Concrete foundation wall reinforcing

While reinforcing concrete foundation walls may not be the most challenging part of a build, it can often be a sticking point at inspection time. This article will provides some tips to get it right, the first time

NZS 3604:2011 Timber-framed buildings describes reinforcing for in-situ concrete and concrete masonry foundation walls with a piled foundation system supporting lightweight timber-framed construction (refer to paragraph 6.11.7 and Figures 6.13, 6.14 and 6.15 of the Standard).

Reinforcing generally consists of 12mm diameter deformed (D12) bars. The use of deformed bars, which have an irregular surface, creates a good bond between the reinforcing and the concrete. They are installed both horizontally and vertically at certain spacings depending on:

• The height of the wall.
• Whether the wall is in-situ concrete or concrete masonry.
• Whether the wall is to support one or two-storey construction.
• Whether the wall is cantilevered or not.

The details for reinforcing in-situ concrete and concrete masonry foundation walls are summarised in Table 1 and shown in Figures 1 and 2.

Reinforcing for foundation walls combined with concrete slab-on-ground floors

NZS 3604 contains examples of reinforcing for foundation walls combined with concrete slab-on-ground floors supporting light-weight construction (refer Figures 7.13(B), 7.14(B) and 7.14(C)).

The Standard also contains examples of the reinforcing if the combined foundation/concrete slab-on-ground floor also supports masonry veneer cladding (refer Figures 7.15(B), 7.16(B) and 7.16(C)).

The details for reinforcing combined footing/concrete floor slabs are summarised in Table 2 and shown in Figures 3 and 4.

Note that B1/AS1 Amendment 11 removed the untied slab/footing details in Figures 7.13(A), 7.14(A), 7.15(A) and 7.16(A) of the Acceptable Solution. All concrete slab-on-ground floors must now be reinforced and the slab reinforcing tied into the foundation wall reinforcing.

Laps and changes in direction

Where horizontal reinforcing bars change direction, and in other situations where they must be lapped, the overlaps must be a minimum of 500mm. At corners, the laps must be at least 500mm in each direction as shown in NZS 3604: Figure 6.15(a).

Lapped reinforcing should be tied with 1.6mm black annealed steel wire, which is soft and easily bent, at each end of the lap and at regular spacings in between.

Linking horizontal reinforcing bars

Where pairs of horizontal reinforcing bars are required in the footings of combined foundation wall/concrete slab-on-ground floors, they must be linked by stirrups. The stirrups are formed from the R10 reinforcing bars installed at 400mm centres and tied with steel wire ties at the junctions of the reinforcing and the stirrups (see Figure 4).

The bends in the reinforcing that form a hook or create a right angle must be at least five times the diameter of the bar, eg, the minimum bend diameter for 12mm diameter deformed reinforcing is 60mm.

Other reinforcing requirements

Other reinforcing requirements for foundation walls and footings include:

• Stepped footings – these must have additional reinforcing in accordance with NZS 3604: Figure 6.12 (see Figure 5).
• Where concrete or concrete masonry is against ground, reinforcing must have a minimum concrete cover of 75mm.
• Openings in foundation walls larger than 300mm in any direction must have one D12 trimming bar on each side of the opening. These bars must extend at least 600mm past each corner of the opening. Where a lintel is less than 650mm deep, the jamb trimming bars must be bent over 60mm from the top of the concrete.

This article first appeared in Codewords – Issue 85