This book describes concrete curing process; why the need to cure concrete works and how to deal with it. This book is among the site books one must have to perfect his/her work.

1. CONTENTS
Health awareness
Why curing is necessary?
Methods and materials
Exposed concrete
Paving and floor slabs
Further reading
The downloadable booklets in the Concrete
on site series are a continuation of a series
originally issued in 1951 and have become
standard guides to site personnel.
6 Concrete
on site
!Safety on site
Many construction activities are
potentially dangerous so care is needed
at all times. Current legislation requires
all persons to consider the effects of
their actions, or lack of action, on the
health and safety of themselves and
others. Advice on safety legislation can
be obtained from any of the area offices
of the Health Safety Executive.
Concrete on site 6 - Curing
OS006
© The Concrete Society
05/12
Curing

2. Curing
HEALTH awareness
Dry cement powders in normal use
have no harmful effect on dry skin.
As with any dusty material there
may be ill effects from the inhala-
tion or ingestion of cement dust
and suitable precautions should be
taken.
When cement is mixed with water,
alkali is released. Precautions should
therefore be taken to prevent dry
cement entering the eyes, mouth or
nose, and to avoid skin contact with
wet concrete and mortar.
Repeated skin contact with wet
cement over a period of time may
cause irritant contact dermatitis.
The abrasiveness of the concrete or
mortar constituents can aggravate
the effect.
Some skins are sensitive to the small
amount of chromate that may be
present in cements and can develop
allergic contact dermatitis, but this
is rare.
Continued contact with the skin can
result in cement burns with ulceration.
Note that with alkali burns, pain is
not immediate.
Handling precautions
Protection for the eyes, mouth and
nose should be worn in circum-
stances when dry cement may
become airborne.
When working with wet concrete or
mortar, suitable protective clothing
should be worn, such as long-
sleeved shirts, full-length trousers,
waterproof gloves with cotton liners
and wellington boots. Clothing con-
taminated with wet cement, mortar
or concrete should be removed and
washed before further use. Should
concrete or mortar get into boots,
remove them IMMEDIATELY and
thoroughly wash the skin and the
inside of the boots before proceed-
ing with the job.
If cement enters the eye it should be
washed immediately and thorough-
ly with clean water and medical
advice sought.
Concrete or mortar elsewhere on
the skin should also be washed off
immediately. Whenever there is
persistent or severe irritation or pain
a doctor should be consulted.
WHY CURING IS NECESSARY?
Curing is the last, and one of the
most important stages of concrete
construction. No matter how
thorough you have been up to this
point, if the curing is not done prop-
erly, the concrete will not develop its
full potential.
Concrete that has been properly
cured is superior in so many ways.
It is stronger, more resistant to
chemical attack and traffic wear, and
more watertight. Furthermore, it is
less likely to be harmed by frost and
the accidental knocks it is bound to
receive.
The surface is the part of the con-
crete most affected by poor curing.
Yet it is this skin that gives concrete
its ability to withstand wear, and
that protects the reinforcement
and the‘heart’concrete. If it is
Fig 1. Well-cured concrete achieves its
potential

