Recycled Concrete Aggregate and the Need for Sustainability

The interest of including sustainable development in construction is on the rise due to the changing systems of the world based on its economy, its environment and society. The most immediate and obvious way to achieve more sustainable construction today is by conserving new raw materials such as natural aggregates, and reusing construction and industrial wastes. For those who may not know, recycled concrete aggregate, better known as RCA, is an example of a common construction waste that is produced after demolishing concrete structures. Supplementary cementing materials (SCM) such as fly ash and slag are industrial by-products, which have a long history of use with Portland cement (PC) in concrete. Although Portland cement is the primary source used for construction, it has been found that it is no longer feasible and the continuation of its usage will no longer supply benefits in cost management and will further deteriorate the environment, consequentially impacting global society. The study of recycled concrete aggregate is interesting because it is a new mechanism used that could potentially impact society as a whole, economically, environmentally and socially. The aim of this article is to examine and provide an analysis of the technology used when recycling concrete aggregate, instead of using virgin aggregate, as is the current procedure in Canada.

Due to the high urbanization of many cities in Canada, concrete is one of the most popular construction materials used in the country. Environment Canada has identified that two thirds of the concrete’s production is used in basement’s foundations for commercial and residential constructions. The basic components that produce concrete are aggregates, cement, water and some chemical admixtures. Concrete consists of 75% of the weight of all construction materials. Interestingly, construction and demolition waste in Canada amounts to 15-20% of all landfill materials. Due to the increase in waste in general, there needs to be a mechanism in places where waste can be reduced and one place is within the process of recycled concrete aggregate. By finding new applications for concrete waste, and creating a market for its use, we can bypass the need to consume virgin natural aggregate and simultaneously conserve landfill space.

So far, RCA has been used based on a granular base, mixed with natural aggregate in concrete applications used for roads. Research has shown that the use of 30% RCA and 70% natural aggregate in high strength concrete produces concrete of similar strength as that containing only natural aggregate. Although the strength of RCA is similar, the concern is the shrinkage that occurs during the drying of RCA in concrete. New concrete made with RCA experiences creep and drying shrinkage that is 10- 30% greater than that of concrete made from natural aggregate. It has been found that the porosity level in RCA increases the levels of shrinkage when it is in the drying process. Furthermore, RCA is also known to have lower elastic modulus than natural aggregate, which also contributes to drying shrinkage and creep. Although these negative impacts occur, this does not mean that RCA cannot be used entirely.

Previous to the notion of use recycled concrete aggregate, blended Portland cement (type I (SM) ASTM or CEM II-M), river siliceous sand, and granitic crushed stone (nominal size 6-20 mm) were used to make the concretes. Now, as a result of virgin concrete shortage, there is a use of not only RCA but recycled fine aggregate (RFA). For recycled concretes, recycled fine aggregate (RFA) is taken from crushed waste concretes of varying qualities, and made with granitic stone, and is used by replacing its contents (0%, 20%, and 30% by volume) of natural sand. In all concretes, an admixture acting as a water-reducer is used. The physical properties consist of either coarse and fine aggregates, they are determined by their fineness, gravity (saturated and surface dry – SGssd), water absorption, and material finer than 75 lm.

Many countries have begun discovering the use of RCA but RFA has been found to be just as workable and each RCA and RFA have their benefits in varying industries. Several countries have recommendations for the use of recycled coarse aggregates in structural concretes, but the recycled fine aggregates (RFA) are discarded because they may produce modifications on the fresh and hardened concrete properties. It is not only the use of RCA that can reduce waste and help benefit the environment and the economy, but RFA is a precious material and has become more important in concrete production because of economic implications related to the shortage of natural sands suitable for that, and the need for a comprehensive utilization of such wastes, since the fine fraction remains when the recycled coarse aggregates are used, which storage and control is complex. Dr. Khatib, an environmentalist, illustrates that concrete made of 25% and 100% of RFA reduce compressive strength by 15% and 30%. However, another research at hand demonstrate that the compressive strength is not affected by the utilization of RFA, but the most significant changes within concrete with RFA is the higher drying shrinkage and less durability. Previous studies have been made on the use of RFA in concrete which show that the use of RFA over 50% produce significant decreases on the slump of the mixes at the fresh state, similar to those produced in concretes made with natural crushed sand.

With every new process that arises, there are impacts to the economy, the environment and society. It is first interesting to understand the economic impact of RCA. The cost of aggregates typically is between 20 and 30% of the cost of materials and supplies, or 10 to 15% of the total construction cost of a roadway section. Due to the high quality of virgin aggregate and its known shortage, the price is increasing. Since there is a shortage in virgin aggregate and the price is increasing, access to this very important construction material is decreasing and an economical option is crucial. The good news is that concrete pavements are 100% recyclable. Concrete recycling has been used predominantly in Europe since the 1940s and in the U.S. since the 1970s. Concrete recycling for paving applications is now performed in at least 41 states and has the support of the Federal Highway Administration. The FHWA has found that reusing the material used to build the original highway system makes sound economic, environmental, and engineering sense. The cost of aggregate is one of the major costs of highway construction. As the sources of virgin aggregate becomes scarce, haul distances increase, which then cause additional supply costs. As the source becomes scarcer, the more the costs of using virgin aggregate will increase. The cost of producing RCA can be considered to be limited to the costs of crushing the demolished concrete and screening and backhauling the RCA. The cost of removal and demolition of concrete will stay the same, regardless of whether it is natural aggregate or recycled concrete (RC). However, the usage of RCA can produce savings in the hauling and disposal costs, especially if the RCA is produced on site. Moreover, the waste issue comes back when the economical benefits of RCA are discussed. Disposing of demolished concrete slabs in landfills is becoming expensive because the space available in landfills are also becoming limited. The recycling of concrete pavement annihilates the need to dispose the concrete, which results in cost savings and allows more room for our landfills to be filled with waste that cannot be recyclable.

