Effect of Curing Methods on Durability of High-Performance Concrete

Transportation Research Record 1798 ■ Paper No. 02-3305

31

Effect of Curing Methods on Durability of High-Performance Concrete Hani Nassif and Nakin Suksawang Many state departments of transportation are currently either using high-performance concrete (HPC) or developing new mix proportions for the application of HPC to transportation structures, with emphasis on bridge decks. However, many state engineers have observed that curing methods and conditions in the field affect the behavior of HPC structures. Moreover, little is known about the effect of curing on the long-term durability of HPC. Therefore, it is necessary to understand the behavior of HPC under various curing conditions and durations and the effect of pozzolanic material such as fly ash and silica fume on rapid chloride permeability (RCP). These factors were studied as part of a project for the New Jersey Department of Transportation to develop and implement mix design and technical specifications for HPC transportation structures such as pavements and bridges. Several mixes were tested, and the best mix was selected on the basis of strength and shrinkage test performance. The long-term durability was assessed by tests for RCP, creep, and freeze–thaw behavior. Moreover, the effect on HPC of four curing methods—moist curing, air-dry curing, burlap wrap, and curing compound—was investigated. Moist-cured cylinders performed better than those cured with other methods, and a minimum of 14 days of cure was required for HPC to attain its full strength.

According to the National Bridge Inventory, of 575,000 existing bridges, about 30% are structurally deficient (1). Major decisions must be made to allocate the limited funds available for repair, rehabilitation, and replacement. Moreover, infrastructure facilities constitute a major part of national investment, and the use of high-performance concrete (HPC) is emerging as an important alternative for dealing with deteriorating infrastructure. The concept of HPC in the United States was developed under the Strategic Highway Research Program (SHRP) Contract C205. The program identified the initial parameters that are necessary for HPC mix proportions and documented the performance of HPC prepared in the laboratory and in the field (2– 4 ). At the end of the SHRP program, a major thrust was made to implement the results. FHWA has initiated programs for the design and construction of HPC bridges and pavements with the aim of reducing the costs of both initial construction and long-term maintenance. Many projects are complete or under way in several states (2). With the development of concrete technology, many admixtures (e.g., fly ash, fibers, slag, and silica fume) can be added to achieve desired concrete performance in areas in addition to compressive strength. Several definitions of HPC are available. The definition adopted by FHWA covers eight parameters. The four strength-related parameters are compressive strength, modulus of elasticity, creep, and shrinkage. The remaining four parameters are related to durability: H. Nassif, 131, and N. Suksawang, 139, A-Wing, SOE Building, Rutgers, the State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854.

freeze–thaw resistance, abrasion resistance, chloride permeability, and scaling resistance. The acceptable range of these parameters, test methods to be used for measurements, and recommendations for different exposure conditions have been published previously (3 ). All eight parameters are relevant conditions in New Jersey. However, the basic properties needed for HPC mixes will be evaluated under various conditions. The primary objective of this ongoing research project is to identify HPC mix proportions that are suitable for transportation structures in New Jersey. The proposed research plan involves the selection of mix proportions using local aggregates, pozzolanic material such as silica fume and fly ash, and slag cement; the evaluation of trial mixes prepared under laboratory and field (ready-mix batching plants) conditions; and the preparation of specifications for implementing HPC in future projects. However, it has been established that after casting, the compressive strength and other properties (such as shrinkage and chloride permeability) are affected by curing conditions applied in the field. The question being asked by many contractors is whether it is possible to minimize the duration of curing without adversely affecting the short- and long-term properties of HPC. Therefore, it is necessary to understand the behavior of concrete under different curing conditions and durations. A study of the effect on HPC of four different kinds of curing conditions—moist curing, air-dry curing, burlap wrap, and curing compound—is presented. The duration of curing is also considered. Results obtained will be implemented in a technical specification guide for contractors and quality assurance engineers.

