Carson Baker Maturity Essay

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CONCRETE MATURITY:EFFECTS OFAMBIENTAIR TEMPERATUREON EARLYAGE CONCRETE TEMPERATURE

CEE425HONORS REPORT

by

Carson Baker

UNIVERSITY OF WASHINGTON

DEPARTMENT OF CIVIL ENGINEERINGSEATTLE, WASHINGON 98195

3January, 2015


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TABLE OF CONTENTS

List of Figures ............................................................................................................................................... ii

List of Tables ................................................................................................................................................ ii

1 Introduction ........................................................................................................................................... 1

2 Rate Of Strength Gain ........................................................................................................................... 1

2.1 Standard Temperature ................................................................................................................... 1

2.2 Varying Temperature .................................................................................................................... 3

3 Analyses Of Concrete Maturity Vs. Ambient Temperature .................................................................. 4

3.1 Description Of Data ...................................................................................................................... 4

3.1.1 Concrete ................................................................................................................................ 4

3.1.2 Weather ................................................................................................................................. 4

3.2 Analyses ........................................................................................................................................ 5

3.2.1 Initial Observations ............................................................................................................... 5

3.2.2 Correlation between Maturity and Air Temperature ............................................................. 5

4 Conclusion ............................................................................................................................................ 6

4.1 Results of Analyses ....................................................................................................................... 6

4.2 Recommendations ......................................................................................................................... 6

4.3 Future Work .................................................................................................................................. 8

References ..................................................................................................................................................... 9

Appendix A: Stoneway Concrete Mix Properties ....................................................................................... 10

LIST OF FIGURES

Figure 2-A: Effect of ACI Constant a on Concrete Strength..................................................................... 2

Figure 2-B: Sample Temperature Plot versus Standard Maturity ................................................................. 3

Figure 3-A: Sample Concrete Slab and Air Temperature Data (Deck 26 Shown) ....................................... 5

Figure 3-B: Computed Maturity values for Elevated Decks 25-28 .............................................................. 6

Figure 4-A: Computed Maturity values for Elevated Decks 25-28 .............................................................. 7

LIST OF TABLES

Table 2-A: Values of the constant for use in Equation (2-1) ..................................................................... 2


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1 INTRODUCTION

Concrete slabs are widely used in various applications including high rise office buildings and parking

garages. These slabs must be given adequate time to cure before loads may be placed on them, or before

prestressing operations can be performed. Thus an understanding of a concrete slabsability to mature is

critical in determining its suitability in design. The strength of a concrete mix is a complex function of

many variables including the concrete temperature and the allowed curing time. The strength of a given

mix is determined by allowing a specimen of concrete to cure under standardized temperature and

moisture conditions and then subjecting it to a compression test after a specified amount of time in

accordance with ASTM C39. However these standard conditions are not likely to be met at the job site.

The variability in site conditions introduces uncertainty regarding the behavior of the concrete, and thus it

is essential that appropriate cold-weather concreting practices are followed. This ensures that the concrete

can develop a required strength before forms are removed or load is placed on the concrete. This load

may take several forms including re-shoring load and post-tensioning load. As a result the effects of

environmental conditions including ambient air temperature must be taken into account when determiningthe early strength of concrete at the job site.

The purpose of this report is to investigate the correlation between ambient air temperature and the rate of

concrete maturity at the 815 Pine job site. Temperature data from both concrete maturity meters and

weather stations was analyzed. A correlation was found between the air temperature and concrete

maturity levels, suggesting a minimum air temperature when pouring concrete slabs which require high

early strength.

2 RATE OF STRENGTH GAIN

Concrete develops strength as the cement is allowed to hydrate. This chemical reaction is notinstantaneous but rather occurs over time. As more cement is allowed to hydrate the concrete continues to

gain strength. Thus the strength of concrete is closely related to the time given to mature. The concrete

temperature is directly related to the rate of curing and strength gain as well as environmental conditions.

In order to predict the concrete strength at a given time t, standard procedures and methods were

developed to establish a consistent means for estimating concrete strength.

2.1

STANDARD TEMPERATURE

The American Concrete Institute (ACI) provides recommendations for modeling and predicting concrete

strength over time. The ACI 209R-08 report provides a general equation for computing the mean

compressive strength at any time as:

()=[ + ] (2-1)

where: =the time after the casting of the concrete measured in days. =ACI constant measured in days.


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= ACI constant measured in days.