Other than impacts stemming from cement production, transport has a large influence on the environmental load which is then dependent on transportation distances and the transportation vehicle used. Regardless of the transport scenario, the impacts of cement and aggregate production life-cycle phases for RAC are slightly larger than for NAC. In consequence, the results present the fact that the impacts of cement and aggregate production phases are slightly larger for RAC than for NAC. There are varying studies that show that sometimes NAC has higher impacts on the environment and other studies show that it is RAC that could be costing more impact economically, environmentally and ending up affecting the social aspect as well. Total environmental impacts in terms of energy use, global warming, eutrophication, acidification and photochemical oxidant creation depend on transport distances and types. By observing a study done by Dr. Marinkovic on the matter, I understand that transport scenario 1 (transport distances of recycled aggregate are smaller than those of natural river aggregate) the environmental impact of RAC and NAC production in terms of studied impact categories is approximately the same and the benefit from recycling in terms of waste and natural mineral resources depletion minimizing is clearly gained. Whereas scenario 2 indicates (transport distances of natural and recycled aggregates are equal), the total impacts of RAC are larger, increase ranging from 11.3% to 36.6% depending on the impact category.

This then trickles down to having the process of RCA also reduce the emission of greenhouse gasses (GHGs). Every activity that requires fuel and/or electrical

power when producing virgin aggregate is inevitably responsible for the creation of GHGs and other pollutants. Concrete recycling helps to reduce the environmental impact of pavement reconstruction activities while helping to ensure the maintenance of our transportation infrastructure. Another environmental benefit that RCA produces is that it lowers the carbon dioxide (CO2) levels in the air. Research at the University of New Hampshire has shown that RCA has significant value as a sink for carbon dioxide (CO2), a primary greenhouse gas, through the mechanism of spontaneous carbonation, in which atmospheric CO2 reacts with calcium hydroxide (Ca(OH)2), a by-product of the cement hydration, in the concrete mortar to produce calcium carbonate.

In order to continue the wave of recycling, many participants of the Global North have been increasingly adding to their lifestyle. The recycling of concrete aggregate also needs to begin and be woven as a process in Canada and needs to be regulated under its government. One interesting factor about recycled aggregate is its use for an abundant resource at the moment, which is water. However, this precious resource is endangered for beginning to be scarce. Although there is a lot of water on earth, less than 3% is fresh and most of that is either locked up in fast-melting glaciers and ice caps, or is too deep in the earth to retrieve. The status of water being an endangered natural resource is in part due to the rise of global warming and the growth of agricultural, urban, and industrial needs, where water is increasingly used to keep up with the demand of various industries. Therefore, our resources need to be used more efficiently so if we do not cut back on the water usage and concrete in construction, then this industry could be adding to the woes humans will have to face when there is a shortage of water. Concrete companies and constructions sites, being the largest industrial consumers of fresh water, it is crucial that the concrete industry use water more efficiently. Approximately 100 L/m3 (20 gal./yd3) wash-water used by the ready mixed concrete trucks, we’re using too much water for concrete mixing and the yearly global mixing water requirement of 1 trillion L can be cut in half by better aggregate grading and by greatly expanding the use of mineral admixtures and superplasticizers. Drinking water has been identified as being acceptable for making concrete so there is no harm in not using fresh water all of the time for recycled aggregate.

As it stands today, there are no virgin aggregate resources available in Canada. If there are no virgin aggregates available for usage due to the shortage, then there will no longer be any concrete developed for construction. If there are no construction materials being produced, then as a result, there will be a shortage in jobs available not only in the construction industry, but in contracting, renovating and real estate. The effect of this crisis will also cause architects, civil engineers, construction workers, miners, and geologists to suffer cuts in their industries. This negative impact does not only effect society but most importantly, the economy. By replacing virgin aggregate with RCA and RFA, this will not only conserve our resources for future generations, but it would also aid the environment in beginning a cycle of recycling in a new sector where concrete is used. Tapping into a system that introduces a product that is a hundred percent recyclable like recycled concrete aggregate is a large movement forward in saving the environment because as shown, using virgin aggregate is not feasible due to its shortage but the methods used with this product is not helping the environment either. RCA can open up new spaces in landfills, it can conserve our water and disables toxins to disperse in our atmosphere. It is crucial that Canada jump on the bandwagon of recycling concrete aggregate as part of their goal for a “greener” society.