Effect of Accelerated Curing Conditions on the Performance of Precast Concrete Round Robin Phase

Transcription

1 NATIONAL RESEARCH COUNCIL CANADA Effect of Accelerated Curing Conditions on the Performance of Precast Concrete Round Robin Phase A report for the Canadian Precast/Prestressed Concrete Institute March 31, 2014

2 Effect of Accelerated Curing Conditions on the Performance of Precast Concrete Round Robin Phase Project Manager Jon Makar, Ph. D., P.Eng. Approved Ahmed Kashef, Ph. D., P. Eng. Acting Director Civil Engineering Infrastructure Report No: A Report Date: March 31, 2014 Contract No: A Reference: Agreement amended January, pages Copy 14 of 14 copies

3 Executive Summary This report describes the results and analysis of the second stage of a Canadian Precast/Prestressed Concrete Institute (CPCI)/National Research Council Canada (NRC) project to determine the appropriate length of curing for precast concrete. Although CSA A23.4 (Precast concrete Materials and construction) governs the production of precast concrete, the moist curing times for concrete mixes in CSA A23.1 (Concrete Materials) have generally been followed by the precast concrete industry. Reduction of curing times below the 7 days required in CSA A23.1 has the potential to produce significant savings on production costs for CPCI members and also has the potential to reduce the environmental impact of 7 day curing times. The results of a round-robin test using nine different permanent precast concrete plants on the effects of curing time on standard concrete performance tests are presented. Compressive strength and rapid chloride penetration (RCP) tests were carried out on specific CSA A23.1 C-1 and C-XL samples produced in the controlled environments of the permanent precast concrete plants. Two plants produced C-1 samples while 7 produced C-XL samples based on their own standard mixes. Samples were either: air cured immediately after heating; moist cured for 72 hours; or moist cured for 168 hours (the current practice). Samples were tested at 28 and/or 56 days of age according to the requirements of CSA A23.1. Measured strength results reflected those in submitted historical data, indicating that the mixes used in the project were typical of those produced by the companies. All tests produced results that met the CSA A23.1 requirements for the C-1 and C-XL mixes no matter the curing regime. In the case of the C-1 mixes, the results from one plant showed consistent behaviour in most cases at 28 days of age (compressive strength) and in all cases at 56 days of age (compressive strength and RCP), no matter the curing regime. Results from the second plant were found to be dependent on moist curing time. Too few plants produced C-1 samples to allow a statistical analysis of the results. A matched pair comparison using Student s t test was used to statistically evaluate the C-XL results, which were obtained at 56 days of age. Almost all of the samples moist cured for 72 hours produced results that were statistically the same as those moist cured for 168 hours. One set of RCP results was statistically different. The air cured after heating samples also produced RCP test results that were statistically the same as those from the 168 hour moist cured samples. The compressive strength results were not statistically the same for the air cured after heating and the 168 hour moist cured samples, but would be considered to be statistically the same at an acceptance criteria of 0.957, rather than the typical 0.95 value used in the tests. 2

4 Acknowledgements This report was co-authored by Jon Makar, Ph. D., P. Eng. and Mark Arnott of the National Research Council Canada (NRC). The authors gratefully acknowledge technical support provided by Glendon Pye, Gordon Chan and Ken Trischuk during the course of the project, as well as comments and suggestions from Daniel Cusson, Ph. D., P.Eng and Tai Sato, Ph. D., P.Eng. The work in this report was co-funded by the Canadian Precast/Prestressed Concrete Institute and NRC. The support of these organizations is also gratefully acknowledged. 3

5 Contents Executive Summary... 2 Acknowledgements... 3 Figures... 5 Tables... 5 Introduction... 6 Sample Preparation and Test Procedures... 7 Mix Design and Sample Preparation... 7 Test Procedures... 9 Hardened Air Content Analysis... 9 Compression Strength Testing... 9 Rapid Chloride Penetration Testing... 9 Results Air Void Analysis Assessment of Whether Samples were Damaged in Transit Effects of Moist Curing Time on Compressive Strength Rapid Chloride Penetration Results ASTM C1202 Measurements CSA S413 Measurements Discussion Effects of Moist Curing Regime on Sample Performance Comparison to CSA A23.1 Performance Requirements Comparison of Results from Different Curing Regimes Conclusions References

