International Seminar on Durability and Lifecycle Evaluation of Concrete Structures-2007

Thermal Stress Cracking Phenomena of Low-heat Hydration Concrete with
Cement Ingredient Dependency
Seigo NASU
Director of Research Center for Social Management
Professor of Infrastructure Systems Engineering / Entrepreneur Engineering Course
185 Miyanokuchi, Tosayamada-cho, Kami City, Kochi Prefecture 782-0003, JAPANTEL：81-887-57-2792 FAX：81-887-57-2811 E-mail:nasu.seigo@kochi-tech.ac.jp

ABSTRACT：The thermal stress cracking phenomena of low-heat hydration concrete were observed at massive concrete
casting site, and complex phenomena of stress and strain development with changing creep coefficient, changing
modulus of elasticity, and their age dependency were investigated. Trial mixings and their physical tests of wide
range ingredients of blended cement were performed in order to investigate relationships between ingredient of
low-heat hydration cement and thermal stress crack characteristics. Sensitivity for cracking was investigated byanalyzing tensile strength obtained by two methods, ordinary splitting test and direct pull-test. Tensile strength
obtained by these tests were compared by taking ratio of those strength, since direct pull-test was estimated to be
more sensitive to micro-failure of concrete and this ratio is used as an indicator of sensitivity for cracking. And
relationship between sensitivity for cracking, and cement ingredient and its physical characteristics wasinvestigated.
Based on these studies, two results were obtained. First, thermal stress at site cannot be simulated by a simple
thermal stress analysis recommended by the Japan Society for Civil Engineering, since it dose not provide
accurate enough physical model which should vary very much with age of setting, age-dependent modulus ofelasticity, creep coefficient of those especially young age, and they are not usually taken into account accurately.
Secondly, sensitivity for cracking and hydration shrinkage depends on cement ingredient (clinker, blast-furnaceslag, fly ash, and their proportion), and fineness of especially blast-furnace slag. Taking these results into account,more accurate appreciation of thermal crack probability will be achieved.
1. Introduction
The characteristic of thermal stress crack depends on temperature variation, tensile strength of concrete,modulus of elasticity, creep coefficient, which depend on aging and hardening process of concrete. The probabilityof thermal stress crack is usually evaluated by thermal stress analysis, which takes into account all thesecharacteristics. The analyzed thermal stress is compared to the tensile strength the ratio of which the probability ofcracking is estimated. But because of the complexity of the phenomena, this analysis is not always successful.
The low-heat hydration concrete using low-heat hydration cement was applied at the massive concrete
foundation of Akashi-Kaikyou Bridge, which is the world’s largest suspension bridge (see Table.1, Table.2).
Table.1 specimen of low-heat hydration cement (example)(1) physical characteristics
gravityspecific surface area

setting startsetting end

3.00

4.060 cm2 / g

270 minutes

407 minutes(2) main chemical ingredients
ignition

calcium oxide

magnesium oxide

sulfur trioxide

1.4 %48.7 %

4.2%

2.0%(3)mix proportion of ingredients

International Seminar on Durability and Lifecycle Evaluation of Concrete Structures-2007

Portland cement

fine blast-furnace slag

fly ash

30%

70%

0%

(4)heat of hydration
7 days28 days

91 days40.4 cal/g

46.5cal/g50.6 cal/g

Table.2 mix proportion of low-heat hydration concrete and requested physical conditions(1)Mixture proportion (example of 3P foundation)
Chemicalunite quantity(kg/m3)
W/C
s/a
admixture
(%)
(%)
water
cementgypsumsand
aggregate
SP9N
775S
53.5
41.0
140
260
(2)Required physical conditions

30

710

1157

1.1%

2.5A

design strengthslump

area(%)

bleeding

casting temp

adiabatic
temp. rise

240kg/cm28±2.5cm

4±1%

under 5%20deg. centigrade

under 25 degrees

Real phenomena of concrete hydration process were monitored at the site such as temperature, strain, andstress, which were compared to laboratory simulation experiment to investigate and confirm the precise
phenomena. At the initial stage of aging, compressive stress was found smaller than calculated or predicted due tothe smaller modulus of elasticity and higher creep coefficient than estimated by mixing tests, and this makes
concrete easier to crack than estimated. Hydration shrinkage was also measured, which produce additional tensilestress and makes concrete even easier to crack.
Table.3 blended cement for mixing tests
ProducerSpecific surface
area (cm2/g)

