrete, or reinforced plastic) and inducing cure at room temperature with little or no external pressure2s3.

Currently, sheeting material for translucent flat and corrugated panels is made mainly by a continuous

process 3. The glass-reinforcement (strands of chopped

roving or chopped strand mat) is deposited on a film of regenerated cellulose supported by a moving conveyor belt, compactors and/or impregnators force the resin into the mat to thoroughly wet the fibers, then an upper film is applied to encase the material. GRP profiles, such as angles and channels, can also be made by this process. The major advantages of the

continuous process are uniformity of product, efficient use of raw material, higher production capacity and lower labor costs. This process is not yet fully automated; a skilled operator is needed to maintain desirable efficiencies and product quality.

One of the newer processes3.9, allowing for more automation, uses sheet molding compound (SMC) in

matched-die molding. SMC's, which are in a sheet-like

form, consist of a complets blend containing catalysed, unsaturated polyester resin, a reinforcement, filler, and pigment. The SMC is supplied in rolls, with the unfinished sheet-like composite packed between films of polyethylene. It permits molding of large and complicated pieces that may be varied greatly in thick- ness and that have molded-in ribs, inserts, bosses or threads, sharp radii and comers as well as other modifications. The fabricated parts have a smooth molded surface, without fiber pattern, resulting in an improved surface wear layer and therefore superior chemical and weather resistance than in parts made by hand lay-up. Products made by this method are used mainly in automotive applications, but their use is now spreading to furniture and construction applications.

Properties.

The properties of the finished GRP composite material depend on a great number of compositional and fabrication factors, some of the most important being resin formulation, filler, curing conditions, type and amount of reinforcement, coupling agent, fabrication process and workmanship (which governs density, void content, presence of contaminants), and surface finish.

GRP composites based on general purpose resin, a

thermosetting material, are rigid and do not flow

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appropriate choice of ingredients, special properties can be achieved 3.10.H. For example,$re retardance can

be imparted by incorporating appropriate additives, but a preferable way is by modifying the basic unsaturated polyester resin to obtain built-in fire resistancelO. Weathering resistance can be enhanced by the use of neopentyl glycol and methyl methacrylate. To reduce shrinkage during curing, appropriate thermoplastic polymers can be added2.

GRPs that are reinforced with E glass fibers (those with low alkali content) have good resistance to weathering, water and alkali; those with A glass fibers (high alkali) are considered to be more resistant under acid conditions but do not weather well. GRP reinforced with chopped strand mat is essentially isotropic, whereas cloth fabric reinforaement and roving give a material which is anisotropic in character with varying directional properties

'.

The glass content of GRP composites affects strength properties and durability. The higher the glass-fiber content, the stronger the material. However, too high a glass content may result in insufficient impregnation, and therefore poorer bonding. The glass content of GRP reinforced with chopped strand mat generally varies between 25 to 35%; for GRP reinforced with cloth, the glass content ranges from 50 to

63%. Sheet material manufactured by hand lay-up

process will have lower strength properties than those fabricated by a press-molding process.

Because of the great number of factors which define a GRP composite, the range of mechanical and other physical properties is very wide. For example, the tensile strength at room temperature may vary from 69 MPa (lo4 psi) to 896 MPa (13 x lo4 psi) or higher, wet strength retention from 50 to 95% and specific gravity from 1.2 to 1.9. The range of some physical properties given in Table I are typical for . GRP sheet materials produced with normal care from general purpose polyester resin and reinforced -wiA

-

three types of glass-fiber r e i n f o r c e m e n 3 ~ i r g corn-

&?l: et data on physical p r o p g ~ 6 f GRP composites can be-fouiX1n~kErZn~es 3, 8 and 9.

Applications in Construction.

