Micro Concrete Column Jacketing for Structural Repair

[edit] Introduction

Micro concrete column jacketing is a structural strengthening technique used to increase the strength, stiffness and durability of existing reinforced concrete (RC) columns. It is commonly employed during the repair and rehabilitation of buildings and civil engineering structures that have experienced deterioration, increased loading, seismic damage or construction defects. By adding a reinforced concrete jacket around an existing column using a free-flowing micro concrete, the structural capacity of the member can often be increased without the need for demolition and replacement.

[edit] Micro concrete column jacketing

Column jacketing involves constructing a new reinforced concrete layer around an existing reinforced concrete column. The jacket typically contains additional longitudinal reinforcement and transverse reinforcement, which act compositely with the existing column to improve its structural performance. The technique may be used to increase axial load capacity, improve flexural and shear resistance, enhance ductility and restore the performance of deteriorated structural elements.

Micro concrete is commonly used as the infill material because of its high flowability, low shrinkage and ability to fill confined spaces around closely spaced reinforcement without mechanical vibration. It is supplied as a pre-packaged cementitious material formulated for structural repair applications.

Potential improvements resulting from micro concrete column jacketing include:

• Increased load-bearing capacity.
• Improved flexural and shear strength.
• Enhanced ductility and seismic performance.
• Greater stiffness and reduced deformation.
• Increased durability where appropriate repair and protection measures are incorporated.

The effectiveness of the technique depends on appropriate structural assessment, design, detailing, material selection and workmanship.

[edit] Reasons for column jacketing

Column jacketing may be undertaken for several reasons, including:

• Structural deterioration caused by ageing or environmental exposure.
• Corrosion of reinforcing steel leading to concrete cracking and loss of section.
• Increased loading resulting from changes of use, additional storeys or new equipment.
• Damage caused by earthquakes, impact or fire, subject to engineering assessment.
• Inadequate original design or construction.
• Deficiencies identified during structural appraisal.

Before specifying column jacketing, a structural assessment should determine the underlying causes of deterioration and confirm that strengthening is an appropriate solution.

[edit] Properties of micro concrete

Micro concrete is a high-strength, non-shrink cementitious grout or concrete specifically developed for repair and strengthening applications. Compared with conventional concrete, it offers several characteristics that make it suitable for column jacketing:

• High flowability, enabling placement within congested reinforcement.
• Low shrinkage, reducing the risk of shrinkage cracking.
• Good bond with prepared concrete substrates.
• Placement without mechanical vibration under appropriate conditions.
• High early and long-term strength, depending on the product specification.

Material properties vary between manufacturers and products, and should be selected to suit the structural design requirements.

[edit] Applications

Micro concrete column jacketing is used in a wide range of reinforced concrete structures, including:

• Residential buildings.
• Commercial buildings.
• Industrial buildings.
• Hospitals and healthcare facilities.
• Educational buildings.
• Bridges and other civil engineering structures.
• Buildings requiring seismic strengthening.
• Existing structures undergoing refurbishment or change of use.

The technique is particularly appropriate where strengthening can be achieved more economically and with less disruption than demolition and reconstruction.

[edit] Installation process

Although procedures vary depending on project requirements, column jacketing generally includes the following stages:

1. Structural assessment to determine the extent of deterioration, existing reinforcement, loading and strengthening requirements.
2. Removal of unsound concrete and preparation of the existing surface to achieve adequate bond.
3. Cleaning and treatment of corroded reinforcement, together with replacement or supplementation of reinforcement where required.
4. Installation of additional longitudinal reinforcement and transverse reinforcement designed to achieve the required structural performance.
5. Installation of formwork capable of containing the micro concrete without leakage.
6. Placement of the micro concrete in accordance with the manufacturer's recommendations and the project specification.
7. Curing, inspection and quality assurance to verify satisfactory installation.

The design of the jacket, including reinforcement detailing and interface treatment between the existing and new concrete, should be undertaken by a suitably qualified structural engineer in accordance with applicable design standards.

[edit] Comparison with conventional concrete jacketing

Compared with conventional concrete, micro concrete generally offers improved flowability, making it easier to place around congested reinforcement and within narrow jacket sections. It also typically exhibits lower shrinkage and does not normally require mechanical vibration during placement. Conventional concrete may remain appropriate where there is sufficient access for placement and compaction, particularly for larger jacket sections.

The selection between conventional concrete and micro concrete should be based on the structural design, site conditions, access constraints and construction methodology.

[edit] Design considerations

The successful application of micro concrete column jacketing depends on careful structural design and construction. Important considerations include:

• Assessment of the existing structural condition.
• Adequate bond between existing and new concrete.
• Appropriate reinforcement detailing and anchorage.
• Durability requirements, including protection against future corrosion.
• Compatibility of repair materials with the existing structure.
• Quality control during installation and curing.

Strengthening works should comply with relevant structural design codes, repair specifications and applicable national standards.

[edit] Related articles on Designing Buildings

• Reinforced concrete
• Concrete repair
• Structural engineer
• Structural survey
• Structural appraisal
• Reinforced concrete construction
• Corrosion of reinforcement in concrete
• Carbonation
• Concrete
• Building pathology

--Gubbicivilengineers