Frequently Asked
Questions

Cement, when combined with water to hydrate it, is the binding agent between coarse and fine aggregates that creates concrete.

The basic categories of concrete are defined between their aggregate size, compressive strength (mpa) and slump. Sometimes expressed as 32 20 80 for a mix of 32mpa concrete with 20mm aggregate and a slump of 80mm

On top of the basics we can add colours and fancy aggregate for decorative purposes or other chemicals to speed up / slow down setting time or increase the slump without the addition of water for example.

MPa, or megapascal, is a unit pressure used to describe the compressive strength of concrete. The higher the mpa the more cement per m3. For example 20mpa gives a compressive load of 200kg per cm2.

Where is this applicable:
20mpa is a foot path

25mpa is residential grade or vehicles <3t

32mpa is generally commercial, structural and vehicles over 3t

I have placed concrete up to 65 mpa in columns and most grades in-between. High strength mixes deserve a lot of respect with how they are managed as the have a very short workability window.

As a general rule of thumb I prefer 32mpa its finishes far nicer (finish will last longer also) than 25mpa due to the higher cement content and generally you can have a 30 odd percent increase in strength and durability without a 30% increase in costs.

To calculate slump, wet concrete is pulled out of a cone and measured against it in millimetres. Standard slump is 80mm and most guys are going to pour between 80-120 slump depending on the application – some may like it a lot wetter however over slumping the concrete or ‘pissing it up’ reduces the concrete strength in terms of the mpa and increases the likelihood of shrinkage cracks ( the added water will leave at some point ) and a dusty or chalky finish. Slump can be increased with chemicals but they can also have their draw backs however.

It depends. As mentioned above, even low strength concrete can take an impressive load in prefect conditions. There are rarely perfect conditions, perfect concrete or perfect concreters. If we are just discussing paving and alfresco etc, and not an engineered member like your house slab or footings, we don’t really need the reinforcing in the same way, as mentioned concrete performs well in compression however in tension no so well, tension is created in paving slabs due to irregular shapes and shrinkage that is part of the curing process as the slab loses water. Therefore we reinforce slabs and paving to hold the cracks together and not because of the load we are putting on them like to footings and slab holding up your walls and roof.

Cover on reinforcing is the amount of concrete between it and the environment and can be specified somewhere between 30mm to 60mm depending on the application an internal floor might have a top cover of 30mm but an external wall on the coast might have a 60mm cover requirement. The rule of thumb is 50mm. Poor or incorrect concrete cover can result in early failure of the concrete and the reinforcement may rust and expand popping off sections of concrete commonly called spalling.

Ground preparation is definitely important in concrete, for several reasons one is to have accurate estimates of concrete volume therefore costs, uniform concrete thickness should generally yield uniform finishing and curing results, but specifically compaction reduces settling of the structure ( those pesky cracked cornices on the new builds ) and allows the load to be transferred uniformly keeping the slab in compression not tension when under load. Basic paving prep would involve removal of any organic waste grading to desired level adding water as required and compacting, testing can be performed with a Perth sand penetrometer.

Black plastic, builders film, polly, polythene, vapour barrier. Simple answer Australia Standards would say anything inside yes external no. Essentially as the vapour barrier suggests it is water proofing the underside of your slab.

I personally like it in most applications, it holds the moisture in the concrete, stops sand being pushed into the slab, and gives the bar chairs a little more sit on.

Anything with reinforcing wants vibrating on some level however it isn’t effective on a 100mm slab and definitely not suitable for exposed aggregate, edges would be vibrated in this case to seal the edge for cover, all footings, suspended slabs and vertical elements would require some for of vibration to compact the concrete around the reinforcement.

Most concrete cracks, as stated above generally something to do with tension, shrinkage, loading or mix design / slump. However we do try to control it with reinforcement, curing, finish type but also jointing. The main joints you will see are control joints that are either tooled in with a groover while the concrete is wet or saw cut that afternoon or the following morning as the name suggest the control joint ideally is used to weaken the concrete to force the crack in the joint, keeping everything uniform.

Isolation joints is the typical foam joint between the house wall and the slab or a post and the slab, we want the elements separated because they move differently.

A construction joint could be seen as where we stop the slab because its a different finish or need to break down the slab into more manageable areas  there are a could of ways to manage this joint, we can isolate it with foam, carry the mesh through or use a dowel method that creates a true expansion joint allowing the slabs to more independently in one direction.

Screeding and finishing super flat floors is an art and an expensive exercise done in big warehouses at 3am with laser screeds and ride on trowel machines, we aim to get floors within a commonly accepted standard of 5mm variation over a 3m for internal flat work.

Falls for external covered areas have a minimum grade of 1mm every 200mm or 0.5% External uncovered areas 1mm every 100mm or 1%. For reference plumbers fall is 1mm every 60mm or 1.65%.

These are the minimums. Type of finish, expected water volume, drainage type and pad shape size will influence the grade.

Essentially there are two types of finish we do, smooth and non slip.

Smooth can be a hand trowel finish, machine stick troweled finish, a machine burnt polish or a ground polish.

No slip can be a floated / panned hand finish, broom finish, exposed aggregate or a machined non skid.

Where practical a machined finish is preferred as it compacts and hardens the surface and reduces the likelihood of cracking. The finish types can be applied to most concrete mixes however some finishes are best performed with a specific mix.

Curing concrete is essentially trying to trap water in for the initial chemical process a minimum of 7 days however the curing process is commonly know to take 28 days when concrete should reach its full strength. By keeping the concrete wet or trapping the moisture in it reduces shrinkage therefore cracking and increases the strength of the concrete, we will commonly spray  a curing compound once the concrete is finished the compound will seal the moisture in other methods are ponding the slab ( essentially keeping it wet ) or covering it with plastic and or wet hession.

Yes, sealing concrete is highly recommended as the final step. Sealing helps protect the surface from, stains, and other damage. It blocks the pores in the concrete, preventing the absorption of water and mineral salts, and enhances the appearance with a rich, glossy finish. The best time to seal your concrete is 30 days after pouring, once it has fully cured and reached its intended strength.

As an industry standard, we recommend waiting 7 days before driving or parking on newly poured concrete. During this time, the concrete needs to cure and gain strength. After 24 hours, it will be set enough to walk on, and after 48 hours, it can handle light foot traffic. However, keep bicycles, skateboards, and prams off it for a bit longer. After 30 days, the concrete will be fully cured and strong enough for heavier vehicles.

With 20 years of experience, Concrete Contracting has built a proud reputation for quality, reliability, and affordability. We can help you choose the best option based on your style, budget, and needs. You can trust us to deliver lasting results, and we offer a variety of concreting options to fit any project. We’re always happy to customise our services to ensure you get the exact look and function you want.

Proudly Aboriginal owned and operated, we also support the local economy by sourcing materials from Australian suppliers. Even more, as we grow, we are committed to creating training and employment opportunities for Indigenous people.