Specified Strength of Cylinders vs. Cores

On a recent project the concrete column strengths for one pour fell short of the specified 4000psi. Cores were taken in accordance with IBC 2006 Section 1905.6.6 and ACI 318 Section 5.6.5. Due to project schedules and forming techniques cores were taken vertically from the column tops. Subsequent strength testing indicated acceptable results and the project continued with only this small blip. Afterwards, our client questioned our field-testing. They claimed that if the core testing results met the requirements of the Code, the cylinder testing was obviously incorrect and therefore refused to pay for the coring. What is the relationship between the strength indicated by the test cylinders compared with the strength of the concrete in the structure?

Test specimens (cylinders) are made, cured and tested under certain standard conditions that are usually appreciably different from the conditions existing in the structure. The value of field-cast test specimens is that they give a measure of the strength potential (they evaluate the materials and mix as supplied by the producer, to ascertain the concrete meets project specifications).Test specimens are not intended to yield an exact strength of the concrete in the structure, and the actual strength of the concrete in the structure can be appreciably different. Besides variable environmental site conditions and curing, other variables between test specimens and the concrete in the structure include variations of mix components, water content, size, and shape of the structure, workmanship, degree of consolidation, possible presence of defects such as rock pockets, restraint, and combinations of loading in the structure. It is because of these unknowns that the Structural Engineer must consider a factor of safety when the structure is designed.

Variations in cylinder strengths are not always reflective of a problem in the structure. For instance if three sets of specimens are made from one day’s concrete placement and maintained under identical conditions throughout the test duration, there is no assurance they will all fail at the same strength when they are tested at the same age. In fact, each one will almost always break at a different strength. These are normal variations, and they should be expected.

Cored specimens are usually obtained days or weeks, even months, after the laboratory testing of cylinders. This additional time must be taken into account when comparing cylinder and core test results. In addition, cored specimens are tested in a dry or moist condition, but rarely in the saturated condition similar to test cylinders. It is well documented that dry specimens have a higher compressive strength than saturated specimens.

We do know that there are variations in the strength of the structure that are not caused by basic variations in the concrete itself. For example when cores are taken from a column, the cores from the upper portion of the column invariably indicate lower strength than the cores from the bottom portion of the column. The reason is that the concrete near the bottom was compacted by static hydraulic head of the concrete being worked above, yet there was no change in mix or materials.

 Jump to Top

Establishing Concrete Strengths with Core Tests at 85% 

I’m investigating an older concrete building and would like to use ACI 318 Section 5.6.5 to confirm the existing concrete strengths. Can you outline a procedure for specifying the work and explain the rule of 85%?

A nondestructive test meth-od, such as probe pene-tration, impact hammer or ultrasonic pulse velocity may be useful in surveying structural members for areas of lower strength concrete. From this preliminary viewpoint use ASTM C823-00 “Standard Practice for Examination and Sampling of Hardened Concrete on Construction” to formulate specific areas of investigation. The selected areas then can be specified for investigation for concrete strength according to ASTM C42-04 “Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete”. Section 3.2 States “Generally, test specimens are obtained when doubt exists about the in-place concrete quality” and “use of this method is to provide strength information on older structures.”

According to International Building Code IBC Section 1905.6.5.2, three cores will be taken for each strength test. And Section 1905.6.5.4 states, “the average of three cores is equal to at least 85% of f’c”.

The rule of 85% can be best explained by ASTM C42-04 Section 3.5 “There is no universal relationship between the compressive strength of a core and the corresponding compressive strength of standard-cured molded cylinders. The relationship is affected by many factors such as the strength level of the concrete, the in-place temperature and moisture history, and the strength gain characteristics of the concrete. Historically, it has been assumed that core strengths are generally 85 % of the corresponding standard-cured cylinder strengths, but this is not applicable to all situations.”

The commentary of ACI 318 Section R5.6.5 also states “Core tests having an average of 85% of the specified strengths are realistic. To expect core tests to be equal to f’c is not realistic, since differences in the size of specimens, conditions of obtaining samples, and procedures for curing, do not permit equal values to be obtained.”

NOTE: According to ACI 214.4R-03 “Guide for Obtaining Cores & Interpreting Compressive Strength Results” the preceding method is NOT an option when evaluating for structural capacity

For further information ASTM references Neville, A., “Core Tests: Easy to perform, Not easy to Interpret,” Concrete International, Vol.23 No. 11 November 2001, pp. 59-68.

  Jump to Top

Water-Cement Ratio vs. Strength

 Many published articles relate how changing the water-cement ratio has a large effect on concrete strength. Is there a simple explanation for this effect?