6 Figures Figure 1 Core sample locations in 150 mm thick slabs for rapid chloride testing. Cores marked in grey were tested at one time period (28 or 56 days), with cores in white at the other time period Figure 2 Compressive strengths of fully moist cured specimens at 28 days of age Figure 3 - Compressive strengths of fully moist cured specimens at 56 days of age Figure 4 Effects of curing regime on compressive strength for C-1 mixes (Moving the strength requirement to the same value at 56 days of age has been proposed for CSA A ) Figure 5 - Effects of curing regime on compressive strength at 56 days of age for C-XL mixes Figure 6 - Effects of curing regime on ASTM C1202 rapid chloride penetration results for C-1 mixes Figure 7 - Effects of curing regime on rapid chloride penetration according to ASTM C1202 at 56 days age for C-XL mixes Figure 8 - Effects of curing regime on rapid chloride penetration according to CSA S413 for C-1 mixes (top of slab) Figure 9 - Effects of curing regime on CSA S413 rapid chloride penetration results for C-XL mixes at 56 days age (top of slab) Figure 10 - Effects of curing regime on CSA S413 rapid chloride penetration results for C-1 mixes (bottom of slab) Figure 11 - Effects of curing regime on CSA413 rapid chloride penetration results for C-XL mixes at 56 days age (bottom of slab) Tables Table 1 Summary of experimental sample production... 8 Table 2 Air void content analysis for wet and dry samples Table 3 Compressive strength of fully moist cured specimens at 28 days of age Table 4 Compressive strength of fully moist cured specimens at 56 days of age Table 5 Average compressive strengths for C-1 mixes Table 6 Average compressive strengths for C-XL mixes at 56 days of curing Table 7 ASTM C1202 rapid chloride penetration results for C-1 mixes at 56 days age Table 8 ASTM C1202 rapid chloride penetration results for C-XL mixes at 56 days age Table 9 - CSA S413 rapid chloride penetration results for C-1 mixes (top of slab) Table 10 CSA S413 rapid chloride penetration results for C-XL mixes at 56 days of age (top of slab) Table 11 - CSA S413 rapid chloride penetration results for C-1 mixes (bottom of slab) Table 12 CSA S413 rapid chloride penetration results for C-XL mixes at 56 days age (bottom of slab).. 21 Table 13 CSA requirements for tests performed in the project Table 14 - Matched pair test results for 56 day old C-XL samples

7 Introduction This report describes the results of the second phase of an investigation into the effects of different moist curing times on the performance of precast concrete. Adequate curing is essential to produce good concrete performance. Curing procedures promote the hydration of the ordinary Portland cement (OPC) by the water in the concrete and consists of control of temperature and of the movement of moisture in and out of the concrete 1. There is not general agreement on good curing practices. Although CSA A governs the production of precast concrete, the moist curing times for concrete mixes in CSA A23.1 3, which were developed for cast in place concrete, have generally been required by the owners and specifiers of precast concrete. CSA A23.1 specifies 7 day curing times for C-1 and C-XL mixes regardless of the application for the concrete and does not differentiate between different materials or environmental conditions. In contrast, European standards 4 for cast in place concrete specify different curing times based on the rate of strength development of the concrete, the ambient temperature, the relative humidity during curing and degree of exposure to sun and wind. Precast concrete made in permanent plants is produced under controlled, repeatable conditions indoors. Moreover, steam cured concrete is produced with an initial curing period at temperatures ranging from o C, which accelerates both the hydration reactions and strength development of the product. In principle, this process would be expected to reduce the needed duration of the additional curing. There is, however, currently no differentiation in Canada with respect to curing regimes needed for concrete products produced by different manufacturing methods. In part, this lack of differentiation is due to poor structure in the relevant standards, with the Concrete Materials standard 3 referencing the Precast Concrete standard 2 and vice-versa without a clear statement on the curing requirements for precast concrete in either document. There is also a lack of robust, modern data to support different standards for precast versus cast in-place concrete. Most knowledge of the effects of accelerated curing on basic concrete properties used to support current standards in North America were developed during studies funded by the American Federal Highway Administration 5, Portland Cement Association 6-9, and Precast Concrete Institute (PCI) These studies focused on accelerated curing and supported the concept that reduced curing times were needed for accelerated cured precast. They were not, however, undertaken with current mix designs. The most recent substantial work was conducted by Sherman, McDonald and Pfiefer 13, 14. This work used different mix designs for different curing conditions, making it difficult to separate the effects of mix design from those of curing. This report presents the results from the second stage of a CPCI/NRC research project to examine the effects of different curing times on concrete performance. Work was undertaken to determine if evidence could be obtained to support the hypothesis that precast concrete curing times for CSA A23.1 C-1 and CX-L type (high durability structurally reinforced concrete used for moderate and severe environments) mixes made could be reduced from 7 days. The mixes of included typical contents of silica fume, blast furnace slag and/or fly ash, with 6 of 7 plants producing C-XL samples using silica fume. 6

