understanding and preventing incompatibility of concrete...

V. T. Cost Consulting, LLC 1V. T. Cost Consulting, LLC

Understanding and Preventing Incompatibility of Concrete Mix Components

1

69th Annual Concrete ConferenceDecember 5, 2019

Earle Brown Heritage Center,Brooklyn Center, MN

Tim Cost, PE, FACIV. T. Cost Consulting, LLC

[email protected]


Why would concrete mix components be “incompatible”?

Well,… It’s not that they’re inherently “incompatible” It has more to do with problems caused by mix

proportions and certain materials properties Causes various performance issues, some severe

Troubleshooting is complicated by: Multiple chemical admixtures & new formulations Changing SCM properties, sources, types Certain cement properties New materials sources Normal variability of materials properties

Introduction

V. T. Cost Consulting, LLC 3

“Incompatibility” defined

Mild to extreme deviations from normal concrete rheology, setting, and/or strength development that result from abnormal early chemistry interactions (abnormal hydration)

Why would concrete mix components be “incompatible”?Introduction


So what happens with “abnormal hydration”?

Possible unexpected performance trends: Increased slump loss not due to hot weather Extended (delayed) setting on a hot day Severely abnormal delayed set (24 to 48 hours) No measurable strength in 24+ hours Longer set after increasing NC accelerator dosage Dramatic set changes after slight adjustments of

admixture dosage or SCM replacement rate (Extreme cases) flash set, even after extremely

delayed set on previous loads

These issues can occur when no individual component material is out of spec!

Introduction


Often first experienced in the field with a new mix design, but may happen mid-project

A normally performing mix may be “on the edge” for sudden (even severe) abnormal behavior

May result from: A new supply of a material with a slight property change

(normal variability) A slight temperature change A routine deviation in batching or re-tempering A seemingly routine adjustment to an admixture dose,

well within manufacturer’s recommendations Incompatibility has often occurred intermittently

without diagnosis or resolution

Some incompatibility caveatsIntroduction


Topics for discussion

The causes of incompatibility Chemistry background (simplified) Materials that are most commonly involved,

and why What can be done when it happens Testing for, predicting incompatibility danger Routine precautions, related QC checks


So what does cause incompatible behavior?

It’s all about the proportions of otherwise normal, in-spec materials that suffer from insufficient calcium sulfate during initial hydration

Calcium sulfate in the concrete mixture comes from the gypsum in portland or blended cement

When there is starvation of dissolved calcium sulfate prior to set, the normal chemistry of setting and strength gain is interrupted

Hang with me…


Next:

A brief cement and hydration chemistry review…


Gypsum (CaSO4) is added in grinding at the cement plant

Portland cement clinker is produced in a rotary kiln (at left), then cooled, stored, and ultimately introduced into the grinding mill (below) along with some gypsum for set control.


C3S = 3CaO • SiO2(Tricalcium silicate)

C2S = 2CaO • SiO2(Dicalcium silicate)

C3A = 3CaO • Al2O3(Tricalcium aluminate)

C4AF = 4CaO •Al2O3 • Fe2O3(Tetracalcium aluminoferrite)

Compounds in portland cement clinker:

Cement chemistry review


C3S = 3CaO • SiO2(Tricalcium silicate)

C2S = 2CaO • SiO2(Dicalcium silicate)

C3A = 3CaO • Al2O3(Tricalcium aluminate)

C4AF = 4CaO •Al2O3 • Fe2O3(Tetracalcium aluminoferrite)

Hydration of these compounds produces significant heat during the first 24 hours

Compounds in portland cement clinker

Cement chemistry review


Some nomenclature and conventions

C3S = tricalcium silicate (3CaO · SiO2) or silicate

C3A = tricalcium aluminate (3CaO · Al2O3) or aluminate

CaSO4 = calcium sulfate or sulfate (from gypsum)

SO3 level = lab analysis result used as an indicator of CaSO4 level in portland or blended cement

SCMs = supplementary cementitious materials (fly ash, slag cement, natural pozzolans, etc.)


What does the gypsum (CaSO4) do?

