What is Prestressed Concrete?
Summary
TLDRIn this Practical Engineering video, Grady explores prestressed concrete, a method to enhance concrete's structural integrity by applying compressive stress before use. Despite concrete's strength against compression, it's brittle and prone to cracking under tension. Prestressing, achieved by tensioning the reinforcement (usually steel) before the concrete cures, reduces deflection and cracking, improving the material's serviceability without necessarily increasing its strength. The video demonstrates pre-tensioning and post-tensioning techniques, showing how prestressed beams can withstand significantly more force before cracking occurs, highlighting the technique's effectiveness in structures like bridges and buildings.
Takeaways
- 😌 **Concrete's Inherent Flaws**: Concrete is almost guaranteed to crack due to its lack of tensile strength and brittle nature.
- 🔩 **Reinforcement's Role**: Reinforcing concrete with steel can help manage its weaknesses, but it's not enough to prevent cracking.
- 🏗️ **Design Criteria**: Structural design goals include ultimate strength to avoid collapse and serviceability to minimize deflection and cracking.
- 🚧 **Cracks and Perception**: Cracks in concrete structures can affect public perception of safety and can lead to structural issues over time.
- 🔄 **Deflection and Reinforcement**: Deflection in reinforced concrete can cause cracks, which may compromise the integrity of the structure.
- 🏋️♂️ **Pre-stressing Techniques**: Pre-stressing concrete members by applying compressive stress before service can reduce deflection and cracking.
- 🔧 **Pre-tensioning Method**: Pre-tensioning involves stressing steel before the concrete cures, using a frame to maintain tension during curing.
- 🔩 **Post-tensioning Method**: Post-tensioning applies stress to the steel after the concrete has cured, using rods that are tensioned within plastic sleeves.
- 📏 **Testing Prestressed Beams**: Prestressed beams demonstrate significantly less cracking and deflection under load compared to conventionally reinforced beams.
- 🏙️ **Applications of Prestressed Concrete**: Prestressed concrete is widely used in various structures like bridges, buildings, silos, and tanks to enhance performance and durability.
Q & A
What is the first rule of concrete according to the video?
-The first rule of concrete is that it's pretty much guaranteed to crack.
What are the two main weaknesses of concrete as a structural material?
-Concrete has almost no strength against tension and it is brittle, lacking any 'give' or ductility.
Why is understanding where and how much a concrete structure will crack important?
-Understanding where and how much a concrete structure will crack is crucial because it can determine the success or failure of the structure.
What are the two main design criteria for reinforced concrete structures mentioned in the video?
-The two main design criteria are ultimate strength, ensuring the structure doesn't collapse, and serviceability, avoiding excessive deflection or movement under load.
Why is deflection a concern in reinforced concrete structures?
-Deflection is a concern because it can make structures feel unsafe, potentially cause attached materials like plaster or glass to break, and allow water and contaminants to reach the reinforcement, leading to corrosion and failure.
How does prestressing concrete help in reducing deflection?
-Pre-stressing concrete involves putting compressive stress into the structural member before it's put into service, which balances the tensile stresses once in use, thus reducing deflection.
What are the two main methods of prestressing reinforcement within concrete?
-The two main methods are pre-tensioning, where the steel is stressed before the concrete cures, and post-tensioning, where the steel is stressed after the concrete cures but before the member is put into service.
How does pre-tensioning differ from post-tensioning in the context of the video?
-Pre-tensioning involves stressing the steel before the concrete cures, using a frame to hold the steel in tension during curing. Post-tensioning stresses the steel after the concrete has cured, using plastic sleeves to allow the rods to slide and be tensioned after curing.
What was the result of the testing on the pretensioned beam compared to a conventionally reinforced beam?
-The pretensioned beam did not show cracks until double the force (around 8 tons) compared to a conventionally reinforced beam, which started cracking at around 4 tons.
What is the primary benefit of using pre-stressed concrete in structures?
-The primary benefit of using pre-stressed concrete is to minimize cracking and take fuller advantage of the strength of reinforced concrete, thus increasing the serviceability of the member.
