Effect of Extending Concrete Delivery Time on Plastic and Hardened Properties of Concrete

Transcription

1 Effect of Extending Concrete Delivery Time on Plastic and Hardened Properties of Concrete Alexandra Akkari (Corresponding Author) Minnesota Department of Transportation Phone: 1-- Fax: Ally.Akkari@dot.state.mn.us Bernard Izevbekhai Minnesota Department of Transportation 1-- Fax: Bernard.Izevbekhai@dot.state.mn.us Dan Vruno American Engineering Testing Phone: 1--1 dvruno@amengtest.com Dave Rettner American Engineering Testing drettner@amengtest.com Words = 1 Tables: ( 0) = 000 Figures 1( 0) = 0 Total = TRB 01 Annual Meeting

2 ABSTRACT Due to some remotely located projects and successes with admixtures, industry requested the State Department of Transportation (MnDOT) allow longer delivery times. In response to this request, 0 and minute allowable delivery times were proposed as alternatives to the current 0 minutes specification for air entrained concrete to allow for more feasible construction. To investigate the impact this change would have on concrete performance, a laboratory study, controlled plant study, and a field study were conducted. These studies all evaluated critical properties of concrete (slump, plastic and hardened air, freeze thaw durability, and compressive strength) for a variety of mix designs at different times after mixing. Many different combinations of air entrainers, water reducers, retarders and cementitious materials were used in the mix designs. Analysis of the data from the laboratory study, controlled plant study, and field study led to mixed theories on how these properties of concrete are influenced by extended delivery times of 0 and minutes. Because in many cases the analysis showed that slump and plastic air content can be significantly affected by a change in delivery time, it would be beneficial to conduct further research on this topic before any changes were made to current specifications for allowable delivery time of concrete. 1 TRB 01 Annual Meeting

3 INTRODUCTION The transition of concrete from the plastic to hardened state is a both a chemical and physical process that has become even more complex with the use of modern admixtures. Both mineral and chemical admixtures can be used to alter the properties of plastic and hardened concrete. The effect these admixtures have on concrete hydration and setting time has spurred recent interest in allowing extended delivery times to project sites. Project Objectives The concrete industry has been asking the Minnesota Department of Transportation to lengthen the allowable time to deliver concrete from point of batching to project sites. MnDOT is planning on constructing many small bridge projects that are difficult to reach within the existing 0 minute time limit for air-entrained concrete. This 0 minute time limit may unnecessarily increase the cost to construct these bridges, as it would likely require a temporary batch plant to be constructed near the project site. Most pavement projects in ready mix concrete are delivered from plants. On rare occasions concrete mobiles are established on site for certain bridge overlay projects. Statistically, therefore, there is a preponderance of projects where implicating longer transit time is of great consequence. This research investigated the prospects of changing the current 0 minute specification for delivery time to either 0 or minutes through a laboratory study, controlled plant study and a regional field study. In each study, the impact of longer delivery time on concrete was evaluated by measuring important material properties (slump, air, strength, durability, and hardened air) over time. Multiple different mix designs, materials, and admixtures were used in each phase. This paper discusses the testing and analysis of results. It utilizes statistical methods to accentuate comparative implications of increased delivery times. Concrete Setting Time and Portland Cement Hydration Specifications for allowable delivery time of concrete are based off of reasonable estimates for the time it takes for concrete to set. Setting is defined as the beginning of rigidity of fresh concrete. Specifically, initial set refers to the point at which fresh concrete can no longer be properly handled and placed (1). Final set refers the point when hardening begins. Hardening is different from setting, as it is characterized by the development of measurable strength. The time it takes for concrete to set depends primarily on the hydration of cement. The hydration process of pure cement compounds can best be described through the reaction of water with the two calcium silicate compounds in cement (tricalcium silicate and dicalcium silicate). The main hydration product from this reaction is a calcium silicate hydrate. The reaction sequence can be split into stages based on the rate of heat evolution throughout the hydration process. The heat evolution is directly related of the rate of the hydration reaction and is most easily measured in tricalcium silicate (C S). C S is more reactive than dicalcium silicate (C S) and therefore is the major contributor to early strength and set time; C S is responsible for high final strength. The first stage of hydration consists of a period of rapid heat evolution lasting only about 1 minutes (1). The second stage is referred to as the dormant stage when the reaction rate TRB 01 Annual Meeting