8 In this phase, a round robin test to examine the effects of three different curing regimes was conducted on typical accelerated curing concrete mixes used by nine different permanent precast concrete plants from across Canada. Plants were located in British Columbia (1), Alberta (2), Manitoba (2), Ontario (3) and Nova Scotia (1). The plants are labelled by letters in the report and the participating plants only know the letters identifying their own plants. Two plants (A and B) produced mixes to meet the CSA A23.1 C-1 requirements, while the remainder produced C- XL mixes. All samples used accelerated curing. Samples were then either: air cured immediately after demoulding; moist cured for a total of 72 hours; or moist cured for a total of 168 hours (the current CSA A23.1 requirement). Samples were produced in the form of concrete cylinders and concrete slabs using the standard practices at each plant and cured under each regime. Additional cylinders were also fully moist cured. Compressive strength and rapid chloride penetration (RCP) tests were conducted on the samples after aging to meet the requirements of CSA A23.1 and the results of the tests were analyzed to determine the effects of reduced curing time on precast concrete performance. Sample Preparation and Test Procedures Mix Design and Sample Preparation Samples were produced at the participating permanent precast concrete plants during the spring and summer of 2013 using standard, proprietary company mix designs according to standard company practices. Although details of the proprietary mix designs are not included here, C-XL mixes had either silica fume content of between 5-8%, 25% blast furnace slag content and/or fly ash contents of 8-19%. The concrete was mixed in the same mixers used for the companies commercial concrete production, with approximately 0.76 m 3 (1 cubic yard) of concrete being produced in a single batch. All plants followed CPCI s comprehensive quality control procedures and used PCI Level I/II Quality Personnel. Twenty two compression testing and two air void samples were cast in standard 100 mm x 200 mm cylindrical moulds at most plants. Plant I produced fifteen compression testing cylinders. Plants A and B, which used C-1 mixes, both produced 44 cylinders to allow compression testing at 28 and 56 days. Table 1 summarizes the samples and casting conditions. Three 650 mm x 500 mm x 150 mm slabs were cast at each plant for use in coring samples for rapid chloride tests. All of the samples except those of plants D and G were steam cured for between 12 and 16 hours, starting 3-4 hours after casting, consistent with the initial set of the mix. Plant G continuously steam cured the samples until they were removed from the chambers at 16, 72 and 168 hours instead, while Plant D used a quick setting cement to obtain the higher curing temperatures required by the project. The samples reached maximum internal temperatures between 46 and 56 o C during the casting process, meeting the current CSA A23.4 requirement of a maximum temperature of 60 o C for precast concrete. The samples were then demoulded and 6 cylinders per plant were placed in a 100% RH curing room for eventual delivery to NRC (12 cylinders for Plants A and B and three cylinders for Plant I). The cylinders delivered to NRC were used to check on whether the RCP samples were 7

