ASTM C926-98a(2005)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: C 926 – 98a (Reapproved 2005)
Standard Specification for
Application of Portland Cement-Based Plaster
This standard is issued under the fixed designation C 926; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope C 207 Specification for Hydrated Lime for Masonry Pur-
poses
1.1 This specification covers the requirements for the appli-
C 219 Terminology Relating to Hydraulic Cement
cation of full thickness portland cement-based plaster for
C 260 Specification forAir-EntrainingAdmixtures for Con-
exterior (stucco) and interior work.
crete
1.2 This specification sets forth tables for proportioning of
C 595 Specification for Blended Hydraulic Cements
various plaster mixes and plaster thickness.
C 631 Specification for Bonding Compounds for Interior
NOTE 1—General information will be found in Annex A1. Design
Plastering
considerations will be found in Annex A2.
C 897 Specification for Aggregate for Job-Mixed Portland
1.3 The values stated in inch-pound units are to be regarded
Cement-Based Plasters
as the standard.The SI (metric) values given in parentheses are
C 932 Specification for Surface-Applied BondingAgent for
approximate and are provided for information purposes only.
Exterior Plastering
1.4 The text of this specification references notes and
C 1063 Specification for Installation of Lathing and Furring
footnotes that provide explanatory material. These notes and
to Receive Interior and Exterior Portland Cement-Based
footnotes (excluding those in tables and figures) shall not be
Plaster
considered as requirements of the specification.
C 1116 Specification for Fiber-Reinforced Concrete and
1.5 Details of construction for a specific assembly to
Shotcrete
achieve the required fire resistance shall be obtained from
C 1328 Specification for Plastic (Stucco) Cement
reports of fire-resistance tests, engineering evaluations, or
E90 Test Method for Laboratory Measurement of
listings from recognized fire testing laboratories.
Airborne-Sound Transmission Loss of Building Partitions
and Elements
2. Referenced Documents
E119 Test Methods for Fire Tests of Building Construction
2.1 ASTM Standards:
and Materials
C11 Terminology Relating to Gypsum and Related Build-
E 492 Test Method for Laboratory Measurement of Impact
ing Materials and Systems
Sound Transmission Through Floor-Ceiling Assemblies
C25 Test Methods for Chemical Analysis of Limestone,
Using the Tapping Machine
Quicklime, and Hydrated Lime
2.2 ANSI Standard:
C35 Specification for Inorganic Aggregates for Use in
A108.1 Specification for Installation of Ceramic Tile
Gypsum Plaster
3. Terminology
C91 Specification for Masonry Cement
C 150 Specification for Portland Cement
3.1 Terms shall be defined as in Terminologies C11 and
C 206 Specification for Finishing Hydrated Lime
C 219, except as modified herein.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 accelerator—an admixture that will shorten the set-
This specification is under the jurisdiction of ASTM Committee C11 on
ting time of plaster.
GypsumandRelatedBuildingMaterialsandSystemsandisthedirectresponsibility
3.2.2 admixture—a material other than water, aggregate, or
of Subcommittee C11.03 on Specifications for Application of Gypsum and Other
Products on Assemblies.
basic cementitious material added to the batch before or during
Current edition approved July 1, 2005. Published August 2005. Originally
job mixing.
approved in 1981. Last previous edition approved in 1998 as C 926–98a.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
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C 926 – 98a (2005)
3.2.3 acid etching—the cleansing and controlled erosion of 3.2.13 factory prepared (“mill-mixed” or “ready mixed”)—
a solid surface, using an acid wash. pertaining to material combinations that have been formulated
and dry-blended by the manufacturer, requiring only the
3.2.4 air entrainment—the use of an air-entraining admix-
addition of and mixing with water to produce plaster.
ture or air-entraining cementitious material in a plaster mix to
3.2.14 fiber, natural or synthetic—an elongated fiber or
yield a controlled quantity of minute (typically between 10 and
strand admixture added to plaster mix to improve cohesiveness
1000 µm in diameter) disconnected air bubbles in the plaster
or pumpability, or both.
(see entrapped air).
3.2.15 floating—act of compacting and leveling brown-coat
3.2.5 backplaster—plaster applied to the face of metal lath
plaster to a reasonably true surface plane using a float tool or
opposite a previously applied plaster.
the act of bringing the aggregate to the surface of finish-coat
3.2.6 bond—the state of adhesion between plaster coats or
plaster.
between plaster and plaster base.
3.2.16 key (also mechanical key)—plaster that physically
3.2.7 bonding compound or agent—compounds surface ap-
surrounds, penetrates, or deforms to lock onto the perforations
plied or integrally mixed with plaster to improve the quality of
or irregularities of the plaster base or previous coat of plaster.
bond between plaster and plaster base or between plaster coats.
