Joints can be a real pain, from knackered knees to aching ankles, arthritic hips to leaking floor joints – they are all a pain. In this article we are going to look at how good joint design in a silage clamp floor can help avoid potential issues in the future – but it isn’t going to be any help to your skeletal joints as it’s too late to redesign those!

Why do concrete floor slabs need joints?

If you could lay all the slab in one pour, then you wouldn’t need any joints – or would you? It’s common practice for really large warehouse slabs to be laid as one, without any joints, using continuous pouring techniques. These slabs are placed as one but joints are then cut into the concrete once it’s cured. So what is the point of the joint?

It’s all to do with thermal movement. As we all know materials expand and contract with changes in temperature and concrete is no different. It doesn’t change as much, but concrete does expand and contract just like metals.

How much – well about one 10 millionth per degree Celsius. That sounds tiny – and it is – but if you have a concrete slab 30m long (100 feet) it will be 170mm (6 and half inches) shorter on a frosty morning than it was on the hottest day of the year. Whilst that doesn’t sound like a problem, and it wouldn’t be if it just shortened in overall length, the problem is that the concrete won’t be able to stand that sort of torsional stress as it’s trying to shrink so it would be cracked all over its length. And it’s not only shrinking in length but in width too.

Now it is possible to design a reinforced concrete slab to resist this movement but it’s not easy. Firstly there are a lot of forces to consider so you need a lot of reinforcement. It is possible to incorporate steel fibres into the concrete mix to do this, but there is a huge problem with this solution. As soon as the clamp is filled with silage, the effluent attacks and destroys the steel fibres closest to the surface. This makes the concrete surface porous and lets the effluent get further down into the matrix and to more of the steel fibres. Before too long you have a Swiss cheese floor slab. The only solution is an asphalt topping over the concrete slab to protect the fibres.

The thermal movement of the concrete is made worse by how we currently use concrete. We tend to build in warmer conditions of the summer, because concrete is ruined if it gets frosted whilst curing. The curing of the concrete, as it’s “going off” is a process that involves hydration of the cement and this reaction is exothermic - it creates heat. All these things mean the concrete was “created” in warm conditions and it is happy and unstressed in this state. Blow the “Beast from the East” across a floor slab at minus 20C and the concrete is trying to shrink and leads to cracking. So joints are there to allow the floor to expand and contract as the temperature changes.

How do I design concrete floor joints?

As with all good design, there are few golden rules you need to follow, and in the case of silage clamps, perhaps unsurprisingly, there are some regulations to make sure you construct the correct type of joints. We will come to this later but first you need to decide where to put the joints, and how many you need. We will skip the maths and just concentrate on the golden rules and it’s fairly simple. Think about the clamp floor in long strips, don’t make any strip wider than 6m (or 20 feet) but usually 4m to 5m is more common. You need a joint between each strip and these are movement joints. Within the length of each strip, you need joints at about 12m (40 feet) and these are construction joints. That means you end up with slabs of concrete no bigger than 6m x 12m.

So Rule 1 – Slabs no more than 6m wide with Movement joints on the edge

And Rule 2 – Construction joints every 12m within a slab

What’s the difference between construction joints and movement joints?

The main difference is what the joints are designed to do. Movement Joints are designed to let the slab contract and expand, where as Construction joints are only designed for the slabs to contract. The details in the joints are slightly different as you need to create a little bit of space in a sealed Movement joint where as the Construction joint design is only about generating a good seal. And that brings us on to the final rule

Rule 3 – provide 3 barriers to leakage at the joint.

Detailed drawings of suggested Construction and Movement joints can be found in the CIRA report C759B but these leave plenty of room for improvement in my opinion. So let’s start from the top down with the bit you can see when it’s finished. The top of the joint needs to contain a material that can flex and take up the movement between slabs whilst still plugging the gap. Some sort of mastic is the usual solution but the size and shape of the mastic bead is critical. You need to give the mastic a chance to do its job so if you are putting a joint at 5m intervals and expecting it to cope with a 15mm movement, it needs to be 30mm wide to start with and 45mm wide at full stretch. To do that it needs to be about 30mm deep as well, and to allow it to “neck out” in the middle, it can only be stuck on the sides of the joint so needs a bond breaker in the bottom of the joint.

There is quite a bit of maths to calculate the exact joint dimensions but don’t worry – either your engineer or we can do that along with the detail drawings for these sections.

So that’s the top of joint dealt with but that’s only one seal and we want three, where do the other two come from?

The floor construction should include a DPM or damp proof membrane under the slab. In most cases this is to protect the underside of the concrete from damp attack whilst making sure the wet concrete doesn’t lose too much moisture to the sub base as it cures. In a silage clamp floor, the DPM can be designed to act as a containment membrane as well. So that gets us two out of the three; in the CIRA report a waterstop joint is recommended for the third element – but waterstops have all sorts of issues.

In a nutshell, waterstop joints are expensive to buy, difficult to set out, must be “welded” at joints and make it very difficult to fix the concrete shutters – oh and they don’t really work. An advocate of waterstops (the salesman) might be able to argue with the first four points, but they can’t deny that they don’t really work. All they do is slow down any leakage, they don’t seal.

To do that you need a hydrophilic strip.

These are relatively cheap, dead easy to use and they really do give you a seal. All you need to remember is that they have to be buried low enough in the slab or they will “blow” the concrete off as they expand.

 So that’s the outline covered, now you need to look at material selection, but that’s another story all together. If you want to discuss the design and detail of silage clamp floor joints with an independent consultant or to discuss any of the other aspects of silage making covered in this series – contact Jeremy Nash @ jeremynash1@btinternet.com