Development of low-shrinkage high-performance ...

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Development of low-shrinkage high-performance concrete with improved durability

NRCC-52622 Cusson, D.; Margeson, J.C.

July 2010

A version of this document is published in / Une version de ce document se trouve dans: 6th International Conference on Concrete under Severe Conditions, Environment & Loading (CONSEC'10), Mérida, Yucatán, Mexico, June 7-9, 2010, pp. 869-878

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6th Intl Conference on Concrete Under Severe Conditions Environment and Loading, Merida, Mexico, June 7-9, 2010

Development of low-shrinkage high-performance concrete with improved durability D. Cusson & J. Margeson National Research Council Canada, Institute for Research in Construction, Ottawa, Ontario, Canada

ABSTRACT: The main goal of this study is to develop low-shrinkage highperformance concrete in order to reduce early age cracking and premature reinforcement corrosion, and to provide long service lives to concrete structures. This paper presents a summary of the results related to the evaluation of the chemical, mechanical, and durability properties of different formulations of normal-density, air-entrained highperformance concrete with a water-cement ratio of 0.35. The effectiveness of two shrinkage reduction measures is evaluated: internal curing with saturated lightweight aggregate, and a shrinkage-reducing admixture. All high-performance concretes developed in this study that used internal curing or a shrinkage reducing admixture (separately or in combination) successfully met all performance requirements in terms of workability, hydration characteristics, volume stability, strength and potential durability under severe exposure conditions simulated in the laboratory.

1 INTRODUCTION

The use of high-performance concrete (HPC) for the construction of different types of infrastructure systems has increased significantly in the past few decades (Aïtcin 1998). For example, most departments of transportation in the USA are now using HPC in bridge decks for many reasons (FHWA 2006), including the reduction of permeability to water and chlorides in aggressive environments, and/or the need for superior mechanical performance under severe loading conditions. High-performance concrete, however, is prone to early-age cracking when shrinkage is restrained in concrete structures. For instance, early age transverse cracking has been observed in more than 100,000 bridges in USA (TRB 1996). This tendency to crack is mainly due to the low water cement ratio (w/c) used in HPC to achieve low permeability and/or high strength. This low w/c often results in selfdesiccation, leading to autogenous shrinkage of concrete. Although autogenous shrinkage is practically inexistent in ordinary concrete, the high water cement ratio often results in undesired performance such as high permeability, low strength and high drying shrinkage, thus resulting in short service lives of concrete structures. The primary focus of this study (Cusson and Margeson 2009) is to develop lowshrinkage high-performance concrete for the construction of durable reinforced concrete structures. To that effect, three concrete technologies are being investigated, namely:

• • •

Supplementary cementing materials, i.e. silica fume (SF) and slag, to reduce concrete permeability and partially replace the cement used in concrete (Aïtcin 1998); Internal curing (IC) with pre-soaked lightweight aggregate (LWA) sand to provide enhanced concrete curing and reduce early age autogenous shrinkage cracking (Bentz et al. 2005); and Shrinkage-reducing admixture (SRA) to reduce drying shrinkage cracking (Berke et al. 2003).

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6th Intl Conference on Concrete Under Severe Conditions Environment and Loading, Merida, Mexico, June 7-9, 2010

