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Essay: Sustainable materials – concrete

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  • Subject area(s): Engineering essays
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  • Published: 5 August 2022*
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  • Words: 2,334 (approx)
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1.1 Introduction

The need to establish sustainable materials within the engineering environment is a vital and immediate challenge for the ever-growing area of civil engineering. This has come even more to light over the past few years with a growing number of environmental problems from flooding to earthquakes becoming more prominent across the world.

One material that is widely used across the world is concrete. Concrete is used due to its reliable nature of being able to withstand extremes of weather conditions. This however only is to a certain degree with other materials added in to the mix to work against the harshest of the extreme weather. When concrete is freshly made it can be poured into many different moulds to shape it in to the desired shape.

With an ever-increasing population there is increase in need for buildings and structures therefore there is much more demand on the resources. This means that a more sustainable material to replace one of the components of concrete should be found so that there is an alternative and possibly more situational version of concrete when this alternative is used. Rubber is a possible alternative.
Rubber is a hard material to find when it is being recycled as what tyres are made up off. Tyres are among the most difficult items to recycle given that they’re made up of a combination of steel, rubber and textile. There is about as much as half-a-million tonnes of rubber tyres entering the waste stream across the UK each year. (http://www.asm-recycling.co.uk/blog/a-guide-to-tyre-recycling-in-the-uk/ )

Most of the available rubber is created from tyres creating crumb rubber. This means due to its small size it can’t be used for many things. However, there are certain things that it can be used for such as Sports surfaces and safety mats for children’s playgrounds and High-performance running tracks and sports pitches. It has also been used for Rubberised asphalt for road surfaces showing that it can be used in concrete.

From using rubber in roads, it has been shown that it makes them quieter. As it is quitter it has been used in some of Scotland’s busiest roads. The experts say that this is because the rubber helps to thicken the bitumen therefore dispersing the sound waves.

1.2 Objectives

The objective of this individual project was to investigate and find out results of how rubber aggregate would affect concrete when replacing the fine aggregate. This would be done by doing the following tests, slump test, Sieve analysis, drying / shrinkage test, sorpotuity test, Hammer Test, PUNDIT test, and a Compressive strength.

Some of the aims for this report include:

  • To create four mixes of concrete with different percentages of rubber aggregate within the mix design, being 0%, (control mix), 25%, 50% and 100% rubber aggregate.
  • To understand how rubberised concrete is affected through non-destructive testing laboratory testing and evaluation.
  • Compare the results of hardened and fresh properties of concrete from the laboratory with previous experiments done on rubberised concrete to see if there are any trends.
  • Establish if using rubber aggregates within concrete is a sustainable and practical option.

1.3 Scope of work

This research was conducted to investigate the properties of rubberised concrete. The volume of aggregate was replaced with 25%, 50% and100% of crumb rubber. Cubes and prisms were prepared with replacements for certain tests, in addition to a standard control mix to compare the mix to. Each mix will be subjugated to five tests consisting of, a Sorptivity test, drying/ shrinkage test, hammer test, pundit test and a compressive test. Each of these tests was done according to the relevant British standard.

2.1 What is concrete

2.1.2 Components of concrete

Concrete is a mixture formed out of four main ingredients. These ingredients are air, aggregates made up of a combination of fine aggregate and course aggregate, water and cement.

Concrete is made of up to 8% of air, between 7-15% of cement, 14-21% of water and 60-75% of aggregates which are coarse and fine. As shown below;
(https://www.cement.org/cement-concrete-applications/how-concrete-is-made)

The main reason concrete is so widely used is due to its brilliant trait of being plastic and malleable when newly mixed, strong and durable when hardened. Therefore, concrete can be used to create various structures such as skyscrapers, roads and dams.

The main reason that concrete can be used in a wide range of projects is because each mix is created differently to fit its purpose.

2.2 The concrete mix

2.1.2 Water

The quality of water used can influence the quality of concrete produced. Water of poor quality may have impurities such as slit, clay, acids, alkanes and salts, organic matter and sewage. If there are too many impurities within the water this may have an adverse effect on the concrete.

