4 Heat of Hydration PDF
4 Heat of Hydration PDF
4 Heat of Hydration PDF
CONTENTS
Overview
38
Analysis results
38
Overview
The rate and amount of heat generation are important in concrete structures having
considerable mass.
Thermal cracking in a
concrete structure tends to be wide and propagates through the structure. This naturally
has adverse effects on strength, durability and permeability.
structures are cast in many stages with construction joints.
segments exhibit different heat source properties and time dependent properties. Therefore,
construction stages must be incorporated in a heat of hydration analysis model to truly reflect
a real construction process.
Stresses due to heat of hydration are classified as Internal Constraining Stress and
External Constraining Stress.
restraining effect of volumetric changes due to different temperature distributions within the
concrete structure.
between the surface and inner parts result in surface tension. Whereas at a latter stage,
contracting deformations in the inner parts are greater than those at the surface, thereby
resulting in tension stresses in the inner parts. The magnitude of the Internal Constraining
Stress is proportional to the temperature difference between the surface and inner parts.
External Constraining Stress is caused by restraining the volumetric change of fresh
concrete in contact with subsoil or the substrate of previously cast concrete. The change in
concrete heat results in the change of volume, and the restraining effect is dependent on the
contact area and stiffness of the external constraining objects.
Heat of hydration analysis can be accomplished through Heat Transfer Analysis and
Thermal Stress Analysis.
change of nodal temperatures with time due to heat source, convection, conduction, etc.,
which take place in the process of generating heat of hydration of cement. Thermal stress
analysis provides stress calculations for mass concrete at each stage based on the change
of nodal temperature distribution with time resulting from the heat transfer analysis. The
stress calculations also account for time and temperature dependent material property
changes, time dependent shrinkage, time and stress dependent creep, etc.
This tutorial demonstrates the process of construction stage analysis and analyzes the
results for a foundation structure constructed in two stages or pours.
outlines the procedure of generating a construction stage model for heat of hydration
analysis and reviewing the analysis results:
Prescribed Temperature
Pipe Cooling
Construction Stage
Perform analysis
* Pipe cooling is not included in this tutorial for clarity in demonstrating the interaction of the two concrete
parts while analyzing the results of heat of hydration analysis.
It
consists of subsoil mass and two parts of mass concrete cast in two stages as shown in
Figure 1. The 2nd pour takes place after 170 hours of casting the 1st pour. Heat of hydration
analysis is performed for the period of 930 hours after casting the 2nd concrete mass.
If the subsoil mass, that is interfaced with the concrete, is modeled as soil springs to
represent the boundary condition, the transfer of the concrete heat cannot be properly
represented.
properties of specific heat and thermal conductivity, to closely represent the true behavior as
shown in Figure 1.
Subsoil mass
: 24 x 19.2 x 3 m
Cement type
2.4 m
Subsoil mass
2.4 m
3m
9.6 m
14.4 m
19.2 m
24 m
In this tutorial, due to symmetry of the structure, we will model and analyze only one quarter of
the entire structure as shown in Figure 2. The use of symmetry not only reduces the analysis
time, it also provides convenience in checking the internal temperature and stress distribution.
9@0.8=7.2
4@0.6=2.4
4@0.6=2.4
5@0.6=3
6@0.8=4.8
12@0.8=9.6
15@0.8=12
Figure 2 Heat of hydration model for construction stages (1/4 symmetry model)
Part
Lower foundation
Upper Foundation
Subsoil
0.25
0.25
0.2
Density (kgf/m )
2400
2400
1800
2.3
2.3
1.7
12
12
12
12
12
20
20
20
19
270
270
Property
Specific heat (kcal/kg )
3
Convection
Surface exposed
coefficient
to atmosphere
(kcal/m hr)
Steel form
Ambient temperature ()
Casting temperature ()
2
a=13.9
b=0.86
a=13.9
b=0.86
2.7734105
2. 7734105
1.0104
1.010-5
1.010-5
1.010-5
Poissons ratio
0.18
0.18
0.2
320
320
K=33.97
a=0.605
K=33.97
a=0.605
This example uses low heat of hydration cement. The maximum adiabatic temperature rise
(K) and reactive velocity coefficient (a) are based on experimental values pertaining to the unit
cement content.
Analysis modeling
Setting work environment
Open a new file (
File /
New Project
File /
Select a unit system, which is often used for thermal property data, namely m and kgf, as
shown in Figure 3.
Tools / Unit System
Model / Properties /
Material
; Weight Density>(1800)
; Heat Conduction>(1.7)
Name>(Creep/shrinkage) ;
Code> ACI
Refer to Using
MIDAS/Civil > Model
> Properties > Time
Dependent Material
(Elasticity) in the
On-line Manual.
Type>Code
; Code>ACI
Model / Properties /
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Structural modeling
First, generate a plate element representing the base of the subsoil mass by creating a node
at a lower corner and extending it to the remaining corner nodes. This plate element is then
extruded into a solid using Extrude Elements.