A corp.
B corp.
C corp.
D corp.
E corp.
F corp.
G corp.
H corp.
I corp.
2P foundation
3P foundation

3330
3240
3900
4500
3110
3880
3910
4420
3400
4860
4850

Mixture proportion (%)
Clinker

Slag

25
75
40
70
25
35
25
30
20
32
30

75
30
30
75
45
55
50
60
48
70

kind of clinker

blast-furnace

25
30

20
20
20
20
20

moderate portland
low heat
low heat
low heatmoderate portland
moderate portland
moderate portland
low heatmoderate portland
moderate portland
low heat(*)2P,3P are names of main tower foundations of Akashi-Kaikyou Bridge.
Wide range cement-ingredients of trial mixings were performed in order to investigate relationship between
ingredient of low-heat hydration cement and thermal stress crack characteristics, such as age dependent
coefficients, hydration temperature hysteresis, hydration shrinkage and sensitivity for cracking. Here, sensitivity
for cracking was measured by applying ratio of tensile strengths obtained by ordinary splitting test and directpull-test, since direct pull-test was considered to be more sensitive to inside failure of concrete, whose matrix ofspecimen is assumed to be evenly tensile.
Sensitivity for cracking is found dependent to cement ingredient. The smaller proportion of blast-furnace slag
and/or alite and the less fineness of cement particles resulted in less sensitivity for cracking. Hydration shrinkage
was also found dependent to cement ingredient, as the smaller percentage of blast-furnace slag and/or alite,

International Seminar on Durability and Lifecycle Evaluation of Concrete Structures-2007
resulted in smaller hydration shrinkage.

2.Field measurement of thermal stress phenomena
The concrete applied at Akashi-Kaikyou Bridge foundation was a low-heat hydration concrete, whose
maximal temperature rise induced by concrete hydration is 25 degrees. Measuring instrument for stress, strain,temperature were embedded inside two massive concrete foundations of suspension bridge (see Fig.1, Fig.2).
Narrow-width cracks were detected by measuring stress and strain inside the concrete layers, whose heightwas 1.5 meters in standard and were placed with the interval of approximately two weeks on top of previous
layers (see Fig.3). Marrow-width cracks occurred at the early stage of the temperature descending, whilemeasured strain was still compressive and measured tensile stress was small enough to avoid cracking.

Figure.1 concrete foundation of
Akashi-Kaikyou Bridge towers

Figure.2 standard portion of measuring
instruments inside the concrete layer

Figure.3 example of measurement
Figure.4 modeling of concrete; stress strain and temperature
hydration phenomena
The phenomena can be considered as follow (see Fig.4). Concrete stores compressive stress with ascending
temperature, but only after concrete itself became hard enough and so as the modulus of elasticity. Creepcoefficient is large at the early stage, and plastic deformation of concrete makes it seem to be even larger so thatcompressive stress does not store up as expected since concrete stress is usually calculated with physicalproperties obtained by trial mixing test at the age of 28 days.
To discuss about the strain inside restricted concrete, which contribute to produce actual stress, <effective
strain> is defined as follow,

