Civil Engineering Reference
In-Depth Information
a) When thickness of new poured concrete is rather small, retrofitted rebars are welded on
original column bars using short bars (Fig. 3.38(c)). Diameter of short bars should not be
less than 20 mm, the length not less than 5
d
(
d
is the relative small diameter of new adding
longitudinal bars and original longitudinal bars) and spacing between them not larger than
500 mm.
b) When thickness of new poured concrete is rather large, U-shape stirrups are used to
fix longitudinal rebars by welding (Fig. 3.38(d)) or anchorage (Fig. 3.38(e)). In the case
of welding, length of inconel weld is 10
d
and length of double welded joint is 5
d
(
d
is the
diameter of the U-shape stirrup). Procedures of anchorage method is that drilling holes in
the place of original column firstly, the distance between which and the column edge should
not be less than 3
d
and not less than 40 mm. The hole depth should be not less than 10
d
,
and the diameter should be 4 mm larger than stirrup diameter. Then stirrups are anchored
in the drilled holes using epoxy grout or epoxy mortar. In addition, rivets not less than 10
mm indiameter could be anchored in holes, then weld U-shape stirrups on rivets.
Fig. 3.38
Constitution of section enlarging method.
c. The smallest thickness of new adding concrete should not be less than 60 mm, or 50
mm when using shotcrete.
d. It is better to use transformed rebar with a diameter range from 14 mm to 25 mm.
e. New longitudinal rebar should be anchored into foundation and the top end. Tensile
bars should not be cut off in floor slab, while 50% compressive bars should transverse slab
and should be compacted between the top of new poured concrete and the girder bottom.
3. Force characteristics
When retrofitting concrete columns under load, stress and strain of the new concrete
lag behind the original because compression deformation exists in original columns and
shrinkage and creep have occurred. Therefore, the new and the old cannot simultaneously
achieve peak stress, reducing the effect of the new concrete. Along with the actual stress in
columns before retrofitting, reduction degree changes, and the higher the stress, the greater
the degree of reduction.
Effect of new concrete is also correlated with the ratio between post-imposed loads and
residual loads. New concrete doesn't bear original loads, if the loads do not increase after
retrofitting. Only when loads increase, will the stress of the new concrete increase. Therefore,
the new concrete is at lower stress levels and cannot fully play its role to retrofit if original
columns have relative high stress and large deformation.
Experiments show that if the combination of the old and new concrete is reliable, the
old and new concrete strain increments are basically the same; deformations of the entire
section are compliant with the plane cross-section assumption.
