Compressive Strength of Concrete: What Causes Low-Strength Breaks in Concrete Cylinders?

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The compressive strength test results of concrete cylinders are the basis of acceptance of ready mix concrete for construction. 

Low-strength breaks of the concrete cylinder during compression testing indicate the low compressive strength of produced concrete. 

Concrete cylinders for the standard compressive tests are prepared from a sample of fresh concrete cured under standard conditions and tested at a particular age, usually 28 days.

Low-strength breaks of these concrete cylinders may occur due to several reasons, like excess water and air content or other unforeseen errors during sample preparations.

This article discusses the ACI reference on low-strength cylinder test results and their major causes.

Low-Strength Results of Concrete Cylinder- ACI 318 & ACI 301

As mentioned in ACI Building Code ACI 318 and the Standard Specifications for Structural Concrete ACI 301, a concrete mix proportioned to meet the requirements of the standards and under compression test has chances of low-strength results about once or twice in 100 tests due to normal variability.

As per ACI and CSA, a cylinder break during a compression test is considered to be of low strength when:

1. The individual compressive strength test of one or two cylinders is more than 500 psi below the specified strength, or
2. If the average of three consecutive tests is not equal to or exceeds the specified strength.

Causes of Low-Strength Concrete Breaks 

The six key causes of low-strength concrete cylinder breaks are:

1. Poor Concrete Mix Design

Preparing the wrong concrete mix results in delaying the concrete strength gain within the time that has been specified. This results in a compressive strength test value lower than expected. 

2. Incorrect Production of Test Sample

The improper casting of concrete cylinders results in a test specimen that is not prepared for compression testing.

3. Improper Handling

Improper handling and transportation of concrete test samples would result in cracking.

4. Improper Curing Conditions

Field-cured test specimens do not reflect the strength values of in-situ concrete samples. If this is not corrected, the strength value of sample testing won’t be given by the real concrete structure. 

5. Improper Specimen Preparation

The test specimen won’t break properly under the applied test load if the cured specimen is not prepared by grinding or capping its ends.

6. Calculation Errors

Before performing a compression test, the compression testing machine must be properly calibrated.

One way to avoid low-strength concrete breaks is to wait for the complete curing of the concrete and conduct the test on the next break day. If the sample still shows low strength, the mentioned causes must be studied and corrected. 

Nowadays, wireless maturity sensors are used to monitor the strength gain of these ditched cylinders (those that failed the compression test), to monitor the strength over time. 

All standard-cured cylinders are made and tested on 28 days for acceptance purposes and strength with age is determined through maturity sensors. 

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