The use of recycled and secondary aggregates within granular columns

Perkins, Lisa (2023). The use of recycled and secondary aggregates within granular columns. University of Birmingham. Ph.D.

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Abstract

The construction industry utilises 90 % of aggregates. It is recognised that the overuse or mismanagement of a resource can lead to its depletion or exhaustion. The reliance of the construction industry on primary aggregates is unsustainable and the overexploitation of sand and other primary aggregates has depleted reserves in recent decades, causing damage to the environment and creating the potential for shortages. Simultaneously the construction industry is responsible for producing millions of tons of waste, sending approximately 44% to landfill.

The adoption of alternative aggregates, i.e., recycled aggregate (RA) or secondary aggregates (SA), offers a potential solution. However, perceived barriers such as lack of confidence and/or perceived risk with the product (perception that they are inferior to PA, or that there are issues with consistency), a lack of suitable specifications and testing protocols (reliability and quality control issues for certain applications), and certification of the produced product. In addition, there may be a lack of awareness of AA products and issues such as supply–demand and waste management licensing regulations/environmental issues may further hinder their adoption.

There has been a significant research effort to address these barriers through research into the use of alternative aggregates in geotechnics, but a significant amount of this research has proposed lower utility application such as fills, which is a positive step, but is a missed opportunity as these alternative aggregates have potential to be used in higher utility applications such as granular columns.

Granular columns offer a cost-effective, low-carbon ground improvement solution. The technique, suitable for weak cohesive and granular soils, comprises the backfilling of bores with aggregate, forming a composite ground that is stiffer that the original soil.

The use of alternative aggregates within granular columns is not a new concept and has been adopted to some extent in industry but there is scope for increased uptake, and this is hindered by the previously mentioned barriers.

Extensive laboratory research has shown the successful use of alternative aggregates within granular column construction, materials studied include tyre chips, fly ash, bottom ash, steel slag, crushed polypropylene. Waste glass has been suggested as a possible aggregate for granular columns by numerous researchers but has not yet been studied extensively.

In this research the suitability of three alternative aggregates, waste container glass, waste flat glass and APCr (waste ash treated by accelerated carbonation), for use within granular columns is explored. The performance of these alternative aggregates when used to construct granular columns was compared against a primary aggregate, granite.

Two types of tests, static loading and constant rate of strain, were undertaken to evaluate the impact of the type of loading on column behaviour and to assess the suitability of constant rate of strain tests, which are adopted frequently but do not replicate the loading experienced by granular columns in practice.

Laboratory scale granular column tests were completed to assess and compare the performance of each of the materials. It is acknowledged that there are limitations in the use of laboratory scale tests for granular columns, however, there are clear benefits to conducting these tests particularly when determining feasibility such as repeatability, speed and low-cost.

Prior to the model column tests, aggregate index tests that are suggested to indicate the suitability of an aggregate for use within granular columns, including the aggregate crushing value, aggregate impact value, LA value, flakiness, and angle of friction were conducted for each of the materials. The results indicated that the alternative aggregates, APCr and both types of waste glass, would perform poorly if adopted for granular columns.

However, in the model column tests the alternative aggregates performed well and the capacity of APCr was comparable to granite despite the apparent poor quality exhibited in the index tests. The improvement factors achieved in the constant rate of strain tests, based on the average test results, indicate that the APCr and flat glass perform comparably to granite with improvement factors of 2.0 and 1.8 respectively. The container glass performed less well but still achieved an improvement factor of 1.5.

The results for the static load tests, conducted on granite, container glass and APCr columns, corroborated the findings of the constant rate of strain tests as the improvement factors measured for ACPr, granite and container glass were 2.2, 2.0 and 1.5 respectively.

The results of this research are encouraging as the work highlights a potential valuable use for these waste materials. It is hoped that the demonstration of the aggregate index tests being a poor indication of the material behaviour within granular columns inspires investigation into other materials that may ordinarily be dismissed, reducing waste sent to landfill and contributing the achievement of a circular economy.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):