RELATED STUDIES The effects of curing conditions and temperature have been addressed for high-strength and conventional concrete. However, few authors have addressed the effect of curing methods and duration on the durability of HPC. Asselanis et al. (5) and Banthia and Sicard (6) studied the effect of curing condition on the compressive strength and elastic modulus of very-high-strength concrete. The results of both studies indicate that early age strength is lost when the concrete is cured at low temperatures. On the other hand, Killoh et al. (7) studied the influence of curing at different relative humidities on the hydration and porosity of a portland cement–fly ash paste. They used small samples of portland cement–fly ash (70:30) paste with a water-to-binder ratio of 0.59, which was initially cured for 7 days and then exposed to different degrees of relative humidity. They conclude that the relative humidity must be kept at 95%, at least in the early curing age, to decrease the porosity in concrete (7). Thomas et al. (8) studied the effect of curing on the strength, oxygen permeability, and water permeability of fly ash concrete. The

32

Paper No. 02- 3305

Transportation Research Record 1798

results indicate that strength increases as the duration of curing increases and is more noticeable in fly ash concrete than in ordinary portland cement concrete. The same effect was also observed for oxygen and water permeabilities. A longer period of curing was suggested to obtain lower permeability in concrete. Nonetheless, few studies have been initiated to address the effect of curing on HPC and its long-term durability, as represented by rapid chloride permeability (RCP). Furthermore, most of the previous work did not focus on the curing methods that are available in the industry but on the curing temperature and relative humidity. Thus, study of the effect of curing methods and curing duration on the durability of HPC is needed.

EXPERIMENTAL PROGRAM The experimental program was designed to generate the best mix based on strength, shrinkage, chloride permeability, and freeze–thaw performance. A typical strategy for developing mix proportions is to evaluate several possible combinations based on past experience and mixes in the literature. Investigators for the SHRP project evaluated about 100 combinations under laboratory conditions. The laboratory investigation was followed by field trial mixes, and with further refinement, the final combinations were evaluated for strength and durability. In this study, a similar strategy (with some modifications) was followed. The experience gained by other states in the Northeast was used to select an initial array of suitable mix proportions. Successful implementation of HPC depends on identifying mix proportions that can be competitively procured in New Jersey; that provide good workability for placing, compacting, and finishing; and that do not require conditions that are difficult or expensive to enforce. The results developed must be able to be readily implemented with slight modifications under field conditions.

TABLE 1

_ FA _ SF (C+SF+FA) (C+SF+FA)

Group Mix

w/c+p

(%)

(%)

B1 B2 B3 B4 B5 B6 B7 B8

0.39 0.39 0.39 0.39 0.39 0.39 0.39 0.39 0.44 0.44 0.37 0.37 0.35 0.35 0.33 0.33 0.29 0.29

7 5 5 5 5 9 10 15 5 5 5 5 7 5 7 5 7 5

15 10 15 20 25 18 20 20 10 15 10 15 15 10 15 15 15 10

A C

BN D G

Various percentages of silica fume and fly ash were used to generate the trial mixes. The variables considered were the target strengths of 41.4, 55.2, 68.95, and 82.7 MPa with water-to-cementitious materials ratios (w/c+p) of 0.44, 0.39, 0.35, and 0.28, respectively. By weight of cement, silica fume proportions varied from 5% to 10%, and fly ash proportions from 10% to 20%. Information about the other materials used and their characteristics is summarized as follows: • Portland cement Type I with specific gravity = 3.15; • Fine aggregate with specific gravity = 2.56, bulk weight = 16.81 kN/m3 (107.01 lb/ft3), and absorption = 0.36%; • Coarse aggregate from local sources with a nominal maximum size of 9.525 mm (0.375 in.), specific gravity = 2.81, unit weight = 15.40 kN/m3 (98.03 lb/ft3), and absorption = 1.0%; • Available tap water at room temperature; • FORCE-10000D silica fume produced by W. R. Grace with specific gravity = 2.22; • Class F fly ash with specific gravity = 2.49; • DARACEM-19 superplasticizer produced by W. R. Grace with a recommended dosage of 780 to 1565 mL/100 kg of cement (12 to 24 oz/100 lb of cement); and • DARAVAIR-1000 air-entraining agent produced by W. R. Grace.

