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Testing of Type "X" Gypsum Boards and Laths


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Technical Note (National Research Council of Canada. Division of Building

Research), 1968-03-01



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Testing of Type "X" Gypsum Boards and Laths

Harmathy, T. Z.


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March, 1968

Inquiry and record purposes




An improved version of ASTM test method C473 is described. This new test method seems to provide a fairly reliable means of distinguishing between regular and type "X" gypsum board products.

* * * * *

During the past few years there has been a very rapid increase in the use of type "X" gypsum wallboards and laths in constructions

which are required to exhibit some specified fire-resistive character-istics. With the rise of the popularity of these type "X" products, unfortunately, a rather annoying problem also crept up: how to identify, short of an actual fire test, a type "X" gypsum board pro-duct without relying on the manufacturer's label. This problem is complicated by the fact that these products do not necessarily contain any uncommon aggregates. A macroscopic or microscopic examin-ation of the material may thus not yield an unambiguous conclusion. For the time being some type of performance test seems to be the only way of obtaining, in a reasonably short time, some decisive in-formation for identification.





To devise a performance test for the identification of

type "X" gypsum board products it is necessary to know the normal mechanism of failure of gypsum products in fire and the material properties which govern this mechanism.

It is well known that in the lower temperature ranges the superior performance of gypsum in fire is due to its absorption of heat in the dehydration of CaSO .2H O. From dilatometric tests it is also known that this dehydration


accompanied by considerable shrinkage of the material. There is, however, a far more substantial, although more gradual, shrinkage between 280 and 4200

C which is probably associated with the transformation of CaS04 (1). There are

some indications that this latter shrinkage is principally responsible for the cracking and disintegration of the regular gypsum board pro-ducts in fire. With the disintegration of a gypsum wallboard or lath on the fire-affected side of a building element the thermal resistance of the element becomes greatly reduced and at the same time some load-bearing, sometimes combustible, components become directly exposed to the flames. Any additive to the plaster which is capable of retarding this disintegration process to a considerable extent can thus render the gypsum board product type "X" quality.

With the aid of properly selected aggregates the fire-retardant quality of a gypsum product can be imprOled in the following ways:

(a) the overall shrinkage can be reduced, (b) the plasticity (deformation at rupture) can be increased, and (c) the strength can be increased. According to aU. S. patent (2) the addition of raw vermiculite, which expands upon heating (due to dehydration), is capable of compensating to some extent for the shrinkage of the plaster. Further addition of non-combustible fibrous materials (usually glass fibres) will produce higher strength and plasticity. Another patent (3) emphasizes the importance of the uniform distribution of the fibrous material. There are good reasons to believe that all type "X'I gypsurn board products now in common use owe their superior performance in fire to such

"expanding" and "reinforcing" aggregates (with rnajor ernphasis on the latter ones), and perhaps to some other additives which further irnprove the plasticity of the products at elevated ternperatures.



-Prelirrlinary investigations using dilatorrletry (4) indicated that, especially between 280 and 420°C, the shrinkage of type "X" gypSUrrl board products was generally less than that of regular gypSUrrl products. Nevertheless this was rot necessarily so, and on the basis of dilatorrletric tests alone the two groups of gypsUrrl board products could not be separated unarrlbiguously. This finding led to the conclusion that it is the higher エセョウゥャ・ strength and

plasticity, not the dirrlensional stability, of the type "X" products which is prirrlarily responsible for the irrlproved perforrrlance in fire. This conclusion was further corroborated by a recent addition to ASTM standard test rrlethod C473 (5), concerned with the sarrle problerrl: the distinguishing features of the type rrxrr products. According to this test rrlethod 2- by 12-in. strips taken frorrl the

sarrlple rrlaterial are loaded in tension with a lOOO-g weight and heated with two natural gas Meker burners. Those specirrlens that withstand the heating for 30 rrlin without rupture are considered to be type rrx rr


Unfortunately, closer inspection reveals several vague and technically unsound points in this standard. For exarrlple, the load is independent of the thickness of the product. Only the flarrle terrlperature is specified, so the specirrlen terrlperature even at the burner location rrlay attain rrlarkedly different values, depending on

the thickness and SOrrle therrrlal properties of the product. The heat produced by the burning of the paper layers on either side of the strip presents another difficulty in the interpretation of the test result.

Because of these flaws in ASTM test rrlethod C473 it was decided to rrlake an atterrlpt to irrlprove the test procedure, so that

the test results could lend therrlselves to a rrlore rigorous interpretation. SUGGESTED TEST PROCEDURE

In the tests conducted in this laboratory the specirrlen con-sisted of a 5-crrl wide 50-crrl long strip of the gypSUrrl board product, with the 50 -Crrl length along the rrlachine direction. This strip was installed in a vertical cylindrical furnace, 16 in. long and 3 in. 1. D.



