Development of a simple trixial test for characterising bitumen stabilised materials
Date
2009-03
Authors
Mulusa, William Kapya
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
Abstract
The need for a more reliable testing procedure for the characterisation and Quality Assurance/
Control of Bitumen Stabilised Materials (BSMs), besides UCS and ITS testing, has long been
recognised by the roads industry. In fact, at CAPSA 2004 and CAPSA 2007, discussions of improved
test methods for granular materials, i.e. possible replacement tests for CBR procedures, were
conducted in workshops. Triaxial testing for the evaluation of shear parameters is widely recognised
as a reliable method of measuring these critical performance properties of granular and Bitumen
Stabilised Materials (BSMs). However, the triaxial test in its current state as a research test has little
chance of extensive use by practitioners and commercial laboratories, because of complexity, cost
and time issues. Major adaptations to the research triaxial test are necessary, before this useful test
can have a chance of being accepted by road practitioners.
The main aim of this study is to investigate possibilities of developing a simple, affordable, reliable
and robust test for characterizing granular and bitumen stabilized materials thus linking test outcome
with in-situ performance. This is achieved through the innovative design and manufacture of a
prototype triaxial cell capable of accommodating 150 mm diameter by 300 mm deep specimens. The
cell is simpler than the research (geotechnical) triaxial cell and the operational protocols have been
streamlined, thereby reducing the time and steps required in assembling specimens and testing them.
In order to ensure the development of an appropriate triaxial cell for industry, a survey was
conducted aimed at investigating currently available facilities, testing capacity and resources within
civil engineering laboratories in South Africa. Findings of the survey (Appendix 4) have provided
guidance with regard to the nature and sophistication of any new tests to be developed. The survey
highlighted some of the limitations and lack of sophistication of the current loading frames used for
CBR and UCS testing such as lack of electronic LVDTs, limited overhead space, limited loading
capacity and others. Most laboratories would need to invest in new loading facilities to carry out
triaxial tests.
A review of the test procedure for monotonic triaxial test showed that two main factors contribute to
the complexity of the research (geotechnical) triaxial cell namely, time taken to assemble the
specimen accurately in the cell and secondly the inherent design of the cell which makes it water
and/or air tight at relatively high pressures.
The design of the Simple Triaxial Test, therefore, was aimed at overcoming the drawbacks of
research triaxial test e.g. fitting a membrane to each specimen to be tested, through considerable
simplification by means of a new structure design and procedure of assembly of specimen into the
cell. The advantage of addressing these issues would be reduction in the number of steps required in the test procedure and therefore reduction in testing time. The design of the cell particularly was
preceded by a conceptualization process that involved investigation of numerous options. Concepts
such as the bottle, encapsulated-tube, bottle and sandwich concepts were considered and given
reality checks. In addition, available triaxial procedures of a similar nature e.g. Texas Triaxial, were
evaluated and analyzed.
Ultimately, with some trials and innovation, a design was developed for a simple triaxial cell
comprising a steel casing with a latex tube which is then introduced around the specimen sitting on a
base plate. It is based on the ‘tube concept’ in which the specimen acts like a ‘rim’ and the cell acts
like a ‘tyre’ providing confinement to the triaxial specimens for testing, within the tube. This approach
eliminates the use of O-rings and membranes for the specimen and tie-rods for the triaxial cell, thus
reducing testing time considerably. The overall dimensions of the cell are 244 mm diameter by 372
mm height (Appendix 5).
The cell was manufactured at Stellenbosch University Civil Engineering workshop and preliminary
tests were conducted under this study. Parallel tests were also conducted with the Research Triaxial
Test setup at Stellenbosch University in order to determine if preliminary results obtained with the
Simple Triaxial Test setup were comparable therefore providing a means of validating the data.
Results of analysis of variance (ANOVA) show that variability between Simple Triaxial Test (STT) and
Research Triaxial Test (RTT) results is less significant whilst that within samples of STT and RTT
results is quite significant. Comparisons also show that good correlation were obtained from
Reclaimed Asphalt Pavement (RAP) Hornfels + 3.3 % Emulsion + 0 % Cement mix and mixes with
the G2 base course aggregate whilst completely different correlation was obtained from RAP + 3.3 %
Emulsion + 1% Cement. It is evident however that the differences observed stem from material
variability i.e. random variability to one degree or the other and not to the STT apparatus. It is
recommended for future research that more STT versus RTT testing be done especially on a mix with
known mechanical properties when compacted to a specified dry density, e.g. graded crushed stone
(G1) compacted to 100% mod. AASHTO.
In summary, a locally made, low cost, relatively durable triaxial cell with relatively easy and quick
specimen assembly procedures has been developed. It is now possible to perform triaxial tests on
150 mm diameter by 300 mm high specimen relatively easily and quickly. However, the challenge of
validating results obtained, as well as improving the manufacture process of its main component, the
tube, still remains.
Description
Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2009.
Keywords
Bitumen stabilised materials, Theses -- Civil engineering, Dissertations -- Civil engineering