Mix Proportions Five groups (A, B, BN, C, and D), with w/c+p values ranging from 0.44 to 0.28, were considered for the study. For each group, eight trial mixes with varying percentages of silica fume and fly ash were mixed. Table 1 shows typical mixes with percentages of fly ash and

Air Content and Slump Results for Group B Mixes and Selected Mixes

_

B

Material Properties

A2 A3 C2 C3 B1N B2N D1 D3 G1 G2

SP AEA (mL/100 kg cement) (mL/100 kg cement) 651 651 651 651 651 651 651 651 521 521 977 847 1,303 1,303 1,172 1,172 1,433 1,303

104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104 104

Moisture Content (%) Sand

CA

1.24 1.4 1.47 1.4 2.85 2.85 2.76 2.76 0.5 0.5 2.8 2.8

1.67 1.84 1.95 1.84 2.25 2.25 2.28 2.28 1 1 1 1 1 0.9 3 3.52 1.11 1.11

1.41 0.4 1 0.62 0.52 0.52

Air Content Slump (%) (mm) 7.5 6.5 5.25 6 3.5 3 3.5 3 4.25 2.5 5 4 5.5 4.5 4 4 3.5 3

146 121 76 152 81 76 51 51 57 64 76 76 127 76 89 89 102 51

NOTE: S = silica fume; FA = fly ash; SP = superplasticizer; AEA = air-entraining admixture; CA = coarse aggregates; 1 mm = 0.03937 in.; 1 mL /100 kg of cement = 0.01534 oz /100 lb of cement.

Nassif and Suksawang

Paper No. 02- 3305

silica fume for Group B and selected mixes from the other four groups. Superplasticizer and air-entraining agents were added to every mix to obtain an air content of 3% to 7% and a slump of 50.8 to 127 mm (2 to 5 in.), respectively. The dosage of superplasticizer was kept to a minimum. The method used for mix design followed the procedure outlined in a report developed by American Concrete Institute (ACI) Committee on High-Strength Concrete 363 (ACI 211.4R.93, Guide for Selecting Proportions for High-Strength Concrete with Portland Cement and Fly Ash, 1994). Slump and air content of each batch of concrete were measured immediately after mixing in accordance with ASTM test standards C143-90a and C173-94a (volumetric method), respectively. The method of mixing used was based on ASTM C192/C192M.

Rutgers Bridge Engineering laboratories. All selected mixes had coulomb readings below 1,000 at 90 days. • Elastic modulus and stress–strain relationship (ASTM 469-94). The elastic modulus of each specimen was determined by taking its slope at 45% of its ultimate strength. • Shrinkage (ASTM 490-93a). Three samples were moist cured, and the other three were cured at room temperature. The shrinkage measuring apparatus is shown in Figure 2.

EFFECT OF CURING METHOD On the basis of the strength test results, two mixes from each group were selected to study their long-term durability performance. One durability aspect of each mix is the effect on HPC of using four methods of curing. Two similar batches of concrete containing 9% silica fume and 18% fly ash with a w/c+p of 0.39 were mixed. The first mix was divided into specimens that were cured by using different curing methods:

Experimental Results For every test, the specimens consisted of 4 cylinders, 102 × 203 mm (4 × 8 in.), for the RCP test; 3 specimens, 76 × 76 × 254 mm (3 × 3 × 10 in.), for drying shrinkage; 4 cylinders, 102 × 203 mm (4 × 8 in.), for stress–strain relationship; and 18 cylinders, 102 × 203 mm (4 × 8 in.), for compressive strength test. At the completion of mixing, three cylinders, 102 × 203 mm (4 × 8 in.), were tested at 1, 3, 7, 14, 28, and 56 days using a Tinius–Olsen universal testing machine with 1780 kN (400 kips) capacity. Table 1 shows typical slump and air content results for Group B and selected mixes from the other four groups. The best two mix designs were selected from each group on the basis of strength and shrinkage results. Compressive strength results for Group B are illustrated in Figure 1. Mixes B2 and B6 performed best. Mix B2 had the highest early age (1 to 7 days) compressive strength, whereas Mix B6 had the highest later age (14 to 90 days). For each selected mix design, three additional tests were conducted to evaluate the long-term performance:

• Group 1 was moist cured, and specimens were stored in a room maintained at a relative humidity of 95%; • Group 2 was dry cured, and specimens were stored in the laboratory at room temperature; • Group 3 was cured by using burlap wrap, specimens were stored in the laboratory, and the moisture of the burlap wrap was monitored every 2 days; and • Group 4 was cured by using a commercially available curing compound that was applied to the specimens using a spray gun. The other mix used for evaluating the duration of curing was divided into four groups: groups 1 through 3 were cured with burlap wrap only, for 3 days, 7 days, and 14 days, respectively. Group 4 was left without curing. For each group, three 102 × 203-mm (4 × 8-in.) cylinder specimens were tested for compressive strength at 1, 3, 7, 14, 28, and 56 days. Moreover, for each group, three 102 × 203-mm (4 × 8-in.) cylinder specimens were tested for RCP at 28, 56, and 90 days. At each