-After the imposition of a well-defined tensile load on the specimen, the temperature of the furnace was made to rise at the constant rate of 50C/rrtin.*

As the temperature increases, the strength of the plaster matrix in the specimen is expected to decrease. The material is further weakened by the development of microcracks caused by the

shrinkage of the matrix. Since the gypsum board products exhibiting a lesser degree of shrinkage and reinforced with non-combustible fibres can be expected to fracture at higher temperature, the temper-ature of fracture can be regarded as a rough measure of the fire

retardant quality of the material.

Some preliminary tests indicated that after the dehydration of CaSO .2H 0 the burning of the paper cover caused the specimen

4 2 ·

temperature to rise sharply to between 350 and 5500

C. As these

temperatures are often high enough to cause the rupture of not only the regular but also of the type "X" products, it may be rather difficult bo separate these two groups of products on the basis of the temper-ature of rupture. To eliminate this difficulty it was decided to remove the paper cover from the wallboards and laths with a band-saw prior to the cutting of the test specimens.

To achieve sufficiently uniform temperature distribution in the furnace, some bulk ceramic fibre (Fiberfrax) was packed

around the specimen near the top of the furnace, and each end of the furnace was closed with two asbestos plates. These plates, when assembled, left an opening of about 5.25 by


5 cm for the specimen.

In the first few experiments the temperature of the specimen was measured by three gauge 36 chromel-alumel thermocouples, the hot junctions of which were placed in small holes drilled at the mid-length of the specimen and at 2 in. above and below this point. The upper thermocouple always recorded somewhat higher temperatures than the other two, but the maximum deviation from the temperature at the midlength was generally les s than 120 C. For a short period,

however, during a rapid rise following the temperature arrest associated with the dehydration, much higher temperature differences were also observed.


This rate of heating is com.m.only used in dilatometric and thermogravimetric tests.



-In all runs the teITlperature at the midlength was regarded as "the speciITlen temperature." Since the teITlperature along the central third of the furnace seemed to be fairly uniform, the practice of measuring the temperature 2 in. above and below the midlength was soon discontinued.

Also in the first few experiments 5/ 8-in. diameter holes were cut with a cork cutter at 1 in. froITl either end of the specimen (before the removal of the paper cover). Steel studs were inserted in these holes to permit the suspension and loading of the specimen. Unfortun-ately, the cutting operation often caused hardly detectable cracks and thus resulted in the early failure of the specimen. To avoid this dif-ficulty, in subsequent tests IIMyers Foldback No. 1414" paper clips were attached to either end of the specimen to facilitate its suspension and loading. The force exerted by these clips was great enough to provide a firm grip, but not great enough to crack the specimen.

The actual cross-sectional area of the specimen was always carefully measured. The load was selected to produce a 500 g/cm2 tensile stress in the ITlidlength cross-section of the specimen.

Since the sec ond and more substantial shrinkage of the gypsum is completed at about 420° C, one may expect that regular gypSUITl

board products will fracture before reaching this temperature, while type "X" products, which are reinforced with non-combustible fibres, will continue to carry the load to even higher temperatures. To check this assUITlption, 24 tests were performed, using both regular and

type "X" gypsum board products made by various manufacturers. These tests, whose results will be described in a later report, generally

supported the above assUITlption. Only lout of 24 runs brought an unexpected result. A specimen of a regular gypsUITl board product ruptured at 527°C, ITlore than 100°C above the supposedly critical temperature; yet another specimen taken from the same material fractured, as expected, well below 4200


To eliITlinate the possibility of basing a conclusion on some odd result, it is recommended that a gypSUITl board product be qualified as a type "X" product only if at least two out of three specimens taken from the product rupture at temperatures above 4200




Although the above described test procedure 18 definitely

an im.provem.ent over the ASTM C473 test, it still contains a few points which are based on judgem.ent rather than sound knowledge of the m.aterial's behaviour. It m.ust also be em.phasized that from. am.oilg the three m.aterial properties that m.ay result in superior perform.ance at elevated tem.peratures (i, e. shrinkage, plasticity and strength) strength was used as the only criterion in this test.

This testing procedure m.ay serve a useful purpose in providing the answer to an urgent practical problem.. The fact still

rem.ains, however, that any real progress in im.proving the fire-retardant characteristics of gypsum. board products can com.e only from. a clear understanding of the behaviour of these m.aterials at elevated tem.per-atures.


(1) Ljunggren, P. Determ.ination of Mineralogical Trans-form.ations of Gypsum. by Differential Therm.al Analysis.

J. Am.. Ceram.. Soc., 43, p. 227 1960.

(2) U. S. Patent No.2, 526, 066, Plastic Com.position Materials and Pr oduc ts Made Ther efr om., Oc t. 17, 1950.

(3) U. S. Patent No.2, 681, 863, Plaster Com.positions and Products, June 22, 1954.

(4) Galbreath, M. Private com.m.unication.

(5) ASTM Standard Methods for Physical Testing of Gypsum. Board Products and Gypsum. Partition Tile or Block (C4 73 - 66),


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