Formwork and Reinforcement of concrete under materials

Formwork and reinforcements

1. FORM WORK
Formwork and False work Technology
Formwork is a temporary or permanent mold (casing) into which fresh concrete is poured to
harden and achieve a desired shape and size. False work is the temporary support structure
(shores/props) that holds the formwork and the wet concrete until the concrete is strong
enough to support itself. The complete system is often collectively called "formwork".
Purpose and Requirements/Properties of Good Formwork
Main Purposes
oTo give the concrete its required shape and size.
oTo support the wet concrete and all construction loads (e.g., workers, equipment).
oTo provide a desired surface finish (e.g., smooth, textured).
Requirements (Properties)
oStrength and Rigidity: Must safely support all vertical and lateral loads without
breaking or deforming. Any deformation (bulging) will be visible on the finished
concrete.
oWater-Tightness (Leak-proof): Joints must be sealed to prevent cement slurry (grout)
leakage. Grout loss weakens the concrete surface and can cause honeycombing (a
rough, stony surface).
oEase of Removal (Stripping): It must be simple to erect and strip safely without
damaging the concrete structure or the formwork. A release agent (oil or chemical) is
often used to prevent sticking.
oDurability & Reusability: The material should endure multiple uses to be cost-
effective.
oAccuracy: The internal surfaces must conform precisely to the lines and dimensions
specified for the concrete element.
oWell-Braced: Must be stable against side-to-side movement, wind, and other lateral
forces.
Types and Materials of Formwork
Type (material)Key
Characteristics
Advantages DisadvantagesCommon Uses
Timber/PlywoodWood planks or
plywood sheets.
Plywood is used
as sheathing.
Economical for
small jobs and
complex shapes.
Easy to cut, nail,
and remove on
Low reuse value
(plywood 5-20
times, timber
less). Can warp
or swell
Footings,
columns, beams,
slabs for small-to-
medium projects,
and

2. site when exposed to
moisture
Complex
architectural plans
Steel Modular panels
fabricated from
steel plates.
Very high reuse
value (100+).
Provides a very
smooth finish.
Strong, durable,
and
dimensionally
stable.
Heavy, requires
cranes for
handling. High
cost.
Large-scale,
repetitive
structures like
high-rise
buildings, tunnels,
dams, and precast
elements.
Aluminum Made from
aluminum alloys,
similar to steel
panels.
Much lighter
than steel,
allowing for
manual handling.
Good reuse value
(50-100 reuses).
Corrosion
resistant.
Higher initial
cost than timber.
Less strong than
steel.
Residential and
mass housing
projects,
particularly
repetitive wall-
slab construction.
Plastic/PVCLightweight
plastic panels or
modular sections.
Very lightweight,
easy to handle
and clean.
Waterproof and
corrosion-
resistant
. Ideal for
repetitive
elements.
Lower strength
compared to
steel/aluminum.
Circular columns,
waffle slabs, and
small
columns/beams.
Insulating
Concrete Forms
(ICFs)
Made of
Expanded
Polystyrene
(EPS) or similar
materials.
Stays in place
after curing
(sacrificial
formwork).
Provides thermal
Insulation.
Highly
specialized,
mainly used for
walls.
Walls for energy-
efficient buildings.
Fabric Flexible textile
sheets.
Used to create
complex, curved,
or irregular
concrete shapes.
Highly
specialized.
Innovative or
specialized
construction.
Choices of Formwork
Selecting the right formwork involves a comprehensive analysis of several factors:

3. 1.Safety: Must be structurally sound to ensure worker safety during pouring and curing.
2.Total Cost: Considers material cost, labor for erection and stripping, cost of treatment
(release agents), and potential cost savings from reuse.
3.Speed of Construction: Modular systems (steel, aluminum) are faster to assemble and
strip than bespoke timber formwork.
4.Availability of Materials: Local supply and resource constraints can dictate the
material choice.
5.Type and Size of the Structure:
Simple, non-repetitive structures (e.g., a small house foundation): Timber
formwork is usually the most economical choice. ○ Tall, repetitive structures
(e.g., high-rise cores): Slip form (continuously rising) or climbing formwork is
used for speed and efficiency.
Large-scale civil works (e.g., bridges, dams): Engineered steel formwork
systems are preferred for strength and finish. 6. Desired Surface Finish:
Architectural concrete requires high-quality, blemish-free formwork (e.g.,
high-quality phenolic-coated plywood or steel)
Formwork Loads
Formwork must be designed to safely carry all loads imposed during concrete placement and
curing.
1.Vertical Loads
These are the downward forces acting on horizontal formwork surfaces (slabs, beams).
oDead Load (DL): Includes the self-weight of the formwork components, the
fresh concrete (typically 24 kN/m³), and the embedded steel reinforcement.
oLive Load (LL): Temporary loads from construction activities, such as
workers, equipment, vibrators, and impact loads from concrete dumping. A
minimum live load of 2.5 kN/m² is often assumed for design.
2. Lateral Pressure of Concrete
Fresh concrete behaves as a fluid and exerts significant hydrostatic pressure on vertical
formwork surfaces (walls, columns). This is often the most critical load in wall and
column formwork design. The maximum lateral pressure depends on:
oRate of Pour (or Rate of Placement): The faster the concrete is poured, the higher the
pressure due to less time for setting and stiffening.
oConcrete Temperature: Higher temperatures accelerate setting, reducing the duration
of fluid behavior and thus lowering peak pressure.
oMix Design (Slump): More workable (higher slump) concrete exhibits higher fluid
pressure.
oMethod of Vibration: Internal vibrators temporarily liquefy the concrete, significantly
increasing lateral pressure, especially during deep penetration.
oAdmixtures: Retarders can increase pressure duration, while accelerators can reduce

4. it.
oElement Dimensions: Pressure can be higher for taller pours or thinner sections where
side friction is less.

3.Wind Loads
For exposed formwork, especially high-rise construction, wind loads must be considered
and forms adequately braced
Formwork Components and Special Systems
Slab Formwork Components
oSheathing: The surface (typically plywood) that directly supports the wet concrete.
oJoists: Small, parallel beams supporting the sheathing.
oStringers (or Bearers): Larger horizontal beams supporting the joists.
oShores (or Props): Vertical posts supporting the stringers and transferring all loads to
the ground or floor below.
oBracing: Diagonal members providing stability to the shoring system against lateral
forces/swaying.
Wall & Column Formwork Components
oSheathing: The surface resisting the lateral pressure of the concrete.
oStuds: Vertical members supporting the sheathing.
oWales (or Walers): Horizontal members supporting the studs and preventing outward
bending.
oForm Ties: Steel rods passing through the concrete to hold opposite wall sides
together against lateral pressure.
oYokes or Clamps: Used to hold column formwork together. Clamp spacing is tightest
at the base due to highest concrete pressure and is increased towards the top.
Special Formwork Systems
oSlip Formwork: The formwork moves continuously upwards as concrete is poured,
used for tall, seamless vertical structures like silos and cooling towers.
oClimbing Formwork: Large sections are "climbed" from one floor to the next, used
for shear walls and building cores in high-rise construction.
oTunnel Formwork: Room-sized steel form that allow walls and slabs of an entire
room to be cast in one pour, ideal for buildings with repetitive layouts like apartments
Formwork Erection, Stripping and safety
Erection and Inspection
Erection: Formwork must be built on a stable foundation (e.g., mudsills on soil) and
assembled precisely according to the design drawings. Bracing must be installed

5. progressively as the formwork is erected.