If when washing aggregates you use impure water, the final concrete produce may be negatively affected in terms of strength and durability.

2.2.2 Aggregates

Aggregates are granular materials such as crushed stone, sand or gravel. In this experiment crumb rubber will be replacing the fine aggregates. The aggregate of a concrete mix is the main deciding factor for what the concrete mixture is used for. This is because 60 to 75 percent of the total volume of concrete is made up of aggregates. These aggregates are split into two categories: fine aggregates and coarse aggregates. Coarse aggregates are any particles greater than 0.19 inch, but generally range between 3/8 and 1.5 inches in diameter. Whereas fine aggregates generally consist of natural sand or crushed stone with most particles passing through a 3/8-inch sieve. ( https://www.cement.org/cement-concrete-applications/concrete-materials/aggregates) All concrete mixes must have a mixture of coarse and fine aggregates.

Aggregates strongly influence concrete’s freshly mixed and hardened properties this means that depending on the characteristics of the aggregates it can affect the concretes function and usage in different areas of building. To find out the exact size of the aggregates a sieve analysis should be completed.

2.2.3 Sieve analysis

A sieve analysis is an important stage of any experiment using a concrete mix. It is completed before the concrete mixture is formed to determine the size of aggregate that should be used. This is important due to their size impacting on the amount of void spacing in the concrete when it is mixed.

2.2.4 Cement

Cements are finely ground powders within concrete that bind aggregates together. There are many different types of cement, which can be arranged into two categories: non-hydraulic and hydraulic. Non-hydraulic cement is a type of cement that hardens by carbonation with the carbon present in the air. Whereas hydraulic cement hardens due to the addition of water. (https://civiltoday.com/civil-engineering-materials/cement/81-cement-definition-and-full-details)

In the early stages of hydration when concrete is in its plastic stage, cement mortar gives fresh concrete its cohesive properties.
The most commonly use concrete is a hydraulic cement called Portland cement.

2.3 Rubber

2.3.1 What is rubber?

There are two types of rubber: natural and synthetic.

Natural rubber is produced by plants. At least 90% of natural rubber comes from the tree Hevea brasiliensis in the rubber plantations of Indonesia, the Malay Peninsula, and Sri Lanka.

There are many different types of synthetic rubber being produced in the current market. One of the main types of synthetic rubbers commonly used for tyre manufacture is styrene-butadiene rubber and butadiene rubber.

Due to the materials that rubber is made of, when it is made into a certain shape or functions it becomes difficult to break down, so it can be recycled. There is a lot of rubber used across the world. When items made of rubber such as a rubber tyre gets damaged it is cheaper to scrap it than repair it.

The UK reprocessed 479,000 tonnes of waste tyre materials in 2009. This means there is a high wastage of materials taking up space from the waste tyres alone. To counter act this, waste tyres are reused.

In 2010, just over 30 per cent of waste tyres were turned into crumb, 18 per cent were used in energy recovery, nearly 20 per cent were re-used (in the UK or abroad), 16 per cent were specifically used in landfill engineering and 11 per cent were re-treaded, according to the Environment Agency.

2.3.2 Rubber aggregate in concrete

Rubber has been used as aggregates in concrete for the past few years. This has been a replacement for some fine and coarse aggregates. Rubber has been used instead of normal aggregates due to the high demand of normal aggregates compared to the huge amount of waste tyres and other rubber equipment.

One usage of rubber aggregate can be seen to be used in some roads in Scotland. These have been refurbished with asphalt, containing shredded rubber from old tyres. The main advantage of using this type if rubber aggregate in roads is that it is quieter and reduces noise pollution. Another advantage is that it requires less maintenance and still allows for drainage. It has also been claimed to save money as the rubber is thinner than standard roads thus costing less.

As seen below rubber can be used in roads in Britain due to the properties it gives off.

