Estimation of the heat evolutionduring the hydration of concrete,mix design of hot concrete, dryingand coating of concrete structures
Exercise 6
Temporal evolution of the hydrationproducts
Heat evolution and hydrationreactions
C3A hydrationFormation ofettringite
Ettringite coatingretards furtheraluminatehydration
Ettringite tomonosulfatetransformationand furtheraluminatehydration
Steady state ischaracterizedby the slowformation ofhydrationproducts
Heat evolution example
Heat evolution as function of grain size
Heat evolution rateduring hydration ofalite (C3S) asfunction of grainsizee.g. specificsurface area.
Hydration heat in concrete
• Generation of hydration heat– chemical reac on of cement and water →
hydration heat (120 cal/g)– low heat transfer in concrete → thermal gradient→ thermal stress → thermal cracking
– usually issue in mass concrete structures :foundation, dam, pier, nuclear power plant...
Hydration heat in concrete
Thermal cracking
Control thermal cracking– control of maximum temperature rise: admixture, cement
type, reduction of unit cement weight, cooling– control of thermal stress: reduction of thermal gradient ,
reduction of external restraint effect– Pre-stressing, strengthening of tensile resistance...
Thermal transfer at boundary
Estimation of the heat evolution ofconcrete
• Initial temperature• Heat of hydration• Heating• Heat evaporation
Initial temperatureAccording to the Finnish bridge manual (e.g. SILKO):
– The temperature of hardening concrete should not exceed 50°C
– the temperature rise should not exceed 25 °C and– the maximum temperature difference between different parts
of the structure should not be over 20 °C.
According to previous knowledge, general conclusions canbe made when using normal hardening cement:
• The 50 °C value is not generally exceeded when:– the initial temperature of concrete is ≤ 20 °C,– The amount of cement ≤ 350 kg/m3 and– thickness of the structure 0,9 m
Initial temperature• The 50 °C value is generally exceeded when:
– The initial temperature of concrete is ≥ 20 °C– The amount of cement ≥ 400 kg/m3 and– thickness of the structure ≥ 1,2 m
• 25 °C temperature rise is generally exceeded when:– the cement content is ≥ 350 kg/m3,– thickness ≥ 1,0 m and– outside temperature ≥ 0 °C
• 20 °C maximum temperature difference betweendifferent parts of the structure is in generalexceeded when:– the outside temperature is below 5 °C and– the only coating is a protective cover (suojapeite)
When low-heat cements are used:– the maximum temperature does not in general exceed 50
°C nor does the 25 °C temperature rise• if the cement amount is below 400 kg/m3 and• thickness ≤ 1,5 m.
– The only limiting matter is the slow strength developmentof low-heat cements.
When roughly estimating the temperature rise andmaximum temperatures during hydration its beneficial touse:• Previous measurements• Calculate the temperature rise during a so-called
adiabatic state
Initial temperature
• The temperature of hardening concrete can be influencedby lowering the initial temperature of the concreteingredients.
• The temperature of concrete is determined by thetemperatures of its ingredientsafter the following formula:
T =T m c + T m c + T m c
m c + m c + m c
Where: T = temperature [°C] b = concrete (betoniC = specific heat capacity [kj/kg °C] c = cementm = mass of the ingredient [kg] a = aggregates
w = water
Initial temperature
• The specific heat of water is 4,2 kJ/kg °C andfor the aggregate and cement 0,8…0,9 kJ/kg°C.
• As an approximation:– the specific heat capacity of water is 5 [kJ/kg °C]– the specific heats capacity of other ingredients is 1
[kJ/kg °C]
T ≈T m + T m + T m × 5
m + m + m × 5
Initial temperature
The factors affecting the tempareture rise of concrete are:– the cement content– heat evolution of the cement.
The cement content can be lowered by:– Using water reducing admixtures– Using as stiff mix as possible– Increasing the maximum size of the aggregate– Using an aggregate composition with favorable
grading
Initial temperature
By using low-heat cements the temperature risein the structure can be essentially lowered.
The greatest benefit can be achieved if thestrength is evaluated at the age of 91 days.
