Compress for Success

By Eric Carleton, P.E.

Although ASTM C31 published its original Standard Practice for Making and Curing Concrete Test Specimens in the Field in 1920, confusion still exists among some precasters about the proper procedures for casting, curing and testing concrete cylinders for concrete mixes and products. Below is a comprehensive list describing reasons why it is important to conduct concrete cylinder compression tests.ⁱ

Testing:

1. Ensures the concrete was batched properly: that is, the proper ingredients, in the proper proportions, were used and the laboratory mix design was adequate.
2. Indicates the statistical variability on the properties of the concrete being produced. High variability is an indication of poor practice.
3. Reveals problems arising due to inadvertent changes in the materials or environmental conditions.
4. Ensures that the people involved in the production, delivery and placement of the concrete do not become lax or careless in their operations.
5. Helps pinpoint the problem even if structural problems arise. Properly carried out and documented tests will be key.
6. Sets timelines for further construction operations to be carried out. For instance, strength tests may be used as a guide for form removal.

The data available to precast concrete manufacturers through testing leads to quick and economical choices to correct problems should they arise. You may never need concrete cylinder testing information until you really need it; and then you may wish you had lots of it. And, of course, by then it is too late. Additionally, it is an easy method for tracking hardened concrete trends for long-term efficiency adjustments, leading your company to the first steps of continuous improvement.

Standard testing made easy

Within the past decade, major changes to standard testing have made it easier to cast and test concrete cylinders. The first is testing and specifying agencies acceptance of the use of unbounded caps in accordance to ASTM C1231 Standard Practice in Determination of Compressive Strength of Hardened Concrete Cylinders. Though this practice was first published in 1993, many agencies hung on to the established methods of capping by high strength mortar or the cumbersome, and somewhat dangerous, capping method of molten sulfur. When proper testing procedures are met, such as using correctly sized retaining rings and not exceeding the maximum of 100 tests per pad (50 per side), unbounded caps will provide consistent results.

Another change that has eased the burden of cylinder testing is the widespread acceptance of the 4 in. by 8 in. high cylinder shape in lieu of the long-standing 6 in. by 12 in. specimen. Besides the handling weight advantage of the smaller samples, the greatest gain is the ability to take the test to an ultimate failure result. Modern precast concrete mixes produce hardened concrete with compressive strengths that routinely exceed 7,000 psi in a week to 14 days. This requires high ultimate loads to break the 6 in. by 12 in. cylinders. This load can exceed the gauge limit on the testing machine. Constant loading to the maximum loads has proven detrimental to the cylinder break testing frames. Consequently, many testing technicians stop the test to save the equipment. This still verifies the mix far exceeds the original specified requirements, typically 4,000 psi, but denies the ultimate strength data that could be used for consistent observations or mix optimization. The smaller specimen size provides the best compressive strength results.

Some of the critical criteria for the allowance of the smaller diameter is similar to the larger 6 in. by 12 in. specimen, including the maximum size of the course aggregate used in the concrete mix not exceeding a 1/3 of the cylinder diameter. The 4 in. diameter cylinder means 1.33 in. However, some specifying agencies have limited the maximum aggregate size to 1.25 in. or even 1 in., which still complies with a large majority of precast concrete mix designs.

The concrete industry and academia agree that smaller cylinders will produce slightly higher compressive strength results.^{ii, iii} Additionally, original testing results indicate the 4 in. by 8 in. cylinders had greater variability of testing results as compared to the 6 in. by 12 in. cylinders. This has led regulatory agencies to require the breaking of three 4 in. by 8 in. cylinders as a companion set for a result in place of the standard two commonly required for the 6 in. by 12 in. cylinder. This protocol requirement is not universally accepted, however, and there is compelling research showing the variability may not be as great as originally anticipated.^vi

Concrete cylinder specimen curing

Within the precast industry, there has been some misunderstanding about curing concrete test cylinders by precast concrete plants. ASTM C31 describes two methods of curing concrete cylinders.

