Virtual Sieving :

Machine Vision Method for Grading Crushed Aggregates

A. The project background

The construction industry in the UK currently uses in excess of 218 million tones of crushed aggregate per year. Approximately 1.25 billion pounds is spent per annum wining this material and processing it to a quality required by end of user (HM Trsury,1998). In terms of quality control the sector is relatively underdeveloped with only very simple manual tests being applied to the end product. As a result produce may be produced that fails to meet the specification and must be reworked. Similarly excessive, and wasteful, crushing and processing effort may be used to produce the desired product.

From an economic perspective it is therefore essential that maximum economic value is obtained from the quarried stone, which will require wastage to be eliminated from each stage of the processing chain. The quality of the aggregate produced in terms of the consistency of it's size and shape also has a major influence on the quality ( particularly in the relation to workability and durability) of the concrete and blacktop mixes subsequently produced. The proposed introduction of an aggregate tax is also going to drive the need for more efficient usage of the quarried rock.

Round or cubic shape aggregate particle shave traditionally been considered the most suitable in relation to meeting the of industry, although works by Hobeda,1988 and Reinhardt,1969 have suggested that bituminous mixes including non-cubic fractions can lead to better road pavement layer stability.

The development of a rapid and efficient means for classifying aggregate size and shape could therefore enable the beneficial properties of an aggregate to be more fully exploited.

Aggregate sizing is carried out in the industrial context by passing the material over sieves or screens of particulars sizes. Aggregate is a 3-dimensional material and as such need not necessarily meet the screen aperture size in all directions so as to pass through that screen. The British Standard specification (as da American and European Specifications) suggest that any single size aggregate may contain a percentage of larger and smaller sizes, the magnitude of this percentage depending on the use to which the aggregate is to be put. BS 63 Part2, 1987 illustrates typical specifications.

To monitor the range of size of aggregate particles produced from any particular screen, regular laboratory tests is carried out. This involves sampling the aggregate from either the moving conveyor belt or alternatively from the stockpile produced. A sieve analysis test is carried out to assess the range of particle sizes present in accordance with BS 812 Part 103 1985. This test is time consuming and therefore only a relatively small fraction (2 kg per 400-500 tone) of the aggregate produced is ever tested. The quality of the result also relies heavily on good sampling technique, which means that feedback to the quarry operators can be slow and many cases unrepresentative.

Certain shape parameters are also specified for particular uses, the most common being Flakiness and the Elongation indices. BS 812 Section 105.2 1990. These tests are also very labor intensive and time consuming and carried out on an event more limited number of samples.

An ability to measure the size and shape characteristics of an aggregate or mix of aggregate, ideally in real time is therefore desirable to enable the most efficient use to be made of the aggregate and binder available.

B. The project work

C. The published papers

1. X.Qiao, F. Murtagh , Paul Walsh, P.A.M Basheer Adrian Long Danny Crookes " Information Retrieval from Image Databass: The Case of Automated Grading of Industrial Materials", K. Dawson-Howe, A.C. Kokaram and F. Shelvin, Eds., Proc. Irish Machine Vision & Image Processing Conference 2004, IMVIP 2004, Trinity College Dublin, pp. 11-18

2. X. Qiao, F. Murtagh , Paul Walsh, P.A.M Basheer Adrian Long Danny Crookes " Virtual Seive": Content-Based Image Retrieval in Automated Grading of Engineering Materials, Proc. ISSC (Irish Signal and System Conference) 2004, Belfast

3. X. Qiao, F. Murtagh, D. Crookes, P. Walsh, P.A.M. Basheer and A. Long, "Machine vision methods for the grading of crushed aggregate", in: Opto-Ireland 2002: Optical Metrology, Imaging, and Machine Vision. Eds. A. Shearer, F.D. Murtagh, J. Mahon, and P.F. Whelan. Proceedings of the SPIE, Volume 4877, pp. 264-270, 2003.

4. F. Murtagh, X. Qiao, D. Crookes, P. Walsh, P.A.M. Basheer and A. Long, "Benchmarking segmentation results using a Markov model and a Bayes information criterion", in: Opto-Ireland 2002: Optical Metrology, Imaging, and Machine Vision. Eds. A. Shearer, F.D. Murtagh, J. Mahon, and P.F. Whelan. Proceedings of the SPIE, Volume 4877, pp. 248-254, 2003.

5. F. Murtagh, X.Qiao, D. Crookes, P. Walsh, P.A.M. Basheer, A. Long and J.L. Starck, "A machine vision approach to the grading of crushed aggregate" Machine Vision and Applications, under review, 2003.

6. F. Murtagh, X.Qiao, P. Walsh, P.A.M. Basheer, D. Crookes, and A. Long, "Grading of construction aggregate through machine vision: results and prospects" Computers in Industry, submitted, 2004.

7. X. Qiao, F. Murtagh, etc "Image Processing of Coarse and fine aggregate images" International Workshop on Structural Image Analysis in Investigation of Concrete SIAIC'02-(pp 231-238) Warsaw, Oct. 21-23,2002.

D. The contact people.