Good Practice Guide - Concrete Manufacturers Association

Good Practice Guide - Concrete Manufacturers Association
PRECAST CONCRETE SLABS ON
LOAD BEARING MASONRY WALLS
G o o d
P r a c t i c e
G u i d e
1
INTRODUCTION
This technical note covers sound and accepted design and construction principles for masonry walls which
support precast concrete slabs used for floors and roofs. As such the publication is aimed at the developer, the
building professionals, architects, structural engineers, quantity surveyors and the builder. It is based on SABS
standards for the types of materials used and required for design and construction. Each building situation is
unique – these notes are guidelines for all those involved in the building process to determine optimum
conditions – the balance of costs, ease and speed of construction, life costing etc.
BENEFITS OF LOAD BEARING MASONRY WALLS WITH
PRECAST CONCRETE SLABS AS A BUILDING SYSTEM
AND THE IMPORTANCE OF PREPLANNING
The use of masonry walls to support floor and roof slabs is not only based on its ability to carry safely the loads
imposed but to provide the envelope that encloses the building. Building a structural framework to support the
loads and then use exterior masonry infill panels, duplicates the structural requirement, increases costs and is
time consuming.
In the interior of the building the use of structural masonry means that there is improved resistance to the
spread of fire, generally an improvement in acoustic properties (less absorption and transmission of noise) and
thermal comfort (improved capacity to store heat and reduction in heat movement) reducing insurance and
operating costs of the building. Flexibility for possible re-arrangement of structural masonry supports can be
improved by building in initially of continuous masonry bond beams at door/window height permitting some
openings, say up to 2.0m in width, to be made at a subsequent date.
Also at the preplanning stage decisions should be made on imposed loadings on floors and roofs to be used in
design. Though the building might initially be designed for office loading, say 2.5kN/m² it might be more
economical, at some later stage to use the building for storage purposes with imposed loading of 5.0kN/m2.
Normally this would not require any major strengthening of the walls but requires the design of a stronger slab
at a relatively small increase in the total cost of the building.
Initial planning can permit the accommodation and location of services (electrical, water, sewerage, etc)
required initially and possibly in the future in optimum positions in the structure. In masonry walls horizontal
chasing significantly reduces strength and is not normally permitted without special detailing. Installing suitable
ducting is a possible answer to the problem. Holes in precast concrete slabs should be planned for, especially
important in factory made slabs and beams. The coring of precast slabs has become a common and cost
efficient procedure for any additional service holes that may be required. Control joints in walls must be in the
same position in the slab.
The cost effectiveness of masonry/slab structures is in appreciating at the preplanning and design stage the
advantages of the construction in reducing costs and time, improving the soundness and durability of the
structure, and reducing initial operating and maintenance costs. The aesthetically pleasing façade of masonry
buildings is an added bonus.
TYPES OF BUILDINGS AND ASSOCIATED ECONOMICS
The benefits of the use of precast prestressed slabs as compared to in-situ concrete are mainly quicker and
easier construction, reduced site problems and supervision, greater confidence in the slab structure and
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better span to depth ratios.
Suspended slabs are suitable for use in all types of building e.g. residential, commercial, industrial,
educational, recreational etc. Commercial office buildings often have a basement with the slab supported on
a RC or structural steel framework structure based on a car parking module. Longitudinally columns are at 5m
MAX SPAN,
THICKNESS,
metres
mm
6
120
Domestic building
7
150
Office loading / Domestic
8.5
200
School loading/Office
10
250
Offices
12.0
250
Roof loading + any of above
(2 car) or 7.5m (3 car) spacing and
transversally at say 7.5m spacing.
With hollow core slabs a guide to the
relationship between thickness, span
and type of structure is given in
table 1.
NORMAL USE
Table 1
A typical cost comparison of an
RC
office block (Johannesburg 2001/02)
CONSTRUCTION
comparing a reinforced concrete
frame of in-situ cast columns and
Time required for
beams supporting a post tensioned
construction, weeks
slab, with a precast concrete
Cost, R
LOAD BEARING
MASONRY AND
PRECAST SLAB
15
9
931 000
802 000
suspended floor supported on
external masonry walls with
Note: The office block was 3600m² in plan and
internally in-situ cast concrete
comparison covered basement, ground and first floor slabs as
columns supporting reinforced
at June 2002.
precast beams is shown in table 2.
Table 2
SUSPENDED SLABS FOR FLOORS
AND ROOFS: THE FUTURE
In South Africa an overall design consideration is to reduce costs in the building and operation of residential,
educational, community and health, industrial and commercial buildings.
