Curing Methods for Different Types of Concrete Construction

Different types of structures / construction require different types of concrete curing methods. The curing method of RCC Slab cannot be applied to RCC column. Therefore, in this article, we will discuss about the various concrete curing methods which can be applied to different types of concrete construction for effective curing.

The curing of concrete is done to keep the moisture content intact for structure so that hydration process can continue and concrete gains strength without the any surface defects on member.

Curing of Pavements and other Slabs on Ground:

Airfield concrete pavements, highway concrete pavements, canal linings, walkways, driveways, parking lots constructed with concrete, ground floor slabs are examples of slabs on ground. They have very large exposed surface area compared to vertical concrete construction. Due to this, the evaporation rates from these surface are high which may result in plastic shrinkage cracks in very early stage.

The loss of moisture in a concrete slab resting on ground can be from both top and bottom surface. Moisture loss from the bottom surface of concrete is due to absorption by dry subgrade.

This continuous evaporation from slab surface results in slowing down of hydration process and as a result, it has deleterious effect on strength, abrasion resistance, and durability of concrete.

Curing methods for concrete pavements and slabs:

The curing of slabs and pavements should begin as soon as the finishing of surface has finished, but without marring the concrete surface. To prevent the moisture loss at a rapid rate, protective measures such as evaporation reduces, wind breakers, sunshields and fog sprayers can be used immediately after the concrete casting. The exposed surface of concrete slabs and pavements are covered with mats and kept wet until the required strength and properties of concrete have developed.

When the curing of concrete slabs and pavement is done continuously for a period of time, the drying of concrete after stoppage of curing may lead to surface defects such as shrinkage cracks due to rapid loss of moisture. To prevent this, the concrete must be protected by covering it with mats or plastic sheets till the concrete has dried under the sheets.

Curing methods for Concrete Buildings, Bridges and other Structures:

A concrete building, bridges and other structures include members such as columns, walls, beams, slabs, footings, piers, retaining walls, conduits and tunnel linings. These structural members are cured by one or more methods described in concrete curing methods.

When the concrete has hardened then for the vertical faces formwork ties may be loosened in a way that does not damage the concrete, and water is applied from the top surface to make it run down inside of the form to keep the concrete wet.

After the removal of the formwork, the concrete surface must be kept dry by water spraying or using water-saturated fabrics. Additional curing should be provided after striking the formworks when the strength and durability of concrete is of much importance. This can be achieved by continuous application of water or by using membrane forming curing compounds.

Curing Methods for Mass Concrete:

Mass concrete is a large volume cast-in-place concrete construction such as heavy footings, dams, piers, abutments and similar massive construction. The heat generated due to hydration process in these constructions are very high and require the proper heat control arrangements to control the cracking and volume change.

The effect of temperature rise is very high in case of high strength and high cementitious concrete. The recommendations for control of heat in mass concrete can be found in ACI 207.1R and ACI 207.2R.

The curing of mass concrete in horizontal or sloping surfaces can be done by keeping it continuously wet by spraying of water, by using wet sand or by using the water-saturated fabrics. For vertical surfaces water must be allowed to rundown from top to bottom inside the form by loosening the ties of top formwork. Immediately after striking of forms, these concrete surfaces can be covered with water-saturated fabrics or can be kept wet by continuous spray of water.

The difference in temperature between the inside of concrete and water should not be more than 110C. The difference more than this may induce temperature stress in concrete. The care must be taken during cold weather conditions. The liquid membrane forming can be used during cold weather conditions.

Curing for Special Concrete:

Special concrete have different properties and composition form the normal cement concrete. These types of concrete require special curing methods.

Following table provides the ACI Committee reports for curing of different types of special concrete:

Special Concrete	ACI Committee Report
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

References: ACI 308R – Guide to Curing

Concrete Formwork Removal Time, Specifications and Calculations

The removal of concrete formwork also called as strike-off or stripping of formwork should be carried out only after the time when concrete has gained sufficient strength, at least twice the stress to which the concrete may be subjected to when the formworks are removed. It is also necessary to ensure the stability of the remaining formwork during formwork removal.

Concrete Formwork Removal Time

The rate of hardening of concrete or the concrete strength depends on temperature and affects the formwork removal time. For example, time required for removal of concrete in winter will be more than time required during summer.

Special attention is required for formwork removal of flexural members such as beams and slabs. As these members are subjected to self-load as well as live load even during construction, they may deflect if the strength gained is not sufficient to handle to loads.

To estimate the strength of concrete before formwork removal, the tests on concrete cubes or cylinders should be carried out. The concrete cubes or cylinders should be prepared from the same mix as that of the structural members and cured under same circumstances of temperature and moisture as that of structural member.

When it is ensured that the concrete in the structural members has gained sufficient strength to withstand the design load, only then formworks should be removed. If possible, the formworks should be left for longer time as it helps in curing.

Removal of formwork from concrete section should not make the structural element to:

• Collapse under self load or under design load
• deflect the structural member excessively in short or the long term
• physically damage the structural member when formwork is removed.

The following points must be kept in mind during formwork removal whether the structure will be prone to:

• freeze thaw damage
• cracks formation due to thermal contraction of concrete

after formwork striking. If there is a significant risk of any of the above damages, it is better to delay  the removal time of formwork. If formwork have to removed for optimising the concrete construction activities, then these structures must be insulated well to prevent such damages.