GRP composites used in construction can be divided into two main classes: the standard items of manufacture such as single skin sheet (flat or corrugated) or sandwich panels, and the custom structures de- signed for a given application by an architect or en- gineer. As details of the manufacturing processes, properties and structual capabilities for standard products have been established2~7~'2~'"'6 it should be relatively easy to incorporate them into a design Is. Regarding the use of custom-tailored structures, the building designer should consider the basic character- istics, behavior of the material and the manufacturing processes when selecting the appropriate material for a given application ',la.

Light-transmitting Panels.

Because of their relatively high light transmission

(about 85%), light weight, toughness and, where appro- priate, fire retardancy, transparent and translucent GRP panels (usually corrugated) have a variety of uses, including glazing for skylights, luminous ceiling or roofing, inner partitions, canopies for warehouses, rail- way stations, sports arenas, swimming pools and agricultural buildingsl"l9. A special grade of GRP sheeting has been developed for use as glazing in flat-plate solar collectors 20.

Opaque and Sandwich Panels.

GRP sheet materials are used as cladding on other structural materials or as an integral part of either a structural or a non-load-bearing wall panel. In the former, it functions as a decorative cladding on structures of concrete or brick, as it provides a wide range of colored and textured surfaces. The GRP sheet used is usually opaque, but it may also be translucent or transparent.

As an integral part of either a structural or non- load-bearing panel, opaque GRP can be used in a variety of ways in conjunction with other materials as cladding for buildings 15,19,21-24. In these panels,

GRP is invariably the exterior skin. The most popular panel is the sandwich type, with an inner and outer skin of GRP and a foam core of PVC, polystyrene, polyurethane or phenolic p l a s t i ~ ~ . ~ ~ , ~ ~ . The successful use of foamed core prefabricated sandwich panels as exterior cladding for large buildings has been re- p0rted25,~~. The prefabricated panels are light in weight, and thus easy to install with minimum equipment and low labor cost. Another application for foam-cored GRP-faced sandwich panels is as wall panels in mobile homes and in the fabrication of boat hulls.

_i

I

.

.GRP Composite in$e%%Gd-&d-~<dular Units;--'

i.

'. Ease

of

t a b d o n of large components and simplicity

i & r t i d i n addition

9

other desirable properties, _..

-'

_such

as lightness, makes GRP composite materials particularly suitable for use in modular construction 23.26.

Building components or sections are prefabricated and

rapidly assembled on the building site. Such structures are p&ticularly suitable on sites where access is limited or where the ground cannot support traditional structures without excessive cost for foundations. Examples include living accommodation and laboratories at An- tarctic bases, lighthouse towers, desert accommodation, living accommodation for those working on off- shore drilling rigs. The walls of sections or modules usually consist of GRP sandwich panel with a plastic foam or a honeycomb core.

Miscellaneous Applications.

GRP i s a very convenient structural material for the prefabrication of bathroom units and componentsZ6gz7.

GRP

composite made by press molding is used in the fabrication of cold and hot water storage tanks. When

the matrix is made from a special type of polyester resin, thorough testing has shown that the tank can withstand the action of pressure and temperature2. GRP composite is also used in the fabrication of window frames and concrete formwork 2,'6.

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Comment : Durability.

Although considerable progress has been made to improve GRP composites, commercial products of these materials still deteriorate when used out of doors. The deterioration usually starts at the outer surface ; its rate of occurrence and extent depend on the composition of the material, manufacturing method, degree of cure, nature of surface finish and service environment. Recent studies performed at the Division of Building Research, National Research Council of Canada on sheet GRP materials have demonstrated that there are two main types of surface deterioration5.6: breakdown in the glass-resin interface which results in fiber prominence ("fiber pop-out") and surface microcracking of the matrix ("surface microcracking"). Both affect principally the appearance of the GRP sheet and its light transmission. When surface deterioration is severe. mechanical and other ~ r o ~ e r t i e s

.

are also

adversely'affected. Other changes occuning in outdoor Figure 1. Surface of GRP Control Sheet. exposure are discoloration and surface pitting, which

may also affect the physical properties of the sheet. Studies at DBR included subjecting GRP sheets to artificial and natural weathering5.6 and assessing the surface deterioration by the scanning electron microscope(SEM). To illustrate the nature and some of the main stages of surface deteriorgtion, SEM micrographs are presented in Figures 1 to 6.

