Keywords

1 Introduction

Approximately 5–8% of the global CO2 emissions is attributed to the production of ordinary Portland cement (OPC) [1]. To reduce the carbon footprint of the concrete industry, substitution of cement by supplementary cementitious materials (SCM) without compromising performance is an efficient solution [2]. Another indirect way to contribute to sustainability is prolonging the service life of infrastructure by using durable concrete. From these two viewpoints, ground granulated blast-furnace slag (GGBFS) and fly ash (FA) are great choices among other SCMs. One of the major benefits of blending OPC with GGBFS or FA is improved resistance to chloride penetration, which has been evidenced by both short-term laboratory tests and long-term field tests. The former includes the widely used rapid chloride migration test method standardized in ASTM C1202 [3] and the chloride diffusion test given in NT Build 492 [4]. However, for application, concrete structures need to be designed for a specific service life, and this requires long-term quantitative field performance assessment, which is not always practical [5]. Therefore, finding the link between the results from laboratory and field tests is important for promoting efficient use of LCCs, but is nevertheless challenging because a high concentration of deleterious species in laboratory tests could have already altered the deterioration processes and the laboratory curing conditions deviate significantly from on-site conditions. The purpose of this paper was to review of the factors affecting chloride penetration in LCC made with GGBFS or FA and the correlations between results from different test methods.

2 Chloride Penetration in LCC

2.1 Effect of SCM Content on Diffusion Coefficients

The replacement of cement with SCM is usually no more than 50% for GGBFS and 30% for FA, due to the reduction in strength with increasing SCM content, as shown in Fig. 1, in which the short-term laboratory test results from Dhir et al. [6] were adopted to demonstrate the effect of GGBFS or FA content on D as well as the 28 day cube strength. It can be seen that the diffusion coefficients continuously decrease with increasing GGBFS content up to 65%; in the range of 30–50%, D is insignificantly affected by FA content but the strength reduction is more pronounced at higher FA dosages. On the other hand, for high-volume FA (HVFA) concrete with >50% FA as cement replacement and a considerably high amount of superplasticizer, the HVFA concrete has proven to yield higher long-term strength and resistance against chloride penetration than OPC concrete, despite the early-age properties of the former being less competitive [7,8,9,10]. Thomas et al. [11, 12] found that HVFA concrete with 50% FA had a significantly lower D and a slightly higher compressive strength than OPC concrete after being exposed to the field marine environment for up to 10 years. Moreover, Moffatt et al. [13] reported the D of a HVFA concrete with 56–58% FA after 24 years of exposure to a harsh field environment where high tides and freeze–thaw cycles occurred, was only 1.5 × 10–13 m2/s compared with 3.6 × 10–12 m2/s for the counterpart OPC concrete.

Fig. 1
2 graphs depict cubic strength in megapascals, D subscript e times 10 power minus 12 in meters squared per second, chloride binding in milligrams per gram of sample, and intrinsic permeability by airflow in meters squared times 10 power minus 17 versus G G B F S and F A contents in percent by mass.

Effects of a ground granulated blast-furnace slag (GGBFS) and b fly ash (FA) content as cement replacement on chloride penetration and other properties assessed by short-term laboratory tests [6, 14, 15]

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2.2 Effect of Curing Conditions on Chloride Penetration

It has been well established that partial replacement of cement by GGBFS or FA improves the microstructure of concrete and thus the resistance to chloride penetration. In LCC, the GGBFS or FA reacts with calcium hydroxide (formed by the hydration of cement) and water to produce C–S–H and a portion of calcium aluminate phases [16,17,18]. Additionally, depending on the specific surface area of particles, GGBFS or FA can act as a filler to fill pores and as nucleation sites to enhance hydration [19, 20]. Apart from the products formed by normal cement hydration, the additional hydrates due to either enhanced hydration or hydration of GGBFS/FA reduce capillary porosity (>30 nm) and thus block the chloride diffusion paths; however, these benefits of GGBFS and FA depend on the curing conditions, especially for LCC with GGBFS [21]. Moreover, it is possible that blended cement concrete will perform no better than OPC concrete when structures are exposed to prolonged drying and carbonation [22]. Figure 2 compares the D of concretes cured in wet and dry conditions, from which it can be seen that OPC concrete with a low strength grade is more sensitive to the curing conditions, and the influence of curing conditions diminishes with increasing concrete strength and exposure duration [23, 24]. Irrespective of the curing conditions, at the same grade the blended concretes consistently outperform OPC concrete in resisting chloride penetration. In this regard, Bamforth [25] examined the D of dry-cured (indoor), membrane-cured and water-cured concrete blocks (40 MPa at 28 days) located in the splash/spray zone on the south coast of the UK for 8 years, and found that the effect of curing conditions on the D of different concrete mixes was inconsistent, but that 70% GGBFS-blended concrete yielded the lowest average D compared with 30% FA-blended concrete and OPC concrete. Additionally, a lower grade of blended cement concrete is more durable than a higher grade OPC concrete at the later age, which was also confirmed by Thomas et al. [11].