Inspection Checklist:
oBefore Concreting: Check dimensions, alignment, stability, and tightness of joints.
Ensure release agents have been properly applied.

oDuring Concreting: Continuously monitor the formwork for any signs of movement,
bulging, deflection, or leaking slurry.
oAfter Concreting: Continue monitoring until the concrete has set.
Stripping (Striking)
This is the process of removing the formwork without damaging the concrete. The timing is
critical and depends on the concrete gaining sufficient strength.
oNon-Load-Bearing Forms (Wall/Column sides): Can be removed early,
typically after 24 to 48 hours.
oLoad-Bearing Forms (Slab/Beam soffits): Must stay in place until the concrete
has reached the required strength (e.g., 7 to 21 days), often verified by field-
cured cylinder tests.
Reshoring: The critical process in multi-story construction where additional shores are
installed on lower, immature slabs to safely distribute the load from the newly poured upper
floor down to the foundation.
Safety Protocols
These protocols are crucial for preventing accidents on construction sites, particularly
formwork collapse. The safety protocols focus on three main areas:
Design and Inspection
Protocol Detail
Qualified Design Formwork must be designed by a
qualified person (e.g., a licensed
structural engineer). The design must
account for all Dead Loads,
Live Loads, and critical Lateral Pressures.
Pre-Pour Inspection Formwork must be inspected before each
concrete pour. An Inspection Checklist
should be used to verify:
Alignment and Dimensions: Forms are
correctly positioned and sized.
Tightness of Joints: Joints are sealed to
prevent grout leakage.
Stability and Bracing: The system is
adequately braced against lateral forces and
movement.

6. Release Agent: Ensure the release agent has
been properly applied to aid stripping.
During Pour Monitoring The structure must be monitored during
concreting for any signs of movement,
excessive deflection, or leaking.
Post-Pour Monitoring Continue monitoring until the concrete has
set.
Personnel Safety and Equipment
Protocol Detail
Personal Protective Equipment (PPE) Hard hats, safety boots, and gloves are
mandatory for all workers.
Fall Protection When working at height (e.g., assembling
slab formwork), guardrails and safety
harnesses are required to prevent falls.
Worker Training All workers involved must be trained on the
proper procedures for erecting, stripping,
modifying, and inspecting formwork.
Housekeeping The work area must be kept clean and free
of debris to prevent trips and falls.
Stripping (Striking) Safety
Protocol Detail
Critical Timing Formwork must be stripped carefully to
avoid damaging the immature concrete. The
timing is critical and must be based on the
concrete's strength gain.
Strength Verification Load-bearing forms (slab and beam
soffits) must remain in place until the
concrete has gained sufficient strength,
often verified by field-cured concrete
cylinder tests (typically 7 to 21 days).
Reshoring In multi-story construction, a proper
reshoring plan must be followed to safely
distribute the load from upper floors to the
ground/foundation.

7. REINFORCEMENT
Reinforced Concrete (RC) is a composite material where steel reinforcement (rebar)
compensates for concrete's low tensile strength and ductility. The two materials work
together because they have similar coefficients of thermal expansion, which prevents internal
stresses from developing during temperature changes.
Function and Placement
oPrimary Function: To resist tensile forces and shear forces that concrete alone cannot
handle.
oSecondary Function: To resist some compressive stresses and provide confinement to
the concrete core, enhancing its strength and ductility.
oBond: Rebar is intentionally deformed (ribbed or lugged) to increase surface area and
mechanical interlocking, ensuring a strong bond that effectively transfers stresses
between the two materials.
Standard Sizes and Grades
Rebar is typically made from High Yield Strength Deformed (HYSD) steel or
Thermo-Mechanically Treated (TMT) bars.
Metric Sizes (Nominal Diameter in mm)
Nominal Diameter (mm) Common Use
6mm, 8mm Used for stirrups (links) in light
beams/columns and steel mesh.
10mm, 12mm Primary reinforcement in thin slabs and
small beams, or as distribution bars.
16mm, 20mm Main reinforcement in most beams,
columns, and heavily loaded slabs.
25mm, 32mm Main reinforcement for large structural
members like foundation rafts, large
columns, and deep transfer beams
40mm, 50mm Used only for massive civil engineering
structures (dams, large bridge piers).
Reinforcement Grades (Based on Yield Strength)
Rebar is classified by its Yield Strength, the stress at which the bar begins to permanently
deform. Fe 500 / Grade 60 is the most common grade used today.

