July–August 2008 | PCI Journal 108 Editor’s quick points n The study detailed in this paper assessed the influence of mixture type, amount and type of cementitious materials, and the variations among concrete suppliers on the strength and durability characteristics of high-performance concrete. n It was determined that the ACI 318M-05 equation for the elas- tic modulus is applicable to high-performance concrete, while its equation for the modulus of rupture is too conservative. n Based on the research findings, a new equation for the modu- lus of rupture for high-performance concrete is recommended. Mechanical properties and durability of high- performance concrete for bridge decks Mohsen A. Issa, Atef A. Khalil, Shahidul Islam, and Paul D. Krauss High-performance concrete (HPC) is classified as concrete that satisfies certain criteria in order to overcome limita- tions of conventional concretes. It includes concrete that provides either improved resistance to environmental influences or increased structural capacity while maintain- ing adequate durability. The enhanced characteristics of HPC are usually obtained by adding various supplemen- tary cementitious materials and chemical admixtures to conventional concretes. The durability of concrete exposed to severe environments depends largely on its ability to resist the penetration of water and aggressive compounds. Concrete durability is significantly influenced by the mixture proportions, composition of admixtures, chemical composition of the cement and cementitious materials, fine and coarse aggre- gate properties, moisture and deicing chemical exposure, temperature and thermal exposure conditions, and cracks within the concrete. An extensive study to evaluate the performance character- istics (strength and durability) of various HPC mixtures was conducted over a period of more than two years. 1 Based on the research findings, a modified equation (in MPa) for the modulus of rupture f r is recommended for HPC. 108
Transcript
July–August 2008 | PCI Journal108
Editor’s quick points
n The study detailed in this paper assessed the influence of mixture type, amount and type of cementitious materials, and the variations among concrete suppliers on the strength and durability characteristics of high-performance concrete.
n It was determined that the ACI 318M-05 equation for the elas-tic modulus is applicable to high-performance concrete, while its equation for the modulus of rupture is too conservative.
n Based on the research findings, a new equation for the modu-lus of rupture for high-performance concrete is recommended.
Mechanical properties and durability of high- performance concrete for bridge decksMohsen A. Issa, Atef A. Khalil, Shahidul Islam, and Paul D. Krauss
High-performance concrete (HPC) is classified as concrete that satisfies certain criteria in order to overcome limita-tions of conventional concretes. It includes concrete that provides either improved resistance to environmental influences or increased structural capacity while maintain-ing adequate durability. The enhanced characteristics of HPC are usually obtained by adding various supplemen-tary cementitious materials and chemical admixtures to conventional concretes.
The durability of concrete exposed to severe environments depends largely on its ability to resist the penetration of water and aggressive compounds. Concrete durability is significantly influenced by the mixture proportions, composition of admixtures, chemical composition of the cement and cementitious materials, fine and coarse aggre-gate properties, moisture and deicing chemical exposure, temperature and thermal exposure conditions, and cracks within the concrete.
An extensive study to evaluate the performance character-istics (strength and durability) of various HPC mixtures was conducted over a period of more than two years.1 Based on the research findings, a modified equation (in MPa) for the modulus of rupture fr is recommended for HPC.
108
109PCI Journal | July–August 2008
Low permeability can be achieved by various means. The traditional method of decreasing concrete permeability is to reduce the water-cement ratio (w/c). In addition, poz-zolans, slag, silica fume, and chemical admixtures can be used to reduce the concrete permeability. Early-age curing can affect permeability, crack resistance, and durability of concrete. French and Mokhtarzadeh3 found that heat curing improves the performance of microsilica concrete at early ages. Austin and Robins4 reported that 10% silica fume concrete responded favorably in a hot, arid climate, provided that at least a two-day moist curing was applied.
For bridge decks, Ozyildirim5 reported that low permeability is achieved by the use of a low water–cementitious materi-als ratio (w/cm) of 0.45 or less and the use of pozzolans (fly ash or silica fume) or slag. Malhotra6 noticed that the rapid chloride penetration (electrical conductivity) values of 12% silica fume concrete with w/cm of 0.30 were below 300 cou-lombs, suggesting a low permeability for the concretes.
Bayasi and Zhou7 found that silica fume reduced concrete permeability and improved the compressive strength significantly. Naik et al.8 reported that the addition of fly ash showed high resistance to chloride-ion penetration. Mangat and Molly9 found that the chloride penetration in the specimens under marine-cycle exposure was high and that adding 15% silica fume by weight of cement greatly reduced chloride penetration.
On the issue of shrinkage and creep, Shah et al.10 found that, despite the increase in early age and overall strength, HPC containing silica fume was more susceptible to early-age cracking than normal-strength concrete was. Bloom and Bentur11 found that the presence of silica fume increased the free plastic shrinkage of concrete and led to earlier cracking. Wiegrink et al.12 noticed that high-strength silica-fume concrete showed higher shrinkage, and cracking for this concrete developed more quickly
fr = 0.75
fc
'
where
fc' = the compressive strength of concrete
Designing more durable concrete structures is necessary for sustainability and to minimize the maintenance cost and traffic restrictions in the future. This study considers the mechanical properties and durability requirements in de-veloping reasonable HPC mixtures for bridge decks to sat-isfy high-performance mechanical properties and extended durability criteria. To accomplish this goal, the behavior and properties of various precast and ready-mix concretes, batched and cast at six different concrete-producing plants, were investigated.
Background information
Some of the major types of distresses that occur in rein-forced concrete are corrosion of reinforcement, alkali-aggregate reactivity, freezing and thawing deterioration, and sulfate attack.
Aggregates also affect concrete durability. In order for concrete to be high performance, the raw materials used to make the concrete must also be high performance. Pave-ment concretes in the Midwest sometimes suffer from D-cracking, which is caused by poor durability of coarse aggregates. Issa et al.2 tested aggregates from 21 different sources in Illinois using the modified Washington hy-draulic fracture test machine. All of the aggregate types—dolomite, gravel, and auto-clave blast-furnace slag—were successfully classified using the failure modes, with some of the aggregates classified as nondurable. To guarantee durable HPC, both fine and coarse aggregates should be thoroughly evaluated before use in concrete.
Table 1. Proposed high-performance concrete mixture proportions for mixture 1 and mixture 2
Concrete component Mixture 1 Mixture 2
Portland cement Type I or I/II 315 kg/m3 to 345 kg/m3 345 kg/m3 to 373 kg/m3
Coarse aggregate 19 mm maximum size (no. 67/CA11) crushed stone 19 mm maximum size (no. 67/CA11) crushed stone
Fine aggregate Natural siliceous sand Natural siliceous sand
Fly ash Class C or F 10% addition (by weight of cement) 20% addition (by weight of cement)
Silica fume 5% addition (by weight of cement) 5% addition (by weight of cement)
Water–cementitious materials ratio 0.36 to 0.38, including water from HRWRA 0.36 to 0.38, including water from HRWRA
Air-entraining agent 67 mL to 133 mL per 100 kg cement 67 mL to 133 mL per 100 kg cement
HRWRA, AASHTO M194 Type F About 3.98 L/m3 to 5.1 L/m3 About 3.98 L/m3 to 5.1 L/m3
Note: AASHTO = American Association of State Highway and Transportation Officials; HRWRA = high-range water-reducing admixture. 1 mm = 0.0394 in.; 1 kg/m3 = 1.6875 lb/yd;3 1 mL/100 kg = 0.01538 oz/100 lb; 1 L/m3 = 25.8 oz/yd3.
July–August 2008 | PCI Journal110
and was significantly wider than that of normal-strength concrete.
In summary, while materials such as pozzolans and silica fume can dramatically improve the durability of concrete, they can also cause construction and cracking problems if proper precautions are not taken. Transferring laboratory test results to field performance is a primary challenge.
Experimental program
Materials selection and mixture proportions
University of Illinois at Chicago (UIC) and Wiss, Janney, Elstner Associates Inc. (WJE) designed two concretes, mixture 1 and mixture 2, to evaluate HPC behavior in terms of strength and durability. Table 1 shows the suggested mixture proportions for both concretes. The main difference between mixture 1 and mixture 2 was the addition of slag to mixture 1 and 10% more fly ash to mixture 2.
