Road Base Construction Research Articles

Engineering Properties of Loess-Fly Ash Mixtures for Roadbase Construction

Southwest Indiana has large deposits of wind-blown loess. Similar deposits are found in other states, including Illinois, Kentucky, Iowa, Minnesota, Missouri, and Nebraska. These soils consist of uniform silt with a plasticity index ranging from 0 to 10. This material is suitable for road construction if it is compacted dry of optimum. However, the material is difficult to work after it becomes wet, which commonly results in construction delays. Indiana also has large stockpiles of Class C fly ash from coal-burning power plants. The ash has cementitious properties after hydration (because of the high calcium content) and can be mixed with native soil to produce a weakly cemented soil. Significant interest exists at the Indiana Department of Transportation about the possibility of using Class C fly ash to improve the engineering properties of Indiana loess soils. The results of a laboratory testing program on the properties of loess–fly ash mixtures are presented. Various percentages of fly ash were mixed with loess soil and specimens were permitted to cure for 3 h to 28 days. Pure loess also was tested for comparison. Changes in Atterberg limits, moisture-density relationships, swell potential, and unconfined compression strength are presented. Based on this testing program, a simple method was developed to determine the optimum fly ash content for construction of a workable loess roadbed to avoid delays in construction due to wet conditions. The data presented will be useful for evaluating the stabilization of loess soils with Class C fly ash in Indiana and other states with significant loess soil deposits.

Use of Soft Limestone for Road-Base Construction in Belize

The results of a highway experiment, constructed in May 1978 in northern Belize, designed to investigate the suitability of locally occurring calcareous materials, known as marls, for road bases are discussed. The marls comprise high-purity carbonate materials containing mainly siltsized particles and fall outside the grading, plasticity, and strength specifications normally required for road bases. Three marls, each with slightly different characteristics, were substituted as road base for crushed stone. One of the marls was also stabilized with ordinary portland cement. Detailed monitoring was then undertaken periodically to determine their performance. The road pavement was constructed on an embankment to ensure good drainage. A good quality surface dressing seal has been maintained. After 19 years of traffic, measured at 1.3 million equivalent standard axles, the marl road bases have performed at least as well as the crushed stone. The cement-stabilized marl road base performed exceptionally well. Stabilization would enable the use of more plastic marls.

Geogrid Reinforcing of Recycled Aggregate Materials for Road Construction: Finite Element Investigation

Economic advantages and environmental benefits encourage the use of recycled materials for road construction. Usually, however, these materials have lower stiffness and strength characteristics than typically used natural materials. Because of these inferior material properties, use of recycled aggregate materials for unbound road base construction may result in increased rutting, differential settlement, and reflective crack propagation. Placement of reinforcement between the underlying soil layer and the aggregate layer has been proposed to improve the load-carrying capacity of road bases constructed from recycled aggregate materials. To investigate the viability of such an approach, finite element analyses were performed of asphalt concrete pavements with base layers consisting of reinforced unbound recycled aggregate materials. The response of such pavements was compared with that of pavements consisting of unreinforced natural aggregates. The criteria chosen for comparison were the influence of the material characteristics of the recycled aggregate and the reinforcement on the development and speed of propagation of reflective cracking in the body of the pavement. Various combinations of reinforcement and aggregate material characteristics were simulated. It was concluded that the placing of reinforcement can reduce the speed of crack propagation into the top layer, improve load spreading in the unbound base layer, and prolong the economic life of the construction.

Pre-Treatment of MSWI fly ash for useful application

At TNO a feasibility study has been carried out into the possibilities of removing a number of easily leachable elements from MSWI fly ash. The aim of this study was to find a way to useful application of MSWI fly ash (after treatment) as a road base construction material. A combination of a slight washing step and a stabilization/solidification step with cement and other additives appeared to be convenient to meet the severe standards of the Dutch Building Materials Decree. The slight washing step removes more than 90% of the cadmium and chloride that were originally present. Over 50% of zinc and sulphate are removed. The remaining solid material (approximately 70% of the original quantity) could be easily processed into a bound road foundation layer, adding some 20% cement and other additives. The leaching characteristics of the stabilized material meet the most severe standards of the future Building Materials Decree in The Netherlands. This means that the material may be applied without any provisions. In contrast, stabilized but unwashed MSWI fly ash could not even meet the less severe standards. A global cost estimate shows that, at least for the Dutch situation, the processing costs of the combination (washing, processing of the washing liquor and stabilization) equal the costs for disposal of MSWI fly ash.

A mixture of fly ashes as road base construction material

By order of NOVEM, a research was carried out by TNO Environmental and Energy Research into the formation, occurrence and disintegration of ettringite in stabilizations of a mixture of fluidized bed fly ash and powder coal fly ash. Two series of specimens were manufactured, one from fluidized bed fly ash only and one from a mixture of fluidized bed fly ash and powder coal fly ash. From both series of specimens the compressive strength and the formation of minerals (especially ettringite) were determined at different times between 2 and 890 days after manufacture. The quantity of ettringite present in the specimens and the compressive strength both show a maximum. During the first couple of weeks calcium hydroxide, calcium sulphate and aluminium hydroxide are transferred into ettringite (until one of these components has disappeared). The formed ettringite fills the pores and thus contributes to the strength development. However, if too much ettringite is formed, the specimens “explode” due to expansion. In the course of time (100–200 days after manufacture) the formed ettringite again disintegrates, as a consequence of carbon dioxide attack. Then the compressive strength decreases, but stabilizes at a lower level. If the content of calcium oxide in the mixture of raw materials is not beyond 12%, and the mixture is allowed to react before manufacture of the specimens, the formed ettringite does not cause any crack formation in the specimens. It is possible to construct good and cheap stabilizations from a mixture of fluidized bed fly ash, powder coal fly ash and sand, without the addition of binding agents.

