Unidirectional Glass Wrap

The reinforcement of structures is typically carried out to achieve various objectives, including increasing the bearing capacity, addressing corrosion and erosion defects, enhancing structural ductility, accommodating changes in the structure’s use, and compensating for design and execution errors.

Bidirectional glass fibers are highly corrosion-resistant and possess high tensile strength. Their lightweight nature, which aligns with retrofitting goals, has led to increasing utilization of these fibers in seismic retrofitting and improvement projects.

When undertaking reinforcement projects, thoroughly examining the structure’s current condition is essential. This includes assessing the current bearing capacity of elements, identifying defects, reviewing calculation files, and analyzing existing documents and drawings related to the structure.

General principles for the design of UD Glass Wrap reinforcement systems are based on the limit state approach to ensure acceptable performance and safety. Due to the relatively lesser knowledge of FRP composites than other materials like concrete, the application of strength reduction coefficients is necessary, aiming for a design reliability coefficient exceeding 3.5. Additionally, testing methods such as core drilling, compressive strength testing, and ultrasonic testing are employed to measure the properties of existing materials, including compressive strength and modulus of elasticity.

Unidirectional glass fibers exhibit specific behaviors in different loading conditions. Under direct tensile loading, these fibers do not exhibit plastic behavior before failure. The tensile behavior of materials follows an elastic linear stress-strain relationship until sudden failure occurs. Factors such as fiber type, orientation, and quantity play significant roles in determining FRP composites’ tensile strength and hardness.

Regarding compressive behavior, UD Glass Wrap, primarily used as an outer covering, should not be relied upon as compression reinforcement. However, in columns, the confinement behavior of compressive-resistant fibers indirectly contributes to an increased capacity of the column.

Evaluation and acceptance of FRP systems involve assessing factors such as fiber direction, presence of layer separation, adhesion to the surface, layer location, and compliance with design drawings. A supervisory body typically performs these evaluations.

To ensure proper usage of unidirectional glass fibers in projects, control measures should be implemented based on laboratory information provided by the supplier company. Once installed, the strength of CFRP composites is evaluated through tests like the adhesive tensile test (Pull-Off test) conducted on core samples. The adhesive resistance must exceed 1.4 MPa to prevent separation and rupture in concrete. Inadequate strength or separation from concrete indicates system defects and a failure to achieve the expected performance.

Evaluation and approval of installed fibers should consider additional factors, including the occurrence of separation, adherence to specified layer placement direction (with deviations of even 5 degrees impacting resistance), thickness after resin curing, and fiber splice length.

Rehabilitation objectives

Glass fibers have been extensively used in various industries. Their mechanical properties are comparable to carbon fibers and polymers.

In general, the purpose of reinforcement is to enhance the structural capacity to withstand different forces, particularly earthquakes. However, reinforcement is not limited to earthquake resistance and can also address issues like building change of use and material quality deterioration.

Building retrofitting can be categorized into two types:

• Existing and in-use buildings
• Earthquake-damaged buildings

Due to the high cost and time required for demolishing and replacing existing buildings, the importance of building retrofitting has been steadily increasing.

Requirements for building retrofitting

The following scenarios necessitate the development and implementation of retrofitting plans:

Existing damage in old buildings

Common forms of damage in buildings include honeycombing, abnormal cracks, and corrosion in beams, columns, and roofs. Detecting such issues often requires various concrete testing methods, such as coring and rebar scanning.

Execution errors

Retrofitting experts need to investigate errors that occur during the construction phase. These errors can manifest in different ways, such as weak concrete specimens, inconsistency between design strength and actual strength, beam and column buckling, and honeycombing.

Change of building use

Changes in building usage require thorough examination by experts. Altering a building’s purpose introduces new forces on structural members that were not originally designed to bear them. These changes may lead to severe damage and failure in load-bearing elements like beams and columns. Consulting engineers with experience in evaluating existing structures and assessing the feasibility of usage changes are essential to develop accurate and detailed plans.

Detecting defects in a timely manner and selecting the appropriate solutions can save significant costs. It is crucial to seek assistance from experienced professionals in this field, as an improper choice of strengthening, repair, or retrofitting methods can worsen the situation.

