Post Tension Slab | Eliminating cracks and joints in concrete!

Summary

This video, hosted by Tyler Lai, discusses the concept of post-tensioned (PT) slabs on ground (SOG) and how they reduce concrete cracking. Tyler explains the basics of PT technology, which involves using prestressed tendons in the slab to create compression, minimizing cracks and joints. He compares PT slabs to traditional rebar methods, highlighting the benefits of fewer maintenance needs and suitability for expansive soils. Although PT slabs can be more complex and costly, they are effective when properly installed. The video offers an engaging and educational take on modern concrete techniques.

Takeaways

• 🧱 The video is about post-tension slab on ground (SOG) concrete construction.
• 👷‍♂️ Cracks in concrete slabs can be a major problem, but cutting joints after casting can help control where cracks form.
• 🗡️ The Romans used arches to avoid cracks by keeping structures in compression, and a similar concept is applied to slabs using post-tensioning (PT).
• 📏 PT involves inserting tendons, which are prestressing strands, inside the slab to compress the concrete, reducing cracking.
• 🏗️ The tendons are spaced 24-36 inches apart, and after the concrete cures, hydraulic rams are used to pull the tendons, compressing the slab.
• 💥 When the tendons are pulled, an anchor locks the tendon in place, keeping the slab in compression and making it stronger.
• 📉 PT allows for fewer and smaller cracks, with joint spacing between 200 and 500 feet, compared to rebar-based slabs.
• 🌍 PT slabs are especially useful in areas with expansive soils, reducing maintenance for property owners.
• ⚠️ Challenges with PT slabs include higher costs for long runs, risks when cutting tendons, and managing bursting stresses at slab edges.
• 👍 Despite being more complex than rebar, PT slabs offer significant benefits, especially when done correctly.

Q & A

What does SOG stand for in the context of this video?

SOG stands for 'Slab on Ground,' which is the type of concrete slab being discussed in the video.

Why are cracks in concrete slabs considered problematic?

Cracks in concrete slabs can lead to structural weaknesses and maintenance issues, causing fear of costly repairs or reduced durability.

How are cracks controlled in a slab-on-ground (SOG) structure?

Cracks are controlled by cutting joints into the slab about 1/3 of the depth after the slab has been cast, typically 14 to 18 hours after pouring. This allows cracks to form in a controlled manner within the joints.

What ancient technique did the Romans use to minimize cracking in their concrete structures?

The Romans used concrete arches, which were mostly under compression, to minimize cracking and create long-lasting structures.

What is post-tensioning (PT), and how does it work in a slab-on-ground?

Post-tensioning (PT) involves using tendons (prestressed strands) embedded in the concrete, which are pulled tight using hydraulic rams after the concrete is cast. This puts the slab in compression, reducing the likelihood of cracks.

What are tendons in the context of post-tensioning?

Tendons are prestressed strands wrapped in plastic material with grease inside them. They are placed inside the slab and are tightened during the post-tensioning process to apply compression to the concrete.

What is the benefit of post-tensioning a concrete slab compared to using rebar?

Post-tensioning doubles the stress capacity needed to crack the concrete and allows for fewer joints, with larger spacing (200 to 500 feet apart). It is especially beneficial for expansive soils and results in fewer, smaller cracks.

What are some challenges associated with post-tensioning concrete slabs?

Challenges include higher costs for long strand runs, the need to protect tendons and anchors, the danger of cutting into tendons during modifications, and the risk of tendons snapping during stressing.

What are 'bursting stresses' in post-tensioning, and how are they managed?

Bursting stresses occur near the ends of the slab when the tendons are stressed, creating high stresses in those areas. Additional reinforcement is needed at the slab edges to manage these stresses.

What are the advantages of post-tensioned slabs in terms of maintenance?

Post-tensioned slabs significantly reduce the number of cracks and joints, minimizing maintenance issues. This leads to happier owners due to the lower long-term maintenance costs.