Substructure Quality

Quality refers to the standard of workmanship, materials and processes used during construction.

It is important to manage and maintain quality records when constructing the substructure as it is critical for supporting the building.

Also, the substructure is usually very difficult to access once complete and following construction activities progress.

An easy way to think about quality is to break it down into the following three categories:

Before Substructure works start

1. Understand drawings and specification - The key to managing quality on site is to have a firm understanding of what is planned to be built.

In particular, it's worth going through the structural engineers specification to see what concrete types have been specified, how it should be finished and what the construction tolerances are.

2. Quality Documents - Locate the project trade quality plan and inspection test plans. These will outline what quality controls are in place, how they will be managed and who is responsible.

3. Certificate of calibration for surveying equipment - equipment used to set out the works such as total stations require regular calibration (mostly annually).

It's worth checking the certificate of calibration is up to date as if this hasn't been carried out there is a chance the building will be constructed in the wrong place altogether.

During works

1. Reinforcement conformance check - within the specification there should be a clause on what certification is required for the reinforcement steel.

There are a few organisations which can provide these. One of the most commonly used within the UK is CARES.

A certificate for the facility producing the reinforcement for your project should be kept on file.

Note: there may be a requirement to check a certain percentage of reinforcement delivery tickets for delivery source.

2. Concrete quality check - there are a number of checks which can be carried out to ensure the concrete being used on site is as what's been called for in the specification. These include:

- Check delivery ticket: before concrete is offloaded, ask the driver for a copy of the delivery ticket. This should have the characteristics of the concrete including compressive strength and the concrete slump.

Concrete wagon delivery

- Concrete slump tests - the consistency of concrete or the 'slump' contributes greatly to how strong it will be once it cures.

The slump of concrete is tested by filling a cone with fresh concrete, poking it down with a rod, releasing it and measuring how much it's dropped.

There is an agreed method for conducting slump tests which can be found easily through an online search.

How far the concrete sinks puts it into a category from S1 - S5. The higher the slump, the more fluid the concrete is. As an example, a concrete with a slump of S3 falls between 100 to 150mm.

Concrete slump test being conducted

- Concrete cubes - when concrete is delivered and poured into a substructure element, a sample of it should also be poured into a cube mould which is usually 150mm x 100mm x 150mm.

This is stored in a water tank on site at a specified temperature and then load tested to check its compressive strength.

Typically for each batch of concrete, four cubes are taken which can be tested at 7 days, 14 days, 28 days and if strength isn't reached then 56 days.

Concrete cube being compression load testing in a laboratory

- Pre-pour check - outlined in the inspection and test plan, this is usually a hold point. Before any concrete is poured the substructure contractor should notify the main contractor to sign off a check list.

This is done as once concrete is poured the reinforcement is not visible, it is difficult to post-fix anything into the concrete if it should've been cast in and it's expensive and time consuming to break concrete out if its in the wrong position.

It includes checking if reinforcement has been installed as per drawing - our post on how to read a reinforcement drawing may be helpful if you've been tasked with signing a pre-pour checklist.

Other considerations are: concrete cover level, cleanliness of the formwork concrete is poured into and checking drawings to see if anything that should be cast into the concrete is in place.

Concrete being poured

- Monitor pour and vibration - when concrete arrives to site it's important to check conditions are as they should be.

For example, some operatives may add water to concrete in hot temperatures to make it easier to work with. This is not allowed as it alters the specification and can be very problematic.

Also, checking to see if concrete is correctly vibrated once it's poured so there's no air trapped inside.

Vibrating poker being using to release trapped air in concrete

After works complete

Quality cube records - a record should be kept within the site files showing the results from the concrete cube tests conducted throughout the project.

There are many ways this can be done but an excel spreadsheet is a good place to accumulate the data. There are templates available online for these.

It helps to have a large print out of the substructure plans on the wall in the site offices. As concrete is poured areas can be highlighted and dates assigned to each element.

This drawing can be folded and kept with the project quality files and used as a reference file for the concrete cube results.

Example as built record drawing

As built records - As part of the hand over process for the substructure from the substructure contractor to the main contractor, as built drawings should be provided.

These should show the actual position and level of all substructure elements installed. In some cases an independent surveying engineer is employed to double-check everything has been installed within tolerance.

Definitions

Tolerance - acceptable differences from the planned measurements

Trade Quality Plan (TQP) - document outlining the quality system to be used for the works. Including what standards are to be compiled with, the scope of works and roles and responsibilities

Inspection Test Plan (ITP) - could be included in the TQP. Document highlighting when, how and who's responsible for each inspection, test or survey required

Setting out - how the positions of the piles will be transferred from the drawings to the actual construction site

CARES - Certification Authority for Reinforcing Steels. Independent and impartial body who certify the quality of steel being produced

Compressive strength - the ability of a material to withstand compressive or crushing forces without collapsing or undergoing significant deformation. Measured in kN/m squared

Concrete slump - measures the consistency of freshly mixed concrete, indicating its workability and flow characteristics

Cover level - distance between the surface of embedded steel reinforcement and the outer concrete surface

Concrete formwork - a temporary structure used to shape and support poured concrete.

Posts to Follow

We plan to go into more details about piling and add sections on:

·       Programme

·       Commercial

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