Outlines
🏗️ Introduction to Prestressed Concrete
The paragraph introduces the concept of prestressed concrete, highlighting the inherent weaknesses of concrete such as its brittleness and lack of tensile strength. It explains how prestressing can mitigate these issues by applying compressive stress to the structure before it's put into service, thus reducing deflection and cracking. The video discusses the importance of design criteria in structural engineering, emphasizing not only the ultimate strength to prevent collapse but also the serviceability to avoid deflection and cracking. The paragraph also touches on the difference between conventionally reinforced and prestressed concrete, explaining how the latter can better utilize the strength of both concrete and steel.
🔩 Demonstrating Pre-tensioning and Post-tensioning Techniques
This paragraph details the process of creating prestressed concrete beams using both pre-tensioning and post-tensioning methods. The pre-tensioning process involves tensioning steel rods before the concrete cures, using a frame to maintain tension while the concrete hardens. Post-tensioning, on the other hand, involves stressing the steel after the concrete has cured. The paragraph describes the construction of test beams, the use of a concrete vibrator for consolidation, and the subsequent testing of these beams under a hydraulic press. The results show that prestressed beams can withstand significantly more force before cracking compared to conventionally reinforced beams, demonstrating the effectiveness of prestressing in enhancing the serviceability and reducing deflection of concrete structures.
Mindmap
Keywords
💡Concrete
💡Cracking
💡Reinforcement
💡Prestressed Concrete
💡Deflection
💡Tension
💡Pre-tensioning
💡Post-tensioning
💡Serviceability
💡Ultimate Strength
💡Dashlane
Highlights
Concrete is almost guaranteed to crack, but not all cracking is considered equal.
Reinforcing concrete can minimize its negative impacts.
Concrete has almost no strength against tension and is brittle.
Understanding where and how much concrete will crack is crucial for structural success.
Design criteria for structures include ultimate strength and avoiding deflection.
Deflection can cause perception issues and damage to attached materials.
Reinforcement in concrete is usually made from steel, which is more elastic.
Concrete must crack before the rebar can take up tensile stress.
Pre-stressing concrete members can reduce deflection and cracking.
Pre-stressing is achieved by tensioning the reinforcement within the concrete.
Pre-tensioning involves stressing steel before the concrete cures.
Post-tensioning stresses steel after the concrete cures but before service.
Pre-stressed concrete increases serviceability by reducing deflection under load.
Pre-stressed concrete is used in various structures like bridges, buildings, silos, and tanks.
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Transcripts
Talk to any concrete professional and they’ll tell you the first rule of concrete is this:
it’s pretty much guaranteed to crack.
But not all cracking is considered equal, and there is a way to reinforce concrete to
minimize its negative impacts.
Hey I’m Grady and this is Practical Engineering.
Today we’re talking about prestressed concrete.
This video is sponsored by Dashlane, never forget a password again.
More on that later.
Despite its excellent qualities as a structural material, concrete has some weaknesses, too.
One that we’ve discussed in previous videos is that it has almost no strength against
tension.
Concrete can withstand a tremendous amount of compressive stress, but when you try to
pull it apart, it gives up easily.
Concrete’s other weakness is that it’s brittle.
It doesn’t have any “give” or stretch or ductility.
Combine these two weaknesses, and you get cracks.
Concrete loves to crack.
And if you’re designing or building something made of concrete, understanding how much and
where it’s going to crack can be the difference between the success and failure of your structure.
To understand how engineer’s design reinforced concrete structures, we first have to understand
design criteria - or the goals of the structure.
The obvious goal that we all understand is that it shouldn’t fall down.
When a car drives over a bridge and the bridge doesn’t collapse, the structure is achieving
its design criterion of ultimate strength.
But, in many cases in structural engineering, avoiding collapse actually isn’t the limiting
design criteria.
The other important goal is to avoid deflection, or movement under load.
Most structural members deflect quite a bit before they actually fail, and this can be
bad news.
The first reason why is perception.
People don’t feel safe on a structure that flexes and bends.
We want our bridges and buildings to feel sturdy and immovable.
The other reason is that things attached to the structure like plaster or glass might
break if it deflects too much.
In the case of reinforced concrete, deflection has another impact: cracks.
The reinforcement within concrete is usually made from steel, and steel is much more elastic
than concrete.