Fig. 2
Three graphs labeled a, b, and c depict D times 10 power minus 12 in meters per second of grades 40 and 60 in dry and wet conditions for O P C concrete, 50 percent G G B F S concrete, and 30 percent F A concrete.

Effect of curing conditions on the D of different grades of concrete exposed to the tidal zone of Cape Peninsula with water temperature between 12 and 15 °C. The wet-cured (wet) concrete was exposed to 6 days’ moist curing (23 °C and 90% relative humidity) after demolding, while the dry-cured (dry) concrete was stored in an open area (23 °C and 50% relative humidity). The concrete was exposed to marine environment at age 28 days [23]. GGBFS, ground granulated blast-furnace slag; FA, fly ash; OPC, ordinary Portland cement

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2.3 Effect of Test Methods on Chloride Penetration

There are only limited published data on relating laboratory test results to long-term field performance of concrete with regard to resistance to chloride penetration. Figure 3 shows the correlation between D from a long-term field test (Dfield on the x-axis) and counterpart D from laboratory diffusion tests (Dlab on the left y-axis) and coulombs (right y-axis) from the rapid chloride permeability test (RCPT), using data from previous studies [13, 26, 27]. Thomas et al. [26] conducted the RCPT as well as diffusion tests (using 16.5% NaCl as per ASTM C1556) on uncontaminated GGBFS-blended concrete exposed in the field to a tidal zone for 25 years, and calculated the chloride diffusion coefficient (Dfield) from the chloride profiles in the field-exposed concrete. Moffatt et al. [13] obtained Dfield and coulombs (RCPT) of high-volume FA concrete exposed to the marine environment for 19–24 years. Compared with the results from the RCPT, Dlab from laboratory diffusion tests following the procedures given in ASTM C1556, NT Build 443 or other similar procedures, could better indicate the ability of concrete to resist chloride penetration. Although there are synergies between the two diffusion coefficients (Dlab and Dfield), the quantitative relationship between them varies. Figure 4 shows the correlation between Dlab (x-axis) and coulombs from the RCPT (left y-axis), and the non-steady-state (Dnssm) or steady-state (Dssm) chloride migration coefficient (Dm on the right y-axis) from accelerated migration tests reported in previous studies [28,29,30,31,32]. Note that these previous studies used different NaCl concentrations and exposure durations, which are summarized in Table 1. When Dlab was used as the reference, the Dm > 2 × 10–12 (m2/s) and RCPT coulombs >800 could be more reliable for ranking concretes in terms of the resistance to chloride penetration.

Fig. 3
A graph of D subscript lab times 10 power minus 12 in meter square per second from laboratory diffusion tests and coulombs from R C P T versus D subscript field times 10 power minus 12 in meter square per second from marine environments.

Correlation between the D from marine exposure (Dfield) and the D from laboratory diffusion tests (Dlab) and coulombs from the rapid chloride permeability test (RCPT) [13, 26, 27]

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Fig. 4
A graph of coulombs from R C P T and D subscript m times 10 power minus 12 in meter square per second from migration tests versus D subscript lab times 10 power minus 12 in meter square per second from laboratory diffusion tests.

Correlation between the D from laboratory diffusion tests (Dlab) and coulombs from the rapid chloride permeability test (RCPT) and the chloride migration coefficient from migration tests (Dm) [28,29,30,31,32]. The gray lines indicate the RCPT results. The red lines indicate the steady-state chloride migration coefficient (Dssm) and blue lines indicate non-steady-state chloride migration coefficient (Dnssm)

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Table 1 Laboratory test methods for assessing resistance to chloride penetration shown in Fig. 4
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3 Conclusions

  1. (1)

    Replacing cement with up to 65% GGBFS or 30% FA improves the resistance of concrete to chloride penetration but decreases early-age strength development. LCC with GGBFS or FA could achieve higher resistance to chloride penetration at equivalent strength or binder content as compared with OPC concrete, indicating that efficient use of LCC requires a performance-based service life design approach.

  2. (2)

    The influence of curing conditions on chloride diffusion coefficients diminishes with increasing concrete strength grade, but could be significant for low-strength concrete. At strength grade ≥40 MPa, the difference in chloride diffusion coefficients arising from the change in curing conditions is much smaller in LCC than OPC.

  3. (3)

    Although short-term laboratory diffusion and accelerated migration tests are reliable in distinguishing parameters that affect chloride penetration in concrete, the correlations between results from different methods are difficult to establish.