Properties of self-consolidating concrete made utilizing alternative mineral fillers

Construction and Building Materials 68 (2014) 268–276

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Properties of self-consolidating concrete made utilizing alternative mineral fillers Shamsad Ahmad ⇑, Saheed Kolawole Adekunle, Mohammed Maslehuddin, Abul Kalam Azad King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

h i g h l i g h t s  SCC mixtures prepared using four different materials as mineral filler were studied.  All four mineral fillers can be alternatively used to produce high performance SCC.  Information presented in the paper can be used to produce economical mixtures of SCC.

a r t i c l e

i n f o

Article history: Received 11 February 2014 Received in revised form 28 May 2014 Accepted 30 June 2014

Keywords: Self-consolidating concrete (SCC) Mineral fillers Limestone powder Silica fume Metakaolin Natural pozzolana Mechanical properties Durability characteristics

a b s t r a c t Self-consolidating concrete (SCC) is a concrete material possessing an ability to take formwork shapes and pass through congested reinforcement bars without being vibrated, making it a ‘smart concrete’ material. However, the high cost of SCC resulting from the use of mineral fillers and high cement content has been a main factor impeding the widespread use of this smart material. Consequently, there is a need to investigate the use of low cost materials in the production of SCC to ensure adoptability of SCC in concrete construction. This paper presents the results of a study conducted to develop and evaluate the performance of four SCC mixtures using different combinations of filler materials, such as silica fume, natural pozzolana and metakaolin, in conjunction with limestone powder. The developed SCC mixtures exhibited high strength (compressive, tensile, bond and elastic modulus), excellent shrinkage behavior and good durability characteristics (high corrosion resistance and related indices). The findings of this study indicated the possibility of producing cost-effective and high-performance SCC mixtures using the mineral fillers considered in this study. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Unusual construction circumstances in Japan in the 1980s incited a team of engineers to develop a smart concrete material, which was later named as self-consolidating concrete (SCC). SCC possesses the ability to take form shapes and pass flow through congested reinforcement bars without any mechanical aid, thus eliminating the risk of concrete honeycombing and other defects resulting from poor compaction [1]. As a result of its phenomenal fluidity, SCC easily flows through obstructions and narrow sections to fill-in the forms by its self-weight, yet free of any objectionable segregation or bleeding. While the material components of SCC are generally the same as for conventionally vibrated concrete (CVC), the major difference lies in relative quantities of the component materials, such as the

⇑ Corresponding author. Tel.: +966 (3) 860 2572; fax: +966 (3) 860 2879. E-mail address: [email protected] (S. Ahmad). http://dx.doi.org/10.1016/j.conbuildmat.2014.06.096 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.

high amount of superplasticizer (SP) for the required level of flowability, the high powder content to act as ‘‘lubricant’’ for the coarse aggregates, and high fine aggregate content to assist workability and enhance stability of the mixture against segregation [2]. Given the aforementioned peculiarities of SCC, for those in Europe, India and other parts of the world where filler materials, like fly ash (FA) and silica fume (SF) are available at little or no cost, the only cost raising component in SCC remains the SP (and in some cases stabilizer also). However, in the regions where these materials are not available, the cost of SCC increases – thus making it uneconomical. Therefore all attempts to develop SCC with alternative locally available materials will always count as welcome developments towards making the SCC economical. Some of the prospective materials for use as mineral filler for the production of SCC include: limestone powder (LSP), calcined clay/metakaolin (MK), and natural pozzolana (NP). Limestone powder (LSP) is a by-product (quarry dust) of the quarrying process of carbonate rocks; hence its main component