CaSO4 regulates setting of cement – without it, there would be a flash set from uncontrolled C3A hydration

The chemical interactions of dissolved CaSO4 serve to interrupt C3A hydration long enough for C3S to begin hydrating

C3S hydration results in set and early strength gain

If dissolved CaSO4 is depleted and C3A is allowed to resume hydration prior to set, C3S hydration is interrupted until enough additional CaSO4 can go into solution and again interrupt C3A hydration


What does the gypsum (CaSO4) do?

This is easier to understand and study when we document and track the rate of hydration of these compounds during initial hours after mixing

The easiest way to do this is track the heat release associated with these chemical reactions


A “thermal profile” – a plot of the changing temperature of a hydrating sample during initial hours after mixing – serves as an indication of C3A and C3S hydration activity and the interaction of CaSO4 (sulfate).

AB

C

A – initial aluminates(C3A) hydration

B – dormancy periodC – main peak (C3S)

hydration

AB

C

A – initial aluminates(C3A) hydration

B – dormancy periodC – main peak (C3S)

hydration

Approximate timing of initial set of concrete

Normal hydration and heat release


Good aluminate control, normal C3S hydration

Poor aluminate control, abnormal C3S hydration

CaSO4 starvation, little aluminate control, interrupted C3S hydration

Three mixtures with incrementally higher admixture dosages

As less CaSO4 is available in the early solution, C3A is less controlled and C3S hydration delayed and/or attenuated.

Progressively abnormal hydration


So how does this happen if all materials are in spec?

Cement is the only source of soluble CaSO4 in concrete

The CaSO4 content of cement is usually optimized for basic mortar cube strength (no SCMS or admixtures)

Chemical admixtures and SCMs also interact with CaSO4 Some more than others Even slight changes in dosage or cement replacement rates

can have a big impact

Likelihood increases with mix complexity

Some cements are more susceptible than others… Most cements actually contain more CaSO4 than minimally

necessary, but some, not – and this varies widely


These materials can and do make good concrete every day, but can also drive incompatible performance under certain conditions (NOT A COMPLETE LIST, and some incompatible influences are possible from almost any admixture or SCM):

So, what materials / conditions are usually involved?

Calcium nitrate and calcium nitrite admixtures (non-chloride accelerators and corrosion inhibitors)

Certain carbohydrate-based (lignin or corn syrup, etc.) Type A/D or A/B/D water-reducers, especially those containing triethanolamine (TEA) Especially at higher dosages

Class C fly ash, especially if derived from Powder River Basin or similar sub-bituminous coal, high in C3A To a lesser degree, some slag cements and some C/F ashes

Hot weather / higher materials temps Cements that are “undersulfated” (not easily identified)


from - Gypsum and Anhydrite in Portland Cement, 3rd edition, US Gypsum Company, 1988, p 37.

Sulfate solubility influences: form & temperature

Different sulfate forms (hydration states) differ in solubility

Sulfate solubility decreases with higher temperature Especially the most soluble

form (hemihydrate, or plaster)

Higher temps also increase aluminate hydration rates

So MORE soluble sulfate is needed as temps increase!


Other “sulfate demand” influences

Mix designs with higher SCM replacement of cement (sustainable mixes, etc.) result in less total sulfates (due to less cement) for aluminate control Can also bring more C3A to the mix, esp. C ash

Some admixtures may chemically interact with sulfates or affect their availability

Normal and common variability in materials Even slight changes can push an “on the edge” mix into

incompatibility


It’s not really that simple…

So why are some cements “more susceptible”?And why wouldn’t there be enough gypsum in my cement?

The form and solubility of CaSO4 in gypsum sources and finished cement vary

Optimization of SO3 level is not an exact science, and there are specification limits and caveats, but has traditionally has been done via mortar cube-based optimization (without admixtures or SCMs) Too much SO3 can cause deleterious expansion

Complicating matters: you can’t tell how “ample” the SO3 level is by the test certificate result! Because of varying SO3 forms and amounts of insoluble sulfate

present in clinker


Considerations in the selection of SO3 plant targets

Both gypsum sources and grinding mill temps impact actual sulfate forms in cement If less soluble forms predominate, a higher SO3 may be needed

Sulfate optimization and setting of targets should rely on results of performance testing Some plants now do testing with admixtures

There is little downside to targeting SO3 a bit higher than the traditional “optimum” as long as C1038 expansions are within limits (Neat) mortar or concrete strength is not very sensitive to SO3 Performance with admixtures and SCMs may even be improved


So how do I tell if a cement may be “undersulfated”?

