Joint Design For Reinforced Concrete Buildings

JOINT DESIGN FOR REINFORCED CONCRETE BUILDINGS

Volume changes caused by changes in moisture and temperature should be accounted for in the design of reinforced concrete buildings. The magnitude of the forces developed and the amount of movement caused by these volume changes is directly related to building length. Contraction and expansion joints limit the magnitude of forces and movements and cracking induced by moisture or temperature change by dividing buildings into individual segments. Joints can be planes of weakness to control the location of cracks (contraction joints), or lines of total separation between segments (expansion joints).

THE NEED FOR JOINTS

Due to the low tensile capacity of concrete, some cracking in reinforced concrete is unavoidable. Contraction joints provide a weakened plane for cracks to form. Through the use of architectural details, these joints can be located so that cracks will occur in less conspicious locations within a building and possibly be eliminated from view. Expansion joints allow thermally induced movements to occur with a minimum build-up of stress. The greater the spacing between joints, the greater the stresses. TypicallY, these jOints isolate a frame into a series of segments with enough joint width to allow the building to expand with increasing temperature. By isolating the segments, expansion joints also provide relief from cracking due to contraction, and therefore act in a dual role.

Crack control in reinforced concrete buildings is needed for two reasons. The obvious reason is aesthetics. Where cast-in-place concrete is to be the finished product, cracks are unsightly. Cracks in major framing elements such as girders and columns tend to promote questions concerning the structural adequacy of the structure. They may, in fact, pose no structural problems, but to the average person without structural knowledge, they can be cause for alarm. Secondly, cracks of substantial width invite air and moisture into the framework of the structure, possibly having deleterious effects.

CONSTRUCTION JOINTS

Except for very small structures, it is impractical to place concrete in a continuous operation. Construction joints are needed in order to accommodate the construction sequence for placing the concrete. The amount of concrete that can be placed at one time is governed by bat ching and mixing capacity, crew size, and the amount of time allotted. Correctly sited and properly executed construction joints provide limits for successive concrete placements, without adversely affecting the structure. For monolithic concrete, a good construction joint provides a wellbonded watertight surface, which allows for flexural and shear continuity through the joint. Without this continuity, a weakened region results, which may serve as a contraction or expansion joint. A contraction joint is formed by I imi ti ng the percentage of reinforcement through the joint, thus creating a plane of weakness. An expansion joint is formed by leaving a gap in the structure of sufficient width to remain open under extreme temperature conditions. If possible, construction joints should coincide with contraction or expansion joints, which are discussed in the following sections. The balance of this section is devoted to construction joints in regions of monolithic concrete.

JOINT CONSTRUCTION

To achieve a well-bonded watertight joint, a few conditions must be met prior to placement of the fresh concrete. The hardened concrete must be clean and free of all. If only a few hours elapse between successive placements, a visual check is needed to be sure that all loose particles, dirt, and laitance are removed. The new concrete will be adequately bonded to the hardened green concrete, provided that the new concrete is vibrated thoroughly over the area. Older joints need a little more surface preparation. Cleaning by means of an air-water jet or wire brooming can be done when the concrete is still soft enough that any laitance can be removed, but hard enough to prevent aggregate from loosening. Concrete that has set should be prepared using a wet sand blast or ultra-high pressure water jet

JOINT LOCATION

The final consideration is placing the construction joint in the right place. Assuming an adequate production capacity, construction joints should be located where they will least affect the structural integrity of the element under consideration, while at the same time being compatible with the building's appearance. Placement of joints varies, depending on the type of element under construction. For this reason, beams and slabs will be addressed separately from columns and walls.

BEAMS AND SLABS

From the point of view of strength in beam and slab floor systems, desirable locations for joints placed perpendicular to the main reinforcement are at points of minimum shear or at points of contraflexure. Typically, joints are located at mid-span or in the middle third of the span, but locations should be verified by the engineer before placement is shown on the drawings. In beam and girder construction, where a beam intersects a girder at the point of minimum shear, ACI 318 states that the construction joint in the girder should be offset a distance equal to twice the width of the incident beam. Horizontal construction joints in beams and girders are usually not recommended. Common practice is to place beams and girders monolithically. with the slab. In the case of beam and girder construction where the members are of considerable depth, Hunter (1953) recommends placing concrete in the beam section up to the soffit of the slab, then placing the slab in a separate operation. The reasoning behind this is that cracking of the top surface may result due to vertical shrinkage in a deep member. With this procedure, there is a possibility that the two surfaces will slip due to horizontal shear in the member. In this case, adequate shear transfer must

be provided (ACI 318). Construction joints parallel to the slab span can be placed anywhere, except those locations in T-beam construction that rely on a portion of the slab to act with the beam in resisting flexure. The main concern in joint placement is to provide adequate shear transfer and flexural continuity through the jOint. Flexural continuity is achieved by continuing the reinforcement through the joint with enough length past the joint to insure an adequate splice length for the reinforcement. Shear transfer is provided by shear friction between the old and new concrete, and/or dowel action in the reinforcement through the joint. Shear keys are usually undesirable (Fintel 1974), since keyways are possible locations for spalling of the concrete. If proper concreting procedures are followed, the bond between the old and new concrete, plus the effect of the reinforcement crossing the joint, are adequate to provide the necessary shear transfer.

COLUMNS AND WALLS

It is general practice to limit concrete placements to a height of one story. Construction jOints in columns and bearing walls should be located at the undersides of floor slabs and beams, and at the top of floor slabs for columns continuing to the next floor. Column capitals, haunches, drop panels, and brackets, should be placed monolithically with the slab. Depending on the architecture of the structure, the construction joint may be used as an architectural detail, or located to blend in without

being noticeable. Quality form construction is of paramount importance in order to provide the visual detail required (PCA 1982). The placement of fresh concrete on a horizontal surface can affect the joint. Common practice has been to provide a bedding layer of mortar, of the same proportions as that in the concrete, prior to placement of new concrete above the joint. The ACI Manual of Concrete Inspection (ACI Committee 311 1981) recommends using a bedding layer of concrete with somewhat more cement, sand, and water than the design mix for the structure. Aggregate less than 3/4 in. can be left in the bedding layer, but all aggregate larger than 3/4 in. should be removed. This mix should be placed 4 to 6 in. deep and thoroughly vibrated with the regular mix placed above. To avoid settlement cracks in slabs and beams due to vertical shrinkage of previously placed columns and walls, the concrete in the columns and walls should be allowed to stand for at least two hours prior to placement of subsequent floors. Placement of vertical construction joints in walls also needs to be compatible with the architectural flavor of the structure. Construction jOints are often located near reentrant corners of walls, alongside columns, or other locations where they become an architectural feature of the structure. If the building architecture does not dictate where the joints should be placed, placement considerations, such as production capacity of the crew or whether or not one set of forms will be reused along the length of the pour may limit the length between joints. This criteria will usually limit the maximum horizontal length to 40 ft between joints in most buildings (PCA 1982). Due to the critical nature of building corners, it is best