By Reid W. Castrodale, Ph.D., P.E.
Internal curing is rapidly emerging as an effective way to improve curing of concrete. It holds promise for producing concrete with increased compressive strength and reduced permeability and cracking, attributes that lengthen the service life of infrastructure products such as bridge decks. While most projects to date have been cast-in-place concrete, the process can also be applied to products manufactured in a precast plant.
What is internal curing?
The Expanded Shale Clay and Slate Institute published the “ESCSI Guide Specifications for Internally Cured Concrete,” which provides a good practical definition for internal curing: “Pre-wetted expanded shale, clay or slate lightweight aggregate is incorporated into a conventional concrete mixture to provide reservoirs of water within the concrete that slowly release the water after the concrete sets to provide internal curing to the mixture.”1
The ESCSI Guide Specifications also state internal curing is accomplished by “… modifying a conventional normal weight concrete mixture … by replacing a portion of the normal weight fine aggregate with prewetted fine or intermediate … lightweight aggregate.”2
What is lightweight aggregate?
Lightweight aggregate is typically a shale, clay or slate that has been expanded in a rotary kiln at temperatures as high as 2,200 F. This process has been used to manufacture structural lightweight aggregate in the U.S. since 1920.
ASTM C1761, “Standard Specification for Lightweight Aggregate for Internal Curing of Concrete,” provides minimum physical requirements for lightweight aggregate, test methods for absorption and desorption, and a procedure for calculating the quantity of lightweight aggregate required for internal curing.
Why use lightweight aggregate?
Lightweight aggregate is used for internal curing because of its increased absorption, which can range from less than 10% to more than 30% depending on the type of aggregate and how thoroughly it has been prewetted. The higher absorption enables prewetted lightweight aggregate to carry water for internal curing. The moisture is not released from the aggregate until after the concrete has set and the pore size in the paste becomes smaller than the pores in the lightweight aggregate. The absorbed water does not affect the water-to-cementitious materials ratio. The addition of lightweight aggregate to a mix for internal curing will reduce the density of the concrete, but that is not the main goal. However, for precast products, weight reduction is an important benefit.
Any concrete mixture that contains lightweight aggregate provides internal curing if the aggregate has been prewetted prior to batching. However, this discussion is limited to using fine gradations of lightweight aggregate to modify a conventional concrete mix to obtain internal curing. Fine lightweight aggregate is used because its small size allows a more uniform distribution of the water-filled internal curing reservoirs in the concrete.
Other types of absorptive material have been suggested for use in providing internal curing. However, only lightweight aggregate is a structural material.
Proportioning mixtures for internal curing
The weight of prewetted lightweight aggregate required for internal curing,
MPLWA, can be determined using the following equation3:
MPLWA = Cf x CS x (1 + A) / (A x D) (Eq. 1)
MPLWA = mass of prewetted fine lightweight aggregate needed per unit volume of concrete (lb/yd3 or kg/m3)
Cf = cement factor (content) for concrete mixture (lb/yd3 or kg/m3)
CS = chemical shrinkage of cement; usually taken as 7 lb/cwt or 7%
A = absorption of lightweight aggregate, expressed as a percentage of the oven-dry mass (%)
D = desorption of lightweight aggregate, expressed as a percentage of the absorbed water that is released by the aggregate in drying conditions compared to the total absorbed water (%)
For a particular lightweight aggregate, Equation 1 can be simplified to:
MPLWA = Cf x K (Eq. 2)
K = CS x (1 + A) / (A x D)
Using typical lightweight aggregate properties (CS = 7%; A = 15% and D = 95%), K = 0.565. Applying this to a concrete mix with a cement content of 564 lbs/yd3, MPLWA = 564 x 0.565 = 319 lbs/yd3.
Once the weight of prewetted lightweight aggregate for internal curing is determined, the concrete mix is modified by reducing the volume of sand by the volume of prewetted lightweight aggregate, using the ratio of the specific gravities:
MRNWA = MPLWA x SGNWA / SGPLWA (Eq. 3)
MRNWA = weight of sand to be replaced by prewetted lightweight aggregate
SGNWA = specific gravity of sand
SGPLWA = specific gravity of prewetted lightweight aggregate
The prewetted lightweight aggregate is then used very much like an admixture, except that the prewetted lightweight aggregate replaces a portion of the sand. This is illustrated in Figure 1, which shows relative volumes of constituents for a typical mixture. The conventional and internally cured mixtures are identical except for the volume of sand replaced with prewetted lightweight aggregate. For actual mixes, other minor adjustments may be required.