9 damaged in transit and were not part of the curing regime comparison. Four cylinders and one slab were then put aside for air curing (8 cylinders for Plants A and B). The remaining 8 cylinders (16 cylinders for Plants A and B) and two slabs were divided into two groups and moist cured for total times of 72 and 168 hours respectively at the plant. Half the Plant A and B cylinders were then air cured for until 28 days of age had been reached. The remaining samples were air cured until 28 or 56 days of age had been reached. The slabs were cored at either 7 days after curing (for CSA S testing) or approximately 1 week before scheduled testing (ASTM ). The samples cored after 7 days were stored in moist conditions following CSA S413 until testing. Samples to be tested by NRC were shipped by courier in 100% RH conditions to arrive before the required date. Table 1 Summary of experimental sample production Plant Mix Type Heating Method Cylinders sent to NRC for curing and testing A C hours steam curing starting 3-4 hours after casting B C hours steam curing starting 3-4 hours after casting C C-XL hours steam curing starting 3-4 hours after casting D C-XL Self-heating by quick set cement E C-XL hours steam curing starting 3-4 hours after casting F C-XL hours steam curing starting 3-4 hours after casting G C-XL Steam cured for full curing time H C-XL hours steam curing starting 3-4 hours after casting I C-XL hours steam curing starting 3-4 hours after casting Cylinders cured and tested at plant 8

10 Test Procedures Hardened Air Content Analysis Hardened air content determination by microscopy was performed at NRC using a Humboldt Linear Traverse Machine controlled by Linear Traverse II software setup to perform ASTM C457 procedure B, Modified Point Count Method 17. The determination was performed on a single sample that had been moist cured after demolding until to NRC. Once at NRC the samples were kept in a 100% R.H., 22 C humid room until testing. The Air void determinations were made at 56 days from casting. A length of 2.25 m on each sample was measured for the determination. Compression Strength Testing Four samples cured under each regime were tested under compression at each participating permanent precast concrete plant following CSA A23.2-9C 18 at 28 or 56 days after casting, having been air cured after the stated moist curing time. Samples were rubber capped with caps rated for 70 MPa with the exception of the samples from plant G, which were end ground. The results presented here are an average of the four compression tests, with no results excluded from the analysis. Compression tests were also done at NRC on control samples cured at 100% RH at 28 and/or 56 days after casting following CSA A23.2-9C 18. In both cases one set of three cylinders was tested using rubber end caps supplied by the plant, while a second set of three cylinders was tested after end-grinding. The rubber cap test was used to determine whether the samples had been damaged in shipping, since undamaged samples would be expected to have similar strength results whether they were tested at NRC or at the plant. The end ground samples were tested to make sure the rubber caps were not influencing the outcome of the compressive tests, a potential problem identified in NRC s first phase report 19. Rapid Chloride Penetration Testing Chloride ion penetrability was conducted according to ASTM C and CSA S The test cores were obtained at the plants from the slabs according to the pattern shown in Figure 1. The cores were obtained at least 50 mm from an edge of the slab using a water cooled diamond core bit. They were taken through the full depth of the slab. The taking of the ASTM C cores at the plants were timed so that samples could be tested at NRC at approximately 56 days of age. In the case of the tests according to ASTM C , three cores had a 10 mm surface removed from each sample. The next 50 mm of sample was then tested and the results averaged. In the case of the samples for CSA S413 18, 10 mm was removed from the top and bottom surfaces of two cores and 30 mm from the middle, resulting in 50 mm thick top and bottom samples from each core, for a total of 4 tests per curing condition and age. 9

11 The cores were then installed in rapid chloride penetration sample holders, caulked for water tightness and vacuum saturated according to the test procedures. The samples were then placed in plastic containers and connected to the test apparatus. Images of the test equipment were included in the first report for the project. 650 mm 50 mm 500 mm Figure 1 Core sample locations in 150 mm thick slabs for rapid chloride testing. Cores marked in grey were tested at one time period (28 or 56 days), with cores in white at the other time period. During the tests, the potential across the 100 mm diameter cores was maintained at 60 volts for 6 hours and the current was measured every 15 minutes using a precision shunt resistor. The solution level in the sample holders was maintained so that the entire end surface of the sample cores was immersed at all times. Results Air Void Analysis 50 mm Table 2 shows the results of the air void analysis conducted on wet concrete at each plant and on dry cylinder samples according to ASTM C457 method 2 at NRC. Table 2 Air void content analysis for wet and dry samples Plant Wet Air Void Content (Plant) (%) 56 day Air Void Content (NRC) (%) A N/A 11.7 B C D E F G H N/A 8.1 I