Performance testing Some imported cements have had issues Check reported SO3 level and check against

“soft” limits (3.0% when C3A ≤ 8%, etc.) Levels considerably below limits may be an issue

Plants that always report SO3 for their products well below the “soft” limits may bear watching… Doesn’t mean there’s definitely a problem, just something

to watch!


Mild cases: Increased slump loss

and water demand Admixtures are less

effective Set time delays Poor early strengths

Severe cases: No set for days or… flash set (!) No measurable

strength gain for several days

What happens to concrete when sulfate is depleted?Example illustration – effects of slight changes in proportions or temps, no materials changes

Baseline mix: 20% ash, 5oz/cwt WR, 90°F


For an “on the edge” mixture, even a slight change in mix temperature or in the properties of any one material can quickly cause extreme performance issues

Thus, compatibility issues can come and go mid-project

What happens to concrete when sulfate is depleted?

“Threshold” zone is quite narrow!

Example illustration – effects of slight changes in proportions or temps, no materials changes

Baseline mix: 20% ash, 5oz/cwt WR, 90°F


An incompatibility case history (2003)

R/M producer: concrete from multiple plants not setting Summer temps 25% Class C fly ash Type A/D admix, dosage recently increased to max In some cases, no set over night Essentially no strength for 2 or more days

Testing of grab samples shows no materials issues Problem mostly goes away the next day Investigation – mortar cube experiments

Can re-create issue in the lab at higher temps Reducing admix or fly ash remedies the problem Lower temps remedies the problem Higher cement SO3 content remedies the problem


Mortar cube testing comparing influences of cements from 11 different plants simulating the problem mix design 25% Class C fly ash from project Type A/D water reducer from project @ 6 oz/cwt (upper end dose) Mix and cure temps 90° F to approximate field temps

An incompatibility case history (2003)


Recap / review Incompatible behavior may include slump loss, severely

extended set, unexpected admixture affects, very poor early strengths, even flash set (rare)

Can result with no out-of-spec materials Caused by early starvation of sulfates (gypsum) Contributing conditions & materials:

Certain admixtures, esp. at higher dosages High replacement of cement with certain SCMs Hot weather / materials temps Relatively under-sulfated cements

Sudden, even severe abnormal behavior is possible with small changes in conditions if the mix is “on the edge”

Similar cements from different plants may have different performance in the same mix design, no other changes


So what should I do when this happens?

Try reducing WR or NCA dosage or try a different admixture

Try cutting SCM replacement rate or taking the SCM out altogether (100% cement mix)

Chill mix with ice Change batching sequence so as to add

chemical admixtures with tail water Try a different cement or work with cement

producer to increase cement SO3, if possible Doesn’t help immediately, but…

Mid-project:


How to pro-actively prevent?

There is no alternative – prediction of incompatibility based on test certificate-reported properties of materials is NOT possible!

Performance testing in the lab!


How to pro-actively prevent?

Laboratory performance testing of proposed materials and proportions Concrete batches can be done, but… Mortar or paste mixtures are simpler Use aggressive “reference” mixtures to evaluate new

materials or for routine checks Verify new mix designs (with or without aggregates)

Include sensitivity testing: mixtures done with exaggerated conditions Materials temps higher than expected in the project Over-dosed admixtures Higher SCM % If problems experienced, modify mix design!

Pre-project and for routine checks:


Laboratory performance testing methods Traditional laboratory concrete batches Mortar testing via modified C109 or C1810 C191 Vicat setting times Mini-slump paste mixtures Isothermal calorimetry of paste Thermal profile testing, paste or mortar


Thermal profile testing aka “semi-adiabatic calorimetry”


WR @ 3.75 fl oz/100 lb(245 ml/100 kg)

WR @ 5.25 fl oz/100 lb(340 ml/100 kg)

WR @ 7.0 fl oz/100 lb(455 ml/100 kg)

WR @ 2.25 fl oz/100 lb (145 ml/100 kg) + RET @ 2.25 fl oz/100 lb (145 ml/100 kg)

WR @ 3.75 fl oz/100 lb(245 ml/100 kg)

