Engineering resilience needn’t be rocket science. The challenge for designers is to be aware of the issues, and to think ahead, as Jenny Burridge and Guy Thompson from The Concrete Centre explain…
In fire, concrete performs well – both as an engineered structure, and as a material in its own right. Due to concrete’s inherent material properties, it can be used to minimise fire risk for the lowest initial cost while requiring the least in terms of ongoing maintenance. In most cases, concrete does not require any additional fire-protection because of its in-built resistance to fire. It is a non-combustible material (i.e. it does not burn), and has a slow rate of heat transfer.
In flood, concrete performs well – both providing flood protection, keeping water away from a building and flood resistance, keeping water out of a building. For houses effected by flooding – concrete and masonry construction provides good flood resilience as it does not absorb significant amounts of water and, depending upon the design, may not require any finishes, which would need to be stripped off following a flood. Nor will concrete and masonry rot or warp as a result of water damage.
Concrete does not burn
Concrete cannot be set on fire unlike other materials in a building and it does not emit any toxic fumes. Concrete is proven to have a high degree of fire resistance and, in the majority of applications, can be described as virtually fireproof. This excellent performance is due in the main to concrete’s constituent materials (cement and aggregates) which, when chemically combined, form a material that is essentially inert and, importantly for fire safety design, has relatively poor thermal conductivity. It is this slow rate of conductivity (heat transfer) that enables concrete to act as an effective fire shield not only between adjacent spaces, but also to protect itself from fire damage.
Concrete structures and fire engineering
Concrete structures perform well in fire. This is because of the combination of the inherent properties of the concrete itself, along with the appropriate design of the structural elements to give the required fire performance and the design of the overall structure to ensure robustness.
Fire performance is the ability of a particular structural element (as opposed to any particular building material) to fulfil its designed function for a period of time in the event of a fire. These criteria appear in UK and European fire safety codes. For any building or structure, regardless of its complexity, design for fire safety should address the following four principal objectives:
- Ensure stability of the load bearing construction elements over a specific period of time;
- Limit the generation and spread of fire and smoke;
- Assist the evacuation of occupants and ensure the safety of rescue teams;
- Facilitate the intervention of fire fighters and other rescue parties.
Good practice in design for fire safety incorporates these aspects and more, in what is termed ‘fire engineering’ for large, complex structures that warrant additional design effort. From a whole building standpoint, concrete can satisfy the 4 principal objectives of fire safety through its inherent fire resistance and the utilisation of its structural continuity in a fire engineered design.
Concrete proof
The impact of a major fire at Tytherington County High School, Cheshire, was limited due to the fire resistance of the concrete structure. Rather than taking a year to be demolished and replaced, as was the case with an adjacent lightweight structure, the concrete classrooms were repaired ready for the following term.
The insurance benefits of concrete construction have been underlined by reports that insurers could refuse cover for lightweight construction buildings due to growing concerns about their fire safety and the increasing frequency and cost of claims. Andrew Minson, executive director of The Concrete Centre said: “Every fire causes financial loss and in most cases insurers have to pay for the damage and repair. For this reason, insurance companies keep comprehensive databases on the performance of construction materials. In mainland Europe, this information often results in reduced insurance premiums for concrete buildings due to their proven fire protection and resistance.”
Minson pointed to France where insurance premiums for warehouses built from concrete can be reduced by up to 20%. He said: “The growing emphasis on risk avoidance means that the inherent fire resistance of concrete is being increasingly recognised, and it will be of no surprise if this is more widely recognised in lower insurance premiums in the UK.”
Concrete can offer up to 4 hours fire resistance, well beyond the periods often stipulated by the Building Regulations for life safety. It offers this high level of protection for buildings whether under construction or completed, with no need for fire-proof boards or finishes that might be compromised due to poor installation, alterations or refurbishment. “Concrete offers insurers and policy holders the potential for minimal damage, and therefore, smaller claims and lower premiums.”