Concrete manufactured by the precasters was cured using a temperature-controlled chamber to simulate the actual cur-ing conditions (steam curing) in the precast concrete plant. During mixture development, concrete specimens were heat cured for a period of 14 to 16 hours after the initial
setting time. Figure 1 shows a typical heat-curing cycle using a computer-controlled chamber. A period of about 1 to 11/2 hours at the beginning and end of the heat-curing cycle is not shown in Fig. 1. Including this time, the total heat-curing cycle time was about 141/2 hours.
After determining the two test mixtures, two precast con-crete producers (P1 and P2) and four ready-mix concrete producers (R1, R2, R3, and R4) were asked to supply con-cretes based on mixture 1 and mixture 2. In addition, the suppliers were allowed to choose a third concrete, mixture 3, based on their own experience, to evaluate its suitability for meeting the defined HPC characteristics. A total of 14 mixtures were submitted for testing. The ready-mix con-crete suppliers submitted a total of eight mixtures. Each precast concrete plant submitted three mixtures, including the two requested as well as one with corrosion inhibitor. The mixtures with corrosion inhibitor, P1M3 and P2M3, were made without supplementary cementitious materi-als such as fly ash, silica fume, or slag. Table 2 shows the mixture proportions submitted by each producer.
Placing and curing of concrete test specimens
The two precast and four ready-mix concrete producers cast a total of 316 cylinders of 150 mm × 300 mm (6 in. ×
Figure 1. This graph shows a typical heat-curing cycle using a computer-controlled chamber. At the beginning and at the end of the heat-curing cycle, there was a period of 1 to 1½ hours that is not shown on the plot. Considering this time, the total heat-curing cycle time was about 14½ hours. Note: °F = (°C × 1.8) + 32.
under ambient cure conditions for 24 hours. They were then transported to the UIC laboratory. After demolding, the specimens were subjected to wet-mat curing for four more days. Afterward, they were cured in the laboratory environment conditions until the day of testing.
Parameters of investigation
The fresh and hardened properties of the concretes were evaluated according to American Society for Testing and Materials (ASTM) standards. Prior to casting, the slump and air-content tests were conducted according to ASTM C14313 and C138,14 respectively. A universal testing machine, which was digitally controlled with a variable cross-head speed and maximum capacity of 1800 kN (405 kip), was used to test the strength properties. Com-pressive-strength and modulus-of-elasticity tests were conducted at various testing dates of 1, 3, 7, 28, 56, 90, 180, and 365 days after casting, according to ASTM C3915 and ASTM C469.16
12 in.), 54 cylinders of 100 mm × 200 mm (4 in. × 8 in.), 250 flexural beams of 150 mm × 150 mm × 530 mm (6 in. × 6 in. × 21 in.), 27 slabs of 300 mm × 300 mm × 150 mm (12 in. × 12 in. × 6 in.), and 36 shrinkage prisms of 75 mm × 75 mm × 285 mm (3 in. × 3 in. × 11¼ in.).
The two precast concrete producers steam cured their concrete specimens according to the plant regulations and transported them after 24 hours to the structural and concrete laboratory at UIC, where they were demolded and cured for three additional days in the moisture room at a temperature of 23 oC (73.4 oF) and relative humid-ity of 100%. The specimens were then removed from the moisture room and allowed to cure in the environmental chamber at a temperature of 23 oC and relative humidity of 50%. After 28 days, all of the specimens were taken out of the chamber and cured in the laboratory environment conditions.
The ready-mix concrete producers covered their concrete specimens with plastic sheets and stored them indoors
Table 2. Mixture proportions submitted by the suppliers
Note: Specimens P1M3 and P2M3 do not contain fly ash, silica fume, or slag but do contain 10.13 L/m3 (261 oz/yd3) corrosion inhibitor and 0.46 L/m3 (12 oz/yd3) retarder. The quality control and practices of the R4 supplier were not approved at the time of mixing and casting; therefore, R4M2 and R4M3 were not considered high-performance concrete and their strength and durability results were poor. 1 kg/m3 = 1.6875 lb/yd3;
1 L/m3 = 25.8 oz/yd3.
July–August 2008 | PCI Journal112
test according to ASTM C115221 and water-soluble test according to ASTM C1218.22 Samples were collected in accordance with two methods.
In the first method, coring was performed on the slab specimens, and the cored sample was sliced at the desired depth from the top surface of the slab specimen and was crushed to obtain the required amount of powder needed for the analysis. In the second method, powder was collect-ed by drilling three holes in each specimen at the desired depth. In the acid-soluble chloride tests, at least two test runs per sample were conducted. The same powder source that was used in the acid-soluble (total) chloride tests was also used in the water-soluble chloride tests.
Discussion of test results
Fresh concrete properties
Fresh properties of HPC depend on the mixture propor-tioning to combine materials for optimum performance. The slump and air content were calibrated for each con-crete to achieve a slump in the range of 140 mm to 215 mm (5.5 in. to 8.5 in.) and air content in the range of 5.5% to 8.5%. Table 3 shows that the measured slump and air content for all mixtures ranged from 170 mm to 215 mm (6.7 in. to 8.5 in.) and from 5.5% to 8.0%, respec-tively. The fresh concrete properties showed that adequate workability with target air content necessary for the freez-ing and thawing resistance was achieved.
Prior to testing, 150 mm × 300 mm (6 in. × 12 in.) cylin-ders were capped with a sulfur compound. Beams were tested for flexural strength at the ages of 3, 7, 28, 56, 90, 180, and 365 days by applying third-point loading accord-ing to ASTM C78.17 A minimum of three prism specimens of 75 mm × 75 mm × 285 mm (3 in. × 3 in. × 111/4 in.) were cast from each mixture for shrinkage measurements according to ASTM C157.18
An automated concrete-analysis system (CAS 2000) performed a microscopic determination of the hardened concrete air-void parameters in accordance with ASTM C457.19 A diamond-sawing machine cut standard samples of 150 mm × 25 mm (6 in. × 1 in.) from cylinders 28 days after casting. The samples were cut perpendicular to the top surface for an area slightly larger than the area recom-mended for the maximum aggregate size of 19 mm (¾ in.). An area of at least 7700 mm2 (12 in.2) had to be secured for the linear-traverse method. Two cylinders were tested from each mixture.
A permeability investigation was completed in accordance with ASTM C1202.20 It included the determination of the electrical conductance of concrete, providing a rapid electrical indication of its ability to resist the penetra-tion of chloride ions. This coulomb-permeability test was conducted on four samples at the concrete ages of 56, 180, and 365 days. A 15% sodium chloride solution was ponded on the concrete slabs. They were tested at various ages for chloride-ion content using the acid-soluble (total) chloride
Table 3. Slump and percentage of air content
Specimen Slump, mm Fresh air, content, % Hardened air content, % Difference, %
P1M1 203 6.1 5.2 0.9
P1M2 216 7.0 6.5 0.5
P1M3 172 7.7 6.7 1.0
P2M1 203 8.0 5.5 2.5
P2M2 203 6.5 5.2 1.3
P2M3 165 7.1 6.0 1.1
R1M1 191 7.0 5.0 2.0
R1M2 203 8.1 5.8 2.3
R2M1 197 6.3 6.3 0.0
R2M2 203 7.2 6.1 1.1
R2M3 184 8.0 5.8 2.2
R3M3 178 5.5 5.4 0.1
R4M2 172 7.7 9.3 -1.6
R4M3 178 6.5 5.7 0.8
Note: 1 mm = 0.0394 in.
113PCI Journal | July–August 2008
compared with the other mixtures. The only difference be-tween these two mixtures and other mixtures with almost identical proportions was the casting procedures. Such results verify the important influence of quality control and assurance practices.
Concrete compressive strength
Table 3 presents the average concrete compressive strength results of all mixtures. The average compressive strength of the HPC mixtures increased significantly with time. The compressive strengths of the precast HPC of mixture 2 types were higher than those of mixture 1 types at all ages, while the compressive strengths of the ready-mix HPC of mixture 1 types were higher than those of mixture 2 types at all ages. However, the differences are small between mixtures 1 and 2 among precast and ready-mix concretes, which indicates that their compressive strengths were comparable.
