Jim Klinger, The Voice Newsletter February 2023
Full Disclosure: Over the past year, the ASCC Technical Division fielded several Hotline calls from members experiencing difficulties related to ASTM C 595 portland-limestone cement Type1L, hereinafter referred to as "PLC" (as opposed to "OPC", ordinary portland cements, brand X,ASTM C 150 Types I and I/II and so on).
At first glance, it seemed suspect PLC issues might be regional; initial Hotline calls were originating mostly from ASCC members in the southeastern US markets. But once we started investigating, PLC was suspected to be problematic under certain climatic jobsite conditions (e.g. temperatures near 40 degrees F and lower) as well. By the time we met for our ASCC Annual Conference committee meetings in Cleveland (then later, informally, at the ACI Convention in Dallas), it became clear that PLC cement issues reported by our members were neither geographically isolated nor random in nature.
In early December 2022, the ASCC Technical Division emailed a survey to ASCC contractor members asking them to describe their experiences, if any, with the "new" Type 1 PLC. Survey results are still being collected and reviewed. But others in the concrete construction industry (e.g. trade magazines and social media) have noticed and discussed potential problems with PLC as well. Just last month, ASCC attended two educational seminars at the World of Concrete; both highlighted the same types of PLC-related issues reported by our Hotline callers.
What follows is a confected summary of what we know to date about PLC. As you will see, information that should be disseminated to help all concrete contractors and project stakeholders is hard to come by. "Lessons learned" and project-specific accounts are few; in some cases, attorneys and non-disclosure agreements (NDAs) prevent us from legally gaining access to test reports and other valuable technical data. In other words, names and places used below have been changed to protect the innocent.
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Early last summer, the ASCC Technical Division took a Hotline call from a member who reported serious problems with the concrete on a major big-box project. The first sign of trouble had actually gone unnoticed up and down the project food chain. The contractor had submitted concrete mix designs with OPC proportions similar to those proven successful over many years using the same ready mix supplier. These were triaged by the Owner's test agency and approved by the LDP.
The mandatory preconstruction meeting was held on site, and the first slab placement date was set for two weeks out and put on the books.
One week later, the concrete contractor received an unexpected "Dear Valued Customer" letter. The letter stated the ready mix supplier had to resubmit the mix designs "to incorporate an industry-wide change in cement from ASTM C150 Type I/II to ASTM C595 Type 1L."
"Please excuse the inconvenience," the letter stated. "This is not a change we requested...this change is being done as a green initiative by the cement industry to reduce the carbon output of cement production...our cement producers have conducted testing and found no significant change in quality or set time."
During follow-up clarification phone calls with the ready mix supplier, the ASCC contractor was assured that this in fact would be a no-cost change, that the cement powder was being swapped purely "on a one-to-one basis"... that comprehensive industry testing had already been done...and that similar cement has been used successfully in Europe for 25 years or even longer. "It is a matter of plug and play," they were told.
There would be no finishing or strength or durability issues. The ready mix supplier added "we are being told by our suppliers that Type 1L PLC is hereto stay and project stakeholders must learn to adapt."
Armed with that reassurance, the new Type1L PLC mix designs were packaged by our contractor member for resubmittal with the explanations..."emergency resubmittal...sorry for the inconvenience...supply chain and climate change issues...please expedite review to help us maintain the project schedule." The design team approved the new PLC mixes and collected their green points.
Problems started when the first routine 25,000 SF slab placement that historically would take say a 10-man crew in an 8-hr shift now required 12 to 14 finishers working well into the night on overtime.
The problem?
The concrete set too slow, and when it finally did set, it went off all at once. The finishers did what they could to try to save the surface, but still had to show up again early the following morning to rework a section of the slab. Subsequent placements required addition of accelerating admixtures.
Admixture dosages were adjusted over several deck placements before the contractor felt comfortable tapering the finishing crew back to the size anticipated at bid time. Then the cost reports hit the concrete contractor's PM's inbox, red-flagged with unfunded line items for overtime, rework and accelerating admixture costs associated with the changes. The Owner, who did not initiate the change from OPC to PLC was not thrilled with the worked slab and didn't want to hear any part of delays or extra cost issues or infighting associated with the PLC.
As of today, this ASCC member has revealed the decks have been cored and petrographic testing has been undertaken, but the project is now under an NDA gag order. Test results, strength data and other crucial information is now under seal and--for now--the concrete contractor is left holding the bag.
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Another ASCC Hotline caller described a similar scenario that featured a mix design change from OPC to PLC after the initial OPC mixes were approved. Wall and column footings were placed using the new PLC mix. Two weeks later, a second PLC mix was used in slab-on-grade replacements. Strength tests at 7 days were normal for the footing mix, but at 28 days, the tests ran about 1000 psi low. Same thing for the SOG tests, roughly 1000 psi low at 28 days. Before anyone realized it, all project concrete had been placed and all 28-day breaks were low. The LDP directed cores be taken and tested for strength and petrographic analysis. It was apparent PLC concrete was gaining strength up until 21 days, then gaining almost zero strength after that. The PLC was just flat dying on the vine and upsetting the project Owner.