S. Ahmad et al. / Construction and Building Materials 68 (2014) 268–276

is calcium carbonate, CaCO3. Although LSP does not possess pozzolanic property [3,4], its use in concrete offers many technical benefits, among which are increase in early strength and bleeding control [4,5], improvement of the concrete workability and excellent densification of concrete microstructure [4–6]. LSP improves the deformability and viscosity of SCC, as well as reduction of SCC porosity. Because of these positive impacts of LSP on the properties of SCC coupled with its economic benefits, it forms a major component of many SCC mixtures. However, there are little or no published information on the performance of SCC made with the limestone quarry dust (herein denoted as LSP in the paper). Metakaolin (MK), a product of calcination of kaolinitic clays, is another effective pozzolana which improves strength and durability properties of concrete [7–13]. Recent studies on the incorporation of MK in SCC mixtures [8,9] showed that MK improves the compressive strength and durability of SCC mixtures, but may raise the plasticizer requirements when large quantities are used [8]. Natural pozzolana (NP) is a raw or calcined natural material having pozzolanic properties, which are among the oldest construction materials [14]. A natural pozzolana of volcanic origin is usually composed of mainly silica and alumina with low contents of calcium and iron oxides. Though the pozzolanic activity of NP is slow according to several studies [14–16], its beneficial contribution in improving the durability characteristics and ultimate strength [14,17–19] had made it the focus of several studies on the development and performance of concrete mixtures. However only few studies [9,19,20] have recently considered the use of NP in SCC. Silica fume (SF) is a byproduct generated from the carbothermic reduction of quartz and quartzite in electric arc furnaces in the production of silicon and ferrosilicon alloys [21]. This siliceous material, containing 85–95% SiO2 with very fine vitreous particles [21], is so popular with its ability to improve the strength and durability properties of concrete to the extent that many modern high-performance concrete mixtures incorporate SF as an important admixture [22]. Several studies [23–26] on the fresh and mechanical properties of SCC incorporating silica fume have been reported in the literature. However, the cost of SF is very high in regions because of importation factors, and as such many local researchers have sought to get locally available replacements for producing concrete. The aim of this study was to explore the feasibility of producing high performance SCC utilizing the combinations of the filler materials that have not been commonly utilized thus far. This will consequently help in reducing the cost of production of SCC. Four different combinations of the mineral fillers, namely LSP, SF, NP and MK were considered in this study. Since LSP is commonly used as mineral filler in SCC, it was common in each of the four combinations of the mineral fillers. The mechanical properties (compressive strength, splitting-tensile strength, bond strength and modulus of elasticity), drying shrinkage behavior, and durability characteristics (water permeability, rapid chloride permeability (RCP), electrical resistivity and corrosion resistance) of the developed SCC mixtures were investigated and the experimental results presented and discussed. 2. Experimental Program 2.1. Materials Type I cement, complying with ASTM C 150 was used. The SF used in this study was sourced from Saudi Ready Mix Company, a local supplier. The NP used was a volcanic ash, complying with the specifications of ASTM C 618, obtained from local volcanic sites in the eastern province of Saudi Arabia. The LSP was sourced from a local limestone quarry. Raw clay from a local source was calcined to obtain MK. The clay was thermally activated in a furnace at 850 °C and then ground with laboratory pulverizer to a fineness of passing #100 (150 lm) sieve. The properties of powder materials (cement and mineral fillers) used are shown in Table 1.

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The coarse aggregate used in this study was crushed limestone sourced from a local quarry. It had a maximum aggregate size of 20 mm, specific gravity of 2.60 and water absorption of 1.4%. Dune sand was used as fine aggregate. Its specific gravity was 2.56, while its water absorption was 0.4%. Fig. 1 shows the grading of coarse and fine aggregates used. A new generation polycarboxylic-based ether hyperplasticiser (superplasticizer (SP)) was used in all the trial mixtures while the stabilizer/viscosity modifying admixture (VMA) used was an aqueous solution of a high-molecular weight synthetic copolymer. Both the SP and VMA were kindly supplied by Saudi BASF for Building Materials Co., Ltd., Al-Khobar. The SP and VMA properties are also shown in Table 1. 2.2. Mixture parameters In this study, the properties of four SCC mixtures prepared utilizing LSP and other three mineral fillers were studied. Table 2 shows the parameters used in the preparation of four SCC mixtures. As can be seen from Table 2, mix design parameters were fixed for all the four SCC mixtures, except the quantities of superplasticizer and stabilizer dosages required for each mixture to achieve self compactability. These dosages were obtained by trials on the concrete mixtures until the rheological parameters attained satisfactory levels. Table 3 shows the designation used for the selected four SCC mixtures. The weights of constituent materials for producing one cubic meter of SCC mixtures, calculated using the absolute volume method, are presented in Table 4. As indicated in Table 4, the mixture L-20 was used as a control mixture, containing 20% LSP, while the other three mixtures contain other fillers in binary combination with LSP, still maintaining the total filler content of 20% of the total powder. Unlike the other three mixtures in which each of the two fillers were 10% of the total powder content, LM-15-5 was made with only 5% MK (and 15% LSP to keep total filler same 20% like others) due to the low mixture workability observed in the preliminary work with 10% MK. As more than one trial was made on each of the trial mixtures in order to achieve the SCC mixtures with acceptable self-compactability, only the optimum dosages of VMA and SP for each mixture were shown in Table 4. 2.3. Evaluation of self-compactability Three self-compactability tests were performed on each of the SCC mixtures. These tests included: slump flow and V-funnel test for filling-ability and U-box test for passing-ability. All these three tests were conducted in accordance with the guidelines provided by EFNARC for SCC [27]. Segregation resistance was evaluated by visual judgment. According to EFNARC [27], visual observation of a flowing concrete on the flow table can offer some indication of its segregation resistance. Emphasis was laid on observing band of mortar or cement paste without coarse aggregate at the perimeter of the pool of concrete on the flow table. In line with this, ‘mortar band width’ criteria were set, based on long-term segregation behavior of previously tested mixtures. The ‘mortar band width’ criteria employed in this study, for classifying mixtures with respect to their stability, is shown in Table 5. 2.4. Evaluation of hardened properties Mechanical properties were evaluated in terms of compressive strength, splitting-tensile strength, bond (pull-out) strength and static modulus of elasticity. The drying shrinkage of the SCC specimens was also assessed. The durability characteristics of the developed SCC were evaluated by measuring water permeability, rapid chloride permeability (RCP), and electrical resistivity. Further, reinforcement corrosion was monitored by measuring corrosion potential and corrosion current density of reinforced concrete ‘lolly-pop’ specimens frequently for about 15 months while exposing them to chloride solution during this period. Table 6 summarizes the specimen size and test methods for evaluating the mechanical properties and durability characteristics of the selected SCC mixtures.

