Drilling into a post-tensioned slab Another popular myth with post- tensioned slabs is that it is very difficult to drill into an existing slab because of the unknown location of the tendons and anchors. But as long the tendons and the concrete in front of the anchors are not damaged, drilling into a post- tensioned slab is a fairly routine issue.
Can you cut into post tension slab?
The only concern is to always remember not to cut or drill into post-tensioned concrete slabs, since once a tendon has been cut, it is very difficult to repair. Many post-tensioned slabs will be stamped to alert the owner and any renovation contractors that the slab is post tensioned.
Can I drill into a post tension slab?
Yes, you can drill into a PT slab. However, you want to make sure you know where the cables are located. I design PT slabs and repairs. It is rare for a tendon to break out of the slab on a residential house. It will shoot out the plugs around the slab edge. If it hit someone it could break a leg.
How long should I wait before drilling into concrete footing?
Let the concrete cure for at least 3 days before drilling. Waiting 7 days would be best before installing the anchors. Expansion anchors exert a great deal of outward force against the concrete and it takes at least 5-7 days for concrete to cure to 60% of its designed strength.
How to pour concrete over existing slab?
Steps Involved in Pouring Concrete over Existing Concrete Slab
- Cleaning Old Concrete Surface. Sweeping dirt’s debris over the existing concrete slab will allow bonding to take place. ...
- Setting Up Slab Perimeter. Measure the perimeter of the area to pour the concrete over the existing slab. ...
- Pouring a Primer Coat. ...
- Pouring of Concrete. ...
What happens if I drill into 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.
How deep are the cables in a post tension slab?
8"The plan maps show the rebar at 3" depth, and the post-tension cables at 8" depth. Discerning a post-tension cable from rebar generally requires scanning a larger area to properly understand the layout of structural elements in a slab.
How much weight can a post tension slab hold?
Even between footings, a post-tensioned slab can safely bear about 1,000 pounds per inch of thickness, or about 5,000 pounds for a typical 5-inch-thick residential slab. If more bearing is needed, it's possible to cut through the slab and pour a pad footing.
Do post tension slabs have footings?
Now, what makes them different from your conventional slab foundation system is that post tension slab foundations typically don't have standard footings at all. They don't have deep footings around the perimeter of the home or under load-bearing walls within the structure.
What happens if you cut a cable in a post tension slab?
Cutting a tensioned cable, or tendon, can endanger the contractor and compromise the structural integrity of the slab. Building professionals frequently use ground-penetrating radar (GPR) to locate cables within concrete slabs and reinforce the slab during and after cutting operations.
How long does post tension slab last?
about 15-20 yearsPost-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.
Is post tension better than rebar?
Post-tensioning, which is a form of prestressing, has several advantages over standard reinforcing steel (rebars): It reduces or eliminates shrinkage cracking-therefore no joints, or fewer joints, are needed. Cracks that do form are held tightly together. It allows slabs and other structural members to be thinner.
What year did post tension slabs start?
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 much weight can a 4 inch thick concrete slab hold?
A 4-inch thick concrete can support upto 40 pounds of weight. Weight should not exceed 40 lbs/sq ft on an undetermined 4-inch slab. 80 lb/sq ft in isolated areas is fine, but unless you know what the soil bearing and reinforcing is for that slab, you might be looking at cracking.
Are post tension slabs good?
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.
How do 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.
Can a post tension slab be repaired?
0:583:09What's the fix for a broken post tension slab cable? - YouTubeYouTubeStart of suggested clipEnd of suggested clipWell the best fix is to call a post-tension repair company and they'll re-cable. It because there'sMoreWell the best fix is to call a post-tension repair company and they'll re-cable. It because there's a sleeve.
What is a tension cable in a slab?
Post tension cables are used in concrete construction to add strength to thin and long slabs. This allows more span lengths between support columns. The cables have steel wires within plastic sheath. They are tensioned after pouring concrete,hence the name post-tension cables.
What is a post tension cable?
Post–tension cables are used in concrete construction to allow thinner slabs and greater span lengths between support columns. These cables, composed of steel wires inserted into a plastic sheath, are subsequently "tensioned" and grouted after the concrete is poured.
Is post tension better than rebar?
Post-tensioning, which is a form of prestressing, has several advantages over standard reinforcing steel (rebars): It reduces or eliminates shrinkage cracking-therefore no joints, or fewer joints, are needed. Cracks that do form are held tightly together. It allows slabs and other structural members to be thinner.
When should you stress post tension cables?
Post tensioned slabs should be stressed when the concrete strength designated by the engineer's plan is reached. The practice of MLAW has been to apply stress when the concrete reaches 2000 psi.
Why are post tension cables used?
Now, post-tension cables are great because they allow concrete slabs to be thinner and more stable. But these cables are like rubber bands inside of concrete, and if you accidentally cut one, it can snap. When you’re engaging in post-tension slab drilling, cutting, or coring you want to avoid this snap at all costs.
What GHz is a GPR antenna?
Using our 1.6 GHz GPR antenna, we scanned every proposed coring location. This customer didn’t care about cutting embedded conduits — avoiding a PT cable hit was their only concern.
What happened before the recession?
Before the recession, a company bought three lots on one road. They built condos on the center lot with a parking garage underneath. Just as they finished pouring slabs on the parking garage of the two remaining lots, the recession hit.
How long did they leave tar paper and sod over parking garages?
So, they put tar paper and sod over the parking garages and turned them into parks. They left them like this for over five years until they were ready to resume with the original project.
Can you skimp on professionalism on Friday afternoon?
