The future of concrete

Opening the event, Raoul de Parisot, president of the Technical Association of the Hydraulic Binders Industry (ATILH), noted: “Concrete, together with cement, which is one of the main components, is the most common material in our everyday lives.” Celebrating the bicentenary of the discovery of the laws of hydraulicity by Louis Vicat, delegates were taken back to 1806 when he first qualified as an engineer. As part of his work on bridge construction, he looked at how best to prepare and use lime in submerged piles, publishing a report entitled: ‘Experimental studies on ordinary construction limes, concretes and mortars’ in 1817, aged just 26.

Since then we have seen a number of major trends in the concrete industry including the introduction of novel binders, improved production techniques, methods to reduce CO2 emissions and the concept of ‘smart concrete’. All of these trends have culminated in the range of durable and versatile concretes in use today.

High-performance concretes

Commercial production of modern Portland cement started around 1824, but it was another 150 years before reliable production of the current generation of high-performance concretes began. Ultra-high performance concretes emerged about 25 years ago, offering endless possibilities for building structures that use less cement, therefore with less associated CO2 emissions than traditional concretes. They brought with them improved concrete homogeneity, compactness, microstructure and compatibility with microfibres.

Enhanced strength and compactness of cement materials is achieved through a considerable reduction in the water-to-binder ratio, now possible due to the development of superplasticisers and mineral additions. Meanwhile, the correct use of fibres has allowed for post-cracking tensile capacity and pseudo-ductility. This is the result of more than two decades of development of conventional fibre-reinforced concrete, gaining efficiency due to the high-quality matrix and greater number of fibres. By using only a limited size range and grading of aggregates, imperfections can be significantly reduced.
Perhaps the best known high-performance concrete on the market is Ductal®, developed by Lafarge, among others. It offers up to 250MPa compressive strength, 50MPa flexural strength, high durability over a long lifetime, using fibre reinforcement (giving it the acronym UHPFRC). Such materials remove the need for reinforcement bars, allowing concrete to be readily used in place of other building materials. A range of relevant standards has been developed in France.

Deterioration management

Durability is a vital concrete property. The occurrence and rate of deterioration is usually due to the movement of liquids, gases or ions through pores. Therefore, understanding a concrete’s microstructure is key. The presence of water in the environment and in the concrete pores is necessary for most deterioration mechanisms, so that resistance to deterioration very much depends on the binder system.

Concrete cracking is also critical, as the ingress of aggressive agents can accelerate deterioration. Processes that are relevant to sound concrete cannot simply be transferred to cracked concrete as microcracks are an essential element of that product. Various measures of penetrability and permeability are used to determine the potential for a lack of durability, but interpretation requires a thorough understanding of the particular structure and its environment.

Emissions management

When it comes to emissions, during its life, concrete, along with mortar, can absorb about 43 per cent of the carbon resulting from decarbonation during the manufacture of the cement it contains. Although some calculations put the total CO2 absorbed at nearer 10-30 per cent, it is evident that carbonation of cement products represents a substantial carbon sink that is not always adequately allowed for in emissions inventories. A draft European standard, known as ‘Sustainability of construction works – environmental product declarations – product category rules for concrete and concrete elements’, includes a methodology for calculating CO2 uptake during the life of a structure, which will vary depending on the initial concrete specification and the degree of shelter experienced during service life.

Concrete from demolished structures presents an even greater opportunity for reaction, and laboratory tests suggest that 150kg of CO2 can be stored in 1t of crushed demolition concrete. Industrial processes to exploit this possibility are now being developed. Full carbonation may enhance the properties of recycled aggregate in terms of water absorption and strength. The challenge is to develop reactors to carbonate high tonnages quickly and cheaply, possibly using air with added CO2 at pressures above atmospheric.