Unidirectional carbon fiber reinforcement has become increasingly popular due to its high strength, lightweight nature, electrical non-conductivity, cost reduction, and other benefits. Unlike common retrofitting methods such as steel and concrete jackets, which introduce issues like weight gain, architectural changes, disruption to long-term building use, and increased costs, bidirectional carbon fiber reinforcement offers advantages such as corrosion resistance and prevention of recurring problems.

Employers have various retrofitting solutions to choose from, and they must select the most suitable method for their projects.

The implementation details of UD Glass Wrap depend on the structure’s geometry, substrate smoothness, and other factors. The contractor must adhere to the specifications provided by the consulting engineer when using FRP materials. These specifications include fiber type, resin type, number of layers, execution method, and more to ensure proper reinforcement operations.

Steps for implementing UD Glass Wrap:

Concrete repair and substructure preparation:
Before applying the fibers, all defects in the main concrete and underlying concrete that may compromise the integrity of the CFRP composite should be eliminated. Additionally, if rebar corrosion or rust in the concrete is possible, it must be cleaned and repaired.

Mixing resins:

Resins should be mixed according to the manufacturer’s instructions and project specifications. Proper temperature and quantity should be maintained during the mixing process.

Various methods for installing unidirectional glass fibers:
Once the substrate is prepared, the FRP fibers are installed using different methods specified by the consultant. Here are some installation methods:

a. Unidirectional glass fiber wet gluing system:

This method manually installs the fiber system without any dry or corroded resin. The fibers can be impregnated separately using a resin impregnation device and then applied to the concrete surface.

b. pre-impregnated fiber system:

Unidirectional pre-impregnated systems are resin impregnated at the factory but not cured. These fibers are bonded to surfaces with or without additional resin and cured on-site.

Failure to comply with the instructions provided by the consultant and the manufacturer may weaken the system’s load-bearing capacity and, in some cases, result in fiber separation from the surface. To ensure proper installation, the following should be observed:

Minimize sticking bidirectional glass fibers on the inner surface of corners and ensure sufficient development length.
The overlap should be applied to patched areas.
Following these steps and adhering to the guidelines set by the consultant and manufacturer will help achieve the desired UD Glass Wrap implementation quality.

After implementing the FRP system, it is essential to establish quality assurance and control programs led by the quality control unit (QC). Quality control should encompass a series of necessary inspections and tests.

The quality control inspectors should possess comprehensive knowledge of the FRP system and its installation process. During the installation of the UD Glass Wrap system, they should conduct daily inspections, focusing on the following aspects:

• Ambient temperature, relative humidity, and weather conditions.
• Surface dryness.
• Surface preparation methods.
• Observing the resin curing process.
• Identifying any layer separation, air bubbles, or defects.
• The quality control unit should also conduct periodic inspections and evaluations of the implemented CFRP composite.

Evaluation methods for FRP systems:

General inspections:

Visual inspections examine any changes in color, fiber separation, cracking, or other factors. The presence of such factors indicates weaknesses in the system and improper execution. Surfaces covered with FRP materials should be regularly monitored and inspected, as repairs during operation are uncommon or rare.

Evaluation of concrete composite material separation:

Concrete composite material separation is a critical failure in FRP systems. It renders the FRP system practically inefficient. Pull-off tests should be performed at regular intervals per regulations to diagnose this condition. If minor damage is detected, such as resin injection, caution should be exercised during repair to prevent further damage. For more significant damage, such as scaling or widespread layer separation, adhesives, and composites may need to be removed. Defective materials should be removed from the concrete surface, and the cleaned area should be recoated with new materials.

Long-term monitoring and inspection:

Like other structural members, the reinforcement system with UD Glass Wrap requires regular monitoring and inspection. For bridges, such measures are typically conducted annually in general and every six years in detail. However, regular inspections of buildings are seldom performed. Building quality control engineers should advocate for regularly monitoring and inspecting reinforced members. CFRP composite reinforced buildings should be subject to regular monitoring and inspection of the reinforced members.