Influence of Hydration Temperature on the Durability and...

Myers, J.J., Carrasquillo, R.L., “Influence of Hydration Temperature on the Durability and Mechanical Property Performance of HPC Prestressed/Precast Beams”, Journal of the Transportation Research Board, Fifth International Bridge Engineering Conference, pp 131-142, No. 1696, Volume 1, Publication 5B0038, April 2000.

Influence of Hydration Temperature on the Durability and Mechanical Property Performance of HPC Prestressed/Precast Beams

John J. Myers, Ph.D., P.E.

Assistant Professor of Civil Engineering

The University of Missouri at Rolla

Center for Infrastructures Engineering Studies

218 Engineering Research Lab

Rolla, Missouri, USA 65409-0710

Ramon L Carrasquillo, Ph.D., P.E.

Professor of Civil Engineering

The University of Texas at Austin

Construction Materials Research Group

10100 Burnet Road, Building 18B

Austin, Texas, USA 78758

Abstract

Concrete technology has continued to advance throughout the years to meet the demands of

designers and innovative structural systems. With the advent of high performance concrete

(HPC) which contains high contents of cementitious materials, the need to further investigate the

impact of concrete temperature development during hydration relative to concrete performance is

of keen interest. The study conducted herein utilized the use of match curing technology to

investigate the influence of concrete temperature development during hydration on the

mechanical and material property performance of HPC prestressed/precast beams. These

members were fabricated for use in two recently constructed HPC bridges in Texas. The

investigation was conducted in conjunction with the design, instrumentation, and construction of

the Louetta Road Overpass in Houston, Texas and the North Concho River US 87 & S.O. RR

Overpass in San Angelo, Texas. The Federal Highway Administration (FHWA) and the Texas

Department of Transportation (TxDOT) cosponsored these projects as part of the Strategic

Highway Research Program (SHRP) to stimulate the use of HPC and to demonstrate the

suitability of HPC in highway structures. Results of this study indicated that for HPC’s which

contain high contents of cementitious materials, the concrete temperature during hydration can

have a dramatic impact on both the mechanical and material (durability) performance of the


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concrete. Temperature recommendations are provided herein to avoid less than optimal concrete

performance based on the member shapes and mix design investigated within this study.

INTRODUCTION

When portland cement reacts with water an exothermic chemical reaction occurs which

produces heat. The heat that is generated is dependent upon the composition and amount of

each of the cement or cementitious materials within the concrete, and the mass of the concrete

that is poured. Several cement compositions have been developed over the years to

accommodate various casting conditions (rate of hardening) and exposures to improve long-term

concrete performance. With the advent of high performance concrete (HPC), the issue of

concrete performance relative to heat generation raises concerns since many HPC incorporate

very high contents of cementitious materials to attain the desirable level of strength required

either at early or later-ages. This issue of temperature development is particularly important for

many precasters who need to meet high early release strengths in a timely fashion to maintain

plant productivity, but cannot afford detrimental later-age performance.

In 1993 the FHWA initiated a series of bridge projects that included the complete incorporation

of HPC from design to long-term monitoring to investigate and demonstrate the suitability of HPC

for use in highway structures. Two of these bridges, the Louetta Road Overpass in Houston,

Texas and the North Concho River Overpass in San Angelo, Texas, were recently constructed in

the State of Texas. A research program for these bridges was developed to investigate aspects of

mix design, concrete properties, applicability of current design requirements, applicability of

current construction practices, predictability of short and long-term performance as well as

recommendations for a quality control/quality assurance testing and inspection program. As part

of this research study, the influence of concrete temperature development during hydration on the

mechanical and material property performance of HPC prestressed/precast beams was

investigated as discussed herein. The following sections discuss the test methods and

procedures used, the instrumentation and monitoring plan, as well as the in-situ results and

conclusions reached.

TEST METHODS AND PROCEDURES

The following test methods and procedures were observed during the scope of this

investigation.

Compressive Strength ASTM C39-94

The compressive strength tests were performed in accordance with ASTM C39-94 “Standard

Test Method for Compressive Strength of Cylindrical Concrete Specimens” using a 272 ton (600


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kip) capacity hydraulically operated compression test machine. The specimens were loaded at a

rate of 241 ± 103 kPa per second (35 ± 15 psi per second) until failure. Special care was taken to

test the specimens in a moist condition if they were moist-cured cylinders to eliminate any

variation associated with their moisture condition at the time of testing. Neoprene pads inserted in

steel retaining caps were used throughout this study. Neoprene pads used for HS/HPC had a

manufacturer stated minimum hardness of 70 durameter. Specimens for the prestressed/precast

beams were tested at release of pre-tensioning (22 to 26 hours) and at the specification

requirement for design strength (56 days).

Modulus of Elasticity ASTM C469-94

Elastic moduli were determined in accordance with ASTM C469-94 “Standard Test Method for

Static Modulus of Elasticity and Poisson’s Ratio of Concrete

in Compression”. The elastic modulus test set-up is illustrated

in Figure 1. Elastic modulus was evaluated at release and 56

days.

