WHAT IS MPa?
A megapascal (MPa) is a measure of the compressive strength of concrete. It lets inspectors know how much pressure can be applied to the concrete before it cracks or fails. One MPa is equal to one million pascals (Pa); as a pascal is one newton of force per square meter, a megapascal is one million newtons per square meter.
If you have a project which requires a defined strength of concrete - i.e. 20MPa to set the posts for a raised deck - then its time to call in the experts and have concrete delivered from a ready mixed truck – it’s the only way to guarantee the structural strength in the concrete.
You may also see concrete requirements expressed as 'N20/10' - this is where N means normal class concrete, 20 refers to 20MPa of strength and 10 refers to it containing a maximum aggregate size of 10mm.
CLASS CONCRETE
Normal Class concrete
Normal-class concrete that complies with AS 1379 has the following parameters (or properties): ■ A mass per unit volume in the range 2100 to 2800 kg/m3 when determined in the saturated, surface-dry condition in accordance with AS 1012.12 Determination of mass per unit volume of hardened concrete.
■ Acid-soluble chloride and sulfate contents within the limits given in Clause 2.7 of AS 1379 when determined in accordance with Clause 5.5.2 of AS 1379.
■ A drying shrinkage strain after 56 days of drying not exceeding 1000 x 10-6 when determined in accordance with Clause 5.6 of AS 1379. Note: This maximum value is consistent with the use for design purposes of a median basic shrinkage strain value of 850 x 10-6.
■ A mean compressive strength at 7 days, assessed in accordance with Clause 5.7 of AS 1379, of not less than the values shown in Table 1.2 of AS 1379.
■ Cement complying with AS 3972 alone or in combination with one or more supplementary cementitious materials.
■ No lightweight aggregate as defined in AS 2758.1.
What MPa is considered normal class concrete ?
Normal Class concrete is defined as products ranging from 20 to 50 MPa compressive strength at 28 days with a design slump that has a point of acceptance from 20 to 120 mm are available in both 10 mm, 14 mm and 20 mm aggregate sizes.
Normal Class MPa's and Applications
20 MPa and 25 MPa compressive strength
Commonly used for house slabs, driveways, footings and footpaths.
Residential slabs and footings
A default slump of 100 mm has been adopted by the premix concrete industry to reduce the uncontrolled addition of water on site and make placing and compacting of concrete easier for concreters.
32 MPa, 40 MPa, 50 MPa compressive strength
Higher strength is commonly used for concrete that will experience greater loads and traffic. This may be specified by engineers or builders to suit the load requirements which the concrete must support during its life
Special Class Concrete and MPa Comparisons
The parameters and attributes that should be specified for special-class concrete should be as set out in Schedule B with reference to Appendix B and Table B1 of AS 1379 Special-class concrete commonly has the same basic parameters as normal-class concrete with some additions and/or exceptions. Parameters or attributes that are different from, or additional to, those of normal-class concrete should be included in Schedule B. If the requirements of Schedule B for any concrete are inconsistent with those for normal-class concrete then the requirements of Schedule B take precedence for that concrete. Where any parameter other than strength grade requires the specification of a special-class concrete, or the proportions of the mix are specified, the concrete should be identified by an appropriate code agreed to between the supplier and customer that identifies that particular mix.
Special Class MPa's and Applications
High strength 65 MPa, 80 MPa, 100 MPa compressive strength
Commonly used for projects requiring high strength concrete with options in design slump between 140 and 200 mm. 65 MPa concrete is available in 10 mm, 14 mm and 20 mm aggregate sizes while 80 and 100 MPa are only available in 10 mm and 14 mm aggregate size. High strength concrete is usually specified by engineers in applications such as high rise buildings.
Block fill
7 mm and 10 mm flowable product ranging from 20 to 40 MPa compressive strength at 28 days with options in design slump between 140 mm and 230 mm. Block fill product has been designed to pump through a two inch rubber hose line making it easier when placing into block work walls.
Shotcrete
Shotcrete is designed as a 7 mm and 10 mm aggregate sized sprayed product ranging from 20 to 50 MPa compressive strength at 28 days, with a design slump of 60 mm. Ideal for spraying pool walls, embankments, structural and retaining walls.
Concrete Curing, why and how
Aggregate mixed with the cement becomes part of the “rock” mass. The process of transformation is most rapid during the first 28 days. Nothing can stop the transformation except a lack of water or subnormal temperatures. If concrete is allowed to dry out in the initial stages, it becomes permanently poor because water evaporated from the mix in the beginning cannot be forced back into the mix in time to prevent the cement gel from spoiling. Similarly, low curing temperatures can adversely affect concrete strength as shown in a comparison between job site cured and standard cured test cylinders. In this example, the job site curing was in a 200-litre drum with the temperature varying between 7ºC and 9ºC.
