Concrete Curing: Methods, Duration and Common Mistakes (as per IS 456:2000)

 You can use the best mix design and perfect placing, but if the concrete is not cured properly, it will never reach its full strength and durability. Curing is cheap and easy, yet it is the step most often skipped or done poorly on site.

What Is Curing?

Curing means keeping freshly placed concrete moist and at a suitable temperature so that cement hydration continues. Hydration is the chemical reaction between cement and water that makes concrete gain strength. If the surface dries out too early, the reaction slows down or stops, and the concrete ends up weaker than designed.

Why Is Curing Important?

• Strength: Properly cured concrete reaches its design strength. Poorly cured concrete can fall well short of it.
• Durability: Good curing gives a denser surface layer, which resists water and chemical attack.
• Fewer cracks: Curing reduces plastic and drying shrinkage cracks.
• Better abrasion resistance: This matters especially for floors and pavements.

Minimum Curing Period as per IS 456:2000

Clause 13.5 of IS 456:2000 gives these requirements:

Condition	Minimum curing period
Ordinary Portland Cement (OPC)	7 days
Mineral admixtures or blended cements (fly ash, slag, etc.)	10 days
Concrete exposed to dry and hot weather	Not less than 10 days

For blended cements and mineral admixtures, the code recommends extending these minimum periods to 14 days. Always check the latest edition of the code and your project specification.

Common Methods of Curing

1. Ponding
Used for slabs, roofs, and floors. Build small earth or mortar bunds around the slab and fill them with water. This is one of the most effective methods for horizontal surfaces.

2. Wet hessian or gunny bags
Cover the surface with hessian and keep it constantly wet. This suits columns, walls, and other vertical members. The cloth must never be allowed to dry out.

3. Sprinkling or spraying water
Suitable for walls and columns. Water must be applied regularly, because intermittent wetting and drying can cause surface cracks.

4. Membrane curing (curing compounds)
A liquid compound is sprayed on the surface to form a film that stops moisture from escaping. It is useful for pavements and for places where water is scarce.

5. Polythene sheets
Cover the surface with sheets and seal the edges to hold moisture in. This is useful for small repair jobs and in windy conditions.

6. Steam curing
Used mainly in precast plants to speed up strength gain.

Common Curing Mistakes on Site

1. Starting curing late. Curing should begin as soon as the surface is hard enough that it will not be damaged.
2. Letting the surface dry between wetting. Concrete needs continuous moisture, not occasional splashes.
3. Stopping curing early. Many sites stop after 2 to 3 days because of schedule pressure.
4. Ignoring weather. Hot, windy, or low-humidity conditions cause fast moisture loss, so extra care is needed.
5. Removing formwork early without protection. Exposed vertical surfaces need curing right away.
6. Using dirty or contaminated water. Curing water should be clean and free from harmful chemicals and oil.

Quick Site Checklist

• Start curing as soon as the surface will not be damaged
• Keep the surface continuously wet for the required period
• Extend curing for hot weather and blended cements
• Protect curing materials from drying out and wind
• Record the curing start and end dates in your site log

Conclusion

Curing is one of the lowest-cost steps in concrete construction, yet it has a major effect on strength, durability, and service life. A little extra attention to curing can save a lot of repair cost later.

What curing method do you use on your site, and what challenges do you face? Share your experience in the comments!

INDIAN STANDARD CODE IS 456 ONLINE

Click on link below to view IS: 456 


https://drive.google.com/file/d/0By5WViLAgbucd0hjUnQySWZnY00/view?usp=sharing

IS 800

Click on link below to download IS 800

https://drive.google.com/file/d/0By5WViLAgbucQUEwVXM3bS1JaGs/view?usp=sharinging

What does IS:875-1987 covers?

This code covers basic design loads to be assumed in the design of buildings, in 5 parts. Each part is discussed here under:-

IS:875-1987(Part-1)- Dead Loads:-

                                                  This part specifies the unit weights of different types of building materials and the stored materials. The dead load in building shall comprise of the weight walls, partitions, floors etc. It is recommended that the load of partition walls, be assessed, by the designer on the basis of the actual constructional details of the proposed partitions.