International Seminar on Durability and Lifecycle Evaluation of Concrete Structures-2007
Definition of Effective Strain<effective strain> = <temperature variation>×<coefficient of linear expansion> ― <measured real strain>
Effective strain is considered to decrease further with creeping and plastic deformation. And it decreases evenfurther with concrete hydration and development of elastic modulus, since proceeding hydration holds
compressive strain (effective strain) inside the microstructure of concrete matrix and effective strain superficiallyseems to decrease. Due to all these phenomena, even if measured strain was still compressive, concrete stress was
tensile so that concrete cracks occurred.
To verify hydration shrinkage of low-heat concrete, stress-free strain gauge were embedded in the concrete,
which enable to measure strain of non-stress concrete. Figure.5 shows an example how stress-free strain changes
during hydration process or temperature ascending and descending. In the figure, stage.1 shows concrete
hydration process from placed point to setting point, stage.2 shows process from setting point to maximum
temperature point, and stage.3 shows process after maximum temperature point. A gradient of stage.1 isconsidered as a simple coefficient of linear expansion, but gradients of stage.2 and stage.3 vary from stage.1.
Relationship between temperature and strain is linear in stage.1 and in stage.2, while superficial coefficient oflinear expansion at stage.2 is smaller than stage.1, and that of stage.3 is larger. This means concrete expanded or
shrunk linearly with temperature, but there is some other negative strain at stege.2 and stage.3, which is assumed
to be a hydration shrinkage of over 100 x 10－6 in this case.

Figure.5 stress-free strain during hydration process of 3P foundation

3.Recreating thermal stress crack phenomena by laboratory experiment
In order to ascertain thermal stress crack phenomena of massive concrete observed at the site, laboratory
examination was performed at University of Horishima by professor Tasawa. Based on the measured condition of
concrete placed at the site, such as mix proportion, temperature hysteresis, restriction condition, concrete placed at
the laboratory was observed. Two standard specimens were set in the curing box, in which temperature hysteresis
was precisely recreated. One specimen was restricted as same as the concrete at the site, and other specimen was
set without restriction, so that hydration shrinkage with the same temperature hysteresis were able to be observed.
These laboratory experiments recreated the thermal stress crack phenomena what we observed at the site. First,
thermal stress crack occurred at the approximately same age (days after placing) although the strain was observed
compressive which was exactly the same phenomenon at the site. Secondly, the specimen without any physicalrestriction showed shrinkage phenomenon, which was obtained by eliminating thermal strain. Fig.6 is theobserved stress of both at the site and experiment. The phenomena observed are found reproducible andexplainable as an assumed hydration phenomena.
Hydration shrinkage of around 100 x 10－６ was also observed by the definition shown below.
Definition of Observed Hydration Shrinkage

International Seminar on Durability and Lifecycle Evaluation of Concrete Structures-2007<hydration shrinkage>=<non-stress strain variation> ― <temperature variation> × <coefficient of linear
expansion>

Figure.6 Observed stress of both at the site and experiment of 2P foundation

Figure.7 hydration shrinkage observed at the site and laboratory experiment (3P)
4.Mixing tests of low hydration concrete
In order to observe the influence of ingredient of low-heat hydration cement on physical coefficients,
sensitivity for cracking, and hydration shrinkage, mixing tests of eleven ingredient cements were performed (seeTable.3), here to keep the same physical conditions as the concrete placed at the bridge foundations, mix
proportions varied with each low-heat hydration cement.
4.1.Tensile strength and sensitivity for cracking
Both splitting test and direct pull-test were performed to obtain the ratio of these test results (strength ratio),which was used as a parameter of sensitivity for cracking, since direct pull-test was considered to be moresensitive to inside failure of concrete, whose matrix of specimen is assumed to be evenly tensile. While tensile
strength of splitting test is estimated only at the center cross-section of a specimen where in perpendicular
direction stress is compressive and failure in cement could be reinforced by aggregate restriction. Here, it ispossible to assume that the larger a strength ratio is, the larger sensitivity for cracking is. Standard specimen was
loaded under a test device shown in Photo.1. Both end of the specimen were bonded to thick steel plate, whichwere pulled by hinged bearing so that specimen was loaded evenly without any bending moment.