28 =the concrete mean compressive strength at 28 days in psi.The constants and are a function of both the type of cement used and the method of curing used.Varying the value of modifies both the rate of strength gained and the ultimate concrete strength, butdoes not affect the 28-day concrete strength. The effect of the value used for the ACI constants issignificant, but little guidance is provided by ACI for selection of a value, and their recommendations in

ACI 209R-08 do not explicitly consider many important factors. Efforts have been made to link the ACI

constants to mixture properties including cement contents. The following data represents results from

Jake Meaders report entitled Structural Design Parameters of Current WSDOT mixtures where cement

content of several mixes was varied with the paste content remaining the same. Values for the ACI

constants were then found by curve optimization, as provided inTable 2-A.A plot of the generated curve

for Mixture 6 is provided inFigure 2-A.

Table 2-A: Values of the constant for use in Equation(2-1)Type of cement Mixture 1 Mixture 2 Mixture 3 Mixture 4 Mixture 5 Mixture 6 Mixture 7

High Cement 3.6 1.3 1.9 1.9 1.6 1.3 1.3

Low Cement 3.3 3.4 3.7 3.2 2.9 2.6 4.3

Figure 2-A: Effect of ACI Constant a on Concrete Strength.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 7 14 21 28 35 42 49 56

fc28F

actor

Time (days)

High Strength

Low Strength

Cement, a = 2.6

Cement , a = 1.3


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2.2 VARYING TEMPERATURE

The equations described have been developed assuming standardized conditions for the concrete

specimens, including a standard concrete temperature of 73oF. While this temperature is achievable under

idealized lab conditions, this is often impractical at job sites. As a way of measuring the maturity of

concrete given variable temperature conditions the Nurse-Saul maturity function is commonly used

(ASTM C1074):

Maturity (t) = ( )

(2-2)

where: =the temperature of the concrete.= A lower bound on the concrete temperature at which the concrete reaction stops,typically taken at 14

oF.

These equations provide a generally accepted method for approximating a compressive strength using

readily available information. A plot of measured concrete temperature along with standard temperatures

is given inFigure 2-B.(Slab data from Deck 26, Sensor #1 depicted)

Figure 2-B: Sample Temperature Plot versus Standard Maturity


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Thus in order to predict the strength of a sample of concrete over time, its mixture properties must be

established and maturity growth must be understood. When the ambient air temperature is cooler than a

specimens internal temperature, the internal temperature of the sample tends to lower, and thus delay the

development of the concretes maturity. This implies that lower ambient air temperatures may lead to

lower concrete strengths than previously anticipated.

3 ANALYSES OF CONCRETE MATURITY VS. AMBIENT TEMPERATURE

3.1

DESCRIPTION OF DATA

In order to investigate the effect of ambient air temperature on concrete maturity, data was collected from

the 815 Pine job site. It is important to recognize that this data represents a specific mix used in a

particular geographic location and application, and is insufficient to describe all types of concrete in all

conditions.

3.1.1 Concrete

In anticipation of potentially lower rate of strength gain due to expected cold weather HollandConstruction installed thermal sensors in several concrete slabs to monitor their temperatures over time.

Nick Hoffman of Holland Construction provided the thermal data measured by the maturity meters.

All specimens had the same mix properties and were supplied by Stoneway Concrete. A 6ksi normal-

weight concrete mix was utilized, with a cement factor of 6.33 sacks, and effective cement factor of 7.61

sacks. The water to equivalent cement ratio was 0.35, with a paste content of 28%. All slabs were cast

7 thick and were prestressed. Additional information on the mix properties is provided in Figure A.1.

As the maturity is directly related to the internal concrete temperature, it is expected that mixes with

higher cement contents would generate more heat and have higher resistances to low ambient air

temperatures. Similarly, if more cement replacements were used, less heat would be generated and thesample would become more susceptible to cooling from the air. Additionally, thicker slabs have a higher

volume to exposed surface ratio, and would thus be less sensitive to air temperature.

The temperature sensors were installed at four locations across decks 25-30 of the 815 Pine tower. Data

was recorded on hourly intervals.

3.1.2 Weather

A WSDOT weather station within several blocks of the 815 Pine job site recorded air temperatures every

hour. The temperature data was provided by Mark Albright from the University of Washington

Department of Atmospheric Sciences. While it is expected that other air properties including humidity

and wind speed may have an effect on concrete maturity, only air temperatures were considered in this

study. The weather data ranged from November 6 through December 20th.


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3.2 ANALYSES

Temperature profiles were developed for slabs on decks 25-30 of the 815 Pine tower, beginning

immediately after the concrete was placed. Using this information maturity was calculated up to three

days, depending on the available data.