Even if you’re the last job on a Friday afternoon, we’ll never skimp on professionalism or attention to detail. It’s just too critical. Contact us today for help avoiding hits when cutting, coring, or drilling post-tensioned slab (or any other type).
What is a GPR in construction?
Building professionals frequently use ground-penetrating radar (GPR) to locate cables within concrete slabs and reinforce the slab during and after cutting operations. Advertisement.
How to locate post tension tendons?
The operator must determine which is which. Expand the survey area if unsure. Scanning a larger area provides more information to discern the layout of the slab structural elements. This helps discern which elements are PT tendons and which are reinforcement bars.
What is post tension cable?
Post-tension cables are steel wires inserted into plastic sheathing laid in place while pouring the concrete slab. Once the slab cures, qualified personnel apply tension and grout the cables. Contractors sometimes need to drill or cut into a post-tensioned slab for repair or renovation work. Cutting a tensioned cable, or tendon, can endanger ...
How to strengthen a slab?
Some common methods to strengthen the slab include adding new intermediate supports, increasing slab depth, adding concrete beams, creating a composite section by applying externally bonded steel or fiber-reinforced polymer, or adding external post-tensioning.
Do you need post tension tendons for stairwells?
Large openings for stairwells or large duct shafts between floors often require cutting post-tensioned tendons. Determine potential locations for the new opening into the slab. These should avoid major structural members such as beams and bands of post-tension tendons.
Can I Drill into a Post Tension Slab?
Bonded post tensioning simply means that after stressing of the cables has occurred, the PT ducts are fully grouted, this allows for a progressive transfer of the force from the cables to the slab. For this type of post tensioning, it is generally OK to drill or cut into the slab, provided necessary investigation works have been undertaken and advice from a qualified structural engineer is given (see below for more on this).
What is post tensioning?
Post Tensioning: The assembly of the post tensioning cables ducts and embedment systems. This system provides the compressive force to the slab.
What is PT concrete?
A post tensioned (PT) concrete slab uses the compressive force of post tensioning cables to reduce the tension within the slabs cross-section allowing it to span further and have a thinner profile compared with a conventionally reinforced concrete slab.
What happens when you post tension a slab?
The introduction of post tensioning within the slab can result in the full cross section of the slab remaining in compression when it is bending. Taking the simple example from the previous image, we can cut a cross section through a portion of slab to see the stress distribution across the section with and without post-tensioning.
How long after concrete is poured can you use a stressing jack?
24 Hours after the concrete has been poured around 25% stress load is applied to the cables using the stressing jack. This is to control shrinkage cracking.
How does self weight of concrete affect the building?
Therefore reducing the slab thickness can allow reduction in column sizes as well as depth and volume of foundations.
Why add a compressive force to a toilet roll?
Adding a compressive force at either end of the toilet roll system ensures that no net tension exists while it is bending. Tension capacity has not been added to the system but rather the need for tension capacity has been removed.
What is placed on the top of a slab?
Top: To prevent restraint, plastic is placed along the top of a wall that the slab is crossing. Bottom: The black foam rubber around the first few inches of the dowels coming out of the adjacent wall will allow the post-tensioned slab to shorten during tensioning without cracking the slab.
What is 88 blowout?
88© Concrete Construction ©January 2006 This small blowout is a result of a localized low point in the tendons between two higher points. Note the gap beneath the tendons and the slab,indicating how much the tendons lifted. These tendons will create a downward force that is nowhere near a band or a vertical support and that can cause cracking or a small blowout.
Can post tensioning crack concrete?
Bottom: Significant over-balancing of the weight of the concrete with post-tensioning can actually lift and crack the slab.
Is reinforcement discontinuous across a pour strip?
Reinforcement must be discontinuous across a pour strip.
What is the ACI 318 code?
Reality: ACI 318 code states that the size of the drop panel has to be ⅙ of the span in each direction for conventionally reinforced concrete only. This size limitation does not apply to PT slabs. The engineer has tremendous flexibility to size relatively small and efficient drop panels that are effective in shear and flexure.
What does it mean when a steel wedge cracks?
Cracks indicate that the wedges are conforming more tightly and getting a better grip on the steel strands as they hold the prestress force in place. It is common to see cracks in 2-part wedges after stressing. It is less common in 3-part wedges.
How does posttensioning work?
Reality: Okay, this one is true. However, the use of post-tensioning also results in many other cost savings to the owner that are often overlooked. The following is a list of additional cost-savings to the owner: 1 Overall building height is reduced. This can be used to add more leasable space within the same building height, it can be used to increase ceiling height, or it can be a cost savings. 2 Because the building height is reduced, there is a reduction in all building services that are tied to building height such as mechanical, electrical, plumbing, elevators, etc. 3 Because the building height is reduced, there is a savings in the cost of the building façade. 4 Because the building height is reduced, operating costs such as heating and cooling costs are reduced for the life of the building. 5 When used in below-grade applications, excavation costs are reduced. 6 Formwork and reshoring costs are reduced.
Why is there not enough room for PT?
There is a common misconception that there is not enough room to stress the PT in a below-grade situation because of the confines of the surrounding walls. There is also the fear that the surrounding walls will result in restraint to shortening cracking.
Why is there a reduction in the cost of the building façade?
Because the building height is reduced, there is a savings in the cost of the building façade.
Why is building height reduced?
Because the building height is reduced, there is a savings in the cost of the building façade. Because the building height is reduced, operating costs such as heating and cooling costs are reduced for the life of the building. When used in below-grade applications, excavation costs are reduced.
What is general zone?
The general zone encompasses a larger zone around the local zone and is inclusive of the local zone. The design of the general zone is influenced by not only the local zone but also by the overall geometry of the member and the reinforcement that exists in the member.
