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.

An Investigation on the mineralogical and chemical characterization, pore structure, chemical shrinkage and pozzolanic activity of commercially produced rice husk ashes (RHA 1 and 2) and a control silica fume (SF) are presented in this paper. RHA possesses high silica content like silica fume which is used as supplementary cementitious materials (SCM) in the production of concrete. There is a need for an alternative to silica fume in the production of concrete because of its high demand and relatively high cost.

The mineralogical composition of RHA 1 and 2 show high silica content of 77% and 84% respectively which is close to the silica content (˃80%) of class 2 silica fume. The oxides of Ca are 3.53% and 7.68% while Al is 1.19% and 1.29% for RHA 1 and 2 respectively which suggest that RHA is a low Ca⁺² content binder. The results of chemical shrinkage of RHA 1, 2 and SF are 0.42 mL/g, 0.52 mL/g and 0.11 mL/g after 500 hrs of hydration. This indicates that RHA 2 has the highest reactivity (hydration) with water due to its highest Ca⁺² content.

Investigations were conducted to evaluate the effects of synthetic zeolite additive on delayed ettrintite formation (DEF) in heat cured cementitiuous systems. Mortar prisms 25 x 25 x 285 mm (1 x 1 x 11.4 in.) were prepared at water-cement ratio (w/c) = 0.5, using portland cement CEM I 42.5 N,R. Sulphates were introduced into the mixtures by adding 2%, 3% and 5%SO₃. Also, a commercially available synthetic zeolite additive PWC was added to cement at proportions of 1%, 0.5% and 2.5%PWC. The mortar specimen were heat-treated at 95^oC (203^oF) then stored in water.

It was found that heat treatment led to higher early strength gain as expected but increase in sulphate concentration correspondingly caused reduction in early strength development. There was a marked increase in early strength of heat-cured mortars containing PWC. Use of 2.5% PWC in expansive mortars led to five-fold increase in expansion at 90 days. Accordingly, preliminary results indicate a possibility of adverse effect of PWC on DEF in cementitious systems, especially when used in elevated proportions. Further investigations are needed to conduct detailed evaluation of PWC effect on DEF.

For the production of concretes with an extended service life a range of admixtures and additions are available. For these concretes polycarboxylate ether (PCE) based superplasticizers are the product of choice. Specially designed additions are necessary to achieve the required performance and service life. In this paper the tailor-made development of these products is described and their structure activity relationship with cement is discussed.

A well balanced PCE combines a high water reducing effect with a limited influence on the setting time and, therefore, the strength development. Very important for the use of concrete the workability window can be adjusted for the required period of time. It is shown which components of the polymer have to be linked to the required performance in fresh and hardened concrete. Furthermore, the development of calcined clays and their reactivity in highly durable concretes is described and discussed. The results of x-ray investigations show the enhanced performance of calcined clays.

Derived from these investigations the focus is on ready-mix production and use of highly durable concretes on construction site. The benefit for a sustainable development of the construction industry is the durability and therefore the reduction in use of natural resources. As a highlighted example the Emscher Project is described and the use of calcined clays based concretes with enhanced acid resistance in that project. From that project general recommendations on the development, production and use of highly durable concretes are derived.

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.