2 EXPERIMENTAL PROGRAM 2.1 Materials

For an objective comparison of the above technologies, different HPC formulations were developed and based on the same basic proportions, such as: (i) water-cement ratio of 0.35; (ii) cement/sand/stone mass ratio of 1:2:2; and (iii) maximum aggregate size of 10 mm. Table 1 presents four of the six mix formulations developed in this study. The mixing water was adjusted to account for the liquid part contained in the chemical admixtures and the water absorbed by the dry normal sand and dry coarse aggregate during mixing. The commercial blended cement used in this study was a CSA Type GUb-S/SF including 5% silica fume, 20% slag, and 75% of ordinary Portland cement (equivalent to ASTM Type I), and a Blaine value of 575 m2/kg. The aggregates used in concrete were: (i) porous lightweight sand (expanded shale, 5 mm max., 15% water content); (ii) normal-density sand (silica/quartz, 5 mm max.); and (iii) normal-density coarse aggregate (limestone, 10 mm max). Table 1. Concrete mix designs – per kg of cement (total w/c = 0.35). HPC-2 HPC-3 HPC-4 HPC-6 (IC) (SRA) (IC/SRA) (Control) Mix water (kg) 0.36 0.35 0.35 0.37 SF/Slag cement (kg) 1 1 1 1 1.1 2.0 1.1 2.0 Dry normal sand (kg) SSD lightweight sand (kg) 0.5 0.5 2 2 2 2 Dry 10-mm stone (kg) Water reducer (mL) 2 2 2 2 Superplasticiser (mL) 9 10 8.5 9 Air entrainer (mL) 0.14 1.50 0.70 0.15 17 11 Shrinkage reducer (mL) Note: HPC-1 and HPC-5 are outside the scope of this paper.

In HPC-2 and HPC-4, 0.5 kg of saturated LWA per kg of cement was added to provide enough internal curing water to compensate for chemical shrinkage, which is the main factor responsible for the development of autogenous shrinkage in self-desiccating concrete. The quantity of 0.5 kg/kg of LWA was included in the mix as a partial replacement for 0.9 kg/kg of normal sand to keep the same total volume of sand in the concrete. In HPC-3, the dosage rate of 17 mL of SRA per kg of cement (≈ 7.5 L/m3 of concrete) corresponds to the typical maximum dosage rate recommended by the manufacturer. In HPC-4, the SRA dosage rate was reduced to 11 mL/kg, since it was combined with a second shrinkage reducing measure (i.e. internal curing). 2.2 Testing procedures

Characterization of fresh concrete – The following properties were measured: workability (ASTM C143), density/air content (ASTM C231), and time of setting (ASTM C403). Characterization of hydration products – The main products of hydration were determined by thermogravimetric analysis (TGA). Specimens of fresh mortar (sieve from concrete) were cast into 10x50 mm cylindrical containers and stored at room temperature and 100% relative humidity (RH). The hydration reactions were stopped at 4 hrs, 8 hrs, 12 hrs, 1 day, 2 days, 3 days, 5 days, 7 days, 10 days, 14 days, 21 days, 28 days, 60 days, 120 days, and 360 days by vacuum drying under a vacuum pressure of 1 torr for 1 day in order to remove the absorbed water in the specimens. The samples were then finely ground and stored in sealed glass vials until the time of testing. TGA was conducted with a computercontrolled furnace that heats a pair of measuring heads: one containing approximately 50 mg of the test specimen and the other as a reference cell, both heated in a dynamic nitrogen atmosphere. In order to separate the absorbed water from the hydrate water, each specimen was heated at 10°C/minute to 80°C and held isothermally for 1 hour. After this initial treatment, specimens were heated at a rate of 10°C/minute to a final temperature above 1000°C. 2

6th Intl Conference on Concrete Under Severe Conditions Environment and Loading, Merida, Mexico, June 7-9, 2010

