International Concrete Abstracts Portal

Modern construction is unthinkable without concrete, the world production and consumption of which is about 10 billion m3 per year. Given the steady growth of the world’s population by 2050, it is expected to double this volume, which will undoubtedly be significantly affected on energy consumption and increase global CO2 emissions. Concrete is perhaps the most universal building material since the beginning and development of civilization. It is sufficient to recall the Great Wall of China, the palaces and temples of Ancient India, the pyramids of Ancient Egypt, the unique buildings of Romans, made with the use of lime-pozzolanic binders. Universality of concrete is defined by simplicity and convenience of its production, rather low cost, structural integrity and homogeneity, durability and a long service life under various aggressive environments. However, the concrete image is sometimes not favorable. It is associated with greater labor intensity of construction works and dismantlement, massive structures, a large impact on the environment in connection with the s consumption of not renewable natural resources. The same perception is greatly facilitated by the fact that, according to Gigaton Throwdown Initiative, “the cement industry is responsible for about 5 to 7% of total CO2 emissions, or 2.1 Gt per year.” Indeed, when producing cement clinker about 0.9 t CO2 / t clinker are produced. Taking into account the annual increase in the production and use of Portland-based cement (more than 4.1 million tons per year) that is the main binder used in the production of concrete, this fact poses a significant threat to humanity as a whole. According to the Intergovernmental Panel on Climate Change (IPCC), actions are necessary to reduce carbon dioxide emissions because in about 30 years CO2 concentrations is expected to reach 450 ppm – a dangerous point above which irreversible climate change will occur on our planet. Since concrete will remain the main building material in the future, it is expected that if new ways and mechanisms to reduce the environmental burden by at least 50% will be not found, it is not possible to maintain the existing level of impact. This problem is so deep and serious that there is hardly a single way to solve it. There is a need for an integrated approach, several complementary activities that provide some synergy. Until recently, the main efforts were aimed at improving technological processes and reducing the consumption of clinker through the production of blended cements, as well as the creation of new types of binders. Active search for alternative binders has led to the development of sulfoaluminate-based cements; alkali-activated materials and geopolymers (slag, fly ash, metakaolin, etc.), efficient and fairly water-resistant magnesia cements; phosphate cements (ammonium phosphate, silicate phosphate, magnesium phosphate etc.), cements with calcium halogen-aluminate and the so called low water demand binders. With the advent of high-performance concretes and new technologies, the possibility of a radical increase of the cement factor in conventional concrete due to the use of high-performance superplasticizers and other chemical admixtures, dramatically reducing the water consumption of the concrete mixture; active mineral additives such as micro silica, metakaolin, fly ash, finely ground granulated slag, etc., as well as a variety of inert fillers that can improve the functionality of concrete mixtures, such as fine limestone. Strictly speaking, “pozzolanic effect” and “filler effect” are easily combined and provide a certain synergy. The potential for reducing cement consumption in concrete production is still undervalued. This is due to certain fears of decreasing the corrosion resistance of concrete and durability of reinforced concrete structures, since the great bulk of the existing standards is prescriptive and sets the minimum cement content in concrete under specific operating conditions. Reinforced concrete structures of buildings and constructions, as a rule, initially, shall have the design strength and sufficiently long service life because their construction often requires a significant investment. The durability of these structures, however, is determined by different ageing processes and the influence of external actions, so their life will be limited. As a result, many structures need to be repaired or even replaced in fairly short time periods, resulting in additional costs and environmental impacts. Therefore, there is a need to improve the design principles of structures taking into account the parameters of durability and thus achieving a sufficiently long service life. Development of the concept of design of structures based on their life cycle, “environmental design”, including a holistic approach that optimizes material and energy resources in the context of operating costs, allow us to completely revise our ideas about structural concrete construction. It should be noted that many recent developments in the field of life cycle analysis (LCA) are aimed at expanding and deepening traditional approaches and creating a more complete description of the processes with the analysis of sustainable development (LCSA) to cover not only the problems associated mainly with the product (product level), but also complex problems related to the construction sector of the economy (at the sector level) or even the general economic level (economy level). The approach to “environmental design” is based on such models and methods of design, which takes into account a set of factors of their impact on the environment, based on the concept of “full life cycle” or models of accounting for total energy consumption and integrated CO2 emission. All of this could become a basis for the solution of the global problem – to contain the growing burden on the environment, providing a 50% reduction in CO2 emissions and energy consumption in the construction industry. Hence a special sharpness P. K. Mehta’s phrase acquires: “...the future of the cement and concrete industry will largely depend on our ability to link their growth for sustainable development...” The above-mentioned acute and urgent problems form the basis of the agenda of the Second edition of International Workshop on “Durability and Sustainability of Concrete Structures – DSCS-2018,” held in Moscow on 6 – 7 June 2018 under the auspices of the American Concrete Institute, the International Federation on structural concrete and the International Union of experts and laboratories in the field of building materials, systems and structures. The selected papers of this major forum, which brought together more than 150 experts from almost 40 countries of the world, are collected in this ACI SP.