Based on the results of this study, a mechanism was postulated for each of the two main types of surface breakdo~n6.28~29. Fiber prominence occurs long before surface microcracking because the glass-resin interface in the surface region is more susceptible to brpkdown than the more homogeneous matrix. In- deed. owing to great dissimilarities in most properties (inchrdmg thermal k x p a n s i b n i i t e r absorption and strain response), the main compone of the interface respond differently to the action

"&,

of en 'ronmental

factors such as moisture and temnerature. Thus t k -

-

_-

cyclic variations (natural fluctuatioAs) of moisture and ~ i ~ u r c 2 . - ~ a c e of GRP Sheet aged in the weathering temperature in the outdoor environment produce dif- maclfin- 55 cycles.

1- - - - - -

_

-_--

ferential volume changes in the glass and resin at the

.--

interface. This results in alternating stresses which im- pose a stress-fatigue on the matrix of the interface region. These physically induced stresses produce rup- ture of the matrix resin, mostly parallel to the glass fiber. The cracks along the glass fibers (fig. 2) may occur after relatively short periods of exposure, depending on the intensity of stresses at a particular site. Solar radiation causes the resin to become quite brittle, as a result of post-crosslinking ; thus the matrix at the interface may suffer severe fragmentation or spalling (fig. 3). The rate of breakdown is greater in glass-rich regions (multifilament locations) than in resin- rich regions (at single filaments and at sites with few filaments). The glass fibers eventually become com- pletely delaminated. The delamination becomes excessive with aging and results in a great number of fibers lying on the surface of the sheet, but still retained at some points of their length by the matrix (fig. 4)

This type of deterioration is commonly called "fiber _Figure_3. _{Surface of GRP Sheet aged in the weathering} prominence" or "fiber pop-out". The underside of a machine for 300 cycles.

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GRP sheet also undergoes this type of breakdown, but at a considerably slower rate (fig. 5).

Although fiber prominence may be induced just by humidity-temperature cycling, the simultaneous exposure to radiation greatly promotes its formation, especially in the later stages of deterioration.

Surface microcracking generally occurs on the side exposed to solar radiation, usually after fiber prominence has become relatively advanced5.29. Results have indicated that it is caused by radiation and stress fatigue, which is induced by moisture and temperature. In accordance with the proposed mechanismz9, the resin in the exposed surface of the GRP sheet undergoes gradual crosslinking, due to the influence of the ultraviolet portion of the radiation. This results in shrinkage that produces permanent tensile stresses in the surface matrix, with gradients from the surface

Figure 4. Surface of GRP Sheet (top side) weathered h ~ a r d . When the surface resin rw~ches a certain

outdoors for 12 years. degree of rigidity as a result of post-crosslinking, it can no longer deform reversibly under the action of

_-

stress-fatigue, and thus undergoes fracture. The exposed surface of GRP sheet weathered outdoors for 12 years is shown in Figure 6. The cracks grow from the surface inwards and are V-shaped, indicating that the stresses involved are tensile and have a gradient. The cracks are confined to the surface region (5 to 10 pm deep) of the exposed side. These studies have demonstrated that breakdown induced by exposure to outdoor weathering can be reproduced in the laboratory.

The scattered fibers, the very irregular surface of the fractured matrix, and the microcavities diffuse some of the light instead of transmitting it, and thus renders an originally translucent sheet increasingly more opaque. Both types of deterioration. fiber prominence and surface microcracking, impair the appearance of the GRP sheet (with conventional surface finish) and may render it aesthetically unacceptable

Figure 5. Surface of GRP Sheet (under side) weathered after a period of 10 to l5 years, On the

outdoors for 12 years. formulation.

