We Use Recycled Products Daily, Why Not Recycled Concrete?

Every day we engineer building structures using various types of materials: concrete, steel, masonry, wood, etc. But what about recycled materials? Think about it…we use products made from recycled materials all the time. We drink water from recycled plastic bottles. We dry our hands using recycled paper towels. The concrete industry is one of the most environmentally demanding industries in the world given the need for natural aggregates, clean water, and the energy required during the production of cement. If it were possible to reduce the impact of this material on the environment in a safe and potentially cost saving way shouldn’t that be done? At Schaefer, we not only ask these questions, but we study the answers!

Since 2010, I have investigated the use of recycled concrete aggregates (RCA) in new concrete building construction as part of my doctoral dissertation at the University of Notre Dame, and have published several papers (see below) detailing the results of this research.

The natural aggregates that make up the concrete we use in building construction require mining, production, and transportation that, depending upon the scarcity of natural resources in a particular region, can impact concrete production costs in addition to the environmental costs. When a building has reached the end of its serviceable lifespan, the material is often discarded as waste and sent to the local landfill. A portion of the material may be crushed and used as non-structural fill for gravel roads/trails and against basement foundation walls, or as a compacted sub-base for new roads, sidewalks, and concrete slabs, for example. My research has focused on using RCA as aggregate in structural concrete as a replacement for natural aggregates, a topic that has made great strides over the past six years. In addition to research still being conducted at the university level, one of the next steps involves the development of technical standards and testing in real world applications. The goal is that the old crushed concrete can essentially be cleaned and reused either in conjunction with or entirely in place of natural aggregates (i.e., limestone, granite, river rock) without any changes in the other constitutive materials of the concrete (i.e., sand, cement, water, admixtures). With approximately the same crushing and cleaning processes already required for natural aggregates, the recycled concrete can be used as a more sustainable alternative.

With approximately the same crushing and cleaning processes already required for natural aggregates, the recycled concrete can be used as a more sustainable alternative.

Throughout the course of my research, it’s been clear that the use of RCA concrete is certainly within the realm of possibility in actual building structures, although there are some limitations. In general, RCA concrete is capable of achieving the same concrete compressive and tensile strength as natural aggregate (NA) concrete. This in turn means that a concrete beam, for example, made using RCA concrete will have the same ultimate capacity to support structural loads as an equivalent beam made using NA concrete. However, the principle difference observed between RCA and NA concrete is the stiffness of the material. Stiffness is a mechanical property that relates the amount of force applied to an element and the deformation that the element experiences under that force. For example, as you are walking on your floor at home, the floor joists under your feet are actually moving (i.e., deflecting) downward from the weight you are exerting on them. The amount they deflect is a function of the stiffness of the wood joists. RCA concrete has a reduced stiffness as compared to NA concrete, meaning that under the same amount of force, an RCA concrete member will deflect more. The amount of stiffness reduction is directly proportional to the amount of RCA used in place of NA in the concrete mix. If all of the natural aggregate is replaced with RCA it is possible to have as much as an approximate 40% reduction in stiffness, whereas if only 20% RCA is used the reduction is significantly less. This behavior is observed under the influence of an immediate load as well as a long-term application of load. While the average building occupant would likely not notice this difference, building materials sensitive to large deflections in supporting members (e.g., tile, stone, etc.) could be affected if appropriate measures aren’t taken by the engineer to keep these deflections within code limits (or even more restrictive limits imposed by the specific finish materials being used).

Concrete experiences two unique phenomena known as creep and shrinkage. Both phenomena represent increased deformations over time, one regardless of load (shrinkage) and the other proportional to the amount of load applied to the member (creep). Going back to the wooden floor joist example, if the joists were instead made of concrete and you were to stand in that exact same spot for 5 years straight, the joist would deflect slowly over that time period as a result of creep and shrinkage. RCA concrete will generally experience increased creep and shrinkage (i.e., increased time dependent deflections) as compared with NA concrete. Based on testing of full scale beams, if NA is fully replaced by RCA it is possible to see total deflection (i.e., immediate and long-term) as much as 70% greater as compared to an equivalent NA concrete beam. As mentioned previously, this increase is not likely to be noticed by the average building occupant but will affect other materials being supported by the concrete. To help control this long-term deflection, floor slabs or concrete beams, for example, may need increased depth to keep these increased deflections within code limits. Of course, where a smaller proportion of RCA is used these increased deflections are not nearly as pronounced and may not require any modification to the geometry of the structural elements at all.

What does this mean for the use of RCA in new building construction? Based on all of the research that I’ve conducted to date, in addition to research that continues to be conducted at the University of Notre Dame, it is certainly possible to use RCA in new building construction. However, given the decreased stiffness and increased creep and shrinkage of RCA concrete, certain applications should be avoided. For example, podium slab construction is widely used across the country for supporting 4-5 stories of residential or mixed-use occupancy above a thickened concrete slab. Generally in these slabs, long term deflections are principle factors in the design and therefore RCA should either not be used or the amount of natural aggregate replacement should be limited to a small proportion. On the other hand, it is very common in office buildings for the floor system to be made using concrete topped composite metal decking. This would be an excellent application for RCA concrete, where long-term deflections of the concrete are generally not a significant factor in the design. Other examples of locations where RCA would make a good alternative to natural aggregates include concrete footings and pedestals, retaining walls, slabs on grade, lightly loaded concrete columns, slabs in a one-way concrete floor system, concrete shear walls, and tilt up wall panels just to name a few.

There is likely still a significant amount of research to be completed before we will see RCA used in new concrete construction. The research that I’ve conducted has already increased the knowledge regarding the use of RCA in new building construction, and since leaving Notre Dame in 2013, the research on RCA concrete has increased significantly across the country. When the time comes to bring this material from experimental to practical, Schaefer will be there to lead the field.

REFERENCES:
Knaack, A., Kurama, Y., “Creep and Shrinkage of Normal Strength Concrete with Recycled Concrete Aggregates,” ACI Materials Journal, 112(3), 2015, 451-462.

Knaack, A., Kurama, Y., “Sustained Service Load Behavior of Concrete Beams with Recycled Concrete Aggregates,” ACI Structural Journal, 112(5), 2015, 565-578.

Knaack, A., Kurama, Y., “Design of Concrete Mixtures with Recycled Concrete Aggregates,” ACI Materials Journal, 110(5), 2013, 483-494

Knaack, A., Kurama, Y, “Sustainable Concrete Structures using Recycled Concrete Aggregate: Short-Term and Long-Term Behavior Considering Material Variability,” NDSE Report 13-01, 2013, Univ. of Notre Dame.