The primary characteristic that makes mass concrete different from other concrete work is thermal behavior. Cement hydration process is exothermic by nature, in large mass concrete where the heat is not quickly dissipated, temperature rise can be quite high. Tensile stress may develop from the volume change due to the increase and decrease of temperature within the mass. A structure or portion there of may be considered mass concrete if measure are necessary to control thermal behavior to alleviate cracking. Prof Lee in his paper has already investigated that the minimum dimension of 1.50 meter is considered as mass concrete.
History
Prior to 1994, mass concretes are done without any mineral additives. In 1994, fly ash was started to be produced and marketed. Since that time, most mass concrete has been done using 30% fly ash content in mix proportion to reduce heat that is developed during the hydration process.
Temperature
a. Temperature of Normal (Hot Weather) Concrete is 30 'C to 37 'C
To achieve a lower peak temperature of interior mass concrete during hydration period, pre‑cooling concrete materials prior to mixing is practiced. Aggregates in the stockpile are sprayed with water or shaded. Cement in silo was kept at least one night before, and water in tanks can be chilled with ice. For several projects where the initial fresh concrete temperature is very low (max 32oC), crushed ice was introduced into the mixing water. Furthermore, the use of ice in mass concrete would lead to higher production cost and extra supervision should be applied to achieve the most economic result.
b. Calculation of Initial Fresh Concrete Temperature
For example, calculation of fresh concrete temperature:
Concrete Grade K400 (cube,kg/cm2): Mass of cement = 400kg ( Tc=89 'C), Aggregate =1797kg (Ta=29,7 'C), Water=138kg (Tw=29.5 'C), Water in Aggregate=67kg (Twa=29.2 'C).
According to American Concrete Institute, calculation of initial concrete temperature as below formula;
Tf = [0.22 (Ta.Wa + Tc.Wc) + Tw.Ww + Twa.Wwa] / [0.22 (Wa + Wc) + Ww + Wwa]
Tf = 37.3 'C
Where Tf : Fresh Concrete Temperature
Ta : Aggregate Temp ( 29.7 'C)
Tc : Cement Temp ( 89 'C)
Tw : Water Temp ( 29.5 'C )
Twa : Water in Agg Temp ( 29.2 'C )
Wa : Mass of Aggregate ( 1797 Kg )
Wc : Mass of Cement ( 400 Kg)
Ww : Mass of Water ( 138 Kg )
Wwa : Mass of Water in Agg ( 67 Kg)
Calculation of Peak Concrete Temperature :
Tp = {(W xC)+t} /100
TP= 82.6 'C
Where: Tp : Peak Concrete Temperature
W : Mass of Cement ( 400 Kg )
C : Coefficient of Temp Rise 12 'C
t : Ambient Temp ( 34.6 'C)
1. Effect of Temperature on Fresh Concrete
The effects of temperature on fresh concrete are mainly due to its accelerating influence on rate of cement hydration. This results in :
a. Reduction in initial degree of workability for constant mix proportioning about 2 cm slump per 10 'C. Alternatively the water content for equal workability has to be increased about 7 kg per 10 'C. To maintain the W/C ratio for equal strength, cement content is also increased about 10 to 20 kg.
b. Faster rate of workability loss as indicated by shorter setting time of cement or concrete. In addition, the increased rate of evaporation of water produces further stiffening of the mix. Set retarding admixture is needed to extend the setting time and water reducing admixture (plasticizers) for restoring workability. Re-tempering of concrete with water may lead to increase in effective W/C ratio and loss of strength. Re-tempering with admixture or addition of admixture is the recommended practice where necessary.
c. Rapid water evaporation from concrete before placing promotes faster stiffening. After finishing, the loss of bleeding water from large exposed surfaces due to a combination of climatic factors; high temperature, low relative humidity, high wind velocity and high solar radiation of unprotected surface, may lead to potential plastic shrinkage cracking. A rate evaporation exceeding 1kg/m'/hr is considered to be critical.
2. Effect of Temperature on Hardened Concrete
High temperature increases the rate of cement hydration, this produces:
a. Two opposing effects on hardened concrete strength are produced : a faster rate of hydration after setting leads to a higher strength at early ages under hot humid conditions; a high initial rate of reaction during the setting stage of cement produces a non‑uniform paste structure.
b. Potential differential thermal cracking may occur as the heat of hydration developed within the interior of thick section is prevented from dissipating rapidly to the exterior as the conductivity of concrete is low. A differential temperature is created between faster cooling surface and the slow cooling interior. For a minimum dimension of a member in excess of 1.5 to 2.0 meters, the rise in temperature is of the order of 12'C per 100 kg/cum in cement content. A differential temperature of 20'C is often taken as critical value for which the tensile strain capacity of concrete is reached.
Two possible situation for such occurrence are :
• The differential occurring before the peak temperature e.g. no or insufficient surface insulation, for which the subsequent eventual cooling of the interior may produce internal cracking
• The differential occurring after the peak temperature, e.g. when the surface is suddenly cooled by premature removal of insulation, for which external cracking may occur.
3. Methods of Temperature Control There are two stages of temperature control. The first is to control the initial temperature at the time of mixing as estimated from the above equation. The second is to control the rise in temperature due to heat of hydration, i.e. the peak temperature in the hardened concrete.
a. Reduction in initial concrete temperature - Precooling Method ·
. Preventing direct solar radiation on to aggregates with shading and painting cement silos in light color.
