In-Situ Concrete Floors Prestressed and Post Tensioned Floors

slicedhillockUrban and Civil

Nov 25, 2013 (3 years and 6 months ago)

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In
-
Situ Concrete Floors

Pres
tressed and Post Tensioned Floors




A simple reinforced concrete floor (Figure 1) is a simple solution when used in conjunction with an in
-
situ concrete frame and



Whilst a certain amount of deflection is permiss
ible, usually 1/360th of the span in a beam constructed floor
(Riley, M & Cotgrave, A, 2004), one of the inherent problems with concrete is the amount it deflects. Initially
this is due to shrinkage as the concrete cures but the effect of creep continues o
ver time. BS8110 details how
this should be accounted for in the design. Movement joints accommodate for creep and shrinkage aswell as
elastic structural deflections, temperature changes and differential settlements
(Kenward, J
.

et al, 2002),
however the u
se of prestressed and post tensioned concrete members significantly reduce the amount of
deflection.

















is generally poured at the same time as the beams to form an integrated system. They are not usually economically viable for















spans over 5.00m (Chudley, R & Greeno, R. 2003) due to the depth that would be required. To resolve this, beams can be

included within the design to span between the columns (Figure 2 & 3). This does produce a deeper floor construction and it d
oes

require more complex formwork and reinforcement therefore increasing on site labour costs and construction time. The

formwork also requires extensive propping. The slab design can be further refined by incorporating the beam within the

slab depth wh
ich whilst reducing the overall floor depth, increases the slab depth and requires more complex reinforcement

especially at column positions (Figure 4) (Chudley, R & Greeno, R. 2003).





Concrete is not an effective mater
ial under tensile stress as is required for floor structures. It is much more
effective when subjected to compressive forces. Prestressing or post tensioning impart a compressive stress
to the structural member to counteract what would normally be subject
to tensile stress. It can be simply
demonstrated with a row of books. If a row of books is not supported by a shelf or floor, they would collapse.
If one places a hand at each end of the books and applies compressive force, this counteracts the tensile
str
ess cause by the weight of the books and enables the books to be self supporting (
Foster, J. & Harington, R.

2000). Figure 7 describes methods of prestressing.






Figure 1. Simple Reinforced Concrete Floor

Figure 2. One Way Spanning Slab


(Riley, M & Cotgrave, A, 2004) (Chudley, R & Greeno, R. 2003)






Figure 7. Typical Pre
-
tensioning arrangeme
nt Figure 8. Typical Post
-
tensioning arrangement


(Chudley, R & Greeno, R. 2003)

(Chudley, R & Greeno, R. 2003)




Pre
-
tensioned units are
generally
factory
produced so quality is easily controlled.
The wires are placed in the mould and stressed before the concrete is placed.
Hydraulic jacks overstress the wires to 110% and after the concrete is cured, the
wires are cut. The bond between the wires and the conc
rete means that as the
wires try and regain their original length they
put the concrete into
precompression or prestress. A large number of small diameter wires are usually
used to provide greater surface contact.









Figure 3. Two Wa
y Spanning Slab Figure 4. Two Way Spanning Flat Slab



(Chudley, R & Greeno, R. 2003) (Chudley, R & Greeno, R. 2003)







Post
-
tensioning
(Figure 8)
is usually carried out on site after an situ component
has been cast or to join several precast members together. The concrete is cast
around a duct or sheathing and after curing, the tendons are placed within the
duct and then stres
sed by the use of hydraulic jacks at one or both ends of the
member. The anchorages prevent the tendons from regaining their original length
and therefore create the precompression on the member. The remaining space
within the ducts should be filled with g
rout via the grout holes to prevent
corrosion from trapped moisture.



One method of reducing the floor depth is by using ribbed floors. This replaces the widely spaced, deep beams with narrowly
spaced


shallow beams. They can span in one direction as

in a ribbed floor or two ways in a waffle floor (Figure 5). These waffles are
formed by


casting the concrete using glass reinforced plastic formers which are removed after curing. A variation of the ribbed floor i
s the
Hollow


Pot Floor (Figure 6) wh
ich utilizes hollow pots to act as permanent formwork with the concrete floor cast in situ. The voids can be


used
for accommodating services and the flat soffit enables a dry lining or plaster finish to be eas
ily obtained. Large spans can be


obtained

with this method, especially with the use of prestressed beams. Prefabrication of beams means they are quick to install
equire


but requires
cranage due to the weight. Formwork is still required, there is some inflexibility in design and the use of in s
itu


concrete creates
issues of quality control for a critical structural element.




Figure 5. In situ waffle floor Figure 6. Concrete Composite

/ Hollow Pot Floor


There are vari
ous methods of anchoring the tendons
: Magnol
-
Blaton, Gifford
-
Udall, Lee
-
McCall and Freyssinet and Monostra
n
d as shown in Figure 9.



(Riley, M & Cotgrave, A, 2004) (Riley, M & Cotgra
ve, A, 2004)





















Figure 9. Freyssinet and Monostra
n
d Anchorages


(Chudley
, R & Greeno, R. 2003)