3.2.17 metal plaster base—expanded metal lath, or welded
3.2.8 cementitious material—a material that, when mixed
or woven wire lath.
with water and with or without aggregate, provides the
3.2.18 plaster—portland cement-based cementitious mix-
plasticity and the cohesive and adhesive properties necessary
ture (see stucco).
for placement and the formation of a rigid mass.
3.2.19 required—pertaining to a mandatory obligation im-
3.2.9 coat—a thickness of plaster applied in a single opera-
posedbyaforceoutsideofthisspecification,suchasabuilding
tion.
code, project specification, contract, or purchase order.
3.2.9.1 basecoat—all plaster applied before the application
3.2.20 rustication (also “break”)—an interruption or
of the finish coat.
change in plane of a plastered surface.
3.2.9.2 bedding coat—a plaster coat that receives aggregate
3.2.21 scoring (also known as “scratching”)—the grooving
or other decorative material impinged into its surface before it
of the surface of an unset plaster coat to provide a key for a
sets.
subsequent coat.
3.2.9.3 brown coat—in three-coat work, the second coat,
3.2.22 set—thechemicalandphysicalchangeinplasterasit
applied over the scratch coat. In two-coat work, brown coat
goes from a plastic, workable state to a rigid state.
refers to the double-up basecoat. In either use, the brown coat
3.2.23 stucco—portland cement-based plaster used on exte-
is the coat directly beneath the finish coat.
rior locations.
3.2.9.4 dash-bond coat—a thick wet mixture of portland
3.2.24 stucco finish—a factory-prepared, dry blend of ma-
cement and water, with or without aggregate, dashed onto the
terials for finish coat applications.
surface of a plaster base such as smooth monolithic concrete or
3.2.25 temper, v—to mix or restore unset plaster with water
concrete block surfaces to improve the mechanical key for
to a workable consistency.
subsequent plaster coats.
3.2.26 texture—any surface appearance as contrasted to a
3.2.9.5 double-up coat—the brown-coat plaster applied to smooth surface.
the scratch coat plaster before the scratch-coat plaster has set.
4. Materials
3.2.9.6 finish coat—the final layer of plaster applied over
basecoat plaster.
4.1 Materials shall conform to the requirements of the
3.2.9.7 fog coat—a light coat of cement and water, with or
referencedspecificationsandstandardsandtotherequirements
without aggregate or color pigment, applied by machine spray
specified herein.
to improve color consistency.
4.2 Cement:
3.2.9.8 scratch coat—the first coat of plaster applied to a 4.2.1 Portland Cement—SpecificationC 150,TypeI,II,and
plaster base. III, as specified. White where specified.
4.2.2 Air-Entraining Portland Cement—Specification
3.2.9.9 skim coat—a thin finish coat applied to an existing
C 150, type as specified. White where specified.
plaster surface or other substrate to improve appearance.
4.2.3 Masonry Cement—Specification C91, Types N, S,
3.2.9.10 three-coat work—application of plaster in three
and M. White where specified.
successive coats with time between coats for setting or drying,
4.2.4 Blended Hydraulic Cement—Specification C 595,
or both.
Type IP, I(PM), IS, and I(SM) as specified.
3.2.10 cold joint (“joining” or “jointing”)—the juncture of
4.2.5 Air-Entraining Blended Hydraulic Cement—
fresh plaster application adjacent to set plaster, in the same
Specification C 595, Type IP-A, I(PM)-A, IS-A, and I(SM)-A
plane.
as specified.
3.2.11 curing—the act or processes of producing a moisture
4.2.6 Plastic Cement—Plastic Cement shall meet the re-
environment favorable to cement hydration, resulting in the
quirements of Specification C 1328, Standard Specification for
setting or hardening of the plaster.
Plastic (Stucco) Cement.
3.2.12 entrapped air—unintentional air voids in the plaster
generally larger than 1 mm. NOTE 2—Plastic cements are not available nationally.
C 926 – 98a (2005)
4.3 Type “S” Hydrated Lime—A hydrated lime that con- 5.2.2 Smooth or nonabsorbent solid surfaces, such as cast-
tains not more than 8 % unhydrated oxides when tested in in-place or precast concrete, shall be prepared to receive
accordance with Test MethodsC25. See Specifications C 206 portland cement plaster by one of the following methods:
and C 207 for a complete description of a Type “S” hydrated
5.2.2.1 Sandblasting, wire brushing, acid etching, or chip-
lime.
ping or a combination thereof,
4.4 Aggregates:
5.2.2.2 Application of a dash-bond coat applied forcefully
4.4.1 Sand for Base Coats—Specification C 897.Aggregate
against the surface, left untroweled, undisturbed, and moist
failing to meet gradation limits in Specification C 897 shall be
cured for at least 24 h, or
permitted to be used, provided the plaster made with this sand
5.2.2.3 Application of a bonding compound suitable for
has an acceptable demonstrated performance record in similar
exterior or interior exposure solid surfaces in accordance with
construction and climate conditions.
the manufacturer’s written directions.