3. Results and Discussion 3.1. Self-compactability The flow characteristics of the developed mixtures are shown in Table 7. The optimum SP and VMA dosages that gave these acceptable flow properties were used in preparing the respective test specimens for evaluating the mechanical properties and durability characteristics. From the flow results in Table 7, it can be seen that L-20, ML-S10 and LN-10-10 required nearly same volume of SP and VMA. However, LM-15-5 required an increased volume of SP and VMA. Consequently, except a clear benefit that can be seen in this blend over others in terms of mechanical properties and/or durability

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Table 1 Chemical composition and physical properties of powder materials and chemical admixtures. Component

% by mass

CaO SiO2 Al2O3 Fe2O3 K2O Na2O MgO Specific gravity Chloride content

Cement

Silica fume

Natural pozzolana

Limestone powder

Metakaolin

SP

VMA

64.35 22.00 5.64 3.80 0.36 0.19 2.11 3.15 –

0.48 92.5 0.72 0.96 0.84 0.50 1.78 2.25 –

8.06 42.13 15.33 12.21 0.84 2.99 8.50 3.00 –

45.70 11.79 2.17 0.68 0.84 1.72 1.80 2.60 –

– 46.37 15.37 6.66 1.76 0.95 4.58 2.00 –

– – – – – – – 1.08 ± 0.02 60.1%

– – – – – – – 1.1 ± 0.01 <0.1%

Fig. 1. Grading of fine and coarse aggregates.

Table 2 Parameters used in the preparation of four SCC mixtures. Parameter

Amount

Cement content Total mineral filler content Total powder (cement and filler) content Water/powder (w/p) ratio Sand/total aggregate ratio Combinations of mineral fillers Superplasticizer (SP) dosage Stabilizer (VMA) dosage

400 kg/m3 (constant) 100 kg/m3 (constant) 500 kg/m3 (constant) 0.30 by mass (constant) 0.50 by mass (constant) 4 (details shown in Table 4) Determined through trials Determined through trials

Table 3 Designation used for the selected four SCC mixtures. Mixture ID

Mineral fillers

L-20 LS-10-10 LN-10-10 LM-15-5

20% 10% 10% 15%

limestone limestone limestone limestone

powder powder and 10% silica fume powder and 10% natural pozzolana powder and 5% metakaolin

performance, economical SCC utilizing MK may need to have a w/ cm ratio greater than 0.30 used in this study.

