Triassic

Formwork Construction

Recommended Posts

I'm slowly moving forward with my foundations.   Because of the sloping site we went with an L shaped configuration, with a lounge and bedroom above in one wing and a walk in basement with kitchen above in the other wing. Having sent the drawings and a scope of work to six companies and got three quotes, the cheapest is £50,000 and the most expensive £145,000.  However, the cheapest quote doesn't cover everything listed in the scope of work, leaving me thinking I'd be better following Ian's lead @recoveringacademic, and using local trades to do the individual element. A few phone calls today and I've found a local ground worker to do the excavations and the drains, ive also got a local company to do the concrete pour into the insulated slab, I'll be installing that! 

 

Now to the question! I need to do the formwork for the basement walls. Is this something a local joiner could do, or is it a specialist job? I was also wondering if I could DIY it as I have loads of timber left over from the demolition of the old house.

Share this post


Link to post
Share on other sites

Why not consider using an ICF block as the basement as that is DIY and ties in well with your floor ..?? 

  • Like 2

Share this post


Link to post
Share on other sites

I think the word joiner is a bit incorrect if your doing some hunting around, you want a shuttering chippy, your joiner bloke will be more interested in fitting skirting and stuff. 

Share this post


Link to post
Share on other sites

ICF would be a doddle if you have strip founds in place - built it up and pour it in. The most important thing is how you water proof it tho - regardless of which option to shutter you use. I've been learning about different products recently and why damp proof membrane is not suitable as the water can track under it as opposed to others that bond with the concrete and properly waterproof it. Worth bearing in mind?

Share this post


Link to post
Share on other sites
49 minutes ago, jamiehamy said:

ICF would be a doddle if you have strip founds in place - built it up and pour it in. The most important thing is how you water proof it tho - regardless of which option to shutter you use. I've been learning about different products recently and why damp proof membrane is not suitable as the water can track under it as opposed to others that bond with the concrete and properly waterproof it. Worth bearing in mind?

I've always worried about ICF bursting, is this an issue in practice.

 

What is the cost difference between ICF and traditional shuttering?

 

The current spec calls for a waterproofing membrane to be applied to the concrete. How would you go about waterproofing ICF?

 

 

Edited by Triassic

Share this post


Link to post
Share on other sites

We used icf for the basement. Ground workers had never used it before so were learning as they went, with support from the icf manufacturer.

 

Supporting the blocks is key, they need to be braced prior to pour. There is a risk of bursting despite the blocks having plastic bracing inside.

 

No membranes, waterproof concrete called kryton. Also used their products at junctions e.g. slab to wall.

Share this post


Link to post
Share on other sites

How high is your retaining wall? Is it simple ie a U shape with 90' corners? I am a big fan of ICF and it's certainly doable on a DIY basis. Tanking is usually applied to the outside of ICF or a water proof concrete is used, we are planning to use both with the external tanking linked to the dpm from under the passive slab. Solid steel bracing can be hired from the ICF supplier which can expensive also these needs fixings to the slab but our slab already has UFH pipe so a wee bit of imagination is needed here. 

Although we have a digger we took on a local guy with a tracked digger, sit on roller and 3t dumper for the ground work on a day rate this worked out very . 

 

Share this post


Link to post
Share on other sites

Have a look at this thread, and particularly @Bitpipe's stuff about waterproofing: 

 

 

Share this post


Link to post
Share on other sites

There are quite a few admix products, we used Sika, Kryten (mentioned above) is another. What's key is that your concrete pour team are trained in the system and you get the work inspected at each stage by the manufacturer to get the warrantied sign-off. 

 

Our groundwork contractor used two guys who did all the steel work, shuttering and pouring, plus a few labourers. It's quite skilled, especially the formwork construction, which is very heavy duty stuff - panels needed to be moved with their machine. There is a 6m linear pour limit for vertical sections of waterproof concrete, so once the slab and kicker were poured & cured (in one shot), they built two formwork sections (one a corner and one a flat wall), that they rotated around our build to make best use of time - each section took 24 hours before being struck and then 3-4 days to go off before a neighbouring section could be poured. 

 

When they did the 100mm slab kicker, they left a groove in the top (using shuttering) to house the water bar for the wall pours. Ditto, a groove was left at the end of each vertical section for the same reason. Before the bar was applied, each groove was cleaned out, scarafied and dried with a blow torch. They then used an approved Sika mastic to hold the bar in place. All penetrations (150mm sleeves for 110mm drains to follow) were double wrapped in water bar before the pour. Everything was photographed and emailed to Sika rep for approval and on a few days he came in person.

 

BC wanted to inspect the steel before pours also, checking that it matched the engineers spec - the concrete guys called out a few omissions and mistakes in the SEs bar schedule and added extra reinforcement around door and window corners to prevent cracking.

