Concrete aggregate alternatives
A Brief History of Concrete and its UsesEdit
For those who live in the global north, concrete is a common place. One would be hard pressed to go a single day without seeing this material performing some kind of service. As a civil material, engineering use it to solve problems that require high strength such as roads and buildings. According to the EPA, in 2004 the world produced 2 billion metric tons of cement over 150 countries. Cement is only one of the five ingredients used in a concrete mixture. With proper ratios of cement, rock, fine aggregate, air, and water, the EPA further cites that the previously mentioned amount of cement would be enough to produce 14 - 18 Gt of concrete making it the most manufactured material in the world.  The earliest use of concrete in the United States involved the construction of the Erie Canal in the early 19th century. During this time, concrete was novel and not very well understood. Presently, most structures use what is known as reinforcement. Steel bars with ribs or notches are added to the concrete to give it strength in tension. Even though reinforcement of concrete was patented by S.T. Fowler in 1960, it was not accepted as a practice until after 1900 because it was too expensive and not reliable enough. 
Issues With ConcreteEdit
Concrete is a very fickle material. After all, this quality was one of the major reasons why it did not gain acceptance for so long.  Since concrete started being used on a more regular basis, committees such as ACI (American Concrete Institute) have formed over the years to research and document acceptable construction practices. The role these committees have played over the years has proved imperative because in establishing consistent a measurable ways to build with concrete. The way concrete is mixed and how much of each ingredient is added cause large variations in performance. Also, how much steel and where it is placed make a difference in what the concrete is capable of doing. These issues have been slowly engineered and researched to ensure successful constructions but there are still regions where solutions are too costly. Because soil type is hardly consistent throughout the world, fine aggregate or rock can be hard to acquire. Large amounts of steel can be added to concrete in areas with high seismic activity but this does not change the fact that concrete is a brittle material.
Alternatives to the Traditional AggregateEdit
If there were criterion for a solution for the issues held by concrete, it would meet the following: Low tech, widely available, and be an improvement. A solution would need to be low tech because application needs to not require expensive tools. Barriers, such as these, cause implementation to be limited to the well funded or just slow. Wide availability ensures that people will have access to the material and that it will be cheap. Cost is one of the major contributors to the problem of limited access to concrete aggregate and a cheaper solution would help communities anyway. Concrete’s greatest strength of being able to handle high compressive loads also doubles as its greatest weakness in ductility. Not every project would benefit from increase ductility but areas with high seismic loads would view this as a considerable improvement.
Metrics for ConcreteEdit
When a designer works on a concrete project there are several aspects that are taken into account: strength in shear and moment, and serviceability which is the amount of deflection that the element will undergo with load. Compressive strength and Elastic Modulus are two pieces of information that are needed before any analysis can be started. These two data values also act as a nice metric for compare differing concrete mixtures to each other. 
The compressive strength of concrete is useful for calculating very important qualities of a concrete beam or column. This value is typically expressed in terms of psi (pounds per square inch) in the United States and as MPa elsewhere and is denoted by the term f’c. This term shows up in the equation for calculating the shear and moment capacity of a beam section and the axial load capacity of a column. In order to determine this value for a specific concrete mixture, multiple cylinders are cut or formed and placed in a machine that slowly added load to the top until the section fails. Testing more than one cylinder of each type is necessary because the contents of concrete vary based on many factors. The data from these tests are averaged and reported as the mixture’s f’c value. 
The elastic modulus of a material expresses the relationship of how material handles stress, the load within the material, with the strain or the way the material deforms. Measuring this quality of a section is very similar to the way a section is tested in compression in that a machine applies a large load to the top. In this test, the load is applied to a predefined value, released, then the load is reapplied. The goal for this test is to see how the material deforms under the load and to see if it will return to its original shape when the load is removed. This value is used to calculate how much the material will deflect while loaded. 
Currently there is no proper replacement for the traditional aggregates for concrete. As will be discussed in list of materials below, every addition changes the concrete’s properties. At the cost of axial capacity, some add improved thermal, sound, and ductile properties to the mix. The goal of each associated paragraph is to explain how the material changes the characteristics of the concrete and what resource poor regions need to know about its new properties.
Papercrete is a hybrid material made of paper slurry, white lime or sand, and portland cement. To make this type of concrete, add paper pulp, any waste paper will be appropriate, to a basin of water and allow to soak for 24 hours. After this time, mix slurry for ten minutes using a mixer and drain the excess water. The recommended ratio is 1:0.5:4 (portland cement:white lime:paper sludge). Add water as needed to make the mixture workable. This creates a concrete that is not acceptable for structural projects. Walls not used to support have been found to have decreased thermal conductivity and better sound absorption. Because the strain on this type of concrete is so much larger than the stress, walls such as these although being strong will show large amounts of deflection under load.   
When vehicles tires reach the end of their usable life, they can still find some use as a replacement for course aggregate in concrete mixes. Crumb rubber is car or truck tires that are ground up between the sizes of 3 - 10 mm. This mix has very poor compressive strength due to its high air content. It is believed that when this rubber is mixed in with the concrete air becomes trapped in with it. One benefit to the addition of this alternative is the rubber keeps the concrete from shattering in failure.    
