1. Design and Control of Concrete Mixtures CHAPTER 18
Curing Concrete
Design and Control of Concrete Mixtures
CHAPTER 18
Design and Control of Concrete Mixtures, 16th edition, Chapter 18 – Curing Concrete

2. Overview Curing Methods and Materials Curing Period and Temperature
Sealing Compounds
This presentation will discuss curing concrete including the basic principles ofcuring and the various methods and materials used to cure concrete.

3. Curing Concrete
Curing is the maintenance of a satisfactory moisture content and temperature in concrete for a sufficient period of time during and immediately following placing so that the desired properties may develop.

4. Curing Concrete Gonnerman and Shuman 1928
With proper curing, concrete becomes stronger. The strength development is rapid at early ages but continues more slowly thereafter for an indefinite period. The graph shows the strength gain of concrete with age for different most curing periods.
Gonnerman and Shuman 1928

5. Curing Concrete
These graphs show the relative strength gain of concrete cured at different temperatures. The left graph shows effect of curing temperature on strength gain relative to 28-day strength. The right graph shows the same thing relative to the strength of concrete at 23°C (73°F).
Burg 1996

6. Curing Concrete The curing method depends on: materials
method of construction
temperature during placement
intended use
Concrete mixtures with high cement contents and low water-cementing materials ratios (less than 0.40) may require special curing needs. When moist curing is interrupted, the development of strength continues for a short period and then stops after the concrete’s internal relative humidity drops to about 80%. Loss of water will also cause the concrete to shrink. Hydration proceeds at a much slower rate when the concrete temperature is low.

7. Supplying Additional Moisture
Ponding and immersion
Fogging and sprinkling
Wet coverings
Methods that provide additional sources of moisture replace moisture lost through evaporation or hydration and maintain the mixing water in the concrete during the early hardening period.

8. Curing Methods and Materials
Supplying additional moisture
Sealing in the mix water
Accelerated curing
Concrete can be kept moist by three curing methods.

9. Ponding and Immersion Water Earth or sand dikes
More common for laboratory
Ponding is an ideal method for preventing loss of moisture from the concrete
Earth or sand dikes around the perimeter of the concrete surface retain a pond of water
The most thorough method of curing with water consists of total immersion of the finished concrete element most commonly used in the laboratory for concrete test specimens

10. Fogging and Sprinkling
Fogging minimizes moisture loss during and after placing and finishing concrete. A fine fog mist is frequently applied through a system of nozzles or sprayers to raise the relative humidity of the air over flatwork, thus slowing evaporation from the surface.

11. Wet Coverings
Fabric coverings saturated with water are commonly used for curing. Wet, moisture-retaining fabric coverings should be placed as soon as the concrete has hardened sufficiently to prevent surface damage. Here, lawn sprinklers saturating burlap with water keep the concrete continuously moist.

12. Sealing in Mix Water Impervious paper Plastic sheets
Membrane-forming curing compounds
Forms left in place
Covering the concrete with impervious paper or plastic sheets, or applying membrane-forming curing compounds reduces the loss of mixing water from the surface of the concrete.

13. Impervious Paper
Impervious paper is an efficient means of curing horizontal surfaces and structural concrete of relatively simple shapes. Periodic additions of water are not required.

14. Plastic Sheets
Polyethylene film is an effective moisture barrier for curing concrete and easily applied to complex as well as simple shapes. To minimize discoloration, the film should be kept as flat as possible on the concrete surface.

15. Membrane-Forming Compounds
Liquid membrane-forming compounds consist of waxes, resins, and other materials that retard or reduce evaporation of moisture from concrete. Clear or translucent compounds may contain a fugitive dye that makes it easier to check visually for complete coverage of the concrete surface. Pigmented compounds are recommended on hot, sunny days as they reduce solar-heat gain. Curing compounds should be applied immediately after final finishing of the concrete.