Share this post


Link to post
Share on other sites
15 hours ago, Triassic said:

I've always worried about ICF bursting, is this an issue in practice.

 

What is the cost difference between ICF and traditional shuttering?

 

The current spec calls for a waterproofing membrane to be applied to the concrete. How would you go about waterproofing ICF?

 

 

Cost difference - first thing to bear in mind is that it's not ICF vs shuttering - if this is a basement, then the ICF will also provide your insulation which needs to be factored in. Labour wise, with traditional shuttering you build it up, pour and remove, and depending on size, you may need materials for this i.e timber and ply. ICF tends to work our around £50m2, depending on insulation levels. Polarwall will provide bracing for a reasonable fee - and support with build and pour. 

 

Waterproofing - waterproofing ICF is only one part of the big equation you will have for waterproofing the basement - @Bitpipehas provided a really good overview. My mistake with the garage is that I didn't take a holistic view of this i.e i should have consulted a specialist to provide an overall solution. What I'm planning will work, but I would have preferred a more integrated approach and different materials for this. I'm still pondering whether to use waterproof concrete or go with an externally applied product over the ICF (there are a number on the market but do not go with a bitumen based product)- this will lap down and tie in with the slab DPM, however as mentioned above, a traditional DPM under the ground bearing slab was probably not the best option here. 

 

 

Share this post


Link to post
Share on other sites

In my limited experience, quite a few basement specialist contractors will advise an internal membrane draining to a sump & pump arrangement. This can work fine but if the pump fails or there is a power cut.....

 

Starting question for @Triassic is what are your ground conditions like? Your SE should have interpreted these (bearing strength, water table location etc, and designed accordingly - down to bar bending schedule etc. If you have this level of detail then I'm surprised that you're getting such massive variations in quote - ours were within 10% of each other once normalised.

 

if you're essentially above the water table, then WPC alone should suffice. We have a land drain to soakaway at the foot of the slab and backfilled the 1m working space with large clean stone (fist sized), so any surface water quickly drains away.

 

 

Share this post


Link to post
Share on other sites

We're on free draining lime stone, the geological conditions are known, the scope of work was clear, the SE, architect drawings and bar schedule were all supplied,  everyone quoting visited the site, yet we have a huge variation in costs £40k to £145k. Talking to a neighbour, a surveyor, he thinks some of the inflated costings are down to the 'self Build premium'.

 

I'm thinking along @Bitpipe's line, waterproof concrete, as we're on limestone and the basement is open at the front as it's cut Into a hillside.

 

 

Edited by Triassic

Share this post


Link to post
Share on other sites
13 hours ago, Triassic said:

We're on free draining lime stone, the geological conditions are known, the scope of work was clear, the SE, architect drawings and bar schedule were all supplied,  everyone quoting visited the site, yet we have a huge variation in costs £40k to £145k. Talking to a neighbour, a surveyor, he thinks some of the inflated costings are down to the 'self Build premium'.

 

Are the quotes itemised?

 

Both of our contenders for the job gave rates and quantities for excavation, muck away, concrete & steel (these were SE specced) plus formwork rates, crane and pump hire etc, backfill material, drainage materials etc - made it easy to compare like for like and see where the money was going and question some of their assumptions & calcs.

 

The prices came to within £10k of each other on a £200k job, we went with the firm that felt more organised - they usually build underground carparks and the like so our job was small beer.

 

I do think that when you combine 'basement' with 'self build' the £££ signs start to mount up in some contractors eyes.

 

Ground workers are almost always local but the concrete team will travel to jobs (as they are essentially labour) +- tricky part if you split between contractors is deciding who is responsible for supplying the formwork, plant and getting the concrete on site. Depending where you are, I can PM you our concrete guy's details.

Share this post


Link to post
Share on other sites

Looking at the quotes, the highest has not followed the scope of work supplied and has lump a number of elements together without giving any real detail. The one at £40k has been done by a QS and closely matches the price my QS suggested for the job, albeit, the lowest quoter missed a number of items and double counted others, having said that he's the closest to the QS price.

 

I have a groundworker coming this week to look at the digging out of the basement, I'm told his work is good and his prices are keen, we will see.

Share this post


Link to post
Share on other sites

I have been amazed at the complexity of the shuttering process.  I'd be sure this task is placed in the hands of properly experienced contractors.  A lot can go wrong...with massive consequences.

  • Like 1

Share this post


Link to post
Share on other sites
On 13/08/2017 at 22:08, mvincentd said:

I have been amazed at the complexity of the shuttering process.  I'd be sure this task is placed in the hands of properly experienced contractors.  A lot can go wrong...with massive consequences.

Having appointed the ground worker to dig out the basement my next task is to find a suitable formwork contractor. If anyone in the NW of England has any recommendations, please pm me.  @iSelfBuild I recall your work in this particular sector, so again all help greatfully received! 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now