Granular Subbase Research Articles

Experience with end-result specifications for granular base aggregates in Ontario

In 1982, the Ontario Ministry of Transportation (MTO) introduced end-result specifications (ERS) for the acceptance of granular base and subbase aggregates with respect to lot-by-lot statistical testing for grading and percentage of crushed particles. Under ERS, the mean and range of four test results (sublots) were used to determine the payment for specific production quantities (lots). Quality assurance (QA) sampling and testing were conducted at the aggregate source by the MTO. Materials within a lot that were, on average, marginally outside the specification limits or exhibited a wide range would be paid at a reduced contract price. Materials significantly exceeding these requirements would be rejected. In 1997, MTO introduced quality control (QC) requirements that made the contractor responsible for sampling and testing at the source. Quality assurance testing was based on a reduced number of samples taken from materials delivered to the worksite. Price adjustments, if any, were determined solely on QA test results (subject to a referee process). This paper describes in detail the steps taken to introduce these various ERS schemes. Acceptance for granular base materials at full price in 2002 has not changed significantly from 1994 and earlier years; the quantity of rejected materials has also not changed significantly over the years. It is concluded that the introduction of contractor QC testing has not significantly improved the quality of materials supplied, but neither has it had a detrimental effect.Key words: aggregate, construction, end-result specification, granular base, pavement, quality assurance, quality control, statistics, testing.

Evaluation of Optimum Rubblized Depth to Prevent Reflection Cracks

Reflection cracking is a common distress in asphalt overlays on cracked concrete pavements. Rubblization, which can minimize reflection cracks, has been used as an efficient rehabilitation method for aged concrete pavements in the United States. When applying the rubblization method, it is typical for the upper layer of aged concrete pavement to be rubblized to 40–70 mm in size, while the lower layer of the aged concrete pavement remains at a larger size, often over 300 mm . For thick concrete slabs, it is difficult to rubblize the entire depth of thick concrete slabs in terms of both construction and cost. The high impact energy necessary to rubblize the entire depth may cause degradation of the structural capacity of the pavement substructure. Korean highways are typically paved with thick concrete pavement slabs ( 30–33 cm concrete slabs with 15 cm lean concrete bases on granular subbases). This study conducted in Korea determined the minimum depth of effective 40–70 mm size rubblization to deter a ref...

On Forecasting the Resilient Modulus from the CBR Value of Granular Bases

ABSTRACT The unbound granular materials (UGM), base and subbase layers, play an essential role in the overall structural performance of thin pavement structures. They show complex stress dependent elasto-plastic behaviour under external loading. Therefore the UGM are commonly tested using the Repeated Load Triaxial (RLT) testing method to estimate the stiffness of the material by applying haversine loading pulses. The RLT testing method represents the actual stress situation quite adequately and gives satisfactorily estimates of the stiffness characteristics of UGM. A simple test that has been used for a long time in structural design of flexible pavements is the CBR (California Bearing Ratio) test. In the CBR test, the load-deformation curve is acquired while a plunger is penetrated into the material at a constant rate. In the literature one can find a number of relationships for UGM where the CBR value is used to predict the stiffness. These connections usually do not take into account that stiffness of UGM is both stress and moisture dependent. To investigate if a relationship between the two tests exists, twenty materials have been tested with both methods and the test results compared. The materials were of varying quality and were tested at four different moisture contents. The results indicate that a simple power law can be used to forecast the stiffness if the CBR-value is known.

Assessing Damage to Airport Pavement Structure due to Complex Gear Loads

The Federal Aviation Administration’s National Airport Pavement Test Facility (NAPTF) was constructed to generate high-quality, full-scale accelerated test data from pavements subjected to dynamic simulated heavy aircraft traffic. During the first round of testing, the flexible pavement sections were subjected to dual-tridem Boeing 777 gear and dual-tandem Boeing 747 gear trafficking to study and compare their effect on pavement structural responses and rutting performance. This paper presents the results from traffic testing on a low-strength subgrade flexible pavement section with a substantially thick unbound granular subbase layer. Data from in situ instrumentation and heavy weight deflectometer (HWD) testing were analyzed at different stages of trafficking to evaluate the pavement structural deterioration imposed by the test gears. At the end of traffic testing, posttraffic forensic investigation studies were conducted to investigate the failure mechanism of pavement structures. The results showed th...

Severity effects of dual-tandem and dual-tridem repeated heavier aircraft gear loading on pavement rutting performance

In this paper, the rutting behaviour of flexible test pavements subjected to multiple-wheel heavy aircraft gear loading at the National Airport Pavement Test Facility (NAPTF) are characterized. Two series of traffic tests were conducted. During the first series, a Boeing 777 (B777) aircraft gear and a Boeing 747 (B747) gear were trafficked on two low-strength subgrade and two medium-strength subgrade flexible test sections until the test sections were deemed failed. The second series of traffic tests involved repeated loading of six-wheel aircraft gear (the same as B777 gear) and four-wheel gear on low-strength subgrade test sections with variable granular subbase thicknesses. The results showed that the mean rut depths (RD) accumulated under B777 loading and B747 loading were similar. For a similar number of load repetitions, low-strength subgrade test sections with reduced subbase thicknesses yielded larger RD. The NAPTF failure criteria did not yield consistent RD.