Rapid Chloride Permeability AASHTO T277

Rapid chloride permeability testing was performed in

accordance with AASHTO T277 “Standard Method of Test for

Rapid Determination of the Chloride Permeability of

Concrete”. The test measured, in coulombs, the amount of current that flowed through a slice of

saturated concrete when placed between two electrolytic solutions and subjected to a 60-volt DC

voltage. The permeability results were averaged based on a minimum of four slices for any one

casting date under any one curing condition or thermocouple location selected.

INSTRUMENTATION AND CONCRETE TEMPERATURE-TIME MODELING

In order to investigate the influence of concrete temperature during hydration on the

mechanical and material (durability) performance of HPC prestressed/precast members, a

commercially available match curing system was used. The concept of match curing technology

is to produce quality control specimens that closely represent the actual concrete in the member

by monitoring and matching the temperature-time profile of the concrete in the member to the

quality control specimens. Through the use of a data acquisition system, the temperature-time

profile for the actual member was monitored at six-minute intervals. This system was located in

an enclosed temperature controlled building located adjacent to the precasting beds. The data

acquisition system and interface box contained eight controllable channels and six uncontrollable

Figure 1: Static Elastic Modulus

Testing Ring and Set-Up


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channels to monitor temperature-time profiles. Each controllable channel was capable of

controlling the temperatures of six match curing molds. The steel molds were standard 100-mm

dia. x 200-mm long (4-in. dia. x 8-in. long) cylinders and were surrounded by an internal heating

coil that was insulated by a protective material. The match curing steel molds were located in a

room where the temperature of the environment could be controlled (Figure 2) since the match

curing system could not cool the specimens, only heat the specimens.

The room was maintained at a constant

temperature of 21.1 ± 1.7°C (70 ± 3°F), so that

the temperature profile of the match cured

specimens would not “lag” behind the actual

temperature-time profile of the concrete in the

precast member. The temperature-time

development of the concrete was monitored at

four critical locations for the Louetta Road

Overpass U-Beams for each casting date

monitored by the researchers. To determine

these critical locations, thermocouples were stacked at 127-mm (5-in.) intervals vertically in the

member for the first beam monitored. The maximum peak concrete temperatures during

hydration occurred in the two end block locations (square and skewed) 914-mm (36-in.) vertically

from the bottom of the member (Figure 3). End blocks were required for the U-Beams to satisfy

support requirements on the skewed abutments and precast pier caps.

The maximum peak concrete temperatures during hydration, non-end block location, occurred at

mid-depth of the upper flange location while the minimum peak concrete temperatures during

hydration occurred in the web at

mid-depth. The temperature-

time development of the

concrete was monitored at three

critical locations for the North

Concho River Overpass

AASHTO Type IV precast

beams for each casting date

monitored by the researchers.

These critical hydration

Figure 2: Climate Controlled Room Incorporating Match Curing DAS

And Match Curing Molds

460 mm min. to1,220 mm max.at skewed EB

Endblock Cross Section

Upper Flange

End BlockWeb

to match curesystem

to match curesystem

1,400 mm

1370

mm

a.) Texas U-Beam Member

occurred in the web at mid-
Figure 3: Instrumented Thermocouple Locations forPrestressed/Precast Members
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depth. The temperature-time development of the concrete was monitored at three critical

locations for the North Concho River Overpass AASHTO Type IV precast beams for each casting

date monitored by the

researchers. These

critical locations were

determined in the same

fashion as the U-Beam.

The maximum peak

hydration temperatures

occurred in the square

end block location 914-

mm (36-in.) vertically

from the base of the

member. The end

blocks for the AASHTO

Type IV beams were

required as part of the

member’s two-stage stressing operation (partial prestressing and partial post-tension). The

members were redesigned to accommodate a two stage stressing operation to meet the

fabricator’s bed capacity limitations. This was done to avoid a retooling/redesign of the

fabricator’s precasting bed. The maximum peak concrete temperatures during hydration, non-end

block location, occurred in the upper flange location 1168-mm (46-in.) vertically from the base of

the member. The minimum peak concrete temperatures during hydration for the locations

monitored occurred in the web at mid-depth. The configurations for the thermocouples for both

the Texas U-Beam and AASHTO Type IV beam are illustrated in Figure 3. These locations were

selected on the minimum and maximum heat generation location within the member during

hydration. Figure 4 illustrates the concrete temperature profiles recorded during a representative

casting date for the Texas U-Beam. Tables 1 and 2 summarize the maximum temperature

encountered during hydration as well as the maximum temperature rise recorded for the casting

dates monitored. Table 3 summarizes the HPC mix design that was utilized in production for the

precast/prestressed beams.

460 mm min.to 610 mm max.

Endblock Cross Section

Upper FlangeEnd Block

Web

to match curesystem

to match curesystem

660 mm

1370

mm203 mm

b.) AASHTO Type IV Member

Figure 3: Instrumented Thermocouple Locations for Prestressed/Precast Members


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A wide variation in the temperature-time profiles recorded may be noted depending on the

monitoring location and/or curing condition selected. This wide variation was representative for

both the HS/HPC U-Beams and AASHTO Type IV beams [3]. The member-cured cylinder is the

current quality control specimen used in the precast industry in Texas to verify that specification

release and design strengths have been satisfied. The member-cured cylinder not only

underestimates the temperature-time profile of the concrete within the member (Figure 4), but

also the compressive strength at release [3]. For a further discussion on the impact that curing

condition plays on HS/HPC concrete property development, see References 3 and 5.

