Concrete Strength at 7 Days

Concrete strength at 7 days is often measured even though the specified compressive strength of concrete is measured at 28 days. It is often necessary to measure the compressive strength of concrete at 7 days and 28 days which is why a number of concrete cube samples are taken during the pour.

There are a number of methods which can be used to estimate the required compressive strength of concrete at 7 days. The most popular methods are described below. The CivilWeb Concrete Strength at 7 Days spreadsheet has been designed to complete this analysis using three methods. The concrete strength gain curve can be estimated using either two concrete cube test results or estimated from the specified compressive strength at 28 days using two methods.

Concrete Strength at 7 days

The compressive strength of concrete at 7 days is often required for two reasons. Firstly it may be necessary to begin loading a concrete member after only around a week in order to meet the construction programme. This is common in multistorey buildings where one floor will need to be loaded in order to complete the next, or in concrete roads and pavements which need to be opened to traffic as soon as possible to minimize disruption.

The second reason is so that the quality of the concrete can be checked without having to wait 28 days. A favourable concrete cube strength test result after 7 days gives a good indication that the concrete will satisfy the specified compressive strength of the concrete.

Maturity Concept of Concrete

Concrete gains strength due to the ongoing cement hydration process. Concrete will gain strength from the moment it is placed and starts to hydrate. Much of this strength gain will occur in the first few days after placement but the slower chemical reactions will continue for many years after placement, particularly if the concrete has access to moisture from the air for example.

Characteristic compressive strengths are usually specified assuming a 28 day compressive strength. In many applications 28 day strength hits a sweet spot, long enough for the concrete to gain most of it’s strength but before it is expected to accommodate significant load.

In some cases however 28 days may be much too long to wait before the quality of the concrete is known. This happens when a concrete road for example needs to be opened a week or so after pouring or where the strength of a concrete beam needs to be utilized in order to construct the next level in a building. In these cases an assessment of the suitability of the concrete will need to be estimated from concrete cube testing at a much earlier age. This often occurs at 7 or 14 days but can be as early as 3 days. In order for these results to be meaningful, a maturity relationship needs to be developed using a concrete maturity method such as those described below to relate early age and 7 day compressive strength test results with the specified 28 day strength.

Concrete Maturity Testing

Concrete maturity testing can be undertaken using the same concrete cube tests as are used for general compressive strength tests and compliance testing. Maturity testing usually includes for 3 days, 7 days cube strength and 28 days. If required the maturity testing can include longer periods of 60 or 90 days. Further information on measuring the concrete strength at 7 days is included in our Concrete Cube Test post.

Concrete Strength Gain Curves

The results from the concrete maturity testing are then translated into a concrete strength gain curve for this particular concrete mix. This concrete strength gain curve can then be used to estimate the long term compressive strength of the concrete from 7 day compressive strength results. 7 day compressive strength of concrete formula can also be developed from the concrete strength gain curves where this is deemed more appropriate than a graphical solution.

The rate of strength gain of the concrete will depend on the cementitious materials used. Ordinary Portland cement will gain strength in a manner similar to the below graph with the concrete strength after 7 days around 60% of the 28 day strength. Rapidly setting cements are available which can gain significant compressive strength in a few hours. These high early strength concretes are often used in order to open concrete road repairs quickly after placement. Conversely low heat cements and cements blended with GGBS or Fly Ash will take longer to reach their design strengths, sometimes 6 months or more. This will need to be allowed for in the specification.

Rates of strength gain are different for each mix and for this reason a concrete strength gain curve or formula should be provided by the concrete supplier or determined from long term testing of this particular concrete mix. This can then be used to evaluate early strength test results for conformity with the required strength at 28 days. A typical concrete strength gain curve (plotted on a logarithmic scale) can be seen below.