A higher increase in compressive strength at early ages was observed for the precast HPC mixtures compared with the ready-mix HPC mixtures, as a result of the different curing methods (heat curing for precast HPC and moist curing for ready-mix HPC). The overall compressive-strength test results passed the target value of 41.5 MPa (6 ksi) at 28 days. The 365-day strengths ranged from
Table 3 also presents the air-void content in the hardened state for each concrete. The change in air content between the fresh and hardened concrete was a loss from 0.1% to 2.5%, except for mixture R4M2, which experienced higher air content in the hardened state than in the fresh state by 1.6%. This unexpected result for mixture R4M2 could be attributed to inaccurately measured air content in the fresh state. On average, the difference between air voids in the fresh and hardened states was an approximate 1% loss.
Influence of quality control and assurance practices
To produce HPC, strict quality control and assurance prac-tices must be followed during mixing, placing, and curing procedures. These procedures are as important to HPC quality as the mixture proportions are, if not more.
Unfortunately, two of the mixtures (R4M2 and R4M3) from supplier R4 had poor quality control and assurance practices that were not approved at the time of batching, mixing, and casting. These two concretes had low w/cm of 0.37 and 0.39, respectively, and included supplementary cementitious materials, which complied with the typical mixture proportions of HPC. However, these two mixtures had the worst compressive and flexural strengths, elas-tic moduli, permeability, and chloride-ion penetrability
Table 4. Compressive strength f 'c and flexural strength fr values for all specimens
Specimen name
Average compressive strength f 'c and flexural strength fr, MPa
1 day 3 days 7 days 28 days 56 days 90 days 180 days 365 days
f 'c fr f 'c fr f 'c fr f 'c fr f 'c fr f 'c fr f 'c fr f 'c fr
46 MPa to 69 MPa (6.7 ksi to 10 ksi) and averaged 58.7 MPa (8.5 ksi) for the precast concrete mixtures and 58.2 MPa (8.4 ksi) for the ready-mix concrete mixtures, excluding R4M2 and R4M3. In general, the variation in compressive strength for HPC mixtures was related to the type of material, w/cm, HPC mixture supplier (precast or ready-mix), and type and percentage of supplementary cementitious materials.
The compressive strength results showed that all of the HPC related to mixtures 1 and 2 showed satisfactory strength results at all ages, taking into account two major points. They contained different mixture proportions of materials, that is, mixture 2 included a lower percent-age of supplementary cementitious materials (25%) but contained more cement. Mixture 1 included more supple-mentary cementitious materials (30%) but less cement. Also, HPC specimens showed higher strength values than ready-mix HPC specimens showed as a result of different curing methods. These observations were also true for the flexural-strength test results.
Modulus of rupture (flexural strength)
Table 4 presents the test results of the average moduli of rupture of the concretes. The precast concrete of mixture 1
and mixture 2 types had only a slight difference, which was related to the amount and type of cementitious materials used in each mixture. In general, the precast concretes had higher flexural strength at almost all ages than the ready-mix concretes had, except P1M1. The ready-mix concretes R1M1 and R2M1 had higher strengths than the precast concrete mixture P1M1 had at almost all ages. The typical strict and favorable quality control and assurance practices in the precast concrete plants and the use of steam curing were the direct reasons for the favorable performance of the precast concrete. The results showed that all specimens achieved flexural strength of more than 4.48 MPa (650 psi) at 28 days, excluding R4M2 and R4M3. The pre-cast concrete averaged 5.48 MPa (800 psi) at 28 days.
Several equations have been developed to express the relationship between the modulus of rupture of concrete and its compressive strength. ACI 318M-0523 provides the following equation:
fr = 0.62
fc
' (ACI 318M-05, Eq. [9-10])
where
fr = modulus of rupture of concrete, MPa
fc' = compressive strength of concrete, MPa
Table 5. Relationship between the flexural fr and compressive f 'c strengths
Note: The quality control and practices of the R4 supplier were not approved at the time of mixing and casting. As a result, R4M2 and R4M3 were not considered high-performance concrete and their strength and durability results were poor.
115PCI Journal | July–August 2008
Table 5 compares the predicted modulus of rupture using Eq. (9-10) of ACI 318M-05 with the experimental test re-sults at various ages for all of the HPC mixtures. Equation (9-10) significantly underestimates the flexural strength of HPC. This calls for a modification of the equation to be reasonably applicable to HPC. Consequently, this study recommends a new modified equation, as presented below, that leads to a more accurate prediction of the concrete modulus of rupture.
fr = 0.75
fc
'
Furthermore, a straightforward relationship between the concrete modulus of rupture and compressive strength expressed as fr = 0.1
fc' can be applied for HPC.
Static modulus of elasticity
The static modulus of elasticity Ec was measured for each mixture at various testing dates using a compressometer and applying strain control according to ASTM C469. Table 6 presents the average results of the elastic modulus of all mixtures as well as typical values predicted by the
equation in ACI 318M-05 and the American Association of State Highway and Transportation Officials’ AASHTO LRFD Bridge Design Specifications.24 The modulus-of-elasticity values had about the same trend as the compres-sive strengths. The modulus of elasticity was higher for the HPC manufactured in the precasting plant than for the ready-mix HPC, mainly due to the different curing procedures.
The applicability of the elastic modulus equations by ACI 318M-05/AASHTO LRFD specifications, ACI 363R-92,25 CEB-FIB-99/Eurocode2,26 Gardner,27 and Nassif et al.28 for the experimental test results was investigated. The follow-ing equations are provided by the previous references in the International System of Units (SI).
ACI 318M-05 and AASHTO LRFD specifications:
Ec = 0.043w
c
1.5fc
'
where
wc = concrete density
Table 6. Experimental and predicted elastic modulus values from ACI 318M-05
Specimen type
Average experimental and predicted modulus of elasticity, GPa
For concrete density of about 2300 kg/m3 (150 lb/ft3), the previous equation becomes
Ec = 4000
fc
'
Table 7 shows the average differences between the experi-mental and predicted elastic-modulus values for the concretes. The equations that had the best agreement with the experi-mental results were those of Gardner, ACI 318M-05, and AASHTO LRFD specifications. At all ages, the predicted values by the Gardner equation were within 0.1% to 1.7% of the experimental results. The values predicted by the ACI 318M-05 and AASHTO LRFD specifications equations were within 0.3% to 2.3% of the experimental test results.
The ACI 363R-92 and Nassif et al. equations underesti-mated the elastic modulus by 7% to 11% and 14% to 17%, respectively, while the CEB-FIB-99/Eurocode2 equation overestimated the elastic modulus by 8% to 13%. Based on these results, the authors recommend using the Gardner, ACI 318M-05, and AASHTO LRFD specifications equa-tions for predicting the elastic modulus from the compres-sive strength of concrete.
Shrinkage-time response
Shrinkage depends on the amount of cement paste, type and amount of cementitious materials, water content, ad-mixtures, air temperature, and in some cases the aggregate type. The main variations among the mixtures investigated were the casting and curing methods (precast or ready-mix
For concrete density of about 2300 kg/m3 (150 lb/ft3), the previous equation becomes
Ec = 4730
fc
'
According to ACI 363R-92:
Ec =
3320 fc'+ 6900
!"
#$
wc
2320
!
"%#
$&
1.5
where
21 MPa < fc' < 83 MPa
For concrete density of about 2300 kg/m3 (150 lb/ft3), the previous equation becomes
Ec =
3320 fc'+ 6900
!"
#$
wc
2320
!
"%#
$&
1.5
According to CEB-FIB-99/Eurocode2:
Ec =
21,500
fc'
10
13
According to Gardner:
Ec = 3500 + 4300
fc
'
According to Nassif et al.:
Ec = 0.036w
c
1.5fc
'
where
Table 7. Comparison between experimental and predicted elastic modulus values
Elastic modulus equation, SI unitsPercentage difference between the predicted and the experimental results
28 days 56 days 90 days 180 days 365 days
Gardner, Ec = 3500+4300 fc
' 1.6 1.3 1.7 0.4 0.1
ACI 318M-05/AASHTO LRFD,
Ec = 4730 fc
' 1.9 1.5 2.3 -0.3 -0.6
ACI 363R-92,
Ec = 3320 fc
' +6900( ) -7.4 -8.3 -7.6 -10.2 -10.8
CEB-FIB-99/Eurocode2,
Ec = 21,500fc'
10
!
"#
$
%&
1/ 3
12.8 11.4 12 9 8.3
Nassif et al.,
Ec = 400 fc
' -14.4 -14.8 -13.9 -16.3 -16.9
Note: The concrete density that was substituted in the equations is 2300 kg/m3 (150 lb/ft3).
