Concrete Basics

What is Concrete Made of

Pouring Concrete

Concrete Mix Designs

Aggregates in Ready-Mix Concrete

Building Concrete Slabs

Concrete Testing

Seasonal Pouring Tips

Repairing Concrete

Concrete Repair

Fixing Decorative Concrete

Applications

Staining Concrete

Stamped Concrete

Concrete Countertops

Concrete Patios

Concrete Floors

Commercial Concrete

Tying New Concrete to an Existing Foundation

Question:

We are installing a new concrete sidewalk that will abut our home's concrete foundation. The contractor says he wants to pin or "knit" the new pour to the foundation using metal rebar. Is that a good practice, and is it necessary?

Answer:

When pouring a new walkway or small patio that abuts a house or other structure's foundation, always consult local building codes for guidelines on pinning or knitting the new pour to the existing foundation. In most cases, the new slab should be left separate from the foundation to allow slab movement. If the sidewalk is knitted to the foundation, cracking can occur in those areas.

The only reason to consider knitting the two together would be to prevent the new slab from settling back toward the house, creating a drainage problem. I have heard of situations where rebar is installed in the new slab and allowed to slide in and out of greased holes drilled into the foundation. This allows the slab to move without causing cracks, while keeping it from settling toward the house. However, if the sidewalk is placed over a properly prepared and compacted subbase, there should be no settling problems and no reason to tie the slab and foundation together (see Subgrades and Subbases for Concrete Slabs).