Low rise buildings, say up to three storeys where lifts are not normally required, increase the density in the
use of land saving costs on services i.e. roads, water, sewerage, electricity etc.
Low-rise buildings of masonry construction use local material and are a traditional form of construction.
Suspended slabs use readily available materials and techniques of construction that are cost effective, while
the higher strength concrete used with increased prestressing can reduce the mass and handling of suspended
slabs. The overall consideration in development is “buildability” – ease of construction, which reduce costs.
Future developments will involve better preplanning, construction planning, easier and firm access to site,
cheaper and safer lifting equipment, better detailing of the beam/slab/wall intersection, while reducing site work
to a minimum.
Suspended slabs on masonry walls is a recognized and proven system. It is time saving and economical. New
technical developments together with improved construction management techniques will ensure that this is a
sound and viable form of construction for the 21st century.
TYPES OF SUSPENDED SLABS
The hollow core slab (HCS) is a reinforced or prestressed concrete slab, containing cores, generally varying in
thickness from 120mm to 250mm and, depending on loading, spanning up to 12m. The width is normally 900 or
1200mm.
A rib and block slab (RB) is composed of rectangular shaped (generally) precast concrete reinforced or
prestressed ribs supporting rebated filler blocks placed between two ribs. This system is sometimes referred
to as plank and block or beam and block. In-situ concrete is poured between and over the blocks. Slab depths
vary from 170 to 380mm with clear span up to 10m. Beams with a width of 100–200mm and minimum depth
60mm are used with infill blocks 200–250mm long, 440 to 485mm width and 100–355mm deep.
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The choice of which type of suspended slab to use depends on a number of factors and consideration should be
given to the following.
The HCS is manufactured in the quality-controlled conditions of a factory and the only site work involved is the
placing of a levelling screed 30 to 45mm thick. HCS fitting into non-modular widths (module normally 1200mm)
are cut to size in the factory while concrete is fresh. HCS are cut to length to suit as built building dimensions
immediately after the concrete has reached the required strength. Propping is usually not required on HCS and
following trades can start work immediately after erection.
The RB system is more flexible in coping with irregular shapes. Spans are smaller and the lifting capacity
required to place beams is less. It is significantly slower than HCS in construction time as in-situ concrete
must be poured and cured. Propping of the system during construction is required with a RB system.
TYPES OF WALLS
Single leaf, collar jointed, cavity and diaphragm walls and walls with fins are suitable structural walls. Horizontal
and vertical dimensions together with economic considerations will influence the wall type chosen. Single leaf
external walls are normally 140–190mm in thickness and collar-jointed walls 190 and 230mm. With cavity and
diaphragm walls (if the suspended slab rests on the inner leaf only) the external leaf is sometimes of face units
90, 110 or 140mm in thickness, while the inner leaf if supporting the slab, should have a minimum thickness of
110mm. Loading and the eccentricity of loading, and the slenderness ratio, vertical and horizontal, will dictate
wall thickness. Hollow units are easier to reinforce both vertically and horizontally. Solid units can only be
reinforced horizontally, unless the cavity of a cavity wall or special pockets are used to house vertical
reinforcement.
DESIGN OF LOAD BEARING MASONRY WALLS
AND PRECAST CONCRETE SLABS
Structural masonry walls give strength and stability to the structure. To ensure that the structure has
satisfactory resistance to collapse, the layout arrangements of all components and their interaction to resist
destabilising forces must be considered. Sometimes a designer will be responsible only for the structural
masonry, or the suspended floors, or the roof. There should be a single designer responsible for overall design,
including foundations and stability.
Structural components/elements of the structure should be effectively tied together such that in the
longitudinal, transverse and vertical direction the whole structure is of robust construction. Consideration of
the interaction between the masonry elements and other parts of the structure required to provide continuity
is essential. The position of intersecting walls, piers and control (movement) joints must be assessed to ensure
that they do not affect lateral stability adversely.
Overall masonry design covering accommodation of movement, design for stability, exclusion of moisture, fire
resistance, thermal properties, sound absorption, sound insulation and noise reduction are covered in SANS
10021, SANS 10145 and SANS 10249.
Empirical structural design or design of reinforced masonry by simple rules have been developed by SABS (refer
SANS 10400) and the National Home Builders Registration Council (refer Home Building Manual) where a
rigorous design procedure is not warranted. They are normally conservative to ensure that the empirical design
will have an adequate factor of safety against failure. Limitations and the type, size and wall configuration,
which are covered in the design, are stated.
