Low Concrete Cylinder Results And What Happens Next

A low concrete cylinder result means the concrete from that truck, as tested, did not reach the strength the drawings specify, and the element it went into has to be assessed before more is built on it. The answer takes three steps: check the test, check what the element actually needs, and where there is still doubt, test the concrete in place.

The element is not automatically condemned, and it is not automatically fine. It is a question that needs an engineer's answer in writing.

How concrete strength is tested

When concrete is delivered, samples are taken and cast into cylinders, which are cured and crushed at set ages to the AS 1012 test methods. AS 1379 covers how concrete is specified and supplied, and how its strength results are assessed.

The specified strength is the strength at 28 days. Earlier results, often at 7 days, show the trend and give an early warning. The 28 day result is the one the drawings are measured against.

The test itself

A cylinder is a small sample of a large pour, and a lot can happen to it between the truck and the laboratory.

Cylinders left in the sun on a hot site before collection, knocked about in transport, poorly compacted in the mould or cured badly will all crush low while the concrete in the slab is fine. One low cylinder among good results from the same truck points at the cylinder. A full set of low results from one load points at the concrete.

Where the concrete went

Delivery dockets and pour records tie each truck to an element: the columns on level three, or one bay of a slab. Without those records, a single low result can put an entire floor under question.

That is one more reason the paperwork during a pour matters as much as the concrete.

What the element needs

The engineer then re-checks the element at the lower strength.

Many elements have room. A slab sized by deflection or by minimum thickness can carry its loads with lower strength concrete than specified. A heavily loaded column or a transfer element has far less room, because strength is what sized it.

Durability is checked separately. Concrete strength is tied to the exposure classification, so a lower strength can fail the durability requirement even where the structure is strong enough. The link between strength, cover and exposure is explained in [concrete cover and why it decides durability](/blog/concrete-cover-and-why-it-decides-durability).

In-situ testing

Where the numbers do not clear it, the concrete in the element is tested directly.

Non-destructive tests such as the rebound hammer and ultrasonic pulse velocity map the extent of the weaker concrete. Cores are then drilled from the element and tested under AS 1012.14, with a cover meter used first so the drill misses the reinforcement. Core results are read with allowances for drilling damage, core size and age, so a core and a cylinder from the same concrete do not give the same number.

Age helps too. Mixes with fly ash or slag keep gaining strength well past 28 days, and a later test can show the concrete has reached its grade. How mix design affects strength gain is covered in [concrete technology, admixtures, mixes and specifications](/blog/concrete-technology-admixtures-mixes-specifications).

The three outcomes

Accept. The in-situ strength is adequate for what the element carries. The decision is documented with calculations, and certification can rely on it.

Accept with strengthening. The element is adequate with help, such as a steel jacket on a column or fibre-reinforced polymer bonded to a slab soffit. The methods are covered in [strengthening an existing slab for a new load](/blog/strengthening-an-existing-slab-for-a-new-load).

Remove and replace. Where adequate strength cannot be shown, the element comes out. It is the worst outcome, and it is far cheaper caught early than after three more floors are built on top.

The next stage on hold

Until the question is settled, the element is treated with care. Props stay in, stripping waits, and no new floor is poured above it. Stripping and propping depend on measured strength, as explained in [formwork and propping cycles](/blog/formwork-and-propping-cycles).

How we approach it

When a low result arrives, we move quickly, because the programme stops behind it. We review the test records and the pour records, re-check the design at the reported strength, specify any coring, and write the decision up with calculations so the certifier has something solid to rely on.

A low cylinder starts an investigation. The concrete in the element holds the answer, and the job is to go and measure it.

ACSES provides structural engineering and construction phase support across Sydney. Talk to us about a project.

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