Civil Engineering Reference
In-Depth Information
It is better to retrofit beams by using curved prestressed reinforcement. Therefore, the pre-
stress retrofitting method is widely used in beam retrofitting. For example, an 'I' shape
beam of a bridge with 20 m span had a deflection of 5.4 cm at middle of span and widest
crevice of 0.5 mm before being retrofitted; after it was retrofitted by 4
φ
25 lower-supported
prestressed reinforcements, the largest span was not only offsetting the load deformation,
but also upwarp for 0.47 cm. The load of two arrays of car-10 was eccentrically applied
before being retrofitted, while the load of two arrays of car-13 can be eccentrically applied
after being retrofitted. For another example, a lot of cracks appeared in a thin webbed roof
beam of a factory building after one year service; one of the thin web beams had more than
63 items of crack; some cracks were developed into the whole web height, and the largest
crack width even reached up to 0.6 mm. The reasons for this phenomenon were too thin
(100 mm) web, too little steel in the web and too low strength of concrete. After being
retrofitted by lower-supported prestressed reinforcements, oblique cracks and vertical cracks
were closed and the beams worked well.
The prestress retrofitting method, effect of prestress, calculation of bearing capacity of
retrofitted components and numeration of stretch elongation are illustrated in this section.
The discussion below is for beam components; however, the principles and methods are also
applicable to plates.
There are two working types of prestressed reinforcements. The first one is to deliver
force through anchoring point and supporting point externally to an original beam, which is
simple, effective and widely used in engineering; the second one is to deliver force through
bonding effect between the new and the old concrete by pouring concrete after stretching
prestressed reinforcements. The construction and calculation techniques of external prestress
retrofitting method are listed below.
1. Prestress retrofitting techniques
The basic techniques of retrofitting beams and slabs by prestressed reinforcements are:
a. Add prestressed reinforcements in the external tensile region that is in need of being
retrofitted.
b. Stretch prestressed reinforcements and anchor them at the ends of beams (slabs).
The stretching method and anchoring techniques of prestressed reinforcements are de-
picted below.
(1) Stretching prestressed reinforcements
The prestressed reinforcements which are used to retrofit beams are usually put in the
externals of beams, so the stretching process is done. There are various types of stretching
methods and the common methods are:
a. Jack stretching is to stretch and anchor prestressed reinforcements at the tops or the
ends of the beams by jacks, which is especially suitable for curved reinforcements. It is
always impractical for straight reinforcements because it is hard to put jacks at the end of
abeam.
b. Transverse frapping is applying prestress across two directions. The principle is to
fasten reinforcements at both ends, using simple tools including a torque-indicating wrench
and blots to bend them from straight line and produce tensile strain; consequently, prestress
is established in reinforcements.
The techniques of transverse frapping are as follows (shown in Fig. 3.12):
a) Fasten the ends of reinforcements to original beam. Reinforcements can be either
curved lower-supported or straight line, as shown in Fig. 3.15(e).
b) Brace stay bars
(angle steel or thick steel bar) between two reinforcements
at
regular intervals.
