Curing method for Concrete Structures.

Curing plays an important role on strength development and durability of concrete. Curing takes place immediately after concrete placing and finishing, and involves maintenance of desired moisture and temperature conditions, both at depth and near the surface, for extended periods of time. Properly cured concrete has an adequate amount of moisture for continued hydration and development of strength, volume stability, resistance to freezing and thawing, and abrasion and scaling resistance.

METHODS USED FOR CURING OF CONCRETE

There are various methods of curing. The adoption of a particular method will depend upon the nature of work and the climatic conditions. The following methods of curing of concrete are generally adopted.

• Shading concrete work
• Covering concrete surfaces with hessian or gunny bags
• Sprinkling of water
• Ponding method
• Membrane curing
• Steam curing

1. SHADING OF CONCRETE WORK

The object of shading concrete work is to prevent the evaporation of water from the surface even before setting. This is adopted mainly in case of large concrete surfaces such as road slabs. This is essential in dry weather to protect the concrete from heat, direct sun rays and wind. It also protects the surface from rain. In cold weather shading helps in preserving the heat of hydration of cement thereby preventing freezing of concrete under mild frost conditions. Shading may be achieved by using canvas stretched on frames. This method has a limited application only.

2. COVERING CONCRETE SURFACES WITH HESSIAN OR GUNNY BAGS

This is a widely used method of curing, particularly for structural concrete. Thus exposed surface of concrete is prevented from drying out by covering it with hessian, canvas or empty cement bags. The covering over vertical and sloping surfaces should be secured properly. These are periodically wetted. The interval of wetting will depend upon the rate of evaporation of water. It should be ensured that the surface of concrete is not allowed to dry even for a short time during the curing period. Special arrangements for keeping the surface wet must be made at nights and on holidays.

3. SPRINKLING OF WATER

Sprinkling of water continuously on the concrete surface provides an efficient curing. It is mostly used for curing floor slabs. The concrete should be allowed to set sufficiently before sprinkling is started. The spray can be obtained from a perforated plastic box. On small jobs sprinkling of water may be done by hand. Vertical and sloping surfaces can be kept continuously wet by sprinkling water on top surfaces and allowing it to run down between the forms and the concrete. For this method of curing the water requirement is higher.

4. PONDING METHOD

This is the best method of curing. It is suitable for curing horizontal surfaces such as floors, roof slabs, road and air field pavements. The horizontal top surfaces of beams can also be ponded. After placing the concrete, its exposed surface is first covered with moist hessian or canvas. After 24 hours, these covers are removed and small ponds of clay or sand are built across and along the pavements. The area is thus divided into a number of rectangles. The water is filled between the ponds. The filling of water in these ponds is done twice or thrice a day, depending upon the atmospheric conditions. Though this method is very efficient, the water requirement is very heavy. Ponds easily break and water flows out. After curing it is difficult to clean the clay.

5. MEMBRANE CURING

The method of curing described above come under the category of moist curing. Another method of curing is to cover the wetted concrete surface by a layer of water proof material, which is kept in contact with the concrete surface of seven days. This method of curing is termed as membrane curing. A membrane will prevent the evaporation of water from the concrete. The membrane can be either in solid or liquid form. They are also known as sealing compounds. Bituminised water proof papers, wax emulsions, bitumen emulsions and plastic films are the common types of membrane used.

Whenever bitumen is applied over the surface for curing, it should be done only after 24 hours curing with gunny bags. The surface is allowed to dry out so that loose water is not visible and then the liquid asphalt sprayed throughout. The moisture in the concrete is thus preserved. It is quite enough for curing.

This method of curing does not need constant supervision. It is adopted with advantage at places where water is not available in sufficient quantity for wet curing. This method of curing is not efficient as compared with wet curing because rate of hydration is less. Moreover the strength of concrete cured by any membrane is less than the concrete which is moist cured. When membrane is damaged the curing is badly affected.