Expanded Polystyrene plays a very unique role in concrete. Research has shown that concrete with this material behaves very similarly to light weight concretes containing traditional aggregates. This is believed to be the case because the styrofoam fills in the gaps that would normally be filled with air. EPScrete is made using 1mm - 3mm diameter expanded polystyrene beads, sand, cement and water in the following ratio 666.6:500:233.3:1 (cement [kg/m^3]:sand [kg/m^3]:water [L/m^3]:polystyrene [m^3/m^3]). Even though the large aggregate is replaced with polystyrene, t his type of concrete typically reaches a compression strength of 3psi which is very comparable to lightweight concrete and 25% less than normal weight concrete. EPScrete is cheaper than lightweight concrete and has very good acoustic and thermal resistance. The disadvantages of this material are, because of the integration of polystyrene, decrease strength at temperatures above 300C and is generally sensitive materials that are non-polar.   
High-density polyethylene can be added to concrete and sand as a feasible replacement for course aggregate. Research has shown that plastic bags that are tightly packed and then heated, shrink to a size that can easily be used in concrete. But as the name implies, the material is too ductile at full strength to support structural loads at 2500psi. One benefit to this material is its measurable qualities in holding heat within buildings.    
Concrete with glass aggregate is a material mixture that has a lot of potential but also a lot keeping it from reaching this potential. Any type of glass when broken down to about #8 to #4 sieve. With compression strengths comparable to that of normal weight concrete and when glass powder has the same glueing effect as cement, Glascrete is the best aggregate alternative of the list. Under ideal conditions this may be the case but there are reasons this material has not seen wide adoption. Glass is well known for having issues with ASR (alkali-silica reaction). When this silica chemically reacts with naturally occurring hydroxyl ions in the cement, silica gel is formed and causes cracks in the cement at it absorbs water. Glass is also very expensive to properly clean. Any residue leftover on glass such as organics or sugar can also negatively effect the strength of concrete. For those who can overcome these obstacles concrete made from glass is the best option.    
Based on the findings from the list of materials, concrete with expanded polyethylene instead of virgin rock is the best alternative for aggregate replacement. This material was the only one to still maintain enough strength to still be useable for structural projects and does not require machinery to break up tough materials.
An organization named EPS Industry Alliance is currently large blocks of EPScrete to solve geotechnical problems all over the United States. This geofoam can be found under major freeways in Utah and near Lake Michigan to create a strong enough base to support heavy traffic but not overload weak soils. Its use is not just limited to roadways, but it finds most of its press in projects that lightweight solutions for geotechnical problems. 
Concrete and roadways and buildings eventually reaches its defined end of life and is demolished to make room for more appropriate uses of land or bring the structure back up to code. Today is it common practice for this concrete to get processed into fresh concrete again. Demolished concrete goes through a process that crushes it down to small course sized aggregate so that it can be used again in building projects. For many areas including Michigan, this practice makes more sense than using virgin aggregate for every concrete project.     
Faculty and Students at the University of Miami are looking into researching the feasibility of recycling CRT television tube glass in concrete. These tubes contain toxins that they hope can be encapsulated within the concrete. 
In the field of Masonry, the allowable axial load for bearing walls are significantly reduced because of slenderness effects. Masonry depends on concrete mixtures for its block forms. Aggregate alternatives used in this field may yield axial loads that are more similar to traditional loading capacities.
- Wight, James K. Reinforced Concrete Mechanics & Design. 6E Person:Boston, 2012
- American Concrete Institute. Building Code Requirements for Structural Concrete (ACI 318-11)
- Hibbeler, R.C. Mechanics Of Materials. 7th Person:New Jersey, 2008
- Sgobba, Sara. USe of Rubber Particles from Recycled Tires as Cocrete Aggregate for Engineering Applications. June 2010. http://www.academia.edu/839680/Use_of_Rubber_Particles_from_Recycled_Tires_as_Concrete_Aggregate_for_Engineering_Applications
- Xu, Yi and Lin Hua Jiang. Investigating Mix Proportions of Lightweight Expanded Polystyrene. July 2011. http://www.scientific.net/AMM.71-78.950
- Al Bakri, A.M. Mustafa et. al. Investigation Of HDPE Plastic Waste Aggregate On The Properties Of Concrete. AESS Publications, 2011. http://ideas.repec.org/a/asi/joasrj/2011p340-345.html
- Verdugo, Cesar and Dr. Mang Tia. The practicality, versatility and feasibility of utilizing recycled glass as a concrete aggregate. University of Florida. http://www.honors.ufl.edu/apps/Thesis.aspx/Details/237
- MDOT. Using Recycled Concrete in MDOT’s Transportation Infrastructure— Manual of Practice. Aug. 2011. http://www.michigan.gov/documents/mdot/MDOT_Research_Report_RC1544_368544_7.pdf
- Kelly, Thomas. Crushed Cement Concrete Substitution for Construction Aggregates— A Materials Flow Analysis. 1998. http://pubs.usgs.gov/circ/1998/c1177/c1177.pdf
- EPA. Introduction To WARM and Concrete. http://www.epa.gov/climatechange/wycd/waste/downloads/concrete-chapter10-28-10.pdf
- MDOT. Using recycled concrete for sustainable roadways. Aug. 2011. http://www.michigan.gov/documents/mdot/MDOT_Research_Spotlight_Recycled_Concrete_382375_7.pdf
- Venere, Emil. Concrete recycling may cut highway construction cost, landfill use. April 4, 2011. http://www.purdue.edu/newsroom/research/2011/110421OlekConcrete.html