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Use Portland-limestone cement (PLC) as part of an approved concrete mix design, not as a reason to adopt a universal shovel ratio. In North American specifications, PLC is generally identified as Type IL(xx) under the ASTM C595/AASHTO M 240 framework reviewed by the Federal Highway Administration (FHWA); the “xx” indicates the limestone percentage. Read the cement bag, delivery ticket and project specification before batching.
The cement designation does not by itself determine the amounts of water, sand, stone, air or admixture. Strength, exposure, aggregate properties, workability, placement method, temperature and the required curing regime all affect the proportions. For structural concrete, foundations, pavements or any consequential placement, use the sealed project mix design or the ready-mix supplier’s approved mixture.
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What “limestone cement” means on the job
Identify Portland-limestone cement correctly
“Limestone cement” is often used informally for Portland-limestone cement. FHWA’s 2023 Portland Limestone Cement technote describes it as Type IL(xx) in the ASTM C595/AASHTO M 240 context. The number in parentheses identifies the limestone content associated with the designation. The standards framework discussed by FHWA permits up to 15 percent limestone in applicable blended cements, but that ceiling is not proof that every Type IL product contains 15 percent or that a particular bag meets your project’s requirements.
Confirm all of the following before unloading or batching:
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- The cement type, such as Type IL, and any parenthetical percentage or suffix printed on the bag or ticket.
- The applicable ASTM, AASHTO, local standard or contract requirement.
- Any special properties required for the exposure, such as air entrainment, sulfate resistance or an early-strength requirement.
- Whether the supplied material is the same cement named in the approved mix submittal.
If the ticket, silo label and design do not agree, stop and resolve the discrepancy with the supplier, engineer or responsible concrete professional.
PLC is not a separate recipe
The available FHWA guidance explains how cementitious-material properties, mixture proportions, setting, bleeding, strength gain and climate influence concrete behavior, but it does not prescribe one cement-sand-aggregate-water ratio for all PLC concrete. A ratio that works for one quarry, strength class or placement can produce unacceptable concrete elsewhere.
| Design decision | Why it changes the mixture |
|---|---|
| Specified compressive strength and age | Controls the cementitious content, water-to-cementitious-material ratio and sometimes the need for accelerating or water-reducing admixtures. |
| Exposure and durability | May impose limits on water, air content, permeability, sulfate resistance or other performance requirements. |
| Aggregate size, grading and moisture | Changes paste demand, workability and the amount of batch water needed after correcting for aggregate moisture. |
| Placement and finishing method | Determines the required slump, pumpability, set behavior and tolerance for bleeding. |
| Weather and curing conditions | Influences evaporation, setting, finishing time and the method needed to retain moisture. |
| Admixtures | Can adjust slump, air, set or water demand without simply adding uncontrolled water. |
Plan the placement before batching
Obtain the right mix design
Tell the ready-mix producer or designer the intended strength, exposure class, placement date, required slump or flow, air requirement, aggregate size, delivery method and finishing constraints. The approved design should state quantities per batch or cubic yard (or cubic metre), the allowable water and admixture ranges, and acceptance criteria. Do not translate the phrase “Type IL” into a fixed water ratio.
For a small nonstructural repair, use the manufacturer’s or supplier’s packaged-concrete instructions rather than substituting a homemade PLC ratio. For a footing, wall, slab-on-grade carrying significant loads, pavement or structural element, follow the project documents and engineer’s instructions.
Check materials and equipment
- Keep cement dry and protected from contamination or moisture intrusion.
- Verify aggregate stockpiles are separated, clean and represented by the moisture data used in the design.
- Check that the mixer, truck, pump, forms and finishing tools are ready before concrete arrives.
- Have the specified curing materials on site before discharge: water-spray or fogging equipment, wet fabric, sheets or an accepted curing compound, as applicable.
- Plan who will record ticket numbers, batch adjustments, weather observations, slump, air and test specimens.
Account for weather
FHWA advises anticipating curing needs from the cementitious materials, mixture proportions, expected setting and strength gain, bleeding behavior and the climate during placement and the following days. A hot, dry, windy placement can lose surface moisture rapidly; a cool placement may set slowly and require a different protection plan. Do not wait until the concrete is already drying to search for curing supplies.
How to mix PLC concrete
1. Use the approved quantities and correct aggregate moisture
Batch cementitious material, aggregates, water and admixtures by the approved design. Aggregate stockpiles contain either absorbed moisture or free surface water. The batch water must be adjusted using the supplier’s current moisture information so that the total effective water remains within the design limit. A wet stockpile can contribute substantial water; a dry aggregate can remove water from the paste and reduce workability.