4 is reduced to relative inactivity. This stage generally lasts a couple of hours and allows concrete to remain in the plastic state. Concrete does not reach initial set until the beginning of stage three when the reaction rate increases rapidly. The maximum rate is reached at the end of this period and marks the time of final set. Stage four occurs after four to eight hours, when final set has been accomplished, and the rate of reaction begins to decrease as early hardening and strength gain begin. At 1 to hours, stage five marks the period of steady state reaction contributing towards final hardening and later strength gain. Effect of Admixtures on Set Time Although the previously described reactions do play a major role in determining the concrete setting time, it is also commonly dependent on the different admixtures used. An admixture is defined as any material added to concrete, or mortar, immediately before or during mixing other than water, aggregates and cement (). The admixtures used in this research and their effect on setting time will be discussed presently. It is important to note many different admixtures react uniquely with each other, sometimes altering the effect they have on concretes properties. Set-Retarding Set retarding admixtures are used to prolong the time for cement to hydrate and are beneficial in concrete in many ways. By delaying the time of setting, concrete can be hauled from farther mixing plants. They can help counter the effect of increased set from high temperatures. They are especially beneficial when concrete requires difficult placement or timely finishing. Set retarders work by forming a coating around cement compounds through absorption that slows the reaction with water. In time, depending on the thickness of the coating, it will break down to allow the normal hydration process to occur (). Water Reducers Water reducers are soluble organic materials that can be added to concrete for multiple reasons: to reduce the amount of mixing water necessary to achieve a specific slump, to reduce water to cement ratio, to reduce cement content, or to increase slump (). There are different types of water reducers categorized based on their effect on set time; some accelerate set, some decelerate set, and some have little effect on set. Any water reducer which extends the initial set time of concrete more than three hours is classified as a retarding water reducer. Mid range water reducers are considered to have a lesser effect on set times and achieve more consistent set times than standard water reducers (). Air-Entrainment Air entraining admixtures are used to stabilize microscopic air bubbles distributed throughout the hardened cement paste in concrete. Research has shown that air entraining agents do not significantly affect the rate of the cement hydration reaction and also do not affect the hydration products chemical composition (). It has been found that less air entraining admixture is needed to reach a desired air content when water reducing and set retarding admixtures are also used (). TRB 01 Annual Meeting

5 Fly Ash Fly ash is a very finely graded residue produced from the combustion of either ground or powdered coal. The setting time of concrete can be increased with the use of most Class C fly ashes and all Class F fly ashes (). Some consider the magnitude of the change in set time between a fly ah mix and one without fly ash of the same materials to not necessarily require changes in construction techniques (). Some have found that the use of class C fly ash causes unpredictable differences in the time between initial and final set, and can also cause flash or false set in field construction (). EXPERIMENTAL DESIGN This research included a laboratory study, a controlled plant mixing program, and a regional testing program which will be discussed in detail presently. Laboratory Study A large scale laboratory study evaluated the response of different mix designs to a change in delivery time. Specific admixtures, cementitious materials, and dosage rates were chosen to reflect those to be further evaluated in the field testing program. Any mix design which showed inadequate properties was to be either adjusted and reevaluated or entirely eliminated from the field testing program. Suggestions and recommendations from ready mix suppliers were used to choose cement, fly ash and admixture combinations to be used in the mix designs that were representative of those used in current practice. This avoided the study of rare materials or those that were incompatible (such as unwanted synergistic effects between admixtures). All mixes had a water to cementitious ratio of 0. and a 0% Fly Ash replacement. Table 1 summarizes the composition and variations in the mix designs used. TRB 01 Annual Meeting