6. STEAM CURING

Steam curing and hot water curing is sometimes adopted. With these methods of curing, the strength development of concrete is very rapid.

These methods can best be used in pre cast concrete work. In steam curing the temperature of steam should be restricted to a maximum of 750C as in the absence of proper humidity (about 90%) the concrete may dry too soon. In case of hot water curing, temperature may be raised to any limit, ay 1000C.

At this temperature, the development of strength is about 70% of 28 days strength after 4 to 5 hours. In both cases, the temperature should be fully controlled to avoid non-uniformity. The concrete should be prevented from rapid drying and cooling which would form cracks.

Miscellaneous Methods of Curing

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Calcium chloride is used either as a surface coating or as an admixture. It has been used satisfactorily as a curing medium. Both these methods are based on the fact that calcium chloride being a salt shows affinity for moisture. The salt not only absorbs moisture from atmosphere but also retains it at the surface. This moisture held at the surface prevents the mixing water from evaporation and thereby keeps the concrete wet for a long time to promote hydration. Formwork prevents escaping of moisture from the concrete, particularly, in the case of beams and columns.

Keeping the form work intact and sealing the joint with wax or any other sealing compound prevents the evaporation of moisture from the concrete. This procedure of promoting hydration can be considered as one of the miscellaneous methods of curing.

When to Start Curing and how Long to Cure?

Many a time an engineer at site wonders, how early he should start curing by way of application of water. This problem arises, particularly, in case of hot weather concreting. In an arid region, concrete placed as a road slab or roof slab gets dried up in a very short time, say within 2 hours. Often questions are asked whether water can be poured over the above concrete within two hours to prevent the drying. The associated problem is, if water is applied within say two hours, whether it will interfere with the water/cement ratio and cause harmful effects. In other words, question is how early water can be applied over concrete surface so that uninterrupted and continued hydration takes place, without causing interference with the water/cement ratio. The answer is that first of all, concrete should not be allowed to dry fast in any situation. Concrete that are liable to quick drying is required to be covered with wet gunny bag or wet hessian cloth properly squeezed, so that the water does not drip and at the same time, does not allow the concrete to dry. This condition should be maintained for 24 hours or at least till the final setting time of cement at which duration the concrete will have assumed the final volume. Even if water is poured, after this time, it is not going to interfere with the water/cement ratio.
However, the best practice is to keep the concrete under the wet gunny bag for 24 hours and then commence water curing by way of ponding or spraying. Of course, when curing compound is used immediately after bleeding water, if any, dries up, the question of when to start water curing does not arise at all. There is a wrong concept with common builders that commencement of curing should be done only on the following day after concreting. Even on the next day they make arrangements and build bunds with mud or lean mortar to retain water. This further delays the curing. Such practice is followed for concrete road construction by municipal corporations also. It is a bad practice. It is difficult to set time frame how early water curing can be started. It depends on, prevailing temperature, humidity, wind velocity, type of cement, fineness of cement, w/c used and size of member etc. The point to observe is that, the top surface of concrete should not be allowed to dry. Enough moisture must be present to promote hydration. Regarding how long to cure, it is again difficult to set a limit. Since all the desirable properties of concrete are improved by curing, the curing period should be as long as practical. For general guidance, concrete must be cured till it attains about 70% of specified strength. At lower temperature curing period must be increased. Since the rate of hydration is influenced by cement composition and fineness, the curing period should be prolonged for concretes made with cements of slow strength gain characteristics. Pozzolanic cement or concrete admixed with pozzolanic material is required to be cured for longer duration. Mass concrete, heavy footings, large piers, abutments, should be cured for at least 2 weeks. METHOD ADVANTAGE    DISADVANTAGES Sprinkling with Water or Covering with Burlap Excellent results if kept constantly wet Likelihood  of  drying  between sprinklings;  difficult  on  vertical walls Straw Insulator  in  winter Can dry out, blow away, or burn Moist Earth Cheap but messy Stains   concrete;   can   dry   out; removal   problem Pending on Flat Surfaces Excellent  results,  maintains  uni- form  temperature Requires   considerable   labor;   un- desirable  in  freezing  weather Curing  Compounds Easy  to  apply  and  inexpensive Sprayer   needed;   inadequate coverage   allows   drying   out;   film can be broken or tracked off before curing   is   completed;   unless   pig- mented,  can  allow  concrete  to  get too  hot Waterproof Paper Excellent   protection,   prevents Heavy cost can be excessive; must drying be   kept   in   rolls;   storage   and handling  problem Plastic Film Absolutely watertight, excellent protection. Light and easy to handle Should  be  pigmented  for  heat protection;  requires  reasonable  care and tears must be patched; must be weighed  down  to  prevent  blowing away