117PCI Journal | July–August 2008
Figure 2. This graph plots the shrinkage-time results of mixture 1–type specimens P1M1, P2M1, R1M1, and R2M1.
0
100
200
300
400
500
600
700
800
0 10 20 30 40 50 60 70 80 90 100
Shrin
kage
,
R2M1
R1M1
P2M1P1M1
Specimen age, days
Figure 3. This graph plots the shrinkage-time results of mixture 2–type specimens P1M2, P2M2, R1M2, and R2M2.
0
100
200
300
400
500
600
700
800
0 10 20 30 40 50 60 70 80 90 100
Specimen age, days
Shrin
kage
, Sh
rinka
ge,
R1M2
R2M2
P2M2P1M2
July–August 2008 | PCI Journal118
concrete), the supplier, and the amount and type of cemen-titious materials in each mixture.
Figure 2 shows the shrinkage-time curves for P1M1, P2M1, R1M1, and R2M1 (mixture 1 types), while Fig-ure 3 shows the shrinkage-time curves for P1M2, P2M2, R1M2, and R2M2 (mixture 2 types). These specimens provided adequate data to make obvious comparisons between mixtures 1 and 2 as well as between the precast and the ready-mix concrete in terms of the shrinkage-time response. The variations in the admixtures and aggregate type were almost negligible among these mixtures.
The common trend in Fig. 2 and 3 showed that the shrink-age increased rapidly at an early age and then continued to increase at a slower rate after 28 days. Table 8 presents the 28- and 90-day shrinkage data for each mixture, and it compares the difference in shrinkage-time data among the HPC mixtures for different scenarios.
Variation in the mixture suppliers For mixture 1 types, P1M1 experienced shrinkage of 400 µε at 28 days and 470 µε at 90 days, while P2M1 experienced shrinkage of 420 µε at 28 days and 490 µε at 90 days, which were 5% and 4% higher than the 28-day and 90-day shrinkage of P1M1, respectively. Also, R1M1 experienced shrinkage of 430 µε at 28 days and 540 µε at 90 days, while R2M1 experienced shrinkage of 560 µε at 28 days and 640 µε at 90 days, which were 30% and 19% higher than the 28-day and 90-day shrinkage of R1M1, respectively.
For mixture 2 types, P1M2 experienced shrinkage of 390 µε at 28 days and 470 µε at 90 days, while P2M2 experienced shrinkage of 430 µε at 28 days and 500 µε at 90 days, which were 10% and 6% higher than the 28-day and 90-day shrink-age of P1M2, respectively. Also, R2M2 experienced shrink-age of 480 µε at 28 days and 560 µε at 90 days, while R1M2 experienced shrinkage of 530 µε at 28 days and 630 µε at 90 days, which were 10.5% and 12.5% higher than the 28-day and 90-day shrinkage of R2M2, respectively. Such variation in the shrinkage for the same mixture type reveals that the variation in the supplier was minor (less than 10%) when considering precast concrete but significant when consider-ing ready-mix concrete (up to 30%).
Precast concrete versus ready-mix concrete To compare the difference in the shrinkage among the pre-cast and the ready-mix concretes, the shrinkage data of the precast as well as the ready-mix concretes were averaged to eliminate the influence of supplier variation. For mixture 1 types, the shrinkage data of P1M1 and P2M1 averaged 410 µε at 28 days and 480 µε at 90 days, while the shrink-age data of R1M1 and R2M1 averaged 495 µε at 28 days and 590 µε at 90 days.
For mixture 2 types, the shrinkage data of P1M2 and P2M2 averaged 410 µε at 28 days and 485 µε at 90 days, while the shrinkage data of R1M2 and R2M2 averaged 505 µε at 28 days and 595 µε at 90 days. For both mix-ture types, the precast concrete experienced about 20% less shrinkage than the ready-mix concretes at 28 and 90
Table 8. Shrinkage values of the high-performance concrete specimens at 28 and 90 days
Specimen P1M1 P2M1 R1M1 R2M1 P1M2 P2M2 R1M2 R2M2
(R1M1+ R2M1) / (P1M1+ P2M1)
(R1M2+ R2M2) / (P1M2+ P2M2)
(P1M2+ P2M2) / (P1M1+ P2M1)
(R1M2+ R2M2) / (R1M1+ R2M1)
Shrinkage at 28 days, µε
400 420 430 560 390 430 530 480 — — — —
Difference 5% 30% 10% 10% — — — —
Shrinkage at 90 days, µε
470 490 540 640 470 500 630 560 — — — —
Difference 4% 18.5% 6% 12.5% — — — —
Difference between the shrinkage of the precast and the ready-mix HPC at 28 days 20% 23% — —
Difference between the shrinkage of the precast and the ready-mix HPC at 90 days 23% 23% — —
Difference between mixture 1 and mixture 2 HPC at 28 days 0% 2%
Difference between mixture 1 and mixture 2 HPC at 90 days 1% 1%
Notes: Mixture 1 contained fly ash, silica fume, and slag. Mixture 2 contained fly ash and silica fume. The total cementitious materials content in mix-ture 2 was 5% higher than in mixture 1. HPC = high-performance concrete. µε = microstrain = mm/mm or in./in.
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Figure 4. This graph compares the shrinkage results for the high-performance precast concrete specimens.
0
100
200
300
400
500
600
700
0 10 20 30 40 50 60 70 80 90 100
Specimen age, days
High-performance precast concrete
ACI 209 model
Gardner and Lockman model
Hou et al. model
Shrin
kage
,
Figure 5. This graph compares the shrinkage results for the high-performance ready-mix concrete specimens, excluding R1M2 and R2M1.
0
100
200
300
400
500
600
700
0 10 20 30 40 50 60 70 80 90 100
Specimen age, days
High-performance ready-mix concrete
ACI 209 model
Gardner and Lockman model
Hou et al. model
Shrin
kage
,
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For the ready-mix concrete, mixture 2 experienced 17% to 19% greater shrinkage than mixture 1 experienced for supplier R1 and 15% to 17% less shrinkage than mixture 1 (for supplier R2). The average of shrinkage data for all concretes of mixture 1 type was similar to the average shrinkage data of mixture 2 types. Based on these results, it was concluded that for similar mixing, casting, and cur-ing conditions, the shrinkage-time responses of mixture 1 and mixture 2 types were almost equivalent.
Comparison of shrinkage results with other shrinkage models
Figures 4 and 5 show the shrinkage-time curves for the ACI 209-92,29 Gardner and Lockman,30 and Hou et al.31 models. The models were compared with the shrinkage data for precast and ready-mix concretes. The Hou et al. and ACI 209-92 shrinkage-prediction models agreed with the precast and ready-mix concrete shrinkage data at 90 days, but they underestimated the early-age shrinkage of HPC.
The HPC increased rapidly at an early age followed by a continued increase at a slower rate after 28 days. Typical-ly, HPC undergoes significantly greater early-age shrink-age and less long-term shrinkage than normal-strength
days. This was attributed to the better quality control and assurance practices as well as the curing procedure in the precasting plants.
Mixture 1 versus mixture 2 The last scenario of com-parison considered the influence of the amount and type of cementitious materials on the shrinkage-time response. The amounts of cement, fly ash, silica fume, and slag in the mix-ture 1 types were 316 kg/m3, 31 kg/m3, 16 kg/m3, and 47 kg/m3 (19.7 lb/ft3, 1.9 lb/ft3, 0.99 lb/ft3, and 2.9 lb/ft3), respectively. In the mixture 2 types, they were 346 kg/m3, 69 kg/m3 to 73 kg/m3, 17 kg/m3, and 0.0 kg/m3 (21.7 lb/ft3, 4.3 lb/ft3 to 4.6 lb/ft3, 1.1 lb/ft3, and 0.0 lb/ft3), respectively. The total cementitious-materials content in mixture 1 types was 410 kg/m3 (25.6 lb/ft3), and in mixture 2 types it was 432 kg/m3 (27.0 lb/ft3), which was about 5% greater than in mixture 1 types.
