Influence of mineral admixtures on the mechanical properties and corrosion of steel embedded in high strength concrete

Materials Letters 57 (2003) 2037 – 2043 www.elsevier.com/locate/matlet

Influence of mineral admixtures on the mechanical properties and corrosion of steel embedded in high strength concrete I˙brahim Tu¨rkmen a,*, Mehmet Gavgalı b, Ru¨stem Gu¨l a b

a Department of Civil Engineering, Atatu¨rk University, 25240 Erzurum, Turkey Department of Mechanical Engineering, Atatu¨rk University, 25240 Erzurum, Turkey

Received 18 July 2002; received in revised form 29 August 2002; accepted 30 August 2002

Abstract The corrosion rate of steel material in concrete is extremely low when the metal is in passive state at normal conditions. This situation is completely different in the regions where corrosive ions are available. Chlorine ions around steel, at the passive state, destroy the reinforced concrete. To improve the mechanical properties of concrete and to increase the corrosion resistance of steel embedded in concrete, silica fume (SF) and blast furnace slag (BFS) were used at different ratio instead of Portland cement (PC). The dry-unit weight, the compressive strength and the ultrasonic pulse velocity (UPV) of the samples without steel reinforcement were determined. The corrosion current densities of the samples with steel reinforcement in 5% NaCl solution were measured by the linear polarization technique on the 28th, 75th, 150th and 250th days. Finally, it was observed that the samples with 10% SF + 20% BFS had the highest compressive strength, and that the concrete samples with 10% SF + 40% BFS and 0.35 water-binder ratios had the lowest corrosion current density. As a result, it can be concluded that the mineral admixtures improved the compressive strength, UPV and corrosion current density. D 2002 Elsevier Science B.V. All rights reserved. Keywords: High strength concrete; Silica fume; Blast furnace slag; Corrosion; Pulse velocity; Chloride

1. Introduction In most structures, metallic reinforcement provides static-constructional security. Concerning load carrying capacity, corrosion of the metallic material is important. Concrete normally provides excellent corrosion protection for reinforcing steel due to the high pH value of hydrated cement [1]. The long-term durability of this protection against corrosion is connected with the stability of conditions necessary for the passive layer [2]. Depending on concrete quality, thick-

*

Corresponding author.

ness of concrete cover, workmanship and constructional characteristic under certain environmental conditions, the passivation effect of the concrete pore solution can be neutralized [3]. Chloride ions penetrate concrete by capillary and diffusion process [3]. They migrate to the reinforcing steel, break down the passivation film and enhance the corrosion process (Fig. 1). Thus, corrosion reactions on the steel may occur [1 –3]. The high performance concrete is the one which has better properties such as workability, strength, durability and permeability than usual concretes in two cases: fresh and hardened. Generally, high performance concretes have a water – cement ratio of less than 0.45, and they contain silica fume (SF) about 5 –

0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01136-9

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Fig. 1. The deterioration of passive layer and the cycle of chlorine.

10% replacement for cement. A third binder, preferably blast furnace slag (BFS) and fly ash, must be used with them [4]. Concrete deteriorates due to the main reasons of harmful chemical, industrial wastes, freezing –thawing, cracking because of heat effects, shrinkage, humidity effect and the corrosion of steel reinforcements [5]. Those deteriorating factors affect cement due to its weak resistance. Therefore, these negative effects can be avoided by increasing the resistance of concrete with the use of a suitable cement such as Portland cement (PC) with normal and rapid hardened rate, sulfate-resistance and puzzolanic cements, cement with BFS, super sulfate cements and cement with high alumina content, when it is necessary [6]. First of all, the factors such as best mixture ratio, low water – cement ratio, the compression of produced concrete, the control of cement dosage, the suitability of aggregate characteristics and the improvement of laboratory must be taken into consideration for any kind of concrete before taking into account a cement having high resistivity [5,6]. The true compressive strength of concrete is not resistive to chemical effects. However, for the same kind of cement, since the concrete with higher strength has lower permeability, having that will be more resistive to chemical effects [5,6]. A number of studies [3,7 – 11] have been conducted on the corrosion of steel in concretes made with cement containing mineral admixtures. Baweja et al. [3] determined lower corrosion current density of concrete containing 35% BFS than without BFS. Also, Huang et al. [9] determined that concrete with 15% and 30% BFS have high corrosion resistance. Similar findings were also reported by Arya and Xu [10]. Hope and Ip [7] found that corrosion of the steel rods in concretes decreased with increasing BFS content, except during the first 7 days after casting. Baghabra Al-Amoudi et al. [11] investigated the long-term corrosion resistance of steel in 5% sodium chloride solution in the 7th year