6 Batch No. Cement ( Type I/II) TABLE 1 Cementitious Materials and Admixtures Fly Ash ( Type C and 1 Type C/F) Air Entrainment ( Types) Mid-Range Water Reducer ( Types) Set Retarder Water Reducer ( Types) 1 1 Vinsol Resin #1 - # Vinsol Resin #1 - # 1 Synthetic #1 - # Synthetic #1 - # 1 1 Vinsol Resin #1 - # 1 Vinsol Resin #1 - # 1 1 Synthetic #1 - # 1 Synthetic #1 - # 1 1 Vinsol Resin # - # 1 Vinsol Resin # - # 1 1 Vinsol Rosin #1 #1 #1 1 1 Vinsol Rosin #1 #1 #1 1 1 Synthetic # # # 1 Synthetic # # # Synthetic # # # 1 1 Synthetic # # # 1 1 Synthetic # # # Synthetic # # # 1 1 Synthetic # # # Vinsol Rosin # - # 1 1 Vinsol Rosin # - # 1 Vinsol Rosin # # # Vinsol Rosin # # # All mixes tested had identical design weights for coarse aggregate, fine aggregate, cement, fly ash, and water. Aggregate moisture contents were measured before batching, and water and batch weights were adjusted accordingly to maintain a water to cement ratio of 0.. Admixtures were added to separate water buckets before being added to the mixer with previously mixed sand and rock. The initial time of batching was then determined once cement and fly ash were added to the mixer. Initial plastic tests were performed, including air, slump, unit weight and temperature. Initial hardened samples were cast. The remaining concrete was stored in plastic buckets with moist burlene covers. Plastic tests (air and slump) were subsequently performed after 0 and 0 minutes. At minutes, the concrete was placed back in the mixer and remixed for three minutes. More plastic tests (air and slump) were performed immediately after remixing at 0 minutes and then again at minutes. Final hardened samples were cast at minutes. Hardened air content was measured for seven different batches after seven days of curing. Air specimens were prepared after initial mixing and minutes after batching. Compressive strength specimens for each mix prepared after initial mixing and after minutes were tested at 1,, and days. Finally, freeze/thaw specimens for each mix TRB 01 Annual Meeting

7 prepared after initial mixing and minutes were tested for durability according to ASTM C. Controlled Plant Mixing Program A controlled plant mixing program was conducted at a single ready-mix concrete plant as preliminary evaluation of the four concrete mixes that would be used in the field study. Each batch had a combination of admixtures, including air-entertainer, water reducer and hydration stabilizer. MnDOT A (Batches 1 and ) Batch 1: 0% Fly Ash Replacement Batch : % Ground Granulated Blast Furnace Slag (GGBFS) Replacement Air entrained 0. cement to void (water and air) ratio 00 psi compressive strength Greater than 0 lb/cy cementitious material 1- inch slump Class CA- 1 to 0 inclusive MnDOT Y (Batches and ) Batch 1: 0% Fly Ash Replacement Batch : % Ground Granulated Blast Furnace Slag (GGBFS) Replacement Air entrained 0. cement to void (water and air) ratio 000 psi compressive strength Greater than 0 lb/cy cementitious material - inch slump Class CA- to 0 inclusive Slump, air content, unit weight, and temperature of the plastic concrete were tested at 0, 0, 0 and minutes after initial batching. Compressive strength, hardened air content, and freeze/thaw samples were cast at both 0 and minutes after batching. Compressive strength was tested after a day curing period. Regional Field Testing Program The field testing program built upon observations and results from the controlled plant study. The regional testing program was to evaluate the same two concrete mix designs, MnDOT A and A, which were used in the plant study. However, in this program, only fly ash replacement ranging from 1% to 0% was used (no slag). Seven different ready mix suppliers were selected to participate, with three in the metropolitan area and one in each of the four TRB 01 Annual Meeting

8 regions of the state. Each plant produced two batches of concrete to be tested, one of each of the two previously mentioned mixes. Plastic air, unit weight and temperature were tested 0, 0, 0, 0 and minutes after batching. Compressive strength, hardened air, and freeze/thaw samples were cast at 0 and minutes after batching. STATISTICAL ANALYSIS AND RESULTS The experimental design described in the previous section generated an extremely large quantity of data. This data could not be easily interpreted with simple averages or graphs because the multiple different mix designs used in each study resulted in highly variable data. The data was unique in the fact that it was only meaningful when comparing specific pairs between two data sets. Therefore, statistical tests were utilized to conduct a meaningful analysis of the data: The Paired Student s T-Test and the Wilcoxon Signed Rank Test. The Paired Student s T-Test is a parametric test, while the Wilcoxon Signed Rank Test is a non-parametric test. These two procedures will be discussed in detail in the following sections. Paired Student s T-Test Description As stated before, the Paired Student s T-Test is a parametric statistical procedure. It requires two data sets of matched pairs. The test assumes that the sample populations follow a normal distribution and that the two populations have equal variances. This test is particularly useful in determining if there is likely to be a change between two paired populations to a certain confidence percentage. The Student s T distribution can also be used to calculate the confidence intervals for the mean difference so that one can determine the probable range for the difference between two paired samples. The analysis was conducted by comparing slump, air, compressive strength, hardened air, and freeze thaw durability for a single mix at two different delivery times. The null hypothesis tested was that the mean difference between the tested property at two delivery times would be zero, and the alternative hypothesis was that the mean difference at two delivery times would be other than zero. For a more complete analysis, the % confidence intervals were also computed for the probable range of the impact changing the delivery time may have on these properties. Results The tables and charts in this section summarize the results from the Student s T-Test. The P statistic can be interpreted as the probability that changing the delivery time will not influence the specific material property. A significance level of 0.0 was used to ascertain if the property was statistically different at different delivery times. A change in delivery time is likely to influence the property when the P statistic is less than 0.0; these instances are highlighted and bold in the tables. Table provides the results for plastic air and slump. TRB 01 Annual Meeting