P1M1 and P1M2, as well as P2M1 and P2M2, experienced similar 28- and 90-day shrinkage (less than 3% difference). The 28- and 90-day shrinkage of R1M2 were about 19% and 17% greater, respectively, than the shrinkage of R1M1, while the 28- and 90-day shrinkage of R2M2 were about 17% and 15% less, respectively, than the shrinkage of R2M1. Mix-tures 1 and 2 had equivalent shrinkage-time response.
Table 9. Air-void system parameters of concrete specimens
Sample source
Void content, % Spacing factor, mm Specific surface, 1/mm Voids per mm
Freezing and thawing tests, in accordance with ASTM C666,32 were conducted on all of the specimens. Four 100 mm × 100 mm × 400 mm (4 in. × 4 in. × 16 in.) speci-mens were cast from each concrete. Two specimens were wet cured for 14 days (regime A curing) prior to testing, while the other two were wet cured for 14 days followed by 14 days of dry curing in the laboratory environment (re-gime B curing) prior to testing. The durability factors for all specimens were determined after 300 and 500 freezing and thawing cycles. The target criteria were durability fac-tors greater than 90% at 300 freezing and thawing cycles and 85% at 500. Table 10 shows the average durability factors determined for each concrete. The durability fac-tors presented in Table 10 were the average of two speci-mens for each concrete. The durability factor for single specimens, instead of the average, was reported when there was a significant difference between the two specimens.
concrete experiences. The high early-age drying shrinkage of concrete is the dominant factor that leads to cracking.
Hardened air-void system and freezing and thawing
The results for the hardened air-void system parameters were obtained from the average of two analyses per specimen. Re-sults were recorded for the distance traversed, area covered, range and size of voids, void content, specific surface, total number of voids, voids per millimeter, and spacing factor. Table 9 shows the comparison of void content, specific sur-face, voids per millimeter, and spacing factor with the allow-able limits of ASTM C457. The overall air-void parameters of the HPC were generally good, with some variability. The void content was generally within the allowable target speci-fications, except in R4M2, which had greater air content due to the inadequate practices of R4.
Table 10. Durability factors for all specimens after freezing and thawing tests, %
Specimen typeRegime A curing Regime B curing
300 cycles 500 cycles 300 cycles 500 cycles
P1M1 100 100 103 103
P1M2 103 103 94 100
P1M3 100 99 104 104
P2M1 99 98 100 98
P2M2 99 99 Not tested
P2M3— —
101 9983* 55*
R1M1 93 76 97 99
R1M2 99 97 99 101
R2M1 24 14 98 93
R2M2 101 101 98 99
R2M3 101 99 9663*
100*
R3M3 9568*
103 10388*
R4M2 94 94 100 99
R4M3 93 94 101 95
*The durability factor for each specimen was reported when there was a significant difference between the two specimens for each concrete.Note: Specimen P2M3 cracked at 300 cycles during regime A curing; relative dynamic modulus was not able to be measured. The durability factor represents the average of two specimens. The durability factor for single specimens was reported instead of the average when there was a significant difference between the two specimens. Regime A curing = 14 days’ wet curing; regime B curing = 14 days’ wet curing and 14 days’ dry curing.
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The durability factor was compared with the hardened air-void system, which is a major factor determining the resistance of concrete to freezing and thawing cycles. For regime A curing, P2M3 specimen had the lowest durability factor of 83% after 300 cycles. This specimen had adequate air-void content (5.95%) but a marginal spacing factor of 0.229 mm (0.009 in.) and specific surface of 21.3 mm2/mm3 (541 in.2/in.3). R1M1 specimens had moderately low air-void content (5%), an adequate spacing factor of 0.160 mm (0.0063 in.), an adequate specific surface of 31 mm2/mm3 (787 in.2/in.3), and a durability factor of 76% after 500 cycles.
R3M3 specimens had marginally low air-void content (5.43%), an adequate spacing factor of 0.193 mm (0.0076 in.), an adequate specific surface of 26.3 mm2/mm3 (668 in.2/in.3), and a durability factor of 88% in one speci-men and 68% in the second specimen after 500 cycles. Conversely, P2M2 had slightly worse hardened air-void parameters than the previous specimens’ (5.24% air voids, 0.307 mm [0.012 in.]) spacing factor and 17.8 mm2/mm3 (452 in.2/in.3) specific surface, but it had greater durability factors than the previous three specimens had.
For the regime A curing procedure, all of the specimens had high durability factors, which ranged from 93% to 103% (except for specimens P2M3 and R2M1) and from 94% to 103% (except for specimens P2M3, R1M1, R2M1, and R3M3) after 300 and 500 freezing and thawing cycles, respectively. P2M3 specimens had durability factors of 83% and 55% after 300 and 500 freezing and thawing cycles, which were below the target value of 90% and 85%, respec-tively. R1M1 specimens had durability factors of 93% after 300 cycles and 76% after 500 cycles. R3M3 specimens had durability factors of 95% after 300 cycles and 68% in one specimen and 88% in the second specimen after 500 cycles.
For the regime B curing procedure, all of the concretes had durability factors ranging from 94% to 104% after 300 freezing and thawing cycles. After 500 freezing and thawing cycles, the specimens had high durability factors ranging from 93% to 103%, except in R2M3 specimens, which had a durability factor of 63% in one specimen and 100% in the second specimen. All specimens met the target durability factor after 300 freezing and thawing cycles. The comparison of the durability factors revealed that the regime A curing procedure was more aggressive than the regime B curing procedure was.
Table 11. Summary of coulomb-permeability results
Specimen typeAverage coulomb value
Remark based on ASTM C120220
56 days 180 days 365 days
P1M1 1009 736 574 Low or very low
P1M2 1850 1524 1480 Low
P1M3 3645 3437 3264 Moderate
P2M1 1356 1567 1488 Low
P2M2 1583 1110 987 Low or very low
P2M3 4122 3354 3570 Moderate
R1M1 1253 1451 1319 Low
R1M2 2021 2528 2253 Moderate
R2M1 1988 1782 1530 Low
R2M2 2334 2215 2048 Moderate
R2M3 2164 2672 2493 Moderate
R3M3 1077 1000 621 Low or very low
R4M2 9944 9075 8871 High or very high
R4M3 9326 8939 9455 High or very high
Notes: Mixtures P1M3 and P2M3 do not contain fly ash, silica fume, or slag. Mixtures R4M2 and R4M3 were not considered high-performance concrete due to the poor quality control and practices of the R4 supplier at the time of mixing and casting.
123PCI Journal | July–August 2008
supplementary cementitious materials showed high cou-lomb values from 3200 to 4100 coulombs (with an average 3884 coulombs), even with a low w/cm of 0.37.
The average coulomb values for the mixtures with the supplementary cementitious materials (fly ash, silica fume, and slag) were low to very low. The average 56-day pre-cast concrete coulomb value was 1513, not including the P1M3 and P2M3 corrosion-inhibitor specimens. The aver-age ready-mix 56-day coulomb value was 1912, excluding R4 specimens.
Precast HPC specimens showed lower conductivity-test values than ready-mix HPC specimens showed, which was likely a result of the heat curing.
Chloride measurement of concrete specimens
Chlorides can result from the initial concrete components: cement, aggregate, chloride water, or the environment. The purpose of chloride limits is to decrease the possibility of reinforcement corrosion. The subject of chloride limits is currently under debate within the concrete industry. The acid-soluble (total) chloride test can be run first because it is the simplest test to run for chlorides in concrete. Water-soluble chloride is a portion of the total.
Also, P2M1 had higher durability factors than the previ-ous three specimens had, even though it had an adequate air-void content (5.53%), a moderately high spacing factor of 0.274 mm (0.011 in.), and a moderately low specific surface of 18.6 mm2/mm3 (472 in.2/in.3). The correlation between the hardened air-void properties and cyclic freez-ing and thawing durability was not always perfect, but generally the concrete specimens performed well when they met all of the target air-void system parameters.
Coulomb-permeability test results
Table 11 shows the coulomb values for all HPC specimens at 56, 180, and 365 days. The average value was obtained from the test results of four samples for each specimen. These values were compared with the recommended ASTM C1202 values. The HPC specification requires the coulomb reading to be less than 2000 at 28 days. However, half of the specimens (R1M2, R2M2, R2M3, R4M2, R4M3, P1M3, and P2M3) did not meet this 2000 coulomb requirement at 56 days. The same seven mixtures still did not meet the required 2000 coulombs at 365 days. The average coulomb values for most of the mixtures were between very low and moderate, except for mixtures R4M2 and R4M3.