Initial temperature
According to the formula all of the following actions lowerthe temperature of concrete by about 1 °C
• The temperature of cement is lowered by 10 °C• The temperature of aggregate is lowered by 1,6 °C• The temperature of water is lowered by 3,6 °C• Part of the water is replaced with ice slush (jäähile) 6
kg/m3
Initial temperature
=
Heat of hydration
Heat of hydration
W = heat of hydration during the period 0-t (kJ/kg cement)
× *
Where, C = amount of cement (kg/m 3)cb = specific heat of concrete (kJ/kg °C )ρb = density (specific gravity) of concrete (kg/m3)W = heat of hydration
during the period 0-t (kJ/kg cement)
= × ×
Where, A1 = area of the heated concrete (m2)cb = specific heat of concrete (kJ/kg °C )ρb = density (specific gravity) of concrete (kg/m3)V = volume of the heated structure(m3)Wu = during the heating the amount of outside
energy brought into the concrete (kJ/m2)
Heating
Wu = during the heating the amount of outside energybrought into the concrete (kJ/m2)
Wu = Q*fQ =( A *ln (t) + B )
Heating
= × − ∆
Where, Tb = average temperature of concrete(°C)cb = specific heat of concrete (kJ/kg °C )ρb = density (specific gravity) of concrete (kg/m3)A2 = area of the cooling concrete (m2)V = volume of the heated structure(m3)kt = heat transfer coefficient of surface A2
at the time of calculation(W/ °C m2)Tu = temperature of outside air(°C)∆t = a period which is chosen according to
the rate of change of the temperature (h)
Heat evaporation
Estimation of the heat evolution ofconcrete
= +×
× −×
− ∆
Initialtemperatureof the freshconcrete
Heat ofhydration
+× *
EnvironmentHeat treatment
Solutions to exersices 1 and 2 werepresented in the EXCEL FILE:
H6 T1 ja T2.xlsx
Proportion the following mix as heated concrete (+ 50 °C)• Cement 325 kg/m3
• Water 188 kg/m3
• Aggregate 1835 kg/m3 (moisture contentof aggregate 4,2 %)
Exersice 3
From empirical data we know that:• a concrete mix at 50 °C is 1-2 consistency classes stiffer than the
same concrete at 20 °C
• From the mix design formwe can see that a change ofone consistency classrequires about 10 l/m3
water.• Thus the temperature
change requires about 15l/m3 extra water
The mix composition is:WATER 188 + 15 = 203 kg/m3
The new cement amount from the mix design formwhen the strength of the concrete stays the same:
188 + 20325
=203 + 20
C
→ C = 348 kg/m
AGGREGATE amount by using the basicequation of concrete665 l → 1775 kg/m
Air
The temperature of the concrete (Tb) canbe estimated using the formula:
(or one can use a more precise formula:
T = )
T ≈T m + T m + T m × 5
m + m + m × 5
s = Cementr = Aggregatesv = Water
m (kg) T (°C) T*m
CEMENT 348 20 6960
AGGREGATE 1775 55 97625
WATER from aggregate 74,55 55 4100,25
WATER added 128,45 65 8349,25
Tb = 6960 + 97625+5*(4100,25+8349,25)348 + 1775 + 203*5
→ 53 °C OK
Choose the temperatures
CONCRETE DRYING
Concrete dryingFactors which affect the time needed for concrete todry to required moisture levels include:
– Type of cement– Type and amount of aggregate– Water/cement ratio– Presence of a vapor barrier
(slab-on-grade)– Curing and drying conditions– Thickness of the concrete slab– Specified moisture condition set forth by flooring
material manufacturer
Concrete drying• Typically, the water/cement ratio is the single most
important determining criteria for the drying ofconcrete.
• For a concrete with a water/cement ratio of 0.50 to0.70, the drying time to reach 90% RH is anywherefrom 3 to 9 months, under suitable dryingconditions.
• Concrete utilizing a water/cement ratio of 0.38 – 0.5typically take 2 to 3 months to reach 90% RH undersuitable drying conditions.