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IS:875-1987(Part-2)-Imposed Loads:-Imposed loads not only include live loads but also loads like that of machinery etc.

Live loads based on floor usage in various types of buildings.

Reduction in floor live load for design of columns.

Live load on Roofs:-

-Live loads on various types of roofs. 

-Snow load.

-Loads due to Rain.

-Loads on members directly supporting the roof coverings.

-Loads on roof coverings.

-Horizontal loads on parapets

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IS:875-1987(Part-3)- Wind Loads:-

                                            This standard gives wind forces and their effects (static and dynamic) that should be taken into account when designing buildings, structures and components. The effect of wind on the structure as a whole is determined by the combined action of external and internal pressures acting upon it. Wind speed in the atmospheric boundary layer increases with height from zero at ground level to a maximum at a height. This code gives:

Wind map: which gives Basic Maximum Wind speed in m/s (peak gust velocity averaged over a short time interval of about 3 seconds duration). These wind speeds have been worked out for 50 years return period.  

Design Wind Speed(Vz) at any height z depends on the

-Probability factor,

-Terrain, height and structure size factor and

-Topography factor.

Design Wind Pressure: It depends on the:

-Coefficient, which depends on the atmospheric pressure and air temperature.

-Design Wind Velocity.

Off Shore Wind Velocity: Cyclonic storms form far away from the sea coast and gradually reduce in speed as they approach the sea coast.

Modification factors to modify the basic wind velocity to take into account the effect of terrain, local topography, size of structure, are included.

Terrain is classified into four categories based on characteristics of the ground surface irregularities. 

Force coefficients (drag coefficients) are given for frames, lattice towers, walls and hoardings.

The calculation of force on circular sections is included incorporating the effects of Reynolds number and surface roughness.

The external and internal pressure coefficients for gable roofs, lean-to roofs, curved roofs and multi- span roofs have been rationalized.

Pressure coefficients are given for combined roofs, roofs with sky light, circular silos, etc.

Dynamic effect.

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 IS: 875-1987(Part-4)- Snow Loads:

                                             This standard deals with the snow loads on roofs of buildings. Roofs should be designed for the actual snow load due to snow or for the imposed loads specified in Part 2 Imposed Loads. Ground snow load at any place depends on the critical combination of the maximum depth of undisturbed aggregate cumulative snowfall and its average density. Ice loads should be taken into account in the design of over- head contact lines for electric traction, aerial masts.

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IS:875-1987 (Part-5)-Special Loads and Loads Combinations:

The different load combinations to be considered for the design of any structure are:

-DL+LL

-DL+WL

          -DL+IL+WL

                               Where DL indicates dead load , LL indicates live load, WL indicates wind load, IL indicates imposed load and EL indicates earthquake load. While considering earthquake effects, EL is substituted at place of WL. It may be noted here that there will be four cases (one from each direction) due to the effect of WL or EL. For symmetrical structures, the number of cases may be reduced accordingly.

What does IS:456 covers?

IS:456-2000-Plain and Reinforced Concrete for Building Construction:

This is the fourth revision of the standard. The common methods of design and construction of plain or reinforced concrete structures is covered in this code. The code is divided into 5 sections, which detail out various guidelines and specifications.

Section 1: 

                        defines the scope, terminology and symbols.

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Section 2:

                        explains about materials, workmanship, inspection and testing. It includes types of cement, use of blended cements with mineral admixtures like flyash and ground blast- furnance slag, quality norms on water, admixtures, well defined exposure conditions, grade of concrete, formula for estimation of modulus of elasticity of concrete, durability of concrete, construction  joint etc.

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Section 3:

                    tells about general design consideration. It covers methods of design, various loads and load combinations, stability of structure against overturning and sliding, fire resistance requirements, Linear Elastic Theory to calculate internal action produced by design loads, recommendation regarding effective length of cantilever, recommendation for deflection due to lateral loads. According to clause no. 18.2, there are two methods of design (a) Working stress method & (b) Limit state method. Clause 26 gives guidelines for " Requirements governing reinforcement and detailing" wherein maximum and minimum spacing between reinforcement and cover requirements for various exposure conditions is specified.