Regular concrete cannot be reinforced with aluminum metal due to high pH that will dissolve the protective oxide layer and evolve hydrogen gas. However, it has been demonstrated that replacement of 50% portland cement with calcined clay can prohibit hydrogen gas evolution in the early stage and will deplete all calcium hydroxide formed by cement hydration in the long run securing that gas evolution will not happen in the future either.

The advantages of such a "reduced pH" aluminum metal reinforced concrete is in many ways environmentally friendly as the reinforcement does not need to be protected by a dense concrete cover. The concrete can be made by higher w/c to achieve the required compressive strength and not over-shoot it for low permeability reasons. After all, most of the concrete made is below 35 MPa characteristic compressive strength. Hence, such high porosity concrete with only required strength according to its use will pick up CO₂ from atmosphere faster and further lower the carbon footprint and secure a stable system. With appropriate Al alloying, even seawater can be used as mixing water, and with such high SCM dosage alkali reactive aggregate can be used, further adding to ecological benefits.

Many performance-based test methods adopted in various national and international standards were adopted decades ago based on short-term evaluations. Many of the durability tests use various methods of acceleration to obtain results in a reasonably short period of time, and then pass/fail criteria are set for these tests and included in standard specifications. If long-term tests conducted in the field, or at least in outdoor exposure, can verify the appropriateness of both the test methods and the test limits, then it provides confidence that the test methods are meaningful and that the specification limits are appropriate. This has been done in the case of ASTM and CSA test methods for sulfate resistance, mitigation of alkali-silica reaction, for de-icer salt scaling resistance and for resistance to chloride ingress for marine and deicer exposures. The potential downside can be that the materials and mix designs used in the long-term tests may no longer be representative of those currently in use. In addition, the precision of all test methods needs to be evaluated in inter-laboratory test programs to provide confidence in the test results obtained. This contribution describes results from several long-term test programs and inter-laboratory studies focused on verifying specific standard test methods for durability.

The mixture proportioning of concrete for sustainability should consider four aspects, without sacrificing affordability: the lowering of the carbon dioxide emissions; the minimization of raw materials required; reduction of energy demand during manufacturing and construction; and the longevity of the structure or other applications. Taking a set of concretes with different binders, including ordinary portland cement (OPC), fly ash (FA) and ground granulated blast furnace slag (GGBS), sustainability is assessed using different types of indicators including those that take into account the binder and clinker content, compressive strength, carbon footprint and energy demand. A new set of indicators called A-indices has been proposed for combining the influence of carbon dioxide emissions obtained from life cycle assessment (LCA) and durability parameter that relate to the service life of a structure. Here, this concept is illustrated by obtaining a parameter based on the chloride migration coefficient of the concrete. It is proposed that the decision-making process for sustainable concrete be made by minimizing both the A-index and the energy intensity, defined as the energy demand for a unit volume of concrete and unit performance parameter, such as 1 MPa of 1-year compressive strength. The best concretes considered here come out as those with ternary binders having 40% of the OPC replaced by a combination of GGBS and FA.

Sustainable concretes, also termed eco-concretes or green concretes, produced with a significantly reduced cement content provide a promising alternative for improving concrete sustainability without using supplementary cementitious materials, such as fly ash or slag. However, the production of such eco-concretes is a challenge in view of concrete technology and concrete properties. In particular, new design concepts as well as new admixtures have to be developed and applied to produce concretes with a cement content of approx. 100 kg/m³ while keeping the concrete performance on a level similar to ordinary structural concrete of today.

To evaluate the sustainability of these new types of concretes not only the very low ecological impact due to the composition may be regarded, but in addition also their technical performance, i.e. their mechanical, physical and chemical properties have to be taken into consideration.

This contribution firstly gives an overview on sustainable approaches for concrete structures, further introduces the Building Material Sustainability Potential as an index, which is applied in combination with the service life prediction for cement-reduced concretes using full probabilistic methods. The composition of these particular structural concretes is discussed and related test results for their performance are presented. The contribution closes with an introduction to graded concrete members as an innovative approach for the improvement of concrete sustainability on a structural level and presents testing results for mechanical and durability properties of graded bending beams.