Fire-retardant GRP sheets have considerably lower resistance to breakdown when exposed to the outdoor environment30. Tests have shown that one common type of fire-retardant GRP sheet (based on tetrachlorophthalic acid polyester) undergoes both types of surface deterioration 2.5 to 3 times faster than sheets based on general purpose (conventional) polyester matrix.

As most of the deterioration is confined to the surface material, the surface region can be modified to increase the resistance of the GRP sheet to breakdown. For example, use of a gel coat as a surface finish of GRP sheets protects the glass-resin interface against the effect of moisture and/or temperature- induced stress-fatigue and thus no fiber prominence occurs 3 1 . Microcracks will form, however, after ap-

proximately the same period of exposure as in sheets with a conventional surface finish, unless a specially

Figure 6 . Surface of GRp Sheet weathered outdoors formulated resin, highly resistant to U.V. light, is used for 12 years. in the gel coat. Good resistance to both types of 3 1

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breakdown is achieved by coating the GRP sheets with a lacquer based on u.v.-stabilized acrylic resin. The acrylic coating protects the glass-resin interface against the effect of stress-fatigue and the underlying matrix against the action of U.V. light3'. This type of coating may be particularly useful for fire-retardant GRP sheets, which are especially susceptible to breakdown in outdoor exposureg0. Similarly, GRP sheets protected with a u.v.-stabilized (in-plant laminated) PVF surfacing film (0.025 mm thick) have remarkable resistance to the effect of weathering3O.

References :

1. Blaga, A., "Thermosetting Plastics, " Canadian Building Digest (CBD) 159, Ottawa, Division of

Building Research, National Research Council of Canada, 1974.

2. Parkyn, B., Editor, Glass Reinforced Plastics,

London, Zliffe Books, 1970.

3. Mohr, J. G., et al., SPI Handbook of Technology and Engineering of Reinforced Plastics/Compos- ites, 2nd Edition, New York, Van Nostrand Rein-

hold Company, 1973.

4. Matthan, J., et al., Editors, Aging and Weathering of Plastics, A review of the Literature, Shawbury,

Rubber and Plastics Research Association of Great-Britain, 1970.

5. Blaga, A., "Weathering Study of Glass-Fibre Reinforced Polyester Sheets by Scanning Electron Microscopy," Polym. Eng. Sci., Vol. 12 ( I ) , 53,

1972, (Available as reprint NRCC 12270).

6. Blaga, A., "Durability of GRP Composites," BC timent InternationallBuildin~ Research and Prac- tice, Vol. 3 ( I ) , 10, 1975, i ~ v a i l a b l e as reprint NRCC 15037).

7. Benjamin, B.S., Structural Design with Plastics,

Polymer Engineering Series, New York, Van Nos- trand Reinhold Company, 1969.

8. Frados, J., SPI Plastics Engineering Handbook, Society of the Plastics Industry, Inc., 4th Edi- tion, New York, Van Nostrand Reinhold Company,

1976.

9. Agranoff, J., Editor, 1975-1976 Modern Plastics Encyclopedia, Vol. 52 (IOA), Oct. 1975, New York,

McGra w-Hill, Znc.

10. Boening, H. V., Unsaturated Polyester: Structure and Properties, Amsterdam, Elsevier Publishing

Company, Holland, 1964.

11. Brydson, J. A., Plastics Materials, London, Butter-

worth and Company (Publishers) Ltd., 1975.

12. Skeist, I., Editor, Plastics in Building, New York,

Reinhold Publishing Carp., 1966.

13. Randolph, A. F., Editor, Plastics Engineering Handbook of the Society of Plastics Industry, Inc.,

New York, Van Nostrand Reinhold Company, 1960.

14. Schwartz, R. T., and H . S . Schwartz, Fundamental Aspects of Fiber Reinforced Plastic Composites,

New York, Intersci. Publish., 1968.

