Choosing the right Concrete.
Concrete has been used for millennia. The Romans made it out of hydrated lime and volcanic ash and produced a recipe which was superior to anything else available. The Romans also invented hydrated concrete which contained volcanic ash and clay. Modern concrete mixes can be complicated. Some are even designed using computer software. The design has to take into account weather conditions and the required design strength. A mix is then arrived at using a cementatious material such as Portland Cement, chemical additives, water and aggregates. Whether you’re building placing concrete foundations, putting up a fencing post or laying a concrete driveway you will need to understand a few basic points to enable you to choose the right concrete mix for the work. There are a number of types of readily available types of concrete with different characteristics. Their suitability depends on the application. Customers should always ask for advice from the supplier because the right concrete mix can sometimes be dependent upon local conditions. And if you’re working from an engineered construction plan you should ask the structural engineer for his opinion on the concrete mix best suited to the project. Be warned however that choosing the wrong concrete mix for your project might ruin it completely.
Typical PSI Standards for Concrete
Concrete comes in varying compositions. The strength of concrete is usually measured by ‘compression testing’ which involves taking standard molded, standard cured cylinder samples,. The result is expressed in pounds per square inch (PSI). Most standard concrete strength ranges between 2500 and 5000 PSI. But special engineered design mixes for specialist applications can go well over 10,000. Lower concrete strengths are available for things like displacement block fill, filling in old utility lines and displacement. Here however we focus on common concrete strengths for use in common applications. The cheapest generally available concrete is 2500PSI and is suitable for uses like walkways, driveways and laying floor slabs on grade. Set on sub-grade (solidly compacted fill material), 2500PSI concrete is usually sufficient for such applications. Owing to warrant requirements however many professional builders however prefer to use a higher strength concrete specification and in some cases building rules specify a higher grade.
- 3000PSI concrete is the usual multipurpose concrete mix for use in general building projects. 3000PSI contains sufficient cement to give a good finish, is durable and can be used whilst still reasonably wet without loss of quality
- 3500 PSI strength concrete is used in applications where higher loading is anticipated and where surface spalling has to be avoided. Applications include laying slabs in environments where heavy loads may be stored or moved around on the surface, for placing grade beams, laying building footings, for applications involving surfaces where heavy traffic might pass, and for paving curbs
- 4000PSI grade concrete is ideal in applications like placing concrete footings and floor slabs where heavy loads are anticipated, and for use in pavements where heavy traffic is expected
- 5000PSI and concrete strengths even higher is usually reserved for exceptional construction enterprises, when extreme conditions, very low wear rates and high impact resistance has to be provided for
In the UK Concrete mixes & strength are often prescribed like this-
Grade C7 C10 & C15 7- 15 Newton strength concrete. Used for non structural elements like paths or patio bases.
C20 & C25 20- 25 Newton strength concrete. Used for strip foundations and ground bearing slabs.
C30 & C35 30- 35 Newton strength concrete. Usually reinforced with the use of steel rods. Used for Structural floor slabs & ground beams.
C40 & C45 40-45 Newton strength concrete. Usually reinforced as above- Used for ground bearing structures that require very high strength like bridges and free standing walls.
Special Concrete Mixes
Special concrete mixes are achieved using various types of aggregates and other ingredients. The variations introduce different qualities into the concrete, and they can be tailored to produce a concrete mix suited to any particular project.
