Recycled Aggregate for Carbon Neutral Concrete

 (Sustainable Construction 1-27-2010) With the development of environmentally conscious construction projects across the world, there are still many demands of building materials that today’s technologies have not yet satisfied.  Looking at the two main structural building materials, steel and concrete, one in particular lacks characteristics of carbon neutrality.  While steel is nearing full recycled production, concrete remains a very carbon intensive process.  The main contributor to this trait is portland cement.  Portland cement exhausts sulfates, soot and nitrogen oxides when it is created (Chandra 518).  In addition, natural aggregate from raw mined material contributes to concrete’s carbon consumption.  These two aspects of concrete are in need of sustainable remediation in order for carbon neutral projects to allow its use in generous quantities.  
 
With increased industrial activity and improvements in society, exists the ironic counteraction that these industrial buildings produce waste and consume natural resources during construction and the lifetime of their production. Advancements in concrete technology have been oriented to correct this deficiency in several ways.  Through developments in alternative aggregates from recycled material and recycled admixtures that address the usage of portland cement, ready-mix concrete can be composed to be used in commercial and industrial infrastructure projects.  First, this new technology must undergo substantial empirical testing and analysis before it can be qualified by building codes. 
 
Construction and demolition waste programs have been established throughout the world to allow the use of recycled concrete as an alternative aggregate and 40% of all construction demolition is concrete (Ridzuan).  Other recovered concrete can come from sources such as site waste and old slump tests, as well as prefabricated concrete factory waste.  The Netherlands is the most responsible nation in this practice with nearly all of its concrete recovered.  Sadly, Europe in total only manages to recover 30% of concrete.   The United States recovers 82% of concrete in construction and demolition waste (CSI 9).  A common misconception is that recycled concrete can have cement extracted out of it but the process in which the clunker is formed is irreversible (Chandra 518).  The remaining use for recycled concrete is replacement of natural coarse aggregate in concrete mixes.  
 
Because recycled concrete does not relieve the use of Portland cement in concrete mix it does not necessarily produce many carbon credits in its function. However, research shows that concrete does carbonate over time, and in particular, crushed concrete does so more efficiently because it has more surface area exposed to air.  An article in the Journal of Life Cycle Assessment shares that during its secondary life, all particles of crushed concrete can achieve full carbonation (Collins 552).  As a common application indicated by many sources it is most effective when used as a road sub base as 1.5-2 inch aggregate (PCA).
 
While others say it is best used as road sub base because its structural properties are undetermined, Frank Collins’s research indicates that recycled concrete in road sub base application can carbonate up to 41% compared to the carbon emissions produced when the Portland cement used is created (Collins 556).  In this respect, it is understood that depending on application of the recycled concrete, the carbon neutral credits could be assigned.  Nevertheless, crushed recycled concrete does carbonate to some extent when recovered and stored and thus consumes CO2.  
 
In addition to road construction, structural concrete applications of recycled concrete pertain to aggregate substitution in various proportions.  As mentioned before, cutting back on raw mined material used as aggregate in concrete is beneficial to the environment.  However, this mostly pertains to reductions in green-house gas emissions, not carbon emissions.  In one study, the use of recycled concrete aggregate matched the structural properties attained using natural concrete aggregate.  The recycled concrete aggregate even produced concrete that was capable of exceeding standard compressive strengths by 2-20%.  
 
This data alone is not enough to consider its value as structural concrete.  The recycled aggregate size used in the study was slightly smaller and lighter than natural aggregate.  The 20 mm nominal aggregate diameter had a loose bulk density of 1255 kg/m3 while the natural aggregate attained a loose bulk density of 1390 kg/m3.  This difference allows almost 30% more water to infiltrate between the aggregate and increase workability of the recycled aggregate concrete; however this leaves the shrinkage of the mix when cured to suspect.  The common trend in most recycled concrete aggregate studies was that 3000 psi concrete could be developed for structural purposes.  The most common succeeding proportion of mix included 50% of 20mm recycled coarse aggregate, 30% fine granite aggregate (10mm) and 20% coarse granite aggregate (Ridzuan).
 
In a further study on the qualities of recycled concrete aggregate, the pore size and distribution of pores is analyzed. Variations in water absorption of previous studies discussed were due to variation in pore sizes.  Pore sizes are contingent on what type of cement was used in its primary application.  As discussed before different admixtures are being used to achieve carbon cutting ready-mix cement.  These chemicals range from by-products of industrial production such as fly ash, silica fume, or slag (Ngab).  
 
These chemicals, among others, will affect the porous properties of the aggregate and thus the water absorption ratio.  Results of this study revealed that the pore volume of concrete with silica fume was reduced by 30% compared to a 50 nanometer to 2 micrometer standard Portland cement pore volume due to the dense properties of the adhered mortar (Moon).  In order to make a consistently accurate mix, it is important to know what the primary concrete mix was composed of and thus the porous volume in the aggregate.  
 
With water absorption ratios and compressive strengths confirmed viable by several experiments, one must consider the resultant concrete’s seismic performance.  In one study in China, researchers built three types of portal frames to perform seismic tests on.  One was constructed using concrete comprising of natural aggregate while the other was 50/50 recycled aggregate to natural aggregate and the final mix was 100% recycled concrete aggregate.  
 
The results of the study revealed no evident difference in the cyclic lateral loading and the bearing strength of the recycled concrete aggregate compared to normal coarse aggregate.  Further conclusions of the study reported that additional studies needed to be performed in order to test the durability and serviceability of using recycled aggregate (Sun).  Through research studies it seems feasible to replace natural aggregate with recycled concrete aggregate, but a major factor to its use in construction is practicality.  
 
In fact, due to several aspects of the necessary process, recycled concrete aggregate may not be very practical on large scale construction projects or as a component of ready-mix concrete.  In addition, at this time, the economic disadvantages are enough to outweigh any environmental contributions that recycled concrete may add to any project.  Sorting through construction demolition and waste is not an easy process (Chandra 514).  Not to mention, once the steel, glass, wood and concrete is separated there is still various granulated material mixed in to each batch.  These unknown constituent materials can cause impurities in the aggregate that could affect how the concrete cures (Limbachiya 129).  Once the concrete is separated it must be crushed into 10mm- 20mm pieces for fine and coarse aggregate.  This process is often cost deficient and could be considered more harm than good as units of recovered concrete are relocated several times from the demolition site to its secondary use as recycled aggregate.  
 
This is not to say that this applies to all measures of use of recycled concrete aggregate.  An example of proficient recycled aggregate recovery is demolition of bridges and highways as well as airport pavement.  In these instances, the aggregate needs only to be separated from steel reinforcement and has no unknown constituent material (Limbachiya).
 
While the concrete industry has made advances to develop mixes using recycled concrete as aggregate, it has made no tremendous gains in replacing carbon intensive Portland cement with an environmentally friendly alternative.  Through research of recent studies, it can be concluded that recycled concrete aggregate is an acceptable replacement to natural coarse aggregate in bearing and compressive strength.  The variation in pore size has led coarse aggregate to have a lower bulk density and thus higher water absorption, but the increased slump and workability does not affect its strength after it cures for 28 days (Ridzuan).  
 
The practicality of using recycled concrete aggregate on large scale projects seems to be its limiting factor.  Nevertheless, structural properties achieved in a mix using recycled concrete aggregate are sufficient in complying to specifications and could be used.
 
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