The two precast concrete mixtures (P1M3 and P2M3) that included the corrosion inhibitor but did not contain any
Figure 6. This graph plots the chloride concentration profiles for all specimens at one year. From these profiles, the chloride concentration at any depth can be estimated. Note: 1 mm = 0.0397 in.
0.001
0.01
0.1
1
0 10 20 30 40 50 60 70 80 90 100
Depth, mm
Ch
lori
de
con
ten
t, %
by
wei
gh
t o
f co
ncr
ete
P1M1P1M2
P1M3P2M1
P2M2P2M3
R1M1R1M2R2M1
R2M2R2M3
R3M3R4M3
R4M3
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From these chloride concentration profiles, the surface chloride concentration at 10 mm depth and the chloride concentration at any depth can be estimated. Figure 6 reveals that the chloride content is the highest at a depth of 10 mm and decreases sharply with increasing specimen depth.
Table 12 shows the acid-soluble chloride results at 10 mm and 25 mm (0.4 in. and 1 in.) depths. At a depth of 10 mm, all of the specimens showed a higher chloride content than 0.03% at 90 days and 0.07% at six months, except for R3M3 past the 90-day limit. At a depth of 25 mm, most of the specimens satisfied the 0.03% require-ment at 90 days, except for P1M3, P2M3, R4M2, and R4M3. At six months, only four specimens (P2M2, R1M1, R2M2, and R3M3) met the defined 0.07% HPC character-istics. Consequently, mixtures P1M3, P2M3, R4M2, and R4M3 cannot be classified as HPC in reference to chloride permeability. These results provided a valid reason to run the water-soluble chloride test. The supplier-designed specimens P1M3 and P2M3 had a corrosion inhibitor admixture but no fly ash, silica fume, or slag.
Work at the Federal Highway Administration (FHWA) demonstrated that the conversion factor from acid-soluble to water-soluble chloride could range from 0.35 to 0.9, depending on the particular constituents and history of the concrete. The most widely used conversion factor is 0.75.33 The established specification of the HPC specimens in this study required the chloride content at a depth of 12.5 mm to 25 mm (0.5 in. to 1.0 in.) to be less than 0.03% chloride by weight of concrete at 90 days and less than 0.07% chlo-ride by weight of concrete at six months.34 If the materials met either of the acid-soluble chloride or water-soluble chloride limits, they were considered acceptable.
Acid-soluble chloride test results The acid- soluble chloride test was performed according to ASTM C1152 for each specimen at testing ages of 90, 180, 365, and 720 days. The acid-soluble chloride contents were measured at depths of 10 mm, 25 mm, 38 mm, 50 mm, and 63 mm (0.4 in., 1.0 in., 1.5 in., 2.0 in., and 2.5 in.), and the results were plotted. Figure 6 shows the acid-soluble chlo-ride concentration profiles obtained at one year. The shape of these curves is similar to those found in the literature.
Table 12. Percentage of acid-soluble and water-soluble chlorides by weight of concrete
Specimen
Acid-soluble (total) chloride content at a depth of 10 mm
Acid-soluble (total) chloride content at a depth of 25 mm
Water-soluble chlorides at a depth of 12.5 mm to 25 mm
90 days 180 days 1 year 90 days 6 months 1 year 180 days 1 year
Note: Specimens P1M3 and P2M3 do not contain fly ash, silica fume, or slag and are not considered high-performance concrete. Specimens R4M2 and R4M3 were not considered high-performance concrete due to the poor quality control and practices of the R4 supplier at the time of mixing and casting. 1 mm = 0.0394 in.
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The w/cm varied slightly, from 0.37 to 0.39. This variation of 0.02 did not significantly affect the durability results be-cause some mixtures with greater w/cm values showed less chloride content than those mixtures with lesser w/cm val-ues. Consequently, the effect of the slight differences in the w/cm was not used in the following durability correlations.
Comparison with the previous research Significant corrosion research papers24,35,36,37 covered chloride-diffusion coefficients and their relationship to the ASTM C1202 and AASHTO T27734 coulomb-passing test data following 1- or 2 1/2-year saltwater-ponding tests. These tests at the Virginia Department of Transportation5 (VDOT) on 24 mixtures and the PCI-funded one-year ponding tests at WJE33 on 15 mixtures subjected to water, wet burlap, and heat curing procedures demonstrated that low-permeability HPCs were produced in both studies with coulomb-passing values of less than 2000. These two stud-ies were reviewed in detail by Sherman et al.35
The VDOT data clearly showed that the conventional 0.35 w/c concrete had a low diffusion coefficient of 0.2 × 10-6 mm2/s (3.1 × 10-10 in.2/s) even though the coulomb value was nearly 2000. The data also clearly showed that essentially all of the 21 pozzolan- or slag-modified concretes with coulomb levels generally less than 1000 had higher diffusion coefficients than the 0.35 w/c conven-tional concretes had.25
Water-soluble chloride test results WJE mea-sured the water-soluble chloride contents for the same 14 mixtures at a depth of 12.5 mm to 25 mm (0.5 in. to 1.0 in.) according to ASTM C1218, using the same powder source that was used in the acid-soluble (total) chloride tests. Table 12 shows the results at six months and one year. All of the specimens met the six-month 0.07% limit (defined for HPC) except for P1M3, R4M2, and R4M3. Specimen R1M2 was at the limit.
Chloride content versus coulomb value
The durability characteristics of concrete were affected by many factors, including variation in the source of raw materials, mixture proportions, supplementary cementi-tious materials, quality control practices of each supplier during the batching and mixing procedures, and the curing method. As observed in the coulomb and chloride ponding tests, the effect of the mixture proportions was obvious in the results of P1M3 and P2M3 specimens containing corro-sion inhibitors that were designed by the suppliers without the addition of fly ash, silica fume, or slag. For the same mixture type, the steam-cured precast concrete specimens had lower chloride content than the moist-cured ready-mix concrete specimens.
Table 13. Ordered coulomb-passing data and average water-soluble chloride data
Sample sourceAverage coulomb
valueAverage water-soluble
chloride*Chloride percentage
less than R4 mixturesChloride percent less than the DCI mixtures
Special ingredients in each mixture
R4M2 and R4M3
9268 0.319 — —FA, SF (R4M2)
FA (R4M3)
P1M3 and P2M3
3565 0.120 62 — DCI (both)
R2M3 2443 0.081 75 33 FA, SF, slag
R1M2 2267 0.106 67 12 FA, SF
R2M2 2199 0.091 71 24 FA, SF
R2M1 1767 0.065 80 46 FA, SF, slag
P1M2 1618 0.059 82 51 FA, SF
P2M1 1470 0.040 87 67 FA, SF, slag
R1M1 1341 0.073 77 39 FA, SF, slag
P2M2 1227 0.015 95 88 FA, SF
R3M3 899 0.059 82 51 FA, SF, slag
P1M1 773 0.066 79 45 FA, SF, slag
* Percentage by concrete weight.Note: The first two rows were combined because they were not considered high-performance concrete. DCI = Darex corrosion inhibitor; FA = fly ash; SF = silica fume.
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The data in Table 13 indicate that low- and constant-chlo-ride-permeability concretes were produced with 7 of the 10 HPC specimens, which had coulombs ranging from 800 to 1800. They had an average chloride-content reduction of about 83% compared with the 9200 coulomb non-HPC and 55% reduction compared with the 3500 coulomb corrosion-inhibitor specimens. These seven HPC speci-mens were in agreement with VDOT5 and PCI33 data. As the permeability of concrete approached zero, the concrete had constant, low chloride content (diffusion) that was not fully reflected in the coulomb permeability test method. In addition, as the permeability of concrete approached zero, the benefit of heat curing disappeared.
A 1987 FHWA corrosion report35 showed that after severe, cyclic saltwater exposure, an overnight heat-cured concrete (63 °C [145 °F]) with 0.44 w/cm had a 50% to 60% reduc-tion in chloride at a depth of 25 mm (1.0 in.) compared with a three-day water-cured concrete. The 1996 PCI study5 showed similar chloride reductions in water-cured and heat-cured concretes after severe one-year saltwater ponding. The heat-cured, 0.46 w/c conventional concretes in the PCI study had a 43% lower diffusion coefficient of 6.19 × 10-6 mm2/s (1.9 × 10-9 in.2/sec) and were about 92% less than the 0.46 w/c water-cured concrete diffusion coefficient of 10.9 × 10-6 mm2/sec (3.4 × 10-9 in.2/sec). The observations from the study in this paper also support this trend.
