The above blog title is the most frequently asked question I am receiving from contractors as of late. Since the revised standard specifications (RSS) addition of compressive strength cylinders for concrete pavement acceptance, I am being asked, do we still need to make flexural beams? The quick answer, it depends on what the contractor wants to achieve prior to acceptance.
The Standard Specification Revisions
Based on the October 2021 RSS, the replacement of the third paragraph of section 40-1.01D(5) Mix Designs is shown below:
The 2018 version
To determine the minimum content of cementitious materials or the maximum ratio of water to cementitious materials, use modulus of rupture values of at least 570 psi for 28 days age and at least 650 psi for 42 days age.
The RSS (10/21) 2018 version
To determine the minimum content of cementitious materials or the maximum ratio of water to cementitious materials, use the compressive strength equivalent to modulus of rupture values of at least 570 psi for 28 days age and at least 650 psi for 42 days age. Compressive strength must be tested under CT 521.
With modulus of rupture (MOR) change from 570 at 28 days and 650 at 42 days, to the new compressive strength equivalent of modulus of rupture, Quality Assurance (Q/A) now will be making compressive strength cylinders for acceptance.
The New Compressive Strength Acceptance Testing (Q/A)
The RSS (10/21) also has a change in Section 40-1.01D(8)(a) replacing the modulus of rupture at 28 days with:
Compressive strength at 42 days | California Test 521 | 1,000 cu yd
With this change, the average of 3 compressive strength cylinders will be now fabricated for acceptance at 42 days or sooner to be equal to 650 psi MOR as per 40-1.01D(8)(c)(i)
Compressive strength at 42 days (min, psi) | California Test 521 | 650b equivalent modulus of rupture to compressive strength
b Average of the individual test results of 3 cylinders
If the contractor does not wish to open to traffic until the 42 days, then according to the above changes to the standard specifications, no MOR flexural beams will be fabricated and tested.
The Use of MOR Flexural Beams for Early Age Opening
So now this brings us to if the contractor would like to schedule opening the pavement to construction or vehicle traffic prior to the 42 days, then MOR flexural beams will need to be fabricated and tested. This is based on Section 40-1.03J, the fourth paragraph, “Do not allow traffic or use equipment on concrete pavement before the concrete has attained a modulus of rupture of 550 psi, or equivalent compressive strength if using maturity per section 40-1.0L, based on your testing unless:”
How to Fabricate and Cure MOR Beams in the Field
To open prior to the acceptance age the contractor will fabricate beams as per CTM 523 (ASTM C31) for field curing, as per Section 10.2.2 which states:
As nearly as practicable, cure beams in the same manner as the concrete in the structure. At the end of 48 +/- 4 h after molding, take the molded specimens to the storage location and remove from the molds. Store specimens representing pavements of slabs on grade by placing them on the ground as molded, with their top surfaces up. Bank the sides and ends of the specimens with earth or sand that shall be kept damp, leaving the top surfaces exposed to the specified curing treatment. Store specimens representing structure concrete as near the point in the structure they represent as possible and afford them the same temperature protection and moisture environment as the structure. At the end of the curing period leave the specimens in place exposed to the weather in the same manner as the structure. Remove all beam specimens from field storage and store in water saturated with calcium hydroxide at 23.0 °C +/- 2.0 °C [73.5 °F +/- 3.5 °F] for 24 +/- 4 h immediately before time of testing to ensure uniform moisture condition from specimen to specimen. Observe the precautions given in 10.1.3.2 to guard against drying between time of removal from curing to testing.
Based on the field condition curing beams, the specimens must test at 550 psi or higher in order to allow traffic on the pavement. For a single wheel of the paver, 350 psi or higher must be achieved prior to allowing construction equipment on the pavement. It is critical for the proper curing of the field specimens, kept in a uniform moist state surrounding the beams, in order to guard against drying of the specimens. Once the MOR flexural beams dry out, the hydration will be arrested and the strength gain will be highly variable when tested in the laboratory.
An Option to MOR Beams, Section 40-1.03L
The last part of 40-1.03J suggests, “…or equivalent compressive strength if using maturity per section 40-1.03L, based on your testing unless:” At the contractor’s option, a new section has been added for the use of maturity testing in lieu of MOR beams. This will be the subject of our next blog.
Partner With Us
G3 Quality, Inc. is here to provide any further information you may need and how to implement these revisions on your current or upcoming projects. For more information, reach out to our Technical Services Director, Marc Robert by email at firstname.lastname@example.org or by phone at 562.321.5561.
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