This document discusses mix design considerations for concrete placed in hot weather. Key factors that affect concrete quality in hot weather include high ambient temperature, concrete temperature, low relative humidity, and solar radiation. These conditions can increase water demand, accelerate setting and slump loss, and increase cracking risks. The document recommends ways to control these risks, including cooling concrete ingredients, using admixtures to reduce water demand and extend workability, limiting cement content, and promptly placing and curing concrete. Trial batches should be used to establish suitable mix designs for local hot weather conditions.

1. Design mix for Hot
Weather concrete
Libyan academy / Misurata
By :Ahmed Elgasayer.

2. Introduction
 The design of a concrete mix is a complex
procedure. It depends on many factors such as
properties of materials Method of preparation,
placement and curing Of concrete, as well as the
requirements of a construction

3.  This procedure becomes more Difficult to achieve for
an optimum mix design in hot Weather conditions.
Why ?

4. What is the hot weather
 Hot weather is defined in the Aci 305 is any
combination of the following condition that tends to
impair the quality of freshly mixed or hardened
concrete by accelerating the rate of moisture loss and
rate of cement hydration

5. - High ambient temperature.
- High concrete temperature.
- Low relative humidity.
- High wind speed.
- Solar radiation.
- Rate of slump loss and corresponding

6. potential problems in hot weather
concreting
o freshly mixed state
 Increased water demand.
 increased rate of slump loss
 Increased rate of setting
 Increased tendency for plastic shrinkage and
Thermal cracking

7. o concrete in the hardened state
 Decreased strengths resulting from higher water
demand (Addition of 20 kg of water could
the strength by 12 to 15%)
 Decreased durability resulting from cracking

8.  In order to develop a suitable method for concrete mix
Design in hot weather, the effects of the following factors
Were considered.
1. Water
2. Cement
3. Aggregate
4. Admixtures

9. 1-Water
 Water, as an ingredient of concrete, greatly influences
many of its significant properties, both in the freshly
mixed and hardened state.
 High water temperatures cause higher concrete
temperatures, and as the concrete temperature increases,
more water is needed to obtain the same slump.

10. 1-1 Effect of concrete temperature on
water requirement

11.  The extra water increases the w/c and will decrease the
strength, durability, water tightness. and other related
properties of the concrete this should be accounted for
during mixture proportioning.
 (Addition of 20 kg of water could reduce the strength by
12 to 15%)

12. 1-2 Effect on drying shrinkage
 Drying shrinkage generally increases with total water
content increased.
 Rapid slump loss in hot weather often increases the
demand for water. increasing total water content and.
therefore. Increasing the potential for subsequent drying
shrinkage.
 Concrete cast in hot weather is also susceptible to
thermal shrinkage

13. 1-3 Effect on Temperature of concrete
 Because water has a specific heat of about four to five times that of
cement or aggregates. the temperature of the mixing water has the
greatest effect per unit weight on the temperature of concrete, even
though water is used in smaller quantities than the other ingredients.
 In general, lowering the temperature of the batch water by 3.5 to 4°F
(1.9 to 2.2°C) will reduce the concrete temperature approximately I °F
(0.5°C), but the quantity of cooled water should not exceed the batch
water requirement

14. which will depend on the mixture proportions and the
moisture content of aggregates.
 Water can be cooled to as low as 33°F {1°C) using water
chillers, ice, heat pump technology. or liquid nitrogen.

15.  To keep it cold. tanks. pipes. and trucks used for storing or transporting
water should be insulated and painted white.
 Using ice as part of the mixing water is a major means of reducing
concrete temperature
 When greater temperature reductions are required. Cooling by
injection of liquid nitrogen into the mixer holding mixed concrete can
be the most expedient means Injected liquid nitrogen does not affect
the mixing water

17. What if we make the water content remain constant ?

18.  when the amount of mixing water is held constant. it
indicates that an increase of 20°F in temperature can be
expected to decrease the slump by about I in. (25 mm)
 The mixing water required to change slump will be less
when a water-reducing. mid range water-reducing. or
high-range water-reducing admixture is properly used.

19. 2 cement
 selection of a particular cement can have a decided effect on the hot
weather
 The use of a slower-setting Type II Portland cement or type IP or IS
blended cement can improve the handling characteristics of
concrete in hot weather .
 Concrete that contains the slower-setting cements is more likely to
exhibit plastic shrinkage cracking, if not protected

20.  When using slower-hydrating cements, the slower rate of heat development
and that will effect the concrete temperature by resulting in lower peak
temperatures.
 There is less thermal expansion, and the risk of thermal cracking
 cooling of the concrete is reduced.
 The temperature increase from hydration of cement in a given concrete
mixture is proportional to its cement content. Therefore, the cement content
should be limited to the amount required to provide strength and durability.