Benefits of internal curing
The main benefits of internal curing for precast products are reduced shrinkage, cracking tendency and permeability. These factors, which are discussed below, are important in preventing or delaying reinforcement corrosion. Other benefits of internal curing can also contribute to improved long-term performance of concrete:
- Reduced modulus of elasticity
- Reduced coefficient of thermal expansion
- More effective use of cementitious materials
- Reduced density
- Reduced curling and warping
Reduced shrinkage and cracking tendency
Reduced shrinkage of internally cured mortar mixtures was observed by researchers at Purdue University for sealed mortar mixtures, as shown in Figure 2.4 The conventional mixture (blue line) experienced significant shrinkage in the first seven days. However, as sand was replaced with increasing quantities of prewetted lightweight aggregate, shrinkage was reduced or even eliminated. Percentages shown in the figure indicate the fraction of the total mortar volume occupied by the lightweight aggregate fines rather than the percentage of sand replaced. Reductions are most significant for low w/cm mixes, but benefits still exist for mixtures with ratios of 0.5 or greater.
Because of the decreased shrinkage, cracking in restrained concrete with internal curing is reduced or delayed, as shown in Figure 3.4 In the figure, cracking occurs when strain suddenly decreases, as indicated by vertical lines. The conventional mixture cracked six days after mixing, while the internally cured mixtures cracked later or not at all. Factors other than reduced shrinkage also affect cracking tendency, such as the reduced modulus of elasticity and coefficient of thermal expansion.
Another important benefit of internally cured concrete is its reduced permeability as a result of improved cement hydration and an improved interfacial transition zone at the surface of lightweight aggregate particles. The better-hydrated cement provides a denser and less porous paste, while the improved interfacial transition zone restricts movement of fluids along aggregate particles. This is usually a major pathway for water penetration into conventional mixtures.
Resistivity data from two bridge decks constructed in 2010 with and without internal curing are shown in Figure 4.5 Increased surface resistivity of concrete indicates lower permeability. The data show the internally cured bridge deck concrete (blue line) has significantly greater resistivity (lower permeability) than the conventional concrete mixture (red line). After one year, the internally cured concrete had 75% higher resistivity than the conventional concrete mixture. This large increase in surface resistivity should not be expected in all cases.
Cost of internal curing
Lightweight aggregate costs more than conventional aggregate because of the high temperature processing required to achieve expansion. Transportation costs are also usually higher because of the limited number of manufacturing facilities available in the U.S. An additional increment in cost for internally cured concrete may also be required to account for prewetting and testing the aggregate. However, the higher cost is a relatively small investment compared to the increased service life and other performance benefits of internally cured concrete.
A striking example of the effects of internal curing
In 2012, Denver Water Co. used lightweight aggregate to internally cure concrete for the floor and roof slabs of a 10-million-gallon water storage tank. Two trial slabs with conventional concrete and internally cured concrete were placed at the same time in the hot and dry environment near Denver. No curing compound was applied to either slab. A photo (Figure 5) was taken the morning after the concrete was placed. It shows the internally cured concrete is still moist, while the conventional concrete is dried out.
After the contractor discovered the internally cured mixture performed well and had more rapid strength gain with significantly reduced cracking, he asked to also use it to construct walls and columns. The owner plans to construct future tanks using internal curing.6
A new approach for the precast industry
Internal curing can aid in manufacturing more durable precast products, especially when used in conjunction with other sound production processes. Producers must also continue to practice proper quality control to assess aggregate moisture levels within the concrete mix design. Overall, internal curing provides the precast concrete industry with a new approach to reduce the risk of early-age cracking and increase the longevity and sustainable properties of precast products.
Reid W. Castrodale, Ph.D., P.E., president of Castrodale Engineering Consultants, is a structural engineering consultant who provides services related to prestressed concrete and lightweight concrete. He has more than 30 years of bridge-related experience in research, a bridge design firm, the Portland Cement Association, and the lightweight aggregate industry.
1 Expanded Shale Clay and Slate Institute. “ESCSI Guide Specifications for Internally Cured Concrete,” Information Sheet 4001.1, ESCSI, 2012.
2 Additional information on internal curing can be found on the ESCSI website (escsi.org).
3 The equation is derived from the widely-accepted method in Bentz, D. P., Lura, P., and Roberts, J. W., Mixture Proportioning for Internal Curing, Concrete International, Vol. 27, No. 2, February 2009, pp. 35-40.
4 Henkensiefken, R., Bentz, Dale, Nantung, Tommy, and Weiss, Jason. Volume change and cracking in internally cured mixtures made with saturated lightweight aggregate under sealed and unsealed conditions. Cement & Concrete Composites, 2009. 31(7), pp. 427-437.
5 Di Bella, et al. “Documenting the Construction of a Plain Concrete Bridge Deck and an Internally Cured Bridge Deck,” Report TR-1-2012, IN LTAP Center, June 2012.
6 Bates, Robert T., Holck, Erik, Dee, Miles, King, Michael. “Design and Construction of an Internally Cured Slab,” SP290-05, The Economics, Performance and Sustainability of Internally Cured Concrete, Ed. Schindler, Grygar and Weiss, American Concrete Institute, 2012.
7 Expanded Shale Clay and Slate Institute, “Internal Curing: Helping Concrete Realize its Maximum Potential,” Publication No. 4362.1, ESCSI, 2012.