Characterization of free shrinkage – The free autogenous shrinkage strain of concrete was measured on a set of three sealed concrete prisms (75x75x300 mm), which were kept at constant room temperature and monitored for a minimum period of 28 days. The test apparatus used for this experiment was similar to that suggested in ASTM C157 for length change measurement. The monitoring started shortly after the casting of concrete. Longterm weight loss was measured on specimens that were initially used for the short-term autogenous shrinkage measurements. The specimens were unwrapped and left fully exposed to the ambient conditions of the room, in which the relative humidity was monitored at 65%. Characterization of mechanical properties – The compressive strength was determined at 7 days, 28 days, 56 days, and 90 days on wet-cured 100x200 mm cylinders according to ASTM C39. The elastic modulus was determined at 28 days on wet-cured 100x200 mm cylinders, which were equipped with two LVDTs measuring axial displacements. The cylinders were loaded up to 50% of their strength to remain in the linear range of the stress-strain behaviour, according to ASTM C469. Characterization of durability – The permeance was determined by measuring water vapour transmission according to ASTM E96, in which a plastic cup containing distilled water was capped with a 13x100 mm concrete disk and weighted regularly over 40 days to determine the rate of vapour movement through the specimen to the ambient atmosphere (RH = 65%). The specimens were wet-cured for 28 days prior to testing. Chloride permeability of concrete was evaluated according to ASTM C1202, consisting of monitoring the electrical current passing through a 50-mm thick concrete disk for a period of 6 hours. A potential difference of 60 V dc was maintained across the ends of the specimen, one of which was immersed in a sodium chloride solution, and the other in a sodium hydroxide solution. The specimens were wet-cured for 28 days prior to testing. The resistance of concrete to freeze-thaw cycles was assessed by subjecting 75x75x300 mm prisms to rapidly repeated cycles of freezing in water and thawing in water at a rate of four cycles per day, with minimum and maximum temperatures of -18°C and +4°C, respectively, according to ASTM C666. The concrete specimens were wet-cured for 28 days prior to testing, and were monitored over 300 F/T cycles. The salt scaling resistance of concrete was conducted according to the ASTM C672 test method, which determines the resistance to scaling of a horizontal concrete surface exposed to slow freezing and thawing cycles in the presence of de-icing chemicals. The specimens consisted of large 355x355x65 mm concrete slabs, with the top surface immersed with a solution of calcium chloride (4g per 100 mL of water). The slabs were wet-cured for 14 days followed by 14 days of drying prior to testing and monitored over 50 slow freeze-thaw cycles at a rate of approximately 5 cycles per week, with minimum and maximum temperatures of -18°C and +23°C, respectively. After the test, the weight change was assessed and pictures of exposed and unexposed surfaces were taken. 3 RESULTS AND ANALYSIS 3.1 Properties of fresh concrete

High slumps of 200 mm ± 50 mm were targeted in order to obtain workable concrete with minimum consolidation required for placement in reinforced concrete structures. Most slump values were within the desired range. High air contents were also desired in this study to achieve high resistance to freezing and thawing in the presence of de-icing chemicals. Most entrained air contents varied from 4.5% to 5.5%. In fact, Aïtcin (1998) suggested that a total air content between 3.5% and 4.5% is sufficient to protect most highperformance concretes from frost damage in most field conditions. The fresh concrete densities varied from 2247 kg/m3 to 2369 kg/m3, with the lower values measured for HPC-2 and HPC-4, which included lightweight aggregate sand for internal curing. Most initial set times varied from 6 hours to 7 hours after adding cement to water, while most final set times varied from 8 hours to 9 hours.

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6th Intl Conference on Concrete Under Severe Conditions Environment and Loading, Merida, Mexico, June 7-9, 2010

3.2 Amount of hydration products

16

16

12

12

C-S-H content (%)

Absorbed water content (%)

Thermo-gravimetric analyses of finely crushed mortar specimens (sieve from concrete) were conducted to characterize the hydration process and major products of hydration, including contents of absorbed water, and calcium silicate hydrate (C-S-H), which is the main compound in hardening cement responsible for strength increase. According to our procedure, the absorbed water content was determined from the total mass loss measured when the sample was heated from 20°C to 80°C, the C-S-H content was determined from the total mass loss measured from 80°C to 400°C, and the calcium hydroxide (CH) content was determined from the total mass loss measured from 400°C to 520°C. Figure 1 illustrates the absorbed water content in the concrete developing over time. As expected, the absorbed water content decreased over time due to further cement hydration. A very interesting trend can be observed, whereas HPC-2 and HPC-4 with internal curing had significantly higher contents of absorbed water at any given time (with more than 20% increase). This is an indication that internal curing does actually provide extra water in the system available for cement hydration. Figure 2 confirms this observation where it is shown that higher contents of C-S-H formed in internally-cured concretes compared to non-internally cured concretes (20% increase), especially at early ages (from 7 days to 14 days). It is noted that HPC-3 with SRA alone yielded slightly less absorbed water and C-S-H contents than those of the control concrete, respectively, which may be due to a slightly slower hydration of cement at early ages or a slightly lower maximum achievable degree of hydration, since SRA was added in the mixture by replacing an equal volume of water. However, when both IC and SRA were used in the same concrete (HPC-4), the thermogravimetric analysis indicated high contents of absorbed water and C-S-H at any given time.