22. 3 - Chemical admixtures
 By the admixtures we can solve some of the undesirable characteristics
of concrete placed during periods of high ambient temperatures.
 The benefits can include lower mixing water demand, extended periods
of use. and strengths comparable with, or higher than. concrete without
admixtures placed at lower temperatures.
 Admixtures without a history of satisfactory performance at the
expected hot weather conditions s should be evaluated before their use.

23. 3-1 types of chemical admixtures
 Water-reducing and retarding
Because water reducing and retarding admixtures generally increase
concrete strength, they can be used, with proper mixture adjustments, to
avoid strength losses that would otherwise result from high concrete
temperatures.
Type (D) can increase the early bleeding and rate of bleeding of concrete.
This admixture-induced early bleeding can be helpful in preventing drying
of the surface of concrete placed at high ambient temperature and low
humidity.

24.  Flowing concrete ( super plasticizer )
At higher slumps, heat gain from internal friction during mixing of the
concrete will be less.
The improved handling characteristics of flowing concrete permit more
placement and consolidation, and the period between mixing and initial
finishing can therefore be reduced
concrete strengths are generally found to be substantially higher than
of comparable concrete without admixtures and with the same cement
content.

25.  Extended slump
Some water-reducing admixtures can cause the concrete to
extend its working time by up to 2 hours. followed by
acceleration of strength gain.
 Mid-range water reducing admixtures.
 Extended set-control

26.  concrete containing an admixture should be evaluated for consistency of
performance in regard to the desired characteristics in hot weather
construction. such as workability. permeability, early strength
development. Placing and finishing characteristics
 in addition to the basic properties of slump retention, setting time. and
strength. These characteristics can influence selection of an admixture and
its dosage more than properties usually covered by most specifications.

27. 4- Aggregate
Aggregate usually account for 60 to 80% of the volume of normal weight
concrete. Therefore. the properties of the aggregate affect the quality of
concrete significantly. For Example Crushed coarse aggregate contributes
to higher water demand than rounded gravels, but is reported to provide
better resistance to cracking.
Coarse aggregate is the ingredient with the greatest mass in concrete:
changes in its temperature have a considerable effect on concrete
temperatures.
Cooling the coarse aggregate can be an effective supplementary means to
achieve desired lower concrete temperature

28. proportioning
 Mixture proportions should be established or adjusted on the basis of
field performance records indicating the effect of expected seasonal
temperatures and delivery times.
 The selection of ingredients and their proportions should be guided by
their contribution to satisfactory performance of the concrete under
hot weather conditions
 The cement content should be kept as low as possible, but sufficient to
meet strength and durability requirements

29.  Inclusion of supplementary cementations materials, such as fly ash
or slag cement. should be considered to delay setting and to
reduce the temperature rise from heat of hydration.
 The use of various types of water-reducing admixtures can offset
increased water demand and strength loss that could otherwise be
caused by higher concrete temperatures.

30.  High-range water-reducing retarders formulated for
extended slump retention should be considered where longer
delivery periods are anticipated.
 concrete should be proportioned for a slump of not less than
3 in. (75 mm) to permit prompt placement and effective
consolidation in the form
 Trial batches have to be made.

31. Temperature control of concrete
 Concrete can be produced in hot weather without maximum limits on
placing temperature. but an effort should be made to keep the
temperature of the fresh concrete as low as possible.
 concrete can typically be reduced by 1 °F (O.5C) if any of the following
reductions are made in material temperatures:
1. (4°C) reduction in cement temperature;
2. 4°F (2°C) reduction in water temperature
3. 2°F reduction in the temperature of the aggregates.

32.  As the greatest portion of concrete is aggregate , reduction of
aggregate temperature brings about the greatest reduction in
concrete temperature.
 Above-ground storage tanks for mixing water should be provided
with shade and thermal insulation.
 Mixer drum cooling.
 water chillers or heat pump technology as well as other methods.
such as substituting crushed or flaked ice for part of the mixing
water, or cooling by liquid nitrogen .

34. Curing in Hot Weather
 For mass concrete, curing should be by water for the basic curing
period when the air temperature is at or above 20°C, in order to
minimize the temperature rise of the concrete.
 If approved, the application of the curing compound should be
preceded by 24 hours of moist curing.

35. Precautions.
 Use materials and mix proportions that have a good record in hot
weather conditions.
 Cool the concrete or one or more of its ingredients.
 Using of the admixtures, and carefully design of the concrete mix .
 Use a concrete consistency that allows rapid placement.
 Reduce the time of transporting, placing, and finishing as possible.

36.  Schedule concrete placements to avoid extreme weather, such as at
night or during favorable weather conditions.
 Consider the methods to limit moisture loss during placing and
finishing such as sunshades, wind screens, fogging, and spraying.

37. References
1. Reported by ACI Committee 305 .Guide to Hot Weather
Concreting.
2. Concrete mix design for hot weather report
By: Al-gahtany , Husain jubran.