For any structural engineer who designs concrete, the fundamental basics are usually handled by applying simple formulas. Design codes vary around the world, but – for the most part – you can very quickly determine the bending strength or moment capacity of a concrete beam armed only with the section’s dimensions (i.e. D x B), the depth to the tensile steel (d), the area of that steel (Ast), and the concrete’s compressive strength (f’c). Similarly, applying a formula or two will allow you to roughly determine a slab’s effective stiffness and thus how much it will deflect under the design load. Fortunately for music lovers around the world, such parameters and formulae haven’t (yet) been worked in to any song lyrics.
But there are many aspects to concrete design that go beyond the formulas and beyond the maths. Such aspects are less empirical, but no less tangible. Because they’re less mathematical, they’re often left out of design guides or textbooks, or they simply get overlooked by young engineers still learning the ropes. It can take many years at the coal face and, critically, many years of observing how concrete actually behaves in real-world structures to appreciate the nuances of concrete design that extend beyond the maths. Here are four rock songs to sing to yourself next time you’re embarking on concrete design…
- It’s hip to be square (Huey Lewis and The News, 1986)
When engineers design concrete beams and slabs, the design process steers you down the path of satisfying strength and serviceability – well, serviceability in terms of deflection. Many engineers often overlook the serviceability checks that relate to drying shrinkage and crack control – particularly when it comes to pavement slab design.
Pavement slabs – whether footpaths, driveways, or industrial slabs for heavy vehicles – are typically anywhere between 75mm and 180mm thick, usually with a top layer of reinforcement, e.g. a single sheet of fabric mesh. In order to control drying shrinkage and mitigate the risk of unsightly cracking, the pavement is typically split up into smaller-sized panels using control joints. Notwithstanding the architectural constraints and the physical size of the overall pavement, engineers should design and configure their joint layouts so that the resulting panels are as square as possible.
If the geometry is such that rectangular-shaped panels can’t be avoided, the maximum aspect ratio of each panel (that is, the panel’s length-to-width ratio) shouldn’t exceed 1.33:1. (Many guidelines and handbooks often suggest a maximum aspect ratio of 1.5:1, but plenty of designers – this author included – have noted such dimensioning can still result in an unwanted drying shrinkage crack forming in the middle of the panel). If you want to adopt best practice and minimise the risk of shrinkage cracks appearing – keep your panels square.
For many decades, a common practice worldwide was to adopt panel sizes of 6m x 6m – or at least to set 6m as the maximum dimension of any slab panel. (A carry-over from the imperial days of joints at 20-foot centres). However, it was observed in many countries that such spacings often failed to adequately prevent cracking. This was finally addressed in Australia in 2016 with the new release of AS3727.1 that year, which now stipulates a maximum joint spacing or panel dimension of 4.5m. What this means is that no slab panel can be larger than 4.5m x 4.5m. In order not to fall foul of the aspect ratio limit of 1.33:1, another way to look at this is that if your joints are spaced at 4.5m centres in one direction, the shorter side of your panels can’t be any less than 3.4m!
- Rip this joint (The Rolling Stones, 1972)
Similar to the discussion above with our first song, concrete shrinks…and engineers need to account for this in their designs. Most buildings – and certainly domestic/residential architecture – aren’t conveniently square in shape, and so suspended concrete slabs are often irregular in their layout and configuration. L-shapes, acute angles, folds, rebates, block-outs, re-entrant corners around voids, stairs, service-cores and the like are all features that will trigger shrinkage cracks if the engineer does not account for the likely strains and movement. Similarly, the size of the concrete pour needs to be considered. Notwithstanding what can realistically be poured and finished in a single day’s work, a concrete pour can only be so large before shrinkage strains will start to pull the slab apart. This can have nasty implications for the slab’s watertightness, durability, strength, and – of course – the finished aesthetic.
Accordingly, engineers should employ pour breaks, construction joints, or other jointed solutions to manage those shrinkage strains. Before you set about specifying and detailing your slab reinforcement, “zoom out” and look at the big picture, taking into account where the slab is restrained and where it is unrestrained. Where will the slab “lock up”? Is it restrained in both directions, or free to move in one direction? Is the slab restrained by the supporting walls under (e.g. cogged starter bars coming out of reinforced core-filled blockwork?) or is it on brickwork and free to move over slip joints? If the slab’s function and/or architecture limit the solutions available to you, calculate the shrinkage strains and ensure your reinforcement design and bar layout provides adequate resistance.
