The expansion from ASR does not solely affect the mechanical properties of the concrete, it may also impose additional forces in structures. For reinforced concrete, the expansion due to ASR is to be considered as a long term load.
A reinforced concrete structure exposed to ASR expansion gets an internal re-straint from the reinforcement. If the structure is statically indeterminate it will also have an external restraint as deflection and displacement get prevented. In addition, if parts of the structure expand more than other parts, this will give imposed forces in the structure. Following, these three cases are explained in detail.
Internal Restraint from Reinforcement
Only the concrete will expand when exposed to ASR. Due to the bond between the concrete and the reinforcement, the steel bars will be strained as it is fol-lowing the concrete. This results in stresses both in the reinforcement and the concrete. Internal equilibrium is obtained such that the force in the steel is equal to the force over the reinforced cross-section. This is shown in the form of an axial-load in the gravity center and an eccentric moment shown in figure 2.4. This is an internal rearrangement of the forces and for a situation with statically determined structure it will not give external loading [1].
Figure 2.4: ASR-loads on reinforced concrete [1]
Internal stresses are developing as long as the material is linear elastic. Since the ultimate moment capacity is calculated when the steel is yielding the initial strain from ASR give little impact on the capacity of the cross-section. In this way, it is possible to look at these internal stresses as an inner pre-tension effect.
Even though the internal stresses give a small impact, the stage of the cross-section has a significant impact. Whether it is in stage I or II is very important for how the cross-section is responding [1]. Usually a structure, during its ser-vice life, will be in stage II as it is exposed to dead load and live load. Being in stage II means that only the reinforcement is obtaining the tensile stresses. In some cases, structures may have parts in stage I and stage II at the same time, and this should be taken into account in capacity controls.
9 External Restraint
Once the structure is fixed for elongation or rotation, it gets an external re-straint. The prevented expansion induced by ASR will lead to an external load contribution.
This can be understood by looking at a cross-section where the free expansion has a linear distribution over the height. Typically the upper part of a beam is more exposed to ASR than the lower part, which results in a higher expansion in the top giving a uniform gradient in the free expansion, ε0. This induces a greater tensile force in the top, making the beam tend to curve upwards. If the beam is without external constraints, the beam would be subjected to such curvature and no external loads would appear.
In a statically indeterminate structure, the beam would be prevented to curve due to the external constraints. This is occurring in the form of a secondary moment giving compression in the upper part and tension in the lower part.
This provides stress-contributing strains, εσ, in the cross section. To get the actual resulting strain in the beam,ε, the stress-contributing strains are added to the free expansion.
ε=ε0+εσ
This is illustrated in figure 2.5:
Figure 2.5: Effect of external constraints
The beam gets an expansion due to the ASR-loads, but since the curvature is prevented, the elongation is uniform and a secondary moment is present. This external moment imposes stresses on the concrete and is defined by equation (2.1).
My = Z
A
z·εσEdA (2.1)
10 CHAPTER 2. ALKALI SILICA REACTIONS External Restraint from Adjoining Parts
The expansion of a structure due to ASR is often not uniform throughout the whole structure. Factors as more humidity and reactive aggregates locally in the structure could make some parts expand more than adjoining parts.
Figure 2.6 displays two connected beams, where beam 1 is subjected to an initial strain ε0 from ASR and beam 2 is not. If both beams were subjected to the sameε0, they would both expand equally and there would not be any forces in the beams due to the expansion ofε0. When beam 1 expands more compared to beam 2 it will strain beam 2, since this beam is forced to follow, to maintain compatibility in the section. This strain will subject beam 2 to a tension force N2 from the adjoining beam. Furthermore, beam 2 will resist the strain from beam 1, which will give a compression force in beam 1,N1. The resulting strain of the beamsεis illustrated in figure 2.6 by the blue line.
Figure 2.6: Effect of external restrain from adjoining parts
An important aspect of this situation is the difference in the resulting strain ε and the stress contributing strain εσ. The stress contributing strain is an imaginary strain state that represents the stresses in the cross section. The initial strain must be subtracted from the resulting strain to achieve the stress contributing strain. Such that:
εσ=ε−ε0
The initial strain does not contribute to any stresses itself, as the expansion is due to ASR. It is the retaining forces from the section trying to be in compati-bility that introduce these forces. Figure 2.7 illustrates the stress contributing strain of the sections.
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Figure 2.7: Strain of beam 1 and 2
The resulting axial forces in the section is found by the following equation:
Nx= Z
A
εσEdA (2.2)
Resulting Loads form ASR
Depending on the boundary conditions, different load effects can be obtained.
The effects caused by the constraints also depend on the stress state in the concrete. In addition, the support system, the size of the affected parts, the reinforcement layout and the variation in expansion over the structure parts decide the resulting load effects from ASR.
The resulting moment coming from ASR includes the effects of both the internal and external constraints. These effects result in a positive moment, leading to an increase in field moments and a reduction over the supports. The shift in the moment may introduce significant moments in places originally designed with poor reinforcement such as zero moment sections illustrated in figure 2.8. Since such sections are not designed for high bending moments they are possibly very critical. At these places, the elastic moment capacity can be exceeded leading to the formation of plastic hinges. The static system changes and capacities may be exceeded elsewhere as well.
12 CHAPTER 2. ALKALI SILICA REACTIONS
Figure 2.8: Shift in moment distribution due to ASR-loads
If a part within the structure is imposed to a higher ASR-expansion, the part will be compressed, whereas the less exposed parts are set in tension. This may occur locally in the beam or extend over the entire structure. The axial forces occurring due to different expansions can become very high and impact the load bearing capacity of the structure.
