STRENGTHENING OF FOUNDATIONS
7.6 STRENGTHENING OF FOUNDATIONS
Columns foundations need strengthening in the case of applying additional
loads. Widening and strengthening of existing foundations may be carried out by constructing a concrete jacket to the existing footings. The new jacket
should be properly anchored to the existing footing and column neck in order
to guarantee proper transfer of loads. The size of the “jacket” shall be selected such that the average maximum foundation pressure does not exceed the recommended allowable value. Attention shall be given during construction in
order that the excavations for the new “jackets” do not affect the existing
An isolated footing is strengthened by increasing the size of the footing and the reinforcement steel bars as follows:
Excavating around the footing _
Cleaning and roughening the concrete surface_
Installing dowels at 25-30cm spacing in both directions using an appropriate epoxy material_
Fastening the new steel bars with the dowels using steel wires. The_
diameter and number of steel bars should be according to the design_
Coating the footing surface with a bonding agent in order to achieve the_
required bond between old and new concrete
6_Pouring the new concrete before the bonding agent dries. The new
concrete should contain a non-shrinkage material.
The previous steps are illustrated in Fig 7-11.
The following photos (13-16) illustrate the practical way of jacketing a footing by reinforced concrete.
7.7 ADVANCED STRENGTHENING TECHNIQUES USING FRP
7.7.1 SPRAYED-UP FRP STRENGTHENING FOR CONCRETE
A new repair and strengthening method by spraying carbon or glass chopped
fiber with vinyl ester resin upon concrete structures is introduced. This method is engineered systemically for the purpose of repair and strengthening of concrete structures using sprayed-up FRP composites. Carbon or glass fiber
chopped with 1.5 or 2 inch-length is sprayed with vinyl ester resin using the air-compressed spray machine on the surface of concrete structures directly. The anti-symmetrical loading test of rectangular columns was carried out for the purpose of confirming fundamental behaviors of RC structures strengthened
by sprayed-up FRP system. As the result, it can be recognized that the similar
shear reinforcing effect is obtained in either way spraying or fiber sheet. Anchor method by the advantage of sprayed FRP ’s flexibility is investigated. Anchor bolts or slits are set on the concrete surface, FRP sprayed in and on them.
Compared with fiber sheet, the bond strength by sprayed FRP has over the
equivalent of one by sheet, when the thickness is set having equal rigidity. By filling the FRP to the slit, the mechanical bearing can be expected.
Nowadays, strengthening by post casting concrete, steel plate jacketing, fiber reinforcements such as carbon, armed , and glass are utilized as seismic strengthening methods for concrete structures. Recently, a seismic strengthening method by wrapping continuous fiber sheets has often been
used, since the constructability and durability is superior. However, materials using continuous fibers are expensive. On the spread of seismic strengthening for buildings and infrastructures in future, simple methods of strengthening with low cost should not only be suggested, but also seismic behaviors should be cleared.
In this study, a new, inexpensive, and simple strengthening method for
concrete structures is discussed and suggested in order to improve future
seismic strengthening. This method using short fibers with vinyl ester is a new combination of materials as seismic strengthening. Chopped short fibers of carbon and glass with vinyl ester resin are sprayed in place on the concrete structures. It is called “Sprayed-Up FRP (Fiber Reinforced Polymer).”Benefits of using vinyl ester resin in this strengthening method are that it takes shorter time to harden the resin than epoxy resin. In addition, the mechanical properties of vinyl ester resin are the same as the one of epoxy resin. In the following sections, the outlines of this method and the results of column test under the anti-symmetrical loading are reported. In addition, the bond behavior between FRP and concrete, and anchoring behavior using slit (groove) are reported.
188.8.131.52 OUTLINE OF SPRAYED-UP FRP STRENGTHENING METHOD
Figure 7-12 illustrates the idea of the sprayed-up FRP strengthening method for reinforced concrete buildings. Photos 17 and 18 show the construction site of sprayed column specimens. In this method, resin is carried through a narrow hose by an air compressor. The resin is mixed with short fibers such as carbon or glass at a tip of the narrow hose. The mixed materials are sprayed directly on a surface to be reinforced. After that, the surface is made flat by a roller. The resin will be hardened and the whole sprayed structure will be reinforced with FRP. This method makes seismic strengthening possible that all structure members ,which are columns, beams, walls, and slabs ,are monolithic since it is possible to reinforce an entire interior structures in building structures.
The installing procedure of the sprayed-up strengthening is as follows:
1_Base arrangement; Surface of concrete is polished by a disc-sander and
cleaned by air.
2_ Primer resin coating; Primer resin is applied to the surface in order to
make highly adhesive between concrete and putty/resin.
3_Putty arrangement ;Dent areas and steps on concrete surface are filled
with putty and make the surface flat in order to prevent from partial stresses of FRP and air voids on concrete. After putty dried, the surface is sanded.
4_ Resin coat; In order to make fibers more adhesive, resin is coated first by
a spray gun.
5_Spraying (Photo 17); Resin and short fiber are sprayed on concrete at a same time by a spray gun. The lengths of the carbon fiber and glass fiber are 2_0 inches and 1.5 inches, respectively.
6_Impregnation (Photo 18); Entrapped air is rolled out.
7.7.2 STRENGTHENING AND REPAIR OF REINFORCED CONCRETE
STRUCTURES USING COMPOSITE MATERIAL
Advanced Composite Materials (ACM)are considered the new material of this century. They are used in structures in the form of fiber-reinforced polymers (FRP).The advantages of FRP materials such as their high strength, light weight, resistance to corrosion…etc, expand their use in reinforcing or repair and strengthening of structural elements.
In order to investigate the appropriateness of FRP applications in different structural systems, a comprehensive research program of the usage of FRP as a repair and strengthening material is performed in Housing and Building Research Center (HBRC)in Egypt. The results of the research program will be under consideration of FRP committee responsible of preparing the first Egyptian Code of Composite Material. FRP systems are very effective and powerful when used in some applications such as confinement of columns and joints to increase ductility for seismic loading; strengthening of beams in shear; strengthening of slabs in flexure… etc.
7.7.2_2 TESTS ON COLUMNS STRENGTHENED WITH FRP
Using of FRP sheets increases the lateral confinement of the reinforced concrete sections. This means that the axial load capacity as well as the ductility of the reinforced concrete columns will be enhanced. The axial carrying capacity and ductility of the rectangular columns can be increased up to 95% and 207%, respectively, depending on the strengthening scheme. Moreover, the axial carrying capacity and ductility of the square columns were increased up to 60%and 330%, respectively, depending on the strengthening scheme. Fig.7-13 shows the failure mode of some columns strengthened with FRP sheets.
7.7.2_3 TESTES ON BEAMS STRENGTHENED WITH FRP
RC beams strengthened with CFRP (Carbon Fiber-Reinforced Polymer) strips with well-anchored ends had higher flexural capacity.Fig.7-14 show the crack pattern of some specimens and the strengthening arrangement.
7.7.2_4 TESTES ON EXTERIOR BEAM-COLUMN CONNECTIONS STRENGTHENED
Using GFRP sheets in repairing of exterior beam-column connection could
improve the ultimate strength of these joints with about 50%compared to the
original joint. The displacement ductility increased considerable and the
required specimen lies in the zone of medium to high ductility. GFRP could
recover the stiffness of the original specimen, and reduction of about 35%was observed compared with the original joint.
Fig.7-15 shows the retrofitted exterior slab-column connection.