WR @ 5.25 fl oz/100 lb(340 ml/100 kg)

WR @ 7.0 fl oz/100 lb(455 ml/100 kg)

WR @ 2.25 fl oz/100 lb (145 ml/100 kg) + RET @ 2.25 fl oz/100 lb (145 ml/100 kg)

Normal hydration –Series of otherwise identical mixtures comparing admixtures and dosages for retardation effects, robustness of hydration

Abnormal hydration –Series of mixtures with varying admix dosage or fly ash replacement driving varying stages of incompatibility (sulfate depletion) effects, with corresponding 1-day strengths

Profiles of both normal and abnormal hydration are useful for evaluation of materials and proportions


Simple testing for evaluation of cementitious mixtures or component materials (C1753)

Using mix temperatures plotted vs. time during early hydration

Various applications for evaluating setting, hydration efficiency, incompatibility, and mix optimization


The Appendix is 12 of the 18 pages, with examples and recommendations for many applications.

C1753 – one of very few ASTM standards with extensive guidance on applications & examples


Equipment variations, manufactured and adapted


Paste mixtures for incompatibility evaluation

Paste batching with two technicians: A batch every 4 to 6 minutes 48 or more batches in a morning


General recommendations for a mix series All mixes – same procedures, same conditions Pre-weigh all cementitious combinations Use a mix timer (60 sec. works for paste) Assure consistent thermal sensor position Begin data collection ASAP Control curing environment to initial mix temps

Temp-controlled cylinder curing tanks work well Avoid any air movement around samples, HVAC

vents, areas near equipment cooling fans, etc. Include a reference channel – a dedicated channel

with an inert sample (sand & water – similar total mass), as a record of test ambient temperatures


Strength data from each mixture is also useful

Hardened test specimens can be tested for strength after thermal profile testing Extra specimens can also be

tested at different ages

Essentially via ASTM C39o Neoprene capso Sulfur compoundo Machined ends without caps

Parallel mortar cubes –another option

Strength bar charts presented with thermal profiles may be helpful with data interpretation


Profile shapes provide indications of the extent of sulfate imbalance, recurring aluminate activity, interruptions of normal hydration

5 paste mixtures w/ 25% C ash, incremental WRA dosage

Thermal profile shapes relating to incompatibility


1-day strength

1-day strength

Examples

Effects of incremental fly ash replacement

Effects of C and F ash compared in otherwise identical mixtures using a type A/D WR admix known to have high sulfate-demand Single sample of Type I/II

cement w/cm = 0.40 upper-limit dosage of

admixture 32°C (90°F) mix and cure

temps


Examples

Effects of NCA dosage in high volume (50%) ash mixes

Mixtures with incremental dosages of non-chloride accelerator (NCA) in pursuit of a target set performance

Moderate dosages result in acceptable set for F ash

NCA less effective with C ash, slight incompatibility suspected at higher dosages

Development of a high-sustainability mix design


Examples

Verification of incompatibility using sulfate additions

The affected mixture with C ash and 30 oz/cwt NCA and incremental CaSO4 additions, using Terra Alba gypsum (PoP would also work)

Profile shapes and 1-day strengths improve with additions Confirms sulfate balance (incompatibility) issues


Otherwise identical paste mixtures, aggressive mix with 25% Class C fly ash, upper-limit dose of Type A/D WR, and 35°C (95°F) mix and cure temps

Examples

Comparing 7 cements for sulfates adequacy


A1, SO3= 3.08

A5, SO3= 3.06

B, (control) optimized SO3

A3, SO3= 3.21

A4, SO3= 3.28

A2, SO3= 3.16

Cement source “A” is being evaluated.