European concrete standards contain recommendations on the use of coarse recycled aggregates in terms of replacement rates as well as physical and chemical aspects. When it comes to standards for lifecycle assessments, questions have been raised as to how much CO2 is incorporated in granulated slag and how the benefit or debit should be shared between the blastfurnace operator and the cement producer?

Reduce, reuse, recycle

From an environmental point of view, concrete recycling not only reduces the use of non-renewable resources but also keeps debris out of landfills. Despite this, the environmental impact of producing a concrete with recycled aggregates often exceeds that produced using natural materials. This is due to the additional cement demand required to obtain the same concrete compressive strength with recycled aggregate and the additional processes, such as screening, that are required.

However, studies in France have shown that recycle rates below 30 per cent have little effect on concrete strength, while other work coordinated by ATILH has successfully used recycled concrete fines as 15 per cent of the raw feed to a dry-process cement kiln. Promising results have also emerged from laboratory tests using recycled concrete fines to replace limestone in CEM II binders. Meanwhile, if immediate reuse for a fresh roadbed is possible, this is the preferred recycling route.

While it is recognised that large concrete buildings can store energy, increasing renewable energy penetration to the power grid and avoiding grid peaks, the state of readiness varies greatly country-to-country. HeidelbergCement has been involved in the Norwegian EnergyNest initiative to develop a thermal energy storage system with ‘thermal batteries’ capable of delivering heat at 150-500˚C.

Consisting of a series of integrated heat exchanger tubes in a steel casing, these easy-to-transport ‘batteries’ can be used when solar power is unavailable to feed a power turbine or to produce super-hot water. A special concrete known as Heatcrete® has been developed with enhanced thermal capacity and conductivity to store heat at around 400˚C.

Canada-based CarbonCure Technologies has developed a process to inject CO2 into ready-mixed concrete during batch mixing to capture the carbon and improve concrete performance, allowing the amount of hydraulic binder to be reduced. The retrofit technology, which is now in commercial use, simply bolts on to existing concrete plants.

Novel and innovative concretes

A variety of novel concretes have appeared on the market. For example, translucent panels using pre-inserted thermoplastic polymer resin that offer a much cheaper option than optical fibres and a greater ability to capture light. Or concrete using cement with a titania additive that becomes powerfully reactive in daylight, breaking down pollutants that come into contact with it.

Innovations such as self-compacting, self-placing and ultra-fluid materials are being widely used on building sites. And it does not stop there. In the future, self-adjusting concrete will regulate humidity, temperature or dissipate seismic energy. Self-repairing concrete will automatically repair damage, limiting the ingress of chemicals. Conductive concretes could be used in electrical networks, while reflective or self-cooling concretes could cool surfaces by 20-30˚more than conventional materials. The introduction of appropriate fibres could improve a material’s fire resistance, or enhance electrical or thermal properties, enabling heat dissipation or preventing ice formation.

3D concrete printing

Progress is also being made in 3D concrete printing. For example, a storm surge basin weighing over 5t and measuring more than 2m in diameter was produced in less than nine hours. Controlling both the rheology and hydration kinetics is a common challenge in the printing process with layered extrusion being the most popular digital fabrication technique. Material placement, hydration control, the formation of cold joints and their impact on durability, all present further challenges, while properties of fresh concrete, such as extrudability and buildability, need to be measured and controlled.

‘Smart concrete’

‘Smart concrete’ is another current buzz word, describing intelligent concretes that adapt to their surroundings. They can behave first as sensors to detect signals, then as actuators to act upon their environment, before modifying their physical properties in response. At the Paris symposium, delegates received badges made of smart concrete. Measuring 11cm x 7cm and weighing around 100g, they gave the participants access to digital services around the event programme by interfacing with mobile phones and notepads via an integrated electronic chip.

It is encouraging that many architects now see concrete as their material of choice for a wide range of applications. The next challenge is to educate the general public about how concrete chemistry has developed over the years and the innovations that are becoming mainstream to enhance our everyday lives and help protect our environment.