COMPARISON OF BUILDING WASTE AND FLY ASHES FROST SUSCEPTIBILITY
DOI:10.30825/5.EJPAU.167.2018.21.4

Daniel Garlikowski, Andrzej Pawłowski
Institute of Building Engineering, Wrocław University of Environmental and Life Sciences, Poland

ABSTRACT

Tests results of frost susceptibility of clayey gravelly sand obtained from processing of building waste and of fly ashes are presented in the article. Firstly, basic physical properties of both materials were determined and then  frost heave was investigated using a laboratory test permitting to evaluate the frost susceptibility of the soil on the basis of measurement of  frozen sample height increase  The freezing susceptibility of investigated soils  is very distinct, and the research methods  used permitted to evaluate it  quantitative and qualitative way.

Key words: building waste, fly ashes, frost heave.

INTRODUCTION

In recent years, an increase in the use of anthropogenic soils known as industrial waste has been observed. These are metallurgic waste, coal spoil, building waste, fly ash, flotation waste and others. An enormous amount of such waste has been lately embedded in road embankments, yet the question arises, whether appropriate  care had been exercised during the evaluation of their physical and chemical properties, and in particular if an examination of frost heave has been done. Research on the influence of low temperatures on soils has been conducted for many years. The process of freezing in some types of soils results in the emergence of frost heave. While freezing front is penetrating into the soil, the water contained in it gradually freezes, creating small crystals of ice. This phenomenon, progressing deep into the ground with a simultaneous  rising of water from deeper layers, causes the growth of ice crystals which finally form ice lenses.  Later, when the ground is thawing, the places where the ice melts accumulate large amounts of water which cannot flow out and thus cause over-humidity and weakening of the ground strength parameters. The destructive impact of frost heave observed in earth engineering, in particular in road  construction, has led to the need for the development of tests that would make it possible to identify soils which are particularly susceptible to the  negative impact of low temperatures. The criteria of frost heave allow to take into account the potential likelihood of frost heave occurring in the soil, based on its physical properties. These criteria most often take into consideration the granulation, passive capillary head,  and less often the water relations in the area and the geological origins of the soil. The most often applied criteria include those by Casagrande and Dücker, Beskow and many others [3]. However, those criteria are ambiguous. The same soil can, according to different criteria, present different susceptibilities to low temperatures. In earth engineering there also exists a mandatory standard [7] which uses the sand equivalent  beside curves of grain size distribution and passive capillary head. Experience has resulted in increasing popularity of criteria that enable to evaluate the susceptibility of soils to frost heave directly on the basis of  laboratory tests. By the end of the 1960s, Croney and Jakobs created and Roe and Webster developed in the 1980s, for the Transport and Road Research Laboratory (TRRL), a test enabling to evaluate the frost heave of soil, on the basis of height increase  of frozen samples [4, 8]. Based on this test the British Norm was introduced [2]. According to the TRRL criterion, frost heave unsusceptible soils are those where the height of the sample after 4 days of testing increases by less than 9 mm. If the height increase is  within the range  9–15 mm the soil is uncertain, and if the increase exceeds 15 mm it is susceptible to frost heave. The authors of the paper decided to use this criterion in order to perform such evaluation of selected industrial wastes. The subject of this frost heave study was the fly ash of sandy silt granulation and construction waste of clayey gravelly sand granulation.

EXPERIMENTAL PROCEDURES

The tests of frost heave of waste were conducted in accordance with the procedure specified in „Specification for the TRRL Frost-heave test” [8]. In Sutherland and Gaskin’s opinion [9], this test is one of the best laboratory methods for frost heave testing. The test equipment consists of a  cooling chamber, which enables to maintain inside it a freezing temperature of -17ºC. According to the guidelines, the test lasts approx. 120 hours. During the first 24 hours the equipment is starting to achieve the required temperature of -17ºC and the next 96 h are for the test proper. In order to describe the process of frost heave more precisely, the examination of the soil in question lasted 240 h. Test samples were prepared by means of dynamic compaction of soil, as in the Proctor test, in a tripartite cylindrical form of a diameter of 10.16 cm and height of 15.24 cm. The application of such procedure prevents deformation of samples during removing from the form  [5, 6]. Space between the samples in the cooling chamber was filled with coarse-grained sand, and the freezing below zero progressed in downward direction. The tests were conducted in the so-called open soil and water system that provides constant capillary pulling of water. The adopted test methodology enables to track the process of frost heave development in time, and with different initial conditions. Samples of tested materials were formed by different initial water content and different dry density. Basic physical properties of tested soils are presented in Table 1, and the grain size distribution curves – in Figure 1.