12 Where both sets of results were available, the expected decline in air void content from wet to dry measurements was observed. In most cases, the mixes met the requirements under CSA A23.1 for concrete exposed to freeze/thaw conditions. The two plants that did not provide wet air void measurements had 56 day dry air void contents that exceeded the allowable limits of CSA Assessment of Whether Samples were Damaged in Transit The comparison between the compressive strength measurements performed by the plants on fully moist cured samples and measurements made at NRC are shown in Figures 2 and 3. Average strengths, standard deviations and coefficients of variation (COV) are shown in Tables 3 and 4. Variations between the results from the different plants are due to the different mix designs used at each plant. No consistent overall trend was seen across the measurements made on samples from each plant. At 28 days of age (Figure 2 and Table 3), all of the Plant A samples were within one standard deviation of each other. The results from the Plant B samples measured at NRC were lower in value by more than one standard deviation, but did not show large differences in absolute terms. Figure 2 Compressive strengths of fully moist cured specimens at 28 days of age. Table 3 Compressive strength of fully moist cured specimens at 28 days of age Plant Plant tested samples Samples tested at NRC with rubber caps Samples tested at NRC after end grinding Average Compressive Strength Standard Deviatio n COV Average Compressive Strength Standard COV Average Compressive Strength Standard COV 11

13 A B At 56 days of age (Figure 3 and Table 4), plants E, F, G and I all produced samples where the difference between the plant measurements and either of the NRC measurements was greater than the total of the corresponding combined standard deviations. Plant G had higher average results in the NRC tests than the plant tests, while the remainder had lower results. The largest differences were seen for Plant F (8.4 MPa difference and 6.2 MPa total standard deviation for the rubber capped NRC samples; 7.3 MPa difference and 6.5MPa total standard deviation for the end ground NRC samples). Three of the plants (C, D and G) had NRC rubber capped tests that had higher values than those at the plant, while the remainder had lower values. Figure 3 - Compressive strengths of fully moist cured specimens at 56 days of age. Table 4 Compressive strength of fully moist cured specimens at 56 days of age Plant Plant tested samples Samples tested at NRC with rubber caps Samples tested at NRC after end grinding Average Standard COV Average Standard COV Average Standard COV Compressive Strength Compressive Strength Compressive Strength A B C D E

14 F G H n/a n/a n/a I n/a n/a n/a The observed lack of a trend between the NRC measurements and those produced at the plants indicated that, overall, the samples were not being damaged in transit. Despite this, the rapid chloride penetration results for the individual cases where NRC measurements were significantly lower than the plant measurements were examined particularly carefully for evidence of damage in transit. All of the samples more than met all the requirements of CSA A23.1, suggesting that any damage that may have occurred in transit was minimal. In addition, measured strength results reflected those in submitted historical data, indicating that the mixes used in the project were typical of those produced by the companies. While early measurements on samples from Plant I raised concerns that the use of rubber caps on the samples might have affected the results for the higher strength concrete 19, no evidence of a bias produced by the rubber caps was seen for the other plants. Effects of Moist Curing Time on Compressive Strength The compressive strength results from the test program are shown in Figures 4 to 6. Figure 4 shows all of the CSA A23.1 C-1 concrete mix results, while Figures 5 and 6 show the C-XL concrete mix design results at 28 and 56 days respectively. Tables 5 to 7 show the corresponding average strengths and their standard deviations. In the case of compressive strength testing the requirements for C-1 concrete is 35 MPa at 28 days of aging, while in the case of C-XL the requirement is 50 MPa at 56 days of aging. Changing the age of testing for C- 1 samples to 56 days is under consideration for the 2014 edition of CSA A23.1. All of the measured values exceeded the relevant CSA requirements, regardless of curing time. The results in Figure 4 and Table 5 show that all of the compressive strength results for the C-1 samples moist cured for 72 hours were within one standard deviation of the average values for the samples moist cured for 168 hours. The air cured after heating results for plant B at 56 days of age are also within one standard deviation of the average value for the samples moist cured for 168 days. The remaining air cured after heating are either two (Plant A, 28 and 56 days) or three standard deviations away (Plant B, 28 days) from the 168 hour moist cured results. 13