Create Nodes
Coordinates (0,0,0)
Model>Elements>
(12,0,0)
(12,9.6,0)
(0,9.6,0)
Create Elements
Elements Type>Plate
Type>Thick (on)
Material>1 : Mat Foundation
Nodal Connectivity>(1, 2, 3, 4)
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Iso View
Model>Elements>
Extrude Elements
Select All
Extrude Type>Planar Elem. Solid Elem.
Source>Remove (on)
Element Type>Solid ; Material>1: Mat Foundation
General Type>Translate ; Number of Times = 1
Translation>Equal Distance (on) ; dx,dy,dz>(0, 0, 7.8)
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Division of element
Next we divide the element using Divide Elements. The size of mesh depends on the total
configuration.
changes in stresses, for fine meshing. The subsoil part does not need fine meshing, and
yet it needs to be meshed such a way that no significant change in stresses takes place
within an element. For the sake of simplicity, we will divide the element uniformly as shown
in Figure 9.
Model / Elements /
Divide Elements
Select All
Divide Elements>Element Type>Solid
; Equal Distance
y: (12)
z: (13)
13
Now that we created a mesh consisting of brick elements using Extrude Elements and
Divide Elements, we will now delete unnecessary elements from the overall model.
Front View
Shrink
Model>Elements>
Delete Elements
Front View
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to the model.
Left View
Model>Elements>
Delete Elements
Iso View
Left View
15
When we created the 3-D solid element using Extrude Elements, we assigned it as a
concrete material. We will now revise the material to that corresponding to the soil material.
Change
Element
Parameters can be
also used to change
the
properties
of
elements.
16
17
18
Assign Structure Groups for the Mat Foundation, 1st poured lower part and the 2nd poured
upper part.
Figure 15 Defining Structure Groups for Mat Foundation (Lower & Upper Parts)
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Boundary Surface
group represents the
construction
joint
surface between the
1st and 2nd pours.
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Left View
Hidden
;
Shrink
Model View
Window / Tile Horizontally
Zoom Fit (Model View & Model View : 1)
Model / Boundary / Supports
Select Window ( in Figure 17)
Select Window ( in Figure 17)
Boundary Group Name>CS1
Options>Add
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Since it is a 1/4 symmetrical model, we need to specify the symmetric boundary condition.
First, we will enter the symmetry condition pertaining to the 1st pour.
Options>Add
Options>Add
Front View
X axis
symmetric condition
Left View
Y axis
symmetric condition
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Options>Add
Options>Add
Front View
X axis
symmetric condition
Left View
Y axis
symmetric condition
23
Maximize
24
Heat of hydration analysis can consider static load cases for construction stage analysis.
First, self weight is assigned.
Load>Self Weight
Load Case Name>Self
Load Group Name>Self
Z : (-1)
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integration factor, initial temperature & stress output location) for heat transfer analysis.
Analysis / Heat of Hydration Analysis Control
Final Stage>Last Stage
Integration Factor>(0.5)
Initial Temperature>(20)
If creep is to be
considered by reducing
the modulus of elasticity
without using general
creep functions, select
Effective Modulus.
If a general creep
function is to be used,
define the function and
select General.
26
If ambient temperature
varies
at
different
locations due to exposure
to the atmosphere, being
partly immersed in water,
etc., a number of Ambient
Temperature
Functions
can be defined and
applied.
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28
Boundary Surface
group represents the
construction
joint
surface between the
1st and 2nd pours.
We now assign the previously defined ambient temperature and convection coefficient function
to the concrete surface, which is exposed to the atmosphere.
Front View
Left View
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We now define the convection boundary condition at the surface joining the 1st and 2nd pours.
Load / Heat of Hydration Analysis Data / Element Convection Boundary
Select Window ( in Figure 26)
Boundary Group Name>CS1-Boundary Surface
Option>Add/Replace
Convection Boundary>Convection Coefficient Function>Convection Coeff
Ambient Temperature Function>Ambient Temperature
Selection>By Selected Nodes
Front View
Left View
Figure 26
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We now move on to define the convection boundary surface of the 2nd pour.
Load / Heat of Hydration Analysis Data / Element Convection Boundary
Select Window ( in Figure 27)
Boundary Group Name>CS2
Option>Add/Replace
Convection Boundary>Convection Coefficient Function>Convection Coeff
Ambient Temperature Function>Ambient Temperature
Selection>By Selected Nodes
Select Window ( in Figure 27)
Front View
Left View
Figure 27
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assigned the symmetric boundary condition or the convection boundary condition (for
example, boundary surface in contact with the soil).