2.4 Fresh properties of concrete

2.4.1 slump test

A slump test is a simple test to find out the workability of a material. The workability is the relative ease with which a fresh mix can be handled, placed, compacted, and finished without segregation or separation of the individual ingredients. To find out the workability a slump test is done finding out how much the fresh concrete mix slides down along an inclined plane. There are four different types of slumps they are called a true slump, a shear slump and a zero slump. A true slump is when the slump is even when slumping down the inclined plane, this means that this mix is even and can be used. A shear slump is when the slump is uneven when slumping down the inclined plane this means that this mix is not able to be used as it.

A high-slump concrete is one that is very fluid, and a low-slump concrete is drier and stiffer. A high-slump mix may cause excessive bleeding, shrinkage, cracking, and dusting of the hardened concrete. (https://theconstructor.org/concrete/properties-of-fresh-concrete/6490/#Properties_of_Fresh_Concrete_for_Construction_Works)

There are a few different types of slumps:

An analysis of a slump test using rubber aggregates

2.5 Hardened properties of concrete

2.5.1 Sorptivity test

A Sorptivity test is used to indicate the porosity in concrete. To do this, cubes are needed to be cured for 28 days then dried. During the process it is noted how much water can get into the gaps between the aggregates, known as the pore spaces of concrete. The reason this is so important is that the test results are used to show and predict the service life of the concrete.
In previous experiments with concrete containing rubber aggregates it is shown that the higher the amount of percentage of rubber aggregate the higher the Sorptivity coefficient is in the mix. This means that there is a correlation in the percentage of voids in the concrete to the percentage of rubber used.

This experiment has not been used on rubberised concrete many times in the past. It could be used to show that there is a correlation between rubber aggregates and the sorptivity coefficient. This is not certain due to the limited amount of results available.

2.5.2 Drying/ shrinkage test

a drying and shrinkage test is used to indicate

it can be affected by many things such as;

  • Material Selection
  • Water cement Ratio
  • Environmental Conditions
  • Cement Content
  • Aggregates
  • Type of Cement used in the mix
  • Admixture in Concrete
  • Size and Shape of the Specimens

The water cement Ratio selection is important as higher the water to cement ratio of the concrete mix, more is the chances for drying shrinkage. As the water to cement ratio increases, the strength of the paste and the whole stiffness will decrease. Hence shrinkage increase with the increase of water.

The rate of shrinkage will decrease the lower the humidity in the area therefor making environmental conditions important.

The cement content is important as the higher as the increase in the cement content, the more rate of shrinkage will increase.

The size of aggregates is important as the bigger the maximum size the lower the shrinkage. and if it is a rougher aggregate it will also resit shrinkage.

The surface to the volume ratio is a factor that can influence the rate of shrinkage. the magnitude of shrinkage will decrease with the increase of surface to volume ratio.

Hammer Test

A hammer test is done to assess the relative compressive strength of concrete based on the hardness at or near its exposed surface. The rebound hammer test graph describes the correlation between the strength of concrete and rebound number.

The rebound hammer that will be used in this experiment is the Silver Schmidt.

Previous experiments of rubber aggregates in concrete

PUNDIT test

A pundit test is an ultrasonic pulse velocity (UPV) test to examine the quality of concrete.
The pundit that will be used in this experiment is the Pundit 200. The measurement of pulse velocity may be used to determine the homogeneity of the concrete, the presence of voids, cracks or other imperfections and, even changes which may occur over time or through external influences.
The pundit consists of a control unit and two transducers. One of the transducers is used to send an ultrasonic pulse and the other is to receive an ultrasonic pulse.

Experiment with pundit test

Compressive strength

A compressive strength test is a basic test and is given by the ratio between load and area of the specimen. This means that it lets you know the strength of the concrete when hardened. compressive strength is the maximum compressive stress which a material can sustain before failure.

To do a compressive test it must be conducted for 7 days or 28 days after curing. When doing this test, it must comply with the relevant British standard.

As seen below from previous experiments of concrete with rubber aggregates.

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