Tag Archives: precast construction

Tag Archives: precast construction

How precast concrete can overcome offsite construction risks

Adopting offsite exposes designers and clients to risk.  Precast concrete overcomes these concerns.

Precast concrete can be used for all building types and infrastructure projects.  It overcomes risks associated with other offsite construction methods and it is a local product with strong sustainability and performance credentials backed by many years of experience.  Typical risks of offsite construction are addressed below and an explanation is given of how precast concrete mitigates them.

Precast concrete wall

Product standards – precast concrete has long-established standards for products through BSI and many of these are harmonised across Europe through CEN.  Therefore, many precast products are CE marked as normal practice. This enables designers and project teams to specify their offsite products with greater confidence.  This also equips contractors with the information to receive offsite products with the right quality and performance parameters.

Design Codes – The Eurocode suite of design codes by CEN, with UK National Annexes by BSI, have now replaced the British Codes, which have been withdrawn and are no longer maintained.

Designers can use these Eurocodes with offsite precast products because the precast products standards and design codes have been designed simultaneously, with the purpose of being consistent with each other.

A responsible chain – British Precast members provide responsibly sourced products.  With the BRE BES 6001 scheme the British Precast Charters for sustainability and health and safety – and all members operating under UK employment laws – clients can fully address risk by choosing from the British Precast supply chain.

A robust chain – The membership of British Precast is extensive, with many members producing a wide range of products.  This provides project teams and contractors with a competitive supply chain.  This is beneficial at tender stage and reassuring so that the project is not reliant on one possible supplier.

Local supply – Precast concrete is a local product.  The majority of precast used in the UK is made in the UK, from materials sourced in the UK.  This reduces the risk to exchange rate fluctuation, transport problems, communication problems and difficulties in inspecting products prior to leaving the factory.

Durability/robustness during construction – Is there a better material to deliver to a building site than concrete when it comes to being durable and robust?  It does not require the same protection from the weather and impacts as lightweight solutions do.

Longevity during operation – The precast concrete properties of durability and robustness also deliver a low-maintenance, long-lasting solution that reduces risks during operation.

Fire resistance during construction –  The HSE view is clear – timber frame solutions pose higher risks.  The timber industry has introduced additional measures to be adopted by clients, designers and manufacturers that seek to address these risks.  However, the HSE advises that there is a duty of care to reduce risk through design.  This can most effectively be done by choosing concrete and avoiding unnecessary fire load.

Fire resistance during occupation – Government statistics state timber frame solutions have a higher risk of more extensive fires.  To address this, the chief fire officers are trying to register all new timber framed buildings but the scheme is voluntary.  Some local authorities are trying to introduce information plate for buildings that are timber framed.  These initiatives highlight the problem and loos to mitigate the risks, but they do not reduce the potential wider liabilities arising from CDM obligations.  Non- combustible solutions reduce these risks.

Overheating resilience – The Zero Carbon Hub may have to be would up in 2016, but its warnings to government of the risks of overheating should be headed.  Responsible developers and informed building owners are increasingly aware of the major risk of current and future overheating.  With the right design and good ventilation, the inherent thermal mass of precast concrete is ideally suited to absorb heat to reduce peak temperatures.  This overheating risk is recognised in the Building Regulations and the recent housing white paper.

Long lead-in – Offsite solutions have the disadvantage of requiring a longer lead in period.  During this lead-in period, onsite in situ concrete works can be carried out.  For example, foundations and the ground floor.  In situ concrete and precast concrete can be designed and constructed together seamlessly.  Same design codes, same material properties and same jointing principles.

Non-repetitive elements – With precast concrete offsite solutions, the one-offs can be cost effectively created with in situ concrete as it is wholly compatible.  For example, a foyer in a hotel may not be suitable for factory production but can be cast onsite while precast elements are installed elsewhere on the project.

Source

BPA Construction News Precast Supplement – March 2017

http://digitalissues.emap.com/CN/PRECAST2017/#

 

Tag Archives: precast construction

Design and Standards Steel Grades – Non Stainless and Stainless

Untreated non-stainless steel is not commonly used in precast construction due to the long-term danger of corrosion.   It may however be galvanised to give better performance.  It is often used for beams, lifting devices, and temporary fixings where the risk of corrosion is not critical.  In such instances, non-stainless is preferable since it is not only far cheaper but also stronger than stainless steel.

Steel grades:

‘Normal’ steel members were for many years covered by BS 4360:1990 “Specification for weldable structural steels” but this is now withdrawn and superseded by a host of other standards too numerous to list

Members such as beams and columns are generally available in two ‘standard’ grades of material.  These were traditionally known as grade 43 and grade 50.  Grade 43 is the basic steel grade commonly available.  Grade 50 is higher yield steel, but if specified then care should be taken that the material is actually available within the required delivery period.  There is also a grade 55, but this has a cost implication and is used less.

The current steel design standard BS5950-1:2000 uses a different terminology to specify steel grades.  Grade 43 is now known as S275, grade 50 is S355, and grade 55 is S460.  These are based on the design strength of the material. Continue reading