Load / Heat of Hydration Analysis Data / Prescribed Temperature
Select Window ( in Figure 28)
Select Window ( in Figure 28)
Boundary Group Name>CS1
Option>Add
Temperature> Temperature (20)
Front View
Left View
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mix design, maximum adiabatic temperature rise and reactive velocity coefficient are
automatically calculated based on experimental equations and entered if the cement type,
casting temperature and unit cement content are specified.
Load / Heat of Hydration Analysis Data / Heat Source Functions
Function Name>(Heat Source Function)
Function Type>Code
Refer
to
Heat
of
Hydration Analysis in the
Analysis Manual.
33
34
the construction stage CS1 for the stage of 1st concrete pour.
Load / Heat of Hydration Analysis Data / Define Construction Stage for Hydration
Stage> Add> Name>(CS1)
Initial Temperature>(20)
Times inputted in
Step are accumulative,
not incremental.
Load>Group List>Self
Activation>
35
We then define the construction stage CS2 for the 2nd concrete pour. The duration for the
heat of hydration analysis will be 930 hours after the 2nd pour.
Load / Heat of Hydration Analysis Data / Define Construction Stage for Hydration
Stage>
Name>(CS2)
Initial Temperature>(19)
Figure 32 Defining element and boundary groups for the 2nd pour stage
36
From the Model View, we can check if the Construction Stages are properly defined.
Stage>CS1
Display
Misc tab
37
Structural analysis
We have thus far completed a construction stage model for heat of hydration analysis. We
can begin the analysis.
Analysis /
Perform Analysis
Analysis results
In this example, the major cause for thermal stresses is due to the temperature differences
within the concrete mass resulting in internal constraints. Recapping the overview, Internal
Constraints are caused by unequal volume changes. Initially, cooling surface and warm inner
parts cause tension at the surface and compression at the inner parts.
At a later stage,
after the rise in temperature due to heat of hydration reaches the peak level, the cooling
(contracting) inner parts relative to the surface cause tension in the inner parts and
compression at the surface.
temperature differences between the inner parts and surface. It is also anticipated that the
two concrete masses of two separate pours of different ages will exhibit different heat
transfer characteristics.
We will analyze the characteristics of thermal stresses in concrete by reviewing the results of
heat of hydration analysis reflecting construction stages by graphics, tables, graphs,
animations, etc.
38
; Legend (on)
39
Next, we will check the temperature distribution at the construction stage 2. The fact that
the analysis accounted for construction stages, we note in Figure 35 that heat source action
progresses in the lower part of the mat foundation, which was already cast.
Stage Toolbar>CS2
Result / Heat of Hydration Analysis / Temperature
Step>HY Step 4, 220 Hr
Type of Display>Contour (on)
; Legend (on)
40
cm
Stage Toolbar>CS1
Result / Heat of Hydration Analysis / Stress
Step>HY Step 6, 120 Hr
Stress Option>Global ;
Avg.Nodal
Components>Sig-XX
Type of Display>Contour (on)
; Legend (on)
Status Bar
41
boundary surface of the first pour shows tension stresses at the early stage of the 2nd pour.
The tension stresses at the boundary surface are caused by the increase in volume due to
increased temperature in the 2nd pour. This exerts tension on the previously cast concrete.
Stage Toolbar>CS2
Result / Heat of Hydration Analysis / Stress
Step>HY Step 4, 220 Hr
Stress Option>Global ;
Avg.Nodal
Components>Sig-XX
Type of Display>Contour (on)
; Legend (on)
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convenience to sufficiently demonstrate the trend of the analysis results as shown in Figure
38. We will first assign the nodes for generating results.
1st pour concrete: Interior (1476), Surface (1988)
2nd pour concrete: Interior (2308), Surface (2818)
Result / Heat of Hydration Analysis / Graph
>Node Define>Node (1476) ; Stress Components> Sig-XX
>Node Define>Node (1988) ; Stress Components> Sig-XX
>Node Define>Node (2308) ; Stress Components> Sig-XX
>Node Define>Node (2818) ; Stress Components> Sig- XX
2818
1988
2308
1476
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The time history graph for an interior point (node: 1476) during the 1st pour is shown below.
Result / Heat of Hydration Analysis / Graph
Defined Nodes>N1476-X(on)
Graph Type> Stress + Alw. Stress Graph (on)
1st Pour
Figure 39 Time history graph of stresses at an interior point of the 1st pour
44
Next, we will review the results of time history of a point (node: 1988) on the construction
joint surface between the 1st and 2nd pours.
1st Pour
Figure 40 Time history graph of stresses at a surface point of the 1st pour
45
We will finally check the temperature time history of the interior and surface points during the
1st pour.
Result / Hear of Hydration Analysis / Graph
Defined Nodes>N1476-X(on)
; N1988-X(on)
Interior (1476)
1st Pour
Surface (1988)
Figure 41 Temperature history graphs of interior and surface points of the 1st pour
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; Legend (on) ;
Animate
Close
In order to save the animation in a file, click the
Close
Save
Record
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