Did you know that concrete is the second-most consumed substance globally after water? It may be surprising to learn that the concrete industry is a huge global industry, worth over $137 billion.
If you take a walk through either a city or town, there is a high probability that the buildings, roads, and bridges around you are made of concrete. Concrete is a versatile ingredient that plays a significant part in modern infrastructures.
Table of Contents
Worldwide, it is estimated that approximately 10 billion tons of concrete are produced annually because of its strength and flexibility.
The basic ingredients of concrete are a combination of cement, fine aggregates, coarse aggregates, and water.
In the discussion that follows, we will explain some sustainable and cost-effective alternative aggregates for concrete and (hopefully!) provide a pathway for more sustainable construction processes.
Why Source Alternative Aggregates for Concrete?
Construction activities are increasing around the world, and as a result, concrete demand is increasing at an increasing rate. This exponential increase is leading to shortages in aggregate (natural raw materials). Concrete production produces approximately 10% of all greenhouse gas emissions.
Because we are living with the consequences of material depletion and environmental degradation, sourcing sustainable and alternative aggregates for concrete is are necessary step towards a more sustainable concrete ecosystem.
Natural aggregate sources, such as river beds and quarries, are being depleted faster than nature can replace aggregate materials, and there are significant environmental consequences due to this continuing extraction, including erosion, habitat loss, and polluted waterways.
Alternative aggregates are a viable solution. Not only can alternative aggregates contribute to the conservation of natural resources, but many alternative aggregates are used in the process of reuse, meaning they were already diverted from a landfill, making them sustainable, environmentally friendly, and likely less expensive than natural aggregates.
Overview of Replacing Concrete Aggregates
Concrete materials may partially replace some of their natural materials using recycled or waste materials, which can help reduce their harmful effects on the environment.
Example replacements for cement are: fly ash, silica fume, wood ash, and blast furnace slag. Concrete developed with recycled materials can be termed Green Concrete.
Similarly, it is important to understand that coarse aggregates and fine aggregates can be replaced with concrete materials that are not from a natural source.
However, the replacement with concrete materials can reduce emissions, which may warrant a more robust consideration of the circular economy. Various alternatives exist, including rubber, plastics, and by-products from the agricultural and industrial sectors.
Below are examples of five significant alternatives.
1. Papercrete: Fibrous Concrete Using Waste Paper
Materials Required:
- Waste paper slurry
- White lime or sand
- Portland cement
Manufacturing Process:
- Soak shredded waste paper (1 inch or smaller) in water for 24 hours.
- Mix in a slurry with a mechanical mixer for 10 minutes.
- Drain excess water.
- Combine Portland cement, lime, and paper slurry in a 1:0.5:4 ratio.
- Add water for workability as needed.
Advantages:
- Cost-effective and created with low-tech tools.
- Lightweight and easy to form.
- Some limit in low-load-bearing uses.
Disadvantages:
- Less rigid when compared to conventional concrete.
- Low compressive strength (roughly 96-1.1 MPa), much lower than conventional concrete (15 – 70 MPa).
- Weak against moisture.
Best Use Cases:
- Non-load-bearing walls
- Insulation panels
- Garden art or décor
Papercrete offers an interesting way to recycle paper waste while generating a useable building product, however, for it to be widely used in construction, it would need enhancement either with additives or as a hybrid formulation.
2. Plastic Waste Concrete
Raw Materials:
- Plastic waste
- Cement
- Fly ash
- How it’s made (Based on MIT Study):
- Shred the plastic into small flakes.
- Gamma radiation exposure for a very short time.
- Grind into a fine powder.
- Combine with cement and fly ash.
Advantages:
- Very flexible and customizable.
- Lightweight, resistant to impact.
- Excellent water, chemical, and thermal resistance.
- Low-cost to produce.
Disadvantages:
- Poor bonding properties lead to lower strength.
- Low melting point indicates not all uses.
- Currently not ready for structural applications.
Best Use cases:
- Pavements
- Sound barriers
- Non-structural panels
Plastic waste concrete demonstrates a new way to deal with plastic landfill waste while decreasing the overall costs of construction. Further studies and improvements on the bonding strength will help facilitate more widespread use.
3. Glasscrete: Recycled Glass as Fine Aggregate
Raw Materials:
Glass aggregates crushed to sizes #8 to #4 Cement Coarse aggregates
Manufacturing Process: Clean broken glass to remove organic contaminants. Dry and cool the glass particles. Grind the glass particles to glass sand in a Los Angeles abrasion machine. Mix with cement and aggregates similar to concrete.
Advantages: Better workability than natural sand. Improves durability and concrete efficiency. Improves the aesthetics of the concrete with a polished/translucent finish.
Disadvantages:
- Possible Alkali-Silica Reaction (ASR) causing cracking.
- Cleaning the glass is a labor-intensive and expensive task.
- Glass may not be best used in high-stress applications.
Best Use Cases:
- Decorative concrete surfaces
- Tiles
- Countertops
Glasscrete provides a particularly valuable input in producing aesthetically pleasing architectural features. As glass recycling methodologies improve, the potential use in construction is not formatted.