• Rapid chloride permeability (ASTM C1202-9). Tests were performed at the New Jersey Department of Transportation and at the

80 1 day 3 days 7 days 14 days 28 days 56 days

Compressive Strength (MPa)

70 60 50 40 30 20 10 0 B1

B2

B3

33

B4

B5

B6

B7

Mix

FIGURE 1 Histogram of compressive strength for Group B mixes (w /c p 0.39).

B8

34

Paper No. 02- 3305


FIGURE 3

Curing methods applied to cylinder specimens.

age, one cylinder was sliced into four 102 × 50.8-mm (4 × 2-in.) specimens. The average value from all four specimens was recorded for each age. Typical cylinder specimens cured using the various curing methods are shown in Figure 3.

Compressive Strength

Shrinkage test apparatus (comparator).

60

50 Compressive Strength (MPa)

FIGURE 2

At 1 to 7 days, the compressive strengths are similar for all curing methods (Figure 4). However, at 28 days, the compressive strength of specimens cured with dry cure (room temperature), curing compound, or burlap wrap is about 12% less than that of moist-cured specimens. At 56 days, the results are similar, with a difference of

40

30

Moist Curing No Curing Burlap Curing

20

Curing Compound 10

0 0

10

20

30

40

50

Time (days)

FIGURE 4

Comparison of compressive strengths with different curing methods.

60


Paper No. 02- 3305

35

80

Compressive Strength (MPa)

70 60 50 40 30

Burlap 3 days Burlap 7 days Burlap 14 days No curing

20 10 0 0

10

20

30

40

50

60

Time (days)

FIGURE 5

Variation of compressive strength with different curing durations.

about 10%. Moreover, a minimum of 14 days’ curing is needed to ensure a compressive strength close to the specified 28-day strength (Figure 5). Curing for only 3 and 7 days resulted in reductions of about 10% and 5%, respectively.

erence bar using a comparator (Figure 2). When the comparator is used, the specimen is slowly rotated such that the minimum reading is recorded. The length change was recorded for each specimen at various ages. Figure 6 shows the variation in length for specimens cured under different conditions.

Drying Shrinkage Rapid Chloride Permeability

Three specimens of 76 × 76 × 254 mm (3 × 3 × 10 in.) for each curing method were molded, and gauge studs were screwed into the plates at each end of the mold using a hex screw. The length between the two gauge studs was measured as well as the length of the ref-

For each curing method, four specimens were tested three times: at 28 (or in some cases, 36), 56, and 90 days. Each cylinder was cut into three 50.8-mm (2-in.) slices using a water-cooled diamond saw,

0.06 Moist Curing Dry Curing Curing Compound Burlap 3 Days Burlap 7 Days Burlap 14 Days

0.05

Length Change (%)

0.04 0.03 0.02 0.01 0 -0.01 0

5

10

15

20

25

30

Time (days)

FIGURE 6

Variation of drying shrinkage for specimens cured using different methods.