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Rational design is based on assumed loading (SANS 10160) and resistance of the structural masonry to this
loading (SANS 10164-1 and 2), codes of practice, which cover un-reinforced, reinforced and prestressed
masonry. Suspended precast concrete slabs should comply with the requirements of SANS 1879 and the
design code SANS 10100.
MATERIALS
Masonry
Masonry units, solid or hollow, brick or block size, of burnt clay or concrete of adequate compressive
strength (usually 10.5 to 21MPa for solid units or 7.0 to 14.0MPa for hollow units) are satisfactory. Class II
mortar (50kg common cement, 0–40 l lime, 200 l sand) is normally used. Fine and coarse aggregates should
comply with the relevant SABS standard, particularly in respect of the fines content.
Precast slabs, ribs and blocks
Ribs, infill blocks and slabs should be manufactured to satisfy the requirements of SANS 1879. The
standard specifies requirements for tolerances of dimensions of ribs, blocks and hollow core slab. The design
and manufacture of ribs and slabs must ensure that under proof load the deflection, and recovery after removal
of the test load satisfy the requirements of SANS 1879.
CONSTRUCTION
Masonry walls
Accuracy in the building of walls in the plan position to ensure satisfactory bearing length support for beams
and slabs, and to the designated level (bedding joints thicker than 15mm reduce wall strength significantly) are
vital for the success of the structure to perform its intended service. Packing pieces of fibre cement sheeting
can be used under slabs or beams to achieve the correct level but gaps between shims must be filled with a
suitable mortar mix. SANS 10155 covers accuracy in building and SANS 10164-1 states permissible deviations
in accuracy for structural walls.
Hollow core slabs
Slabs are placed on the masonry walls with a minimum bearing of 100mm as per drawing details. On roofs or
exposed balconies, install the specified material to accommodate thermal movement (e.g. bituminised softboard
or similar). Such provision must make allowance for changes in camber or deflection, particularly where light
parapet walls are built on prestressed HCS. In such situations a light mesh reinforcement should also be placed
in the finishing screed or topping.
Rib and block system
Ribs are placed on the masonry walls with a minimum bearing of 100mm as per drawing details at approximate
centres, their position being finally adjusted to suit the width of the filler block with a 35mm minimum bearing of
block on rib. Closed end filler blocks are placed at the end of each line. Temporary propping of beams not
exceeding 1800mm centres are erected to suitable level and camber. If transverse stiffener ribs are detailed
then blocks are left out to accommodate reinforcement and concrete. Services should be installed over blocks
and not ribs and the specified mesh is placed throughout. Before grade 25 (minimum) concrete is cast, all
rubble should be removed and the blocks thoroughly wetted. Concreting should be continuous. Removal of the
temporary propping before the compressive strength of the in-situ concrete reaches 17MPa will lead to an
increase in the long term deflections.
Transport and lifting equipment
For HCS a crane is required to place the slabs into position directly from the delivery truck. Tower cranes give
maximum reach but on normal 2 and 3 storey buildings mobile cranes are used. They have a lifting capacity of
30 tonnes with a 31m boom, and operate easily over a 17m radius. Larger cranes are available where required.
With HCS the placing of 600m2 to 700m2 of finished floor area per shift can be achieved. With the RB system
the ribs and blocks are usually stored on site and placed in position by hand when required.
Details of some intersections of masonry slabs
The connection between the masonry and the slabs is obviously critical to the structural strength, aesthetics
and water resistant properties of the building.
5
It is part of the role of the professional team in conjunction with the technical department of the manufacturer
to develop details that best suit the needs of the various situations. A few of the typical details most commonly
used follow.
Roof slab parapet wall detail
Slope to inside to avoid
streaking of paint
Waterproofing membrane
DPC on top of slab
Expansion joint
Screed/foam
cement to fall
Ref 100 mesh
Brickforce in every course
Jointex
V-joint in external plaster
Approved
stretch paint
Slip joint
2 layers DPC or 2 ply Malthoid
V-joint in
plaster
Brickforce in every course above
door/window height
NB A 110mm wide parapet wall is recommended, but a 230mm
wide parapet wall can be used on the height of the parapet wall.
Suspended floors on external 140mm walls
140
140
Plaster
Topping if required
Concrete slab
V-shaped
joint in
plaster
Slip joint – see note
Slip joint– see note
Bond beam/U-beam
Bedding reinforcement
to suit – maximum
diameter 6mm
NOTE:
1. If floor slab span exceeds 6m
spanning on to wall and large
movements expected consider
a slip joint on top of wall, such
as two layers of DPC or 2 ply
Malthoid. Structural stability &
robustness may preclude use
of slip joint.