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Section 4:

                         (Clause 28 to 34) relates o " Special Design Requirements for Structural Member & Systems". Guidelines for Deep Beams, Flat Slabs, Walls, Stairs, Stairs, and footings etc are covered under this section.

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Section 5:

                               (Clause 35 to 43) covers " Strutural Design ( Limit State Method)" in detail. Various limit states including that of flexure, compression, shear, Torsion, Deflection, Cracking etc are discussed.

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Annex B, discusses the requirements of " Working Stress Method".

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Annex D, gives moment coefficients for rectangular slab panels.

 

 

                   

IS: 1893: Design of Earthquake Resistant Structures

This standard is intended for earthquake resistant design of normal structures. It is applicable to buildings, elevated structures, bridges, concrete, embankments and retaining wall. It has been endeavored to ensure that, as far as possible, structures are able to respond, without structural damage to shocks of moderate intensities and without total collapse to shocks of heavy intensities. It takes into account:

a) Seismic zone factor. (Fo)

b) Importance factor, to account for the varying degrees of importance for various structures. (I)

c) The coefficient of Flexibility for design of multi- storeyed building is given in form of a curve with respect to period of buildings.

d) Performance factor depending on the structural framing system and brittleness or ductility of construction.

e) Intensity of shock due to earthquake.

f) The seismic zone map, the object of this map is to classify area of the country into five seismic zones.

Earthquake cause random motion of ground, which can be resolved in any three mutually perpendicular directions. This motion causes the structure to vibrate. The vibration intensity of ground expected at any location depends upon the magnitude of earthquake, the depth of focus, distance from epicenter and the strata on which the structure stands. There might be cases in which structure have less importance factor and relatively small structure for which no analysis need be made. There is Clause in code which gives permissible increase in allowable bearing pressure or resistance of soils. 

Use of IS:13920- 1993

IS:13920- 1993- Detailing of reinforced concrete structures subjected to seismic forces

This standard covers the requirements for designing and detailing of monolithic reinforced concrete buildings so as to give them adequate toughness and ductility.

Flexural Member

Clause gives the general requirement, which flexural members should satisfy.

a) Longitudinal Reinforcement:

-The top as well as bottom reinforcement shall consist of at least two bars throughout the member length.

- The positive steel at a joint face must be atleast equal to half the negative steel at that face.

- The steel provided at each of the top and bottom face of the member at any section along its length shall be at least equal to one- fourth of the maximum negative moment steel provided at the face of either joint.

- in an external joint, both the top and the bottom bars of the beam shall be provided with anchorage length, beyond the inner face of the column, equal to the development length in tension plus 10 times the bar diameter minus the allowance for 90 degree bend.

- Clause no. 6.2.6 gives about the lap, splice in beam.

IS:5525- Detailing of reinforcement in reinforced concrete works

This standard deals with the general requirements of detailing of reinforcement in reinforced concrete structures.

• According to it, all reinforcement bars used in structures shall be suitably designated & numbered both in drawing & in schedule.

• The use of the same type of line for the same purpose considerably enhances the clarity & usefulness of the drawing. 

• Different structural members of a structure shall be marked using symbols, abbreviations & notations.

• A key framing plan shall be prepared to a convenient scale & the two axes marked, one side with alphabets A, B, C etc & other with numbers. The same key framing plan may be used for all floors if the arrangement of beams is same at different floors.

• Columns & Foundations shall be specified by grid arrangement giving reference to the floor.

• Beams, Slabs & Lintels, Tie Beams shall be numbered from left hand top corner.

• Scales shall be so chosen to bring out the details clearly. No general recommendations can be given in this respect, although commonly used scales are given below as examples:

IS: 962: Architectural and Building Drawing

IS: 962 deal with architectural and building drawing. IS:962 designate drawing sheet sizes into six different types i.e. A0, A1, A2, A3, A4 and A5. Surface area of basic size A0 is 1 m2. The surface area of two successive sizes is in ratio 1:2.