3.2.1

Initial ObservationsBy plotting both concrete temperature and air temperature, it was found that the recorded concrete

temperatures converged to the measured air temperatures, as shown inFigure 3-A.(Slab data from Deck

26, Sensor #1 depicted). This trend tends to converge roughly 5 days after pouring. This is not

unexpected, as by this point in time the chemical reaction has largely finished occurring, and thus the heat

produced internally is insignificant. The observation that the data converges with time reveals two things.

Firstly, the correlation of the data indicates it is reliable and suitable for comparison. Secondly, the data

confirms that the air temperature plays a significant role in the thermal history of the concrete with time.

Figure 3-A: Sample Concrete Slab and Air Temperature Data (Deck 26 Shown)

3.2.2 Correlation between Maturity and Air Temperature

Following computations of maturity for all samples, these values were plotted based on the average

measured air temperature surrounding the samples. It was discovered that 24-hour maturity values werethose most significantly and directly affected by the ambient air temperature, as the temperature

difference between the air and the concrete was most significant. Thus the 24 hour maturity values are

those most relevant for determining effects on early strength. A plot comparing computed maturity with

standard maturity is provided inFigure 3-B.


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Figure 3-B: Computed Maturity values for Elevated Decks 25-28

4 CONCLUSION

4.1 RESULTS OF ANALYSES

While data is dependent on a variety of factors, for the specific concrete mix used for PT slabs in the 815

Pine project, and for a slab thickness of 7 , the concrete maturity was found to be lower than standard

maturity when the average 24-hour ambient air temperature fell below 50 oFas suggested byFigure 3-B.

This finding is similar to the definition of cold-weather as defined by ACI 306-R88, which states that

when temperatures above 50oFoccur during more than half of any 24 hour duration, the period is no

longer regarded as cold weather.

The trendline through the plotted points nearly passes through the x-axis at 14oF. This is consistent with

the Nurse-Saul maturity function (Eq. 2-2) which defines , the point where the concrete reactionstops and thus the concrete does not mature, at 14oFunder ASTM C1074.

This data is only applicable for the conditions described. It is expected that the slope of the trendline

would change with changes to cement content, slab thickness, slab type, relative humidity, and other

conditions.

4.2 RECOMMENDATIONS

Cold weather can reduce rate of strength-gain, which can lead to problems for activities that require early

strength such as stressing PT strands and stripping deck forms. This work analyzed air and slab

temperatures for a specific construction project, and found that air temperatures below 50oFcould lead to


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early rates of strength gain in slabs that would be lower than corresponding to standard curing (73oF).

While the data provided is representative of a specific mix of concrete, in general it may be understood

that as ambient air temperature approaches 50oFit is expected that the maturity of the concrete begins to

be adversely affected by the cold weather. This is consistent with recommendations from ACI 306-R88.

The language is different but similar to ACI 306-R10, which defines cold weather as the point at which

the air temperature drops to 40oF

for any amount of time during the time required to protect the concretefrom exposure effects. This revised definition relies on an absolute measurement instead of an average

measurement over a given period. Following the language used in ACI 306-R10, the plot inFigure 4-A

was generated comparing minimum temperatures to maturity. Similar results were found to those given in

Figure 3-B which uses an average temperature metric.

Figure 4-A: Computed Maturity values for Elevated Decks 25-28

The concept of maturity is that of a chemical process occurring over a period of time. While the definition

of cold weather in ACI 306-10 uses an absolute threshold to define cold weather, an average temperature

method is expected to yield more accurate and context-specific results. This method also places less

emphasis on brief atypical or inaccurate measurements.

To mitigate the effects of cold weather, several preventative actions may be appropriate. It is

recommended that a cold-weather mix be utilized. This may be achieved by increasing the cement content

of the mix, thus increasing the heat produced during hydration and thereby increasing the resistance to the

effects of ambient air temperature. Mixes should have an appropriate amount of entrained air to allow for

the relief of hydraulic pressure. The effectiveness of entrained air varies with the amount of air and the

spacing and size of voids. Chemical admixtures such as Type C or Type E chloride or non/chloride

accelerators may be utilized, however the effects of corrosion, discoloration, as well as sulfate and alkali-

silica reactions must be considered. See ASTM C494 for additional information on admixture

classification.


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REFERENCES

ACI 209R-08 Strength gain with time

ACI 306-R10 Cold weather concreting

ACI 306-R88 Cold weather concreting

ASTM C39Compression testing spec

ASTM C494Admixtures

ASTM C1074 - Maturity

Jake Meader


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APPENDIX A: STONEWAY CONCRETE MIX PROPERTIES

Figure A-1: Stoneway Concrete Mix Properties for 815 Pine PT Slabs

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