If the slab is too large or the shapes are too irregular, add a construction joint and split the slab into two separate pour operations. Or leave a pour strip behind to be infilled later down the track. The builder won’t thank you for having to do two pours instead of one, but your client will thank you and appreciate getting the quality product they expected and paid for.
- Steel bars (Michael Bolton, 1991)
Reinforced concrete, of course, has steel reinforcing bars to provide strength and ductility – particularly in zones subject to tension. Most countries around the world have a standard grade or type of reinforcement that is applied for most situations. In Australia, our standard deformed, normal ductility bar is the “N bar” with a yield stress of 500MPa, and Australian engineers will typically reach for N12, N16, N20, N24 and larger bars as their default when designing and specifying their reinforcement.
However, if you looked over a typical set of reinforcement plans produced by most consultants, you’d be forgiven for thinking the N-bar was the only tool at our disposal. Many engineers forget that other bar types (and bar diameters) are available to them. We have lower grade steels (e.g. 250MPa); we have plain or non-deformed bars (e.g. R6, R10, R16, etc); and we have softer bars designed to be more easily bent by hand on site (e.g. S12). There are many instances where other bar types can and should be used, often enabling us to achieve savings in tonnage and costs.
For example, think about shear ties or stirrups for beams: It’s amazing how many engineers default to N12 stirrups as their smallest option – even for regions where the shear is low and the reinforcement can be relaxed. Can you not specify R10’s in these regions? As a 250MPa grade plain bar, it’s cheaper to produce and purchase, and the smaller diameter will also achieve savings in tonnage. You’ve saved your client money and you’ve also actually reduced the carbon footprint of the building!
Many Australian engineers also forget that N12 is no longer the smallest normal ductility bar on the market. The N10 bar was introduced to the Australian market back in 2001, and can be well utilised in situations such as trimmers, ties, stirrups, transverse bars, crack-control bars, and so on. N10 bars can also be useful in narrow profile sections where cover is tight and/or congestion is an issue.
- Have you ever seen the rain? (Creedence Clearwater Revival, 1971)
Water is a key ingredient of concrete, and a good mix design will ensure appropriate proportions of water are provided to achieve workability in the plastic stage, and then long-term strength in the hardened stage.
But once the concrete has hardened, water – particularly rainwater – becomes the enemy. Rainwater can carry chemicals (e.g. acid rain), dissolved airborne salts, and other contaminants that can have a deleterious effect on the concrete’s matrix and reinforcement. In fact, water – together with air and chlorides – is the golden recipe that can trigger corrosion of the embedded steel reinforcement, leading to concrete cancer.
So what’s the point here? Keep the water out! And how do you do that? Well, you can start by following the guidelines already discussed above – give careful consideration to your detailing to reduce the risk of drying shrinkage and movement cracks that can let the water in. Taking a leaf out of the Eurocode, the Australian design code for concrete, AS3600, now also has provision to design/check for flexural cracking by limiting the stresses in the rebars and calculating crack widths.
And as your back-up? Specify a waterproofing admixture. There are several on the market – many claiming to do the same thing, but they work differently and have different degrees of effectiveness. Long-established additives such as Caltite are hydrophobic and repel water from the surface, making them great for situations such as basement walls and lift pit bases. More recently-developed products utilise crystalline technology to form a barrier and prevent entering water from permeating further through the concrete matrix. Some products, such as Penetron, can even exhibit self-healing properties and repair hairline cracks through the matrix.
However, you’ll note we used the word “back-up” when describing concrete additives and admixtures. Such tools should be your last line of defence, not your first. No amount of concrete admixture will stop water entering a 2.0mm wide crack, so ensure you employ sound reasoning and clever detailing to reduce the risk of shrinkage and/or flexural cracking. (Consider that the two most popular crystalline waterproofing admixtures on the market only warrant their product for crack widths up to a maximum of 0.4 and 0.5mm respectively!) As such, where possible and practical, ensure an external waterproofing membrane is part of the project’s specification – particularly for any external roof slab or planter box slab that acts as the roof to a habitable interior space below.
– – – – – – – – –
So…get humming at the office and design away. And, if you enjoyed this correlation between engineering and music, you might also like our other article, What Engineers Can Learn From The Beatles.
Cheers,
AD
PS….got any other songs or tips you’d like to add? Put them in the Comments section below for others to benefit from!