International Seminar on Durability and Lifecycle Evaluation of Concrete Structures-2007

Photo.1 test device for direct pull-test
Table.4 explanatory valuables adopted for multiple regression analysis
objective valuable

explanatory valuables

sensitivity for cracking(* )mix proportion of blast-furnace slagof blast-furnace slag and Aliteof clinker
finenessof cementof blast-furnace slag
compressive strength of concrete(* )which is obtained as a tensile strength ratio of ordinary splitting test ,and direct pull-test
Relationship between strength ratio and variation of cement ingredient was evaluated at several concrete ages
by multiple regression analysis, and coefficients of multiple correlations were obtained. Here objective valuable
was strength ratio, and explanatory valuable were the percentage of ingredients shown in Table.4, of which
combinations of two valuables were adopted.
Figure.8 shows an example of relationship between strength ratio and percentage and fineness of each
ingredient at the concrete age of 14 days, from which it is found that the concrete of low-heat hydration cementwith higher percentage of slag and/or alite has a tendency to have higher strength ratio.

Figure.8 relationship between strength ratio and cement ingredient
The calculated coefficients of multiple correlations by multiple regression analysis are shown in Table.5. Firstof all, with growing age of concrete, coefficients of multiple correlations become smaller in any case of two
explanatory valuables. Secondly, as already seen at Figure.8, in case percentage of slag is chosen as one of a
explanatory valuable, coefficients of multiple correlations are relatively higher than other cases. And the
combinations of explanatory valuables, as follow, have highest coefficients, which is significant enough to use
their regression function to predict sensitivity for cracking at certain age of concrete. Especially, in case of slag
percentage and its fineness, normalized coefficients of both valuables at the regression function are steady at all

International Seminar on Durability and Lifecycle Evaluation of Concrete Structures-2007
age (see Table.5).
○Essential combinations of explanatory valuables for sensitivity for cracking
1)slag percentage
and slag fineness2)(slag + alite) percentage andcement fineness3)(slag + alite) percentage andslag fineness

Table.5 calculated coefficients of multiple correlations by multiple regression analysis

4.2.Hydration shrinkage
The same concrete and same cement was tested to obtain hydration shrinkage of several ages. At the age oftwo days, specimens were wrapped with vinyl chloride film to avoid drying out, and contact chips were set on thespecimen body to measure hydration shrinkage. With these tests, unfortunately measurement of concreteshrinkage was failed so that we analyzed the shrinkage of cement only.

Figure.9 relationship between hydration shrinkage and cement ingredientFigure.9 shows an example of relationship between hydration shrinkage and percentage and fineness of each
ingredient at the concrete age of 14 days, and the same tendency found for sensitivity for cracking come to beevident, that the low-heat hydration cement with higher percentage of slag and/or alite, has a tendency to havehigher strength ratio. But there was no evident tendency found for fineness.
Relationship between hydration shrinkage and variation of cement ingredient was evaluated at several ages bymultiple regression analysis, and coefficients of multiple correlations were obtained. Here objective valuable washydration shrinkage, and combinations of explanatory valuables were the same for the analysis of sensitivity forcracking (strength ratio). The calculated coefficients of multiple correlations by multiple regression analysis areshown in Table.6, which shows the combinations of explanatory valuables, as we have already seen for sensitivity
for cracking, indicate its significance at all age of cement, except that the normalized coefficients of fineness ofeach ingredients are small enough to neglect for prediction of hydration shrinkage. Consequently, it is foundhydration shrinkage of low-heat hydration cement simply depends on ingredients, especially slag and alite.

International Seminar on Durability and Lifecycle Evaluation of Concrete Structures-2007

Table.6 calculated coefficients of multiple correlations by multiple regression analysis

4.3.Other physical coefficients(1)Modulus of elasticity
Figure.10 shows results of modulus of elasticity, which indicates that tangent modulus of elasticity obtained bydirect tensile pull-test is larger enough than gradient modulus of elasticity at 1/3 of compression strength
ordinarily used for thermal stress analysis, so as to influence the calculated result of stress analysis.
(2)Creep coefficients
Figure.11 shows laboratory results of creep coefficients of the same concrete placed at the foundations site at
the age of 40 hours, 3 days, and 7 days. The results indicate that the ordinary creep coefficients used for thermal
stress analysis are significantly smaller than those of young age of low-heat hydration concrete.