So, in order to mobilize the strength of the steel, first it has to stretch a little.
But, unlike steel, concrete is brittle - it’s doesn’t stretch, it cracks.
So that often means that concrete has to crack before the rebar can take up any of the tensile
stress of the member.
This demonstration is from a test in a previous video showing a conventionally reinforced
concrete beam.
Go back and check that video out if you haven't seen it yet.
Notice how this beam is resisting the load on it, even though it is cracked at the bottom.
It’s meeting design criterion number 1 - it’s holding the load (in this case 6 tons) without
failing.
But it’s not meeting design criterion number 2 (serviceability) - it’s deflecting too
much and the concrete is cracked.
Those cracks not only look bad, but in an actual structure, they could allow water and
contaminants into contact with the reinforcement, eventually causing it to corrode, weaken,
and even fail.
One solution to this problem of deflection in concrete members is pre-stressing, or putting
compressive stress into the structural member before it’s put into service.
This is normally accomplished by tensioning the reinforcement within the concrete.
This gives the member a compressive stress that will balance the tensile stresses imposed
in the member once it is put into service.
A conventionally reinforced concrete member doesn’t have any compression to start with,
so it will deflect too much well before it’s in any danger of not being strong enough to
hold the load.
So with conventional reinforcement, you don’t even get to take full advantage of the structural
strength of the member.
When you prestress the reinforcement within concrete, you don’t necessarily increase
its strength, but you do reduce its deflection.
This balances out the maximum load allowed under each of the structural design criteria,
allowing you to take fuller advantage of the strength of each material.
There are two main ways to prestress reinforcement within concrete, and of course I built a couple
of beams to demonstrate.
The first method is pre-tensioning.
And yes that terminology is a little confusing.
It’s pre-stressed because the steel is stressed before the member is put into service, but
pre-tensioned because the steel is stressed before the concrete cures.
To make this work, I had to build a little frame to go around my concrete beam.
This frame will hold the steel in tension while the concrete cures.
I installed threaded rods through the mold and frame, and then tensioned these rods by
tightening the nuts.
I tried to use the pitch of the ringing to get them at around the same tension, and you
can see how much my frame is flexing from the force in these steel rods.
The other method for pre-stressing steel is post-tensioning.
In post-tensioning, the steel is stressed after the concrete cures, but still before
the member is put into service.
In this beam I cast in smooth plastic sleeves in the mold.
The steel rods can slide easily within the sleeves.
Once both molds were prepared, I filled them up with concrete.
And I finally got a construction grade concrete vibrator as well.
This machine helps get all the air bubbles out of fresh concrete before it cures, a process
called consolidation.
After the concrete’s has had some time to cure, it’s time to test the beams out.
On the pretensioned beam, I can unscrew the nuts and take off this frame.
Because the concrete hardened around the bolts, the steel rods are still under tension inside
this beam.
I put it under the hydraulic press for testing, and the results are easy to see.
In a conventionally reinforced beam where the steel is simply cast into the concrete
without any tension, cracks start forming at around 4 tons.
In the pretensioned beam, the cracks didn’t appear until double that force at around 8
tons.
The tension already in the steel is able to take up the force of the press without requiring
the beam to flex.
For the post-tensioned beam, I inserted the steel reinforcement after the concrete had
cured.
Then I tightened the bolts on the rods to pre-stress the steel.
Under the hydraulic press, the results are nearly identical.
The tension in the steel held beam in compression for much longer than a conventionally reinforced
member could.
Of course, the cracks eventually appear, but it takes much more force before they do.
That’s because, adding force to the beam is not creating tension, but just reducing
the compression that’s already been introduced through the tension in the steel rods.
It’s important to point out that we didn’t necessarily make these beams stronger.
Both the steel and concrete have the same strength as they would without prestressing
the steel.
But, we did increase the serviceability of member by reducing the amount of deflection
under load.
Of course, none of these examples actually failed because of the reinforcement, and that
wasn’t the point of the demo.
But, it’s still more fun to test everything to failure.
Pre-stressed concrete is used in all kinds of structures from bridges to buildings to
silos and tanks.
It’s a great way to minimize cracking and take fuller advantage of the incredible strength
of reinforced concrete.
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