Long-Lasting Concrete

Introduction

Hardened concrete has two primary expected properties: strength and durability. Strength is typically defined by the 28-day characteristic compressive strength as C30, C40, and so on—a property familiar to everyone. Durability means that concrete does not suffer significant damage from deteriorating effects from within or from the environment over time, and that it has a long economic life requiring no costly repairs. Damage can affect either the concrete directly or the steel reinforcement within it. Concrete production that uses the correct cement, aggregates, water, and admixtures in compliance with relevant standards, and that cures concrete in the proper temperature and moisture environment, will not experience harmful internal effects (alkali-silica reaction, delayed ettringite formation, and so on). Harmful effects on concrete from the environment are clearly defined in TS EN 206 [1] and in the national standards that supplement it (TS 13515, BS 8500, and others). These can be grouped into two main categories: • Physical effects such as freeze-thaw and mechanical wear that damage concrete, or chemical effects from soil, groundwater, sulfates, acids, and similar sources. • Effects such as carbonation and chlorides that cause reinforcement corrosion. One way to protect concrete and its reinforcement against external effects is to make the concrete sufficiently impermeable to prevent external factors from penetrating it, or to delay their penetration for the foreseen period (economic life). Another approach is to coat it with an impermeable layer (waterproofing, plaster, paint) or to add impermeability admixture to concrete to achieve the same goal. In foundation concrete in contact with soil and exterior wall concrete in high-rise buildings and similar large projects, both methods are frequently employed to protect against harmful environmental effects. Reinforcement can also be given corrosion-protective treatment (paint, cathodic protection).

Economic Life – Cover to Reinforcement

The concrete-specific solution is to protect the reinforcement and the core concrete (the cross-section of the structural element between the outer reinforcements) with a sufficiently thick and sufficiently impermeable concrete cover (cover to reinforcement). The thicker and more impermeable the cover to reinforcement, the longer the economic life; conversely, the more permeable and thinner it is, the shorter the economic life. TS 500 [2] defines cover thickness as the net concrete cover measured from the outer face of the outermost reinforcement and specifies a minimum of 50 mm for elements in direct contact with soil, a minimum of 25 mm for columns and beams exposed to weather, a minimum of 20 mm for columns and beams inside buildings not exposed to external effects, a minimum of 15 mm for wall panels and slabs, and requires increases as deemed necessary where fire, corrosion, and other harmful external effects are a concern.Improving concrete impermeability depends on establishing an aggregate skeleton with minimum voids between particles (proper grading), filling these voids with sufficient cement paste (minimum cement content requirement), and ensuring that the cement paste itself is also impermeable (maximum water/cement ratio requirement). Minimum cement content and maximum w/c ratio also determine the concrete's compressive strength class.

Environmental Effects

TS EN 206 classifies environmental factors affecting concrete's economic life by type and level of harm: XO: No corrosion or harmful effect hazard. XC1, XC2, XC3, XC4: Corrosion caused by carbonation. XD1, XD2, XD3: Corrosion caused by chlorides other than seawater. XS1, XS2, XS3: Corrosion caused by seawater chlorides. XF1, XF2, XF3, XF4: Freeze-thaw effect. XA1, XA2, XA3: Chemical effects. The standard has proposed in Table F1 the maximum w/c ratio, minimum cement content, and smallest strength class needed for the structure to have 50 years of service life under these environmental effects, and has left the detailed regulation to complementary national standards. The complementary Turkish Standard TS 13515 [3] added mechanical wear and alkali-silica reaction to environmental effect factors; expanded and revised Table F1 to make it mandatory; determined the cement types that can be used; but did not provide for an economic life other than 50 years.