9 to 0 minutes 0 to minutes TABLE Paired Student s T-Test Slump and Air Change in Slump (in) Change in Air (%) P stat Average Change % Upper % Lower P stat Average Change % Upper % Lower Lab Plant Field Lab Plant Field In the lab study, the remixing at 0 minutes actually caused an increase in slump and air from 0 to 0 minutes that is significant to the % confidence level. The % confidence levels suggest slump is likely to increase anywhere from 0. inches to 1. inches with this activity. Measurements at minutes, however, did not show any significant difference from those taken at 0 minutes, for either slump or air. The plant study data sets only consist of samples at each time. Therefore, these results do not support any significant change in slump or air for either 0 or minutes, except for a decrease in slump around 1. inches from 0 to minutes. Finally, results from the field study, with 1 samples per delivery time, show significant change in slump and air at both 0 and minutes. Because of the extremely low P statistic for all four cases, and the reasonable sample size, the field study provides substantial evidence that changing delivery time from 0 to 0 or minutes will change slump and air content. However, it is also important to note the magnitude of this change. The mean difference between slump at 0 and 0 minutes can be expected to fall within the range of 0. inches and 1. inches with % confidence, and is even more for 0 to minutes at 1. to inches. The effect is similar for air, where a change to 0 minutes can decrease air by 0. to 1%, and by 1. to 1.% at minutes. The figures 1 and illustrate the % confidence levels for the difference in slump and air with a change in delivery time. TRB 01 Annual Meeting

10 Difference in Slump -% Confidence [in] Lab 0 to 0 Plant 0 to 0 Field 0 to 0 Lab 0 to Plant 0 to Field 0 to % Upper % Lower Average Change Difference in Slump - % Confidence [in] Change in Delivery Time FIGURE 1a % Confidence intervals for difference in slump. Lab 0 to 0 Plant 0 to 0 Field 0 to 0 Lab 0 to Change in Delivery Time Plant 0 to Field 0 to FIGURE 1b % Confidence intervals for difference in air. FIGURE 1 % Confidence intervals for difference in air and slump. % Upper % Lower Average Change TRB 01 Annual Meeting

11 The table summarizes the Paired T-Test results from freeze thaw testing done in accordance with ASTM C after 00 cycles, and from hardened air content measurements. The only significant change in durability was in lab study, where specimens cast after both 0 and minutes were compared. The only significant change in hardened air content was in the field study specimens cast at 0 and minutes. The wide % confidence range in the plant study from 0 to minutes is a result of only pairs of data to analyze. The field study, which compared samples of 1 measurements, shows a much narrower range. The % confidence limits suggest the mean difference in air content is likely to be anywhere from 0.% to 1.%. Time Change TABLE Paired Student s T-Test Durability Factor and Hardened Air P Stat Change in Durability Factor Change in Hardened Air Content (%) Average Change % Upper % Lower P Stat Average Change % Upper % Lower Lab: 0 to minutes Plant: 0 to minutes Field: 0 to minutes Number of Freeze/Thaw Paired Samples: Lab = Plant = Field = 1 Number of Air Paired Samples: Lab = Plant = Field = 1 Table presents the Paired T-Test results from compressive strength measurements. The only significant change in strength is found in the lab study, where 1-day and -day specimens cast after 0 and minutes were compared. These were the only two cases with entirely negative % confidence intervals. TABLE Paired Student s T-Test Compressive Strength Time Change Study Age P Stat Average Change % Lower % Upper Lab to minutes Lab Lab to 0 minutes Plant Field to minutes Plant Field Lab = 1 paired samples, Plant = paired samples, Field = 1 samples paired 1 1 TRB 01 Annual Meeting

