While concrete is strong in compression, it is weak in tension. Steel is strong under forces of tension, so combining the two elements results in the creation of very strong concrete components. Post-tensioning can help create innovative concrete components that are thinner, longer, and stronger than ever before.

What is the purpose of a post tension slab?

Post-tension concrete slabs are used to create a monolithic (single pour) slab that is stronger than a traditional slab without reinforcement. This is achieved by laying out high-strength steel cables in a crisscrossed grid pattern throughout the foundation prior to pouring the concrete.

Is a post tension slab better?

First off, one could argue that post-tensioning is superior to conventionally reinforced concrete in a pure engineering sense. The reason is because post-tensioning is a method of pre-compression, which means the concrete is put into a state of compression, which allows the concrete to take on flexure without cracking.

Should a post tension slab crack?

Post-tensioned concrete slabs are typically expected to crack less than other slabs because post-tensioning produces a compressive stress that must be exceeded before enough tensile stress develops to cause cracking.

What happens if you drill into a post tension slab?

Post-tension slab drilling, cutting, and coring can be incredibly dangerous. Not only is there a risk of structural collapse, but you also risk severe injury and death. It's definitely NOT something you want to play around with.

Post Tension Slab Basics

17 related questions found

What is the principle of post-tensioning?

Working Principle of Post Tensioning

When these post tensioned steel tendons are stressed, the concrete is squeezed, in other terms, the concrete is compacted which increases the compressive strength of the concrete and at the same time the steel tendons that are pulled increase the tensile strength.

Why do shrinkage cracks happen?

Plastic shrinkage cracks occur when there is rapid loss of water from the surface of freshly poured concrete (before it has fully set). This causes the top of the concrete slab to dry more quickly than the bottom, and they pull apart.

When did post tension slabs begin?

The first post-tensioning in U.S. building construction was in the mid- to late 1950s in buildings using the lift- slab construction method.

How can you tell if a slab is post tensioned?

Most homes that have post tension slabs have a sign that is stamped into a concrete garage floor, often near the edge of the garage door right in plain sight. Older homes with post tension slabs may have a plastic or paper sign fastened to the wall.

Do post tension slabs have rebar?

With a post-tension foundation, the contractor digs the perimeter trench in much the same way as in the conventional foundation. ... Rebar and fill sand are used in a similar way as conventional foundation, but a post-tension foundation also includes steel cables that form a grid pattern across the entire slab.

What is the difference between normal slab and post tension slab?

ØNormal slab is reinforced concrete(RCC) structural element whereas post-tension slab is per-stressed concrete (PSC) structure. In Post tension slab the reinforcement is replaced with cables/ steel tendons. ... ØIt requires more formwork when compared with the flat slab.

Is it possible to have 0% crack in a building?

Explanation: Concrete expands and shrinks when the temperature changes which directly leads to cracking in the building. 4. Is it possible to have 0% crack in our building? Explanation: It is not possible to have 0 % cracks in any building after construction.

Why does bleeding occur in concrete?

Bleed in concrete is the result of excess water being forced to the surface of the concrete. ... Any excess water not absorbed by the sand and cement rise to the surface. This leaves a pool of water on the surface of the concrete, which we call bleed water.

What is plastic settlement?

Plastic settlement cracks are so-called because they form while the concrete is still plastic, ie has not set. The settling concrete is restrained and cracks form at the surface. They may become visible very early, ie while finishing is proceeding, but are often not noticed until some hours after placement.

Where is post tensioned concrete used?

Common uses and advantages

Post-tensioning now is used extensively in bridges, elevated slabs (parking structures and residential or commercial buildings), residential foundations, walls, and columns. Jim Rogers Bending and flexing creates high tensile forces that can cause the concrete floor slab to crack.

What are the advantages of prestressed concrete?

Advantages of Prestressed Concrete

Longer span length increases untroubled floor space and parking facilities. Longer spans results in fewer joints, as a result, less maintenance is needed. Possibility of steel corrosion and subsequent concrete deterioration is declined because concrete is crack-free.

What is the difference between post tensioned and prestressed?

Pretension is the technique in which we are imparting tension in strands before placing the concrete. Post tensioning is done by forming a duct in which strands are pulled (tensioned) after the concrete gains it's full strength. ... This method is done due to bonding between concrete and steel.

What is difference between segregation and bleeding?

Bleeding is a form of segregation where some of the water in the concrete tends to rise to the surface of the freshly placed material. ... The presence of an adequate proportion of very fine aggregate (smaller than 150µm) reduces bleeding. Similarly polypropylene micro-fibres are known to reduce bleeding.

Why does honeycomb occur in concrete?

Honeycomb is a rough and stony surface that appears on concrete when there are air voids between the course and the aggregate. This happens when insufficient fine material gets into the concrete mixture due to poor mixing or incorrect aggregate grading.

What is the permeability of concrete?

Permeability is a measure of the amount of water, air, and other substances that can enter the concrete matrix. Concrete contains pores that can allow these substances to enter or depart.

Are concrete cracks normal?

While cracking is very normal in freshly poured concrete, cracks usually become undetectable as the work settles. It's alarming to notice thin cracks forming in concrete when you just paid for the cost of a new driveway, concrete slab, walkway, or garage floor.

Does concrete react with air?

Rather, it sets as it dries and reacts with carbon dioxide in the air. It is resistant to attack by chemicals after setting. Hydraulic cements (e.g., Portland cement) set and become adhesive due to a chemical reaction between the dry ingredients and water.

Why do Walls crack in buildings?

Cracks can occur due to various reasons chemical reactions in construction materials, foundation movements and settling of buildings, changes in temperature and climate, environmental stresses like the nearby movement of trains, earthquakes etc.

How long does post tension slab last?

Conventional concrete & rebar foundations last about 20-30 years before failure begins to occur. Post-tension slabs are lasting about 15-20 years and are failing at a much higher rate. Post-tension slabs are created with concrete trenches around the perimeter of the house where the external walls will be placed.

What are the disadvantages of Post Tensioning?

Here are a few disadvantages of post tension slabs:

• Since there are a number of tendons and wires spread inside the post tension slab, it can result in corrosion. ...
• Complexity of work: The post tension slab can be made only by skillful professionals.