Optimizing Concrete Construction Joints for Durability and Performance

Optimizing Concrete Construction Joints

Why Concrete Joints Are Essential

• Ideally, concrete is placed continuously to achieve a monolithic structure, avoiding the need for joints.
• In practice, joints are often necessary to facilitate continuous concreting, especially when work extends over multiple days.
• Joints are also crucial for controlling shrinkage cracking. Without them, continuous and rapidly placed concrete can develop uncontrolled cracks due to drying shrinkage.

Key Considerations for Concrete Joints

Careful attention must be paid to the following aspects:

• Joint Location and Design
• Joint Surface Preparation
• Concreting Continuation Methods

Joint Location and Design Principles

• Joints must be specified in the project plans.
• Orient joints as perpendicularly as possible to compressive stresses, placing them where their effect will be least damaging.
• Avoid placing joints in areas subject to maximum stress, particularly where structural elements are under significant tension.
• In beams, concrete joints should be located near the quarter or fifth points of the span, where shear and bending stresses are moderate. A sloped joint between 30° and 45° is often recommended.

Joint Surface Preparation Methods

Joint surfaces must be treated carefully to ensure that a construction discontinuity does not become a structural discontinuity. Common methods for preparing joint surfaces include:

Formwork or Smoothed Surfaces

• Difficult to achieve optimal bond.
• Exhibit low adhesive capacity.

Brushed Surfaces

• Achieved by removing the surface mortar grout to expose the coarse aggregate.

Natural Surfaces

• No specific action is performed on the surface.
• Excessive vibration can lead to significant loss of grout adhesion on the surface.
• Adhesion is comparable to planed surfaces.

Expanded Metal Lath for Joints

• Its durability should match the expected lifespan of the structure.
• Galvanized metal is recommended.

Special Procedures for Joint Preparation

• Sandblasting
• Resin Primers

Prohibited Joint Preparation Methods

• Avoid mechanical chopping or chiseling: These methods cause microcracking in the concrete, leading to a general weakening of the joint and loss of adhesion.

Best Practices for Concreting Continuation

The treatment to apply to the joint surface before pouring new concrete is straightforward:

• Clean dust and dirt: Even small amounts of dust can reduce adhesion by up to 30%. Dust suppression can be achieved by blowing air or brushing.
• Wet the surface of the old concrete: This prevents the old concrete from absorbing water from the new mix.
• Place new concrete when the surface is visibly dry: Recent studies indicate that keeping the surface moist until new concrete placement can reduce bond strength.
• Do not concrete on frozen surfaces.
• For wall brackets and similar elements: Wait until the concrete is no longer plastic before concreting the beam or slab. This allows initial contraction to occur, minimizing subsequent cracking.