how to repair concrete columns with carbon fiber composites?
Carbon Fiber Application Expertise
The strength and durability of reinforced concrete columns can be affected by four main factors: corrosion of internal metal rebar, freeze-thaw deterioration, chemical attack, and persistent localized damage from extreme load conditions and natural disasters such as earthquakes and floods .
As the structure ages, concrete spalling can occur, exposing the rebar to the environment, which can accelerate corrosion of the rebar. If the concrete columns are not properly repaired in time, the deterioration of the concrete will increase, and the risk of collapse will be greatly increased. Therefore, reinforcement may be required as soon as possible to prevent further corrosion and concrete degradation, and to restore the original structural bearing capacity of reinforced concrete.
01. Concrete column reinforcement and repair technology
For decades, researchers have worked to improve methods of strengthening and repairing concrete columns to address many possible defects, including reinforcement and repair techniques that balance structural requirements with nonstructural issues such as minimizing cost, Maintain functionality and aesthetics, keep workers safe, and make repairs in short order.
In recent years, an innovative technology using high-strength fiber-reinforced polymers (FRPs) is rapidly emerging as an alternative to traditional retrofit techniques in the construction industry.
FRP strength-to-weight ratio and adhesion, corrosion resistance, ease of operation and installation speed are unmatched by traditional repairs (such as column tie rods, supplemental vertical reinforcement, etc.). Compared to unconfined reinforced concrete members, FRP-wrapped columns increase ductility while providing excellent shear, compressive bearing capacity, and energy redistribution.
02. Traditional concrete column repair method
Repairs to columns can be divided into two categories: cosmetic repairs and structural repairs. When properly designed, surface repair can reduce the rate of further concrete degradation by preventing delamination and stopping reinforcement corrosion.
2.1. Appearance/surface repair
Cosmetic or surface repairs are for minor localized deterioration such as cracks and pitting in affected columns and concrete delamination. Corrosion accelerates significantly when rebar is exposed. Some examples of surface repairs include changing mortar, post cages, using protective coatings and composite wraps.
2.2. Structural/Comprehensive Maintenance
Structural or comprehensive repairs restore or enhance the integrity of the concrete column and increase the overall strength of the column. Reinforced concrete cover can be used successfully if aging does not significantly reduce the cross-section.
2.3. Reinforced concrete protective layer
The application of reinforced concrete cover is a traditional repair technique used to reinforce and repair defective and damaged concrete columns. It involves covering the existing columns with a new layer of concrete, which can either be poured around the rebar supports or bolted directly to the base of the existing columns. Steel sheathing is also an effective repair method, increasing load and shear capacity while increasing resistance to bending stresses on the column.
Although this column repair technique is an effective method for protecting and strengthening columns, it is expensive and time-consuming in terms of axial bearing capacity, flexural strength, and ductility. In addition, the improvement in ductility is small because the cover material is often brittle and does not always bond effectively to aged concrete. In addition, it changes the cross-sectional area of the column, thereby changing the mass and stiffness.
03. The increase in the application of carbon fiber composite repair
When the bearing capacity of existing columns is insufficient, construction companies are increasingly using composite materials such as carbon fiber/epoxy and fiberglass to provide external restraint to damaged and fragile concrete cores. These techniques offer the added advantage of preventing further degradation of corroded rebar.
Carbon fiber-based composite repairs can also increase horizontal loads and provide shear reinforcement for concrete structures.
3.1. Why use carbon fiber for structural repair and reinforcement
Carbon Fiber Reinforced Polymers (CFRPs) have been the main material in the aerospace industry for decades and have been successfully used in many other industries. CFRP is stronger, lighter and offers better fatigue resistance than any other typical building material.
CFRP combines a high strength-to-weight ratio, excellent stiffness, ductility, high temperature resistance, minimal added weight, and excellent corrosion resistance. They can also be used to repair and prevent cracks, straighten beams, upgrade bridge columns, strengthen walls and piers, enhance seismic performance, strengthen existing structures after structural modifications, increase lateral bearing capacity, and increase compressive strength.
3.2. Excellent properties of carbon fiber composites
The main advantages of carbon fiber composite materials for building structural repair and reinforcement are as follows:
High strength to weight ratio
Lightweight and corrosion resistant
Acid resistance
More economical than supplementary column installation
Easy and fast installation
Reduce shipping and installation costs
Prevent column buckling
Increased column load-carrying capacity
Enhances the flexural strength of the repaired part
04. How to repair concrete columns using carbon fiber composites
When damaged or seismically deficient columns require repair, strengthening existing columns with fiber-reinforced polymers such as carbon fiber composites can restore structural integrity in accordance with DOT regulations.
Column repair is an ideal application for carbon fiber composites because the material is lightweight and extremely strong. The properties of FRP composites reduce the overall thickness of the repair and minimize any added weight. A hand-applied system will conform perfectly to any size and shape, preventing further corrosion to the rebar and covering any exposed metal.
The general process of using carbon fiber composite materials to repair concrete columns is as follows:
Surface Treatment. When repairing damaged concrete, it is important to first prepare the surface, remove any loose material from the posts and footings, and ensure proper bonding of the composite repair.
Replace the missing concrete. In advanced FRP systems, polymer cement is often used to replace any missing concrete prior to carbon fiber repair. Concrete is treated with a penetrating primer immediately before installation to improve material properties. The primer will increase the adhesion of the carbon fiber composite and strengthen the concrete itself.
Wrap the Carbon Fiber Composite around the Post. Wrapped with carbon fiber composite material, the overlap is interleaved with the subsequently wound composite material to increase bond strength. Once the specified number of carbon fiber composite layers have been applied to the concrete column, the material can be cured and a UV-resistant topcoat applied.
Together, the carbon fiber reinforcement system, along with the appropriate mortar or polymer cement, provides unmatched strength, elasticity and longevity for new restorations. Depending on the environment, carbon fiber can last up to 50+ years without additional maintenance.