8 HPC-2 HPC-3 HPC-4 HPC-6

4

0 0

10

(IC) (SRA) (IC/SRA) (CTRL)

20 30 40 50 Time after casting (days)

8 HPC-2 HPC-3 HPC-4 HPC-6

4

(IC) (SRA) (IC/SRA) (CTRL)

0 60

Figure 1. Absorbed water content measured by TGA.

0

10

20 30 40 50 Time after casting (days)

60

Figure 2. C-S-H content measured by TGA.

3.3 Mechanical properties

The compressive strength was measured on 100x200 mm concrete cylinders at 7 days, 28 days, 56 days and 90 days after casting. Figure 3 shows the development of compressive strength over time for the different concretes. It can be seen that all concretes exceeded strengths of 40 MPa at 7 days, 55 MPa at 28 days, and 60 MPa at 90 days, which is an indication of high quality concrete with high early strength and high long-term strength. The concretes with internal curing (HPC-2 and HPC-4) developed 20% higher strengths at 56 days than the concretes without internal curing, which can be attributed to the improved cement hydration at early ages (see Figure 2). HPC-3 with the SRA alone seems to have developed slightly lower compressive strengths (up to 10% decrease before 56 days) than those of the control concrete, which is also in agreement with the observations in Figures 1 and 2. The compressive modulus of elasticity was determined by measuring the axial strain as a response to the axial stress applied on 100x200 mm cylinders loaded up to 50% of their strength. The average modulus of elasticity ranged from 30,000 MPa to 35,000 MPa, which are considered to be similar and typical of high performance concrete. 4

6th Intl Conference on Concrete Under Severe Conditions Environment and Loading, Merida, Mexico, June 7-9, 2010

80

Compressive strength (MPa)

Average of 3 tests 70 60 50 40 30

HPC-2 (IC)

20

HPC-3 (SRA)

10

HPC-4 (IC/SRA) HPC-6 (CTRL)

0 0

10

20

30 40 50 60 70 Time after casting (days) Figure 3. Compressive strength measured on φ100x200mm cylinders.

80

90

3.4 Short-term autogenous shrinkage and long-term drying

The free autogenous shrinkage of sealed concrete specimens was determined by measuring the total strain and subtracting the thermal strain obtained from measurements of temperature and thermal expansion coefficient. Figure 4 presents the average autogenous shrinkage strain measured on a set of three 75x75x300 mm prismatic specimens over a period of 28 days. It is shown that the free autogenous shrinkage strain of HPC-6 (with no shrinkage prevention measures) is quite high and is most likely large enough to produce cracking if shrinkage had been fully restrained in these specimens. When used individually, internal curing (HPC-2) and SRA (HPC-3) yielded similarly low autogenous shrinkage strains. For instance, a large 60% reduction in the autogenous shrinkage strain at 28 days was measured for HPC-2 and HPC-3 when compared to the control concrete HPC-6. It is interesting to note that the concretes with internal curing produced typical early age expansion during the first two days as opposed to concretes with no internal curing (Cusson & Hoogeveen 2008). This expansion, however, was neither beneficial nor detrimental, since it was offset shortly after the peak by further development of autogenous shrinkage. The combined use internal curing and shrinkage reducing admixture in HPC-4 proved to be much more effective in reducing autogenous shrinkage (than the single use of either internal curing or shrinkage-reducing admixture) providing early age expansion close to 100 με at one day followed by very low shrinkage developing afterward. The sealed specimens (used to monitor autogenous shrinkage with LVDTs for approximately one month) were re-used to monitor long-term drying shrinkage after unwrapping the specimens and exposing them to a drying environment for a year. Figure 5 presents the weight loss of each specimen measured over time. Note that the test durations are different because these specimens were cast and tested at different times. The results show that HPC3 (made with the highest SRA dosage rate) had a relatively lower drying rate, and HPC-2 (with internal curing alone) had a relatively higher drying rate when compared to the other specimens. Linear strain measurements of long-term drying shrinkage (not shown here) also resulted in trends that are similar to those presented in Figure 5. These trends may be explained as follows: since SRA is designed to reduce shrinkage by lowering the surface tension of water in concrete pores, reductions in both autogenous shrinkage and drying shrinkage can be expected, which is supported by the experimental results shown in Figures 4 and 5. With internal curing, the primary goal is to eliminate early age autogenous shrinkage by providing a certain quantity of internal curing water equal to that needed for the compensation of chemical shrinkage. Since the water supply in the lightweight aggregate is not infinite, and the fact that there is slightly more dryable water in the internally-cured system, internal curing is usually not expected to reduce long-term drying shrinkage, which is also supported by the test results in Fig. 5. 5

