This video (at the end) explains the production of a prestressed concrete unit (Double T) on a prestressing concrete plant.
Double T is a versatile prestressing unit which are mostly used in parking structures but can be used in different buildings including office buildings, industrial buildings etc. The Double Ts have the capability of supporting span ranging from 60ft and can even go to upto 120ft.
First step of the process after cleaning is setting up the forms required (in this case of double T). In this video, a bed is setup for 5 double T units of about 60ft long. Self stressing form is used for prestressing which is capable of taking prestressing compression force and also the eccentricity of the strands.
Strands are placed in the form by crew members and then are passed through prestressing plate. Strands are laid continuously over 5 units and are later cut using the space provided by the dividers.
Strands which are placed in 2nd step are then tensioned to about 2 to 5 kips based on gauge pressure reading. In this case a mark is made on strands after the preload is applied. The strands is then tensioned and noted using gauge pressure on hydraulic pump. Quality control Technician than check elongation of strands against theoretical elongation. There is a requirement that force on gauge and elongation measured must match within 5% tolerance limit. If not then strand may have to be detensioned and then tensioned again.
After tensioning of strands remaining embed is placed. This includes stem reinforcement, lifting devices, web reinforcement, Flange connectors etc. After which again a quality control check is performed to ensure that everything is in the right place before placing concrete.
After embed is set up concrete is poured. Concrete used mostly in this case is self-consolidating concrete because of its properties such as ease of placing and finishing. Concrete surface is then finished and sprays are applied to reduce the effect of plastic shrinkage.
Final step is curing. Accelerated steam curing is done in this case which is roughly equal to 7 days of conventional curing. After one day at bed, these units are then stored in garages where conventional curing is done for about 7 days.