S.No	Designated	Trimmed Size	Untrimmed Size
1	A0	841*1189	880*1280
2	A1	594*841	625*880
3	A2	420*594	450*625
4	A3	297*420	330*450
5	A4	210*297	240*330
6	A5	148*210	165*240

Layout of drawing

•  General 

  Sufficient margins are left from the edges of finished drawings to facilitate filing and binding where necessary.

• Margins 

Finished prints, which have been trimmed, will then be slightly less in size than the original drawings but not less then the frame or border line size of the drawings.

• Title Block

Title block show the details of drawing like title of drawing, name of organization or firm, scale, date of drawing etc in definite manner. The title block is placed at the bottom right- hand corner of sheet.

Scale	Name of office
DRN
TCD
Revisions	CHKD
ARCHITECT
DATE
DRG. NO.
Title of Drawing	Drg. No.

OR

	North direction
	Notes (From top downwards)
	Revisions (from bottom upwards)
	Title of Drawing
	Details given in the drawing
	Scale
DRN	DATE	checked
Signature		Signature
Name		Name
	Architect/ Engineer
	Drawing No

Typical layout of title blocks

• Additional information:

It is included to make the drawing complete. It contains:

a) Job no.

b)Material List:- it include item like schedule of reinforcement, quantity required etc. 

c)North point should be indicated.

• Reproduction of Drawings:

Original drawings and tracings are normally preserved carefully and copies are used on sites. The blue print copy is developed by immersion in water. Only blue print can provide translucent copy from which further copies can be made.

• Selection of scales for Drawings:

 It is estimated that drawing to scale 1: 1000 can be read to accuracy of 500 mm and drawing to 1: 400 scale can be read to accuracy of 200mm. If more than one detail drawn on different scale occurs on a sheet, the corresponding scale shall be shown under each relevant detail. Typical method of expressing numerical scales on drawing is 1 cm = 10mm or 1 : 1000 , 1 cm = 2.5 km or 1: 250000. Metric scales for architectural and building drawing is given in code IS: 962 on 12th page.

• Line Work:

All lines should be dense, clean and black to produce good print. There are different types of lines examples center-lines, hidden lines etc. 

What is use of SP:20?

This code is based on structural safety of buildings: Masonary Walls.

• Masonary

An assemblage of masonary units properly bonded together with mortar.

• Design by calculated masonary method 

The thickness of masonary for different spans, storey heights and openings given by Nomograms are worked out for three occupancies as given below:

S.NO.	OCCUPANCY	LIVE LOADING
a)	Residential buildings	200 kg/m2
b)	Office buildings	300 kg/m2
c)	Office buildings	400 kg/m2

• Structure of Nomograms

The Nomograms for thickness of brick wall consist of nine vertical lines. From left to right, the vertical lines represent:

1. Basic stress: of Masonry depends on the crushing strength of masonry unit and mortar used.
2. Storeys: In case of multi- storeyed buildings, the wall thickness at each floor is found by passing the line through the no. of storey above that section.
3. Reference line 1: It fixes a point on the line for any combination of values for Basic stress and storey.
4. Span point: Fourth line has a span point, through which all lines shall pass through for arriving at the thickness.
5. Reference line 2: It fixes a point on the line for any combination of values for Basic stress and Storeys.
6. Percentage of openings and thickness of walls for spans of 3m, 3.6m & 4.2m: The openings provided on the walls for windows, ventilators, doors etc are taken care of in the nomograms by this line.

What information is needed to start design of bridge as per IRC:5?

According to it, following information is needed before starting design of bridge:

• Collection of the data:

1. General data including maps, plans & topographic features.
2. Index map showing location of bridge.
3. Contour survey plan.
4. Site Plan (showing all relevant details).
5.  Cross- section of channel.
6. Hydraulic data for particular bridge site selected.
7. Geological data.
8. Loading data.

•  Determination of design discharge.
• Determination of linear waterway and effective linear waterway.
• Spacing and location of piers and abutments.
• Vertical clearance.
• Freeboard.
• Obstruction and river training.
• Determination of maximum depth of scour.
• Culverts.
• Kerbs.
• Width of Roadway and foot-way.
• Super elevation.
• Drainage of roadway.
• Expansions joints.
• Bridge foundations.