of immersion using regression analyses by electrochemical technique. The results of regression analyses indicated excellent correlation between corrosion resistance and porosity for both plain and blended cement concretes. The corrosion rate of the steel in BFS containing concrete specimens was about between one-half and one-twelfth of those in plain concrete specimens. Lorentz and French [8] indicated that condensed SF increased the specimen resistance and decreased the corrosion current of specimens. In the design of high durability buildings, it is necessary to reinforce the adherence of concrete and steel, to determine the factors affecting the corrosion of concrete and to improve the precautions for the elimination of the corrosion factors. In these works, the effect of BFS and SF was investigated on corrosion of reinforcement concrete and mechanical properties of concrete in the 5% NaCl. To investigate the Table 1 Physical properties and chemical analysis of mineral admixtures used Portland cement

Blast furnace slag

Silica fume

314 3.08

400 2.86

20 000 2.25

– –

– –

– 24.4 42.4

– 18.0 –

– – –

Chemical analysis SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2 O Loss of ignition

19.80 5.61 3.42 62.97 1.76 2.95 0.47 0.87 2.17

39.56 10.82 0.33 37.68 6.79 0.33 – – –

85 – 95 1–3 0.5 – 1.0 0.8 – 1.2 1.0 – 2.0 – – – 0.5 – 1.0

Bogue composition C3S C2S C3A C4AF

54.88 15.37 9.08 10.41

– – – –

– – – –

Physical properties Blaine surface area (m2/kg) Specific gravity (kg/m3) Setting time (min) -Initial -Final Compressive strength (MPa) -2 days -7 days -28 days

135 200

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Fig. 2. Reinforced concrete specimen.

corrosion of reinforcement, it was tried to determine the effect of mineral admixtures and water-binder ratio by recording current densities at different periods.

2. Materials and methods The PC 42.5, the BFS and SF was provided from Ankara SET cement, OYSA Iskenderun and EtibankAntalya Electrometallurgy Industry Plant, in Turkey, respectively. The chemical composition and some physical properties of the PC, BFS and SF are listed in Table 1. The Bogue compound composition of the PC was calculated according to ASTM C 150 and is presented in Table 1. In the experiments, sikament FF´ KA, was N superplasticizier (SP), supplied from SY used as 2% of cement weight. Density of SP was 1.21 kg/l, its pH 9 and proposed amount was 0.8– 3% of cement weight. The regular tap water was used in the whole tests. The coarse aggregate was 16-mm maximum size of basalt of bulk specific gravity 2.64 with water absorption capacity of 2%. The fine aggregate was the beach sand of specific gravity 2.31 with water absorption capacity of 4%. Each value reported is the

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average of three readings obtained from three different specimens. A 70  70  70 mm cube (see Fig. 2) and 100-mm diameter and 200-mm height cylindrical forms were used. The steel bars were cleaned with ethanol and then dried prior to embedding into concrete. Later, the steel samples were kept in desiccators to protect from corrosion until experimental time. The bar embedded in the center of the prepared specimens was used as a working electrode, and another two steel sheets produced from the same material were used as counter electrodes having the size of 50  50  1 mm as shown in Fig. 3. In the concrete specimens, that three different water-binder ratio [0.35(C), 0.40(D) and 0.45(E)] was used. The dosage was 450 kg/m3. In the concrete mixes, SF and BFS were used instead of cement in the ratios of 10%, 20% and 40%. The mixture proportions of all groups were presented in Table 2. All concrete samples were first cured 24 h in laboratory conditions, and then 13 days in the water saturated by lime followed by the cure of 14 days in an atmosphere of 75 –80% humidity and 30 jC. After the 28th day, the control samples were cured in limesaturated water till 250th day, while the samples for experiments were cured in 5% NaCl solution. The linear polarization method uses the smallest potential spectrum of all the direct corrosion current measurement methods. The main advantage of the linear polarization over other direct corrosion current measurement methods is that it has so small potential spectrum and is essentially a nondestructive test. Linear polarization measurements can be repeatedly made on the same test electrode. The test electrode’s open circuit potential (OCP) was measured prior to

Fig. 3. Experimental setup for corrosion measurement.