3. Curing
inadequately cured, the effective-
ness and the lifespan of the con-
crete will be affected.
What exactly does curing do?
When cement is mixed with water, a
chemical reaction known as hydra-
tion occurs. It is this that causes
the cement to harden and then
develop its strength and durability.
But hydration can take place only if
the concrete is kept moist and at a
favourable temperature in its early
life after placing — the process we
call curing.
If the concrete dries out, the hydra-
tion reaction comes to a halt. The
earlier this happens, the more seri-
ous is the effect on the concrete’s
potential quality.
In this booklet, we tell you how
to ensure that the concrete stays
moist and at a suitable temperature
during the first few days in normal
conditions. For information about
the extra precautions that need to
be taken during cold weather, see
Concrete on site 11: Winter Working.
METHODS AND MATERIALS
The Methods
There are two basic ways of curing
concrete. The first - let us call it
Group One - involves keeping the
surface of the concrete moist by the
use of ponding, spraying/sprinkling,
damp sand or damp hessian.
The second — Group Two —
prevents the loss of moisture from
the concrete by covering it with
polythene sheeting, spraying-on a
curing membrane, or leaving the
formwork in place.
Group One methods are undoubt-
edly the most efficient -and the
most appropriate for some types of
work.
However, they suffer from the
practical disadvantage that they are
expensive since they are labour-in-
tensive. Moreover if they are not car-
ried out properly- and it is difficult to
ensure this- they may do more harm
than good.
Group two methods, while not so ef-
ficient, are usually satisfactory for all
except very special work, and they
have the big plus point that they
can be carried out more easily.
Therefore, it is these methods that
we shall describe in this booklet.
Polythenesheet
A simple, yet effective, way of keep-
ing moisture in the concrete is to
cover it with 125 micron (500 gauge)
polythene sheeting.
Polythene sheet is a material that is
easy to handle and store, and after-
wards it can be re-used for so many
other purposes on site- including,
of course, curing other concrete at a
later date.
Usingpolythene
The polythene should be placed
is position as soon as possible- on
vertical surfaces within half an hour
of the removal of the formwork; on
slabs as soon as the concrete is hard
enough to prevent surface damage
or marking. Wait until the water
sheen has evaporated, but not so
long that the concrete starts to dry.
Fig 2. Hessian requires frequent
wetting
Fig 3. A roll of 125 micron polythene,
4m wide x 25m long

4. Curing
When the surface of a slab has a
special finish - such as a texture - the
polythene should be spread over
light frames above the concrete.
With such finishes there is no
need to wait for any bleedwater to
evaporate before you position the
polythene; the curing can start im-
mediately the slab
is finished.
It is important that the exposed
edges and corners of the sheet
should be well fixed down. Other-
wise wind and draughts would get
underneath and create a wind-tun-
nel effect that would dry the surface
of the concrete.
Good fixing also, of course, stops
the polythene sheet from blowing
about. If you have to use more than
one sheet, overlap them by at least
300mm, and tape the joins. Use tape
or string for tying round beams and
columns.
If there is no chance of frost, and the
concrete is hard enough, it will assist
the curing if you spray on water
before placing the polythene.
Spray-on compounds
Curing compounds are sprayed on
to the surface- a hand-held garden
spray is suitable for most jobs- and
can be used on both vertical and
horizontal surfaces. It is essential
always to choose the correct type of
compound.
Mostly, the compounds consist of
a resin in a solvent that evaporates
after it has been applied, leaving
behind a thin, continuous film or
membrane that seals-in most of the
moisture. The film peels off under
the action of wind and sun after
about four weeks (or foot traffic
when applied internally). Water
based compounds are available
for internal use where solvent
fumes may cause Health and Safety
concerns.
Most of the compounds come in
two grades, a standard grade having
a curing efficiency of 75% and a su-
per grade with 90% efficiency. Both
are usually available with either a
white or aluminized pigment, or
with a fugitive dye.
The white or aluminized pigmented
compounds are for external paved
areas because they reflect the sun’s
rays, thus keeping the concrete
cooler.
Those with a dye allow you to check
at a glance that a uniform, all-over
layer has been applied. The dye then
quickly disappears without causing
stains, provided it is not applied to a
dry surface.
In Britain a super-grade compound
with white or aluminized pigment
is advised for external paving, but
a lower, non-pigmented grade is
satisfactory for structural concrete.
In tropical climates, the higher
efficiency grade should be used in
every case.
If a surface is to receive any addi-
tional material that requires a posi-
tive bond, such as: screeds, render-
ing, paint type finishes or a further
layer of concrete, then a standard
curing compound should not be
used. Special curing compounds
are available and their compatibility
with the proposed additional mate-
rial should be checked.
Fig 4. Protect the finish of a textured
slab by raising the polythene on a
frame
Fig 5. Paved areas are cured with a
super-grade white, or aluminized,
spray-on compound