Cement sulfates evaluation: 0.40 w/cm mixtures @ 95° F, 25% C ash, 4 oz/cwt WR, cement grab samples @ various SO3 levels

Examples


A1, SO3= 3.08

A5, SO3= 3.06

A3, SO3= 3.21

A4, SO3= 3.28

A2, SO3= 3.16 B, (control) optimized SO3

Cement sulfates evaluation: admixture sensitivity comparison, same mixtures with WR admix dosage increased to 8 oz/cwt

Examples

Conclusion: When SO3 approaches the lower range of normal variability, cement source “A” may be under-sulfated for such aggressive mixes


A) 100% cement, no WR, w/cm = 0.45B) 25% C ash, no WR, w/cm = 0.45C) 25% C ash, 4 oz/cwt WR, w/cm = 0.40D) 25% C ash, 6 oz/cwt WR, w/cm = 0.40

Higher temps alone drive incompatible behavior in mixtures with WRA

Examples

Temperature influences on incompatibility potential

4 paste mixtures compared at different temperatures: 70°F (21°C) vs. 93°F (34°C)


Evaluation & resolution using lab testing

Confirm that issue is related to sulfates Incremental sulfate demand approach (overdose

admixtures, increase SAC %, higher mix temps) Incremental sulfate supply approach (different cement

samples at varied SO3 or sulfate additions to mixtures)

Change one or more of the key influences: Replacement rate of Class C fly ash Admixture dosage or type Review / evaluate retardation strategy Cement SO3 level Mix temperatures

Re-evaluate at the most extreme mix & cure temps envisioned in the field

In summary…


Other recommendations – evaluation and QC

Evaluate unfamiliar materials and proposed mixes under highest envisioned project temperatures Compare against controls (familiar materials) Check setting and main peak variability with temperature

Check materials sensitivities to incompatibility Include overdoses of admixtures and SCM’s Compare against familiar mixtures and materials

Test new mix designs and repeat whenever any materials sources are changed

Repeated testing can be used to check materials variability as supplies are replenished

In summary…


• Bauset, R., “Abnormally Delayed Setting of a Low-Heat Portland Cement with Calcium Lignosulfonate Admixtures,” 5th International Symposium on Cement Chemistry, Vol. IV, 1968, p 53-57.

• Cost, V. T., “Incompatibility of Common Concrete Materials – Influential Factors, Effects, and Prevention,” National Concrete Bridge Conference 2006, National Concrete Bridge Council, Skokie, IL, 2006.

• Cost, V. T., and Knight, G., “Use of Thermal Measurements to Detect Potential Incompatibilities of Common Concrete Materials,” Concrete Heat Development: Monitoring, Prediction, and Management, ACI SP-241-4, Atlanta, GA, April 2007, 39-58.

• Cost, V. T., and Gardiner, A., “Practical Concrete Mixture Evaluation via Semi-Adiabatic Calorimetry,” 2009 Concrete Technology Forum – Focus on Performance Prediction, Cincinnati, OH, May 2009.

• Cost, Tim, “Thermal Measurements of Hydrating Concrete Mixtures – A Useful Quality Control Tool for Concrete Producers,” NRMCA Publication 2PE004, National Ready Mixed Concrete Association, 900 Spring Street, Silver Spring, MD, August 2009.

• Cost, V. T., “Concrete Sustainability versus Constructability – Closing the Gap,” 2011 International Concrete Sustainability Conference, Boston, MA, August, 2011.

• Cost, Tim, “Optimization of Concrete Paving Mixtures for Sustainability and Performance,” accepted for the 10thInternational Conference on Concrete Pavements, Quebec City, Quebec, July 8-12, 2012

• Helmuth, R., Hills, L. M., Whiting, D. A., and Bhattacharja, S., Abnormal Concrete Performance in the Presence of Admixtures, Portland Cement Association No. RP333, Skokie, IL, 1995, p 1, 3, 8-9, 11-12, 14, 17-18.

• Hills, L., and Tang, F., “Manufacturing Solutions for Concrete Performance,” IEEE-IAS/PCA Cement Industry Technical Conference, Chattanooga, TN, 2004, p1-6.

• Khalil, S. M., and Ward, M. A., “Influence of SO3 and C3A on the Early Reaction Rates of Portland Cement in the Presence of Calcium lignosulfonate,” CeramicBulletin, Vol. 57, No. 12, 1978, p 1116-1122.

Suggested reading, papers on incompatibility


• Roberts, L. R., and Taylor, P. C., “Understanding Cement-SCM-Admixture Interaction Issues,” Concrete International, V. 29, No. 1, January 2007, pp. 33-41.

• Sandberg, P., and Liberman, S., “Monitoring and Evaluation of Cement Hydration by Semi-Adiabatic Field Calorimetry,” Concrete Heat Development: Monitoring, Prediction, and Management, ACI SP-241-2, Atlanta, GA, April 2007, pp. 13-24.

