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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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