5. Curing
Curing compounds are generally
non-toxic but approval should be
obtained before using them on
structures that will be holding drink-
ing water.
Whentospray
The compounds should never be ap-
plied to dry surfaces since these will
absorb the compounds, and stain-
ing or discoloration may result. If the
surface is dry, that is a sign that the
curing has been left too late.
On vertical surfaces, apply the
compound- standard or high
grade- almost immediately after the
formwork has been struck. If the sur-
face has dried out, wet it down with
clean water, and then spray while it
is still moist.
On slabs, apply the compound as
soon as possible after any bleed
water has evaporated, and certainly
no later than half an hour after the
sheen caused by the water has
gone. Curing compounds do not
work if sprayed on top of water.
In damp conditions the water may
take a few hours to disappear, but
much less on a drying day.
Usingthecompounds
There are various points to watch
when you are applying a curing
compound.
Before you start, make sure that you
have enough in stock for the job in
hand-you will need about a litre for
each 4 m2
of surface area — and suf-
ficient of the cleaning solvent, too.
Always wear protective goggles,
gloves and mask. You should avoid
getting the compound on your skin,
in eyes or in your mouth, for it might
cause harm.
Make sure you read beforehand,
and understand, the manufacturer’s
health and safety notes so that you
will know what to do if anything
goes wrong.
Stir the compound before use, espe-
cially if it is a pigmented one, since
the pigment tends to settle at the
bottom of the drum. Fill the spray
reservoir with the compound, and
then you can start work.
You should hold the spray about 300
to 500mm from the surface, prefer-
ably even closer when there is wind.
It is vital that you cover all the con-
crete, and with a uniform layer. This
is best ensured by first moving the
nozzle back and forth to apply half
the quantity required, then applying
the remainder by moving the nozzle
at right angles to the initial strokes.
Clean out the equipment with
solvent after you have finished- and
whenever you break off spraying for
more than, say, half an hour- paying
particular attention to the nozzle
and hose.
Protection given by formwork
While it is in position, formwork
protects concrete against loss of
moisture, and thus allows curing to
proceed. It is only after it has been
struck that further curing of col-
umns, walls and beams, for instance,
may be necessary.
Fig 6. Fugitive dye shows whether the
application has been thorough
Fig 7. Equipment for applying a
spray-on curing compound

6. Curing
Concrete that is in an exposed posi-
tion must be thoroughly cured even
after prolonged protection by the
formwork.
But in a mild, protected environ-
ment, and provided that the
formwork has been in position for
at least four days, there might be no
need for further curing, even in dry
weather. However you should check
this point with the engineer or clerk
of works.
If the formwork is struck in less
than four days, and the surfaces are
designed to be a uniform colour- as
in the case of a row of columns, or
a continuous wall- thorough and
uniform curing is essential.
Even when the formwork is left in
place, exposed top surfaces must be
cured in the normal way. The top of
a wall, for example, will be exposed
to more severe weather conditions
than will the face. The top will prove
more durable if it is sprayed with a
high-grade curing compound.
Where steel reinforcement projects
from the top of a wall or column or
through a horizontal construction
joint-the top surface of the concrete
should be covered with polythene
sheet pressed into place over the
ends of the bars. The polythene
should be well taped to the top of
the formwork.
When concrete will receive a final
applied decorative treatment, such
as rendering, plaster or paint, in
conditions of mild exposure (inside
a building, for instance) further cur-
ing may not be necessary, no matter
how early the formwork is struck,
but this is something you need to
check first.
EXPOSED CONCRETE
All concrete that will be perma-
nently exposed to the weather
needs to be cured with extra care,
and for at least seven days. This is
particularly important for visual con-
crete, including white and coloured
concrete, plus special architectural
finishes created by abrasive blasting
or tooling.
Well-cured concrete will be more
impermeable and better able to
withstand freezing and thawing,
wetting and drying. It will also have
a better long-term appearance,
since dirt will not collect easily.
Polythene sheeting can be used, but
a spray-on curing compound is usu-
ally more convenient, though its use
may be subject to approval in the
case of special visual requirements
for colour and uniformity.
Colour uniformity
The colour of concrete can vary ac-
cording to how long the formwork
is left in position after placing, and
whether the day on which it is struck
is dry or wet.
Where uniformity of colour is impor-
tant, e.g. with as-struck or textured
surfaces, you should either leave the
formwork in position for four days,
or, where it is struck in less than that
time, cover or wrap the concrete in
polythene sheeting for at least an-
other three days. Curing compounds
might discolour the concrete, even
if only temporarily, and might leave
some uneven staining.
Fig 8. Formwork gives some protec-
tion, but tops of walls and columns
still need to be cured
Fig 9. All exposed concrete must be
thoroughly cured