15 Figure 4 Effects of curing regime on compressive strength for C-1 mixes (Moving the strength requirement to the same value at 56 days of age has been proposed for CSA A ) Table 5 Average compressive strengths for C-1 mixes Plant Air cured after heating Moist cured for 72 hours Moist cured for 168 hours Average Compressive Strength Standard COV Average Compressive Strength Standard COV Average Compressive Strength Standard COV A 28 days B 28 days A 56 days B 56 days In the case of the C-XL samples, Plant D continued to have results for the reduced curing times that were within one standard deviation of the average results for the 168 hour moist cured samples (Table 7). The shorter moist curing time samples had greater strengths for Plants E and F than did the 168 hour moist cured samples. The results for Plants C, G, H and I showed declining strengths with reduced moist curing times. A statistical analysis of these results is shown later in the report. 14

16 Figure 5 - Effects of curing regime on compressive strength at 56 days of age for C-XL mixes. Table 6 Average compressive strengths for C-XL mixes at 56 days of curing Plant Air cured after heating Moist cured for 72 hours Moist cured for 168 hours Average Standard COV Average Standard COV Average Standard COV Compressive Strength Compressive Strength Compressive Strength C D E F G H I Rapid Chloride Penetration Results Data from the rapid chloride penetration (RCP) tests are presented using the same groupings as were used for the compressive strength measurements. In the case of the RCP tests, CSA A23.1 requires a result of less than 1500 coloumbs at or before 56 days of aging for C-1 mixes and of 1000 coloumbs at or before 56 days of aging for C-XL mixes. 15

17 ASTM C1202 Measurements Figures 6 and 7 show the experimental results from the ASTM C1202 tests carried out by NRC, while Tables 7 and 8 show the corresponding data. The C-1 results in Figure 6 and Table 7 show that all of the samples met the CSA A day performance requirement. The average results from the samples from both plants subjected to reduced moist curing times were within one standard deviation of the results from the samples that were moist cured for 168 hours. Figure 6 - Effects of curing regime on ASTM C1202 rapid chloride penetration results for C-1 mixes Table 7 ASTM C1202 rapid chloride penetration results for C-1 mixes at 56 days age Plant Air cured after heating Moist cured for 72 hours Moist cured for 168 hours Average Standard COV Average Standard COV Average Standard COV A B The ASTM C1202 RCP results at 56 days of age for the C-XL samples (Figure 7 and Table 8) also show similar results from the different curing regimes. All plants except E and I produced samples that had average results after moist curing for 72 hours that are within one standard deviation of those for samples cured for 168 hours. The results for Plants D and I at 72 hours of moist curing are better than those for the 168 hour moist cured samples. In the case of the air cured after heating samples, Plants C, D and H had results that were better than those of the corresponding samples moist cured for 168 hours, while the remaining plants had results that 16

18 were more than one standard deviation higher than corresponding 168 hour moist cured specimens. Figure 7 - Effects of curing regime on rapid chloride penetration according to ASTM C1202 at 56 days age for C-XL mixes Table 8 ASTM C1202 rapid chloride penetration results for C-XL mixes at 56 days age Plant Air cured after heating Moist cured for 72 hours Moist cured for 168 hours Average Standard COV Average Standard COV Average Standard COV C D E F G H I CSA S413 Measurements CSA S413 requires separate measurements of top and bottom sections of the slab. The results from these tests are therefore presented as separate graphs for the top section measurements and bottom section measurements. These are shown as Figures 8 and 9 and Figures 10 and 11, 17

19 respectively. Tables 9-10 and Tables show the corresponding RCP averages, standard deviations and coefficients of variation. The top of the slab C-1 results (Figure 8, Table 9) were similar to those in Figure 7 in that the samples again met the 56 day performance requirements regardless of curing regime. All of the C-1 samples, however, showed increasing RCP values with decreasing curing time. Only the Plant B results at 72 hours moist curing were within one standard deviation of the corresponding 168 hour moist curing average values. Figure 8 - Effects of curing regime on rapid chloride penetration according to CSA S413 for C-1 mixes (top of slab). Table 9 - CSA S413 rapid chloride penetration results for C-1 mixes (top of slab) Plant Air cured after heating Moist cured for 72 hours Moist cured for 168 hours Average Standard COV Average Standard COV Average Standard COV A B The C-XL samples (Figure 9, Table 10) showed fewer effects due to the changes in moist curing time. At 56 days of age, all of the reduced moist curing time results from Plants D, E, G, H and I had values within one standard deviation of the results from the samples moist cured for 168 hours. Plants C and F had results from the 72 hour moist curing samples that were within one standard deviation of those from the 168 hour moist cured samples 18

