The Internal Curing Team Approach
Table of Contents
The quest for more robust and longer lasting concrete has given rise to extensive research of the age-old technology of internally cured concrete using expanded shale, clay and slate (ESCS) lightweight aggregate. As a result, we now have a more complete understanding of how internal curing (IC) works and a way to design for IC in conventional concrete. We also have a better understanding of why IC increases the durability and service life of concrete in an economical and simple way.
The only difference between a conventional concrete mixture and an internal curing mixture is that a small portion of the normal weight concrete sand is replaced with a prewetted fine ESCS aggregate by volume. Nothing else changes...
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.
Using this information, ESCS producers, ready-mix producers and concrete contractors are working in concert with designers and engineers in a true "team approach" to the efficient use of internal curing for the long-term performance of concrete. These applications include commercial, industrial, concrete pavement, bridges and other infrastructure uses. With this team approach, internally cured concrete can be produced, delivered and placed without complications.
The following guidelines are intended to contribute to the success of the internally cured concrete project.
Aggregate Selection for IC
The porous nature of ESCS lightweight aggregate allows for a high degree of water absorption compared to normal weight aggregates. Equally important to the internal curing process is the aggregate's rate of desorption, its ability to release the internal moisture into the concrete paste at the proper time to support cementitious hydration. Any aggregate intended for the use of IC must 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.
Concrete Mixture Considerations
The amount of prewetted ESCS aggregate needed for internal curing 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 higher a water demand during their reaction than cement alone, it may be reasonable to vary dosages of IC aggregate above and below the 7 lb value in some applications with high SCM replacement.
Knowing the target amount of IC water needed, and the aggregate's absorption and desorption, 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.
Aggregate Saturation and Stockpile Management
Construct ESCS lightweight aggregate stockpiles at the concrete batching facility so as to maintain uniform moisture throughout the pile. Using an approved sprinkler system, continuously and uniformly water the stockpiles. Turning or mixing the aggregate pile will help to reduce moisture variation. Watering should continue for a minimum of 48 hours and until the absorbed moisture content of the aggregate, as determined by ASTM C 1761 and the ESCS producer's recommendation, is achieved. All fine materials with excessive surface moisture will cause handling issues at the batch plant. At the end of the wetting period, allow the stockpiles to drain for at least 12 to 15 hours 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 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. For this reason, it is recommended that the loader operator keep the bucket elevated approximately 6" to 9" to avoid charging the batch plant with material containing an excessive amount of surface moisture.
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.
Batch Plant Adjustments to IC Aggregates
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.
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.
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.
The Team Approach
ESCS Lightweight Aggregate Supplier
- Supply ESCS lightweight aggregate for internal curing that meets ASTM C1761 in the grading specified. Provide the physical properties of the material that are appropriate and provide assistance and recommendation on using the material.
- Internally cured concrete is not structural lightweight concrete. Internally cured concrete mixture design are the same as NWC mixtures, with the exception that a small portion of normal weight sand aggregates is replaced with prewetted fine lightweight aggregate. Nothing else changes within the mixture.
- Specify a pre-placement meeting with the following present: engineer, architect, contractor, concrete supplier, ESCS aggregate supplier, testing agency, admixture supplier and pumping contractor, if applicable.
- On large jobs, these same people should be present at the first concrete placement.
- Specify exactly where concrete shall be tested, preferably at the point of final discharge as per ACI 304.2R.
- Realize that absorbed IC water does not affect the water/cement ratio, as defined in ASTM C 125.
General and/or Concrete Contractor
- Keep everyone communicating. (This is a team effort).
- Specify to the concrete supplier the number of yards needed per hour, not how many trucks.
- Make an agreement with the concrete supplier as to how the quantity of concrete delivered will be determined.
- Schedule placement days in advance so the concrete supplier has the ability to prepare and manage IC aggregate stockpiles.
- The fine or intermediate lightweight aggregate that is used for internal curing must be prewetted. Consult with the ESCS lightweight aggregate supplier and the section titled "Aggregate Saturation and Stockpile Management" in this document.
- Manage and prepare lightweight aggregate stockpiles to be ready for IC use. A consistent amount of surface (free) moisture, as low as possible, is the key to success. Stay ahead of the demand.
- Keep two stockpiles: one already prepared for the current day's use and the other for prewetting for future use.
- Covering the prewetted stockpiles with a tarp or non-permeable cover will help to reduce evaporation and improve the moisture consistency.
- Designate an individual to be responsible for IC stockpile prewetting and management.
- Remix stockpiles prior to testing and use.
- Perform moisture checks prior to batching. If necessary, make adjustments to assure that the free moisture is accounted for and the proper amount of internal moisture is present for optimal curing. Also see the section in this document titled "Batch Plant Adjustments to IC Aggregates."
- Give yourself time to perform the required steps to assure a successful placement.
- The field inspector shall be ACI Field Technician Grade 1 (or equivalent) per ASTM C 94.
- Make sure the inspector has the proper tools, understanding and directions for the job.
- On large jobs, use the same inspector for all concrete placements.
ACI 213R-14, Guide for Structural Lightweight-Aggregate Concrete
ACI 301 Specifications for Structural Concrete
ACI 304.2 R Placing Concrete by the Pumping Method
ACI (308-213)R-13, Report on Internally Cured Concrete Using Prewetted Absorptive Lightweight Aggregate
ASTM C 94 Standard Specification for Ready Mix Concrete
ASTM C 125 Standard Terminology Relating to Concrete and Concrete Aggregates
ASTM C 138 Standard Test Method for Unit Weight, Yield, and Air Content (Gravimetric) of Concrete
ASTM C 172 Standard Practice for Sampling Freshly Mixed Concrete
ASTM C 231 Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method
ASTM C 1761/C 1761M Standard Specification for Lightweight Aggregate for Internal Curing of Concrete
Bentz, D.P. and Weiss, W.J., Internal Curing: A 2010 State-of-the-Art Review, NISTIR 7765, U.S. Department of Commerce, February 2011
ESCSI's Guide for Calculating the Quantity of Prewetted ESCS Lightweight Aggregates for Internal Curing (IC Calculator)
Rao, C. and Darter, M., Evaluation of Internally Cured Concrete For Paving Applications, September 2013