36

Paper No. 02- 3305

and the top and bottom 25.4-mm (1-in.) slices were discarded. The samples were stored in the curing room until the test date. On the test date, one slice from each sample (four slices total) was allowed to dry for about 1 h. After drying, the sides of the specimens were coated with high-strength epoxy. The coated specimens were left for 24 h to allow the epoxy to harden (Figure 7), then subjected to vacuum in a desiccator for 3 h. The deaerated water was drained through a separator funnel, and the vacuum pump was turned on for an additional 1 h and then turned off to allow air to reenter the desiccator. The specimens were soaked under water for 18 h, then removed from the water and dried with a towel. Both sides of the cells were coated with silicon material to prevent the leakage of chemical solutions. One side of each cell (negative terminal) was filled with 3.0% NaCl solution, and the other side (positive terminal) was filled with 0.3 N NaOH solution. The wires from the RCP test apparatus were connected to the cells (Figure 8). The apparatus was turned on, and its power was set to 60 V. The passing current was recorded every 30 min for 6 h. Typical values for the range of coulomb readings correlated with RCP are listed in Table 2. The average values of four readings considered for each age using low and high percentages of fly ash are shown in Figure 9. Results indicate that adding fly ash enhances the permeability of HPC. The variation of the charge passed in 6 h (coulomb) readings with time for various w/c+p values is shown in Figure 10. The readings are consistent for the same w/c+p at 56 and 90 days, with a slight variation at 28 days. Similar trends were observed for other mixes. The study was extended for the samples with various curing methods. Figure 11 shows the charge passed in 6-h (coulomb) readings for specimens cured using different methods. The coulomb readings at 56 and 90 days rather than 28 days are used to compare the performance of each specimen with regard to chloride permeability. Next to moist curing, curing with burlap wrap for 14 days is essential to achieve a specified value 4000 2000 – 4000 1000 – 2000 100 – 1000 < 100

Chloride Permeability High Moderate Low Very Low Negligible


Paper No. 02- 3305

3000

Charge Passed in 6 h (coulomb)

2500

w/c+p = 0.40, FA = 30%

2000

w/c+p = 0.37, FA = 10% 1500

1000

500

0 20

30

40

50

60

70

80

90

100

Time (days)

FIGURE 9

Variation of coulomb readings with percentage of fly ash (FA).

3000 0.44 0.37 0.365 0.29-1 0.29-2


2500

2000

1500

1000

500

0 20

30

40

50

60

70

80

90

100

Time (days)

FIGURE 10 Variation of coulomb readings for mixes with different values for w/cp. All mixes contain 5% silica fume, 10% fly ash except Mix 0.29-1 (7% silica fume and 15% fly ash) and Mix 0.29-2 (5% silica fume and 10% fly ash).

37

38

Paper No. 02- 3305


4000 28 Days


3500

56 Days 3000

90 Days

2500 2000 1500 1000 500 0 Dry

Burlap 3 Days

Burlap 7 Days

Curing Compound

Burlap 14 Days

Moist

FIGURE 11 Variation of average coulomb readings for specimens cured using different methods.

REFERENCES 1. The State of the Nation’s Highway and Bridges: Condition and Performance and Highway Bridge Replacement and Rehabilitation Program. FHWA, U.S. Department of Transportation, 1998. 2. New Hampshire Department of Transportation and FHWA. Strategic Highway Research Program High-Performance Concrete Showcase, workshop at the University of New Hampshire, Durham, Sept. 22–23, 1997. 3. Goodspeed, C. H. High-Performance Concrete Defined for Highway Structures. Concrete International, Vol. 18, No. 2, Feb. 1996, pp. 62–67. 4. Zia, P. High-Performance Concretes: A State-of-the-Art Report. SHRP C-317. Strategic Highway Research Program, National Research Council, Washington, D.C., 1991. 5. Asselanis, J. G., O.-C. Aitcin, and P. K. Mehta. Effect of Curing Conditions on the Compressive Strength and Elastic Modulus of Very HighStrength Concrete. Cement, Concrete, and Aggregates, Vol. 11, No. 1, Summer 1989, pp. 80–83.

6. Banthia, N., and V. Sicard. Effect of Curing Temperature on the Complete Stress–Strain Plots for High Strength Concrete. In Fracture of Concrete and Rock: Recent Developments (S. P. Shah, S. E. Swartz, and B. Barr, eds.), Elsevier Applied Science, London, 1989, pp. 587–595. 7. Killoh, D. C., L. J. Parrott, and R. G. Patel. Influence of Curing at Different Relative Humidities on the Hydration and Porosity of a Portland/Fly Ash Cement Paste. Special Publication 114. American Concrete Institute, Farmington, Mich., 1989, pp. 157–174. 8. Thomas, M. D. A., J. D. Matthews, and C. A. Haynes. The Effect of Curing on the Strength and Permeability of Fly Ash Concrete. Special Publication 114. American Concrete Institute, Farmington, Mich., 1989, pp. 191–217. The findings expressed in this article are those of the authors and do not necessarily reflect the view of the New Jersey Department of Transportation. Publication of this paper sponsored by Committee on Properties of Concrete.