16. Internal Curing
Hydration continues because an internal water supply is available in addition to the mixing water
Internal Curing
Allows concrete to gain additional strength and also results in a reduction of permeability due to a significant extension in the time of curing
Avoids early cracking of concrete
Avoids early shrinkage
Fine aggregate in a mixture can be replaced with saturated lightweight fine aggregate to maximize process

17. Forms Left in Place
Forms provide satisfactory protection against loss of moisture
Forms should be left as long as practical
Wood forms left in place should be kept moist by sprinkling, especially during hot, dry weather. Color variations may occur from formwork and uneven water curing of walls.

18. Accelerated Curing Steam curing Insulating blankets or covers
Electrical Oil
Microwaves
Infrared curing
Self-annealing concretes

19. Steam Curing
Steam curing is advantageous where early strength gain in concrete is important or where additional heat is required to accomplish hydration, as in cold weather. Two methods of steam curing are used: live steam at atmospheric pressure (for enclosed cast-in-place structures and large precast concrete units) and high-pressure steam in autoclaves (for small manufactured units). A typical atmospheric steam-curing cycle is shown in this graph.

20. Steam Curing
Shown in the graph is the relationship between strength at 18 hours and delay period prior to steaming. In each case, the delay period plus the steaming period totaled 18 hours. A 3 to 5-hour delay period prior to steaming will achieve maximum early strength.
Hanson 1963

21. Insulating Blankets or Covers
Layers of dry, porous material such as straw or hay
Discussed more in Chapter 20

22. Electrical, Oil, Microwave, and Infrared Curing
Concrete as electrical conductor
Reinforcing steel as heating element
Heating elements
Electric blankets
Electrically heated steel forms
Electrical heating is especially useful in cold-weather concreting. Hot oil, hot water, or closed loop steam may be circulated through pipes surrounding the steel forms to heat the concrete. Infrared rays and microwave have had limited use in accelerated curing of concrete. Concrete that is cured by infrared methods is usually under a covering or enclosed in steel forms. These methods are used primarily in precast industry.

23. Self-Annealing Concrete
Self-annealing concrete refers to the process in which the mixing water and internal heat of hydration generated by concrete are retained within the formwork to accelerate the curing process. As shown in the graph, concrete placed in a standard form gains initial heat of hydration then undergoes rapid thermal loss through the formwork within hours.
(Stanley 2005)

24. Strength Development of Different Curing Methods
Compressive Strength of Different Curing Methods
The self-annealing process utilizes the heat generated by the cement reaction resulting in an autocatalytic reaction where the temperature is elevated until most cement particles are hydrated. The cement hydration rate is increased to more fully hydrate cement at earlier ages. This accelerates strength gain and maturity while maintaining the moisture, then gradually allows concrete to cool to ambient temperature or to a point where the concrete strength can withstand thermal stresses prior to exposing it to the fluctuating environment as shown in the table.
(Stanley 2005)

25. Curing Period and Temperature
A high curing temperature provides earlier strength gain in concrete than a lower temperature but it may decrease 28-day strength. One-day strength increases with increasing curing temperature, but 28-day strength decreases with increasing curing temperature.
Verbeck and Helmuth 1968

26. Curing Period and Temperature
Field Cured Cylinders
Match Curing
Field Cured Cylinders:
fabricated in the field to establish the time when curing can cease or forms can be removed
Match Curing:
Using equipment that can monitor internal concrete temperatures and match that temperature in the concrete cylinder box

27. Sealers
Sealing compounds are liquids applied to the surface of hardened concrete to reduce the penetration of liquids or gases such as water, deicing solutions, and carbon dioxide to protect concrete from freeze-thaw damage, corrosion of reinforcing steel, and acid attack. They are fundamentally different than curing compounds. Sealants shown are effective at reducing the amount of water into concrete as shown in this graph.
(Golias 2010)

28. The effectiveness of film-forming sealers depends on the continuity of the layer formed. Penetrating sealers help protect reinforcing steel in bridge decks from corrosion due to chloride infiltration. Sealers darken the concrete and periodic reapplication is necessary for long-term protection.

29. Summary
Curing Methods and Materials
Curing Period and Temperature
Sealing Compounds

30. Questions ?