7. Curing
Whiteandcolouredconcrete
It is best to use the polythene sheet-
ing method when curing white or
coloured concrete.
Polythene also has the advantage
that, if left in place, it will protect the
concrete from dirt caused by activi-
ties that are proceeding on the site.
Removing stains and dirt at a later
stage is both time consuming and
expensive.
Hessian or a curing compound
should only be used if these will not
cause staining of the concrete.
Rust Staining
The action of rain on reinforcement
left projecting from the tops of walls
and columns e.g. for continuity with
concrete that will be placed later
on, can create rust. This will form
stains as it is washed down the face
of the cast concrete. Rust stains can
be prevented by applying a neat
cement grout wash, or taping plastic
sheaths, to the exposed reinforce-
ment.
PAVING AND FLOOR SLABS
It is essential for most horizontal
surfaces to be well cured, especially
if they will have to stand up to foot
or vehicular traffic.
Always start the curing soon after
the water sheen has disappeared.
However, if you are using polythene
sheeting, allow the concrete to
stiffen slightly first, so that the sur-
face will not be marked or damaged
as the sheet is placed on top of it.
Major concrete roads are usually
cured by a membrane sprayed from
a machine that is part of the paving
train. They are not considered in this
booklet.
Combined curing and sealing
compounds are available that can
reduce the incidence of dusting
from floor slabs.
Minor roads, paths and drives
A curing membrane, applied by a
hand-operated garden-type spray,
is most convenient for small paved
areas built by semi-manual methods
of construction.
Always use white pigmented or
aluminized super grade of com-
pound, because this will reflect the
sun’s rays. You must also take care
to ensure that an even, uniform
coat of the membrane is applied,
particular care being needed in
windy weather, when the stream of
compound could easily be blown
‘off course’.
If the road is fairly wide, you will not
be able to reach far enough to spray
the middle. You will therefore need
to work from a movable walkway
that spans the road. A stout plank
well supported at both ends will be
sufficient.
However, you might find it more
convenient to use polythene sheet-
ing, and this is just as good. Keep
the sheet in place for at least seven
days, and make sure it is well fixed at
the edges, where there is a tendency
for the concrete to dry out more
quickly.
Fig 10. By leaving polythene sheeting
in place, you give added protection to
the concrete
Fig 11. Paths and drives are cured
with a white or aluminized spray-on
compound

8. Curing
Direct finishes and toppings
Special attention to curing is essen-
tial for power-floated and power-
trowelled finishes.
Immediately after the final finishing,
the surface should be firm enough
to be either covered with polythene
sheeting or similar in the case of
small areas, or treated with a curing
compound. Polythene should be
ideally kept in place for at least
seven days. Some loss of moisture
may occur at the edges and joints of
the sheets, and it will then be neces-
sary to turn them back and spray the
surface with water every other day.
Cement-sand screeds
Curing compounds are not
recommended for cement-sand
screeds. The screed should be kept
continuously damp for seven days,
preferably by being covered with
polythene.
!USEFUL CONTACTS
British Precast Concrete Federation
www.britishprecast.org
BSI Quality Assurance
www.bsi-global.com
Cement Admixture Association
www.admixtures.org.uk
Cementitious Slag Makers Association
www.ukcsma.co.uk
Construction Plant Association
www.cpa.uk.net
Meteorological office
www.metoffice.gov.uk.
Mineral Products Association
www.mineralproducts.org
Quality Scheme for Ready Mixed Concrete
www.qsrmc.co.uk
Sprayed Concrete Association
www.sca.org.uk
The Concrete Centre
www.concretecentre.com
The Concrete Society
www.concrete.org.uk
UK Cares (reinforcement)
www.ukcares.co.uk
UK Quality Ash Association
www.ukqaa.org.uk