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Keep cementitious-material quantities and water records with the batch ticket. If the design calls for supplementary cementitious materials, use only the stated material and dosage; do not treat limestone in PLC as a substitute for an independently specified addition.
2. Follow the producer’s batching sequence
For ready-mixed concrete, the producer controls the plant sequence and mixer operation. At a jobsite mixer, follow the equipment and mix-design instructions for loading aggregates, cement, water and admixtures. The objective is a uniform mixture without dry pockets, cement balls or segregated stone. Do not alter the sequence simply because the cement contains limestone.
Admixtures must be measured and introduced as directed by their supplier and the mix design. A water-reducing admixture can be an appropriate way to obtain the required slump while preserving the designed water limit, but its dosage and compatibility must be established for the mixture.
3. Check concrete before making an adjustment
Use the delivery ticket and the specified field tests to determine whether the load meets the required slump, air and temperature limits. Concrete that appears stiff is not automatically deficient: slump, air, temperature, elapsed time and the project’s acceptance criteria determine whether an adjustment is permitted.
4. Control any permitted jobsite water addition
NRMCA’s summary of ASTM C94 permits adding water when the measured slump is below the specified requirement and the total water based on the mix design will not be exceeded. The addition must be documented. This is a controlled correction, not permission to add water by eye.
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- Obtain authorization from the person responsible for the concrete and calculate the amount allowed by the design.
- Add only the permitted quantity, or use the approved water-reducing admixture when that is the specified adjustment.
- Record the amount, time and person authorizing the change on the ticket or placement record.
- After adding water or admixture, mix for at least 30 revolutions at mixing speed so the batch becomes homogeneous, as summarized by NRMCA from ASTM C94.
If the load would exceed its designed water limit, do not try to rescue it with extra water. Contact the supplier or responsible concrete professional for the disposition of the load.
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5. Confirm uniformity and discharge promptly
After the required mixing, look for consistent color, texture and aggregate distribution. Discharge in a way that limits segregation and avoids unnecessary rehandling. Keep the crew, forms and curing operation ready so finishing does not delay moisture protection.
Place and finish without creating a curing problem
PLC does not eliminate ordinary concrete risks. Excessive vibration can encourage segregation; adding water to make finishing easier can weaken the surface; and finishing while bleed water is present can trap water near the surface or create a weak layer. Use the specified placement and finishing practice, and coordinate the finishing crew with the curing crew.
Exposed slabs and pavements deserve particular attention because their large surface area exchanges moisture rapidly and experiences temperature cycling. FHWA notes that, in high-evaporation conditions, protection may be needed before concrete has fully set. Windbreaks, sunshades, fogging or other approved measures may be needed to reduce evaporation while the concrete is still plastic. These measures do not replace the final curing system.
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How to cure concrete made with PLC
What curing accomplishes
Curing manages moisture and temperature so cement hydration can continue and the concrete can develop the intended strength and durability. The required approach depends on the cementitious materials, water-to-cementitious-material ratio, section size, setting time, bleeding, ambient conditions and the contract’s acceptance requirement. A project may specify a calendar period, a strength threshold or both.
There is no single PLC-specific wet-curing duration that applies to every footing, wall, slab or pavement. Follow the current project specification, governing standard and supplier or engineer guidance. FHWA’s broad curing references are useful for method selection, but several of those reports focus on pavements and should not be treated as a universal code requirement for every structure.
Choose a moisture-preserving method
| Method | How it works | Planning and compatibility issues |
|---|---|---|
| Ponding | Maintains a layer of water over a horizontal surface. | Needs a watertight boundary, reliable water supply and a surface strong enough to avoid damage. Temperature differences between water and concrete must be considered. |
| Fogging or sprinkling | Adds moisture to the surface and surrounding air. | Requires continuous attention and enough capacity to prevent the surface from drying between applications; useful where ponding is impractical. |
| Wet mats or fabric | Keeps a saturated covering in contact with the concrete. | Must be installed without marring the surface and kept continuously wet. Gaps, wind displacement or drying fabric interrupt curing. |
| Sheet materials | Retain moisture already in the concrete by covering the surface. | Edges, joints and penetrations must be sealed or secured. Sheets need protection from wind and must be compatible with the finished appearance and specification. |
| Liquid membrane-forming compound | Forms a film that reduces moisture loss. | Product type, coverage, timing and acceptance matter. Some compounds can act as bond breakers for later overlays, coatings or toppings. |
FHWA distinguishes methods that add water from those that retain moisture already present. Select the method the contract accepts and that the crew can maintain continuously under the actual weather conditions.