The length of adequate curing time is dependent on the following factors:

• Mixture proportions
• Specified strength
• Size and shape of concrete member
• Ambient weather conditions
• Future exposure conditions

Slabs on ground (e.g. pavements, sidewalks, parking lots, driveways, floors, canal linings) and structural concrete (e.g. bridge decks, piers, columns, beams, slabs, small footings, cast-in-place walls, retaining walls) require a minimum curing period of seven days for ambient temperatures above 40 degrees Fahrenheit1.

American Concrete Institute (ACI) Committee 301 recommends a minimum curing period corresponding to concrete attaining 70 percent of the specified compressive strength2. The often specified seven-day curing commonly corresponds to approximately 70 percent of the specified compressive strengths. The 70 percent strength level can be reached sooner when concrete cures at higher temperatures or when certain cement/admixture combinations are used. Similarly, longer time may be needed for different material combinations and/or lower curing temperatures. For this reason, ACI Committee 308 recommends the following minimum curing periods3:

• ASTM C 150 Type I cement seven days
• ASTM C 150 Type II cement ten days
• ASTM C 150 Type III cement three days 
• ASTM C 150 Type IV or V cement 14 days
• ASTM C 595, C 845, C 1157 cements variable

Effect of curing duration on compressive strength development is presented in Figure 1.

Figure 1. Moist Curing Time and Compressive Strength Gain

Higher curing temperatures promote an early strength gain in concrete but may decrease its 28-day strength. Effect of curing temperature on compressive strength development is presented in Figure 2.

Figure 2. Effect of Curing Temperature on Compressive Strength

There are three main functions of curing: 

1) Maintaining mixing water in concrete during the early hardening process

Ponding and immersion
Ponding is typically used to cure flat surfaces on smaller jobs. Care should be taken to maintain curing water temperature at not more than 20 degrees Fahrenheit cooler than the concrete to prevent cracking due to thermal stresses. Immersion is mainly used in the laboratory for curing concrete test specimens.

Spraying and fogging
Spraying and fogging are used when the ambient temperatures are well above freezing and the humidity is low. Fogging can minimize plastic shrinkage cracking until the concrete attains final set.

Saturated wet coverings
Wet coverings saturated with water should be used after concrete has hardened enough to prevent surface damage. They should be kept constantly wet. 

Left in Place Forms
Left in place forms usually provide satisfactory protection against moisture loss for formed concrete surfaces. The forms are usually left in place as long as the construction schedule allows. If the forms are made of wood, they should be kept moist, especially during hot, dry weather.

2) Reducing the loss of mixing water from the surface of the concrete

Covering concrete with impervious paper or plastic sheets
Impervious paper and plastic sheets can be applied on thoroughly wetted concrete. The concrete surface should be hard enough to prevent surface damage from placement activities. 

Applying membrane-forming curing compounds
Membrane-forming curing compounds are used to retard or reduce evaporation of moisture from concrete. They can be clear or translucent and white pigmented. White-pigmented compounds are recommended for hot and sunny weather conditions to reflect solar radiation. Curing compounds should be applied immediately after final finishing. Curing compound shall comply with ASTM C3094 or ASTM C13155.

