SCM Research - Part 3

SCM Research - Part 3
Durability Performance of SCM Concrete

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A wide range of cementitious materials available in New Zealand were assessed for strength and durability. Research findings are presented from concrete containing GP cement from two cement suppliers (Golden Bay Cement and Holcim New Zealand) together with established supplementary cementitious materials (SCMs) such as fly ash and slag and natural pozzolans such as perlite, pozzolana and calcined clay.

A summary of the different concrete mixes used for durability testing is shown in Table 1 together with compressive strengths. All concrete mixes were designed with a total binder content of 350 kg/m³ and water/binder ratio of 0.45 and SCM details are given below:

S1 GP cement from either Golden Bay or Holcim cement
S2 Huntly fly ash (ASTM class C)
S3 Adani fly ash (ASTM class F)
S4 New Zealand pozzolana (Golden Bay Cement)
S5 New Zealand perlite (Blue Pacific Minerals)
S6 Calcined clay (55 percent kaolinite clay from Geraldine)

Table 1: Concrete mix designs and compressive strength results (Golden Bay Cement and Holcim New Zealand)

Concrete was tested for a range of durability properties with the following being reported in the article – chloride resistance, carbonation resistance and prevention of expansion associated with alkali silica reaction. All concrete was cured in water until tested with the exception of accelerated carbonation, which had either 3, 7 or 28 days wet curing before drying for 7-days and then exposure to 2.5 percent carbon dioxide.

MICROSTRUCTURAL ANALYSIS
Concrete was tested at 28 and 90 days for microstructural properties affecting transport mechanisms affecting durability. A summary of properties is shown in Table 1 and includes:

Porosity measured by drying and vacuum saturation in water with dense concrete typically having values below 10 percent while every dense concrete has values below 7.5 percent.
Oxygen permeability index (OPI) is a log scale where low permeability concrete has values typically above 10.0 while values below 11.0 indicates virtual impermeability.
Resistivity is measured electrically with dense concrete having values greater than 20 kOhm.cm that may restrict the rate of corrosion in reinforced concrete.

Table 1: Microstructural properties of concrete after 28 or 90 days curing

From this analysis some overall trends can be seen and are discussed below:

Fly ash concrete showed good microstructural quality after 90 days.
Concrete with pozzolana or perlite showed similar performance to fly ash concrete.
Concrete made with calcined clay had excellent microstructural properties.

CHLORIDE RESISTANCE

The chloride resistance of concrete was assessed using the rapid chloride migration test (NTB 492) with 28 and 90 day results compared in Figure 1. High levels of chloride resistance are typically achieved when concrete has migration coefficients below 5x10 ^-12 m²/s.

Figure 1: Rapid chloride migration coefficients for SCM concrete

Longer-term chloride migration coefficients of SCM concrete was generally much lower than that of the control concrete (except for mixes containing perlite). Concrete containing calcined clay (mixes H6 & H16) had consistently better chloride resistance than other SCM concretes at 28 and 90 days.

CARBONATION RESISTANCE

The carbonation resistance of concrete was assessed using EN 1920 that uses a high carbon dioxide concentration to accelerate the carbonation reaction in concrete. Concrete samples were exposed to 2.5 percent carbon dioxide for 56 days before being split open and the depth of carbonation using phenolphthalein indicator solution. Figure 2 shows a comparison of carbonation depth measured in the different concrete types exposed to three different levels of curing (e.g. 3, 7 or 28 days wet curing).

Figure 2: Accelerated carbonation depths after 56 days exposure

Carbonation depths were higher for SCM concrete than PC concrete controls regardless of curing duration. Extending wet curing from 3 days to 7 days had more beneficial effect in terms of carbonation resistance for SCM concrete than for similar PC concrete. Concrete containing SCMs has slower rates of maturity than PC concrete, which makes these materials more susceptible to poor curing practices.

ALKALI SILICA REACTION MITIGATION

Mitigation of expansion by SCM in concrete associated with alkali silica reaction was assessed in accordance with ASTM C1293. For this ASR testing, a high strength concrete mix was used with Portland cement content of 450 kg/m³, reactive andesite crusher sand and total alkali level of 5.25 kg/m³, achieved using admixed sodium hydroxide.

Figure 3: Expansion due to alkali silica reaction of concrete prisms stored at 50^oC

Interim results from this testing found that some SCMs where able to suppress expansion associated with alkali silica reaction. Assessing the ASR expansion results, the following was found after seven months testing with CPT50:

Concrete made with GP cement and no other SCM had significant expansion in the first three months that indicated no ASR mitigation.
Concrete made with class C fly ash or perlite allowed slow expansion that required several months to develop but showed significant expansion after six months.
Concrete made with other SCMs (class F fly ash, pozzolana, calcined clay or slag) showed insignificant expansion or slight shrinkage after seven months that showed excellent ASR mitigation.

CONCLUSIONS

The incorporation of SCMs into grade 40-50 MPa concrete mix designs was found to produce a range of performance when microstructural and durability properties were compared. The durability performance of SCM concrete significantly improved with additional moist curing and when tested at greater ages. Having said that, no generalised trend was found for all concrete types as the materials produce a range of properties due to physical and chemical differences.

All SCM showed the ability to improve chloride resistance over time provide concrete was well cured. Carbonation resistance of SCM concrete was generally poorer than concrete made with GP cement but it should be noted that comparisons were not made on the basis of standardised strength but were rather done using similar binder contents. ASR mitigation was improved when using SCM concrete especially when using class F fly ash, natural pozzolana or calcined clay (e.g. S3, S4 or S6).