Chapter 3
Carbon Fiber Reinforced Polymers
The performance requirements for many existing civil engineering structures do not satisfy today’s demands. The need for upgrading structures has become an arising issue that needs to be solved. This has led to an innovation in methods used for strengthening. During the last 20-30 years a new technique has become common for strengthening concrete structures. This method consists of using externally bonded fiber reinforced polymer composites, also known as FRP [12].
Figure 3.1: CFRP fabric [13]
3.1 Products and Application Techniques
The fibers can either be of glass, aramid, basalt or carbon and are combined with a polymer matrix. Together they make a strong composite material working with the original material. The matrix is typically a polymer of thermosetting and has the function to protect and bind the fibers and to distribute the loads.
The fibers together with a matrix define the FRP material and its mechanical properties [14].
Fibers made of carbon is preferable when dealing with concrete structures and is featured as CFRP. The properties of the fibers depend on how they are made and the amount of carbon. All types of carbon fiber offer high yield strength-and modulus materials [12].
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14 CHAPTER 3. CARBON FIBER REINFORCED POLYMERS
(a) FRP Sheets[15] (b) FRP Strips [16]
(c) FRP Rods [17]
Figure 3.2: FRP Products
The Application of FRP can be divided into different categories in the field of civil engineering. Further in this report, only the strengthening of structures with externally bonded FRP will be treated. The FRPs are mainly used for strengthening structures and are available in the form of thin unidirectional strips, flexible sheets/fabrics or rods. The strips are approximately 1 mm thick and the rods have a diameter of a few millimeters. Both are made by pultrusion.
The sheets have fibers in one or several directions [18].
There are different ways to apply the FRP strengthening and the type of method must be carefully chosen by evaluating the aspect of the situation. The struc-ture part must be considered as well as the purpose of the strengthening. The different methods are mainly separated into two categories:
• Cured in-situ systems
• Pre-cured (prefabricated) systems
When providing flexural strength, the fibers must be placed in the longitudinal direction on the tensioned area, see figure 3.3. It is also beneficial to attach fibers at the sides of a beam. Flexural strengthening FRP is often in the form of long strips, fiber sheets or laminates.
For increasing the shear capacity, FRP shear-reinforcement is installed in three different configurations; side bonding covering two sides, U-wrapped and
com-15
Figure 3.3: Flexural strengthening on the tension side [19]
pletely wrapped beams. Completely wrapped beams have the highest strength-ening effect, but can be challenging to install due to availability. U-wrapping of a beam provides a certain possibility of debonding which reduces the effective-ness of the material. Even though, one can almost achieve the same strength as a complete wrap by sufficient anchoring which is preventing delamination of the U-wrap’s ends. In the same way as for steel stirrups, the shear reinforce-ment can be laid orthogonal on the member’s longitudinal axis or in an inclined position [19].
Figure 3.4: Configurations of shear reinforcement [19]
Figure 3.5: Lateral view of FRP strengthening [19]
The most common and basic technique of applying external bonded FRP is a manually bonding of the reinforcement onto the surface [18]. Three elements need to be evaluated; the substrate, the adhesive and the FRP reinforcement.
The surface must be controlled for unevenness, cracks and imperfections. The
16 CHAPTER 3. CARBON FIBER REINFORCED POLYMERS levels of humidity, chloride and sulphate must be considered, as well as the strength and carbonization of the existing parts.
The deteriorated concrete must be removed and restored with shrink-free ce-ment and measures to prevent corrosion of existing steel must be done. If the surface is too rough it can be leveled by using epoxy paste. The next step is to sandblast the concrete surface to a suitable roughness degree. Furthermore, the surface must be cleaned to remove any dust, foreign particles, oil or other materials that could prevent the bonding. When treating a concrete member with sharp corners or edges, these have to be rounded to a radius of minimum 20 mm [19].
Afterwards a suitable bonding agent must be chosen. This can be a multiple-component system or a single bonding agent. Eventually the FRP reinforcement can be connected [18].
There are some general requirements for the conditions around the applica-tion of FRP concerning temperature and humidity. Even though FRP can be installed on structural elements in both dry and humid environments, a very moist environment can delay the curing of the resin [18]. In the case where fire can occur, protection for fire must also be provided. Fire insulation can be done by using protective plasters or panels [19].
When installing FRP outdoors it is necessary to prevent chemical-physical re-actions in the matrix and the reinforcement must therefore be protected against sunlight. This can be done by using a protective acrylic paint [19].
The FRP material needs sufficient anchoring to maintain the desired capacity both for bending and shear. This can either be done by different applications of FRP-material, bolts or steel plates. Concerning flexural strengthening, the most common method is the use of U-shaped straps placed orthogonal to the flexural strengthening. This solution maintains the position of the longitudinal strip and prevents debonding [19].
3.1.1 Assumptions for Application and Design
The right application of the FRP materials is extremely important as it is the basis of the design rules. Some assumptions of the application and the properties of the composites are made to be able to design the strengthening and these must be fulfilled [18]:
• Steel stirrups have sufficient deformation capacity such that the FRP can reach its design strength
• Slip between FRP and the concrete is negligible. This is provided for as long as the adhesive used is of high quality and has at least a thickness of 1.0 mm. In this way it is justified that viscoelastic phenomena are negligible.
• Interlaminar shear strength of FRP is higher than the adhesive bond shear strength. To ensure this, it is important to choose a resin of high quality.
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• Preparation done at the surface is good enough to achieve the required bond strength.
• An elastic analysis method of a cracked cross section can be used to de-termine the strain in the existing steel reinforcement at the time of FRP application.