Calculation of Safe Formwork Striking Times:

Structural members are constructed based on designed load. But before a structure is complete and subjected to all loads assumed during structural design, the structural members are subjected to its self weight and construction loads during construction process.

So, to proceed with construction activities at a quicker rate, it is essential to calculate the behaviour of structure under is self load and construction load. If this can be done and structural member is found to be safe, formwork can be stripped-off.

If these calculations are not possible, then following formula can be used for calculation of safe formwork striking times:

Characteristic strength of cube of equal of maturity to the structure required at time of formwork removal

This formula was given by Harrison (1995) which describes in detail the background of determination of formwork removal times.

Other method to determine the strength of concrete structure is to conduct the non-destructive tests on structural member.

Factors Affecting Concrete Formwork Striking Times

The striking time of concrete formwork depends on the strength of structural member. The strength development of concrete member depends on:

• Grade of concrete – higher the grade of concrete, the rate of development of strength is higher and thus concrete achieves the strength in shorter time.
• Grade of cement – Higher cement grade makes the concrete achieve higher strength in shorter time.
• Type of Cement – Type of cement affects the strength development of concrete. For example, rapid hardening cement have higher strength gain in shorter period than the Ordinary Portland Cement. Low heat cement takes more time to gain sufficient strength than OPC.
• Temperature – The higher temperature of concrete during placement makes it achieve higher strength in shorter times. During winter, the concrete strength gain time gets prolonged.
• A higher ambient temperature makes the concrete gain strength faster.
• Formwork helps the concrete to insulate it from surrounding, so longer the formwork remain with concrete, the less is the loss of heat of hydration and rate of strength gain is high.
• Size of the concrete member also affects the gain of concrete strength. Larger concrete section members gain strength in shorter time than smaller sections.
• Accelerated curing is also a method to increase the strength gain rate with the application of heat.

Generally following values of concrete strength is considered for removal of formwork for various types of concrete structural members.

Table – 1: Strength of concrete vs. Structural Member Type & Span for Formwork Removal

Concrete Strength	Structural Member Type and Span
2.5 N/mm2	Lateral parts of the formwork for all structural members can be removed
70% of design strength	Interior parts of formwork of slabs and beams with a span of up to 6m can be removed
85% of design strength	Interior parts of formwork of slabs and beams with a span of more than 6m can be removed

Table – 2: Formwork Stripping Time (When Ordinary Portland Cement is used):

Type of Formwork	Formwork Removal Time
Sides of Walls, Columns and Vertical faces of beam	24 hours to 48 hours (as per engineer’s decision)
Slabs (props left under)	3 days
Beam soffits (props left under)	7 days
Removal of Props of Slabs:
i) Slabs spanning up to 4.5m	14 days
ii) Slabs spanning over 4.5m	14 days
Removal of props for beams and arches
i) Span up to 6m	14 days
ii) Span over 6m	21 days

Important Note:

It is important to note that the time for formwork removal shown above in Table -2 is only when Ordinary Portland Cement is used. In normal construction process Portland Pozzolana cement is used. So, the time shown in Table-2 should be modified.

For cements other than Ordinary Portland cement, the time required for formwork removal should be as:

• Portland Pozzolana Cement – stripping time will be 10/7 of the time stated above (Table-2)
• Low heat cement – stripping time will be 10/7 of the time stated above (Table-2)
• Rapid Hardening Cement – stripping time of 3/7 of the time stated above (Table-2) will be sufficient in all cases except for vertical sides of slabs, beams and columns which should be retained at least for 24 hours.

Concrete Formwork Removal Specification:

During stripping of formwork, following points must be remembered:

• Formwork should not be removed until the concrete has developed sufficiently strength to support all loads placed upon it. The time required before formwork removal depends on the structural function of the member and the rate of strength gain of the concrete. The grade of concrete, type of cement, water/cement ratio, temperature during curing etc. influence the rate of strength gain of concrete.
• The formwork parts and connections should be arranged in a way that makes formwork removal easy and simple, prevents damage to concrete and formwork panels so that it can be reused without extensive repair.
• The formwork removal procedure should be supervised by the engineer to ensure that quality of hardened concrete in structural member, i.e. it should be free from or has minimum casting defects such as honeycombing, size and shape defects etc. These defects in concrete influence the strength and stability of structure. Thus immediate repair works can be done or the members can be rejected.
• The separation of forms should not be done by forcing crowbars against the concrete. It may damage the hardened concrete. This should be achieved by using wooden wedges.
• Beam and joist bottoms should remain in place until final removal of all shoring under them are done.
• Joist forms should be designed and removed so that the shores may be removed temporarily to permit removal of joist forms but must be replaced at once. The shores and joists will be dismantled beginning from the middle of the member’s span, continuing symmetrically up the supports.
• The approval from the engineer should be obtained for the sequence and pattern of formwork removal.

Reference:

• ACI (1995) In-place methods to estimate concrete strength. ACI 228.1R-95.
• ASTM (1987) Standard practice for estimating concrete strength by the maturity method. ASTM C1074–87
• BS 8110 – code of practice for the structural use of concrete
• IS-456 – Plain and Reinforced Concrete – Code of Practice