12 Wilcoxon Signed Rank Test Description The Wilcoxon Signed Rank Test is a non-parametric alternative to the Paired Student s T-Test. Although results are sometimes considered less powerful than those from the Paired Student s T- Test, it requires much less strict restrictions on the sample population. It does not assume the sample population to be normally distributed. This procedure uses the differences of between matched pairs of two sample populations to test the null hypothesis that the median difference is equal to zero. For sample sizes smaller than 0, the exact statistical probability (P value) of obtaining a particular test statistic can be calculated by determining all the different possible distributions of the ranks. For sample sizes larger than 0, the exact probability becomes tedious to compute. However, the distribution of possible ranks becomes more normally distributed as sample size increases, and the normal approximation of the probability can be used. As with the Student s T-Test, slump, air, freeze thaw, and compressive strength were compared at 0 to 0 minutes, and at 0 to minutes, using measurements from a single mix as a pair. Results at difference delivery times were considered to be significantly different if the resulting two-tailed probability from the Wilcoxon Signed Rank Test was less than 0.0 (the % confidence level). Results Unlike the T-Test, the Wilcoxon does not give a probable range for the mean difference between the two delivery times. However, the P statistic may be more appropriate for the data in this research as it is not based off the assumption of a particular sample distribution and the many different mixes used in this study make the sample sets highly variable. As was done with the Paired Student s T-Test, a significance level of 0.0 is used to determine if the results of a particular property are statistically different at different delivery times. These cases are highlighted and bold in the tables, and. Table shows the results from the Wilcoxon Signed Rank Test of all air and slump changes from 0 minutes to 0 and minutes. The test found a significant difference in slump and air in both the lab and field study at 0 minutes and in the field study at minutes. Again, the small sample sizes in the plant study make the results of the test statistically insignificant. TRB 01 Annual Meeting

13 TABLE Wilcoxon Signed Rank Test Slump and Air Property Change 0 to 0 minutes 0 to minutes Study Lab Plant Field Lab Plant Field Sum of Negative Ranks Sum of Positive Ranks. Exact P Value P Value for Normal Approximation Total Ties (assigned an average rank) 1 1 Number of Zero Differences Dropped Cases Property Air Slump Change 0 to 0 minutes 0 to minutes Study Lab Plant Field Lab Plant Field Sum of Negative Ranks Sum of Positive Ranks 1 1 Exact P Value P Value for Normal Approximation Total Ties (assigned an average rank) 1 1 Number of Zero Differences Dropped Cases Table provides the Wilcoxon Test results for durability factor and hardened air content. The test shows that durability factors are only significantly different between 0 to 0 minutes in the lab study, and that hardened air content is only significantly different between 0 and minutes in the field study. TABLE Wilcoxon Signed Rank Test Durability and Hardened Air Property Durability Factor Hardened Air Change 0 to minutes 0 to minutes 0 to minutes 0 to minutes 0 to minutes 0 to minutes Study Lab Plant Field Lab Plant Field Sum of Negative Ranks Sum of Positive Ranks 1 1 Exact P Value P Value for Normal Approximation Total Ties (assigned an average rank) Zero Differences Dropped Cases TRB 01 Annual Meeting

