Precast or cast in-situ concrete silage clamp walls?

The walls of a silage clamp - if you’re having walls - will probably be made of concrete. That wasn’t always the case because there was a time when the timber railway sleeper was the wall plank of choice for most silage clamps. Railway sleepers or may be prefab timber panels were the default if you were planning a silage clamp. There must be thousands of clamps that were built with these wooden walls, so why has it all changed?

Can you still build a silage clamp with railway sleepers?

The clamps of the past were just like the farms of the past, they were smaller in just about every sense. Walls were commonly 6 to 8 feet high, or 2.0m - 2.4m in new money, and the tractors that rolled the clamps were positively anorexic compared to todays plus sized supermodels. And the result of all these increases mean that the loads that the walls need to sustain are many many times greater that those of the past.

So does that mean you can’t build a clamp with sleepers anymore? Well not exactly, but it is a bit more complicated than it used to be. I guess that no one will be surprised to learn that it’s all because of the regulations…. The SSAFO regulation bible decrees that the walls of any silage clamp must be designed to retain the loads as described in British Standard 5502 part 22. And while some timber sleepers might be able to resist these loads, you have to prove it. So today someone with an engineering qualification and the requisite professional indemnity insurance has to provide calculations to prove that every sleeper you are going to use can meet these loads.

So it’s concrete walls for the clamp then?

Timber is a variable product and its performance depends on the tree species, the climatic conditions it grew in and the disease stresses it experienced in its lifetime amongst other things. Concrete isn’t so variable and engineers can simply do one set of calculations to prove it works. So it’s not surprising that all the clamps we build today seem to have concrete walls.

Concrete comes in all sorts of shapes and sizes, and in liquid and solid forms too and it’s this last bit I am going to explore here because I’m often asked if precast is better than cast on site or cast in-situ walls.

Is precast better than cast in-situ for silage walls?

Thats a more complicated question than it might at first appear to be because “better” needs to be qualified. I think there are a few ways to look at this, “cheaper”, “stronger”, “more suitable”, “more durable” are some of the ways I am going to look at it.

Lets start with stronger because that’s often quoted when I discuss this with builders. Cast in-situ (or cast on site) walls are usually much thicker than precast wall panels, and most people believe that makes them stronger. I am going to disagree because the strength, or capacity of a wall is dependant on quite a few factors and thickness is only one of them.  Strength really depends on the thickness, concrete grade, reinforcing type and reinforcing position. Thickness really helps because it enables the reinforcing to be positioned in a more advantageous position.

“What the hell does that mean” I hear you shout. Well in essence the load on the reinforcing depends how far it is away from the load (i.e the silage) and how far it is away from the middle of the wall and the outside edge. Having a thicker wall allows the reinforcing to be further away from the these loads so the “leverage” is greater resulting in more wall capacity.

Is stronger concrete better for silo walls?

Concretes are not all equal, some are greater than others. The design strength of any particular concrete mix is usually described by its C number. If you phone the local ready mix plant and ask for a C35 concrete, then the resultant concrete strength should be the same irrespective of which company supplies it or whereever in the country it comes from.

Stronger concrete has a higher C number, because the number is a way of describing the compressive strength of the concrete in N/mm. If you get a builder to cast an in-situ wall for you, they will probably use a C40-C50 mix. If you buy in some precast units they will probably be made in C60 concrete so does that make them stronger? It might do, but it depends on the reinforcing.

Whats the best sort of reinforcing for silage clamp walls?

There are three groups of reinforcing the are likely to be used in a silage wall.

• Steel mesh or fabric

• Bent or straight bar

• Prestressed steel strands or cable

A wall that is built by being cast on site will be reinforced with mesh or bar, or a combination of the two. Prestressed strand is only really used in precast factory conditions because the strand or wire is tensioned with 5-10t of force on each cable before the concrete is poured around it. Once the concrete has “gone off” or cured, then the tension is released, or more accurately, transferred into the concrete.

Why go to all that trouble of prestressing? Well it’s all to do with compression and tension forces and the fact that steel is very good at dealing with tension and rubbish at compression. A tensioned 9mm diameter strand has similar performance to an 80mm diameter steel bar, so prestressing is efficient.

What about durability, are precast walls durable?

There are lots of horror stories about silage wall collapses, but every one that I have investigated is usually due to either poor design, or some sort of overload rather than a failure due to lack of durability. In essence I guess precast units are usually made in a better quality concrete, but usually have less excess design capacity than in-situ walls.

Cast walls can be better suited though because sometimes precast panels are a bit too thin. This is usually the case for a dividing wall where a cast wall will fill the web of the support columns making the wall flush on each side. Dividing wall options are the subject for another in this series so I will cover this in more detail elsewhere.

What is the cheapest concrete wall for silage?

So we’ve done stronger, most durable and most suitable, but what about cheapest? This one will vary depending on your location and the current market conditions but overall, we can say that precast is generally cheaper than cast in-situ walls. This is because in precast there is less overall material (as they are thinner), less reinforcing (because they can be prestressed) and less waste.  There should also be less labour because the moulds are used for thousands of walls and designed to be stripped and re-set up quickly. There should also be less engineering because each in-situ wall should have its own set of calculations whereas the precast manufacturers just check your application against the load table for each panel type.

Is cast on site a more risky option?

There is one key area of difference however, and that comes down to risk. You should never see the precast manufacturers mistakes because they will never leave the factory. An error with concrete compaction on an in-situ silo wall will probably be there all all to see forever more.  There are also factors outside of your, or the builders control that make cast in-situ more risky - if the ready mix doesn’t turn up on time, or if the weather turns really cold, for example.

It is not that one wall type is “better” than another, it’s just that they are different. Precast might suit lots of applications but in-situ might be better for you. If you want to discuss what type of wall is best for your clamp or would like to discuss any other aspects covered in this series, contact me at jeremy@silageconsultant.co.uk