- To keep down costs, Flexible concretes can be made for laying large paved areas, or to produce high structural strength without resorting to extensive reinforcing. These special concrete mixes are categorised according to their flexible strength e.g. 650, 750 and 850 ‘flex’
- ‘Entrained air’, or ‘cellular concrete’, mixes are made by bringing air in to the process during production. The resultant small air bubbles left in the finished concrete product permit more opportunity for expansion in the set concrete and helps avoid it cracking This characteristic is particularly useful in structures which will be exposed to significant variations in temperature
- ‘Exposed aggregate’ concrete mixes are achieved by introducing graded crushed limestone aggregate, smooth gravel or river rock into the concrete mixture in higher than normal proportions Water washing or sandblasting removes the finished grout surface before the concrete is ‘cured’ (green), leaving a regular layer of mixed aggregate left on view
- ‘Pump mixes’ of concrete are formulated to make possible placement using a high pressure hose and mechanical pump whilst minimizing the risk of the hose becoming blocked. This is achieved by making the concrete mix wetter than usual and using smaller aggregates than standard concrete mixes
- ‘High strength’ and ‘high performance’ concretes are available for specialist use. All high strength concrete is high performance but not all high performance concrete is high strength. High strength concrete is made by lowering the water/cement (WC) ratio. Higher quality aggregates are chosen, and sometimes silica fume is added to prevent calcium hydroxide crystals (which reduce the strength at the cement- aggregate bond) from forming in the cement matrix. But low WC ratios make the concrete difficult to work and so super-plasticizers are sometimes added. In some modern mixes the design criteria for high strength concrete is the ‘elastic modulus’, which measures the concrete’s capacity to be deformed elastically when force is applied to it. ‘Ultra High Performance’ concretes with special characteristics and benefitting from state of the art design are available for specialist applications
- ‘Pervious concrete’ which allows water to drain through it is available. It’s produced by omitting all or some of the fine aggregates from the concrete mix. What’s left is then bound with a relatively small amount of Portland cement. A significant volume of the final set concrete consists of voids which allows water to drain through it
- ‘Glass Concrete’ uses recycled glass in place of some of the aggregates and has, in addition to its aesthetic appeal, been shown to have better long term strength and thermal properties than other types of concrete
- ‘Rapid strength concrete’ and ‘polymer concretes’, (where polymers are used to bind the aggregates, are designed to produce high resistance results soon after being manufactured. The characteristic makes them suitable where the building process needs to be speeded up such as in the repair of road surfaces, where the road needs to be in use within hours
- ‘Geopolymer concrete’ provides for better thermal and chemical resistance, and in certain atmospheric and extreme conditions it exhibits better mechanical properties. Geopolymer cement is an alternative to Portland cement and is used as part of a geopolymer cement slurry, mixed with regular aggregates. Inorganic aluminosilicate polymer compounds uutilize 100% industrial waste like as slag and fly ash and can achieve as much as 80% reduction in associated carbon dioxide emissions compared with standard concrete mixes
- ‘Refractory concretes’ are available for use in ovens, and contain ingredients like ganister, fire clay, and calcium aluminate cement
- ‘Gypsum concrete’, a mix of Gypsum, Portland cement and sand, and is used in concrete and wood frame construction in the interests of fire rating. Gypsum concrete is used as a floor underlayment and has further beneficial effects associated with radiant heating, sound reduction and floor levelling.
There are a number of characteristics you will need to appreciate in order to ensure that concrete is placed and finished to best effect.
- Chemical admixtures involve adding chemical products to concrete with the effect of improving the performance and engineering the correct characteristics into the concrete. Addition of plasticizers make the concrete more fluid permitting it to be placed where reinforcing considerations has made it awkward to achieve a good fill, or where other difficulties require a wetter concrete mix which is easier to use. Water reducing mixes limit the amount of water necessary and improve the workability of the concrete without loss of strength. Shrinkage reducing concrete admixes are also available which reduce the necessity for undesirable construction and expansion joints
- Fly ash is introduced into some concrete products to augment the Portland Cement contained in some design mixes. Fly ash itself is an industrial byproduct and reduces the cost of the mix. Addition of fly ash into concrete also makes repair of surface imperfections and spalling easier and slows down the speed at which the concrete sets so more time is available to work the concrete during finishing
- ‘Slump’ is a measure of what the concrete’s ‘plastic’ condition will be when the concrete is placed. ‘Slump’ is measured using a metal conical mold filled with concrete. The concrete is rodded in stages to ensure its thorough consolidation in the mold. Subsequently the mold is removed by carefully lifting it off and the intervals between the concrete ‘slumps’ are calculated. If the distance is (say) 4 inches, the concrete would be described as ‘4 inch slump’. To achieve best performance, design concrete mixtures commonly state a ‘slump range’. However adding, (for example) excessive water to the concrete mix can although making it easier to pour out, and seriously weaken the strength of the concrete.
- Concrete doesn’t just come in the standard grey colour. A variety of integrally coloured mixes of concrete are readily available
To ensure that your concrete lives up to expectations ensure that you choose the right type. But it’s also advisable to focus on three other points
- When placing slabs on grade ensure that the sub-grade is of sufficient structural quality to support the building and the concrete. To achieve the loadbearing capacity required of the concrete it may be necessary to remove unstable material and replace it with structural fill
- Ensure that the reinforcing material is properly installed. If you intend to use a rebar it should be supported and tied in the correct position to enable it to provide the strength required
- Ensure that the concrete is correctly formed, by ensuring that the forms are rigidly constructed, plumb and level as required. Using sufficient bracing and strong materials is essential to placing concrete successfully
Check out or forthcoming KCS Advice article on Concrete finishes