Internal Curing FAQs
What is internal curing (IC)?
ACI defines internal curing as “a process by which the hydration of cement [cementitious] continues because of the availability of internal water that is not part of the mixing water." In general conversation, IC is often referred to as “curing concrete from the inside out.” IC is a practical way of providing additional curing throughout the concrete section.
Why use IC?
It makes concrete more robust resulting in a longer service life.
Is IC new technology?
No. The IC technology dates back to the Roman period and was identified and written about in 1952. With new laboratory equipment and extensive research, we now have a more complete understanding of how internal curing works and a way to design for IC in conventional concrete.
What is different between conventional and internal curing concrete mixtures?
In an IC mixture, a small portion of the normal weight concrete sand is replaced with a prewetted fine ESCS lightweight aggregate by volume.
How much research has been done?
Over 150 documented studies using ESCS for internal curing of concrete have been published, showing how IC makes all concrete better. Visit ESCSI's online Technical Docs and Reports & Papers sections for IC research.
What applications can IC be used in?
IC can be used in almost all residential, commercial, industrial, pavement, bridges, concrete repair patches and other infrastructure uses. It works especially well in concrete which utilizes SCM’s.
Does IC really work to reduce shrinkage in concrete?
Yes! Internal curing provides something that most concrete needs and conventional curing cannot provide: additional internal water that helps prevent early age shrinkage (reducing early age cracking) and increases the hydration of cementitious materials throughout the concrete.
Concrete Producer - Mixture Design, Batching and Testing
Is ESCS fine aggregate difficult to handle?
No. ESCS material used for IC will need to be pre-saturated but once saturated, will maintain a high level of internal moisture for quite some time. This can be further enhanced by covering the saturated stockpile when not in use. Build stockpiles for pre-saturation of a size that allows for ease of handling. Allow for adequate drain time for these stockpiles. Always maintain typical free moisture contents as seen with normal weight sands used in concrete, to avoid handling issues seen with any high, free moisture sands at the batch plant. Pre-planning is the key to success!
Does the use of ESCS for IC concrete have any effect on chemical admixtures?
No. A slight increase in air entrainment may be seen, depending on the type of entrainment used.
How much LWA is needed in an IC concrete mixture?
The amount of prewetted ESCS aggregate needed is based on the absorption and desorption of the material being used. For almost all applications, scientific evidence indicates that 7 lbs of IC water per 100 lbs of cementitious material provides the appropriate amount of IC moisture needed for enhanced hydration. Since SCMs like fly ash and slag are known to have a higher water demand during their reaction than cement alone, it may be reasonable to vary dosages of IC aggregate above the 7 lbs in some applications with high SCM replacement.
The amount of prewetted ESCS aggregate can be determined through the use of ESCSI Guide for Calculating the Quantity of Prewetted ESCS Lightweight Aggregates for Internal Curing (IC Calculator). It is also highly recommended to contact the ESCS lightweight aggregate suppliers for specific material properties.
Do I need to pre-wet the lightweight aggregate and if so how often?
Yes. The LWA pile needs to be uniformly wetted for a minimum of 48 hours or as recommended by the aggregate supplier. Turning or mixing the aggregate pile will help to reduce moisture variation. At the end of the wetting period, allow the stockpiles to drain for at least 12 to 15 hours (longer is better) immediately prior to use. Once the stockpiles have sufficiently drained to a typical free moisture range, the fine lightweight aggregate (FLWA) will handle and flow very similarly to normal weight concrete sand. Even after the recommended drain time, there still may be a high degree of free water on the aggregate at the base of the pile.
Rewetting the pile may not be necessary for several days, weeks or longer. The frequency for wetting the stockpiles will depend on the size of the piles and the weather conditions. During hot, dry and windy conditions, wetting the stockpile will be needed more often than during cooler and more humid conditions.
Under steady operations, two stockpiles may need to be maintained. One pile is receiving new FLWA to be prewetted and prepared for use. The second pile is already prepared and being used for batching. Covering stockpiles with a non-permeable cover after watering will help reduce evaporation and help improve moisture consistency in the stockpile. Effort should be taken to not overwet the stockpiles. FLWA in the stockpiles will maintain high levels of internal moisture for an extended period of time. Keeping the internal moisture of the FLWA uniform at the recommended moisture and without excess surface moisture is the goal. Periodic moisture checks and possible rewetting of the FLWA stockpile are necessary. Moisture checks of the FLWA should always be conducted prior to batching. If changes in moisture levels occur, adjustments to the concrete mixture should be made.
What is different about batching internally cured concrete?
It’s almost identical except some quality control measure MUST be done. Prior to batching, the internal moisture and the external or "free" moisture of the ESCS aggregate must be tested as per the ESCS producer recommendation and determined by ESCSI 4362.3 Test Method to Field Determine the Moisture Content of Lightweight Fine Aggregate in Stockpiles. If necessary, adjustments must be made to the concrete batch weights. This will insure the proper amount of internal moisture is available to the concrete mixture for optimal internal curing. This is done by simply replacing more or less normal weight sand with the prewetted ESCS aggregate. To achieve proper yield of the concrete mixture, be sure to make adjustments by volume, not by weight.
What about concrete testing?
Concrete testing is the same as with any normal weight concrete mix. Sample concrete as per ASTM C 172. If testing for entrained air, test as per ASTM C 231. The use of a volumetric air meter is unecessary. Concrete should be regularly tested as per ASTM C 138 to assure consistency of the mixture.
Will I need an available aggregate bin in the batch plant?
General and Concrete Contractor - Construction
Does IC concrete need special testing equipment?
No. Though IC concretes use ESCS lightweight aggregates to carry the moisture needed for internal curing, it’s not lightweight concrete. Plastic concrete testing is performed using the same equipment as would be used with any standard concrete. If testing for entrained air, test as per ASTM C 231. The use of a volumetric air meter is unecessary. Sample concrete as per ASTM C 172. Concrete should be regularly tested as per ASTM C 138 to assure consistency of the mixture.
Does IC concrete finish differently?
No. If not told, concrete finishers will not know the difference.
Can I pump internally cured concrete?
Yes. When NWC mixtures that are designed for pumping are converted to an internally cured NWC mixture, the concrete pumping will be the same. Since internally cured concrete mixtures are not lightweight concrete, no additional steps or precautions are necessary to successfully pump the IC concrete mixture.
Will I still need to cure the concrete?
Yes. Internal curing does not take the place of proper curing techniques. Proper curing practices as recommended by ACI should always be followed.
Will it cost more?
Initial concrete costs will typically be slightly higher. The lifecycle cost savings from reduced repair and in the concrete’s sustainability is where internal curing shows a great potential in cost savings.
Aggregate Selection and Supply
How do I select the LWA to be used for IC?
The aggregate needs to meet the requirements of ASTM C 1761/C 1761M Standard Specification for Lightweight Aggregate for Internal Curing of Concrete. Fine graded ESCS lightweight aggregate (FLWA) is most commonly used because it provides the best IC water distribution through out the system and it is an easy volumetric replacement of the natural sand in the mixture. On occasion, an intermediate gradation can be used to improve the overall grading of the aggregates in the mixture.
Where do I get ESCS Lightweight Aggregate?
Visit ESCSI.org or contact your local ESCS producer.
Will there be enough aggregate?
ESCS materials are widely available and shortages of ESCS fine aggregates typically used in internal curing have never occurred in the industry's 95-year history.
Where can I get more info?
Visit ESCSI.org or contact your local ESCS producer.