If you’re a structural engineer and you’ve just been asked to inspect a suspended concrete slab before it is poured, now is definitely a good time to go through the things you should be looking for when you carry out your inspection.
Here are my 5 most important things to check when inspecting a suspended concrete slab:
1. Reinforcement at Maximum Moment
At the most basic level, you are checking that the reinforcement is placed in the suspended slab in accordance with the design engineer’s plans.
That is the minimum standard and you should be aiming for better than that. Things change on site and just maybe the communication between the engineer and the builder hasn’t been perfect. That’s no excuse. You can’t just check for compliance with the drawings. You need to be doing more.
Always be considering the shape of the bending moment diagram when inspecting a suspended slab. Is the maximum amount of suspended slab reinforcement located to correspond with the highest bending moments?
If you don’t know what a bending moment diagram is and/or you don’t know where the maximum moments are in a span you have no reason to be inspecting a suspended slab. It is just too critical.
I’m not saying you MUST be a registered engineer to inspect a suspended slab, but before you attempt to check slab reinforcement on site you should know where the most amount of reinforcement should be. You are the builder and engineer’s back up plan and last chance to pick up any errors.
If you see something wrong or at the very least unusual – make the call. Ask the design engineer and get confirmation that the engineer and builder are both working off the latest set of plans.
2. Supports are Where the Drawings Say They Are
When you’re walking around on top of a suspended slab deck, make sure you check the slab supports are where the design engineer thinks they are.
It’s all too common for misunderstandings about half-height walls to make it through to construction and a builder to blindly build a slab with fewer supports than anticipated by the engineer.
Check the loadbearing walls are built to the underside of the suspended slab. Make sure the slab and walls are engaged or isolated in accordance with the design engineer’s drawings. Check the reinforcement coming out of columns is handled as per the drawings.
Simple things that really need to be done before a suspended slab is poured.
3, What Concrete Strength has the Builder Ordered?
The drawings will be fairly explicit about what concrete strength the design engineer needs the suspended slab concrete to be.
My number one preferred way of ensuring the right concrete is going to be poured is to ask the builder, “What grade concrete are you ordering?”
If the builder can’t answer my question off the top of his/her head then I start to worry. Go to the drawings, point out the concrete grade to the builder and make sure they understand.
The concrete grade is the basis of so many aspects of the concrete pour. Surface durability, porosity, beam and slab strength, shear strength, serviceability, and creep are all affected by concrete grade.
It is so important that the right grade of concrete is used I’m surprised I didn’t put this item first.
4. Which Reinforcement Layers Were Placed First and Last?
Design engineers know that the closer the steel reinforcement is to the tension face the more effective it is. When a design engineer specifies the order that slab reinforcement layers are placed it is your job to ensure the builder has placed the steel in the right order.
A different amount of cover affects the forces in the steel reinforcement. This is item number four because if the layers are done wrong extra steel can be added before the pour to fix any mistakes.
Check the drawings. Ensure you know which layers of steel reinforcement were meant to be placed first and last. Then make sure the builder got it right.
5. Slab Reinforcement Cover
Number five of things to check when checking a suspended concrete slab before a concrete pour is to ensure that the correct cover is achieved to the slab reinforcement.
This is the last item in importance because it can pretty easily be rectified on site. However, it is super-critical that you check that suspended slab reinforcement is positioned correctly in the slab.
Too close to the surface and the steel could corrode and fail the slab prematurely.
Too far from the concrete surface and the steel reinforcement may be overloaded or the slab could crack befoe the steel takes up the tensile forces in the concrete.
Don’t be afraid to pull the tape measure out and check the top and bottom cover. Ask where the top of the concrete slab is if you don’t know. Get down low and look for slab mesh that is sticking up higher than the top of slab.
Don’t forget to check the cover to the edge forms and base formwork too. Slab reinforcement hanging out of a slab is a bad look and you are singularly responsible for getting it right.
Get it right.
5 Things to Check When Checking a Suspended Slab
These 5 things are critical aspects of checking a suspended concrete slab.
If these elements are beyond you then don’t be afraid to put your hand up and say you’re not experienced in checking suspended concrete slabs. It is far better to get an experienced structural engineer out to check a suspended slab before it is poured than to be out of your depth and pass a slab that is not ready to be poured.