4.4.2 Perlite—SpecificationC35.
5.2.3 Wherebondcannotbeobtainedovertheentiresurface
4.4.3 Sand for Job-Mixed Finish Coats—Specification
to receive plaster by one or more of the methods in 5.2.2,or
C 897.
where total plaster thickness will exceed the total thickness
4.5 Water—Water used in mixing, application, and finishing
specified in Table 1 for types of solid bases, furred or
of plaster shall be clean, fresh, suitable for domestic water
self-furring metal plaster base shall be installed in accordance
consumption, and free of such amounts of mineral or organic
with Specification C 1063.
substances as would affect the set, the plaster, or any metal in
the system.
6. Plaster Proportions and Mixing
4.6 Admixtures—See 3.2.2 and .
6.1 Proportions:
4.7 Fibers—Specification C 1116 on alkali-resistant fibers,
6.1.1 All portland cement plasters shall be mixed and
glass fibers, nylon, polypropylene or carbon fibers.
proportioned in accordance with the following tables and
accompanying requirements, using measuring devices of
5. Requirements for Bases to Receive Portland Cement-
known volume with successive batches proportioned alike.
Based Plaster
6.1.2 Plaster mix used shall be as designated and referenced
5.1 Metal bases and accessories used to receive plaster shall
to Table 2.
be installed in conformance with Specification C 1063, except
6.1.3 Base-coat proportions shall be as shown inTable 3 for
as otherwise specified.
the mix specified from Table 2.
NOTE 3—All metal, or PVC, or CPVC plastic members should be free
6.1.3.1 Measurement of Materials—The method of measur-
of deleterious amounts of rust, oil, or other foreign matter, which could
ing materials for the plaster shall be such that the specified
cause bond failure or unsightly discoloration.
proportions are controlled and accurately maintained. The
5.2 Surfaces of solid bases to receive plaster, such as
weights per cubic foot of the materials are considered to be as
masonry, stone, cast-in-place or precast concrete shall be
follows:
straight and true within ⁄4 in. in 10 ft (2.1 mm/m) and shall be
3 3
Material Weight, lb/ft (kg/m )
free of form oil or other elements, which would interfere with
bonding. Form ties or other obstructions shall be removed or Portland cement 94 (1505)
Blended cement Weight printed on bag
trimmed back even with the surface of the solid base.
Masonry or plastic cement Weight printed on bag
5.2.1 Solid surfaces shall have the suction (ability to absorb
Hydrated Lime 40 (640)
water) or surface roughness, or both, to provide the bond Lime Putty 80 (1280)
Sand, Damp and Loose (6.1.3.2) 80 (1280) of dry sand
required for the plaster.
A
TABLE 1 Nominal Plaster Thickness for Three- and Two-Coat Work, in. (mm)
Vertical Horizontal
B B
BASE 1st Coat 2nd Coat 3rd Coat Total 1st Coat 2nd Coat 3rd Coat Total
Interior/Exterior
C
Three-coat work:
3 3 1 7 1 1 1 5
Metal plaster base ⁄8 (9.5) ⁄8 (9.5) ⁄8 (3) ⁄8 (22) ⁄4 (6) ⁄4 (6) ⁄8 (3) ⁄8 (16)
Solid plaster base:
1 1 1 5
Unit masonry ⁄4 (4) ⁄4 (6) ⁄8 (3) ⁄8 (16) Use two-coat work
1 1 1 5 3
Cast-in-place or precast ⁄4 (6) ⁄4 (6) ⁄8 (3) ⁄8 (16) ⁄8 (9.5), max
concrete
1 1 1 7 1 1 1 7
Metal plaster base over solid ⁄2 (12.5) ⁄4 (6) ⁄8 (3) ⁄8 (22) ⁄2 (12.5) ⁄4 (6) ⁄8 (3) ⁄8 (22)
base
Two-coat work:
Solid plaster base:
3 1 1 3
Unit masonry ⁄8 (9.5) ⁄8 (3) ⁄2 (12.5) ⁄8 (9.5)
1 1 3 3
Cast-in-place or pre-cast ⁄4 (6) ⁄8 (3) ⁄8 (9.5) ⁄8 (9.5)
concrete
A
Exclusive of texture.
B
For solid plaster partitions, additional coats shall be applied to meet the finished thickness specified.
C
For exposed aggregate finishes, the second (brown) coat shall become the “bedding” coat and shall be of sufficient thickness to receive and hold the aggregate.
C 926 – 98a (2005)
TABLE 2 Plaster Bases—Permissible Mixes NOTE 5—On horizontal ceiling supports or roof soffits protected by a
dripedge,gypsumboardproductsshallbepermittedtobeusedasbacking
NOTE—See Table 3 for plaster mix symbols.
for metal base to receive portland cement plaster.
Mixes for Plaster Coats
Pro
...