CONCRETE GROUND FLOORS.ppt

CONSOLIDATED CONSULTANTS
E N G I N E E R I N G & E N V I R O N M E N T
CONCRETE
GROUND
FLOORS
A.N. OMARI
18 Nov. 2007
Design, Construction and Finish

Concrete Ground Floors:
 Include:
 Side walkways
 Drive ways
 Residential floors
 Heavy duty industrial
floors
 Ware houses
 Streets
 Highways
 Airport pavements.
Properties:
 Hard
 Wear resistance
 Free of cracks
 Durable

SUBGRADE
SUB BASE
Elements of a floor

Elements of a floor
Sub-grade : Naturally occurring ground excavated down to
formation level, or, on made-up ground,
imported fill material.
Sub-base : Selected material imported to form a level,
smooth working platform on which to construct
the slab.
Slip
membrane
(Vapor
barrier)
: Essentially used to reduce friction under the
slab, prevent loss of cement fines from the wet
concrete into sub-base material, and prevent
moisture passing through concrete inside.
Slab : Main structural concrete element forming the
floor.
Wearing
surface
: Upper surface of the slab suitably finished or an
applied topping or other flooring material.

General Principles
Designer should first consider the overall design
data, starting from the surface downwards.
Table (1) : Minimum specifications of concrete for various duties
Category Duty
Minimum
Cement
content
(kg/m³)
Equivalent
Grade
(N/mm²)
Type of finish
or flooring
1
Light foot
or trolley traffic
e.g. in offices, shops
275 30
Thin sheet
or tile flooring
or carpets
2
General industrial
use; vehicles with
pneumatic tyres;
mild chemical
conditions
325 40
Structural slab
finished as wearing
surface. The minimum
cement content is
necessary to ensure
wear resistance.
Also for use with
sprinkle finishes.

Continue Table (1) : Minimum specifications of concrete for various duties
3
As 2, but heavy
abrasive conditions
(e.g. vehicles with
solid wheels) or strong
chemical attack.
300 35
Applied toppings,
to suit conditions.
4
Heavy industrial use;
moderate chemical
conditions.
400 50
Structural slab finished
as wearing surface.
Abrasion resistance
increases with
strength. Strength
level according
to degree of wear
anticipated.
5
Heavy industrial use;
heavy abrasion
(e.g. by steel shod
wheels); impact;
strong chemical
attach.
300 35
Applied toppings, to
suit conditions.

Table (2) : Preliminary design data appraisal
Data
Factors for
consideration
Action required
Surface
characteristics
Impact abrasion
resistance?
Specify appropriate concrete
grade
Surface sealants,
coatings, toppings
needed?
Specify slab surface finish
appropriate to applied
treatment: specify surface
preparation appropriate to
topping
Surface regularity?
May determine need for ground
stabilization, or special
construction methods

Loading
Type – moving, static?
Spacing of point loads,
area of uniformly
distributed load?
Maximum values?
Data required for determination
of thickness of construction
and design details
Design life Years?
Ground
conditions
Bearing capacity?
Support conditions?
Risk of settlement?
Data required for design
}
May require ground
Stabilization, pilling
Continue Table (2) : Preliminary design data appraisal

Loadings
A slab will be subjected to one or more of the
following types of loading:
 Wheel loads.
 Leg loads.
 Uniformly distributed load.