MECHANICAL PROPERTY PERFORMANCE

The impact of concrete temperature during hydration on the compressive strength and elastic

modulus was investigated for both the Louetta Road Overpass U-Beams and the North Concho

River Overpass AASHTO Type IV beams. The following sub-sections discuss the influence of

concrete temperature (during hydration) and temperature rise on both the early and later-age

property development.

Compressive Strength Development Results

The compressive strength development was monitored both at early and later-ages to

investigate the influence of concrete temperature. Results in terms of the maximum concrete

temperature encountered during hydration as well as the extreme temperature rise encountered

during hydration were investigated.

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16 18 20 22 24 26Time (hours)

Tem

pera

ture

(deg

. C)

Skewed End Block(914-mm)

Peak = 86 deg. C

Web (787-mm)Peak = 57 deg. C

Regular End Block(914-mm)

Peak = 78 deg. C

Ambient TemperatureASTM Moist Cure Room

Flange (1168-mm)Peak = 59 deg. C

Max. = 27 deg. C(X-mm) - indicates height of thermocouple from base of member

Release of Formwork

Approx. 8 HourPreset Time

Peak = 53 deg. CMember Cured Cylinder

No Steam CuringdegF = (9/5degF) + 321 mm = 0.0397 inches

Figure 4: Concrete Temperature Profile during Hydration for U-Beam Cast 2-15-96


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Influence of Concrete Temperature during Hydration on Compressive Strength Development

The compressive strength development was monitored both at early and later-ages. Figure 5

illustrates the early-age compressive strength versus the maximum concrete temperature (during

hydration) monitored for all of the casting dates and curing locations investigated. A second order

polynomial trendline best represented the scatter of data at release of prestressing. The general

trend indicates an improvement in compressive strength up to a concrete temperature of

approximately 77°C (170°F), beyond which compressive strength degrades. Specimens, which

were subjected to concrete temperatures above 77°C (170°F) during hydration, experienced

more severe cracking on the microstructure level. The increased microcracking precipitated the

propagation of cracking in the transition zones and thereby resulted in failure of the cylinder at a

lower compressive strength value. This issue of microstructure cracking related to concrete

temperature development is discussed at greater length in subsequent sections of this paper.

R2 = 0.600

30

40

50

60

70

80

90

100

110

0 20 40 60 80 100 120Maximum Concrete Temperature, deg C

Com

pres

sive

Str

engt

h, M

Pa HS/HPC Precast BeamsU-Beam and AASHTO Type IV

Second Order PolynomialTrendline

1 MPa = 145 psi

degF = (9/5 degC) + 32 Figure 5: Early-Age (at Release) Compressive Strength Development

Versus Maximum Concrete Temperature during Hydration

Figure 6 illustrates the later-age compressive strength versus the maximum concrete

temperature (during hydration) monitored for all of the casting dates and curing locations

monitored. A second order polynomial trendline best represented the scatter of data at 56 days.

The general trend indicates a linear decrease in compressive strength at later-ages with an

increase in the maximum concrete temperature (during hydration). Clearly, the higher the

maximum concrete temperature, the poorer the later-age performance.


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R2 = 0.487

70

80

90

100

110

120

130

0 20 40 60 80 100 120Maximum Concrete Temperature, deg C

Com

pres

sive

Str

engt

h, M

Pa HS/HPC Precast BeamsU-Beam and AASHTO Type IV Second Order

PolynomialTrendline

degF = (9/5 degC) + 32

1 MPa = 145 psi

Figure 6: Later-Age (56-day) Compressive Strength Development


Influence of Temperature Rise on Compressive Strength Development

Figure 7 illustrates the early-age compressive strength versus the temperature rise of the

concrete during hydration. When investigating the influence of temperature in terms of the

temperature rise of the concrete, the impact of concrete placement temperature and ambient

conditions are minimized.

20

30

40

50

60

70

80

90

100

110

0 10 20 30 40 50 60 70Concrete Temperature Rise, deg C

Com

pres

sive

Str

engt

h, M

Pa

HS/HPC Precast BeamsU-Beam and AASHTO Type IV

Second OrderPolynomialTrendline

R2 = 0.666

Temp Rise degF = (9/5)(degC)

1 MPa = 145 psi

Figure 7: Early-Age (at Release) Compressive Strength Development

Versus Concrete Temperature Rise

The ASTM moist cured cylinders exhibited the lowest temperature rise, from 4°C to 9°C (6°F to

16°F), due to the controlled curing environment and high surface area to volume ratio (SA/V) of

the cylinders. The member cured cylinders experienced the second lowest temperature rise,

generally in the 17°C and 28°C (30°F to 50°F) temperature range. The end block locations


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exhibited the highest temperature rise followed by the flange and web locations. The end block

locations for the U-Beams experienced a temperature rise between 51°C and 61°C (92°F to

110°F), while the AASHTO Type IV’s varied between 27°C and 47°C (49°F to 84°F). The

maximum heat generation for the U-Beam and AASHTO Type IV end blocks was 10.45 and

7.98°C/100 kg of cementitious material per cubic meter of concrete (11.14 and 8.55°F/100 lb.

cementitious material per cubic yard of concrete), respectively. Tables 1 and 2 summarize the

peak temperatures and heat generation for each precast member casting date. The general trend

for the temperature rise of the concrete indicates an improvement in compressive strength up to a

temperature rise of approximately 39°C (70°F). Beyond this temperature rise, a decrease in the

release strength is exhibited. As noted previously, this may be attributed to increased

microcracking resulting in failure of the cylinder at a lower compressive strength value. The only

location that experienced temperature rises above this level were the end block locations

primarily on the U-Beams. Due to the higher SA/V ratio of the AASHTO Type IV’s, excessive

temperature development during casting did not appear to be as large of a concern since these

members could dissipate heat faster due to the larger SA/V ratio of these members.