We asked our member to send us the mix designs and test reports, including the results of coring and petrographic analysis when available. At this point, the ready mix producer had embarked on a similar companion test regimen with coring and petrographic analysis. In a follow-up phone call with the contractor, it was revealed this project, too, was now under an NDA. In other words, strength test data and the petrographic analyses performed by the Owner's test agency and the companion testing done by the ready mix supplier are now under seal. The contractor, the GC and the ready mix supplier have all hired their own consultants, prompted by Owner posturing for total removal and replacement of the concrete. The ready mix supplier is saying the Owner's test agency mishandled the cylinders. Once again--we will most likely never know the true cause of the low strength and who will be financially responsible.
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Question: What exactly is ASTM C 595 Type 1 PLC? Is limestone considered to be a supplementary cementitious material (SCM) with pozzolanic or hydraulic properties such as flyash or slag?
Answer: ASTM C 150 currently allows up to 5 percent limestone to be blended with OPC.
With PLC, ASTM C 595 allows more straight cement to be pulled out and replaced with between 10 and15 percent limestone. Limestone filler is not considered to be an SCM, and not everyone is convinced limestone fillers exhibit appreciable concrete strength or other technical properties. The addition of limestone has been a subject of considerable controversy within ASTM and industry circles for many years as summarized in 1988 by L.L. Mayfield in ASTM document STP 1064Carbonate Additions to Cement: "There are strong arguments on both sides of the question. The proponents claim a significant savings of energy during production without a degradation in quality and even cite improvements in some cement and concrete characteristics. The opponents charge that limestone merely acts as an adulterant, that strengths are reduced, and that the proposal should be abandoned on ethical grounds."
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Question: Type 1 PLC is being advertised as a new cement intended to help reduce carbon in the environment. How new is this, and how has ASCC addressed concerns about PLC?
Answer: The cement industry in the US has been involved with substituting limestone for straight cement since long before the turn of the century.
The original idea was to allow up to 5 percent limestone maximum in ASTM C 150. ASCC Hotline Operator Emeritus Ward Malisch wrote an article titled "Limestone Additions to Portland Cement: Pros and Cons...Revised ASTM Proposal Still Concerns Some Producers" for the Aberdeen Group in 1998 that accurately predicted the finger-pointing, NDAs and potential lawsuits we are seeing on jobsites today.
Ward wrote "Although the National Ready Mixed Concrete Association hasn't taken a position on limestone additions, some individual producers have. Producers' major concern can be summarized in one statement: They will obtain no benefit from the revised cement specification, but will bear the brunt of customer complaints if the cements don't perform as claimed. The biggest worry is that strengths of cements containing limestone additions won't be equivalent to the currently available C 150 cements."
In addition to the 1998 Malisch article, ASCC has expended considerable effort addressing potential problems associated with so-called "green" concrete that features reduction in OPC and replacement with SCM materials including fly ash, slag, and silica fume (not limestone). In 2011,ASCC Technical Division writers Ward Malisch and Bruce Suprenant described issues associated with high SCM mixes including slow set time, slow strength gain and finishing problems ("Be Wary of "Lean, Green" Concrete...Lowering the amount of portland cement in concrete can raise a number of issues", Challenging the System, Concrete Contractor Magazine).
Recognizing these problems, ASCC collaborated with an industry advisory team including a concrete producer, a structural engineer, a concrete contractor and an admixture manufacturer in a study sponsored by the Charles Pankow Foundation titled "Assessing the Impacts of "Green" Concrete Mixtures on Building Construction."
Published in 2013, the ASCC/Pankow study reported results of comprehensive cylinder and core testing performed on several "green" concrete mixes. As it turns out, the issues studied for the Pankow report are almost identical to the issues with PLC being reported today in ASCC Hotline calls and industry seminars.
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Question: What are the typical problems with PLC being reported from the field by ASCC concrete contractors today?
Answer: According to our recent survey, suspected side effects of added limestone include increased water demand, slow set time (need for adding accelerator admixtures), low strength, crusting of top surface, more shrinkage cracks, more labor required to finish. Anecdotal reports of problems with sawcutting and adhesives not sticking are being vetted by the ASCC Technical Division. Longer-term issues with durability (e.g. wear resistance, polishing issues) are being investigated as well. To date, no issues with shotcrete applications were reported.
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Question: How do we know the PLC is not being placed and finished improperly by the concrete contractor?
Answer: We don't. It is entirely possible that some of the reported problems are actually due to contractor error. This is the major downside to the introduction of NDAs on projects where PLC performance is suspect. We won't ever know the test results, who caused what, what it cost, and who got stuck cutting the checks. Many of the reports come from contractors who have been placing the same slabs for years, making the likelihood of error low.
