Kolos śmierci.pdf

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Lecture 1
1. Concrete structures - weaknesses:
-loads provoking tension locally
-cracking
-deflection
-weight
-span limitation
-durability limitation
2. Prestressing
- carefully planned and controlled introduction of permanent forces into a structural
element, aimed to create opposite action to unfavourable stresses and deformations provoked by
external loads. It is divided into:
-pre-tension (strunobetonowe)
-post-tension (kablobetonowe) - the holes are made into ready concrete plate and after that the
wires are installed and prestressed
3. Internal prestresses (bonded tendons)
External prestressing
4. Prestressed concrete:
-Stress field in the member is a composition of two fields:
-stress field generated by the external loads
-stress field generated by prestress
-Final effect:
-normal force in the member may be greater
-bending moments and stresses may be limited
5. Tendon can be composed of wires or strands.
Cables composed of number of strands. Strands composed of wires.
6. Prestressed technologies:
a) use of steel tendons, inside or outside of concrete cross-section
b) use of hydraulic jacks and expansion joints
c) use of FRP (fibreglass reinforced plastics) bands tensioned to get prestress.
-shape memory alloy- material which can come back to the previous form after deflection
1. Prestress effects (prestress allows to reach several profits):
-possibility to counteract the external forces generated by loads
-possibility to adjust deformation (deflection) of the structural member
-limitation of the size of the structural member
2. Concrete creep - permanent / constant progressive deformation of the material under constant
load/stress:
-one of the most important behaviours in prestressed concrete
-causes higher deflection
Concrete creep
- creep strain of concrete - develops in time in non-linear manner - initially rate of
strain is high and slows down with time to gain the final value theoretically in the infinite future -
practically after several years.
Creep is provoked by series of processes
-colloidal planes slip in cementous gel on H20 particles
-elastic deformation of gravel and gel crystals during slip
-local damages in cementous gel in effect of load micro cracking
-moisture transport in capillary pores under pressure
Cementous Gel - cement hydrated + cement non-hydrated +water (no aggregate)
-pressure to pores - water (moisture ) excapes to the place where the pressure is smaller (outside)
slow movement because of in thermal forces in the material + water theory of diffusion
3. Concrete creep depends on various paramters:
Variation of strain in time:
-mechanical properties
-material properties
-curing
-loading conditions
-loading history
-stress conditions
-conditions
-environmental
What influences creep?
a)quantity of H20 particles (w/c ratio; water-cement) -> more water -higher creep
b) center of cross-section -> surface distance -> the larger distance - the longer process duration
c) fcm~Ecm -> if they are lower, the creep is higher
d) h0 - generalized size of the cross-section -> small cross-section thickness-> higher creep
e) RH - relative humidity value and temperature (external environment), surrounding conditions
4. Shrinkage - decrease in volume (3-D effect) - only longitudinal shrinkage is important, call
provoke crack there are limit assesed
-there cannot be allowed to dry too fast -> graph ->compression in core, tension outside -> cracks
-volume deformation (vertical and horizontal)
-shrinkage cracking, leading to lower durability
-member deformations in the longitudinal direction
-non linear behaviour
Taking shrinkage into account -> important for structural analysis:
-shrinkage cracking -> leading to lower durability
-member deformations in longitudinal direction

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