8. Grade (Metric) Minimum Yield
Stress
(N/mm
2
)
Common Grade
(Imperial)
Fe 415 415 Grade 60
Fe 500 500 Grade 75

Reinforcement Tests
The quality of steel rebar must be verified through laboratory testing to ensure it meets
design specifications.
Tensile Tests (Strength and Ductility)
These are the most critical tests, performed using a Universal Testing Machine (UTM).
Procedure: A standard-length sample bar is mounted in the UTM and pulled until it fractures.
The load and corresponding deformation are continuously recorded to plot the stress-strain
curve.
oYield Strength Test: Determines the stress at which the steel permanently yields. This
value (e.g., 500 N/mm
2
for Fe 500) is the one used in structural design calculations.
oUltimate Tensile Strength (UTS) test: This is the maximum stress the bar can
withstand before breaking.
oElongation test: Measured as the percentage increase in the bar's original length after
fracture. This is a vital measure of ductility, which is essential for energy absorption
during seismic events.
Bend and Re-Bend Test
These tests assess the bar's ability to be bent without cracking, ensuring quality for on-site
fabrication.
oBend Test Procedure: A sample bar is bent through a specified angle (often 180
o
)
around a mandrel of a specified diameter. The bar is then inspected and must not
exhibit any surface cracks or fractures.
oRe-Bend Test Procedure: This test simulates cold working and strain aging. The bar is
bent, then aged (sometimes by boiling in water), and finally re-bent back to a final
angle. Failure to crack during the re-bend indicates excellent material quality and
resistance to embrittlement.
Chemical Analysis Test
This test confirms the percentages of key chemical components in the steel.
Procedure: A sample is analyzed using a spectrometer.
Key Elements Monitored: Carbon (C), Sulfur (S), and Phosphorus (P).
Significance: High carbon content makes the steel brittle and difficult to weld. High sulfur
and phosphorus are generally undesirable as they can weaken the steel.

9. Protection of Reinforcement (Corrosion Prevention)
Protecting the embedded steel is paramount because corrosion (rusting) causes the steel to
expand up to six times its original volume, leading to cracking and spalling of the concrete
cover.
Adequate Concrete Cover
The thickness of concrete covering the outermost steel bar is the primary protective barrier.
Mechanism: Concrete's high pH (approximately 12.5) forms a thin, passive oxide film on the
steel surface, preventing rust.
Minimum Cover Examples: Slabs: 20mm, Beams: 25mm, Columns: 40mm
Durability Enhancements
oLow Water/Cement Ratio: Creates a dense concrete matrix with low permeability,
significantly slowing the penetration of corrosive ions.
oMinimizing Carbonation: Adequate cover and low permeability prevent atmospheric
CO2 from lowering the concrete's pH and breaking down the passive film.
oCorrosion Inhibitors: Chemical admixtures (e.g., Calcium Nitrite) can be added to the
concrete mix to maintain the passive layer or chemically slow the corrosion reaction.
Specialty Coatings (For Severe Environments)
oEpoxy-Coated Rebar (Green Rebar): A Fusion Bonded Epoxy (FBE) layer acts as a
physical barrier against chlorides and moisture, often used in bridge decks and
parking garages.
oGalvanized Rebar: A zinc coating provides sacrificial (cathodic) protection by
corroding preferentially to the steel.
oStainless Steel Rebar: Used for structures with an expected life of 100+ years and
extreme exposure, offering superior corrosion resistance but at a high cos