In general, there exists a fundamental inverse relationship between porosity and strength of solids. This strength-porosity relationship is applicable to a wide range of materials, such as iron, stainless steel and granite. Think of examining a concrete core, which exhibits voids created by a lack of consolidation. You can imagine, why with a lack of internal structure, the compressive strength would be lower than expected. On a much smaller scale, there is a theoretical volume of water (based on curing conditions) required to hydrate a given volume of cement. Once you have added more than that amount it creates capillary porosity (i.e. microscopic cavities or voids). The higher the water-cement ratio the more porous the weaker the strength. Generally, to maximize strength and durability, the water-cement ratio should be the lowest possible to hydrate the cement while maintaining its workability.

  Jump to Top

Low Concrete Strengths on California School Projects

We have a school project in California where the specified concrete strength is 4000 psi at 28-days. On one specific pour the following strengths were obtained:                 

7-day strength = 2780 psi

28-day strength =3890 psi (average of 2 cylinders)

56-day strength = 4150 psi (1 cylinder)

Do you report the results as meeting the requirements of the DSA approved document?

California Building Code, Title 24, Part 2, Chapter 1905A.6.3 Strength Test Specimens states “Strength test acceptance criteria shall comply with the provisions of ACI 318, Section 5.6.3.”  Section 5.6.3.3 notes “Concrete shall be considered satisfactory if both of the following requirements are met:

A) Every arithmetic average of any three consecutive strength tests equals or exceeds f’c. 

B) No strength test falls below f’c by more than 500 psi when f’c is 5000 psi or less.”

Using this guideline, the results above would be acceptable if the 28-day cylinders, when averaged with three consecutive strength test results on the project are equal to or greater than 4000 PSI. This assumes that no individual test was less than 3500 PSI.

The Division of the State Architect holds a different position regarding low strength concrete test results.  DSA believes that the LEA approved laboratory should report all failing test results immediately as a non-conformance.  It is then up to the design professional and DSA to determine a corrective action plan.  If an approved stamped change order is not received from DSA, the failing results should be reported on your laboratory verified report, DSA Form 291.   In the 2007 California Administrative Code, Title 24, Part 1, section 4-335b, Performance of Tests, it states, “Where a sample has failed to pass the required tests the architect or engineer, subject to the approval of DSA, may permit retest of the sampled material.”   Section 4-335d, Test Reports also notes “Reports of test results of materials not found to be in compliance with the requirements of the plans and specifications shall be forwarded immediately to DSA, the architect, the structural engineer, and the project inspector.”

So although the 56-day strength test met the 28-day f’c requirements, DSA does not consider the results to be valid.  The test report must be distributed noting, “the results did not meet the requirements of the DSA approved documents.”   There are no provisions in the California Building/Administrative Code, Title 24 that allow the use of a 56-day test result in lieu of the required 28-day test result.  However a 56-day test result may be useful to the design professional and DSA in arriving at a corrective action plan.

Reference Documents

2007 California Administrative Code, California Code of Regulations, Title 24, Part 1

2007 California Building Code, California Code of Regulations, Title 24, Part 2, Volume 2

Building Code Requirements for Structural Concrete (ACI 318-08) and Commentary

  Jump to Top

Mortar Testing by Cylinders or Cubes

 When testing for compressive strength of mortar the 2007 CBC (California Building Code) specifies “Average Compressive Strength at 28 days” as noted in Table                  2103A.8(2). Is this strength tested and computed by cylinders or cubes?

Mortar testing is normally required for schools and hospitals, hence your reference to table 2103A.8(2) in 2007 CBC. In the title block of the reference table under Average Compressive Strength there is a small b notation, referring the note at the bottom, which reads “b. Average of three 2-inch cubes of laboratory-prepared mortar, in accordance with ASTM C270.”So, the specified strength is based on 2-inch cubes prepared in the laboratory.

Section 2105A.5 specifies “Test specimens for mortar shall be made as set forth in ASTM C1586”. As we were informed in FAQ 10.043, C1586 refers us to C780 Annexes A.7, which describes specimens made as cylinders or cubes.

A further clarification is found in “Reinforced Concrete Masonry Construction Inspector’s Handbook” Fourth Edition, which indicates “The 2-inch cube is typically used for laboratory prepared mortar while the 2 inch x 4 inch cylindrical specimen is used for field cast mortar”.  To obtain an equivalency of a 2” x 4” cylinder field test specimen to a 2” cube specimen, divide the compression test result of the cylinder specimen by 0.85. The factor of 0.85 is the normal correction h/d found in ASTM C780 5.2.6 Note 3.

When testing for compressive strength of mortar in the field you could use either 2-inch cube molds or 2 inch x 4 inch cylindrical molds. The typical standard of practice that most testing laboratories follow is to test field mortar by preparing specimens in a 2 inch x 4 inch cylindrical mold and if required showing a correction factor when the specimens are tested depending on which specimen, cubes or cylinders, was specified for the project.