It’s an unfortunate truth but silage and concrete are not really a compatible couple and don’t make happy bedfellows. It’s all because of the basic chemistry – silage is acidic and concrete is alkaline meaning that the two get on just like United and City fans after a evening in the pub.

So why do we continue to build silage stores in a material that is so completely unsuitable? Well that’s because there isn’t really much else to choose. Iron and steel are subject to rusting (or oxidation) as well as acid attack unless its well protected. The usual system to protect vulnerable steel is to cover it in zinc or galvanize it. Whilst galvanised steel columns will look good as new for decades in a grain store or cattle shed, unfortunately galvanizing breaks down at around pH 5.0 so silage juice at pH 4.0 or less will eat straight through it.

Meanwhile concrete is cheap, readily available, robust and flexible – in that we can make it into whatever shape we need it to be. If money was no object, there are some other materials we could consider like stainless steel or glass  - and if we are getting ridiculous, gold and diamonds can resist low pH too. Glass used to be employed as protection to steel sheets in cylindrical tower silos but in the UK these are now mostly part of farm building history. The dark blue and green towers might still be used elsewhere in the world but in the UK the clamp is king when it comes to precision chopped forage storage.

Concrete walls and floors to clamps

Chances are the walls of a new clamp will be concrete in some form as timber walls have too many limitations. Be the walls precast or site cast, free-standing or supported by columns, concrete has become the default choice. And then there are the floors; this topic has been explored elsewhere but again, it’s likely that concrete is going to be on your short list of materials for any silage clamp floor slab. So if your clamp is going to have a high proportion of concrete, what can you do to protect that investment?

Protecting concrete from silage acids

The best possible protection for concrete is to make sure it’s good concrete in the first place. The old sage will tell you the loads of concrete that “didn’t have enough cement in ‘em” because they are the areas of the yard that start to break up first. But whilst the cement content is critical to concrete life, there is far more to it than that, and the area of disintegrating concrete probably has exactly the same amount of cement as the other areas laid on the same day. Good concrete needs the correct ingredients but the one we all take for granted is perhaps the most important – the water. The amount of water in the concrete is super critical to its life and resistance to attacks. Those bits of the yard that are breaking up probably had too much water rather than too little cement!

 What is water doing when you mix concrete?

The water in concrete is there for two reasons; a small amount – around 250ml for every kilogramme of cement - is bound up in the chemical hydration process. The rest is just there for lubrication. As the concrete sets, or cures, it generates some heat and that drives the lubrication water out of the concrete as it “dries out”. As soon as the concrete starts to turn from a liquid into a solid, any water that leaves during the drying out will leave a void in the concrete matrix. The drying out process also takes a long time, some tests on Roman concrete suggest that it is still losing water 2000 years after it was mixed!

How to cure good concrete

The important thing is to make sure there is no more water than you really need in the mix and that it doesn’t dry out too quickly. As we know, there must be at least 0.25 : 1.0 water to cement ratio but concrete this dry is unworkable. Most concrete needs to be 0.45 : 1.0 or above to make it flow and placable. You can also consider using plasticizer admixtures to increase the concretes flow whilst keeping the water content down. However the jury is still out on admixtures that increase concrete durability in clamps, to date I have not seen anything that can withstand long term silage effluent attack.

You will need to compact the concrete (ideally with a vibrating poker) to get all the air out of it and make sure it’s dense. Once you have produced the surface finish of your choice you need to slow the water loss down as it cures to make sure all the cement gets hydrated before the water has gone. So in a building, close the doors. If the slab is outside then cover it with a sheet or spray on semi-permeable curing membrane. In really hot dry conditions, even consider spraying the surface with a water mist.

Chemical solutions to concrete protection

So you have a good concrete surface and now you need to protect it. Your options are huge as there are so many products that promise so much. These fall into one of two camps, those that form a surface coating and those that penetrate the concrete surface. You should really seek some specialist advice from a company (such as Quattro or New Guard Coating) that are experts in this field rather than choosing products off the shelf. I would suggest products with some form of penetration are a better option in a silage clamp. If a surface paint or membrane is damaged at all, then effluent will get behind it and attack the concrete underneath without you being aware. Penetrating concrete protections still give you projection even when the fork tines do hit the concrete.

In choosing a chemical protection you also need to consider how you can re-apply or touch up the protection in subsequent years because it will be necessary.

If you want to discuss protecting your silage clamp or to discuss any of the other aspects of silage making covered in this series – contact Jeremy Nash at jeremy@silageconsultant.co.uk

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