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2040 Table 2 Mix proportions of all concrete groups Materials

C (water-binder ratio: 0.35) 3

PC (kg/m ) Water (kg/m3) SF (kg/m3) BFS (kg/m3) SP (kg/m3) Sand (kg/m3) Crushed stone (kg/m3) Fresh unit weight (kg/m3)

D (water-binder ratio: 0.40)

E (water-binder ratio: 0.45)

C1

C2

C3

C4

D1

D2

D3

D4

E1

E2

E3

E4

450 186 – – 5.4 798 770 2583

405 186 45 – 5.4 798 770 2576

315 186 45 90 5.4 798 770 2565

225 186 45 180 5.4 798 770 2586

450 209 – – 5.4 771 744 2554

405 209 45 – 5.4 771 744 2539

315 209 45 90 5.4 771 744 2523

225 209 45 180 5.4 771 744 2496

450 230.5 – – 5.4 775 719 2478

405 230.5 45 – 5.4 775 719 2470

315 230.5 45 90 5.4 775 719 2468

225 230.5 45 180 5.4 775 719 2464

with ASTM C 349 at 28 and 250 days. The ultrasonic pulse velocity (UPV) of the control specimens was determined in accordance with ASTM C 597 at 28 days. The test results were illustrated in Table 3.

polarization. The potentiodynamic tests begun at approximately 20 mV from OCP and potential was increased in small 1-mV step until the final electrode potential was approximately + 20 mV from OCP. Electrical current was recorded for each potential step. Corrosion current was calculated using the linear polarization equation with the experimentally determined value of polarization resistance and an assumed value of 100 mV/decade for both the anodic and cathodic Tafel slopes [7]. The test results were illustrated in Table 4. The compressive strength of the SF – BFS – PC based mortar modified by SP determined in accordance

3. Results and discussion 3.1. Dry-unit weight It was observed that the dry-unit weight of the concrete hardened for 28 days decreased with increasing water-binder ratio and with the use of BFS and SF

Table 3 Experimental results of some groups Concrete groups

C (water-binder ratio: 0.35)

D (water-binder ratio: 0.40)

E (water-binder ratio: 0.45)

C1

D1

E1

E2

E3

E4

2393

2382

2346

2296

2

2

3

5

44.5

46.2

47.0

44.45

22

19

18

22

4200

4370

4400

4300

3

+1

+1

1

58.1

58.8

56.4

6

5

9

53.3

56.1

49.7

C2

C3

C4

D2

D3

D4

28-day dry-unit 2474 2467 2457 2402 2440 2420 2412 2356 Weight (kg/m3) Reduction ( ) or 0 0 0 2 1 1 2 3 increase (+) (%) 28-day compressive 57.2 59.5 62.1 53.5 55.5 58.7 59.8 53.2 strength (MPa) Reduction( ) or 0 +4 +8 7 3 +2 +4 7 increase (+) (%) 28-day ultrasonic 4340 4550 4570 4420 4320 4450 4490 4380 pulse velocity (m/s) Reduction ( ) or 0 +5 +5 2 1 +3 +3 +1 increase (+) (%) 250-day compressive strength 61.7 67.4 71.3 66.3 60.8 61.3 62.8 60.9 in curing room (MPa) Reduction ( ) or 0 +9 + 16 +7 2 1 +2 1 increase (+) (%) 250-day compressive strength 54.3 64.1 67.8 57.8 50.8 60.6 61.0 52.2 in 5% NaCl solution (MPa) Reduction ( ) or 0 + 18 + 24 +6 7 + 12 + 12 4 increase (+) (%)

51.9 16 46.5 15

2

+3

9

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in concrete mixture. This behavior can be explained by the fact that the specific weight of BFS and SF are lower than that of PC. When higher water-binder ratios were used, the evaporating residual water also caused a decrease in the unit weight of concrete, resulting in the formation of porous structure. The highest unit weight was obtained for C1 group specimens as 2474 kg/m3 and the lowest one was for E4 group specimens as 2296 kg/m3. The unit weights of the specimens changed between these two values. The results are exhibited in Table 3. 3.2. The effect of BFS and SF on the ultrasonic pulse velocity To determine the strength of concretes with different admixtures, UPV of the specimens was measured. The strength of concrete is also low, in the case the UPV is low [12]. In the measurements after water cure of 28 days, the UPV for the concrete samples with no mineral admixture was low as shown in Table 3. The UPV values were in the range of 4200 – 4570 m/s; the lowest value belongs to the E1 group specimens and the highest one to the C3 group specimens. Since BFS and SF mineral admixtures were much higher than PC, they filled the micropores in cement paste. They were improved by the mechanical properties and durability of concrete by reducing permeability and porosity [13,14]. 3.3. The effect of SF and BFS on the compressive strength The compressive strength values were measured on the 28th and 250th days and the results were given in Table 3. The compressive strength of the specimens increased by about 4% on the 28th day when 10% SF was used instead of PC. The C4 group’s 28th day compressive strength decreased by about 7%. C3 concrete group specimens had the highest compressive strength of 62.1 MPa, while E4 group had the lowest value of 44.45 MPa for the 28th day compressive strength. In case 10% SF was used instead of PC, the compressive strength of the concretes cured at air curing and in 5% NaCl solution measured at the 250th day increased by about 18% and 9%, respectively.