RESULT AND DISCUSSION

Clayey gravelly sand contains 23% of gravel fraction, 64% of the dominant sand fraction, 9% silt fraction and 4% clay, grain uniformity coefficient U = 23, curvature coefficient C = 3, and on the basis of these data it can be classified as coarse, low-cohesive, well-graded soil. Fly ash granulation is equivalent to sandy silt. It contains 53% sand fraction and 37% silt fraction. The uniformity coefficient U = 0.61 and  curvature coefficient C = 3.6 characterise it as a uniform, poorly graded soil. The specific density of fly ash is at  the level of  2.03 g·cm^-3. This value is significantly lower than the value for construction waste 2.661 g·cm^-3, which is comparable to the values characteristic for quartz mineral soils. These differences result from different structure of ash particles, as well as significantly higher content of organic particles in ash, over 10.6%. The compaction ability of both materials is significantly different. Clayey gravelly sand is characterised by  over twice as high maximum dry density, and over four times lower  optimum water content as those for fly ash. The values of capillary head, which  are from 1.79 m for fly ash to 0.92 m for building waste, confirm the differences in the physical properties of the materials tested. Specific surface of ash particles determined with the use of Blein’s method showed a nearly four times higher value than that of construction waste. The ash is also characterised by a permeability coefficient which is nearly by half lower.

Results of the test on frost heave of construction waste and fly ash are presented in Tables 2 and 3.

The tables contain initial densities of soil samples, the respective values of water content at which the samples were frozen, and values of sample height increase  after 120 and 240 hours of freezing. A graphical interpretation is presented in Figures 2 and 3 by means of charting the investigated soil compaction test values of frost heave Δh for specific dry densities and initial water content. The presented results  comprise only the values measured after 120 h  testing, because they refer to the frost heave criterion of TRRL. The points specified in Table 2 allowed to draw the isolines of frost heave for building waste, their arrangement showing that the largest frost heave occurs at compaction lower than Is = 0.92 and water content higher than 10%. The isoline of frost heave equal to 15 mm, which is treated as the borderline in this method, occupies a very small area below the curve determining the compaction ability of clayey gravelly sand. The arrangement of the lines shows that height increase of frozen samples is lowest for highest dry density  when initial water contents were the same.  For a specific level of compaction the susceptibility to frost heave decreases with decreasing initial water content.

For fly ash the arrangement of isolines is different. Frost heave increases with the increasing compaction, and for a specific level of dry density the susceptibility to frost heave decreases with increasing water content.

Because investigated clay gravelly sand contained 7.4% lime, to the fly ash containing 0.35% free Ca  and 1.25% bounded Ca,  6 or 9% lime was added in order to determine its influence on the value of frost heave. At 6% lime addition the isolines presented in Figure 4 show a higher height increase of frozen samples. The intensity of this process increased with increased  compaction, and the frost heave of the samples was much higher than that of samples without lime addition. Similar observations were made by Brandl [1] from tests on cohesive soils, and the authors concluded that the action of lime compounds causes a change in the structure of particles which facilitates the migration of water in the soil environment. In our study the 9% lime addition has completely changed the tendency in the progress of the phenomenon (Fig. 5). In spite of the initial height increase of samples with low compaction, increased compaction reduced the frost heave process similarly to what was observed for clay loams from processing building waste.

We conclude that , in the case of tested fly ash with 6 and 9% lime added, there is a certain limit value at which the binding effect of lime compounds changes the relation between compaction and frost heave.

CONCLUSIONS

1. The evaluation of susceptibility to frost heave of anthropogenic soils, based on classical frost heave criteria, may be insufficient and should be supplemented by tests modelling the process of soil freezing.
2. The content of lime in both tested materials is the reason for the change in the value of frost heave. For fly ash with 6% lime addition the frost heave increase was higher compared with  material not treated with lime. If 9% of lime was added, some reduction in frost heave of fly ash samples was observed.

Accepted for print: 21.12.2018

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