Hundred-Year Service Life: BS 8500-1

The complementary British Standard BS 8500-1 [4] provides, in greater detail, criteria for both hundred-year economic life and varies them depending on cover thickness. The permitted cement types are also specified. The following examples are taken from this standard. XC1, the mildest class of corrosion caused by carbonation (for example, concrete in an environment with very low humidity), with 15 mm cover and a w/c ratio of 0.70, and a dose of 240 kg/m³ C20 concrete, is sufficient for 50 years economic life and also for 100 years of life. Because harmful effects are very limited. Increasing cover to 20 mm provides little benefit; XC3 and XC4 environmental classes can be met for 50 years economic life only with a w/c ratio of 0.45 and a dose of 340 C40 concrete, and are not sufficient for 100 years life. No other environmental classes are met. Increasing cover to 25 mm can meet the XC2 class for 50 years life with a w/c ratio of 0.65 and a dose of 260 C25 concrete; XC3 and XC4 classes with a w/c ratio of 0.55 and a dose of 300 C30 concrete; no other environmental class is sufficient. For 100 years life, only the XC2 class (wet, occasionally dry—for example, water tanks, foundations completely buried in soil with no harmful effects) can be met. Therefore, the 25 mm cover thickness value that TS 500 specifies for columns and beams exposed to weather will not even protect the reinforcement of concrete structures on or near the coast against airborne salts (XS1 environmental class) for 50 years.

XC3, for corrosion caused by carbonation, with 30 mm cover, C28 concrete with a w/c ratio of 0.60 and a dose of 280 is sufficient for 50 years life, while for 100 years life it is necessary to upgrade to a C40 concrete with a w/c ratio of 0.45 and a dose of 340.

However, if cover is increased to 50 mm, a C25 concrete (lower quality) with a w/c ratio of 0.65 and a dose of 260 is also sufficient for 100 years life. XD1, for corrosion caused by chlorides other than seawater, with 30 mm cover, a C32 concrete with a w/c ratio of 0.55 is sufficient for 50 years life, while for 100 years life it is necessary to upgrade to a C45 concrete with a w/c ratio of 0.40 and a dose of 380. However, the same goal can be achieved with the same C32 concrete by increasing the cover from 30 mm to 45 mm. More examples could be given, but are unnecessary. In recent years, projects such as Marmaray have put "hundred-year life – legacy concrete" on the agenda. From the comparisons above, it can be understood that hundred-year economic life concrete design is not excessively difficult: Extending economic life from 50 to 100 years requires sufficient increase in cover and/or sufficient increase in strength class. For example, C50 and C60 concretes commonly used in high-rise projects in Istanbul, with generally w/c ratios below 0.40 and equivalent cement doses above 380 kg/m³, if given a cover of 45 mm, meet the 100-year economic life conditions for all XC, XS, or XD environmental classes.

Application

Correct design depends on correctly defining environmental effects and on correct and compatible selection of cover thickness and concrete w/c ratio, dose, and strength class. However, correct design is insufficient; proper and careful execution is also required: Correct placement of reinforcement in formwork to maintain cover; proper placement and compaction of concrete in formwork; proper curing; obtaining flawless, void-free, non-segregated cover concrete and protection from cracking are important stages. Otherwise, defective cover concrete as seen in the photographs, whether C40, C50, or even C100, will have no potential to protect reinforcement or core concrete.

Conclusion

Legacy concrete—hundred-year economic life—does not appear as difficult as it might seem at first glance. The key to lasting success appears to be correct determination of environmental effect classes and harmonious and optimal technical-economic placement of cover thickness and concrete type (w/c ratio, equivalent cement dose, strength class, binder selection). It would be beneficial for TS 500 to review cover thickness values, enrich them with more detailed and comprehensive provisions, and emphasize long economic life. Perfect harmony between the thickness and impermeability of cover concrete does not appear sufficient for final success; the quality of application must also be elevated to the same level.   Prof. Dr. Erbil Öztekin Chryso Technical Advisor    

References 1- TS EN 206 / February 2014: "Concrete – Specification, performance, production and conformity". 2-TS 500 / February 2000: "Design and Construction Rules for Reinforced Concrete Structures". 3-TS 13515 / June 2014: "Complementary Standard for the Application of TS EN 206". 4-BS 8500-1: 2006: "Concrete – Complementary British Standard to BS EN 206-1 – Part 1: Method of specifying and guidance for the specifier".