 Loads imposed by forklift, trucks,
trolleys and other vehicles which
traverse the floor.
 Smaller ranges of truck up to about 2
tones rating have a negligible effect
upon the slab, but as axle load
increase, their damaging effects
increase rapidly and thicker slabs
become necessary.
 Wheel Loads :

 From warehouse racking systems.
 Two common forms of racking.
 Back to back arrangement, the loads are
relatively closely spaced, racks about 300mm
apart causing high stresses in the slab.
 Mezzanine layout, the leg loads are usually
spaced at 3m apart.
 Leg Loads :

 Imposed on the floor by materials,
storage, containers and other items
being placed directly on to the floor slab.
 Uniformly Distributed Loads :

Classifications of sub- grade and
thickness of sub-base:
Table (3): Recommendations for thickness of sub-base
Classification
types
Typical soil
Minimum sub-base
thickness
(mm)
Poor
(CBR < 10%)
Clays Sits Silty
clays Sandy clays
200
Good
(CBR > 10%)
Sands Sandy gravels 130

 Sub-base will carry construction traffic in the form of
small dumpers.
 If heavier vehicles are to be used and there is a risk of
damaging the sub-grade a thicker sub-base should be
specified.
 In case of wheel and rack loading, the sub-base assists
in reducing the vertical stress on the sub-grade.
 Uniformly distributed loading, very little load
spreading is possible; bearing capacity of the sub-grade
may limit the maximum unit loading on the floor.
 Slabs supported on sub-grades such as organic soils,
heavy clays and loose sands or where substantial
thickness of fill have been used, anticipated long term
settlement are high.

 Some geotechnical process such as:
 Soil stabilization.
 Drainage.
 Compaction.
 Use of pile foundations.

Slab Thickness
 The required thickness of floor slab depends on:
 Type of load.
 Magnitude of load.
 Grade of concrete.
 Sub-grade / sub-base.
Floors are generally subjected to fluctuating loading.
Maximum tensile stress induced in the slab should be limited
to 50% of the ultimate flexural strength of the concrete.

Fork-
lift
truck
rating
(kg)
Max.
axle
load
(kg)
8 h working day 24 h working day
Movements per hour
4 10 20 4 10 20
Life (years)
10 20 30 10 20 30 10 20
3
0
10 20 30 10 20 30 10 20 30
2000 5500 125 150 125 150
3000 7000 125 150 175
3750 8500 150 175 150 175 200
4500 10000 150 175 200 175 200 225
7000 15500 175 200 225 200 225 250 275
Table (4): Guide to thickness of slab (mm) or typical loadings from fork-lift
trucks
Note :
Table (4) for good sub grade, add 25 mm for poor sub grades.

Wheel Loads

Leg Loads

Bay Layout, Joints and Reinforcement
Subdivided in to smaller areas for two reasons:
1- Control tensile stresses due to thermal variations and
shrinkage of concrete.
2- Due to construction limitation (plant and labor capability).
 Floors are constructed using long-strip technique.
 A strip width of about 4.5m has been found to be the most
practical, as it facilitates the placing and accurate finishing of
the concrete and the size of compacting beam is casing
handled by two men.
 The strips may be divided into bays by means of induced
joints,
either by saw shallow grooves in the surface after the concrete
has hardened or by inserting crack inducing strips into the
concrete while it is still plastic.

LONGITUDINAL JOINTS
INDUCED
JOINTS
(OPTIONAL)
EDGE STRIP
TO FACILITATE
CONSTRUCTION
OF SLAB
CONTRACTION
JOINTS
ISOLATION JOINT
ON PERIMETER
IF AGAINST WALLS
AND ROUND
COLUMNS
STRIPS TO FOLLOW DIRECTION OF MAXIMUM
TRAFFIC FLOW AND / OR TO FACILITATE
CONCRETE ENTRY DURING CONSTRUCTION
LAYOUT OF JOINTS

Control Joints
Function of Control Joints :
 Control / minimize cracking by permitting
shrinkage and other volume changes.
 Permit relative movements of adjacent portions
of the structure.

Types of Joints
a)Tied joints :
 Longitudinal joint
Main construction joint formed in the longitudinal direction
between adjacent strips.
 Joint relives warping stresses.
 The bars (unless de bonded) limit direct contraction
and provide load transfer.
 Debonded joints may be provided at about 18m
centers to allow lateral movement of the slab.