International Seminar on Durability and Lifecycle Evaluation of Concrete Structures-2007
Figure.10 tensile tangent modulus of elasticity and gradient modulus of elasticity at 1/3 compression strength

Figure.11 laboratory results of creep coefficients
5.Discussion
The cement and concrete adopted at the massive concrete foundations of Akashi-Kaikyou Bridge, whose
maximum temperature rise was limited to 25 degrees, was considered not to cause any thermal stress crack basedon the thermal stress analysis. Although it is found that both at the site and laboratory tests, the phenomena are
more complicated than what we estimate by thermal stress analysis. The real compressive stress and strain ofyoung age at the site is not exactly what it is measured, and it is found that they are smaller than estimated byanalysis due to the larger creep coefficient and aging concrete which contains compressive strain inside concrete
microstructure and makes effective compressive strain smaller for real stress. It is also evaluated that negative
strain grows during concrete aging, and it is found as hydration shrinkage at laboratory experiment.
Modulus of elasticity used for thermal stress analysis is usually a gradient modulus of elasticity at 1/3 ofcompression strength. However tangent modulus of elasticity obtained by direct tensile pull-test is larger andapparently it is a proper valuable in case thermal crack occurs in a negative stress concrete.
And all of these coefficients applied for thermal stress analysis are usually obtained by mixing test of specimen
cured at the temperature of 25 degrees centigrade, while these coefficients at the site are affected with thetemperature hysteresis, which makes the results of the analysis even more inaccurate.
As shown before, sensitivity for cracking is able to estimate by strength ratio (ratio of tensile strengths
obtained by ordinary splitting test and direct pull-test) and it depends on percentage and fineness of each
ingredient, especially those combination of slag and alite as follow, so that the ingredients of blended cement haveto be taken into account for thermal stress analysis to estimate probability for cracking. And it is found possible tomodel and apply strength ratio of a function of valuables below as a safety factor for thermal stress analysis, sincecalculated coefficient of multiple correlation for these valuable at young age is large enough to indicate possibility
for modeling.
1) slag percentage
andslag fineness2) (slag + alite) percentage and cement fineness3) (slag + alite) percentage and slag finenessHydration shrinkage of low-heat hydration cement simply depends on ingredients percentage, especially slag
and alite at each age. It is also possible to model and apply this relationship as a shrinkage function of valuables
below. Here it is found that fineness of each ingredient does not affect hydration shrinkage as much as sensitivity
for cracking. It is also found that up to a certain percentage of these valuables they do not affect hydration
shrinkage, which means less percentage of these valuable improve hydration shrinkage.

1)
2)
3)

International Seminar on Durability and Lifecycle Evaluation of Concrete Structures-2007
slag percentage
alite percentage(slag + alite) percentage

6. Conclusion
For the thermal stress analysis to obtain thermal stress, it is recommended to recreate the phenomena precisely
as measured at actual concrete placement site, especially at young age. Coefficients such as modulus of elasticity,
creep coefficient, tensile strength also have to be precisely modeled at young age of concrete, and temperaturehysteresis have to be taken into account, which apparently affect these coefficients. Percentage of ingredients ofblended cement and their fineness, mainly slag and alite, affect sensitivity for cracking and hydration shrinkage.
These characteristics have to be modeled for thermal stress analysis. Modeling of sensitivity for cracking andhydration shrinkage were tried but still many problems exist to make it consistent enough. In order to avoid
thermal crack, best blend of ingredients and their fineness have to be selected such as less percentage of slag
and/or alite and less fineness of these ingredients based on the required conditions from the thermal stress analysis.
These results are evaluated for the low-heat hydration cement and concrete. And it is assumed that other kinds of
cement and concrete commonly have the same characteristics, but this assumption still has to be proved.

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Sato, R., Xu, M. and Yang, Y. (1999) Stress Due to Autogenous Shrinkage in High Strength Concrete and Its
Prediction, in Autogenous Shrinkage of Concrete, (ed. E. Tazawa), E & FN Spon, London, pp.351-362.
Suzuki, K., Nishikawa, T. and Ito, S. (1985) Formation and Carbonation of C-S-H in Water, Cement and Concrete
Research, Vol.15, No.2, pp.213-224.