6th Intl Conference on Concrete Under Severe Conditions Environment and Loading, Merida, Mexico, June 7-9, 2010 100

0.0

Unwrapped specimens (ambient RH = 65%)

HPC-4 (IC/SRA)

Strain (x10-6)

HPC-2 (IC) -100 HPC-3 (SRA) -200

Weight change (%)

-0.5

0

Note: Test durations are different because the specimens were fabricated and tested at different times.

-1.0

-1.5 HPC-2 (IC) HPC-3 (SRA)

-2.0 Sealed specimens (average of 3 tests)

-300 0

5

HPC-6 (CTRL)

10 15 20 25 Time after setting (days)

HPC-4 (IC/SRA) HPC-6 (CTRL)

-2.5 30

0

Figure 4. Autogenous shrinkage strain measured on 75x75x300mm prisms.

28

56

84 112 140 168 196 224 Drying time (days)

Figure 5. Weight change measured on 75x75x300mm prisms.

3.5 Permeability to water vapour and chloride ions

The permeability to water vapour was evaluated by measuring the weight of vapour transmitted through 13x100 mm disks of concrete over time. The calculated values of permeance are shown in Figure 6 and found to be quite low, with the control concrete HPC-6 having a slightly higher permeance and HPC-3 (SRA) having a slightly lower permeance than the rest of the group. The chloride permeability was determined by measuring the current passing through 50x100 mm concrete disks over a period of 6 hours. The end results are shown in Figure 7 where it can be seen that all concretes yielded values well below the 1000-Coulomb threshold, under which a concrete is considered to have very low chloride permeability, according to ASTM C1202. The range of permeability values of the different concretes is also very narrow, going from 398 Coulombs for HPC-3 (SRA) to 553 Coulombs for HPC-6 (control). Bentz (2009) also observed that the use of a shrinkage reducing admixture in mortar can effectively reduce chloride penetration. 5.E-07

1000

Average of 3 tests

3.E-07

2.E-07

1.6E-07

1.4E-07 1.E-07

6.5E-08

8.8E-08

Charge passed (Coulombs)

Permeance (perm)

4.E-07

Average of 3 tests 800 553

600 492 415

398

HPC-2

HPC-3

400 200 0

0.E+00 HPC-2

HPC-3

HPC-4

Figure 6. Permeance measured on φ100x13mm disks.

HPC-6

HPC-4

HPC-6

Figure 7. Chloride ion penetrability measured on φ100x50mm disks.

