Mike Hernandez, Technical Director, The VOICE Newsletter, December 2023

ASCC just issued Alert! #2 on portland limestone cement (PLC) (members click here to view). In it, ASCC recommends two tests, one for bleed rate and one for initial set. Because reduced bleed is a common issue raised about PLC, this test was recommended, per ASTM C232. One response from experts regarding our recommendations is that even if tests are done, lab results and field results will likely be different due primarily to field environmental conditions. Similar caution on initial set time. If you have a receptive ready mixed producer both tests can be modified to higher temperature, near 90 F, and low temperature, close to 50F to provide a performance vs temperature comparison. This would be consistent with the 2012 Prescription-to-Performance (P2P) Checklist for Concrete Producer-Concrete Contractor Fresh Concrete Performance Expectations, in which target slump and air content are tested at “Hot”, 70F and “Cold” with the contractor defining anticipated hot and cold conditions. The bleed results desired are more difficult to define. Not too little with evaporation faster than bleeding so little or no bleed water. Not bleeding so much there is deep water ponding. Want that Goldilocks “just right” which can be in the 6% - 10% range for slabs or .15 - .3 lbs./sf/hr. Your feedback is welcomed.”

Cement and ready mixed production have their own tolerances. The cement’s limestone content tolerance is +/- 2%. Even if the cement mill certs are near current, the result is the monthly average of daily averages so there can be day to day variability in the cement in addition to the ready mixed operationally tolerances for batch weights and admixture dosages, per ASTM C94. Ideally, your local producer will receive the cement mill certs monthly and share the results with you, their customer, so if the target cement limestone percentage increases, you will soon be aware of the increase. 

There were other options considered but not recommended. Instead of just wishing members good luck, ASCC staff wanted to recommend actionable tests. A few other test options:

Calorimetry – per ASTM C1753, also known as semi-adiabatic calorimetry. It is most often used to compare mixes with different dosages of admixtures or SCMs. This test was excluded because it is not a concept familiar to day-to-day performance like bleed and set time. It is the most recommended test when dealing with very challenging mix criteria and you can estimate changes in set time with calorimetry.

Evaluating Plastic Shrinkage Cracking of Restrained Fiber Reinforced Concrete – ASTM C1579. This test has been used to evaluate mixes for shrinkage cracking potential. The test exposes the concrete sample to extreme evaporation conditions in a repeatable process but is only useful in mixes with a high enough water cement ratio to have adequate bleeding and cracking. PLC mixes or any mix with low bleed water would not be good mix for this test.

ASCC will continue to seek best practice for our members with PLC and all low carbon concrete options. Look for the World of Concrete session on Best Practice for PLC (Type IL) on Wednesday 1/24/24 at 2:00pm. WOC Session ID: PLCWE 

Topics of the presentations and discussion include:

• Planning for sustainable concrete construction using Type IL cement.
• Meeting both engineering requirements and constructability requirements.
• Examples from multiple projects which implemented Type IL cement.