Pre-stressed concrete beams, what exactly are they??
How are they made?
Why are they useful?
Before we explain, it’s necessary to provide a brief overview of traditionally reinforced
concrete beams for comparison.
Concrete is very strong in compression, but only about one tenth as strong in tension.
So if we make a beam of pure concrete and apply a bending load, the top half gets compressed
while the bottom half is in tension.
This tension zone in the bottom half will form cracks that propagate until the whole
beam fails.
This is where reinforcing steel comes in; primary structural bars are generally placed
in the tension zone—so when the concrete cracks the reinforcing steel takes the tension
load.
There are other types of bars such as for controlling thermal effects, but that’s
beyond the scope of this video.
It’s also worth keeping in mind that the tension zone is not necessarily always on
the bottom; for example near a column or at a cantilever, the tension would be in the
top and compression in the bottom.
Traditionally reinforced concrete essentially combines steel and concrete, then lets it
act as it would passively, compared to pre-stressed concrete which actively engages the materials.
Now let’s contrast the traditionally reinforced concrete beam to a pre-stressed one.
We’ll look first at a pre-cast, pre-tensioned concrete beam.
First the cables are placed in a form, and then they are tensioned.
Next the concrete is poured, and then allowed to cure.
Once cured, the cables can be released from the tensioning forms.
Now, the pre-stressed cables will contract; this induces compression on the bottom half
as part of manufacturing, like a cambering effect.
Once the beam begins taking load, it still applies tension to the bottom half, but this
time that tension has to cancel out some pre-compression before creating a zone of net-tension; in
this way, the pre-stressed beam is able to take more load than the traditionally reinforced
concrete one (all else equal).
The amount of pre-stressing has to be carefully calibrated to avoid tensile cracking on the
top with too much pre-stressing, but also must be enough to avoid concrete shrinkage
depleting the pre-stress over time as the concrete cures.
Early pre-stressed concrete designs made the mistake of not properly accounting for concrete
shrinkage.
As concrete cures, the water inside it evaporates and this causes shrinkage, which can occur
for up to 18 months of curing.
Let’s examine how this shrinkage would impact our pre-stressed beam: As the concrete shrinks,
so too would the pre-stressed steel bonded to it—which reduces the tension force in
the steel, and thus reduces the pre-stressing load.
So in order to offset the losses of shrinkage, you need more pre-stressing; but, at a certain
point you’ll rupture the steel if you pre-stress too much—that’s where high strength reinforcing
comes in to play.
About 100 years ago, designers of concrete structures realized that in order to properly
utilize high strength reinforcing, it needed to be pre-stressed.
This led to a lot of development in material science for high strength reinforcing, as
well as for high strength concrete.
Today, pre-stressed concrete generally uses both high strength reinforcing and high strength
concrete in order to maximize the benefits of pre-stressing.
We can’t discuss pre-stressing without also discussing post-tensioning.
Post-tensioned concrete is a similar active combination of concrete and steel, also generally
using high strength materials.
But, for post-tensioning the “pre-stress” is applied on site after concrete has cured
but before the structure sees its anticipated design loads.
Post-tensioned reinforcing tendons are carefully draped throughout slab and beams to achieve
desired bending and deflection criteria.
Post-tensioned tendons can be either sheathed and greased or unsheathed, which have different
benefits for load transfer, ease of construction or replacement, and weatherability—but that’s
a deep dive for a whole separate video.
The main benefits of prestressing generally stem from the fact that it actively engages
the steel and concrete.
This results in a stiffer section that presents options to use less material, allow longer
spans, or carry additional load.
Of course, these benefits come at the cost of added design and construction complexity,
so use depends on the specific application.
If this brief introduction to pre-stressed beams was interesting for you, let us know
your thoughts in the comments.
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