The fact is most of the treated pine sleepers on the market are produced from the core of the Radiata Pine tree, where the early growth is quite dramatic. This is evidenced by the growth or annual rings which are generally a long way apart.
In the formation of the pine tree is early growth, usually the first 8 to 10 years - this is usually referred to as the juvenile wood, or in wood technology terms, called the heartwood. Then there is a demarcation line and all further growth is called the sapwood, where the annual rings progressively get closer together and the wood is more mature.
From a wood technology point of view, it is important to understand the difference, because the cell structure of the heartwood does not or will not easily accept the treatment solution that gives Radiata Pine, which naturally has a poor durability rating, the durability required for products to last and stand the test of time.
Conversely, the sapwood has a cell structure that will easily accept the treatment solution. By looking at the ends of treated timbers it is not difficult to see the heartwood, which has a light green colour, whereas the sapwood has a dark green colour.
The Australian Standard AS 1604.1 to hazard level H4, clearly covers this by stating;
"the preservative shall penetrate all the sapwood and, in addition the following requirements shall apply;
- "the preservative shall penetrate not less than 10 millimetres of any surface; OR
- "unpenetrated heartwood shall be permitted, provided that it comprises less than 20% of the cross-section of the piece AND does not exceed 50% of the width of the surface on which it occurs".
From my experience many of the treated pine sleepers I see do not meet the standard and therefore have a limited in ground life span.
The other factor that users should be aware of with this product, is strength, or perhaps lack of. As stated above, the heartwood grows very quickly and as a consequence lacks density which could be an important factor, particularly if the application is structural. Also, knot size is important, particularly if they are large.
My information is retaining walls 1.2 metres or more in height should be engineered designed and a building permit required.
If the application is structural, such as a retaining wall and over 1.2 metres in height and treated pine has been specified, it is mandated by the Building Code of Australia (BCA) that treated timbers used for these purposes be treated to AS 1604.1 hazard level H5. Which means that the sapwood must be fully treated and the heartwood must be treated to a depth of 20 millimetres, otherwise untreated heartwood is permitted provided it does not make up more than 20% of the cross sectional area, does not extend from one face to the opposite one, nor extend more than 50% along any one surface.
I would also expect the pine would have to be structurally graded.
Accordingly, I believe treated pine sleepers (that most yards stock) applications should be confined to garden/landscaping situations or non-structural purposes.
SUGGESTIONS:
- Perhaps consider affixing 200UM plastic to the inside of the sleepers before backfi lling to act as a waterproofi ng membrane
- Install appropriate drainage in the backfilled earth
- Perhaps bring the vertical posts closer together
- Coat all cut ends and bore holes with a proprietary brand wood preserver
If you require any addition information, please feel free to email me at feedback@bowens.com.au or alternatively, on the mobile 0412-550-740.