You are on page 1of 78 Search inside document This self-guided presentation covers the use of externally bonded FRP systems for strengthening existing concrete structures. The content of the presentation follows the guidelines given in the ACI It is possible to increase both positive and negative moment capacity and both reinforced and prestressed or post-tensioned concrete members can be strengthened for flexure. The document also does not give specific guidelines on strengthening for flexural loads due to seismic forces. Furthermore, it is generally recognized that FRP The objective for any flexural strengthening application is to provide a design moment capacity greater than the moment demand.
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You are on page 1of 78 Search inside document This self-guided presentation covers the use of externally bonded FRP systems for strengthening existing concrete structures. The content of the presentation follows the guidelines given in the ACI It is possible to increase both positive and negative moment capacity and both reinforced and prestressed or post-tensioned concrete members can be strengthened for flexure.
The document also does not give specific guidelines on strengthening for flexural loads due to seismic forces. Furthermore, it is generally recognized that FRP The objective for any flexural strengthening application is to provide a design moment capacity greater than the moment demand. This is expressed as Eqn and is similar to the general requirements given in ACI Note that increases in FRP reinforcement do result in additional flexural strength being attained.
However, increases in FRP reinforcement also result in reduced deformation capacity and ductility. It is also important to note here that increases in FRP reinforcement do not necessarily result in proportional increases in strength.
As such, the assumptions shown are made in developing the equations for ACI Note that many of these assumptions are similar to the assumptions used to develop ACI The ultimate strength of the beam will be determined based on simultaneously satisfying strain compatibility and internal force equilibrium.
The flexural strength of the section will be gained from the contribution of a compressive resultant force in the concrete, the tensile force from the existing steel reinforcement, and the tensile force contribution from the FRP system. This again is very similar to regular steel reinforced concrete design.
All of the failure modes listed must be considered. It is important to note that both failure modes 1 and 4 are brittle, sudden failure modes. It is most often advisable to avoid these failure modes. Failure mode 4, cover delamination, is a unique failure mode and will be dealt with in more depth in the design detailing portion of this presentation.
Note that with this failure mode there is still significant deformation and therefore warning of failure. This is due to the existing steel reinforcement undergoing significant deformation after yielding. Again significant deformation is attained by significant post-yield elongation of the existing steel reinforcement.
Also note with this failure mode that once the FRP fails, the beam does still have some residual strength and deflection capacity based on the original unstrengthened section.
These equations, shown, yield a debonding strain value. This strain is a function of the concrete strength and stiffness of the FRP reinforcement — higher stiffness materials debonding at lower levels of stress than lower stiffness materials.
If FRP debonding controls failure, only the percentage of ultimate strength will be attained. If concrete crushing controls failure, the concrete will reach its maximum compressive strain before the FRP reaches its rupture strain.
The stress, particularly in the FRP, must be determined through an iterative process of simultaneously satisfying strain compatibility and force equilibrium. It is important to realize that the FRP is usually bonded to surfaces that are already stressed. For example when bonding FRP to the bottom of a beam, the bottom of the beam may already be under tension due to its self weight and dead loads.
Since the FRP is installed unstressed, it is not capable of resisting these loads that are already in place. For calculation purposes, the state of strain on the substrate when the FRP is being installed should be calculated so that it can be subtracted from the strain in the FRP at increasing levels of load.
The initial substrate strain can be computed from the equation shown where M ip is the bending moment in the section due to the existing loads on the member. The estimated neutral axis depth will be checked to see if it satisfies both strain compatibility and internal force equilibrium.
If these two conditions are not satisfied, the neutral axis depth will need to be revised and checked again. It will first be assumed that strain compatibility is satisfied. Force equilibrium may then be checked. This equation will also indicate which mode of failure will govern. For the steel reinforcement which is idealized as elastic-perfectly plastic , Eqn will indicate the stress level in the steel. For the FRP which is idealized as perfectly elastic , the stress level can be determined from Eqn This model is only valid when concrete crushing is governing failure.
If FRP failure governs failure, the strain level in the concrete may be substantially lower than 0. The Whitney stress block will not give an equivalent stress distribution for this condition. The actual non-linear stress distribution in the concrete must be considered or an alternative equivalent stress block model must be employed. If the neutral axis depth, c, determined by the equation shown is different from the estimated neutral axis depth, then force equilibrium is not satisfied.
The neutral axis depth must then be revised and the iterative process repeated until force equilibrium is satisfied.
Seminario Online ACI 440.2R-08.pdf
Issue: Appears on pages s : 76 Keywords: chemical attack; concrete durability; corrosion; cracking; deterioration; discoloration; environments; joints; oxidation; popouts; scaling; serviceability; spalling; staining; surface defects. This document was replaced by FRP strengthening systems use FRP composite materials as supplemental externally bonded reinforcement. FRP systems offer advantages over traditional strengthening techniques: they are lightweight, relatively easy to install, and are noncorrosive.