compressive strengths at all ages higher than that of the mixture with LSP alone as mineral filler (L-20), which was considered as control mixture in this study for comparison. The better performance of the SCC mixtures with blends of more than one mineral filler can be attributed to the fact that LSP alone does not possess sufficient pozzolanic property [3,4]. On the contrary, when half the quantity of LSP in L-20 was replaced by other fillers, the improvement in compressive strength at all ages is obvious. The fact that other mixtures show better performance indicates that the other filler materials (SF, NP and MK) do possess pozzolanic properties, though to varying degrees and activity, concurring with the findings of previous studies [7–14,17–19,28]. The LSP-SF mixture (LS-10-10) can be seen to have the overall best performance. This is in agreement with the established fact in the literature that silica fume is not only highly pozzolanic, but its pozzolanic reaction is fast [22–26]. The LSP-MK mixture (LM-15-5) showed a potential of performing close to LS-10-10 at 7 days, but its activity receded afterwards. However, at 90 days the compressive strength of LM-15-5 is similar to that of LS-1010. Thus, it can be speculated that if the MK content is increased to 10% like in LS-10-10, it may have a similar strength behavior. Unfortunately, the SP requirement for 10% MK was too high in the preliminary studies, and that was why its quantity was limited to 5% of the total powder. The post 28-day strength gain rate in LM-15-5 is quite encouraging, as its 90-day compressive strength slightly exceeds that of LS-10-10. This shows that LSP may improve the hydration reaction after 28 days. Therefore, the use of LSP considerably increased the concrete strength once it is used in conjunction with a pozzolanic material, and its other benefits, such as bleeding control [4,5], improvement in concrete deformability in the fresh stage [4–6], in addition to its low cost makes LSP important in formulating SCC mixtures. It was also shown by MIP analysis [3] that its filling effect in concrete microstructure (by making the ITZ denser) is so enormous that the porosity of LSP containing concretes was lower than that of traditional concrete. High pozzolanic activity of MK has been reported in the literature [8,10,11,29]. It improves both mechanical properties and durability characteristics of concrete, though its performance is determined by the kaolinitic purity level of the source clay. 3.3. Splitting-tensile strength

3.2. Compressive strength Fig. 2 shows a graph of compressive strength evolution with curing time. The plotted values of compressive strength are averages of three specimens prepared from each mixture. It should be stated that the variation in the strength of the individual specimens was within the acceptable range (with standard deviation within 3 MPa). As can be seen from Fig. 2, all three SCC mixtures with blends of LSP with other mineral fillers (SF, NP and MK) have

The splitting-tensile strength of concrete is an important mechanical property that greatly affects the size and extent of tension related failure behavior, such as flexural cracking in beams, inclined cracking from shear and torsion, and splitting resulting from rebar interaction with surrounding concrete [30]. Fig. 3 shows the 28-day splitting tensile strengths of the SCC specimens. The values in boxes close to the top of each bar represent the percentages of the control (L-20) for each mix. From Fig. 3, it is clear

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S. Ahmad et al. / Construction and Building Materials 68 (2014) 268–276 Table 4 Weights of the constituent materials for one cubic meter of the SCC mixtures. Materials 3

Cement (kg/m ) Silica fume (kg/m3) Natural pozzolana (kg/m3) Limestone powder (kg/m3) Metakaolin (kg/m3) Total powder (kg/m3) w/p Water (kg/m3) Paste content (% by volume) Coarse aggregate (kg/m3) Dune sand (kg/m3) SP (% of powder) VMA (% of powder) SP (kg/m3) VMA (kg/m3)

L-20 (control)

LS-10-10

LN-10-10

LM-15-5

400 – – 100 – 500 0.3 165.1 34.59 837 837 2.00 1.25 10.0 6.25

400 50 – 50 – 500 0.3 165.0 34.91 833 833 1.80 1.50 9.0 7.5

400 – 50 50 – 500 0.3 165.1 34.41 840 840 1.90 1.50 9.5 7.5

400 – – 75 25 500 0.3 164.9 35.30 828 828 3.25 1.00 16.25 5.0

that the splitting-tensile strengths of these mixtures are not too sensitive to variations in the compressive strength. This can be explained by the fact that each mixture contains unique combinations of fillers and so variations in the resulting tensile strengths may not reflect proportionately to the variations in compressive strengths. Nevertheless, the pure LSP blend (L-20) with the least compressive strength has the least tensile strength, as expected. Fig. 4 shows the relationship between the 28-day splitting-tensile strength, fct, and the 28 days compressive strength, f0 c, for the developed SCC mixtures. From Fig. 4, it can be seen that the values of cylinder splitting-strength range between 5.9 and 6.4 MPa, corresponding to compressive strengths (cylinder equivalent) of 52.9– 65.3 MPa. These splitting-strength values are obviously higher than the values for CVC of similar compressive strengths, as represented by upper limit of ACI 318 [31]. However, it was established elsewhere [32], that the split-tensile strength of SCC is higher than that of CVC of equivalent compressive strength. SCC cylinder splitting-strengths of about 2–6 MPa have been reported in the literature, corresponding to compressive strengths of about 20– 80 MPa [33]. Also from Fig. 4, the model presented by Felekog˘lu et al. [34] produces about 21% lower values of splitting-tensile strengths, as compared to the experimental values obtained in this study. This may be due to factors like differences in component material types and the limited data they used as a basis for their model. With the experimental data obtained in the present study, a power relationship exists between the compressive and splitting-tensile strengths, as shown in Fig. 4.

