HOW TO INSTALL A STEEL FENCE POST INTO THE GROUND USING CONCRETE
When it comes to installing steel fence posts in concrete many people take a cavalier attitude. The assumption that the concrete will protect and support your steel fence posts until the next ice age can prove dangerously false.
Too often steel fence posts corrode at the base as a result of incorrect installation leading to expensive premature failure and repairs. Follow this guide and get it right the first time.
Steel posts need to comply with and be installed in accordance with appropriate Australian standards and these standards may (among others) include:
- AS 1725 – Chainlink fabric fencing – Security fences & gates
- AS/NZS 4680:2006 — Hot-dip galvanized fabricated articles
- AS/NZS 4792:2006 – Galvanized hollow sections
- AS/NZS 4791:2006 – In-line galvanized open sections
- AS 4312 – environmental corrosivity
- AS/NZS 1170.2.2011 Structural design loads (wind loads)
Why Do Steel Fence Posts Fail?
The lifespan of galvanised products does vary considerably.
According to the Australasian Wire Industry Association the durability of galvanised posts installed near most Australian coastal areas can range from as little as 3 years to more than 50 years.
The Wire Association adds that
“a simple rule of thumb is AS/NZS 4792 HDG300 galvanised posts will last 3 times longer than AS/NZS 4792 ZB100/ZB100 pre- galvanized posts, while batch galvanized posts to AS/NZS 4680 will last 5 times longer than pre-galvanized posts in the same environment.”
It’s important to remember that particular micro-climates may need to be taken into account. These micro-climatic conditions can add layers of complexity to the C1 to CZ corrosion categories as identified by the ISO 9223 standard.
For example, critical microclimates may exist where the post is exposed to high levels of salinity but is sheltered from the cleansing (and corrosion reducing) effects of rain.
Likewise, sports ground installations that are frequently watered may display climatic corrosion levels much higher than the surrounding environment.
Probably the most common area where steel posts corrode to the point of failure is at ground level. This corrosion occurs due to incorrect concrete application. Concrete must be domed around posts to eliminate water pooling.
In the case of base plated posts, plates should be bolted and levelled above the finished ground level and finished by grouting under the plate. Where water and soil collects, corrosion levels rise considerably.
So how do you avoid problems with premature steel post failure?
Are My Steel Posts Adequate For The Job
Both the AWIA and the Galvanizer’s Association of Australia make it quite clear that the service life of any galvanizing coating is directly proportional to the thickness of the coating with posts prepared to AS/NZS 4680 lasting 5 times longer than pre-galvanized posts in the same environment.
In other words, you get what you pay for; do it cheap and it will probably cost you more in the long run.
How Do I Calculate The Correct Fence Post Hole Depth?
Fence stability starts at the bottom. The depth of your fence post footings will vary according to fence height, wind loading, and soil type as well as other factors.
For example, an 1800mm fence in wind zone A will require footings of a minimum of 600mm deep by 200mm wide whereas the footing dimensions for the same fence in wind zone D will need to be at least 700—1100mm deep by250mm wide.
A solid security fence such as a Diamond Chainwire fence will require 800mm deep by 300mm wide footings.
Ensure that the soil at the bottom is adequately compacted to ensure adequate weight bearing support and that the top soil layer is not included in your footing depth.
How Do I Prepare The Posts For Concreting?
Ensure that you have chosen adequate quality steel and galvanised posts for the job.
The Australasian Wire Industry Association recommends the application of an epoxy mastic paint 100 to 200mm above and below the ground level mark on the post. This will help prevent contact with any moisture or soil build-up at the base of the post.
The bottom of the post should be situated 40mm minimum from the bottom of the concrete footer.
This concrete bed is critical to prevent contact between soil, moisture, and the bottom of the pipe. This is a sadly common fault and will severely limit the life of your posts. Equally important is doming the top of footing to eliminate water pooling.
Is Your Concrete Adequate For The Job?
Pour concrete with a strength of 20MPA and taper off at the top to enable water runoff. Make sure there is adequate concrete underneath the post.
Check posts are at the correct level and plumb. You may need to brace the posts at the correct placement until the concrete is set.
How Do I Fix Posts That Are Already Rusting?
If previously installed posts are showing signs of corrosion you may be able to take remedial action to extend the life of the steel. The AS/NZS 2312 standard notes that maintenance painting should not be deferred beyond 2% of the post displaying red rust.
Properly preparing the affected posts and painting with an industry approved coating system can significantly increase the lifespan of affected posts.
In reality, galvanised steel is one of Australia’s most successful building products with service life records commonly recording 50 years of valuable service and as many as 110 years in some cases.
Protective is proud to be part of that great Australian tradition of creating products that won’t let you down.
Taking shortcuts in the installation process will not give you the best results so get it right and your well-designed, correctly installed fence will be part of the heritage you hand over to the next generation.