ON THIS EPISODE OF THE CONCRETE LOGIC PODCAST
What happens when a concrete mix designed to meet sustainability goals starts producing cracks and low strength breaks?

Dr. Jon Belkowitz of Intelligent Concrete returns to discuss a contractor’s concern about a specification requiring significant cement replacement with fly ash.

Jon explains why he believes changes in fly ash sourcing and processing deserve closer scrutiny. He connects residual carbon and added chemistry to potential changes in water demand, admixture performance, and bleeding, while acknowledging that the reported problems require investigation before assigning blame.

Seth pushes for earlier contractor and producer involvement and more flexibility in specifications. Jon challenges whether a global warming potential limit can allow that flexibility when it indirectly restricts cement content.

The conversation raises a practical question: Do you understand the materials going into your concrete, or are you relying on what the specification calls them?

WHAT YOU’LL LEARN
• Why cracking and low breaks alone do not establish fly ash as the cause
• How Jon describes the shift from freshly collected fly ash to reclaimed sources
• Why changing fly ash sources deserves another look at the concrete mix
• How residual carbon can affect water demand and admixture performance
• Why bleeding and evaporation matter to finishing and early cracking
• How Class C and Class F fly ash differ in their reactions
• Why Jon challenges the approach behind ACI 323
• Where Seth and Jon disagree about carbon limits and performance specifications
• How earlier contractor and producer involvement can expose conflicting requirements
• Why reducing concrete quantities and avoiding wasted loads belong in the carbon conversation

CHAPTERS
(01:10) Cracking, low breaks, and sustainability goals
(02:16) Supporting the podcast and Concrete Logic Academy
(05:59) The contractor’s fly ash problem
(07:58) What fly ash is and why it was used
(12:56) Reclaimed fly ash and a changing supply
(17:04) Why fly ash source changes matter
(19:40) Water demand, admixtures, and bleeding
(22:41) Class C versus Class F fly ash
(25:03) Investigating possible causes before assigning blame
(30:18) Jon’s challenge to carbon requirements and ACI 323
(34:25) Carbon limits and performance specifications
(37:23) Reducing concrete quantities and avoiding waste
(40:23) Know what you’re putting in your concrete
(40:45) Connecting with Dr. Jon Belkowitz

GUEST INFO
Dr. Jon Belkowitz
Intelligent Concrete
Guest link:
https://www.concretelogicpodcast.com/guests/dr-jon-belkowitz/

CONCRETE LOGIC ACADEMY
Your concrete materials are changing. Has your understanding kept up?
Concrete Logic Academy offers practical concrete education, professional-development courses, and early access to Concrete Logic Podcast episodes.

Join Concrete Logic Academy: https://concreteschool.co

SUPPORT THE PODCAST
The Concrete Logic Podcast is independent and bootstrapped. If it gives you value, send a little value back.

Support the podcast:
https://www.concretelogicpodcast.com/support/

You can also help by sharing this episode with a colleague or sending Seth a topic or guest suggestion.

KUIU LISTENER OFFER
Get 10% off KUIU with code LOGICPOD and support the Concrete Logic Podcast through our affiliate link.

Shop KUIU: https://www.concretelogicpodcast.com/kuiu

Interested in sponsoring the podcast or working with Concrete Logic Media?
Email Seth: seth@concretelogicpodcast.com

CREDITS
Producers: Jodi Tandett and Concrete Logic Media
Music by: Mike Dunton
https://www.mdunton.com/

WHERE TO FIND SETH
Concrete Logic Podcast:
https://www.concretelogicpodcast.com/
YouTube:
https://www.youtube.com/@concretelogicpodcast
LinkedIn:
https://www.linkedin.com/in/seth-tandett/
Concrete Logic Academy:
https://concreteschool.co

Until next time, let’s keep it concrete.