14 Finally, Table gives the test results from compressive strength measurements. This property showed the least significant change due to a change in delivery time, with the only one day strength measurements from the lab study being statistically different at 0 and minutes TABLE Wilcoxon Signed Rank Test Compressive Strength 0 to 0 to 0 to 0 to 0 Change minutes minutes minutes minutes 0 to minutes Study Lab Lab Lab Plant Field Age (days) 1 Sum of Negative Ranks Sum of Positive Ranks Exact P Value P Value for Normal Approximation Total Ties (assigned an average rank) Zero Differences Dropped Cases 1 1 The discussion provided in the next section will interpret the results from this statistical analysis. It will present any recommendations regarding the influence of delivery time on concrete slump, air, compressive strength, hardened air, and durability factor and how it may impact concretes performance in pavements. DISCUSSION The results for the Paired Student s T-Test and Wilcoxon Signed Rank Sum Test summarized in the last section provide a means to evaluate the impact of changing delivery time on five important concrete properties: slump, air, freeze thaw durability, hardened air, and compressive strength. The P statistic from the two tests is interpreted as the probability that the mean or median of the paired differences of the measured property at two delivery times is zero. It is used to gauge whether or not these differences can be considered statistically significant. In addition, in the case that the test P statistic shows significance, the % confidence levels from the Paired T-Test can be addressed to establish a probable or expected range for the difference. Table compares the results from the Paired T-Test and the Wilcoxon Test, with the designation significant referring to a P statistic less than 0.0. Results from the lab study suggest that remixing at 0 minutes can actually cause a significant increase in slump and air from 0 minutes. The mixed results between tests for 0 to minutes in this study suggest that there is no significant difference in air or slump. Results from both tests for the plant study were uniform for a difference in air at both 0 and minutes, and only significant by the T-Test at minutes for slump. The data from this study does not suggest any considerable change in either of these two properties when changing delivery time from 0 minutes to either 0 or minutes. Finally, results from the field study for both 0 to 0 minutes and 0 to minutes 1 TRB 01 Annual Meeting

15 were uniform between the two tests, showing a significant change in both air and slump. Because this study consisted of reasonable sample sizes, results may be most representative and accurately portray the influence delivery time has on air and slump of plastic concrete. The % confidence levels suggest that when changing delivery time from 0 to minutes, the decrease in slump will be at least 1. inches and can be up to.1 inches, and the decrease in air content will range from 1. to 1. percent. These decreases are substantial and may reduce the concretes performance. However, when changing from 0 to 0 minutes the % confidence limits are less drastic with a maximum decrease in slump at 1. inches and maximum decrease in air at only 1 percent. TABLE Paired T-Test and Wilcoxon Comparison Air Slump Task Change Paired T Wilcoxon Paired T Wilcoxon 0 to 0 significant significant significant significant 1 0 to no significant no no 0 to 0 no no no no 0 to no no significant no 0 to 0 significant significant significant significant 0 to significant significant significant significant Durability Factor Hardened Air Task Change Paired T Wilcoxon Paired T Wilcoxon 1 0 to significant significant no no 0 to no no no no 0 to no no significant significant The Paired T-Test and the Wilcoxon Test produced agreeing results in all cases when performed on both the change in durability factor and change in hardened air. The durability factor was statistically different by both tests only in the lab study, with a change in delivery time from 0 to minutes. There was not a significant difference in durability factor in either the plant or field study with a change from 0 to minutes. The hardened air content was statistically different by both tests only in the field study, with a change in delivery time from 0 to minutes. The Paired Student s T-Test for this study found a % probable decrease between 0.% and 1.% in air content. The upper limit of this range could be significant enough to impact the concretes performance. The analysis of compressive strength measurements from the lab, plant and field data does not prove any significant change in strength due to a change in delivery time. The Paired T- Test only found a significant difference in 1 and day strength measurements between 0 and minutes in the lab study, while only the 1 day strength between 0 and minutes in the lab study was significantly different by the Wilcoxon Test. 1 TRB 01 Annual Meeting