Figure 8 illustrates the later-age compressive strength versus the temperature rise of the

concrete. A second order polynomial trendline also best represented the scatter of data at 56

days. Consistent with the maximum concrete temperature (during hydration), the general trend

indicates a decrease in compressive strength at later-ages with an increase in temperature rise.

70

80

90

100

110

120

130


Com

pres

sive

Str

engt

h, M

Pa HS/HPC Precast BeamsU-Beam and AASHTO Type IV

Second OrderPolynomialTrendlineR2 = 0.555

Temp Rise degF= (9/5)(degC)

1 MPa = 145 psi

Figure 8: Later-Age (56-day) Compressive Strength Development


Influence of Surface Area to Volume Ratio (SA/V) on Compressive Strength Development

Clearly, the SA/V ratio affected the early-age strength characteristics of the concrete (Figure

9). The more massive the section (lower SA/V), the higher the early-age strength up to a SA/V


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ratio of approximately 0.2 for the data accumulated. For the mix design investigated, SA/V ratios

less than 0.2 resulted in excessive temperature development within the member and additional

cracking on the microstructure level as discussed herein.

R2 = 0.641

20

40

60

80

100

120

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Surface Area to Volume Ratio (SA/V)

Com

pres

sive

Str

engt

h, M

PaHS/HPC Precast Beams

U-Beam and AASHTO Type IVSecond Order

PolynomialTrendline

End Block

Web

Flange

Cylinder

1 MPa = 145 psi

Figure 9: Early-Age (at Release) Compressive Strength Development

Versus Surface Area to Volume Ratio

Elastic Modulus Development Results

The elastic modulus development was monitored both at early (release, 22-26 hours) and

later-ages (56 days).

Influence of Concrete Temperature during Hydration on Elastic Modulus Development

Figure 10 illustrates the early-age elastic modulus versus the maximum hydration temperature

monitored for the casting dates and curing locations monitored. A second order polynomial

trendline best represented the scatter of data at release of prestressing. The general trend

indicates an improvement in elastic modulus up to a concrete temperature of approximately 77°C

(170°F) during hydration, beyond which the elastic modulus appears to degrade. It appears that

this reduction in stiffness may be attributed to the interconnected microcracks that are caused by

extreme temperature development although sufficient study of microcracking was not performed

within the scope of this study to confirm this behavior.


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R2 = 0.361

30

35

40

45

50

55

0 20 40 60 80 100Maximum Concrete Temperature, deg C

Elas

tic M

odul

us, G

Pa


Second Order PolynomialTrendline

1 GPa = 145 ksi

degF = (9/5 degC) + 32 Figure 10: Early-Age (at Release) Elastic Modulus Development


Figure 11 illustrates the later-age elastic modulus versus the maximum hydration temperature

measured for all of the casting dates and curing locations monitored. A linear trendline best

represented the scatter of data at 56 days. The general trend indicates a linear decrease in

elastic modulus at later-ages with an increase in maximum concrete temperature during

hydration.

R2 = 0.3972

35

40

45

50

55

60

0 20 40 60 80 100Maximum Concrete Temperature, deg C

Elas

tic M

odul

us, G

Pa


LinearTrendline

1 GPa = 145 ksi

degF = (9/5 degC) + 32 Figure 11: Later-Age (56-day) Elastic Modulus Development Versus Maximum Concrete Temperature during Hydration


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Influence of Temperature Rise on Elastic Modulus Development

In terms of temperature rise, the early-age elastic modulus appears to degrade beyond a

temperature rise of 39°C (70°F), consistent with the results reported in terms of compressive

strength development. This is illustrated in Figure 12.

30

35

40

45

50

55


Elas

tic M

odul

us, G

Pa

HS/HPC Precast BeamsU-Beam and AASHTO Type IV Second Order

PolynomialTrendlineR2 = 0.413


1 GPa = 145 ksi

Figure 12: Early-Age (at Release) Elastic Modulus Development


Figure 13 illustrates the later-age elastic modulus versus the maximum temperature rise

monitored during hydration for the casting dates and curing locations monitored. A linear trendline

best represented the scatter of data at 56 days. The general trend indicates a linear decrease in

elastic modulus at later-ages with an increase in maximum temperature rise during hydration.