20 Figure 9 - Effects of curing regime on CSA S413 rapid chloride penetration results for C-XL mixes at 56 days age (top of slab). Table 10 CSA S413 rapid chloride penetration results for C-XL mixes at 56 days of age (top of slab) Plant Air cured after heating Moist cured for 72 hours Moist cured for 168 hours Average Standard COV Average Standard COV Average Standard COV C D E F G H I The bottom of the slab CSA S413 samples all show lower variability both between the different curing regimes. All of both the C-1 mixes (Figure 10 and Table 11) and the C-XL mixes (Figure11 and Table 12) met the 56 day CSA A23.1 performance requirement. The results from Plant A C-1 samples differed from each other by more than one standard deviation, but the shorter moist curing time samples from Plant B had results that were within one standard deviation of those from the 168 hour moist curing time samples. 19

21 Figure 10 - Effects of curing regime on CSA S413 rapid chloride penetration results for C-1 mixes (bottom of slab) Table 11 - CSA S413 rapid chloride penetration results for C-1 mixes (bottom of slab) Plant Air cured after heating Moist cured for 72 hours Moist cured for 168 hours Average Standard COV Average Standard COV Average Standard COV A B The C-XL results (Figure 11 and Table 12) from the bottom of the slab were consistent across the curing regimes. All of the plants except Plant C produced samples using both reduced curing regimes that had results within one standard deviation of the values measured from the corresponding 168 hour moist cured samples. 20

22 Figure 11 - Effects of curing regime on CSA413 rapid chloride penetration results for C-XL mixes at 56 days age (bottom of slab). Table 12 CSA S413 rapid chloride penetration results for C-XL mixes at 56 days age (bottom of slab) Plant Air cured after heating Moist cured for 72 hours Moist cured for 168 hours Average Standard COV Average Standard COV Average Standard COV C D E F G H I

23 Discussion Effects of Moist Curing Regime on Sample Performance Comparison to CSA A23.1 Performance Requirements The project examined the effect of changing moist regimes in two different ways. First, the results of the tests conducted on the samples were compared to the relevant CSA A23.1 requirements (Table 13). All of the samples tested met those requirements, independent of the curing regime. This was the case for the compressive strength tests as well as both types of RCP tests. Table 13 CSA requirements for tests performed in the project CSA A23.1 Plants Compressive RCP Mix strength C-1 A, B >35 MPa at 28 days age <1500 Coulombs at C-XL C, D, E, F, G, H, I >50 MPa at 56 days age 56 days age <1000 Coulombs at 56 days age Comparison of Results from Different Curing Regimes The second comparison was between the performance produced by the different curing regimes. The results shown in Figures 4 to 11 and Tables 5 to 12 show a variety of relationships between the different curing regimes. In general, the RCP results show more similarities between the curing regimes than do the compressive strength results. The number of samples provided by each plant for each age and curing condition was representative of the numbers typically tested for concrete performance. These values were, however, too few to allow a mean comparison test 20 to be undertaken on the samples. Only two plants produced C-1 samples, which also limits other forms of statistical comparison. An examination of the results showed, however, that of these samples only the 56 day old samples produced by Plant B show consistent behaviour between the different curing regimes, with all differences in results between the different sample sets being less than a standard deviation. It was possible, however, to perform statistical analysis on the results from the C-XL samples by using small sample matched pair analysis based on Student s t test. 20 This statistical approach analyzes the differences between the matched pairs under the hypothesis that the average of the test data (i.e. the air cured after heating or the 72 hour moist cure results) is the same as the average of the control data (i.e. the 168 hour moist cure results). A value T is calculated for each comparison and compared to a standard t-test distribution table using the formula: 22