2. Consider use of U- or bond
beams below slab.
3. If designer assumes wall
laterally restrained by slab then
slip joint not advisable.
TYPICAL MASONRY SLAB CONNECTION DETAILS
Control joints in walls and slabs
At the design stage the position of control joints, to reduce the likelihood of cracking due to movement (mainly
due to temperature changes) is determined by the project engineer. Joints in slabs must coincide with the
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control joints in the masonry walls. Long lengths of freestanding walls are most susceptible. The addition of
brickwork reinforcement and a concrete slab gives some restraint and the interruption of long facades by deep
recesses also helps to reduce the incidence of cracking. The inclusion of a continuous reinforcing mesh in a
screed or topping on a precast slab will increase the degree of restraint provided by the slab.
SUPERVISION AND INSPECTION
Supervision and inspection during construction will ensure that the final structure is of acceptable quality. This involves
Ensuring walls built to acceptable plumb, line and level
Ensuring position of holes or slots for services will not compromise structural integrity or durability.
Adequate bearing length for beams/slabs
Bearing surface is smooth and plane.
Thickness of mortar joint is less than 15mm otherwise packing pieces to be used.
Placing of ribs/slabs so walls are not damaged and ensure continuity of wall control joints into slabs
Ensure that placing and compaction of concrete in a RB slab and structural topping in a reinforced hollow core
slab is satisfactory. Prestressed hollow core slabs only require a 40mm levelling screed of river sand and
cement. For guidance on the correct application of screeds and toppings see the C & CI publication
“Recommendations for screeds and toppings”.
Standards and references on precast concrete slab construction
SANS number
Title
50197-1
Common cement composition, specification and conformity
227
Burnt clay masonry units
1083
Aggregates from natural sources – aggregates for concrete
1090
Aggregates from natural sources – fine aggregates for plaster and mortar
1215
Concrete masonry units
1879
Precast concrete suspended slabs
10145
Concrete masonry constructions
10155
Accuracy in building
10100
The structural use of concrete
10160
The general procedures and loadings to be adopted for the design of buildings
10164-1
The structural use of masonry un-reinforced masonry walling.
10164-2
The structural use of masonry – reinforced and prestressed masonry.
10249
Masonry walling
10400
Application of the National Building Regulations
National Home Builders Registration Council’s Home Building manual
Concrete Manufacturers Association publications:
Detailing of concrete masonry: Volume 1
Solid units – 140
Volume 2
Hollow units – 140/190
Volume 3
Cavity wall – 240/290
Masonry manual – 2000
Structural concrete masonry. A design guide. F.S. Crofts and J.W. Lane
Some definitions
Masonry:
An assemblage of masonry units bonded together with mortar
Solid masonry unit:
A masonry unit containing cores not exceeding 25% of the gross
volume of the unit.
Hollow masonry unit:
A masonry unit containing cores that exceed 25% but do not exceed
60% of the gross volume of the unit.
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APPENDIX A
Accuracy in building masonry walls and manufacture of precast
concrete suspended slabs
Masonry
PERMISSIBLE DEVIATION
ITEM
mm
Straightness, line
10 in 5m
Level, bed joints
± 5 in < 5m
± 10 in 5m to 10m
±15 in 10m to 20m
Plumb, vertical
± 5 in 3m
± 10 in 3m to 6m
± 20 in over 6m
Surface of supporting elements
– 10 + 5
Position on plan of any edge or marking measured
from the nearest grid line or agreed centreline
± 10
Level (deviation) from designed level with reference
to the nearest transferred datum of the average top
surface of an element.
– 10 + 0
Bed joint thickness (normally 10 – 13mm)
±3
• First joint above supporting element
(includes foundation)
– 5 + 10
Suspended slabs
To l e r a n c e s o n d i m e n s i o n s o f r i b s ,
blocks and hollow core slabs
UNIT
DIMENSION
Rib
Block
Hollow core slab (full panel)
Width:
TOLERANCES
mm
Length
± 20
Width
±3
Depth
±3
Length
±5
Width
±3
Depth
±3
Length
± 10
Width
±4
splitter (a)
± 20
Depth: 120 – 150mm
±5
200 – 250mm
±7
(a) A splitter is any slab less than the product standard width
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Portland Park, Old Pretoria Road, Halfway House 1685, South Africa.
PO Box 168 Halfway House 1685
Tel +27 11 805 6742, Fax +27 11 315 4683
e-mail: cma@cis.co.za
website: www.cma.org.za
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