9. Curing
FURTHER READING
For information on Standards and other publications, refer to the Concrete
Book Shop, www.concretebookshop.com
British Standards
BS 1881: Testing Concrete.
Part 113: Method for making and curing no-fines test cubes.
Part 130: Methodfortemperature-matchedcuringofconcretespecimens.
BS 4449: Steel for the reinforcement of concrete. Weldable reinforcing steel. Bar,
coil and decoiled product. Specification.
BS 4482: Steel wire for the reinforcement of concrete products. Specification.
BS 4483: Steel fabric for the reinforcement of concrete. Specification.
BS 5975: Code of practice for temporary works procedures and the permissible
stress design of falsework.
BS 7542: Method of test for curing compounds for concrete.
BS 7973: Spacers and chairs for steel reinforcement and their specification.
BS 8443: Specification for establishing the suitability of special purpose concrete
admixtures.
BS 8500: Concrete – complementary British Standard to BS EN 206–1: 2000,
Part 1: Method of specification and guidance for the specifier.
Part 2: Specification for constituent materials and concrete.
BS 8666: Scheduling, dimensioning, bending and cutting of steel reinforcement
for concrete. Specification.
BS EN 206-1: Concrete.
Part 1: Specification, performance, production and conformity.
BS EN 12350: Testing fresh concrete.
Part 1: Sampling.
Part 2: Slump test .
Part 4: Degree of compactability.
Part 5: Flow table test.
Part 7: Air content. Pressure methods.
BS EN 12390: Testing hardened concrete.
Part 2: Making and curing specimens for strength tests.
Part 3: Compressive strength of test specimens.
BS EN 13670: Execution of concrete structures.
BS EN 13286-41 Unbound and hydraulically bound mixtures: Part 41: Test
method for determination of the compressive strength of hydraulically bound
mixtures.
BS EN 14227-1 Unbound and Hydraulically bound mixtures – Specifications
– Part 1 Cement bound granular mixtures.

10. 10
Curing
Further reading continued.
Building Research Establishment
Design of normal concrete mixes, 2nd ed, 1997
Formwork for modern, efficient concrete construction, BR495, 2007
The Concrete Society
Technical Report 52, Plain formed concrete finishes, 1999
Technical Report 62, Self-compacting concrete, 2005
Formwork – a guide to good practice, 2nd edition, 1995
Good Concrete Guide 2: Pumping concrete, 2005
Good Concrete Guide 6: Slipforming of vertical structures, 2008
Good Concrete Guide 7: Foamed concrete, application and specification, 2009
Good Concrete Guide 8: Concrete practice, Guidance on the practical aspects of
concreting, 2008
Checklist for erecting and dismantling falsework, 1999
Checklist for assembly, use and striking of formwork, 2003
Concrete Advice no 16, Assessing as struck in situ concrete surfaces, 2003
Concrete Advice no 20, Curing concrete, 2005
Concrete Advice no 30, Identity testing for strength in accordance with
BS EN 206-1 and BS 8500-1, 2007
Concrete Advice no 31, Identity testing of fresh concrete for properties other
than strength, 2007
Concrete Advice no 37, Mould release agents, 2008
Construction Industry Research and Information Association (CIRIA)
Concrete pressure on formwork, Report 108, 1985
Formwork striking times – criteria, prediction and methods of assessment,
Report 136, 1995
Action in the case of non-conformity of concrete structures, Report C519, 1999
Construct
Guide to flat slab formwork and falsework, 2003
A guide to the safe transportation of formwork and falsework equipment, 2005
A guide to the safe use of formwork and falsework, 2008
Health and Safety Executive (HSE)
Hand-vibration – The control of vibration at work regulations, 2005
The work at height regulations, 2005
The Control of Substances Hazardous to Health Regulations, 2002
Management of Health and Safety at Work Regulations, 1999
Managing Health and Safety in Construction, 2007
The Personal Protective Equipment at Work Regulations, 1992
The Manual Handling Operations Regulations, 1992
Highways Agency, Her Majesty’s Stationary Office (HMSO)
Manual of contract documents for highway works, vol 1, Specification for High-
way Works

11. 11
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OS006
Published May 2010
© The Concrete Society
The Concrete Society
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Station Approach, Blackwater, Camberley
Surrey GU17 9AB
Tel: +44(0)1276 607140 FAX: +44(0)1276 607140
www.concrete.org.uk
Other publications in this series are available from the
Concrete Bookshop at www.concretebookshop.com
Tel: +44(0)7004 607777
Curing
1. Ready-mixed concrete
2. Reinforcement
3. Formwork
4. Moving fresh concrete
5. Placing and compacting
6. Curing
7. Construction joints
8. Making-good and finishing
9. Sampling testing fresh concrete
10. Making test cubes
11.Winter working