• Sandberg, P., and Roberts, L. R., “Studies of Cement-Admixture Interactions Related to Aluminate Hydration Control by Isothermal Calorimetry, Seventh CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete, Berlin, 2003, 12 pp.

• Sandberg, P. J., and Roberts, L. R., “Cement-Admixture Interactions Related to Aluminate Control,” Journal of ASTM International, V. 2, No. 6, June 2005, pp. 219-232.

• Tuthill, L., Adams, R., Bailey, S., and Smith, R., “A Case of Abnormally Slow Hardening Concrete for Tunnel Lining,” Proceedings, American Concrete Institute,Vol. 57, March, 1961, p 1091-1109.

• Wang, H., Qi, C., Farzam, H., and Turici, J., “Interaction of Materials Used in Concrete,” Concrete International, V. 28, No. 4, April 2006, pp.47-52.

• Wang, K., Ge, Z., Grove, J., Ruiz, M., Rasmussen, R., and Ferragut, T., Developing a Simple and Rapid Test for Monitoring the Heat Evolution of Concrete Mixtures for Both Laboratory and Field Applications, National Concrete Pavement Technology Center, Ames, IA, January 2007, 58 pp.

Suggested reading, papers on incompatibility, cont.


• Cost, V. T., and Gardiner, A., “Practical Concrete Mixture Evaluation via Semi-Adiabatic Calorimetry,” 2009 Concrete Technology Forum – Focus on Perfor¬mance Prediction, Cincinnati, OH, May 2009, 21 pp.

• Cost, Tim, “Thermal Measurements of Hydrating Concrete Mixtures – A Useful Quality Control Tool for Concrete Producers,” NRMCA Publication 2PE004, National Ready Mixed Concrete Association, 900 Spring Street, Silver Spring, MD, August 2009.

• Cost, V. T., “Concrete Sustainability versus Constructability – Closing the Gap,” 2011 International Concrete Sustainability Conference, Boston, MA, August, 2011.

• Cost, Tim, “Preliminary Optimization of Concrete Paving Mixtures for Sustainability and Performance,” 10th International Conference on Concrete Pavements, Quebec City, Quebec, July 8-12, 2012, 11 pp.

• Sullivan, G., Cost, T., and Howard, I., “Measurement of Cementitiously Stabilized Soil Slurry Thermal Profiles,” Geo-Congress 2012 – State of the Art and Practice in Geotechnical Engineering, American Society of Civil Engineers, Oakland, CA, March 25-29, 2012.

• Cost, V. T., Howard, I. L., and Shannon, J., “Improving Concrete Sustainability and Performance with Use of Portland-Limestone Cement Synergies,” Transportation Research Record: Journal of the Transportation Research Board, No. 2342, Washington, D.C., 2013, pp 26-34.

• Howard, I.L., Cost, T. (2014). “Curing Temperature Effects on Soils Stabilized With Portland Cement Having Different Sulfate Contents,”Proc. of GeoCongress 2014 (GSP 234), Feb 23-26, Atlanta, GA, pp. 2159-2168.

• Cost, V. T., Matschei, T., Shannon, J., and Howard, I. L., “Extending the Use of Fly Ash and Slag Cement in Concrete Through the Use of Portland-Limestone Cement,” 2014 International Concrete Sustainability Conference, Boston, MA, May 12-14, 2014, 15 pp.

• Howard, I.L., Sullivan, W.G., Anderson, B.K., Shannon, J., Cost, T. (2013). Design and Construction Control Guidance for Chemically Stabilized Pavement Base Layers. Report FHWA/MS-DOT-RD-13-206, Mississippi Department of Transportation, pp. 162.

Other papers featuring thermal profile testing


Predicting incompatible behavior of concrete requires testing!

At the end of the day…

V. T. Cost Consulting, LLC 55V. T. Cost Consulting, LLC 55

69th Annual Concrete ConferenceDecember 5, 2019

Earle Brown Heritage Center,Brooklyn Center, MN

Tim Cost, PE, FACIV. T. Cost Consulting, LLC

[email protected]

Questions?

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understanding and preventing incompatibility of concrete...

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