Concrete is the world’s most common building material. Over 2,000 years ago the Romans combined gravel, coarse sand, hot lime and water to build structures like the Pantheon. Today, virtually every structure in the developed world has some concrete content.
Concrete is prized for its durability, versatility and availability, but traditionally it has come at an environmental price. The quarrying of virgin stone for use in Portland cement or as aggregate can destroy ecosystems and natural habitats. Emissions from cement plants produce approximately eight percent of all carbon dioxide emissions created worldwide. Enormous amounts of energy are also consumed in the manufacture of cement, accounting for 0.6 percent of the total energy use in North America.
With careful attention to environmentally conscious concepts, architects and engineers can reduce concrete’s environmental impact while adding value to their clients’ projects and saving money. Using supplementary cementing materials, recycling concrete, and increasing concrete durability help reduce the need for new raw materials and also divert old materials from landfill. Concrete’s thermal mass can also be incorporated into passive solar systems, helping to reduce our reliance on energy-consuming mechanical systems for heating and cooling.
Supplementary cementing materials
One of the most environmentally beneficial developments in concrete manufacture is the use of supplementary cementing materials (SCMs) as partial replacements for Portland cement. SCMs are waste products from other industrial processes. Fly ash, blast furnace slag and silica fume, which would otherwise go to landfill, can replace up to 50% of the Portland cement in concrete. The resulting energy reductions are considerable.
Concrete made with SCMs is stronger and more durable, increasing a structure’s expected service life. Use of SCMs in the Confederation Bridge linking New Brunswick and Prince Edward Island helped to produce concrete with an expected service life of about 100 years.
Fly ash and slag can also give concrete a more desirable appearance. Both materials provide a more uniform texture, while concrete made with fly ash has a light beige hue, and slag is darker. When concrete is attractive, there is no need to cover it with superfluous finishes, reducing material use and overall energy used to create a building.
There are also disadvantages to SCMs. Tighter quality control systems are required when SCM contents in concrete are high. The colour and texture of concrete in different batches can vary considerably if SCMs from different sources are used, or if mix proportioning is inconsistent. Shrinkage cracks can form more readily in concrete with high SCM content, and carbonation, or “dusting,” can occur in concrete containing incorrect proportions of fly ash or slag. In addition, strength gain is slower for concrete containing fly ash and slag, which can be a problem in the case of projects with tight schedules.
Reclaimed concrete materials
Demolition and construction have been the pattern for renewal of building stock, resulting in the constant creation of demolition waste for landfill and requiring the constant procurement of raw materials. However, demolished concrete can be recycled as reclaimed concrete materials (RCMs).
RCMs lessen the demand for virgin aggregate and divert concrete from landfill, which is particularly expensive for concrete because the cost of disposal is based on weight. RCMs are half the cost of virgin aggregate and are readily available. By 2030, the concrete industry hopes to achieve zero net waste from concrete and its constituent materials, partly due to RCM use.
Physical properties such as grading, shape, and weight of RCMs are comparable to those of virgin material, making them especially suitable for base courses for foundations and slabs-on-grades, or for use as free draining backfill material. RCMs are also used as asphalt aggregate, shoreline protection, and gabion basket fill.
The strength of RCMs is slightly less than that of virgin aggregate, making it more suitable for low strength concrete applications. Concrete containing RCMs tends to have increased creep and shrinkage and decreased elasticity. Workability decreases in concrete containing RCMs, due to higher total moisture absorption, and RCM sources may contain chloride ions, sulfates, or aggregate that may be susceptible to alkali-silica reactions.
The term thermal mass describes the ability of a material to “trap” heat and release it at a later time. Concrete provides excellent thermal mass because it can hold large quantities of heat, does not release heat rapidly, and is a ubiquitous building material. Incorporating thermal mass into a building moderates indoor temperature fluctuations as the outdoor temperature cycles over a 24-hour period. This reduces energy consumption in both hot and cold climates, achieving more temperature management passively.
One way to use thermal mass is to expose concrete surfaces to solar radiation. Exposed concrete interior walls or floor slabs in areas enveloped with glazing will absorb much of the solar radiation, reducing the immediate heat gain. The absorbed heat is released later, offsetting cooling requirements during the day, or heating requirements at night. Poorly placed insulation or finishing materials will interfere with these effects.
Thermal mass can also be used to “store” cooling. By naturally ventilating a building at night, unwanted heat will be flushed with cool night air and the temperature of the thermal mass will drop, providing a cooling effect during the day.
Building in durability
Careful attention to building design can improve the durability of concrete. For example, use of type C1 concrete, with higher cement content and entrained air, makes a structure less vulnerable to damage caused by freeze/thaw cycles and chlorides. Higher cement contents increase the bond strength of concrete paste, allowing it to take greater expansive forces. Air entrainment, or the intentional introduction to concrete of tiny air bubbles, helps to relieve internal stresses produced by the expansive forces of water freezing in the concrete’s pores.
The proper selection of aggregates in concrete also markedly improves durability, which can be drastically compromised if weak, reactive or freeze/thaw-susceptible aggregate is used. Some aggregates react with the alkaline cement, resulting in expansive products capable of damaging the concrete from the inside out.
The corrosion of embedded reinforcing steel is one of the most common causes of concrete deterioration. Corrosion products expand to approximately four times the volume of the original steel, initiating cracking in the concrete covering the reinforcing steel. Reducing the water/cement ratio minimizes the porosity of concrete and the rate of chloride ingress, increasing service life. Minimizing shrinkage cracking by adjusting concrete mix proportions also slows the rate of moisture and chloride ingress.
An often-overlooked factor in concrete durability is proper detailing, which helps sustain concrete in service. Bad design or construction habits can result in poor consolidation of the concrete, cracking, poor water shedding, or reduced cover over reinforcing steel. These characteristics increase infiltration of water, chlorides, and other agents into concrete and promote corrosion of embedded reinforcement or deterioration of the concrete.
Other protective measures, such as the application of protective membranes and waterproofing sealers, reduce the ingress of water and chlorides into concrete. Corrosion of reinforcing steel can also be reduced with epoxy coatings or cathodic protection systems that prevent the electrochemical processes causing corrosion.
What is the industry doing?
To reduce fuel consumption and carbon dioxide emissions, most modern cement plants have switched to dry process kilns that are up to 30% more energy efficient than the former wet process kilns. To further reduce energy requirements, modern plants preheat
raw materials with waste heat from exhaust gases. Coal fuel is sometimes replaced with waste materials such as old tires, used oil, or petroleum coke (a waste product from refineries). Concrete producers claim that the extreme heat of the kiln completely incinerates these alternative fuels, minimizing pollution. Filters for capturing particulate emissions, released as lime cools, have also been improved. Kiln dust is captured in a closed loop system and reintroduced as raw material.
The role of the design professions
Because our economy is based on abundant sources of inexpensive energy, North America lags behind the world in efforts towards sustainability. We are the world’s highest energy users. As a result, we produce 5.5 tonnes of CO2 per person per year, compared to 2.7 tonnes for Europe and 0.8 for China. Besides endangering our future, this also renders us uncompetitive. Buildings are responsible for a major part of our energy consumption–about 40%–so architects and engineers are more than by-standers in the context of required changes. It takes knowledge and commitment, but there are good business reasons for sustainable building design: both our Earth and our clients will benefit.
Brennan Vollering, M.A.Sc., P.Eng. is Project Engineer at Halsall Associates Limited, a leading Canadian structural engineering and building sciences firm. For more ideas on adding value to clients’ projects, visit the Halsall web site at www.halsall.com.