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The compressive strength of the concrete specimens decreased with increasing water-binder ratios. The highest compressive strength was obtained as 71.3 MPa for the C3 group concrete specimens cured at room temperature for 250 days. The lowest value of 46.5 MPa was obtained for E1 group specimens in the 5% NaCl solution on the 250th. The compressive strength of the specimens cured in NaCl solution was less than those at room temperature. The compressive strength of C3 group specimens cured in NaCl solution was 67.8 MPa. The compressive strength of the concrete with BFS was higher than those containing SF and PC concrete at the 250th day (see Table 3). Although the compressive strength of the specimens with BFS of 28 days is lower, the improvement in the compressive strength of the concrete with BFS after the 28th day increased its value to a higher value than that of normal PC. However, the initial compressive strength value of the concrete with SF is higher than those with BFS and PC [15]. For this reason, for the situation which required early compressive strength, the concrete with SF must be preferred while the concrete with BFS must be preferred when the compressive strength must be developed late. Both these admixtures relatively increased the final compressive strength of the concrete compared to PC. 3.4. Corrosion current density The corrosion current density values measured on the 28th, 75th, 150th and 250th days were given in Table 4. At the beginning, on the 28th day, the current density values of all specimens were higher than those measured on the other days. This can be explained by the fact that the protective passive layer on the steel surface still does not term. Since no pronounced effect of time on the corrosion current density was observed between 75th and 250th days, it can be concluded that the passive protective layer started to form after the 75th day following the complete price of the concrete [16]. It was observed that the mineral admixtures had a pronounced effect on the corrosion of the embedded steel. The use of BFS instead of PC because of higher concrete electrical resistivity and lower chloride diffusion rate, OH-concentration of the solution, can explain this situation [7] and decreased

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Table 4 Corrosion current densities of experimental specimens Concrete groups

C (water-binder ratio: 0.35) C1

28-day corrosion current density in 5% NaCl solution (mA/m2) Reduction ( ) or increase (+) (%) 75-day corrosion current density in 5% NaCl solution (mA/m2) Reduction ( ) or increase (+) (%) 150-day corrosion current density in 5% NaCl solution (mA/m2) Reduction ( ) or increase (+) (%) 250-day corrosion current density in 5% NaCl solution (mA/m2) Reduction ( ) or increase (+) (%)

C2

C3

20.2

18.6

0

9

12.9

0 13.6

0 13.3

0

8.3

36 8.6

37 8.6

35

D (water-binder ratio: 0.40) C4

D1

15.7

13.3

29

52

6.9

47 6.9

49 6.8

49

5.4

58 5.4

60 5.2

62

the corrosion of the reinforcing steel by about 60%. The use of both mineral admixtures, BFS and SF, decreased the corrosion of steel. The highest corrosion value for the 28th day was reached for E1 group specimens as 28.2 mA/m2, while the lowest one was reached for C4 group as 5.2 mA/m2. It was determined that the concretes with BFS admixtures had higher corrosion resistance against chloride ions than the other concrete samples [15,17].

4. Conclusions From the findings of the present work, the following conclusions can be drawn: The dry-unit weight of the concretes decreased with increasing of water-binder ratio and by use of BFS and SF mineral admixtures. The use of 20% BFS and 10% SF instead of PC increased the compressive strength of the specimens by about 8% on the 28th day and 16% on the 250th day. The increase of water-binder ratio from 0.35 to 0.45 decreased the compressive strength approximately by 22% on the 28th day. It was observed that

D2

24.8

+ 19 12.3

5 13.6

0

21.4

D3 19.8

+7

2

11.0

7.8

15 11.0

19

13.4

10.9

+1

18

E (water-binder ratio: 0.45) D4

40

14.9

36 6.1

53

7.8

43

6.1

55

7.8

41

E1

6.1

54

28.2

+ 28 17.6

+ 36 17.3

+ 27 16.0

+ 20

E2 25.1

+ 20

E3

E4 22.7

+ 11

13.8

11.5

+7

11

13.7

11.6

+1

15

13.4

11.5

+1

14

16.8

20 7

46 7.2

47 7.1

47

increasing cure period increased the compressive strength of the concrete with BFS more than other specimens. It was observed that there was similar effect between water-binder ratio – compressive strength and waterbinder ratio– UPV. Increasing water-binder ratio resulted in a decrease in the compressive strength and UPV. The corrosion rate of BFS specimens was lower than those of SF and PC. The corrosion current density increased with the increase of the water-binder ratio and decreased with the increase of the mineral additives percent value of the specimens up to 50%, in which the situation may be related to low permeability characteristics of the specimens. The use of BFS and SF which is very fine compared to PC decreased the corrosion current density of the specimens.

Acknowledgements This work was partially funded by the Atatu¨rk University Research Foundation (the number of project: 2000-22).

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