The first is described in section 10.1.3.1 Final Curing Cylinders, and is described by ASTM C39 to be used when acceptance testing for specified strength, checking adequacy of mixture proportions for strength and quality control. These types of specimens are often described as the “mix potential” of the concrete and are cured in a temperature-controlled water bath for a specified period of time prior to testing. The intent is to eliminate all the variables except the mix properties and batching. Many precasters are not aware this provision permits the use of a moist curing room in place of the water bath. The standard suggests the testing times for these type of cylinders also be uniform and consistent to eliminate another variable. Though this curing method is not required by many specifying agencies for precast concrete products, a precast manufacturer can obtain the concrete mix data that is useful in troubleshooting or process optimization to maintain a uniform frequency of cylinder testing specimens.

The other curing method is the field curing procedure described by ASTM C39. When determining whether a structure is capable of being put into service, this method compares test results of standard cured specimens or with various in-place test methods, adequacy of curing and protection of concrete in structure, and form or shoring removal time requirements. This curing method is often used by specifying agencies and requires cylinders to be cured similarly to the product they represent. After initial curing, if the product is taken out of the plant to the yard, the concrete cylinders go with it. The expectation is the test cylinders will be broken to verify compliance or design minimums prior to installation.

Dry mix concrete cylinder molding

The proper way to cast a cylinder for “zero-slump” concrete or mixes has never been clearly defined by the concrete industry and specifying agencies. The traditional consolidation methods of rodding or internal vibration were deemed inappropriate for these stiff concrete mixes; however, at the time, the use of these dry mix processes was employed by a small segment of the concrete industry. Consequently, the issue was not addressed by the same ASTM C9 Concrete and Concrete Aggregates Committee that developed the C31 standard. It was left to the individual product specifications committees that produced the precast products, which led to a lack of correlation between different product committees on the procedures of dry mix concrete cylinder molding.

For precast concrete products, ASTM Committee C13 Concrete Pipe took it upon itself to clear up the confusion by including a provision within ASTM C497 Standard Test Methods for Concrete Pipe, Manhole Sections, or Tile. Section 11, Cylinder Strength Test Methods, removed the wide variety of dry mix mold making from each of the individual material standards. The provision in C497 now describes the specific procedure to make testing cylinders for precast concrete products other than those specified in ASTM C31 when, “the concrete consistency is too stiff for compaction by rodding or internal vibrations.” See Table 1 for compressive strength testing guidelines.

ACI Level I Testing Technician Certification programs do not cover the method to consolidate dry mix concrete cylinders. It is the manufacturer’s job to properly train their concrete QC personnel on the methods described within ASTM C497 to provide consistent and reliable test results.

Frequency of testing

The final item that has confused the industry is the frequency of making and breaking concrete cylinders. The answer lies with many variables including:

• Variable mixes produced per day and cubic yard output
• The minimum requirements within their own unique quality control manual
• Minimums required under any nationally recognized certification program, such as the NPCA Certified Plant Program
• Unique testing requirements of specifying agencies
• Compliance to the requirements stated within a specific material standard seen in Table 2

One or all of these will determine what minimum testing obligation is required of the precast manufacturer.

Concrete cylinder testing is one of the primary methods of specifying agencies and precast product purchasers to verify compliance with specifications and standards. It can also be a valuable tool employed by the precaster to provide a more efficient and economical manufacturing process. The routine task of making and breaking concrete cylinders, and keeping track of endless testing reports and data should be considered a valuable duty rather than a mundane chore.

Eric Carleton, P.E., is NPCA’s vice president of Technical Services.

References:
ⁱ Concrete, Second Edition by Mindess, Young, and Darwin (section 14.2 Significance of Test, page 365)
ⁱⁱ Concrete and Concrete-Making Materials ASTM STP169C (Chapter 10 Making and Curing of Concrete Specimens), Klieger and Lamond.
ⁱⁱⁱ MODOT Research Report 04-005 Comparison of Compressive Strengths Using 4×8 vs. 6×12 Cylinders for Prestess Concrete.
^vi Variability of 4×8 in. Cylinder Tests; Are three cylinders really necessary?, by Detwiler, Thomas, Stangebye, Urahn, (Concrete International May 2009).