Concrete provides the best fire resistance of any building material. It does not burn, it cannot be ‘set on fire’ like other materials in a building and it does not emit any toxic fumes, smoke or drip molten particles when exposed to fire. Concrete and its mineral constituents enjoy the highest fire resistance classification (class A1) under EN 13501-1.
Concrete and flooding
Flooding and flood risk has become increasingly common in the UK. As an island with a mild climate, the UK has always experienced high rainfall, coastal erosion, and fast flowing rivers, all of which can cause flooding.
The prime reasons to protect against flooding are to prevent human suffering, property and infrastructure damage, the spread of disease and financial distress caused by future large insurance excesses and high premiums. With the impact of climate change, flooding and the risk of flooding is expected to increase significantly. Flooding is not only a major threat to future land development and civil infrastructure, but also for building owners and their design and construction teams.
Concrete products can play an important role in helping to prevent and overcome flooding issues through a number of varied solutions.
Flood protection
Flood protection can either be permanent or temporary and is predominantly used to protect infrastructure. Concrete’s robustness and durability are demonstrated by structures in the marine environment and concrete is the material of choice for permanent sea defences.
However, concrete products are also available for temporary structures – for example:
Barriers including products for blocking air bricks, window openings, doorway openings etc;
Skirt-type flood protection systems that can be attached to the building, rising with the flood water and lifting a continuous membrane to exclude flood water;
Free-standing flood protection systems are available in a range of systems; some of which do require sufficient time and expertise to be erected. There are systems for any scale and interlocking precast concrete units can provide protection to key infrastructure such as power stations and other installations.
Flood resistance
Flood resistance is focussed on excluding flood water from buildings including the consideration of water management as well as good practice in the design of buildings. Water management can be achieved using Sustainable Drainage Systems (SUDS) – SUDS aim to mimic as closely as possible the natural drainage of a site in order to manage the inundation of water and reduce flooding and water pollution. A SUDS system uses concrete in many elements:
Source control – allowing surface water to enter drainage systems (including paved areas). Examples of source control techniques include permeable paving, green roofs, vegetative solutions such as swales etc. Concrete pervious and/or permeable paving also gives a level of basic treatment to reduce concentrations of pollutants.
Attenuation of flows – using devices such as storage tanks, swales and storage capacity underneath permeable paving to reduce the total storm flow and peak flows, and to provide treatment benefits.
Passive treatment – to treat water at the end of the pipe before discharge into a watercourse or aquifer. These methods can range from ponds that allow settlement, to wetlands or underground filtration tanks.
A flood resistance design strategy may allow flooding below ground floor level but would generally seek to stop flooding reaching above ground floor level where major damage generally occurs. Flood resistance for new build is best achieved in most cases by elevating the ground floor, ensuring that the most sensitive parts of a building are above the flood level to a standard of risk which is appropriate to meet at least 1 in 200 year return period. A recommended solution is suitably designed solid concrete floors with an effective damp proof membrane.
A flood resilience strategy to manage flood risk accepts the likelihood that property and structures might flood occasionally, using a flood return period that is appropriate for the location. This demands a building construction which is easy, straightforward and quick to clean up when damaged in a flood.
The main ways that buildings can be made more flood resilient are:
By designing them to dry out quicker and be easier to repair, thereby reducing the length of time the occupants have to stay out of their homes following a flood event.
Using materials resistant to water damage. Concrete and masonry construction does not absorb significant amounts of water and, depending upon the design, may not require any finishes, which would need to be stripped off following a flood. Nor will concrete and masonry rot or warp as a result of water damage. The proven durability of concrete structures means that they can also have an extremely long life. As a result, concrete can provide a cost-effective, high performance, long-term solution to flooding and fire.
Jenny Burridge
Head of Structural Engineering
Guy Thompson
Head of Architecture, Housing and Sustainability
The Concrete Centre