Fly Ash Concrete

Strength Specifications

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Fly Ash
Need for extending Strength Specifications to beyond 28 days – Restraints on use of Fly Ash in Highway Constructions

Strength Specifications of Fly Ash Concrete

It is common for concrete specifications to require strength in 28 days, but often strength is not needed at that time. In fact, 28-day strength requirements are a limiting factor to durable concrete.

Organizations like Headwaters Resources, USA are working to promote the concept of extending the 28-day strength requirement in the governing specifications to exceed 28 days (e.g. 42 day strength, 56 day strength etc).

Typically, concrete made with fly ash will be slightly lower in strength than straight cement concrete up to 28 days, equal strength at 28 days, and substantially higher strength within a year’s time.

Thus, fly ash concrete achieves significantly higher ultimate strength than can be achieved with conventional concrete.

fly ash concrete: strength gain with time

(1 psi = 0.007 N/mm2 )

Fly Ash concrete is more durable than normal concrete made with ordinary Portland Cement. However, present specifications of 28 day strength do not allow taking full advantage of benefits that Fly Ash concrete offers.

Developing sustainable concrete structures to last 100 years or more can require extending the common 28-day strength specifications. With extended age strength parameters, better, more durable concrete can be achieved.

This extended age acceptance has been approved in some countries (like in USA’s “Greenbook, Standard Specifications for Public Works Construction”, which is recognized completely or in part across Southern California and into San Diego); where previously only 28-day specifications were allowed, now 35, 42, and up to 56 day strengths are accepted.)

It is important to note that extended strength parameters are not suitable for every application.

However, if extended age strengths are acceptable, higher percentages of fly ash can be used.

With the right expertise, mix designs can be amended to reflect a percentage of strength at designated time-frames with an ultimate strength overall. Utilizing the experience of the local ready mix producer, proper mix designs can be developed to optimize the projects timeline in order to achieve the highest quality concrete for the project.

Listed below are just a few projects (in U.S.A) driven by the need for long lasting, high quality concrete that have allowed for 35-, 42-, and 56-day or more strength specifications/requirements.

fly ash concrete: list of projects allowing higher than 28 days strength specifications


Concrete-Techgroup thanks Headwaters Resources , USA for granting them permission to use their resources in developing this article.