15. Read, T., and T. O'Brien, "Principles of Detail-

ing GRP Cladding, Technical Study 1-6," Archi- tects' J., Vol. 160, pp. 699-701, 815-817, 945-946, 1061-1063, 1121-1122, 1289-1291, 1974. This is a series of six papers dealing with details of instal- lation, fixing, jointing and local strengthening of GRP panels used in cladding applications.

16. The Use of Plastics Materials in Building, The En-

About the Author :

Dr. Aur& Blaga is a Research Oficer in the Building Materials Section of the Division of Building Research, National Research Council of Canada. Dr. Blaga holds a degree in Chemical Engineering (University of Caen, France) and a PhD in Organic Chemistry ( M c Gill University). Prior t o joining NRC, Dr. Blaga worked in industrial research organizations. Curren- tly, he is conducting research into the durability of plastics and composites and has published numerous papers on this subject.

gineering Equipment Users Association Handbook No. 31, London, Constable and Company Ltd.,

1973.

17. Read, T. and T. O'Brien, "Glass Fiber Reinforced Plastics for Building Claddings. Part I. The Ma- terial and Its Uses," Architects' J., Vol. 157, 699, 1973.

18. Read, T., and T. O'Brien, "Glass Fiber Reinforced

Plastics, Part 2. Translucent GRP," Architects'

J., Vol. 157, 817, 1973.

19. Saechtling, H., Bauen d t Kunstoffen, Miinchen,

Carl Hans Verlag, 1973.

20. White, J. S., "Weatherability of Fiberglass Solar

Collector Covers," Polym. News, Vol. 3 (5), 239,

1977.

21. Read, T. and T. O'Brien, "Glass Fiber Reinforced

Plastics for Building Claddings, Part 3 . Pigmented GRP," Architects' J., Vol. 157, 1035, 1973. 22. Blake, H. V., "Reinforced Plastics Cladding

Panels for High-rise Buildings," 5th International Reinf. Plast. Cunf., Paper 22, London, Brit. Plast.

Fed., Nov. 1958.

23. Speidel, E. O., "Poly-Pod Building Systems," SPI Roc. 31st Annual Conf., Reinf. Plast./Composites Institute, Section 3-F, Washington, D. C., The

Soc. of the Plast. Ind., Feb. 1976.

24. Dietz, A.G.H., Plastics for Architects and Build- ers, Cambridge, Mass., The MZT Press, 1969. 25. Girin-Lajoie, G., " A n Architect's Experience in

FRP Building and Housing Construction," Proc. 28th Ann. Conf., Washington, D.C., Reinforced

Plast./Composite Institute, 1973.

26. Fusco, A. F., "The Growing Role of Polyester in

Construction," SPE J., Vol., 28, 41, 1973.

27. Sprow, T. K., et al., "Filled Polyester Spray-Up

Systems Offering Improved Fire Hazard Classifica- tions," Proc. 28th Ann. Conf. Section 7-D,

Washington D.C., Reinf. Plast./Composites Znsti- tute, Feb. 1973.

28. Blaga, A., and R . S . Yamasaki, "Mechanism of

Breakdown in the Interface Region of Glass Rein- forced Polyester by ArtiJicial Weathering," J. Mat.

Sci., Vol. 8, 654, 1973.

29. Blaga, A., and R. Yamasaki, "Mechanism of

Surface Microcracking in Glass-Reinforced Polyes- ter by Artijicial Weathering," J . Mat. Sci., Vol. 8 ,

1331, 1973.

30. Blaga, A., and R. S. Yamasaki, "Outdoor Durab-

ility of a Common Type (Tetrachlorophthalic Acid- Based) Fire Retardant Glass Fiber Reinforced Polyester (GRP) Sheet, " RILEM Materials and

Structures, Vol. 10, 59, 1977, p. 289.

31. Blaga, A., and R. S. Yamasalti,"Effect o f Surface

Finish on the Durabilit-v of G R P Sheets". RILEM Matkriaux et Structures, Vol. 11, 63, 1 9 7 8 , ~ . 175.