3.4. Modulus of elasticity Because of the concerns that SCC may have a lower elastic modulus [35,36], which may constitute deflection problems and prestress losses in prestressed elements [37], it is a common practice to assess the elastic modulus of the developed SCC mixtures. This research also covered the investigation of modulus of elasticity of the developed SCC mixtures. Fig. 5 shows the 28-day modulus of elasticity (chord modulus) of the developed SCC mixtures, Ec. The value on each bar represents the modulus elasticity, while those in boxes are their corresponding percentages of the control mix. A look at Fig. 5 reveals that all the SCC mixtures studied have similar modulus of elasticity, in spite of their different compressive strengths. The same explanation given previously for the tensile strength variations (Section 3.3) holds for this case as well. The relationship between the modulus of elasticity of the SCC mixtures, Ec, and the corresponding compressive strengths, fc0 , both

at 28 days is presented in Fig. 6. From Fig. 6, the experimental values of the modulus of elasticity can be seen as being similar to those obtained from the relationship presented by Felekog˘lu et al. [34]. Both show values of the moduli of elasticity higher than those of CVC of similar compressive strengths, on the basis of the ACI 318 recommendations [31]. This is a positive outcome, as the concerns of lower elastic modulus exhibited by some SCC mixtures [35,36] is not an issue here.

3.5. Bond strength Since the assumption of strain compatibility between steel and concrete forms an integral part of the foundation upon which the design of reinforced concrete structures are based, concrete-steel bond is an important parameter to be given due attention in the development of any concrete material. Further, defects in concrete-steel bond may also expose the steel to corrosion [33]. The bond strength developed by a rebar embedded in concrete is greatly controlled by the quality of the concrete and its compressive and tensile strengths [38]. Fig. 7 shows the 28-day bond strength of SCC specimens for the mixtures studied, while Fig. 8 shows the 28-day compressive strength (fc0 ) and 28-day bond strength (fcb) of the SCC mixtures as percentage of the control mixture. From Fig. 7, it can be seen that LSP-SF blend (LS-10-10) exhibited the highest bond capacity. This would be expected for these other mixtures, owing to their lower compressive and tensile strengths than the LSP-SF blend, having stated earlier that the compressive and tensile strengths of concrete both play roles in controlling its bond behavior. Looking at Figs. 7 and 8, it is easy to see why the pure LSP blend (L-s20) develops lower bond strength than the rest, as both the tensile and compressive strengths are obviously lower. But in the case of the LSP-NP blend (LN-10-10), the extent of reduction in ultimate bond capacity is not justified by its compressive and tensile strengths. Hence, it may be right to think in the direction of top-bar effect, since it has been established to be a major player in a mixture’s bond behavior [33,38–41]. A check on its plastic stability reveals it to be one of the most stable mixtures. Thus, it may be noted that the short-term stability observed in the fresh state was probably not sustained in the long run, since segregation is sometimes a time dependent phenomenon [27]. The same explanation may be offered for the LSP-SF blend (LS-10-10) whose slight extra bond strength cannot be justified by its large margin of compressive strength above that of the control mixture (Fig. 6). Fig. 9 shows the relationship between the 28-day bond strength, fcb, and the corresponding compressive strengths, fc0 , for

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Table 5 Segregation resistance criteria used for screening the mixtures. Mortar band width

Segregation class

No mortar band observed Band width 6 10 mm 10 mm < Band width 6 20 mm 20 mm < Band width 6 50 mm Large mortar band > 50 mm

None Negligible Low High Severe

the SCC mixtures. Also in Fig. 9 are the plots of relationship between fcb and fc0 obtained by the models of Orangun et al. [42] and Chapman and Shah [40] for CVC. From Fig. 9, it can be seen that, although there seems to be no good correlation between the measured values of fcb and fc0 , the bond strength values are higher for all mixtures than the predicted values computed using the equations of Orangun et al. [42] and Chapman and Shah [40]. This observation is in conformity with the information obtainable from the literature. SCC has been said to have better bond behavior than CVC [2], which is attributable to the good interlocking of aggregates and higher volume of paste in SCC [43]. SCC bond behavior is not only better than that of CVC, but the so called ‘top-bar effect’ is also lower in SCC than in CVC [44]. It is expected that a well formulated SCC will be so stable that the amount of bleeding will be very minimal [33], and plastic settlement under the bars will be lower, owing to its self compactability properties.