· Cooling of fine and/or coarse aggregates with chilled water, liquid nitrogen or dry ice (solidified carbon dioxide).
· Cooling of mixing water until below 10'C, and using ice block as part or total mixing water (+/- 2 ice blocks/m3)
· Sprinkling water on coarse aggregate to induce evaporative cooling of surface moisture.
· Use of liquid nitrogen to produce chilled water (Water Chiller), or slush (loose ice) or direct injection into fresh concrete.
b. Reduction in peak temperature ‑ Cooling Method and Material Selection.
· Reduction of initial concrete temperature by precooling methods
· Use water reduction admixture to reduce cement content
· Use of Low Heat Portland Cement (Type IV), Moderate Heat Portland Cement (Type II), or Sulphate Resisting Portland Cement (Type V)
· Use partial replacement of cement with pozzolans e.g fly ash to reduce cement content.
· POSTCOOLING by circulating cooling water or other liquid through thin‑walled pipes embedded in the concrete.
· Construction Management: Effort are made to protect structure from excessive temperature differentials by knowledgeable employment of concrete handling, construction scheduling and construction procedure.
Planning
The successful completion of a mass pour is largely determined by continuity of concrete supply, placement and compaction. Care must therefore be taken to ensure that :
a. The concrete supplier is able to meet the demand and that alternative sources are available in the event of breakdown.
b. The placing equipment has sufficient capacity and back up equipment is available.
c. The labor resources can handle the rate of concrete delivery.
It is also important to ensure compatibility between concrete production, transportation, placing rate, compaction rate and finishing. A deficiency in any one of these processes can lead to unacceptable delays.
Furthermore, the casting sequence and rate of supply must be such that a live working face is always maintained with the avoidance of Cold Joint.InsulationIf the most suitable concreting material are not available and it is expected that the temperature differential will be excessive and cause cracking, insulation can be applied. This prevents rapid heat loss from the surface and hence minimizes the temperature differential between the surface and the core.
If only modest insulation is needed, tenting may be sufficient. In its simple form, this will consist of polythene sheeting laid on the surface and fixed in such a way as to prevent evaporative cooling. To increase the insulating value, the sheeting can be raised on timbers, but care must be taken to ensure that the system is windproof.
For more effective insulation, quilts or foam mats, or soft board or sand laid on polythene sheets are methods which have been employed. Quilts or foam mats are probably the easiest to apply and remove and allow greatest flexibility.
The insulation should remain in place until the center of the pour has cooled to a temperature level which is low enough to avoid limiting differential being exceeded even if the surface should cool to ambient (below 20'C ). This period will vary with the size of pour (dimensions) and mix used. In usual, period of insulation + 14 to 21 days.
Plywood formwork is also good insulator and if left in place will allow the surface temperature to increase by 20'C or more thereby decreasing temperature differentials, if this system is adopted, however, care should be taken not to remove the formwork when the pour is at its peak temperature after 48 hours to increase or so, this could lead to rapid surface cooling and cracking. If the formwork has to be removed, it is best to loosen the shutters initially, but keep them in place for a period of say, 24 hours. This allows the surface to cool slowly, resulting in less severe thermal gradient.
Temperature Measurement
To determine when insulation or formwork can be removed, it is advisable to in thermocouples to measure directly the in situ temperature. This is relatively cheap and simple, and gives a direct measurement of temperature differentials. The thermocouple should be located at the center and at the surface to measure the temperature extremes and hence the maximum differential. Monitoring can be either manual or automatic.
The measurement can be done on 2 directions, i.e. horizontal and vertical. Horizontal line is purposed to get how concrete differs from I point to other point horizontally, in usually the distance between the points is about 10 meters. Vertical line is purposed to get the picture how concrete differs among the bottom, center and top layers.
Vertically, concrete will be observed at 3 points, where the exact locations are :
o Top layer is at 15 to 25 cm below the upper rebars
o Center point is in between top and bottom points.
o Bottom layer is at 15 to 25 cm above the lower rebars.
Top layer will contact direct to weather, and assumed there will be heat leaks from concrete to air. Bottom layer will contact to ground lean concrete, and assumed there will be small heat leaks from concrete to ground.
The thermocouple is located at the position and tied up with wire to make sure it will not move during concreting. 3 cables at the same position horizontally will be collected and inserted into a PVC pipe (V2 inch diameter ) to protect any cable jointing from concrete fall. The PVC pipe is installed vertical and supported by a steel bar. The steel bar is tied up properly and pipe is tied up to the bar.
The measurement can be done on 2 directions, i.e. horizontal and vertical. Horizontal line is purposed to get how concrete differs from I point to other point horizontally, in usually the distance between the points is about 10 meters. Vertical line is purposed to get the picture how concrete differs among the bottom, center and top layers.
Vertically, concrete will be observed at 3 points, where the exact locations are :
o Top layer is at 15 to 25 cm below the upper rebars
o Center point is in between top and bottom points.
o Bottom layer is at 15 to 25 cm above the lower rebars.
Top layer will contact direct to weather, and assumed there will be heat leaks from concrete to air. Bottom layer will contact to ground lean concrete, and assumed there will be small heat leaks from concrete to ground.
The thermocouple is located at the position and tied up with wire to make sure it will not move during concreting. 3 cables at the same position horizontally will be collected and inserted into a PVC pipe (V2 inch diameter ) to protect any cable jointing from concrete fall. The PVC pipe is installed vertical and supported by a steel bar. The steel bar is tied up properly and pipe is tied up to the bar.
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