Understanding Concrete Slabs

As part of our Pre-construction Exposed segment, Barry Quon, National Technical and Training Manager for Termseal, provides an overview of the various concrete slab construction methods.

Concrete slab buildings are by far the most common type of construction. Understanding the different types of concrete slab and the potential entry points is vital in selecting and installing an appropriate termite management system.

For such buildings, the starting point of a termite management system is the concrete floor slab itself. Australian Standard 3660.1 New Building Work states that a concrete slab constructed to AS 2870 or AS 3600 can be used as a component of a termite management system.

It is important to note that the slab is not a complete barrier but a component of an integrated termite management system. Consideration must be given to slab penetrations, construction joints, step-downs and the perimeter slab edge, in order to complete the barrier.

Slab construction across Australia varies greatly. The different types include knock out block (typically found in North Queensland), stiffened raft slab or monolithic rebated edge slab (either on ground or form void e.g. waffle pod), in fill slab, suspended slab and slab on footing beams, which is predominate in Western Australia.

Figure 1: Stiffened raft (typical convention slab). This picture shows it poured as one, with rebated edge beams and slab thickenings

Understanding the slab construction and points of weakness/potential termite entry points will determine the protection required. For example, slab edge protection on the typical stiffened raft slab (Figure 1) or waffle pod slab (Figure 2) can be achieved easily by installing any of the Termseal systems. For example, Ura-Fen Major within the cavity, or Ura-Fen TWB Shield across the cavity either top loaded before the frame and bricks are installed, or side loaded after the frame and bricks are in place (see previous page). Termseal’s newest cavity barrier system utilising PRM can also be used in a top or side load installation.

Figure 2: Waffle raft slab. Originally designed for sites with highly reactive soils. Void pod material usually Styrofoam (EPS). Slab area starts as flat surface (no trenches), only plumbing/electrical pipes protruding. Strength from use of steel reinforcing in ribs and slab top. May or may not be related from masonry wall. N.B. No footing used

Timber-framed clad homes are becoming a popular alternative to brick veneer (see right-hand side of Figure 2). Although typically constructed on a non-rebated slab edge, there is still a requirement for termite protection. In such a situation, the use of Termseal PRM Active Cord and Capping Strip installed on the surface of the slab and overhanging the slab edge (to be folded down with the cladding when fitted) provides a compliant and effective barrier system.

Homes featuring a knock out block design (Figure 3) are protected using Termseal Multi-Purpose Active in conjunction with Termseal Reo-Band in the same manner as you would install a construction joint on the surface. Termseal PRM Active could be an alternative product in this construction type.

Figure 3: Knock out block

Becoming familiar with the various construction types will allow the experienced termite professional to recognise potential weaknesses in designs or indeed in the Standards. For example, Australian Standard 3660.1 recognises that the stiffened raft slab with edge beams, where the slab and footing are tied together, are monolithic and therefore no termite control measures in this area are required. However, this building technique is specifically for continuous concrete (footing beam) tied to continuous concrete (slab perimeter turndown) that is smooth and clean. Can we really guarantee such a perfect pour? We would always recommend using a Termseal product or system to protect this joint. Why would we, as pre-construction termite management professionals, leave this joint to other trades (i.e. the concreters) to protect? The knock out block (Figure 3) and the left-hand side of the stiffened raft slab with edge beams diagram (Figure 4), illustrates the area of potential construction weakness that could arise as a result of a poor concrete pour.

Figure 4: Stiffened raft with edge beams. Trenches dug for footings and thickenings – width and depth dependent on soil types and building size. Brick veneer on left-hand side of picture shows footing ‘tied’ to slab with reinforcing steel making it ‘monolithic’ (may be one or two pours). Right-hand side of picture shows masonry used to reach slab height. Non-monolithic because masonry on top of strip footing

Understanding slab perimeter construction types and termite entry points is just the start of protecting your client’s home. Be aware of the slab penetrations, such as wastewater pipes and electrical conduits, which also need protection. Be very alert to the fact that even with good planning it is not uncommon for there to be on-site variations that you may not be informed about! Always talk to the builder during the job, especially about any variations and any construction joints, as they may not be readily visible.

Barry QuonNational Technical and Training Manager, Termseal

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