3.6. Water permeability The water penetration depth is a reliable durability assessment test [45]. A lower durability is expected for a concrete material with a higher water penetration depth. Fig. 10 shows the water penetration depth measured after 28 days of curing of the specimens prepared using the developed SCC mixtures. The values on each bar represent the water penetration depth in mm, while those in boxes are their corresponding percentages of the control mix. The low water penetration depth of L-20 L-S-10-10 can be justified easily from the constituent properties. LSP has an excellent filling effect in the concrete microstructure (by making the ITZ denser) [3,4], and that is why the porosity of LSP containing concretes is lower than that of traditional concrete, as confirmed by MIP analysis [3]. Hence, a lower water penetration depth is expected in an LSP containing concrete [18]. SF on the other hand is a highly pozzolanic siliceous material with very fine vitreous particles, which improves strength and durability properties of concrete [21]. Furthermore, both NP [18,45] and MK [7,10–12] have been found to improve the durability properties of concrete. Therefore, all these mixtures are expected to have low water penetration as a result of the durability improvement characteristics of their fillers. The 162% penetration relative to the control obtained in LM15-5 cannot be seen as too high if the actual value itself is considered (11.7 mm). This value is less than half way the limit for a low penetration class of concrete.

3.7. Chloride permeability Because of long testing time of the traditional chloride penetration test, rapid testing methods have been formulated over time. The most commonly used of them is the rapid chloride penetration test (RCPT) method of ASTM C1202 [46]. Although it is been criticized over time [47], as being misleading in some cases, it is still very popularly used and a strong correlation between the RCPT results and the 90-day ponding test have been established for many scenarios [10,48].

Fig. 11 shows the rapid chloride permeability of the developed SCC specimens. The values on each bar represent the charge passed, in Coulombs, while those in boxes are their corresponding percentages of the control mix. From Fig. 11, it can be seen that the LSP-SF blend (LS-10-10) showed a ‘negligible’ level of chloride permeability, which is consistent with the expected action of SF. Also, LN-10-10 with higher water penetration depth (Fig. 10) than the control is seen here to also display more quantity of chloride ions passing. However, the LSP-MK blend (LM-15-5) showed the highest water penetration depth, while it shows a better resistance to chloride ion penetration than the control here. This anomaly can easily be attributed to the fact that the chemistry of concrete pore solution of these mixtures varies drastically, and such an anomaly as noticed here is inevitable since the result of this type of chloride permeability test is highly dependent on the pore solution chemistry [47]. Therefore, the important observation here is that all the developed SCC mixtures are highly durable (in terms of resistance to chloride penetration), as they belong to ‘very low’ (and for LS-10-10, ‘negligible’) chloride permeability class as per ASTM C1202. 3.8. Electrical resistivity Since corrosion is an electro-chemical process, the flow rate of the ions through concrete between the anodic and cathodic areas of a depassivated rebar embedded in concrete determines the rate at which corrosion can occur in that rebar. This flow rate of ions is affected by the resistivity of concrete [49]. Therefore, measuring the electrical resistivity of concrete could provide an indication as to the likelihood of corrosion taking place [50]. Fig. 12 shows the electrical resistivity of the SCC specimens at 3% moisture content. The arrow with dashed line on the graph shows the lower limit for ‘Low to Negligible’ likelihood of reinforcement corrosion. Going by the criteria [49,51], all the blends studied have ‘low to negligible’ likelihood of corrosion on a depassivated steel at 3% moisture content. This agrees with the previous observations made on the corrosion performance of these mixtures in terms of water permeability and chloride ion permeability. With reference to Fig. 12, obviously the highly resistive mixture (LS-10-10) is expected, as it was noticed in other durability indices. However, the higher resistivity of LM-15-5 than all others is unexpected, based on the water permeability results. Consequently, both the RCPT and electrical resistivity for LM-15-5 are favorable, in spite of unfavorable water permeability values. This discrepancy will not be surprising given the fact that the values of electrical parameters, such as resistivity, is affected by the chemistry of the concrete pore solution [47], and since the other fillers, apart from LSP, are not the same in each mixture, the resulting concrete pore solution are bound to be chemically different from each other, and consequently, should be expected. Attempting to correlate the measured resistivity to the corresponding moisture content, an exponential relationship, of the form: q(m) = Cekm, was found to give the best fitting relationship. C and k are constant for each mixture, and m is the moisture content. The electrical resistivity – moisture content curves are shown in Fig. 13 for the developed SCC mixtures. Table 8 shows the summary of the obtained correlation parameters and corresponding electrical resistivity in kO cm. 3.9. Corrosion potentials Fig. 14 shows the variation of corrosion potentials with exposure time on steel in the SCC specimens exposed to 5% NaCl solution. Just on the basis of the ASTM C876 [52] threshold potential of 270 mV SCE, the corrosion potentials curves in Fig. 14 indicate around 310 days for the corrosion initiation of the rebar embedded