 Induced joint:
 Used to break along strip into a series of smaller
bays.
 Relieves warping stresses.
 Fabric reinforcement a cross the joint limits
direct contraction.
 Joint may be used without reinforcement in
lightly loaded unreinforced slabs.

b) Movement Joints :
 Contraction Joint (Transverse Joint):
 Designed to accommodate the contraction of a
long strip laid in one operation.
 Dowel-bars provide load transfer and should be
fitted with a plastic sleeve to permit full sliding
action, or coated with an efficient de bonding
compound.

 Isolation joint :
Provide freedom of movement for the slab at all fixed
features within the slab.
 Around perimeter of the slab against solid walls.
 Around plant bases.
 Around internal stanchion or columns.
 The joint relives warping and contraction movement
but provides no load transfer.
 When a slab is anchored into ground beams, it is
restrained against contraction.
 The recommendations for joint spacing and
reinforcement follow do not apply.

 Expansion Joint
 Rarely needed in a floor slab
 Over all drying shrinkage contraction is
normally more than sufficient to offset any
temperature expansion.
 Used for slabs on poor ground if uneven
settlement is expected.

Longitudinal joint

Induced joint

Contraction Joint

Expansion Joint

Wall Isolation Joint

Spacing of Joints
a) Un-reinforced slabs
 Joint spacing should not exceed 6m in either
direction.
 If movement joints are used with de bonded
or full contraction joint spacing of 18 m can
be used.
b) Reinforced slabs:
 Induced joints generally at spacing not
greater than 10m.

Reinforcement
The steel is assumed to carry the tensile
force developed in the concrete as a result
of the contraction of the slab being
restrained by friction on the sub – base.

Fabric
reinforcement
according to
BS 4483
Longitudinal direction of slab Transverse direction of slab
Maximum spacing of free
movement joints (m)
Maximum spacing of free
movement joints (m)
Slab thickness (mm) Slab thickness (mm)
125 150 175 200 225 125 150 175 200 225
Unreinforced -
6 6 6 6
-
6 6 6 6
18* 18* 18* 18* 18+ 18+ 18+ 18+
A142 25 21 18 16 14 25 21 18 16 14
A193
34 28 25 21 19 34 28 25 21 19
B196
A252 44 37 31 28 25 44 37 31 28 25
B283
49 42 36 31 28
34 28 25 21 19
C283 18+ 18+ 18+ 18+ 18+
B385
67 56 48 43 37
34 28 25 21 19
C385 18+ 18+ 18+ 18+ 18+
A393 69 58 49 44 38 69 58 49 44 38
B503
- 74 64 55 49 -
37 31 28 25
C503 18+ 18+ 18+ 18+
C636 - - 81 74 65 - - 18+ 18+ 18+
Table (5): Free movement joint spacing and fabric reinforcement for slabs
•Induced joints at 6
m maximum centers.
+ 6 m maximum
strip width.
 With these fabrics
high yield tie bars of
equivalent cross
section to the fabric
should be used in
longitudinal joints
when maximum
spacing of movement
joints is adopted in
the transverse
direction.

Available in three types in accordance with BS 4483 as
follows :
a) Square mesh, prefix "A" wire size and spacing are the
same in both directions.
b) Structural mesh, prefix "B" main wires are longitudinal,
but cross wires are more widely spaced although
providing a substantial area of steel.
c) Long mesh, prefix "C" this is the most commonly used
mesh for traditional long strip construction and is the
most economical on a weight basis. The main wires
should be placed in the direction of the slab strip.
Fabric Reinforcement

References
1) Concrete ground floors, R. Colin Deacon,
Cement and Concrete Association.
2) Concrete Industrial Ground Floors – Technical
Report No 34, Concrete Society, 1994.
3) Concrete Manual, Fifth edition, ICBO.
4) The design of ground supported concrete
industrial floor slabs, Interim Technical Note 11,
British Cement Association.

THANKS

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