4. EPScrete: Lightweight Polystyrene Concrete
Raw Materials: Expanded Polystyrene (EPS) beads 1mm-3mm Cement Sand Water
Manufacturing Process: Mix water and colour (optional). Add Portland cement and sand and mix well. Slowly add in the EPS beads and continue mixing. Add or remove water for a workable consistency. Pour into molds and cover with plastic. Initial set in 24 hours, curing for 1 month while wet.
Advantages:
- 82% lighter than traditional concrete.
- Less reinforcement is needed, and good insulating properties.
- Great for thermal and acoustic insulating applications.
Cons:
- Not as strong as stone-based concrete.
- Only appropriate for light structures or as a decorative.
Best Uses:
- Wall partitions
- Rooftop insulation
- Garden walls
EPScrete is great for applications where lightweight and insulation materials are critical. It also has non-toxic properties and is recyclable, which makes it a more sustainable material in construction applications.
5. Crumb Rubber from Used Tires
Raw Material:
Vehicle tires ground into crumb rubber (3 to 10mm)
Manufacturing Process:
- Crumb rubber is used as a partial substitution for coarse aggregate.
- The crumb rubber is blended with conventional concrete materials.
Advantages:
Increased flexibility of concrete.
- Limits the cracking or shattering of concrete under stress conditions.
- Environmental waste from scrap tires is being reused.
Disadvantages:
- Results in air pockets, which reduce compressive strength in concrete.
- Completely unsuitable to be used in structural or load-bearing applications.
Best Uses:
- Sidewalk construction
- Sports courts
- Playground surface
Rubberized concrete gives added life to used tires and also provides additional shock absorption, ideal for surfaces with a lot of foot traffic or where play occurs.
Other Promising Alternatives
With continual advancements, even more materials are currently being investigated as eco-friendly aggregate replacements, including:
- Rice husks
- Sawdust
- Oyster shells
- Tobacco waste
- Groundnut shells
- Cork
- Sugarcane bagasse ash
While many of these materials are potentially available and sustainable, their impact as a concrete material varies. Much work is still experimental and still needs to be researched for strength, durability, and practical applications.
In rural and agricultural areas, any locally available waste now serves as an effective alternative aggregate that could save considerable amounts of transport, reduce environmental impact, and support the circular economy.
Compressive Strength of Concrete
Before selecting any alternative aggregate for concrete, it is important to understand: Compressive strength ( how much load concrete can carry before it fails)
Compressive strength is typically measured in either MPa (Mega Pascals), or psi (pounds per square inch). The compressive strength of concrete usually lies between 2500 psi to 6000 psi (150 to 400 kg/cm²). If alternative aggregates are included, compressive strength should be a determining factor based on your application.
For example, if you are completing a residential building wall, lower compressive strength could be acceptable. For roadways, bridges, and commercial buildings, higher compressive strength is a must.
Why Aggregate Quality Matters?
You may be asking: what is the significance of aggregates in the concrete strength development?
According to the Portland Cement Association (PCA), 70% to 80% of the concrete volume is aggregates. The characteristics of aggregates actually affect:
- Workability (better workability with smooth, rounded aggregates)
- Strength and durability (related to the size, shape, and texture of the aggregates)
- Water-cement ratio (influenced by moisture)
The moisture content also impacts the water needed for the mix. For instance, if an aggregate has too much moisture, it can adversely affect the water-to-cement ratio, reducing the strength and final structure. Thus, it is important to monitor moisture and grading.
If grading and quality control are appropriate, then it would be possible to seek an efficient and reliable result when using recycled or waste based aggregates.
Types of Aggregates
Aggregates are usually categorized into two groups:
Coarse aggregates:
– Made of crushed stones or gravel
– Size ranges from 9.5mm to 37.5mm
Fine aggregate:
– Made of crushed stones or natural sand
– Size is less than 5mm
Optimal performance is only achieved when there is a balanced combination of both coarse and fine aggregates.
Mixed aggregates that are poorly rated or aggregates containing contaminants will restrict the ultimate performance of concrete, even with the best cement available for use.
Specialty aggregates, like lightweight aggregates or recycled aggregates, can also be selected for a multitude of reasons, such as design considerations relating to strength, insulation, and aesthetics.
Final Thoughts
As the construction industry continues to grow, it is now time to embrace a more alternative approach to the aggregates that are used for concrete. This idea is no longer a trend; it is now a necessity.
These sustainable alternatives will provide users with more sustainable mediums to sacrifice resources, reduce carbon emissions, and allow new avenues for innovation.
We have lots of opportunities for alternatives, from recycled paper and plastics to crushed glass and crumb rubber. But we need to begin weighing the strength, durability, and practicality of these media before we can expect them to be used widely in structural construction.
The future of concrete is about making performance and sustainability coexist in harmony. With continued research and utilization of these materials, we may potentially change how we construct the world around us, one concrete slab at a time.
Incorporating these materials not only helps to save our planet, but it may also open new doors for innovation, saving costs, and creating greener options for the next generations. The construction industry is at the precipice of change.
If we are to make the change for not just our industry but also our planet, we should start pouring the foundation today.
Sagar Telrandhe is a Construction Engineer with a B.Tech in Construction Engineering & Management. Passionate about infrastructure development, project planning, and sustainable construction, he specializes in modern construction techniques, project execution, and quality management, contributing to efficient and innovative building.