3.6 Freeze-thaw resistance

The freeze-thaw (F/T) resistance was determined by subjecting 75x75x300 mm concrete prisms to 300 rapid F/T cycles. The following data were measured during this test: (i) weight change, as weight normally decreases due to spalling of concrete; (ii) length change (Fig. 8), as length normally increases due to ice lens formation and microcracking; and (iii) change in frequency of sound transmission, as it normally decreases with damage accumulation. The frequency data were also used to calculate the relative dynamic modulus of elasticity for each HPC (Fig. 9). In general, all concretes provided excellent results as far as weight change, length change, and frequency change are concerned. For instance, the most important length change measured was only 0.017%, which is far below the maximum value of 0.1% permit6

6th Intl Conference on Concrete Under Severe Conditions Environment and Loading, Merida, Mexico, June 7-9, 2010

ted by ASTM C666. Similarly, the lowest relative dynamic modulus of elasticity measured in this study was 95.8%, which is far above the minimum permitted value of 60%. This is an excellent indication of long-term concrete durability (provided that concrete does not crack). 100 Required: LC < 0.1% (average of 3 tests)

0.08 0.06 0.04 0.02

0.017 0.010

0.009

Dynamic modulus (%)

Length change (%)

0.10

96.6

96.8

97.1

95.8

90

80

70 Required: DM > 60% (average of 3 tests)

0.003

60

0.00 HPC-2

HPC-3

HPC-4

HPC-6

Figure 8. Length change measured on 75x75x300mm prisms after 300 F/T cycles.

HPC-2

HPC-3

HPC-4

HPC-6

Figure 9. Dynamic modulus of elasticity measured on 75x75x300 mm prisms after 300 F/T cycles.

3.7 Salt scaling resistance

Resistance to salt scaling was determined by subjecting 355x355x65 mm concrete slabs to 50 cycles of freezing and thawing with the top surface exposed to de-icing chemicals. Figure 10 shows the total weight loss of the specimens after 50 freeze-thaw cycles. Again, the weight losses were found to be very small, with the largest value being 0.46% (HPC-6). Visual observations were made to rate the condition of the specimens after 50 cycles (Fig. 11). According to the observations, all concretes had very slight scaling, which corresponds to a rating of 1 on a linear scale going from 0 (no scaling) to 5 (severe scaling) defined by ASTM C672. RILEM TC 117-FDC offers a quantitative criterion for determining the suitability of concrete against salt scaling, which must not lose more than 1500 g/m2 after 28 freeze-thaw cycles exposed to a 3% NaCl solution. In our test, the largest measured weigh loss was about 700 g/m2 achieved under much more severe conditions (i.e. 50 F/T cycles and 4% CaCl2). 1.0 Average of 3 tests

Weight loss (%)

0.8

0.6 0.46 0.4

0.31

0.30 0.24

0.2

(a) HPC-4 (IC/SRA)

0.0 HPC-2

HPC-3

HPC-4

(b) HPC-6 (Control)

HPC-6

Figure 10. Weight loss measured on 355x355x65mm slabs after 50 F/T cycles with salt.

Figure 11. Photographs of top surfaces of slab specimens after 50 F/T cycles with salt.

4 SUMMARY AND CONCLUSIONS

Normal-density, air-entrained high-performance concretes with water-cement ratio of 0.35 were developed using IC and SRA, and were evaluated in terms of shrinkage devel7

6th Intl Conference on Concrete Under Severe Conditions Environment and Loading, Merida, Mexico, June 7-9, 2010

opment, mechanical performance and potential durability. The following conclusions can be drawn: • • • • • • • • • •

Internal curing, compared to sealed curing, provided 20% more absorbed water and 20% more C-S-H in the system measured by thermo-gravimetric analysis over a 120-day hydration period; The use of IC or SRA separately showed a large 60% reduction in autogenous shrinkage at 28d; The combined use of IC and SRA produced early expansion and very low shrinkage thereafter; Long-term drying patterns were found to be similar in the concretes, except for a small reduction in the drying rate observed for the concrete made with the highest dosage rate of SRA. Internal curing was not found to reduce long-term drying shrinkage; Compressive strengths exceeded 40 MPa at 7 days, 55 MPa at 28 days, and 60 MPa at 90 days for all concretes. Internal curing increased the 56-day strength by 20% on average; Chloride permeability was very low in all concretes, with values from 400 to 550 Coulombs; Resistance to freezing and thawing was excellent for all concretes, with the largest length change of 0.017% measured after 300 F/T cycles (