A conventional concrete structure dries slowly• When the height of the structure is 100 mm and it
can dry to both direction about half of the structuralhumidity exits during 3 to 12 months, depending onthe density of the structure
A conventional concrete structure dries slowly
• The time to dry quadruples when thethickness of the structure doubled!
• The time to dry quadruples when thestructure can dry to only one direction
dry to only one direction
• Raising the temperature +20 °C → +50 °C speedsup the drying process 2 to 4 times.
• However the relative humidity of the surroundingair must be kept sufficiently low.
• By choosing the right concrete composition, thedrying can be speeded up by 2 to 10 times
• Especially harmful for concrete drying is if excesswater (from wet curing, exposure to the weather)comes into contact with the structure aftercasting
A preliminarydrawing ofequilibrium moisturecontents of differentconcretes
Picture from by 45 / BLY 7Betonilattiat 2002page 140
Moi
stur
eco
nten
t[w
eigh
t%]
Relative humidity of concrete [%]
The hysteresis phenomenon causesthe equilibrium moisture content inweight -% to be higher while drying
Picture from by 45 / BLY 7Betonilattiat 2002page 140
Moi
stur
eco
nten
tin
wei
ght-
%
Relative humidity [%]
Exersice 4The flooring is to be done with plastic/vinyl plates (muovilaatta).How long must one wait from the casting until the flooring canbe installed? The structure, environment and the concretespecifics are:
• Concrete slab, thickness 80 mm, strength K30• Under the slab 50 mm cellular plastic, plastic film and
gravel• The slab is not wetted, curing is done with plastic sheets
for 2 weeks• At the time of drying the temperature is estimated at +16
°C and relative humidity at 60 %• The maximum size of aggregate in the concrete is 8 mm,
binder 50 % CEM II A 42,5 R and 50 % GGBS, consistency1...2 sVB
Exersice 4plastic/vinyl plates
(muovilaatta)
80 mmK30
50 mm cellular plastic,plastic film and gravel
curing is done with plastic sheetsfor 2 weeksà NO WATER
T = +16°CRH = 60%
Agg. max. # = 8 mmCEM II A 42.5 RCem / GGBS = 50/50Consistency = 1 .. 2 sVB
60 days is drying time for thebase concrete slab (BY 45)
1. Concrete no air entrainment(1,0), K30 (1,0)
2. The age of concrete at thebeginining of the drying process 2wks, thickness of the slab 80mm(< 150mm) (0,8)
3. At the time of drying thetemperature is estimated at +16°C (about 1,2) and relativehumidity at 60 % (1,2)
4. thickness of the slab 80mm (0,7)5. Under the slab 50 mm cellular
plastic, plastic film and gravel(1,0)
6. Effect of the concretecomposition:• Maximum size of aggregate
8mm (1,0)• Binder (1,0)• consistency (1,2)
The max. relative humidity of theconcrete slab before covered byplastic/vinyl plates (muovilaatta)should be 90%
From by 45 / BLY 7Betonilattiat 2002 page 132
Time from casting
14 + (1*0,8*1,2*1,2*0,7*1,0*1,0*1,2)*60
= 14 + 0,97*60 = 72 days≈2,5 months !
Curing
60 days is drying timefor the base concrete
slab (BY 45)
Factors from table 4.9(BY45)
How long would it take for the concrete to dryso that parquet/hard wood floors could beinstalled?
From practice it is known that drying concreteto RH of 80 % takes 2 to 4 times longer than to90 %.Thus it would take about 4...8 months!
How to shorten the time for concrete drying?
The rule of thumb is that you’ll need to allow 28days of drying time for each 25 mm of concretethickness, if the slab is under ideal drying conditions(an enclosed area with the HVAC on, meaningthere’s air circulation and a low ambient relativehumidity)
≈ 1 mm depth drying / day
How to shorten the time for concrete drying?
To speed thedrying process
After castingSolutions
PreventativeSteps
• low W/B ratio mixes• self-desiccation agents• admixtures, like silica fume
• Dehumidification• Desiccant-based dehumidifiers• The condensation process uses
cooling-based dehumidifiers