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Apply curing compound at the correct time
If a membrane-forming compound is specified, follow its product data and the project instructions rather than an assumed universal application rate. The FHWA curing appendix says application should occur after finishing is complete and the surface water sheen has disappeared. It cautions against applying compound while bleeding is occurring: a premature film can interfere with bleed-water escape and damage the surface.
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Keep the curing system intact
Inspect the surface and covering during the required curing period. Re-wet fabric before it dries, repair displaced or torn sheets, maintain ponding depth and protect compound films from traffic or damage. If the curing system fails, notify the responsible concrete professional promptly; do not assume that a later splash of water restores the lost performance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Weather-specific decisions
Hot, dry or windy conditions
- Schedule placement to reduce the most severe evaporation period when the project permits.
- Stage fogging, wind protection, shade or other approved evaporation controls before discharge.
- Begin the specified curing operation as soon as finishing and surface conditions allow.
- Watch for rapid sheen loss, plastic-shrinkage cracking or edges drying ahead of the middle of the slab.
Cool conditions
Cool weather can delay setting and strength gain. Protect the placement from damaging temperature changes and use only the accelerators, insulation or heating procedures allowed by the design and specification. Do not apply a curing compound over a surface that is still bleeding simply because the air temperature is low.
Rain during placement or early curing
Protect fresh concrete from washout and surface damage. Do not finish rainwater into the surface or add water to compensate for a wet top layer. Once the surface can accept the specified curing method, restore the planned moisture protection and document any interruption.
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The concrete is too stiff at delivery
First verify the measured slump and the ticket. If the slump is below the requirement, the design water limit has not been exceeded and the responsible person authorizes it, apply only the documented adjustment and complete the required 30 mixer revolutions at mixing speed. Otherwise, consult the supplier about an approved water-reducing admixture or the disposition of the load.
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The surface dries before finishing or curing
Reduce evaporation with the approved pre-set protection, coordinate finishing and curing crews, and begin the curing method as soon as the surface condition permits. A dry surface is a signal to investigate weather, timing, bleeding and protection; it is not evidence that more batch water should have been used.
Bleed water remains when the crew wants to apply compound
Wait until finishing is complete and the water sheen has disappeared, as described in FHWA’s guidance. Applying a membrane while bleeding continues can trap water or weaken the surface.
A bonded topping or coating is planned later
Check the curing product’s compatibility before placement. If the compound can inhibit adhesion, select the curing method or surface-preparation procedure accepted by the project specification.
The cement on the ticket does not match the submittal
Hold the load and contact the supplier, engineer or responsible concrete professional. Type IL(xx) identifies a cement category, but the exact designation, suffixes, performance requirements and approved source still matter.
What PLC changes environmentally—and what it does not prove
Reducing clinker-related emissions is the principal reason for moving from ordinary Portland cement (OPC) to PLC. FHWA’s 2023 technote states: “The main motivator for switching from OPC to PLC is to reduce CO2 emissions from the cement and concrete industries.”
FHWA reports that cement production accounted for 0.86 percent of United States CO₂ emissions in 2020. It also reports an average 8.3 percent global-warming-potential reduction for PLC compared with OPC, calculated from industry-average environmental product declarations. That is an EPD-based average, not a guaranteed saving for every project; plant energy, transport, raw materials, mixture proportions and the declared product determine the actual result.
Use statistics with their dates and scope. FHWA reported approximately 890,000 metric tons of PLC produced in the United States in 2016, based on a Portland Cement Association survey. It also cited a U.S. Geological Survey estimate of 24 million metric tons of blended cements imported into or manufactured in the United States from January through November 2022, with the vast majority expected to be PLC. The latter figure is for blended cements overall, not PLC-only production.
Quick Recap
A practical decision sequence
- Identify the cement: verify Type IL(xx), the applicable standard and the approved source.
- Confirm the mixture: use the project-specific quantities and limits for strength, exposure, slump, air, aggregates, admixtures and water.
- Prepare for conditions: correct aggregate moisture, inspect equipment and stage the selected curing materials before the truck or mixer arrives.
- Control adjustments: measure slump; add water only when the specified condition and water limit permit it; document every addition and mix at least 30 revolutions afterward.
- Protect the concrete: manage evaporation during placement and install the accepted curing method at the correct surface condition.
- Maintain and verify: keep coverings wet or intact, protect membrane films, and follow the project’s time- or strength-based curing requirement.
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