CURING OF CONCRETE

Today, Mr Neeraj Sumersingh, Seinor Technical Executive of my team from Delhi (my junior) asked me, why should not we use plastic sheets for curing purpose. To make it clear and what are the other method of curing, I prepared this blog. Many thanks to Mr Neeraj for asking this query…

Curing is the process of controlling the rate and extent of moisture loss from concrete during cement hydration. It may be either after it has been placed in position (or during the manufacture of concrete products), thereby providing time for the hydration of the cement to occur. Since the hydration of cement does take time – days, even weeks rather than hours – curing must be undertaken for a reasonable period. Curing plays an important role on durability of concrete, development of strength, volume stability, resistance to freezing and thawing, abrasion & scaling resistance, prevents cracking due to thermal stresses, can minimize plastic shrinkage, protection against moisture loss.

Let see - Strength of concrete and effects of temperature along with time on concrete

 

Curing time is dependent on the following factors:

1. Mixture proportions

2. Specified strength

3. Size and shape of concrete member

4. Ambient weather conditions

5. Future exposure conditions

Let see what does curing process makes the concrete & how?

a. Curing maintains mixed water in concrete during it’s early hardening process

1. Ponding OR immersion – around flat and small jobs water will be made to stand for specific height by constructing temporary bund. Immersion is mainly used in the laboratory for curing concrete test specimens.

2. Water spraying and fogging - are used when the ambient temperatures are well above freezing and the humidity is low.

3. Saturated wet coverings – wet gunny bags should be used after concrete has hardened enough. They should be kept constantly wet.

4. Left in Place Forms – shuttering forms usually left in place, if the construction schedule allows. If the forms are made of wood, they should be kept moist, especially during hot, dry weather. Extra cost for no’s of forms – Obstruction in working areas – metal are good conductor of heat, they transfer heat from out to in. Note: As per IS 456 – minimum time for removing the form / shuttering for RCC works has been established.

What is the difference between Formwork, Shuttering, Staging, Centering and Scaffolding

Formwork: It is a temporary structure which is used as a mould to pour the concrete. It is a vertical or horizontal arrangement made to keep concrete in position until it gains strength & shape.

Shuttering: It is a is a vertical temporary arrangement which is arranged to bring concrete in a desired shape. In a technical point of view, Formwork for columns, footings, retaining walls is called as a Shuttering.

Centering: It is a temporary arrangement & part of formwork which is arranged to support horizontal members. In a technical point of view, the formwork for floor beams & Slabs is called as a Centering.

Staging: It is a temporary member which is used to support formwork (either it may be for centering or shuttering). It is done by props, jacks, H frames, cup lock system, wooden ballies, etc. (Refer below image for clear understanding)

Scaffolding: Formwork is arranged to support the structural members, whereas scaffolding is provided as a workers platform around the building to work at heights, Scaffolding is a movable/fixed platform.

b. Reduces the loss of mixed water from the surface of the concrete

1. Covering concrete with impervious paper or plastic sheets – In case of non-normal temperature or it is high, impervious paper and plastic sheets can be shall be installed thoroughly wetted concrete till it hardened enough to prevent surface damage from the other working activities. As soon as concrete hardened enough, curing shall be started. Sheets shall be covered thoroughly, color of sheet must be light, holes shall not be there in sheet. Sheet shall be maintained in place by placing the load on it. Air tightness maintenance may be the challenge here. Suitable for floor area. All vertical structures may not be covered perfectly.

2. Applying membrane-forming curing compounds - Membrane-forming curing compounds are used to retard or reduce evaporation of moisture from concrete. Curing compound shall comply with ASTM C3094 or ASTM C13155 or BS 7542.

I came across one of the best curing compound which can be applied using brush / roller / spray is C 20 tested as per BS 7542. Below are the details:

Hint: As per BS 7542, The mortar (you can prepare the concrete) cubes as per the said dimensions, after setting time, weight the cubes, apply curing compound as per manufacturer's specification to some cubes and some do not apply. Allow them to be in specified condition for 72 Hrs and again weight them, Find out the loss of water in both applied with curing compound (test sample) and non applied (control sample). then express the curing efficiency.

c. Accelerating strength gain using heat and additional moisture

1. Live steam - Live steam at atmospheric pressure or high-pressure steam will be sent on the concrete in autoclaves over the concrete. Best used for pre cast products.

2. 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.

3. Electrical heated forms or pads: Are primarily used by precast concrete producers. Let us understand this with video:

Video curtesy of : https://www.youtube.com/watch?v=1RCqAd1ah3M

4. Concrete blankets - Concrete insulation blankets are used to cover and insulate concrete surfaces subjected to freezing temperatures during the curing period.

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 Concrete. For specialty concretes, it is recommended to refer to other ACI reports as follows:

1.    Refractory concrete ACI 547.1R

2.    Refractory concrete ACI 547.1R

3.    Insulating concrete ACI 523.1R

4.    Expansive cement concrete ACI 223

5.    Roller-compacted concrete ACI 207.5R

6.    Architectural concrete ACI 303R

7.    Shotcrete ACI 506.2

8.    Fiber-reinforced concrete ACI 544.3R

9.    Vertical slip form construction ACI 313

General comparison of different types of curing methods:

As per standards, American Concrete Institute (ACI) Committee 301 recommends a minimum curing period corresponding to concrete attaining 70% of designed compressive strength. 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 periods:

A. ASTM C 150 Type I cement seven days

B. ASTM C 150 Type II cement ten days

C. ASTM C 150 Type III cement three days

D. ASTM C 150 Type IV or V cement 14 days

E. ASTM C 595, C 845, C 1157 cements variable

According to the National Ready Mix Concrete Association (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.

So, it is always to better to use the curing admixture or polymer modified products which does not requires the curing at all.

Details curtesy of : CEMENT CONCRETE & AGGREGATES AUSTRALIA – CIP 11 - concurring of concrete.