R2 = 0.2119

35

40

45

50

55

60


Elas

tic M

odul

us, G

Pa


LinearTrendline


1 GPa = 145 ksi

Figure 13: Later-Age (56-day) Elastic Modulus Development



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MATERIAL (DURABILITY) PERFORMANCE

Precast/prestressed concrete has long had the reputation of being a very durable long lasting

concrete as evidenced by performance of many P/C bridges to date. Much of this reputation has

been attributed to the prestressing, which reduces any crack widths, thereby slowing the ingress

of chlorides and other harmful chemicals. Furthermore, the durability performance has also been

attributed to the higher quality concretes and the tighter quality control measures instituted by

many precast plants. This investigation was undertaken to examine if the higher concrete

temperatures during hydration associated with the precast/prestressed concrete members

influenced the permeability of the concrete and thereby the durability performance.

Rapid Chloride Permeability (AASHTO T-277) Results

Rapid Chloride Permeability was monitored at 56 days for all casting dates for the precast U-

Beams and AASHTO Type IV beams. Figure 14 illustrates the rapid chloride permeability test

(RCPT) results versus the maximum concrete temperature during hydration. It may be noted that

the test results presented are for the same mix design and constituents. Therefore, it is unlikely

that any concerns with chloride binding that have been associated with fly ash and the RCPT test

would impact the general trend of the results presented in Figure 14, since the mixture

characteristics and materials are identical. It is clearly evident from the data collected for these

HS/HPC beams that higher concrete temperatures during hydration reduce the chloride

permeability up to a threshold temperature of approximately 88°C (190°F). Above this

temperature range, the RCPT dramatically increased. It may be noted that when simply

examining the temperature rise of the concrete, no trends were observed in terms of permeability

performance based on the scatter of the data collected herein. Clearly, it was the maximum

0

500

1000

1500

2000

10 20 30 40 50 60 70 80 90 100 110Maximum Concrete Temperature, deg C

Perm

eabi

lity,

Cou

lom

bs


degF = (9/5 degC) + 32

ApproximateThreshold Temperature

Figure 14: Rapid Chloride Ion Permeability at 56 Days



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concrete temperature during hydration that affected the permeability performance rather than a

specific temperature rise of the concrete.

Microscopic Examinations

In order to determine what caused the behavior exhibited in the RCPT, the authors conducted

a three phase microscopic investigation. This included a low-level microscopic petrographic

investigation, a high-level microscopic microstructure examination, and a BSE scanning electron

microscope microstructure examination. Concrete from the same casting date and specimens

were used for each of these three investigations for consistency in sample evaluation. The results

from each of these investigations are presented below.

Petrographic Examination – Low Level Microscopic Examination

The first phase was a low-level microscopic petrographic examination. This investigation was

conducted by an independent private consultant [4] to examine three representative specimens

from the same casting date. The only variation in the three specimens was the maximum

concrete temperature during hydration encountered during casting. The three specimens attained

a maximum concrete temperature during hydration of 29°C, 71°C, and 93°C (85°F, 160°F, and

200°F). To conduct an unbiased investigation, no preconditions were specified to the

petrographer other than to optically inspect the specimens and report any discerning features.

The cylinders were examined using methods given in ASTM C856, “Petrographic Examination of

Hardened Concrete”.

To the unaided eye, the only distinguishing feature that was observed by the petrographer

was a slightly lighter color of the paste in specimen 3 [maximum concrete temperature during

hydration of 93°C (200°F)]. Close-up photomicrographs of the paste matrix did not show

distinguishing features. Powdered paste studies showed cement and fly ash particles, as well as

normal hydrated products, but there were no distinguishing features that might relate to curing

conditions or maximum hydration temperatures. The petrographer’s paste characteristics are

summarized in Table 4.

In summary, the petrographer noted that there were no outstanding optical or material

features that appeared to be influenced by the maximum concrete temperature during hydration.

The results of the low-level microscopic investigation did not indicate any reason for the variation

in permeability, as both the paste matrix and material features optically appeared similar.

Microstructure Analysis – High Level Microscopic Analysis

A higher level microscopic analysis was undertaken to more precisely optically examine the

microstructure of the concrete specimens. As noted previously, the same concrete specimens

were investigated. Thin-slices were prepared and examined at the Texas Department of


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Transportation Materials and Tests Division in Austin, Texas. These slices were impregnated with

epoxy and mounted for optical examination. The epoxy impregnated any void space or

microcracks present on the thin slice. Table 5 summarizes the accumulated point-count data and

the resulting modified point-count results for the thin-sliced specimens after they were inspected

optically. Volume percentages were on the same order for each of the thin slices. Minor variations

in the counts may be attributed to sample location and the surface area size of the thin slices

examined.

Two types of imaging were used to inspect and

analyze the microstructure. This included

polarized imaging and ephifluorescences

imaging. The polarized imaging reflects polarized

light on the thin-sliced specimens such that the

mineralogical characteristics of the aggregates

can be identified. Geologists and material

scientists primarily use this method to identify the

mineral composition of the coarse and fine

aggregates. The ephifluorescences imaging

allows for the identification of all air voids and

microcracks within the paste matrix. This was the

primary imaging used to identify and

quantitatively determine the microcrack widths

and magnitude of microcracks. Two methods

were selected to quantitatively examine the

magnitude of microcracks.