It is entirely possible the PLC needs to be cured a certain way, and that way has not been adequately specified in construction documents. We hear the industry has done a good deal of testing and preparing end users for the transition from OPC to PLC. But was that a full-scale 25,000 SF test pour representing a 3rd floor metal deck in the shade when the ambient air temperature is 40 degrees F?
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Question: How can ASCC concrete contractors protect themselves from cost impacts due to PLC being substituted for OPC?
Answer: More and more contractors are told "It's here to stay, get used to it" and "We know there’s a learning curve". There are some regions in the US where Type 1 PLC is not being used yet ("but it's coming", they say).
Sometimes it is easy to forget that one of our prime directives at ASCC is to be the best we can be in service to the Owner. If PLC is going to be mandated into a project, the Owner should be informed and prepared to carry an allowance to cover any repair and replacement costs associated with non-performance of the new material. If there is a "learning curve" that needs to be navigated, fair enough. But the Owner needs to be advised what preconstruction testing is required, how much time it is going to take, what test results constitute acceptance, and what the cost impacts are likely to be.
For example, guidance for ASCC contractors related to cost impacts caused by slow concrete set time is provided in ASCC Position Statement #15: Setting Time Expectations for Hard-Trowel Finishing. In many cases, today's concrete contractor has sufficient experience with fly ash properties to be able to fairly estimate probable costs at bid time. But forward-pricing may not be possible for new, untested mixes, schedule changes, or cement changes made after a project is underway.
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Question: How can PLC present a potential risk to the end user? According to the industry, PLC has been used in Europe for 25 years and comprehensive testing has been performed. What does a contractor need to know about the PLC material risks?
Answer: There are two issues with PLC on the manufacturing side that need to be considered. One, of course, is the quarry source of the limestone material. ASTM requires the limestone must meet certain chemical composition requirements before it can even be considered for use. The second issue arises in the grinding process. If the limestone is not ground to a certain fineness (a “Blaine number" for fineness, much higher than OPC), then all bets are off and the coarser-ground limestone will tend to act as an inert filler.
According to a recent seminar presentation at World of Concrete, the grinding process for limestone "slows production and uses energy." In some cases, we learned, increasing the Blaine number "can cut production by as much as 40 percent." Could some grinding facilities be inclined to skimp on the grinding at the expense of performance? Is every grinding facility technically able to produce the required fineness? As Malisch pointed out 25 years ago..."Cement manufacturers acknowledge that manufacturing cement with limestone additions must be done properly to produce strength equivalents but say that the steps needed to optimize the process are achievable." Translation: You can't skimp on the grind.
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Question
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How will concrete contractors be able to anticipate PLC issues in advance of placing a bid? Once a project starts, the chances of getting compensated for non-performing PLC are limited.
Mix designs are typically not prepared prior to bid. Preconstruction testing during a two-week bid period is another pipe dream. Testing could easily require at least 56 days. How does the contractor proactively prevent a "learning curve" from turning into a costly "burning curve?"
Answer
:
The best way to approach this is to ask your ready mix supplier if they are quoting concrete with Type 1L PLC additions. If they are, ask them for performance backup they propose to include with their mix design submittals, and how they intend to satisfy the ACI 301requirements for mix proportioning, prior break history, trial batch information and so on. Ask if they have been involved in projects involving suspect PLC. In many cases, the "new" PLC supplier will not have such backup. That is the time pre-bid RFI's (and bid qualifications) should be submitted to make the Owner aware there may be unexpected delays and testing costs associated with this "new" material that are beyond the contractor's control and unknown at bid time.
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Question: If PLC is indeed "here to stay", what can ASCC members do as the contractor part of this equation to help mitigate PLC transition issues on behalf of all project stakeholders?
Answer: The most important part right now for ASCC members is collection of data and accounts of problems currently being experienced in the field.
Please double check and make sure your ASCC survey has been completed and returned to the ASCC Technical Division.
As we learned in the World of Concrete seminar, the concrete industry is being squeezed by a "rushed learning curve." Much more testing is required at the industry level to make PLC ready for prime time. In addition, seminar attendees were cautioned there will likely be a need for extensive end-user testing and experimenting, including mockup testing at the project level as well. "Special cautions" were given to attendees to beware of significant changes to PLC mix performance incases of "high-spec floor finishes, other bleed water-sensitive applications, interactions with ash &admixtures, water demand, and colder weather." (ASCC members please include accounts of any preconstruction testing efforts in your survey responses.)
The question then becomes "Who is responsible for specifying, funding and testing the new PLC mixes?"
It appears that much of the research performed for the 2013 ASCC/Pankow study maybe able to be applied to the PLC transition issues.
Perhaps the cement industry will fund a similar collaborative study to help build on what we already know.
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In response to the growing focus on reducing embodied carbon in the construction industry, ASCC is in the process of forming a Sustainability Committee to assist contractors in understanding and managing the upshots contractors are experiencing due primarily to material alterations. ASCC is working collaboratively with the Portland Cement Association and the National Ready Mixed Concrete Association to ensure the reduction of the carbon footprint of the concrete industry.