Answers to some of the most common concrete questions

How much concrete do I need?

Place the dimensions of the unit in the concrete calculator and it will work out the volume you need.

Use the Stevenson Concrete Calculator

What types of finishes are available?

How much does concrete weigh?

Typically concrete weighs about 2350 kilograms per cubic metre. Special mixes may have different ratios of materials or additives which affect the weight.

How much does a truck of concrete weigh?

If a truck is carrying 5 cubic metres, then the weight of the concrete is approximately 2.35 tonnes/m³ (5m³ X 2.35 t = 11.75 t) and the truck weighs 10 tonnes giving a total of about 22 tonnes.

Do I need to remove topsoil before placing concrete?

Yes.  Topsoil and any other organic matter must be removed before placing concrete.  Organic materials are unstable and unable to provide uniform support for concrete slabs.  A properly prepared sub grade contains no organic material and is crucial to constructing a quality concrete pavement or slab on grade

How should I prepare my sub-grade?

The sub-grade should be well-compacted aggregate, uniform in depth and well draining. Ensure that you dampen this base before placing and finishing concrete.

Remove all topsoil and soft soil or clay before placing base course.

How thick should a concrete driveway be?

A minimum of 100 mm is recommended. Thickness is a major factor in determining the driveway’s structural capacity. Increasing your driveway’s thickness from 100mm to 125 mm will add to your concrete cost, but the additional 25mm of concrete will add almost 50% to the load carrying capacity of your driveway.

What is reinforcing and why should I use it in my slab?

Concrete is strong in compression (squashing) and weak in tension (stretching). The introduction of steel reinforcement in concrete improves the concrete tensile strength. In a slab-on-ground, reinforcement increases the tensile strength and helps control the width of shrinkage cracks.

How much time do I have to work with the concrete?

There is no typical or specific time frame for working with in-place concrete due to the many variables involved including, but not limited to, mix design, ambient temperature, wind velocity, and placement location (indoors/outside).  We recommend using a member of the NZ Master Placers Association.

What is finishing?

Finishing is the process used to create the surface texture of the completed concrete pour.  Finishing involves several different steps.  Depending on the type of surface finish desired, it could involve striking off, power floating, edging, jointing, trowelling, texturing, and curing. Refer to NZS 3114 Specification for concrete surface finishes.

What does it mean to cure concrete?

Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete’s temperature stays within a suitable range. During the curing period-from five to seven days after placement for conventional concrete-the concrete surface needs to be kept moist to permit the hydration process. New concrete can be maintained in a wet state with soaking hoses, sprinklers, covered with wet hessian, a curing blanket or coated with commercially available curing compounds which seal in moisture.

How far apart should I place my contraction joints?

ACI 224 3R recommend that you place your contraction joints at 24 to 36 times the depth of your slab i.e. 100mmm slab would have joints at 2.4 m spacings to 3.6 meter spacings. Additional planning needs to be considered when planning joints in slabs of odd dimensions, with penetrations, thickenings and re-entrant corners.

All saw cut joints must be a minimum of ¼ the depth of the slab (25mm on a 100mm slab). Also, it is recommended that you attempt to design panels that are as square as possible. (Avoid exceeding length to width ratios of 1.5:1)

When can I walk on my concrete?

BRANZ recommends leaving the concrete to cure for a minimum of two days before allowing foot traffic.

When can I drive on my concrete?

BRANZ recommends leaving the concrete to cure for a minimum of seven days before allowing pneumatic-tyred vehicles onto the slab.

What does 28 day strength mean?

Concrete hardens and gains strength as it hydrates. The hydration process continues over a long period of time. It happens rapidly at first and slows down over time. To measure the ultimate strength of concrete would require a wait of several years. This would be impractical, so a time period of 28 days was selected by specification writing authorities as the age at which all concrete should be tested. At this age, a substantial percentage of the hydration has taken place.

How does hot weather affect concrete placing and finishing?

Higher temperatures affect concrete in several ways, including:

  • Increased water demand which may lead to higher water/cement ratios and lower strengths;
  • Accelerated slump loss resulting in reduced workability.;
  • Faster set time;
  • Increase potential for plastic shrinkage cracking (see Plastic Shrinkage Cracking) and other types of cracking.

What should I do when placing concrete in hot weather?

Hot weather placement of concrete requires the finisher to take certain precautions to minimize the impact of the higher temperatures. Precautions may include:

  • Use a set retarder, take care of possible plastic shrinkage cracking when using this method
  • Avoid pouring concrete during the hottest times of the day
  • Ensure the base is thoroughly wetted
  • Cure the concrete
  • Make sure you have enough manpower and equipment to place the concrete without any delays
  • Take care to schedule the trucks properly and make sure the job site is accessible so trucks can unload and leave faster
  • Take precautions such as setting up windbreaks or sunshades
  • Use an evaporation retardant
  • Consider the use of fibres

What impact does cold weather have on concrete?

Cold temperature adversely affects set time, the colder the temperatures, the longer the time it will take for the concrete to set and harden.

How do I protect fresh concrete in cold weather?

Several precautions can be taken to protect your concrete from freezing, such as:

Use a non-calcium chloride accelerator to speed up the setting time of the fresh concrete

  • Use concrete with a higher cement content
  • Pour the concrete at a lower slump to speed up set time
  • It is recommended to use  insulation on the finished concrete to maintain concrete temperature and to protect against cold wind and evaporation
  • Use insulated blankets.