The first method examined the more severe

microcrack widths and counted the digital pixels

over the width at the same magnification for all

specimens. This was intended to provide a rough

estimate in the variation of crack widths between

the peak concrete (hydration) temperatures

investigated. The second method involved a full

count of void space minus actual air voids over

the identical surface area for each specimen to

quantitatively determine the magnitude of

microcracks over the specimen surface. Figure

Figure 15: 29, 71, & 93 deg. C (85, 160, & 200

deg. F) Thin-Sliced Specimen under Ephifluorescences Imaging


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15 illustrates the ephifluorescences imaging for the 93°C (200°F), 71°C (160°F), and 29°C (85°F)

specimens. Clearly, the specimen subjected to the highest concrete (hydration) temperature

resulted in the most severe microcracking. The microcracks were located through the paste

matrix and the transition zones. Crack widths for the most severe cracks varied from 18 to 31

pixels wide for the 93°C specimen. The 71°C specimen did not exhibit microcracking nearly as

severe as the 93°C specimen. However, the microcracks visually appeared to be wider compared

than the 29°C specimen. The most severe crack widths measured from 11 to 13 pixels in width,

while the 29°C specimen crack widths were not wider than 4 or 5 pixels at any location. The thin

microcracks which are visible in all of the images are associated with drying and shrinkage and

are representative for any concrete produced. The quantitative magnitude of microcracking is

presented in Table 6. The total point count measured confirmed that the higher concrete

(hydration) temperatures resulted in a higher

magnitude of cracking.

Microstructure Analysis – Scanning Electron Microscope Analysis

Finally, a high level BSE SEM microstructure analysis was undertaken to examine the

microstructure of the concrete specimens and the material composition at a higher resolution and

power than the previous investigation. The thin-slices were impregnated with an epoxy to fill any

void spaces prior to examination to further highlight the microcracks. Following this, the

specimens were mounted and polished. The thin slices were 30 to 40 microns in thickness. The

specimens were carbon coated prior to SEM examination to prevent damage to the impregnated

epoxy under the BSE imaging. The SEM work was performed by staff members from the Geology

and Civil Engineering Departments at Texas A & M University in College Station, Texas. Texas A

& M University was selected because of their digital graphics capability. Existing hardware at the

University of Texas at Austin consisted of analog technology with no digital graphics capability.

Figures 16 through 18 illustrate the results of the BSE imaging.


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Figure 16: BSE Imaging for 93 deg. C (200 deg. F) Thin-Sliced Specimen




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Results of the BSE imaging indicated more severe microcracking for the specimens subjected

to the higher concrete (hydration) temperatures, consistent with the TxDOT thin-slice

examination. The transition zones and microcracks were much more detailed at a magnification

of 100 times using this imaging as illustrated in the figures. This examination detailed the cracking

that occurred within the transition zones, which was not visible at 30 times magnification. The

severity of the transition zone and paste matrix microcracks appeared greatest for the 93°C

(200°F) specimen (Figure 16). The 71°C (160°F) specimen also indicated fairly severe

microcracking in the transition zones (Figure 17), however, the cracks appeared to be

interconnected for the 93°C specimen. The 29°C (85°F) specimen clearly indicated the least

amount of microcracking (Figure 18).

It should be noted that microcracks are already present in the transition zones due to drying

and cooling even before concrete is subjected to loads.

Therefore, it is impossible to eliminate microcracking in the

transition zones and consistent to anticipate more transition zone

cracking in specimens subjected to higher hydration

temperatures. In general, the majority of transition zone

microcracking was located around the silicious fine aggregate

material as illustrated in Figure 19. Significantly less

microcracking was noticeable around the burnet dolomitic

limestone coarse aggregate (Figure 19). This is consistent with

previous research studies that report a chemical bonding in the

transition zone between the hydrated cement paste particles and

particles that are largely composed of limestone and dolomitic

minerals. At these locations, it was difficult to distinguish the transition zone even at higher

magnifications (Figure 19).

The higher concrete (hydration) temperatures clearly impacted cracking on the microstructure

level more severely as discussed herein. Based on the mechanical and material property data

collected within this investigation as well as the microstructure analysis, it is recommended that

excessive hydration temperatures be avoided. To accomplish this, a maximum concrete

temperature limitation during hydration in the temperature range of 71 to 82°C (160 to 180°F) is

recommended as a specification requirement for future HPC precast components.

CONCLUSIONS AND RECOMMENDATIONS

Conclusions have been formulated based on the results of the investigations conducted during

this research study.

Figure 19: Aggregate - Paste Transition Zone


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1. The more massive the section (lower SA/V), the higher the early-age strength up to a

SA/V ratio of approximately 0.2 for the mix design investigated. SA/V ratios less than 0.2

resulted in excessive temperature development impacting concrete performance as

discussed herein.

2. HPC hydration temperatures which exceeded 77°C (170°F) during casting resulted in less

than optimal concrete performance both from a mechanical and material performance

standpoint. This includes a reduction in compressive strength and increased

microcracking as well as increased permeability at later-ages. Higher concrete

temperatures during hydration resulted in more extensive and wider cracking on the

microstructure level. The increased level of microcracking precipitated the propagation of

cracking in the transition zones and thereby resulted in failure of the cylinder at a lower

compressive strength. A reduction in elastic modulus was also noted above this

temperature range.