The VDOT data indicated that essentially all of the con-cretes with coulombs less than 2000 had low diffusion coefficients of less than 1.0 × 10-6 mm2/s (1.6 × 10-9 in.2/s) and that their conventional control concretes with w/c of 0.40 and 0.45 had diffusion coefficients 15 and 25 times greater than the 0.35 w/c conventional concrete. The data from both the VDOT and the PCI studies on 39 mixtures clearly demonstrated that the chloride-diffusion values following these severe ponding tests were low and constant when the coulomb values ranged from 500 to 2000. Their data also indicated that chloride diffusion increases rapidly at about 2500 to 3000 coulombs.
The two-year research study discussed in this paper produced two non-HPC specimens with 56- to 360-day coulomb test results averaging about 9200 coulombs, and two non-HPC specimens containing only corrosion inhibi-tor with 56- to 360-day coulomb of about 3500. These four non-HPC specimens had high chloride-diffusion proper-ties. Table 13 lists the average 56- to 360-day concrete coulomb values and the average 180- to 350-day water-sol-uble chloride data after one year of saltwater ponding. The percentage reductions in chloride content compared with the 9200 coulomb and 3500 coulomb non-HPC concretes are also shown, as are the special ingredients used, such as fly ash, silica fume, and slag.
Figure 7. This graph shows the chloride contents versus coulomb values. HPC = high-performance concrete.
The mixture proportions for mixtures 1 and 2 meet •target values of compressive strength, modulus of elasticity, and flexural strength that are appropriate for HPC characteristics.
Heat curing provides better results than wet curing. •The heat-cured precast concrete specimens experi-enced higher compressive and flexural strengths and modulus of elasticity at each testing age than those of the wet-cured ready-mix specimens. Also, the precast concrete specimens showed lower shrinkage values, lower coulomb values, and typically lower chloride contents than those of ready-mix specimens. The re-duction of chloride content due to heat curing was also observed by others.5,35
Heat curing does not produce any adverse effects •on the air-void parameter system. Air-void system parameters obtained from the petrographic examina-tion were variable but generally satisfactory. No direct correlation between the measured fresh-air content (pressure method) and the hardened-air content was observed.
More workable and less permeable HPC can be •achieved by adding high-range water-reducing admix-tures and supplementary cementitious materials.
The equation by Gardner• 27 as well as the current ACI 318M-05 and AASHTO LRFD specifications equa-tions for predicting the modulus of elasticity of con-crete can be used to accurately estimate the modulus of elasticity for HPC.
A modified equation expressed as • fr = 0.75
fc
' (in MPa) for the modulus of rupture provides a more ac-curate expression of the relationship between the flex-ural and compressive strengths and is recommended for HPC at any age.
Exposing concrete to 14 days of wet curing results in •a lower durability factor than that of concrete that is exposed to 14 days of wet curing followed by 14 days of dry curing in a laboratory environment. However, all of the concretes were found to be durable for the freezing and thawing testing.
The correlation between the hardened air-void proper-•ties and the cyclic freezing and thawing durability was not completely satisfactory, but concretes generally perform well when they meet all of the target air-void system parameters.
It is essential to follow good quality control and as-•surance practices and to add supplementary cementi-tious materials in order to achieve HPC. The poor quality control practices of supplier R4 and the lack
The seven heat- and moist-cured HPC specimens in Table 13 had relatively constant and low chloride diffusion when the coulombs ranged from 800 to 1800, that is, 0.015% to 0.073% chloride averaging 0.055%. The four heat-cured precast HPC specimens (P1M1, P1M2, P2M1, and P2M2) had chloride contents ranging from 0.015% to 0.066% and averaged 0.047%. It is interesting that the precast concrete specimen with the lowest average coulomb value of 773 had the highest chloride content among all of the heat-cured precast HPC specimens.
Chloride content–coulomb value correlation Figure 7 explores the relationship between chloride con-tents and the coulomb values. The correlation coefficient R2 for the 10 HPC specimens with coulombs less than 2400 (excluding P1M3, P2M3, R4M2, and R4M3) was 0.14, which indicated poor correlation. This result illus-trated constant low chloride contents for coulomb values from 800 to 2400, but no direct correlation was consistent with the VDOT and PCI studies. Using the results of all specimens (including the non-HPC specimens), better correlation, with R2 equal to 0.873, was observed between the water-soluble chloride contents by weight and the coulomb values as shown in Fig. 7. This revealed that for coulomb values greater than 2400, the chloride contents and the coulomb values were better correlated, showing that the higher the chloride content, the higher the cou-lomb value.
Chloride content and coulomb value versus air-void content The chloride contents and coulomb values were also compared with the hardened air-void con-tents for all mixtures. A weak correlation was observed for the chloride contents versus the hardened air-void contents (R2 = 0.49) and for the coulomb values versus the hardened air-void contents (R2 = 0.45). This was expected because the air voids in the hardened concrete state are disconnected and should not have significantly affected the permeability of concrete. These weak correlations were also observed when the data of the HPC specimens were used without including the non-HPC specimens.
Conclusion
Based on the experimental test results of 316 concrete cylinders (150 mm × 300 mm [6 in. × 12 in.]) and 250 flexural beams (150 mm × 150 mm × 530 mm [6 in. × 6 in. × 21 in.]) for strength tests, 54 concrete cylinders (100 mm × 200 mm [4 in. × 8 in.]) for permeability coulomb tests, 36 prisms (75 mm × 75 mm × 285 mm [3 in. × 3 in. × 111/4 in.]) for shrinkage measurements, and 27 slabs (300 mm × 300 mm × 150 mm [12 in. × 12 in. × 6 in.]) ponded for two years with a 15% sodium chloride solution for chloride-ion penetration, the following conclusions were drawn in terms of strength and durability properties for various precast and ready-mix HPC specimens.
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6. Malhotra, V. M. 1995. Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete: Proceeding. SP-153. Farmington Hills, MI: American Concrete Institute (ACI).
7. Bayasi, S., and J. Zhou. 1993. Properties of Silica Fume Concrete and Mortar. ACI Materials Journal, V. 90, No. 4: pp. 349–356.
8. Naik, T. R., B. W. Ramme, and J. H. Tew. 1995. Pavement Construction with High-Volume Class C and Class F Fly Ash Concrete. ACI Materials Jour-nal, V. 19, No. 1: pp. 200–210.
9. Mangat, P. S., and B. T. Molloy. 1995. Chloride Binding in Concrete Containing PFA, GBS or Silica Fume under Seawater Exposure. Magazine of Con-crete Research, V. 47, No. 171: pp. 129–141.
10. Shah, S. P., W. J. Weiss, and W. Yang. 1997. Shrink-age Cracking in High Performance Concrete. In PCI/FHWA International Symposium on High Perfor-mance Concrete, pp. 148–158. Chicago, IL: PCI.
11. Bloom, R., and A. Bentur. 1995. Free and Restrained Shrinkage of Normal and High-Strength Concretes. ACI Materials Journal, V. 92, No. 2: pp. 211–217.
12. Wiegrink, K., S. Marikunte, and S. P. Shah. 1996. Shrinkage Cracking of High-Strength Concrete. ACI Materials Journal, V. 93, No. 5: pp. 409–415.
13. Subcommittee C09.60. 2005 Standard Test Method for Slump of Hydraulic-Cement Concrete. ASTM C143/C143M-05a. West Conshohocken, PA: Ameri-can Society for Testing and Materials (ASTM).
14. Subcommittee C09.60. 2007. Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete. ASTM C138/C138M-07. West Conshohocken, PA: ASTM.
15. Subcommittee C09.61. 2005. Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. ASTM C39/C39M-05e1. West Consho-hocken, PA: ASTM.
16. Subcommittee C09.61. 2002. Standard Test Method for Static Modulus of Elasticity and Poisson’s Ratio of Concrete in Compression. ASTM C469-02e1. West Conshohocken, PA: ASTM.
