Tuesday, August 26, 2014

Reinforced and Prestressed Concrete

Concrete resists compresive (pushing/squeezing) forces well, but is weak with respect to tensive (pulling) forces. Steel is the opposite. Reinforced, pre-tensioned, and post-tensioned concrete combines these two materials to create a product that is much stronger than either material.

Reinforced Concrete

Concrete and steel in useful, economic quantities were developed in the 1800s. Joseph Monier obtained his second patent related to reinforced concrete in 1877. He used it in the making of flowerpots. Specifically, before concrete is poured into a form, rebar is added inside the form where the resulting structure would experience tensile forces. The rebar is a twisted spiral rod so that the concrete can "grip" it better after it sets. (Other types of reinforced concrete that use other tension resistant materials are being developed, but steel is still the most typical companion material.) Steel and concrete have similar coefficients of thermal expansion so internal stresses remain low as the temperature of the reinforced concrete changes. The concrete must be mixed correctly to make sure the steel does not rust. Rust expands, and it will crack the concrete. (Wikipedia)

When the Delaware, Lackawanna & Western Railroad built a 28.5-mile cutoff between 1908 and 1911, it used reinforced concrete for all of its structures. The Paulinskill Viaduct was 115 ft tall and, at the time it was built, the world's largest reinforced concrete structure.


Pre-Tensioned Prestressed Concrete

For concrete structures that can be constructed from many members of the same size such as the beams or girders in a bridge, precast plants are generally used. Rather than build a lot of complicated form work in the air at the bridge site, a few forms are built in the factory and used over and over again. After the beams have sufficiently cured at the factory, they are trucked to the site, and lifted in to place with cranes.

Having forms in a factory setting that are used to construct many beams made it feasible to prestress the steel members in the beam so that the concrete is always under compression. Specifically, the rebar is replaced by steel cables that are longer than the beam and that stick out of holes in the form's end. Before the concrete is poured into the form, hydraulic jacks are used to pull each rod to 5000-8000 psi of tensive force. After the beam is poured, it stays in the form until it has enough compressive strength to resist the tensive force in the cables. Then the tensive force is slowly released from the cables to compress the concrete. Now when a load is applied to the beam, instead of creating tensile forces below the centroid of the beam, the applied load just reduces the compressive forces. With only designed loads applied, the concrete never experiences tensile forces. Also the compressive forces added to the bottom of the beam causes the middle of the beam to camber (curve) up. (3:17 in video) This prevents the beam from sagging when a load is applied and makes it look safer.

I've seen references to "bulb" prestressed beams. The difference between I-beams and "bulb" beams is that the tension members are more spread out in the soffet (lower part) of the beam.

California I-Beam California Bulb Beam

Virginia Bulb Beam

Post-Tensioned Prestressed Concrete

A limitation with pre-tensioned concrete beams and girders is that the length of a span (distance between piers in a bridge) is limited by the length of what can be trucked to the site. An advantage that steel plate beams had over concrete beams is that several beams can be joined together to create a longer span. For example, four field joints were used for the Seneca Illinois River Road Bridge to create the center span of 364 feet. Post-tensioning is an additional prestressing technique that allows multiple beams, or segments, to be used in a span. The dotted circles in the above California Bulb-Tee design are optional post-tensioning ducts. They are tubes placed in the form before the pour to create holes through the beams. During construction, falsework (temporary scaffolding) is used to hold the beam segments in place while they are being erected. After the segments are in place, tendons that are long enough to go through all of the segments are pulled through the ducts. A tendon consists of 7 high-strength steel wires wound together. Then a hydraulic or screw jack is used to pull the tendons about 4 inches for every 50 feet of length to apply 33,000 pound of load. (ConcreteWork) Bonded post-tensioned concrete means that grout is pumped into the ducts after the tendons have been stressed. And unbonded means that a protective lithium based grease was used instead of grout to protect the steel from corrosion. When pumping the grout, care must be exercised to avoid leaving any voids. Bridges have collapsed because of tendon corrosion. (Wikipedia)

Below is a pick during the construction of the I-355 Des Plaines River Valley Bridge. On the right side of the picture, under the beams that have already been placed, you can see the falsework. And on the far end of the picked beam you can see a segment brace that has been added to support this segment with the end of the segment that has been placed on the false work. And on the near end you can see the four ducts that have been cast in place.

Time 3:08 in video
I was surprised that the ducts were at the top of the beam instead of the bottom. Then I learned that for long beams the ducts are curved to follow the tensional forces of the span. The depth of the beam in the following diagram is exaggerated to illustrate the curved duct in red. So the above segment would be on the south side of a post-tensioned beam.

See jacks for some pictures of the big multi-strand stressing jacks needed for bridge spans. They have a table of jack sizes, and I notice that the largest can apply 150000 kN (or 21,756 psi) to 108 0.6"-strands and stretch them up to a half-meter. And it weighs almost 6 tons! And another manufacture illustrates equipment to cut, wind, and push the tendons as well as to stress them.

Another major application of post-tensioning is in the slabs (ceiling/floor) of buildings. It allows the slabs to be thinner and/or the columns spaced further apart than reinforced concrete would allow. Thinner slabs not only means less concrete needs to be pumped into the building, it means a lower overall building height for the same floor-to-floor height. In turn, this reduces the weight of the building, the pressure needed to pump concrete to the upper floors, and the costs of the mechanical systems and facade materials. The reduced weight also reduces the foundation costs. (DSI) The size of the hydraulic jacks used to tension slab tendons is much more manageable.

Copyleft, Shakespeare at English Wikipedia

Photo posted by Wayne Helms
Update: I found pictures of rigs to place the bridge segments in the Facebook group "Railroad Maintenance of Way Photo's."

Photo by Jim Kissane of the Leroy Selmon Expressway in Tampa in a comment in the above Facebook link
Jim Jacobs -> Rail & Highway Heavy Loads
Update: Jim's comment:
U-beam, 202' total length, 400,000+lbs. Axles spread to 16'. I was the rear steer driver for a dozen of these loads going to I-595/I-95 over passes a month ago. This was on the I-75 South on ramp from Hwy. 27.

Jim posted the following as a comment to his posting.
Jim Jacobs -> Rail & Highway Heavy Loads
Putting one in place.

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