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					                    Temperature problems in concreting

Main Reference

1. Neville A et al "Concrete Technology", revised reprint 1990, Longman Scientific
   and Technical. (Chapters 2, 9, 10 and 13 are particularly relevant).
2. Sample specification concerning the requirement for temperature in concrete
   (attached).


Concept and data related to temperature problem
 Specific heat (capacity): energy (heat) required to raise temperature of a material
  of unit mass by one degree.
                      Normal Concrete: 1~1.5 kJ/kgC.
                      Water         1 Cal/kgC = 4.18 kJ/kgC
 Latent heat: heat lost or gained when a substance change state (e.g. from liquid to
  vapour) without change of temperature.
 Coefficient of thermal expansion (10-6/C):
                      Concrete: gravel 12, granite 9, limestone 6
                      Cement paste: 11 ~ 20


Temperature problems associated with heat from two sources:
     External source: weather
     Internal source: heat of cement hydration

a. Problems in hot weather:
     Strength reduction due to rapid cement hydration
     Loss of workability due to rapid evaporation
     Surface cracking du to plastic shrinkage caused by rapid evaporation
     Strength reduction of air entrained concrete due to swelling of air bubbles

b. Problems associated with heat of cement hydration
     Strength reduction and cracking due to high temperature
     Strength reduction and cracking due to temperature differentials

c. Problems in cold weather:
     Strength reduction due to freezing before concrete hardening, forming porous
        structures
     Damage due to freezing after concrete set
     Delayed settling time
     Delayed strength development




                                       Temp - 1
Heat of cement hydration

Cement hydration is an exothermic process.

  Compound             Typical content       Heat of hydration kJ/kg (Cal/kg)
           C3 A             10.8                        867 (207)
           C3S              54.1                        502 (120)
           C2S              16.6                         260 (62)
         C3AF                9.1                        419 (100)
Minor Compound                -

Heat generated from hydration of 1 kg cement is about

       H = 0.108  867 + 0.541  502 + 0.166  260 + 0.091  419 = 446 kJ/kg

The specific heat of concrete is around, say 1.3 kJ/kg C. Assuming the concrete
cured in an adiabatic condition, cement content per cubic meter of concrete is 100kg,
and density of the concrete 2400kg/m3, the temperature increased due to hydration
heat is

        T
              446kJ / kg 100kg / m 3  14 C              (1)
             2400kg / m 3  1.3kJ / kg  C

The real situation, no matter how big is the concrete pour, concrete is not in adiabatic
condition. Temperature rise due to cement hydration is always lower than the derived
value from the compound hydration heat assuming adiabatic condition. It would be a
rough guess that 100kg OPC per cubic meter of concrete would raise the temperature
inside large concrete pour by around 10C.


Cracks caused by thermal deformation (movement)

Concrete is weak in tension. Tensile strength of grade 20 - 40 concrete is 3- 5N/mm2,
about 1/10 of compressive strength. Modulus of elasticity of hardened concrete ranges
from 15 to 35 kN/mm2.

                                     Rigid support



                                     Concrete prison

                       Fig. 1

Consider a concrete prison shown in Fig. 1, which is firmly restrained at its two ends
(no movement at the ends). Assuming modulus of elasticity of the concrete is
20kN/mm2, coefficient of thermal expansion is 10  10-6 1/C, then a drop of
temperature of 25C would induce a tensile stress of
               (10  10-6 1/C)  25C  20kN/mm2 = 5N/mm2                 (2)



                                         Temp - 2
in the concrete prison. This stress would be high enough to cause tension crack in the
concrete.

Concrete inside a structure subject to restraint from variety of sources: reinforcement,
support, adjacent members, and also from adjacent concrete if differential temperature
exist. Stress due to differential temperature is illustrated in Fig. 2.

Fig. 2 shows the section of a large-sized structural member. The temperature of the
concrete in the core zone is higher than peripheral concrete when the concrete initially
hardens at, say about one day. As the temperature in the member gradually leveled
off, concrete in the core zone tends to shrink more that surrounding concrete, whereas
peripheral concrete tends to resists the shrinkage of the core concrete. Compressive
stress is, therefore, introduced in peripheral concrete and tensile stress is induced in
the core area. Tension crack will result in the core zone if the tensile stress induced
reaches concrete tensile strength.

                       Peripheral zone,          Core zone,             Peripheral zone,
Core zone,             lower temp, say 30C      normal temp, say 25   normal temp, say 25
high temp, say 70C




                                                                                  Tensile cracks


         24 hr after casting                             120 hr after casting
Fig. 2



Typical requirement in Hong Kong concerning concrete temperature

        Limit placing temperature (32C for Hong Kong).
        Limit maximum temperature (85C).
        Limit temperature difference (25C).
        For sections greater than 1.5m, peak temperature be calculated and measured.
        Trials


Measures to achieve temperature requirement in hot weather

         a.   Use low heat cement
         b.   Use pozzolan, PFA, silica fume, etc.
         c.   Use water reducing agent
         d.   Lower mix temperature (ice, cooling aggregates))
         e.   Internal cooling system for mass concrete
         f.   Apply thermal insulation on formwork
         g.   Lower formwork temperature
         h.   Sheltering


                                              Temp - 3
Temperature of fresh mix

Temperature of fresh concrete is calculated by
           0.22 Ta Wa  Tc Wc   Tw Ww  Ta Wwa
      T
                 0.22 Wa  Wc   Ww  Wwa

Temperature of fresh mix using ice as part of free water is calculated by

            0.22 Ta Wa  Tc Wc   Tw Ww  Ta Wwa  LW i
       T
                  0.22 Wa  Wc   Ww  Wwa  Wi


Questions

1. Why the rate of heat development is more important than the total heat of cement
    hydration for practical purpose?
2. If a concrete mix for mass concrete pour contains 500kg OPC per cubic meter of
    concrete and placing temperature is 30C, estimate the possible peak temperature
    of the concrete.
3. If the maximum temperature in the concrete in previous question exceeds
    specified maximum temperature, what measures you may take to bring the
    temperature down?
4. When would you place ice in concrete mix?
5. State the measures used to minimize temperature gradient.
6. How temperature affect the strength development?
7. Discuss the influence of formwork striking time on the risk of thermal cracking.
8. What are the thermal problems in mass concrete?
9. Why insulation sometimes use in placing large concrete pours?
10. What are the effects of hot whether on fresh concrete?
11. Estimate peak temperature for a mass concrete based on the following
    information.
    Cement (OPC): 300kg/m3.
    Density: 2350kg/ m3.
    Specific heat: 1.2 J/kgC
    OPC contains 60% C3S, 20% C2S, 10% C3A and 10% impurities.




                                        Temp - 4

				
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