24 where is the average of the differences between the sample pairs, n the number of sample pairs and the standard deviation of the differences. An acceptance region is chosen for the test such that it represents a standard probability that the hypothesis can be accepted (i.e. that the test data and the control data really represent the same population with the same average). T values are given in standard tables available in statistics texts and handbooks. If T is less than the specified value from the table for the chosen rejection region, then the hypothesis is accepted and there is no statistical difference between the two data sets. The values from the table are determined by the number of degrees of freedom (DF) present in the analysis, which in this case is n 1. There were n=7 plants making C-XL samples, giving DF=6. Using a typical 0.95 acceptance value, the corresponding test value is If the calculated T value is less than that value, then the data set being tested is considered to have no differences from the control data set. The results (Table 14) from most of both the air cured after drying and the 72 hour moist cured samples were statistically the same as those from the 168 hour moist cured samples. The two exceptions were the compressive strength measurements for the air cured after drying samples and the CSA S413 measurements on cores taken from the top of the 72 hour moist curing samples. Table 14 - Matched pair test results for 56 day old C-XL samples Compressive Strength Air Cured after Heating 72 hours moist cure RCP by ASTM C1202 Air 72 Cured hours after moist Heating cure RCP by CSA S413 Top Air 72 Cured hours after moist Heating cure RCP by CSA S413 Bottom Air 72 Cured hours after moist Heating cure T Hypothesis accepted? No* Yes Yes Yes Yes No Yes Yes * A T value of 2.09 corresponds to a probability of acceptance of 0.957, rather than0.95. It is worth noting that removing the results from the two plants that used variant methods of accelerated curing (D and G) from the calculation produces a value of T that would suggest that 56 day compressive strength for the air cured after heating samples was statistically the same as those moist cured for 168 hours, raising the possibility that standard steam curing practices may produce better air cured after heating results. Additional sample testing would be needed to confirm this point. 23

25 Conclusions The round robin described in this report compared the effects of three different curing regimes (air curing after heating, 72 hours moist curing, and 168 hours moist curing) on the compressive strength and rapid chloride penetration properties of precast concrete of samples produced by nine permanent pre-cast concrete plants. The project s goal was to investigate the effects of the curing regimes on a variety of different concretes, rather than using a standard mix for all plants. Each of the nine permanent precast concrete plants participating in the round robin therefore used a mix design that was commonly used to produce concrete for its clients. Compressive strength values measured in the test program were consistent with those typically measured at each plant. Plants tested the compression samples themselves, but shipped the RCP samples and additional fully moist cured compression cylinders for testing. The additional moist cured cylinders were tested for the purposes of ensuring that the shipping process did not damage the RCP samples in transit, producing higher than expected results. Although the compression results for some plants as measured at NRC were found to be lower than those measured at the plant, the RCP results for those plants were well below that required by CSA A23.1, indicating that none of the samples were damaged in transit. The comparison between the plant measurements and the NRC measurements confirmed the accuracy of the other compressive strength results from the Plants. All of the plants involved in the test program produced samples that met the criteria of CSA A23.1 in terms of compressive strength and RCP. This was true for all curing regimes used at each plant. Too few plants produced C-1 mixes to allow a statistical analysis of the results. 56 day old samples from Plant B showed consistent behaviour no matter the curing regime. Samples from Plant A only showed consistent behaviour between the 72 hour moist cured samples and the 168 hour moist cured samples. In the case of the C-XL samples, the results from almost all of the tests on samples moist cured for 72 hours were statistically the same as those moist cured for 168 hours. The exception was the CSA S413 results from the top of the cores, where the two sets of data were found to be statistically not the same. The RCP results for the samples aged for 56 days that were air cured after heating were statistically the same as those from the moist cured for 168 hours. The degree of difference between the results from the two sets of compressive strength samples for the same curing conditions at 56 days of age was statistically significant at the 0.95 acceptance level but would be statistically the same at a acceptance level. 24

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27 18. CSA A23.2-9C, Compressive strength of cylindrical concrete specimens, in Test methods and standard practices for concrete, 2009, Canadian Standards Association: Mississauga, Ontario. 19. Makar, J.M. and Arnott, M., Effect of Curing Conditions on the Performance of Precast Concrete, 2013, Report A , National Research Council Canada: Ottawa, Ontario. 20. Downing, D. and Clark, J., Statistics The Easy Way, 1997,pp , Barron s: Hauppauge, New York. 26