Summary: This method of statement for plastering includes the following;
• Internal Portland cement plaster:
• External Portland Cement plaster:
• Formed in Place plaster grooves in elevation:
• External Formed in place decorative cornices in building elevation
Materials for Application;
1. Portland Cement ASTM C150 Type
2. Sand Aggregate ASTM C 897 White Sand for base coat, red sand for dash coat.
3. Plastering Accessories: Metal Lath, angle beads, special pieces for internal angles and corner.
4. Fiber
5. Trial Mix
6. Details for Metal Lath Location and Installation (Typical) (Submission for Approval) (Trans.# C-0260).
1. Preparation
A.
Clean the surface and remove any dust, contaminated materials on
plaster bases and substrates for direct application of plaster, removing
loose material and substances that may impair the work. Form ties and
other obstruction shall be removed or trimmed back even with the surface
of the solid base.
B. Dissimilar
Backgrounds: where rendering is to be continued without break across
joints between dissimilar solid backgrounds which are in the same plane,
cover joints with 200mm wide galvanized steel lathing as specified
& approved fixed with corrosion resistant fasteners at not more than
600mm centers along both edges, apply mesh over electro-mechanical
embedded conduits.
C. Apply forcefully dash coat on concrete and concrete masonry
surfaces indicated for direct plaster application. Dash coat is to
cover full surface to be plastered and is to provide rough surface of
sharp protrusions ranging 3.0mm and 5.0mm. Dash-Coat Mix: 2 volume parts
Portland cement to 3 volume parts fine sand, 1 part bonding agent to 3
part clean water mixed to a mushy-paste consistency shall be left
untroweled, undisturbed and moist cured for at least 24 hours after
application and before plastering.
D.
Install temporary grounds and screeds to ensure accurate rodding of
plaster to true surfaces. Check the straightness in each corner and add
grout to hold the external corner beads in correct and internal position
to be fixed using corrosion resistant fasteners.
E. Before plastering starts, dampen, by spraying water concrete dash coat surfaces that are to receive plaster with clean water.
2. Application
A.
All plaster shall be prepared in a mechanical mixer mix 1:5 (1 part
cement and 5 part sand) using sufficient water to produce a workable
consistency. Mix fiber 0.6 kg/m3 of the mortar or plaster by first
mixing with some water (30%). After all the ingredients have been added
including the remaining mixing water, the plaster should be mixed for a
further few minutes until uniform fibre dispersion. Plaster not used
within 1.5h from start of mixing shall be discarded.
B. The second coat shall be applied with sufficient material and pressure to ensure tight contact with dash coat.
C.
The surface of the second coat shall be brought to a true, even plane
with a rod or straightedge, filling surface defects in place with
plaster.
D. Applied plaster shall be
floated as smooth as possible and then steel-troweled. Steel troweling
shall be delayed as long as possible and used only to eliminate the
uneven points and to force aggregate particles into the plaster surface.
E.
Each plaster coat shall be applied to an entire wall or ceiling panel
without interruption to avoid cold joints and abrupt changes in the
uniform appearance of succeeding coats. Wet plaster shall abut set
plaster at naturally occurring interruptions in the plane of the
plaster, such as corner angles, openings and control joints where this
is possible.
F. Make internal angles and corners square.
G.
Finished grooves shall be neat, of uniform width and depth, sharp
straight edges and exact vertical and horizontal lines. Elevation
grooves shall be cut before plaster sets by using molded guide or wooden
strip and fixing it in temporary grout and screed. Regarding recessed,
follow the procedures same as grooves.
H.
Form cornices by plastering additional coats against pre-formed wood
templates fixed onto plastered elevation. Make form wood templates to
shapes, dimensions and projection
indicated on drawings. Apply additional under coats in thickness not
more that 10mm until obtaining required projection. Final finishes refer
to item D.
I. Plaster should not
deviate more than plus or minus 3mm in 3m from a true plane in finished
plaster surfaces, as measured by a 3-m straight edge placed in any
location on surface.
3. Curing
A. Provide sufficient moisture in the plaster by spraying water as frequently as required at least twice per day.
4. Cutting and Batching
A.
Cut, patch, replace, repair and point up plaster as necessary to
accommodate other work. Repair cracks and indented surfaces. Repair work
to eliminate blisters, buckles, check cracking, and dry out as
necessary to comply with visual effects.
B. To correct inaccuracies, dubbed out in thickness of not more than 10mm in same mix as base coat and total thickness of
dubbing must not exceed 25mm unless otherwise approved.
In areas where thickness is more than 20mm, apply dash
keying mix before adding new layer.
C.
To correct inaccuracies in beam, column together with block work, chip
carefully the concrete structures by using hammer and chisel to receive
dash coat (follow the procedure in 1.C). For 25mm thick plaster repair
apply it one time as per 2.B. For 45mm thick, apply 20mm first coat and
apply keying mix before adding the new layer. Each layer must not exceed
10 mm, start again same as from 1C.
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