16 The statistical analysis discussed in this section lead to interesting and notable trends in the change of some critical properties of concrete at different delivery times. These observations and resulting recommendations will be provided subsequent final section. RECOMMENDATIONS AND CONCLUSIONS This research is in response to the concrete industry s request to lengthen the MnDOT specification for allowable delivery time of concrete, which is currently placed at 0 minutes for air-entrained concrete. 0 and minute allowable delivery times were proposed as alternative to the current specification. To investigate the impact this change would have on concrete performance, a laboratory study, a controlled plant study, and a field study were conducted. These studies all evaluated critical properties of concrete (slump, plastic and hardened air, freeze thaw durability, and compressive strength) for a variety of mix designs at different times after mixing. Many different combinations of air entrainers, water reducers, retarders and cementitious materials were used. Two statistical tests were utilized to analyze the collected data: the Paired Student s T- Test and the Wilcoxon Signed Rank Test. These tests provided the probability that two paired data sets will be statistically different. In this study, paired data sets included the above property measurements at 0 and 0 minutes after mixing, and at 0 and minutes after mixing. A 0.0 confidence level was used to ascertain a significant difference between delivery times. The % confidence range for the mean difference between a measured property at two delivery times was also found. The following list summarizes the significant results from this analysis. When the laboratory batched concrete was remixed at 0 minutes, there was a significant increase in both slump and plastic air content between 0 and 0 minutes by both the Paired T- Test and the Wilcoxon Test. However, the change in slump and air between 0 and minutes was not statistically significant. This suggests that with remixing at 0 minutes and an extended delivery time of minutes, the minimal change in the plastic properties of concrete would not be detrimental to pavement placement or performance. Without remixing at the plant, there was not a significant difference in air and slump between 0 and 0 minutes or between 0 and minutes by the Paired T or Wilcoxon Tests. It is important to note that the small data sets obtained from the plant study make it difficult to draw very meaningful conclusions. When in the field, both air and slump measurements were statistically different between 0 and 0 minutes and between 0 and minutes according to the Paired T-Test and Wilcoxon test. The % confidence intervals show the decrease in slump and air at longer delivery times is significantly large enough that it may be detrimental to both placement and performance of the concrete. Because the field portion of this study may most closely replicate what would happen during real construction, these findings should be considered before extending delivery time, especially to minutes when the decrease is largest. In general, analysis did not suggest that there would be a significant change to either durability factor or hardened air content with an extended delivery time of 0 or minutes. The only instance where hardened air content was statistically different by both tests was in the 1 TRB 01 Annual Meeting

17 field study, with a change in delivery time from 0 to minutes. In this case, the confidence interval may suggest that air could be substantially reduced with an extended delivery time. However, it is important to note that durability was only measured in the lab and any change in freeze-thaw durability due to an increase in delivery time would need to be evaluated in the longterm field study before any conclusions can be drawn. Analysis of compressive strength measurements at different delivery times returned varying results. Although there was some cases where strength showed significant differences between 0 and minutes, there was no significant difference in any cases between 0 and 0 minutes, and 0 and minutes. The % confidence ranges for differences in compressive strength were considerably wide. Therefore, one can assume the impact of delivery time on compressive strength is highly variable with mix design. The observations and data analysis from the laboratory study, controlled plant study, and field study in this research led to mixed theories on how important properties of concrete are influenced by extended delivery times of 0 and minutes. In many cases, the analysis showed that slump and plastic air content can be significantly affected by a change in delivery time. However, hardened air, durability (measured in the lab) and compressive strength were not reduced in any case at 0 minutes, and in all but one case at minutes. It would be beneficial to conduct further research on this topic before any changes were made to current specifications on the allowable delivery time for concrete. REFERENCES [1] Mindess, Sidney and Young, J.. Concrete Prentice-Hall, Inc., Englewood Cliffs, N.J. [] ASTM Standard C1, 0. Standard Terminology Relating to Concrete and Concrete Aggregates. ASTM International, West Conshohocken, PA, 0, [] Fattuhi, N. I. 1. Influence of air temperature on the setting of concrete containing set retarding admixtures. Cement, Concrete, and Aggregates (1):1-1. [] Portland Cement Association. 1. Design and Control of Concrete Mixtures (1 th ed) (EB-001). p. [] Portland Cement Association. Chemical Admixtures. July 0. [] Ramachandran, V. S., and R. F. Feldman. 1. Cement science. In Concrete admixtures handbook: Properties, science, and technology, ed. V. Ramachandran, 1-. Park Ridge, N.J.: Noyes Publications. [] Whiting, D and Stark, D.. 1. Control of air content in concrete. NCHRP report (May). Washington: Transportation Research Board, National Research Council. 1 TRB 01 Annual Meeting

18 1 1 [] ACI Committee. 1a. Ground granulated blast furnace slag as a cementitious constituent in concrete ACI.AR-. Detroit: American Concrete Institute. [] Halstead, W. J. 1. Use of fly ash in concrete. NCHRP 1 (October). Washington: Transportation Research Board, National Research Council. [] Rupnow, Tyson D. 0. Evaluation of Fly Ash Quality Control Tools. Louisiana Department of Transportation and Development. ACKNOWLEDGEMENTS The Minnesota Department of Transportation provided funding for this research. DISCLAIMER This paper reflects authors opinions on research conducted and does not necessarily reflect the opinion of any agency or institution. 1 TRB 01 Annual Meeting