17. Subcommittee C09.61, 2007. Standard Test Method for Flexural Strength of Concrete Using Simple Beam with Third-Point Loading. ASTM C78-07. West Conshohocken, PA: ASTM.
of supplementary cementitious materials in P1M3 and P2M3 reduced their durability performance.
As the permeability of concrete approaches zero, the •concrete will have constant low chloride content (dif-fusion) that is not reflected in the coulomb- permeability test method. In addition, as the perme-ability of concrete approaches zero, the benefit of heat curing disappears.
The chloride contents are directly related to the •coulomb values, but the chloride contents and the coulomb values are not well correlated to the hardened air-void contents.
Acknowledgments
This paper is based on a research project awarded to Wiss, Janney, Elstner Associates Inc. (WJE) and the University of Illinois at Chicago by the Chicago Department of Trans-portation. Thanks are given to Stan Kaderbeck, former deputy commissioner/chief engineer for the Chicago De-partment of Transportation’s Bureau of Bridges and Tran-sit for his valuable support. Thanks are also given to the ready-mix and precast concrete companies who provided the mixtures. Our sincere thanks go to WJE for being part of the team on this study and to Stephen Boyd, project en-gineer, and Sharon Tracy, senior materials scientist, WJE.
References
1. Khalil, A. M. 2002. Strength and Durability Assess-ment of High Performance Concrete (HPC). PhD thesis, Department of Civil and Materials Engineer-ing, University of Illinois at Chicago.
2. Issa, Mohsen A., Mahmoud A. Issa, and M. Bendok. 2000. Modified Washington Hydraulic Fracture Test to Determine D-Cracking Susceptible Aggregate. Ce-ment, Concrete, & Aggregates, V. 22, No. 2 (Decem-ber): pp. 116–127.
3. French, C. W., and A. Mokhtarzadeh. 1993. High Strength Concrete: Effects of Materials, Curing and Test Procedures on Short-Term Compressive Strength. PCI Journal, V. 38, No. 3 (May–June): pp. 76–87.
4. Austin, S. A., and P. J. Robins. 1997. Influence of Early Curing on the Sub-Surface Permeability and Strength of SF Concrete. Magazine of Concrete Re-search, V. 45, No. 162: pp. 23–24.
5. Ozyildirim, C. 1998. Permeability Specifications for High-Performance Concrete Decks. Transportation Research Record, V. 1610: pp. 1–5.
129PCI Journal | July–August 2008
30. Gardner, N. J., and M. Lockman. 2001. Design Provi-sion for Drying Shrinkage and Creep of Normal-Strength Concrete. ACI Materials Journal, V. 98, No. 2 (March–April): pp. 159–167.
31. Huo, X. S., N. Al-Omaishi, and M. K. Tadros. 2001. Creep, Shrinkage, and Modulus of Elasticity of High-Performance Concrete. ACI Materials Journal, V. 98, No. 6 (November–December): pp. 440–449.
32. Subcommittee: C09.67. 2003. Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing. ASTM C666/C666M-03. West Consho-hocken, PA: ASTM.
33. Whiting, D. A., P. C. Taylor, and M. A. Nagi. 2002. Chloride Limits in Reinforced Concrete. PCA R&D serial no. 2438. Skokie, IL: Portland Cement Asso-ciation.
34. AASHTO T259. 2006. Standard Method of Test for Resistance of Concrete to Chloride Ion Penetration. Washington, DC: AASHTO.
35. Sherman, M. R., D. B. McDonald, and D. W. Pfeifer. 1996. Durability Aspects of Precast Prestressed Concrete, Part 2: Chloride Permeability Study. PCI Journal, V. 41, No. 4 (July–August): pp. 75–95.
36. Pfeifer, D. W. 2000. High Performance Concrete and Reinforcing Steel with a 100-Year Service Life. PCI Journal, V. 45, No. 3 (May–June): pp. 46–54.
37. Pfeifer, D. W., J. R. Landgren, and A. Zoob. 1987. Protective Systems for New Prestressed and Sub-structure Concrete. FHWA final report no. FHWA/RD-86/193. Washington, DC: Federal Highway Administration.
Notation
Ec = modulus of elasticity
fc' = concrete compressive strength
fr = modulus of rupture
R2 = correlation coefficient
wc = density of concrete
w/c = water-cement ratio
w/cm = water–cementitious materials ratio
µε = microstrain, mm/mm or in./in.
18. Subcommittee C09.68. 2006. Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete. ASTM C157/C157M-06. West Conshohocken, PA: ASTM.
19. Subcommittee C09.65. 2006. Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete. ASTM C457-06 S. West Conshohocken, PA: ASTM.
20. Subcommittee C09.66. 2007. Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration. ASTM C1202-07. West Conshohocken, PA: ASTM.
21. Subcommittee C09.69. 2004. Standard Test Method for Acid-Soluble Chloride in Mortar and Concrete. ASTM C1152/C1152M-04e1. West Conshohocken, PA: ASTM.
22. Subcommittee: C09.69. 1999. Standard Test Method for Water-Soluble Chloride in Mortar and Concrete. ASTM C1218/C1218M-99. West Conshohocken, PA: ASTM.
23. ACI Committee 318. 2005 Building Code Require-ments for Structural Concrete (ACI 318M-05) and Commentary (318RM-05). Farmington Hills, MI: ACI.
24. American Association of State Highway and Trans-portation Officials (AASHTO). 1994. LRFD Bridge Design Specifications. Washington, DC: AASHTO.
25. ACI Committee 363. 1997. State-of-the-Art Report on High-Strength Concrete. ACI 363R-92. Farmington Hills, MI: ACI.
26. Suksawang, Nakin, Hani H. Nassif, and Husam S. Najm. 2006. Comparison of Mechanical Properties of Self Consolidating Concrete (SCC). Transportation Research Record no. 1979, pp. 36–45. Washington, DC: Transportation Research Board, National Re-search Council.
27. Gardner, N. J. 2000. Design Provisions for Shrinkage and Creep of Concrete. The Adam Neville Symposium: Creep and Shrinkage—Structural Design Effects. SP-194, pp. 101–134. Farmington Hills, MI: ACI.
28. Nassif, H. H., H. Najm, and N. Suksawang. 2005. Ef-fect of Pozzolanic Materials and Curing Methods on the Elastic Modulus of HPC. Cement and Concrete Composites, V. 27, No. 6 (July): pp. 661–670.
29. ACI Committee 209. 1992. Prediction of Creep, Shrinkage, and Temperature Effects in Concrete Structures. ACI 209R-92. Farmington Hills, MI: ACI.
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About the authors
Mohsen A. Issa is a professor for the Depart-ment of Civil and Materials Engineering at the Univer-sity of Illinois at Chicago.
Shahidul Islam, PhD, is a senior engineer for Dy-widag-Systems Internation-al in Bolingbrook, Ill.
Paul D. Krauss is the unit manager and senior consul-tant for Wiss, Janney, Elstner Associates Inc. in Northbrook, Ill.
Atef A. Khalil is a former research assistant for the Department of Civil and Materials Engineer-ing at the University of Illinois at Chicago.
Synopsis
The fresh, early-age, hardened, and durability properties of six precast and eight ready-mix concretes were determined at various testing ages over a period of two years. The intent was to assess the influence of the mixture type (pre-cast or ready-mix), amount and type of cementi-tious materials, and variation among the concrete suppliers on the strength and durability charac-teristics of high-performance concrete (HPC).
Target compressive strength, flexural strength, and shrinkage values were attained for the selected concretes. In addition, the hardened air-void parameters, conductivity (coulomb), chloride permeability, and freezing and thaw-ing resistance were measured for each concrete. The correlation among chloride ponding results, coulomb values, and hardened air-void content was explored. The ACI 318M-05 equations for the modulus of rupture fr and modulus of elastic-ity Ec were compared with the experimental test results.
It was found that the ACI 318M-05 equation for the elastic modulus is applicable for HPC, while the ACI 318M-05 equation for the modulus of rupture is significantly conservative. Based on the research findings, a new modified equation for the modulus of rupture is recommended for HPC.
This paper was reviewed in accordance with the Precast/Prestressed Concrete Institute’s peer-review process.
Reader comments
Please address any reader comments to PCI Jour-nal editor-in-chief Emily Lorenz at [email protected] or Precast/Prestressed Concrete Institute, c/o PCI Journal, 209 W. Jackson Blvd., Suite 500, Chicago, IL 60606. J