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Table 6 Details of the specimens and test methods utilized to determine the mechanical properties and durability characteristics of the developed SCC mixtures. Property

Test standard

Specimen size

Age at test/exposure

Compressive strength Tensile strength (split) Modulus of elasticity Bond strength Water permeability Chloride permeability Electrical resistivity Drying shrinkage Corrosion potential

BS EN 12390-3 ASTM C 496 ASTM C 469 Pull out test DIN 1048 ASTM C 1202 2-electrode method ASTM C 157 ASTM C 876

Corrosion current density

Linear polarization method

100 mm cube 75  150 mm cylinder 75  150 mm cylinder 12 mm diameter bar centrally embedded in 150 mm cube 100 mm cube 75  150 mm cylinder 75  150 mm cylinder 50  50  250 mm Prism 12 mm diameter bar centrally embedded in 75  150 mm cylinder 12 mm diameter bar centrally embedded in 75  150 mm cylinder

3, 7, 14, 28 and 90 days 28 days 28 days 28 days 28 days 28 days 28 days 28 days Exposed to chloride solution after 28 days of curing Exposed to chloride solution after 28 days of curing

Table 7 Flow characteristics of the SCC mixtures. Mixture ID

L-20

LS-10-10

LN-10-10

LM-15-5

SP (% of powder) VMA (% of powder) Flow table (650–800 mm) V-Funnel time (6–2 s) U-Box (0–30 mm) Bleeding (visual) Segregation (visual)

2.00 1.25 680 11.0 5.0 None Negligible

1.80 1.50 660 7.0 6.0 None None

1.90 1.50 690 11.0 0.0 None None

3.25 1.00 760 9.0 3.0 None None

Fig. 4. Relationship between splitting-tensile and compressive strengths of the SCC mixtures.

Fig. 2. Evolution of compressive strength of the SCC mixtures.

Fig. 5. 28-day modulus of elasticity of the SCC mixtures.

Fig. 3. 28-days splitting-tensile strength of the SCC mixtures.

in the LSP-MK blend, while all other blends are still significantly below the initiation threshold up to the last measurement made after an exposure period of 450 days.

3.10. Corrosion current density The variation of corrosion current density, Icorr, with time of exposure to 5% NaCl solution is presented in Fig. 15. As indicated in Fig. 15, a threshold Icorr value of 0.30 lA/cm2 is taken as the point

Fig. 6. Relationship between Ec and fc0 for the SCC mixtures.

at which the probability of initiation of active corrosion becomes very high. Going by this criterion, all the mixtures are still inactive as the last measurement time (450 days from exposure). However, looking at the continuous steepening of the Icorr – time curve for the LSP-MK blend at around 300 days of exposure, it may be said

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chloride permeability. Below 100 Coulombs chloride permeability.

Fig. 7. 28-day bond strength of the SCC mixtures.

Fig. 11. Rapid chloride permeability of the SCC mixtures.

Greater than 20 kΩ-cm

reinforcement corrosion

Fig. 12. Electrical resistivity of the developed SCC mixtures at 3% moisture content. Fig. 8. fc0 and fcb of the SCC mixtures as % of the control mixture.

Fig. 9. fc0 and fcb of the SCC mixtures as % of the control mixture.

30 mm water penetration depth as upp

presents a strange observation that prompts detailed analyses, as the LSP-NP blend had consistently the least favorable durability indices of rapid chloride permeability (Fig. 13) and electrical resistivity (Fig. 14). This observation can probably be explained by an excellent corrosion resistant pore solution chemistry of the LSPNP blend, a claim that definitely requires further studies for justification. The delayed initiation in the pure LSP mixture is justified on account of the superior filling ability of LSP as discussed earlier, while the inclusion of the excellently fine and pozzolanic SF in the LSP-SF blend would definitely confer it much better corrosion resistance. A reverse explanation to the LSP-NP blend situation discussed above can be offered for the LSP-MK blend, the only mixture that witnessed corrosion initiation so far. Though it has the highest water penetration depth in all the mixtures studied, the actual value of 11.4 mm (Fig. 10) is not too far from the other mixtures. This is an addition to the fact that all other durability indices are in the range of others. This means the chemical make-up of the pore solution has very low corrosion-resistant property. This shows again that the pore solution chemistry is probably more dominant than other indices in the corrosion resistance of these mixtures.