3. For optimal concrete mechanical and material property performance, specifications should

address a maximum concrete temperature during hydration for precast/prestressed

elements. This is particularly important for HPC components where high heat of hydration

is generated. To avoid detrimental temperature effects and increased microcracking, a

maximum hydration temperature of 77°C (170°F) is recommended for the mix design and

section shapes investigated within this research study.

4. Alternatively to a specification requirement in terms of concrete temperature (during

hydration), a limitation on the maximum temperature rise of 39°C (70°F) may be

considered for the mix design and section shapes investigated within this investigation.

5. No clear correlation between maximum temperature rise and permeability (RCPT) was

noted based on the scatter of data collected within this investigation. Rather, the

maximum concrete temperature during hydration appeared to impact permeability

performance.

6. Notably more severe microcracking was noted for specimens subjected to extreme

concrete temperatures during hydration [93°C (200°F)]. The microcrack widths were

significantly wider and the microcracks appeared to be interconnected for concrete

subjected to extreme concrete temperatures during hydration. The interconnection and

wider crack widths of the microcracking appeared to significantly increase the permeability

of the concrete.

The following information may be note worthy for fabricators of High Performance Concrete

Prestressed/Precast Beams based on the research program conducted herein:


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1. Since HPC members generate significant temperature rise during hydration, traditional

accelerated steam curing [66°C (150°F) - TxDOT specification maximum in Texas], may

not be desirable from both an early and later-age property development viewpoint.

2. To avoid excessive temperature development during the casting of HS/HPC beams,

especially during hot weather, traditional hot-weather concreting techniques should be

employed by the fabricator. These include a.) using chilled mixing water or ice

replacement, b.) batching of concrete when stock piles are at minimum daily low

temperature or cooling stock piles prior to the batching process, c.) using mineral

admixtures such as fly ash as a partial cement replacement to curb the initial hydration

process, and/or d.) the use of retarders to offset the setting time to avoid daily maximum

ambient conditions.

The following recommendations are made regarding areas requiring further research:

1. Investigate the impact of peak hydration temperatures on the durability of HS/HPC and

HPC on the microstructure level more fully such that a direct relationship between

concrete temperature during hydration and microstructure damage can be ascertained.

2. Using match-curing technology, investigate the combination of standard and high

performance mix designs combined with various “standard shape” surface area to volume

ratios to determine where match curing technology results in increased plant productivity.

For some combinations of high SA/V ratios and conventional concretes, member cured

cylinders sufficiently represent the concrete properties in the member.

3. Investigate the combination of mix designs with various cementitious contents combined

with various “standard shapes” to predict at what SA/V ratios (for a given cement content)

less than optimal performance results.

ADDITIONAL INFORMATION ON TEXAS HPC BRIDGES

For additional information on the Texas HPC bridges, see the paper entitled Implementation of

High Strength - High Performance Concrete (HS-HPC) in Two Texas Highway Overpass

Structures: A Critical Comparison by Gross and Burns [6].

ACKNOWLEDGEMENTS

The authors wish to thank the joint sponsors of this research project, The Federal Highway

Administration and The Texas Department of Transportation, for their support and

encouragement. In addition, the authors would like to thank the fabricator, Texas Concrete

Company, for their assistance, interest, and involvement in this research study.


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REFERENCES

1. Myers, J.J., Carrasquillo, R.L., Production and Quality Control of High Performance Concrete in Texas Bridge Structures, The University of Texas at Austin, Center for Transportation Research, Preliminary Research Report, Number 580/589-1, December 1998.

2. Myers, J.J., Carrasquillo, R.L., Quality Control & Quality Assurance Program for Precast Plant Produced High Performance Concrete U-Beams, PCI/FHWA International Symposium on High Performance Concrete, Advanced Concrete Solutions for Bridges and Transportation Structures, New Orleans, October 1997, pp. 368-382

3. Patty, T.S., Deno, D.W., “Petrographic Studies of High Performance Concrete Test Cylinders,” Wiss, Janney, Elstner Associates, Inc., WJE No. 981061, July 2, 1998.

4. Gross, S.P., Burns, N.H., Implementation of High Strength - High Performance Concrete (HS-HPC) in Two Texas Highway Overpass Structures: A Critical Comparison, IBEC Conference Proceedings, Tampa, Florida, 2000.


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Table 1: Temperature Development of Prestressed / Precast U-Beams

Casting Date (Mo-Day-Yr)

“Light” Steam Curing

Peak Temperature, deg C (deg F)

Temperature Rise,

deg C (deg F)

Heat Generation °C/100 kg cem mat’l/m3

(°F/100 lbs cem mat’l/yd3) 9-23-94 No 96.7 (206) 61.1 (110) 10.45 (11.14) 9-30-94 No 95.0 (203) 58.9 (106) 10.07 (10.74) 10-7-94 No 95.0 (203) 59.4 (107) 10.16 (10.84)

10-28-94 Yes 85.0 (185) 56.7 (102) 9.69 (10.33) 11-3-94 No 84.4 (184) 55.6 (100) 9.50 (10.13)

11-10-94 Yes 82.8 (181) 52.2 (94) 8.93 (9.52) 2-15-96 No 86.1 (187) 58.9 (106) 10.07 (10.74) 2-26-96 No 87.2 (189) 54.4 (98) 9.31 (9.93) 3-8-96 No 86.7 (188) 54.4 (98) 9.31 (9.93)

