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Definition
pestressing is the process of strengthening concrete with stressed steel in order to overcome the concrete's natural weakness in tension.
Prestressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that offsets the tensile stress that the concrete compression member would otherwise experience due to a bending load. Traditional reinforced concrete is based on the use of steel reinforcement bars, rebars, inside poured concrete.
It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete.
Prestressing can be accomplished in three ways: pre-tensioned concrete, and bonded or unbonded post-tensioned concrete. |
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Term
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Definition
1. tendons are pulled tight (pretensioned)
2. concrete is poured around the tendons
3. concrete cures
4. tendons are released
the tendons are bonded to the concrete directly, which protects them from rusting. However, this means that there is a required length of development (minimum length) for the beam to reach its full bearing capacity. Tendons must be straight due to pretensioning. |
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Term
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Definition
1. tendons are draped
2. concrete is poured
3. tendons are pulled tight (ERRRURRRERRRURR!)
advantages: more efficient because allows use of draped tendons to directly fight the moment where it is most severe. |
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Definition
| tendons used in prestressing of concrete. generally of high tensile steel cable or rods. |
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Definition
| similar to prestressing strands, tendons of high tensile strength so that they might be pulled tight. post tensioning strands, however, are typically run through channels placed in the concrete before pouring. |
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Definition
| concrete that is poured into a desired size and shape in a factory and then delivered to the jobsite. this is cheaper and more reliable because the curing conditions can be more easily controlled. |
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Term
| precast floor plank (solid, hollow) |
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Definition
a precast concrete member intended for a floor. these can be cast as solid pieces or else (more typically) hollow. these are then delivered to the site and bonded by means of a secondary layer of concrete that is forced into small openings between the pieces called shear keys.
hollow is a better use of material since the only material required at the neutral axis is enough to pass the shear and moment forces from the top to the bottom extreme fibers. |
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Definition
| a bed in which precast concrete is poured to cure. this is like a form work, and is usually covered with a device that creates steam so that the concrete cures properly. |
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Definition
| a device is placed over precast concrete after it is cured for the duration of the 28 day strength period. the device continually creates steam so that the concrete always has the proper amount of water to hydrate and cure. |
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Definition
| the intentional curvature of post-tensioned concrete to resist applied forces. |
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Definition
| a thin sheet of material placed between a masonry plate and the masonry-bearing surface. It is used to fill voids caused by imperfection of the masonry plate and bearing surface, to seal the interface, and to aid in even distribution of loads at the interface |
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Term
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Definition
| a type of precast concrete deck which has one stiffening rib integrally cast into slab (at the bottom of the T) this means that there is no need for a bottom flange because the rib is more efficient than the top flange and can do the same job with much less material. |
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Definition
| two single tees connected at the end of one flange. this is better for wide spans because is much more stable. |
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Term
| bonded v. unbonded tendons |
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Definition
for prestressed concrete. in pretensioning, tendons are always bonded. in post-tensioning, either can be done. bonding is typically achieved by filling the cavity with grout after the tendon has been inserted.
unbonded is preferred sometimes because if the tendon breaks it will loosen and can be easily replaced. |
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Term
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Definition
| in post-tensioning tendons can assume a shape instead of using place straight across. This shape typically mimics the european moment diagrams (or the opposite of ours.) once these draped tendons are pulled tight they cause a camber in the beam. |
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Term
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Definition
the inelastic deformation of a material due to applied stresses over time. This cannot be fixed and can cause some silly problems. creep is always exacerbated by higher temperatures.
sometimes moderate creep is welcomed because it can relieve tensile stresses that could otherwise lead to cracking. |
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Term
| segmental post tensioning |
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Definition
| post tensioned concrete pieces are tensioned together several at a time to ultimately create a span that would not be possible with a single post tensioned piece. |
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