3.11. Drying shrinkage

Fig. 10. Water penetration depth of the SCC mixtures.

that the trend noted in corrosion potential measurements (Fig. 14) is repeated here, in which the LSP-MK blend is the only mixture that crossed the threshold value of corrosion initiation, while all other mixtures still remained passive after 450 days of exposure. A noteworthy point at this juncture is the fact that the LSP-NP blend remains inactive to date, at 450 days of exposure. This

High shrinkage of SCC used to be one of its weaknesses, owing to its higher paste volume [36,53], and lower coarse aggregate content [53]. This is because earlier developers of SCC relied on very high paste volume to achieve high flow and stability. However, recent advancements in SP and VMA and the use of optimized particle packing for achieving stability in SCC have led to the improvements in SCC shrinkage behavior. Consequently, Güneyisi et al. [54] obtained the ultimate drying shrinkage for several SCC mixtures in the range of about 350–530 microstrains against a control CVC mixture having about 475 microstrains. The evolution of drying shrinkage with time is depicted in Fig. 16. As can be seen from Fig. 16, right from the beginning up

S. Ahmad et al. / Construction and Building Materials 68 (2014) 268–276

275

Fig. 16. Evolution of drying shrinkage with time.

Fig. 13. Electrical resistivity–moisture content curves for the SCC mixtures.

Table 8 Correlation parameters and electrical resistivity of the SCC mixtures. MIX ID

C

k

R2

q (m) = Cekm, kO cm

L-20 LS-10-10 LN-10-10 LM-15-5

9593 7253.5 22,004 21,246

1.88 1.508 2.173 2.069

0.9843 0.9855 0.9780 0.9982

34 79 32 43

-270 mV SCE is the threshold potential for initiation of active corrosion of reinforcement

concluded that, for a water-cured SCC specimen containing LSP, the high level of filling of the concrete microstructure by LSP tends to cause more shrinkage, since the curing water only penetrates the outer layers of the specimen, exposing the interior part to self-desiccation. This may be further justified by the LSP-NP and LSP-MK blends having somewhat less shrinkage at the early stages. However, at later stages of drying, all the mixtures, except the LSP-SF blend, converge to nearly the same ultimate shrinkage strains. As at 240 days of drying, all the blends have slightly more than 300 microstrain of drying shrinkage, and they are all getting stabilized to their ultimate values. These shrinkage strains are far lower than those of many CVC mixtures. 4. Conclusions Based on the experimental data obtained in this study, the following conclusions can be drawn:

Fig. 14. Variation of corrosion potentials on steel in the SCC mixtures.

to the 240 days of exposure, the LSP-SF blend consistently shows the least drying shrinkage. This can be attributed to the faster pozzolanic reaction of SF which consumes more of the mixing water to form secondary hydration products to fill the concrete microstructure. At the other extreme, the pure LSP blend suffered the highest drying shrinkage at early stages of exposure. Valcuende et al. [55]

2 is the threshold value of Icorr for initiation of active corrosion of reinforcement

Fig. 15. Evolution of corrosion current density (Icorr) with time.

1. The SCC mixtures made with blends LSP and other local fillers gave high early strengths, with 3 day compressive strengths in the range of 39–52 MPa while their 28 day strengths ranged between 64 and 78 MPa. The average elastic moduli of the studied SCC mixtures ranged from 40 to 42 GPa, while the splittingtensile and pull-out bond strengths ranged between 5.9– 6.4 MPa and 31–33 MPa, respectively. 2. For the SCC mixtures studied, results showed that the elastic modulus, tensile strength and bond strengths do not necessarily correlate proportionately with the compressive strength. Although the bond strength increases with compressive and tensile strengths, the long term stability of an SCC mixture is a dominant determinant of its bond strength. 3. All the SCC blends have slightly more than 300 microstrain of drying shrinkage at 240 days of exposure to drying, which are far lower than those of many CVC mixtures. 4. The water penetration depths for all mixtures studied were far below 30 mm on the average, while the RCPT indicated that all the mixtures have ‘very low’ chloride permeability, in addition to their ‘very high’ electrical resistivity, indicating ‘low to negligible’ probability of corrosion taking place on a depassivated steel embedded in these mixtures. 5. Apart from the LSP-MK blend, which showed the time to initiation of corrosion of around 300 days of exposure to 5% NaCl, rebars in all other SCC mixtures remained in a passive state even after 450 days of exposure. 6. Though the LSP-SF blend showed the best performance in mechanical and durability terms, the performance of blends of other fillers were also acceptably very good, showing that the studied fillers can replace silica fume in the future reducing the cost of production of SCC.

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7. Excellent mechanical properties and the high corrosion resistance exhibited by most of the developed mixtures are a pointer to the fact that suitable high-performance SCC mixtures can be developed utilizing the investigated fillers which can be adopted for aggressive exposure conditions.

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