Bold text indicates the maximum values encountered during casting of this member shape. All peak temperature locations were measured in the end blocks. “Light” Steam Curing refers to steam curing [< 38°C (100°F)] that was provided when ambient temperatures approached freezing to serve as a “buffer zone” between the member and the external environment. The researchers and fabricator wanted to protect the members against excessive internal thermal gradients at release. Additionally, since the high cementitious content of the concrete and the low SA/V ratio of the members generated a significant level of heat, it was determined that traditional accelerated steam curing would only be detrimental to the later-age property development of the concrete. deg F = (5/9deg C)+32

Table 2: Temperature Development of Prestressed / Precast AASHTO Type IV’s

Casting Date (Mo-Day-Yr)

“Light” Steam Curing

Peak Temperature, deg C (deg F)

Temperature Rise,

deg C (deg F)

Heat Generation °C/100 kg cem mat’l/m3

(°F/100 lbs cem mat’l/yd3) 4-1-96 No 52.5 (126) ➊ 25.6 (46) 4.37 (4.68)

2-19-97 No 63.9 (147) 36.1 (65) 6.17 (6.61) 2-25-97 Yes 53.9 (129) 30.0 (54) 5.13 (5.49) 3-3-97 No 63.9 (147) 33.3 (60) 5.70 (6.10) 3-8-97 No 61.1 (142) 30.6 (55) 5.22 (5.60)

3-15-97 No 61.1 (142) 30.6 (55) 5.22 (5.60) 3-22-97 No 68.9 (156) 38.3 (69) 6.55 (7.02) 3-29-97 No 71.1 (160) 41.7 (75) 7.12 (7.63) 4-7-97 No 75.0 (167) 46.7 (84) 7.98 (8.55)

4-12-97 No 45.6 (114) 27.2 (49) 4.65 (4.98) 4-18-97 No 66.1 (151) 37.8 (68) 6.46 (6.92) 4-28-97 No 75.0 (167) 43.9 (79) 7.50 (8.04)

Bold text indicates the maximum values encountered during casting of this member shape. All peak temperature locations were measured in the end blocks unless noted otherwise. ➊ Match curing not performed, value based on member cured cylinder. deg F = (5/9deg C)+32

Table 3: High Performance Concrete Mix Design utilized for the Precast/Prestressed Beams

Component Quantity Type Cement 398 kg/m3 (671 pcy) ASTM C150 Type III Fly Ash 187 kg/m3 (316 pcy) ASTM C618 Class C Fine Aggregate 610 kg/m3 (1029 pcy) Natural River Sand Coarse Aggregate 1138 kg/m3 (1918 pcy) Crushed Dolomitic Limestone,

12.7-mm (½”) max, ASTM GR 7 Water 147 kg/m3 (247 pcy) Potable


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Superplasticizer 1.1-1.4 liter/100 kg cem. (17.0-21.6 oz/cwt) ASTM C494 Type F Retarder 0.168 liter/100 kg cem. (2.57 oz/cwt) ASTM C494 Type B

Table 4: Summary of Paste Characteristics for the Specimens Investigated [4]

Specimen Number

Curing Condition Temp °C

(Temp °F)

Color of Fractured Surface (Polished Surface)

Nature of Unhydrated Cementitious Material

Hardness of

Cement Paste

Luster

1 Match 29°C (85°F)

Lt Brn to Tan (Tan)

➊

Numerous unhydrated fly ash and cement particles. Some of the fly ash particles are etched.

Very Hard Sub-vitreous to Vitreous

2 Member 71°C

(160°F)

Tan (Tan)


Very Hard Sub-vitreous

3 Match 93°C

(200°F)

Light Brown (Light Brown)

➋


Very Hard Sub-vitreous

Additional Comments or Notes: ➊ Appears to contain more calcium carbonate crystals than the other samples. ➋ The cement paste on both the polished surface and he fractured surfaces are visibly lighter in color than the other samples. degC=(5/9)(degF-32)

Table 5: Point-Count Data and Material Volumes for Thin-Sliced Specimens

Curing Regime Peak Hydration Temperature - °C (°F)

Match 29°C

(85°F)

Member 71°C

(160°F)

Match 93°C

(200°F) Point-Count ➊

Paste Fine Aggregate Coarse Aggregate Air Void

377 286 506 31

369 252 544 35

386 268 517 29

Paste Volume (%) 31.4 30.8 32.2 Fine Aggregate Volume (%) 23.8 21.0 22.3 Coarse Aggregate Volume (%) 42.2 45.3 43.1 Percent Entrained Air (%) 2.08 1.24 1.41 Spacing Factor 0.6358 0.6459 0.6595 ➊ One point count is equivalent to one digital pixel acquired during digital imaging. degC=(5/9)(degF-32)


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Table 6: Quantitative Magnitude of Microcracking

Curing Condition - Temp °°°°C (°°°°F)

Total Point Count of Microcracking ➊

Maximum Crack Widths

Match - 29°C (85°F) 216 5 pixels Member - 71°C (160°F) 309 13 pixels Match - 93°C (200°F) 500 31 pixels

➊ Point count measured over the same surface area for each specimen and does not include any air voids. One point count is equivalent to one digital pixel acquired during digital imaging.

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