The following are some of the most common pitfalls encountered by builders. If you are the builder, then you should take note of the following and save yourself time and money upfront, it will also enhance both of our reputations as building professionals if you are able to get it right on the first inspection.
Bored piers are indicated on the footing plan with circles, and excavated piers are indicated with rectangles. The depth of the piers is not typically indicated on the footing plan (refer to soil report for pier depth). The bottom of the piers must be clean and free from any loose material, prior to placing concrete. It's not hard to imagine the consequences of pouring a concrete pier on top of a layer of loose disturbed soil in the base of the hole. The post over the pier may well be supporting up to 3 Tonnes, which in time will compact the loose material, leading to settlement. In the case of a Lift and Build Under, it is normal for the existing residence to be lifted and supported on a system of posts cast into bored pier footings. The lower level slab on ground is then constructed. All concrete must be compacted by vibration to remove air voids to achieve a solid homogeneous mass. Core samples of un-vibrated concrete bear a striking resemblance to pumice stone which we have all seen floating at the beach. Uncompacted concrete has dramatically reduced strength.
It is not uncommon to arrive on site to find the lower level slab beam reinforcement cut and removed because the concrete piers for the posts have been overfilled. Obviously the slab support system of beams can't perform correctly if the reinforcement has been cut and merely abuts the face of the pier. For this reason, we specify the top of the piers should finish in the order of 50mm below the soffit of the concrete over. Taking a little care to achieve this means the reinforcement can be correctly placed and is continuous over the pier. It is then a simple matter of cleaning the top of the pier prior to placing the concrete, which flows down to the top of the pier.
Piers extending through fill to support the slab, and in turn the entire building, invariably rely on end bearing only, hence unless the fill overlies rock or similar substantial material, the area of the base of the pier must be sufficient to ensure the bearing capacity of the underlying soil is not exceeded. It is for this reason that we often specify rectangular excavated piers of dimensions 900x350mm.
Note: The area of the base of a 900x350mm excavated pier is 0.3m2, whilst the area of the base of a 350 dia. bored pier is 0.1m2. It follows that three 350 dia bored piers are required to achieve the same end bearing capacity as a single 900x350mm excavated pier.
Timber Wall Framing:
There is always a note on the drawings about jamb studs and how the load needs to be transferred between floors and ultimately down to the ground, even via a sometimes circuitous route of supporting beams. Solid timber blocking needs to be installed to distribute point loads to the floors throughout the joists and to prevent the joists toppling over sideways. This is particularly important with timber I-Beams, and manufacturer's instructions should be followed in this instance. We have, and utilise during inspection, documentation from the manufacturers relating to I-beam installation. You should obtain a copy of the relevant manufacturer's installation guide and keep it on site.
Masonry Retaining Wall Reinforcement:
The location of the vertical reinforcement in relation to the face of the blockwork is critical to the function of a reinforced concrete retaining wall. "So I have to take the whole wall down and move it over 100?" Look at the retaining wall detail drawing very carefully when you are setting out to place the starter bars into the retaining wall footing, especially where the design requires 300 series blocks. It is too late to fix after the footing concrete has been placed. The reinforcement inside the wall needs to be toward one face of the wall inside the masonry core; The relevant face being dependent upon the type of wall and is clearly defined on the drawings.
Clearing Masonry Wall Cores:
Any mortar which has strayed into the block cores must be cleared out so that voids and air-pockets do not form in the concrete cores. The broken out mortar can then be removed from the retaining wall via the clean-out blocks specified at the base of the wall. Clean out the walls prior to our inspection, and make sure that someone has organised a needle vibrator to vibrate the concrete down into the cores when it is being placed. Aerated (uncompacted) concrete in retaining walls leads to substantially reduced strength and a shorter wall lifespan. For more information relating to Masonry, refer to the Concrete and Masonry Association of Australia Manuals.