Ensuring your slats are still up to the job

The majority of gang slats would be expected to have a working life of approximately 25 years but this is not guaranteed and it does not mean that they cannot fail before that. I have come across reports from farmers of slats failing before this so farmers need to be vigilant.

For that reason, farmers should take time to inspect slats thoroughly during the summer in preparation for the housing period. A lot of building work took place on farms in the 1990s and early 2000s and the slats on these sheds may be coming to the end of their working life.

For slats that were produced decades ago, the steel may have been placed too near the outer surface of the concrete and at risk of corrosion, leaving the slat at risk of collapse. Corrosion, or damage during installation, can allow slurry to come in contact with the steel. The steel eventually rusts, weakens and swells, cracking the concrete.

Case study

On the farm that I visited there was a double tank of gang slats that had to be replaced. The original slats had been installed in 1980 and looking at the slats from the top, there was very little visual signs of damage or corrosion. However, when the bottom of the slats were inspected, a different story was told. The farmer had not intended to replace the slats this year.

However, one of them failed during the spring, slipping into the tank with cattle also falling in. As can be seen in the pictures, the slat cracked across the middle, allowing it to sag enough for the edge to slip off the wall. Cattle were recovered from the tank but it was evident that slats were beginning to fail.

It was at this point that Corbett Concrete came out to inspect the remainder of the slats and it was evident that they would need to be replaced.

Initially, it was thought that it would be a simple job to lift the slats and install new replacement ones. The slats were resting on precast centre beams that ran down the middle of the tank. Once the slats were lifted, the beams became visible.

Immediately it was evident that these too were cracked with visible erosion present. It would not have made sense to fit new slats on beams that had become perished, with a limited remaining life.

Purpose-built beams will be prefabricated and installed to support the weight of the new slats.

The existing slats were six inches thick but it was decided that the new ones would be seven inches to provide greater strength.

New slats

It is vital when laying new slats that they are free from any cracks, honeycombing or chips on the top corner. They must have a full bearing of at least 150mm at points of support and the finished slat floor must be level and free from any rocking. It is worth consulting the Department specifications S123 and S123S if you are thinking of replacing slats.

They should be able to be replaced with minimum disturbance as they have a limited working life, much shorter than that of the shed. One issue that farmers have encountered is where cubicle beds have been positioned on slats and the difficulty that then arises when slats need replacing. Standard cubicle beds must not be built directly on to new replacement slats. Where it is necessary to construct cubicle over slats, precast cubicle beds must be used. The slats under the precast cubicle beds must also be heavy duty.

It is advised that machinery is not driven over any slats as some farmers have reported that these slats become weak due to the large weights that may be going over them. Where there is a possiblity of machinery travelling over slats, then heavy-duty slats must be installed. However, where possible, slats should not be located along the travelling routes of machinery.

The design load for heavy-duty slats is an axle load of 80kN (8.16t), or 40kN (4.08t) per wheel at each end. The actual weight of a tractor and trailer combination will depend upon the number of axles on the trailer and the load distribution. Where machinery has to pass over a tank on a regular basis, it is better if possible to use slabs, rather than heavy-duty slats, as they will have a longer working life and are at least the same strength.

When installing new slats, they should not be driven on as this may weaken them. Corbett Concrete will use a crane with an extendable arm that will safely stretch into the shed from outside.

If you do have to replace slats, ensure that the tank has been emptied before any work starts and ensure that nobody enters the tank. If it is necessary to enter the tank, it should be tested for the presence of any harmful gases.

Other signs

While nothing will beat actually viewing the underside of the slats there are still some tell-tale signs to look out for. These can include chunks of concrete out in fields after slurry has been spread. These chunks can fall off and get sucked up into the tanker and spread on the land.

Another indication that they are failing is where they are beginning to sag. This can be seen if a straight edge is laid down across the centre of the slats. Additionally, with more and more slatted sheds being fitted with rubber mats, it can be even more difficult to spot a faulty slat.

For that reason, ensure that slats are fully cleaned and lift the rubber mats if needs be to get a good look at them. Cleaning down between the gaps in the slats may also help to identify any corrosion or faults. It is also vital to prevent slurry filling up over the slats for a period of time as it leads to greater corrosion.

“We would always tell farmers to look at slats earlier in the year when cattle leave the shed,” said Michael Corbett from Corbett Concrete. “When farmers are replacing slats, it also gives them the opportunity to clean out the silt and gravel that has accumulated in the bottom of the tank.”

Manhole covers

The Department has stressed that slats and manhole covers are not lifetime items and need to be replaced before they fail. Slat or manhole cover failure can be an expensive way to discover that a replacement is required. To avoid broken legs, or worse still, animals or yourself ending up in the slurry tank, all slats and manhole covers should be carefully inspected for cracks and sagging, and replaced as necessary. Attempting to rescue animals from slurry tanks is extremely dangerous and has resulted in the loss of multiple human lives so never rush into a tank, placing your own life at risk.