| Technology
The Pros of Pre-Stressed Concrete
Engineer PARTHA PRATIM ROY explains the advantages and economics
of pre-stressed concrete in building
pre-stressed concrete is a method for overcoming concrete’s
natural weakness in tension. For this, pre-stressing tendons
- generally of high tensile steel cable or rods - are used
that produce a compressive stress that offsets the tensile
stress that the concrete compression member would other-wise
experience owing to self weight and gravity loads. Traditional
reinforced concrete is based on the use of steel reinforcement
bars inside poured concrete.
Pre-stressing can be accomplished in two ways: pre-tensioned
concrete and bondedor unbounded post-tensioned concrete.
Pre-tensioned concrete
Pre-tensioned concrete is cast around already tensioned tendons.
This method produces a good bond between the tendon and concrete,
which protects the tendon from corrosion and allows for direct
transfer of tension. The cured concrete adheres and bonds
to the bars; when the tension is released, it is transferred
to the concrete as compression by static friction.
However, it requires stout anchoring points between which
the tendon is to be stretched and the tendons are usually
in a straight line. Thus, most pre-tensioned concrete elements
are prefabricated in a factory and must be transported to
the construction site, which limits their size. Pre-tensioned
elements may be balcony elements, lintels, floor slabs, beams
or foundation piles. Innovative bridge construction is also
possible using prestressing.
Bonded post-tensioned concrete
Bonded post-tensioned concrete is the descriptive term for
a method of applying compression after pouring concrete and
the curing process (in situ). The concrete is cast around
plastic, steel or aluminium curved duct, to follow the area
where tension would otherwise occur in the concrete element.
A set of tendons is fished through the duct and the concrete
is poured. Once the concrete has hardened, the tendons are
tensioned by hydraulic jacks that react against the concrete
member itself. When the tendons have stretched sufficiently,
according to the design specifications (Hooke’s Law),
they are wedged in position and maintain tension after the
jacks are removed, transferring pressure to the concrete.
The duct is then grouted to protect the tendons from corrosion.
This method is commonly used to create monolithic slabs for
house construction in locations where expansive soils (such
as adobe clay) create problems for the typical perimeter foundation.
All stresses from seasonal expansion and contraction of the
underlying soil are taken into the entire tensioned slab,
which supports the building without significant flexure. Post-stressing
is also used in the construction of various bridges; both
after concrete is cured after support by false work and by
the assembly of prefabricated sections, as in the segmental
bridge. The advantages of this system over un-bonded post-tensioning
are:
Large reduction in traditional rein-forcement requirements
as tendons cannot de-stress in accidents.
Tendons can be easily ‘weaved’, allowing a more
efficient design approach.
Higher ultimate strength owing to bond generated between the
strand and concrete.
No long-term issues with maintaining the integrity of the
anchor/dead end.
Un-bonded post-tensioned concrete
Un-bonded post-tensioned concrete differs from bonded post-tensioning
by providing each individual cable permanent freedom of movement
relative to the concrete. To achieve this, each individual
tendon is coated with grease (generally lithium-based) and
covered by plastic sheathing formed in an extrusion process.
The transfer of tension to the concrete is achieved by the
steel cable acting against steel anchors embedded in the perimeter
of the slab.
The main disadvantage over bonded post-tensioning is the
fact that a cable can de-stress itself and burst out of the
slab if damaged (such as during repair on the slab). The advantages
of this system over bonded post-tensioning are:
The ability to individually adjust cables based on poor field
conditions (for example, shifting a group of four cables around
an opening by placing two on either side).
The procedure of post-stress grouting is eliminated.
The ability to de-stress the tendons before attempting repair
work.
Post-tensioning in building structures
The market factors that favour implementing a post-tensioning
system in building structures are:
• Longer spans
• Unique designs: irregular shapes
• Shorter construction cycles
• Cost reduction
• Shorter floor-to-floor heights
• Superior structural performance
Direct cost reduction
Post-tensioning offers direct cost reduction over conventionally
reinforced slabs primarily by reducing concrete
and rebar material quantities as well as rebar installation
labour. Typically, savings between 10 and 20 per cent in direct
cost are achieved.
The following factors contribute to direct cost reduction:
• Less concrete material
• Reduction in slab thickness reduces total building
height and cost
• Less rebar
• Less labour cost for installation of material
• Reduced material handling
• Simplified formwork leads to less labour cost
• Rapid reuse of formwork leads to less formwork on
jobsite
As a rule, the break-even mark between conventional and pre-stressed
solutions is about 7-m spans.
Improved construction efficiency
As post-tensioned slabs are designed to carry their own weight
at the time of stressing, they can significantly improve construction
efficiency and deliver an additional 5-10 per cent of indirect
savings.
The following factors contribute to improved construction
efficiency:
• Shorter construction cycles.
• Less material handling and impact on other trades.
• Simpler slab soffit - less beams and drop caps/panels.
• Quicker removal of shoring gives more access to lower
slabs.
A typical five-day construction cycle schedule for 800-1,000
sq m of slab is shown below. A three-day cycle is also achievable
with early strength concrete and industrial formwork.
Superior structural performance
The pre-stressing in post-tensioned slabs takes optimal advantage
of tendon, rebar and concrete properties to deliver an economical
structural system.
The factors contributing to superior structural performance
are:
• Use of high-strength materials
• Deflection control
• Longer spans are achieved
• Crack control and water-tightness
• Reduced floor-to-floor height
• Lighter structure requires lighter lateral load resisting
system
• Economy in column and footing design
• Reduced noise transmission compared to RC
• Lower total cost of ownership (mainte-nance) compared
to RC alte-rnatives
Typical quantities
Post-tensioning and rebar rates vary greatly depending on
span configuration and loading. Compared to other countries,
PT projects in US are designed with less loading and lower
PT and rebar rates.
Bonded system :
US values (1 kN/sq m SDL & 2.5 kN/sq m LL)
• 3 - 4 kg/sq m of PT
• 5 kg/sq m of rebar With higher loading (3 kN/ sq m
SDL & 3 kN/ sq m LL)
• 3.5 - 5 kg/sq m of PT
• 7 - 9 kg/sq m of rebar
Unbonded system :
US values (1 kN/sq m SDL & 2.5 kN/sqm LL)
• 3.75 kg/sq m of PT
• 6 kg/sq m of rebar
About the author:
Partha Pratim Roy is General Manager (Technical), ADAPT International
Pvt Ltd.
He can be contacted at intl@adaptsoft.com
| 
