In this AHCI news publication, we give information on elements that are inspected to current guidelines and British Standards. May’s issue will cover materials including concrete and masonry in relation to structural design information and specifications.
Concrete, a hard and strong composite material simulating the properties of natural rock, is a common feature of the modern environment. In fact, concrete is the most commonly used man-made material on earth. Concrete has a multitude of applications when used in the construction of buildings. It can be shaped or moulded into virtually any design, prefabricated off-site or cast in-situ. It can be pumped to a considerable height, piled deeply into the earth or spread flat, even and level to tame adverse ground conditions. Such versatile characteristics make concrete ideal for a plethora of construction purposes.
Concrete has a relatively high compressive strength. This quality makes it a widely used material for building foundations. The ability of concrete to cure even when submerged in water negates problems associated with high ground-water tables.
Although concrete is considered to have a relatively low tensile strength, reinforced concrete is freely available and is in common use across the building sector. Steel reinforcement-bar (rebar) is incorporated within the concrete while the mix is wet to fortify the material, making it resistant to tensile stresses, cracking and/or structural failure.
Any specialist work must be supported by structural design information completed by a suitably qualified engineer. To maintain concrete structural stability it is important that the concrete is designed and engineered to withstand designed loads.
Figure 1. Picture above of the famous Pantheon dome an impressive roman build made out of concrete. The dome has stood the test of time. The building was built entirely without steel reinforcing rods to resist tensile cracking, so necessary in concrete members, and for this concrete dome with a long span to last centuries is incredible.
Temperature and freezing point
The minimum working temperature should not fall below 20C. It is therefore essential for any builder to pre-plan and anticipate any cold weather spells. Air temperature is crucially important when working with concrete. Failure to maintain the 20C parameter can cause ice to form within the concrete mix compromising the strength of the concrete when set. This process is known as freeze-thaw action and it occurs when water in moist concrete freezes and produces pressure in the pores of the concrete, reducing tensile strength as ice expands and contracts. The cumulative effect of successive freeze-thaw cycles and consequent disruption of aggregate can eventually cause expansion, cracking, scaling and crumbling of concrete.
It is important that the temperature of fresh concrete should not fall below 50C at the time of delivery. It must be ensured that ‘immature’ concrete is prevented from freezing before sufficient strength has been achieved. For further information refer to BS5800 and BSEN206-1.
Figure 2. A comparison of concrete brick before and after freeze- thaw action.
Expansion and/or movement joints must be provided as specified by the appointed design engineer or as per the requirements of the relevant British Standard and/or Euro code. All concrete, once placed, will contract slightly during the curing process which, in effect, can cause small surface cracks that appear during the curing process. When concrete sets it expands and/or contracts slightly with ambient temperature. It is therefore advisable to incorporate some form of movement joint. Concrete expansion joints are used to separate slabs and concrete from other parts of the structure. Expansion joints allow independent movement between adjoining structural members, minimising cracking when such movements are restrained. It allows for thermal expansion and contraction without inducing stress into the system.
For further information and/or terms of reference, please refer to the following British Standards:
- BS8110- Structural use of concrete
- BSEN 1992-1-1 Design of concrete structures
- BS 8500 Concrete- Commentary British standard to BSEM 206-1
- BSEN 206-1 Concrete, Specification, performance and conformity
Someone who builds with/dresses hard units such as brick, block or stone is referred to as a ‘mason’. Therefore masonry, is defined as the trade of work conducted by masons and what they construct. Masonry is often used to produce solid elements and structures for a building. Whether it be load-bearing for a fundamental part of the structure, or non-load bearing e.g. cladding/partition wall. Masonry is made by laying interlocking units onto one-another and is usually bound together by a form of mortar. Though dry-set masonry can be used where no mortar is necessary as the units rely upon the friction between each other preventing possible movement. In comparison to other materials, masonry is reasonably inexpensive due to it being very durable with no additional finishing/decorating required, which in-turn makes it easy maintain and repair. In addition, it provides good thermal and acoustic values whilst delivering excellent resistance to fire.
Fit for purpose
The masonry selected should be fit for purpose and provide appropriate durability for the job. The type of brick specified will affect the type of mortar to be used due to the compressive strength required. Generally, units with greater durability should be used where there is likely to be a potential for higher exposure to saturation or wind driven rain. Where developments are within 3 Km of the shoreline, structures and/or protective materials should be assessed for potential risk of corrosion and durability.
You will find that masonry units are poor at resisting tension/lateral loading forces, though it delivers robust compression. By incorporating reinforcement the strength of the masonry will increase. Likewise, if you were to increase the overall width of the units through buttresses/piers. There are common methods used to enhance the performance of masonry walls, including; a cavity, hollow sections within the masonry as well as a vapour barrier and external finishes.
Figure 3. Reinforced masonry construction
Strong foundations are needed when using masonry work due to the weight of the units combined. Protection for the units on any new wall is crucial for when temperatures drop below 2 degrees. Protection will help prevent frost damage, staining and disintegration of joints. Unless the structure is frost free, no plastering or screeding should be undertaken. Similarly, external rendering works should only be conducted where outside temperatures are greater than 2 degrees. Efflorescence is frequently caused by cold and damp weather conditions. It is important to take action when efflorescence may be an issue as severe cases can lead to masonry walls weakening, spalling or even crumbling.
Figure 4. Efflorescence in brickwork
For further information and/or terms of reference, please refer to:
- BS 5628 Parts 1, 2 and 3 ‘Code of Practice for use of Masonry’,
- BSEN 998 ‘Specification for mortar for Masonry’,
- BS 7543 ‘Guide to durability of buildings and building elements, products and components’.
Look out for next month’s publication which will focus on Site Investigation Reports and why we need them.
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