3) Accelerating strength gain using heat and additional moisture

Live steam
Live steam at atmospheric pressure and high-pressure steam in autoclaves are the two methods of steam curing. Steam temperature for live steam at atmospheric pressure should be kept at about 140 degrees Fahrenheit or less until the desired concrete strength is achieved.

Heating coils
Heating coils are usually used as embedded elements near the surface of concrete elements. Their purpose is to protect concrete from freezing during cold weather concreting. 

Electrical heated forms or pads
Electrical heated forms or pads are primarily used by precast concrete producers.

Concrete blankets
Concrete insulation blankets are used to cover and insulate concrete surfaces subjected to freezing temperatures during the curing period. The concrete should be hard enough to prevent surface damage when covering with concrete blankets.

Other forms of curing include internal moist curing with lightweight aggregates or absorbent polymer particles. For mass concrete elements (usually thicker than 3 feet), a thermal control plan is usually developed to help control thermal stresses. Additional information can be found in ACI Committee 308 report Guide to Curing Concrete3. For specialty concretes, it is recommended to refer to other ACI reports as follows:

• Refractory concrete ACI 547.1R
• Refractory concrete ACI 547.1R
• Insulating concrete ACI 523.1R
• Expansive cement concrete ACI 223
• Roller-compacted concrete ACI 207.5R
• Architectural concrete ACI 303R
• Shotcrete ACI 506.2
• Fiber-reinforced concrete ACI 544.3R
• Vertical slipform construction ACI 313

Curing in either cold or hot weather requires additional attention. In cold weather, some of the procedures include heated enclosures, evaporation reducers, curing compounds, and insulating blankets. The temperature of fresh concrete shall be above 50 degrees Fahrenheit. The curing period for cold weather concrete is longer than the standard period due to reduced rate of strength gain. Compressive strength of concrete cured and maintained at 50 degrees Fahrenheit is expected to gain strength half as quickly as concrete cured at 73 degrees Fahrenheit. In hot weather, curing and protection are critical due to rapid moisture loss from fresh concrete. The curing actually starts before concrete is placed by wetting substrate surfaces with water. Sunscreens, windscreens, fogging, and evaporation retardants can be used for hot weather concrete placements. Since concrete strength gain in hot weather is faster, curing period may be reduced. Additional information can be found in ACI 306.1, Standard Specification for Cold Weather Concreting, ACI 306R, Cold Weather Concreting, ACI 305.1, Specification for Hot Weather Concreting, and ACI 305R, Hot Weather Concreting

Curing Concrete Test Specimens

Curing of concrete test specimens is usually different from concrete placed during construction. American Society for Testing and Materials (ASTM) has developed two standards for making and curing concrete specimens. ASTM C1926 is intended for laboratory samples while ASTM C317 is intended for field samples. Both documents provide standardized requirements for making, curing, protecting, and transporting concrete test specimens under field or laboratory conditions, respectively.

ASTM C192 provides procedures for evaluation of different mixtures in laboratory conditions. It is usually used in the initial stage of the project, or for research purposes.

ASTM C31 is used for acceptance testing and can also be used as a decision tool for form or shoring removal. Depending on its intended purpose, the standard defines two curing regimes: standard curing for acceptance testing and field curing for form/shoring removal. Variation in standard curing of test specimens can dramatically affect measured concrete properties. According to the National Ready Mix Concrete Association8 (NRMCA), strength for concrete air cured for one day followed by 27 days moist cured will be approximately 8 percent lower than for concrete moist cured for the entire period. The strength reduction is 11 percent and 18 percent for concrete specimens initially cured in air for three days and seven days, respectively. For the same air/moist curing combinations, but 100 degrees Fahrenheit air curing temperature, the 28-day strength will be approximately 11 percent, 22 percent, and 26 percent lower, respectively.