Jacketing and Collars for Strengthening of Concrete Structures
Jacketing is a restoration or strengthening process used to bring an existing structural member back to its original dimensions or increase its size. This is achieved by encasing the damaged section with suitable materials, such as a steel reinforcement cage or a composite material wrap. Shotcrete or cast-in-place concrete is then applied onto the jacket, which can be constructed around the damaged section. Collars, on the other hand, are jackets that only surround a portion of a column or pier. They are commonly used to provide additional support to the slab or beam at the top of the column. Jackets can be made from various materials such as timber, corrugated metal, precast concrete, rubber, fiberglass, or special fabric, and in some cases, they can be permanent forms. It is important to ensure equal clearance between the form and the existing member, which can be achieved by using spacers.
Materials such as conventional concrete, epoxy mortar, grout, and latex-modified mortar and concrete are commonly used as encasement materials for jacketing. Jacketing is a method used for repairing deteriorated columns, piers, and piles, and can be employed in underwater applications. It is also used for protecting concrete, steel, and timber sections against further deterioration and for strengthening purposes. Permanent forms are preferred when weathering, abrasion, and chemical pollution protection is desired. Collars, which provide increased shear capacity for slabs and decrease the effective length of columns, are considered architecturally better than jacketing but perform the same structural function.
Before applying jackets or collars, it is important to remove all deteriorated concrete, repair cracks, clean existing reinforcement, and prepare surfaces. Surface preparation improves the bond between the newly placed materials and the existing structure, which can be challenging for underwater repairs. For underwater conditions, a plastic shell may be applied at the splash zone to minimize abrasion. However, jackets and collars have the drawback of occupying space that was previously available for other uses.
Temporary or permanent forms made of timber, cardboard, corrugated steel, fiberglass, rubber, or fabric can be used for jacketing. Permanent fiberglass, rubber, and fabric forms are gaining popularity due to their resistance to chemical attack after the repair is complete. Jacketing used for purposes other than covering deteriorated concrete and providing lateral confinement, such as bearing longitudinal loads, requires special considerations. The existing column may have undergone full shrinkage, creep, and elastic strains due to carried loads, while the shrinkage and creep of the new material still need to occur. Load transfer to jacketing is also a crucial issue, and it is better to use jacking to release the load on the member before jacketing, use non-shrinking materials for jacketing, and hammer steel shims at the transfer points of the jacketing after curing.
When using cement mortar or concrete for jacketing, the cement content must be exactly according to the requirements, as both excessive and insufficient cement contents can be dangerous. Clean, stable, and the largest possible size aggregates should be used. Shrinkage can be reduced by controlling the temperature of materials and the immediate surroundings during placing and curing. Admixtures such as plasticizers, air-entraining agents, retarders, accelerators, and waterproofing admixtures are more beneficial in repair than in ordinary construction. Expanding mortars/concretes can be made by adding aluminum powder to the matrix to overcome setting shrinkage and some part of drying shrinkage. Iron fillings or powder can also be used for this purpose if moisture and air are available. If grout is used for filling the forms, it should be allowed to settle for about 20 minutes after filling and then filled to an overflowing condition. The top of the jacket must be finished with pneumatically projected or hand-placed concrete.