Physico-chemical principles controlling the emission of dust from coal stockpiles

This paper describes the construction of an ‘environmental’ wind tunnel which enabled measurements of dust lift off as a function of such variables as wind velocity, particle size and moisture content to be performed under conditions of constant temperature and humidity. Aspects of coal-related properties were also investigated. All coals tested showed a strong response in reduction of dust emission with increasing total moisture content. Each coal exhibited a critical moisture content above which no emission occurred. Extensive studies were made of the effectiveness of chemical reagents in preventing dust emissions. Compounds intended to form hardened crusts on the stockpile surface, or to bind the top layer of particles, as in road base construction, were effective but at high cost. So-called agglomerating reagents were found to be ineffective for dust control.

Use of naturally-occurring but non-standard materials in low-cost road construction

This paper briefly outlines the use of non-standard materials in road base construction in Australia. After a short survey of the history of usage and the parallel development of current rigorous standards for base course materials, a selection of examples of the successful use of materials not meeting those standards is given. Such materials include decomposed rocks, soft rocks, fine grained materials, loams and sands. Some comments are made on construction and maintenance practices appropriate to such usage.

Towards understanding the behaviour of naturally-occurring road construction materials

Naturally-occurring road construction materials generally contain a greater amount of fines, and the fines have a higher plasticity than traditional materials such as a crushed rock aggregates. This makes their behaviour more difficult to understand and predict since soil suction and fabric become important controlling factors. The concepts of suction and fabric are outlined, and their role in controlling the behaviour of road construction materials is illustrated by a fundamental laboratory study of a lateritic gravel and two calcrete gravels. These materials have successfully been used as road base construction materials in low-volume bituminous-surfaced roads in Kenya and Botswana. It is concluded that the presence of fines can be an advantage, since they allow significant suctions to develop and also reduce the permeability.

Geotechnical evaluation of residual gravels in pavement construction

The paper discusses the laboratory and pavement constructional evaluation of residual lateritic gravels over phyllite. Laboratory classification, compaction and bearing test results indicate that the natural materials are unsuitable by standard gravel acceptance specifications for road base construction. However, the bearing properties can be improved to meet base-material specifications through treatment with about 5% cement. Results of construction control studies also revealed that the laboratory compaction properties of both the raw and cement-stabilised gravels are easily reproduced in the field using fairly light, smooth-wheel rollers. This means that the complaints by contractors of difficulty in achieving design specifications using similar plants during construction is not justified. Notwithstanding the results of the studies reported herein, it is not possible at the moment to suggest any construction control specifications for these gravels until more data on their in-service performance in the roadway has been collected and analysed over a number of years.

Uniaxial Tension Tests on Cement-Stabilized Granular Materials

Abstract This paper presents details of tests to evaluate the suitability of a double scissor friction grip system, initially developed for conventional concrete, for determining uniaxial tensile strength of cement-stabilized granular materials suitable for use in roadbase construction. The system is examined in terms of the variability of the results, the significance of fracture location, and the distribution of strain, and is judged to show promise. Test results on a cement-stabilized gravel, typical of roadbase materials used in the United Kingdom, are given, including the stress-strain characteristics in tension, the moduli of elasticity in tension and compression, and the strengths in tension, compression, and flexure.

Some engineering properties of cement-stabilized Keuper Marl with particular reference to rapid and repeated compression tests

Summary A review of relevant properties of Keuper Marl indicates that much of the Marl is likely to be suitable for cement stabilization, exceptions being marls which are either too plastic or contain deleterious proportions of sulphates. The results of static, rapid and repeated compression tests carried out on a 'typical' marl stabilized with cement are reported. The stiffness of the material as measured by the modulus of deformation is shown to be a function of cement content, moulding moisture content, rate of strain and stress history. Repeated load tests showed that when the intensity of stress applied was 65 per cent of the initial compressive strength, fatigue failure was imminent after 10 4 applications of load. Of particular significance was the fact that the modulus of resilient deformation was independent of the number of load cycles applied and sensibly constant over a range of stress intensities. Values of Poisson's ratio were of the low order of 0·04 to 0·20 and increased rapidly as samples approached failure. The marl containing 10 per cent cement had a 7-day compressive strength of the order of 300 lb/sq in and would therefore fail to comply with the Ministry of Transport's proposed new specifications requiring 400 lb/sq in. It is concluded that although the stabilized marl may not be acceptable for road base construction, it should serve satisfactorily for sub-bases. Two construction problems previously encountered when stabilizing Keuper Marl, are discussed and suggestions made for overcoming them.

Swelling sand (gatch) from Kuwait

Widespread deposits of well graded, lightly cemented, slightly clayey quartz sands, known locally as “gatch”, are used extensively in Kuwait for road base construction. When compacted dry of optimum water content the gatch produces an extremely good base material with California Bearing Ratio (C.B.R.) values of 40–80% for well graded samples. When wetted, however, the gatch often swells in excess of 15% and loses most of its strength, the C.B.R. values decreasing to about 2% after soaking. This swelling is caused by about 5% montmorillonite in the sandy soil.