Estimation of resource savings due to fly ash utilization in road construction

A methodology for estimation of natural resource savings due to fly ash utilization in road construction in India is presented. Analytical expressions for the savings of various resources namely soil, stone aggregate, stone chips, sand and cement in the embankment, granular sub-base (GSB), water bound macadam (WBM) and pavement quality concrete (PQC) layers of fly ash based road formation with flexible and rigid pavements of a given geometry have been developed. The quantity of fly ash utilized in these layers of different pavements has also been quantified. In the present study, the maximum amount of resource savings is found in GSB followed by WBM and other layers of pavement. The soil quantity saved increases asymptotically with the rise in the embankment height. The results of financial analysis based on Indian fly ash based road construction cost data indicate that the savings in construction cost decrease with the lead and the investment on this alternative is found to be financially attractive only for a lead less than 60 and 90 km for flexible and rigid pavements, respectively.

The cause and influence of self-cementing properties of fine recycled concrete aggregates on the properties of unbound sub-base

The use of coarse recycled concrete aggregates (CRCA) in conjunction with fine recycled concrete aggregates (FRCA) as sub-base materials has been widely studied. Although research results indicate that it is feasible to employ both CRCA and FRCA as granular sub-base, the influence of the unhydrated cement in the adhered mortar of the RCA on the properties of the sub-base materials has not been thoroughly studied. Generally, it is known that the strength of the sub-base materials prepared with RCA increases over time. However, this mechanism, known as the self-cementing properties, is not well understood and is believed to be governed by the properties of the fine portion of the RCA (<5 mm). This paper presents an investigation on the cause of the self-cementing properties by measuring X-ray diffraction patterns, pH values, compressive strength and permeability of various size fractions of the FRCA obtained from a commercially operated construction and demolition waste recycling plant. Their influence on the overall sub-base materials was determined. The results indicate that the size fractions of <0.15 and 0.3–0.6 mm (active fractions) were most likely to be the principal cause of the self-cementing properties of the FRCA. However, the effects on the properties of the overall RCA sub-base materials were minimal if the total quantity of the active fractions was limited to a threshold by weight of the total fine aggregate.

Incorporating a fly-ash stabilised layer into pavement design

This paper describes a case history where the structural support afforded by a fly-ash stabilised layer was accounted for explicitly during the design of two flexible pavements. Pavements were designed and constructed at two sites in southern Wisconsin employing a layer stabilised in situ with fly ash. One pavement is for a residential subdivision. The other is a test section located in a secondary highway that was recently reconstructed. A control test section employing a conventional cut-and-fill approach was also constructed in the secondary highway. Fly ash was used to increase the strength and stiffness of the fine-grained subgrade at both sites, which was soft prior to stabilisation. Pavements at both sites were designed using the 1993 American Association of State Highway and Transportation Officials (AASHTO) method for flexible pavements so that their structural number would be equivalent to that of the conventional pavement originally called for in the design. Measurements of California Bearing Ratio (CBR) and resilient modulus (Mr) were used with the correlation charts for granular sub-base materials in the AASHTO manual to define layer coefficients for the stabilised layers. Tests were also conducted on specimens collected during construction to verify that the in situ mixture had similar properties to those anticipated during design. The pavement at these sites is being monitored seasonally using a falling weight deflectometer and pavement distress surveys. The monitoring programme has indicated that the pavements constructed with fly-ash stabilised layers provide comparable stiffness to conventional pavements employing a cut-and-fill approach. No signs of distress have been observed in the pavements constructed with a stabilised layer. Thus, assigning layer coefficients for fly-ash stabilised soils based on correlations for granular sub-base materials appears reasonable until layer coefficients specific to fly-ash stabilised soils become available.

Incorporating a fly-ash stabilised layer into pavement design

This paper describes a case history where the structural support afforded by a fly-ash stabilised layer was accounted for explicitly during the design of two flexible pavements. Pavements were designed and constructed at two sites in southern Wisconsin employing a layer stabilised in situ with fly ash. One pavement is for a residential subdivision. The other is a test section located in a secondary highway that was recently reconstructed. A control test section employing a conventional cut-and-fill approach was also constructed in the secondary highway. Fly ash was used to increase the strength and stiffness of the fine-grained subgrade at both sites, which was soft prior to stabilisation. Pavements at both sites were designed using the 1993 American Association of State Highway and Transportation Officials (AASHTO) method for flexible pavements so that their structural number would be equivalent to that of the conventional pavement originally called for in the design. Measurements of California Bearing Ratio (CBR) and resilient modulus (Mr) were used with the correlation charts for granular sub-base materials in the AASHTO manual to define layer coefficients for the stabilised layers. Tests were also conducted on specimens collected during construction to verify that the in situ mixture had similar properties to those anticipated during design. The pavement at these sites is being monitored seasonally using a falling weight deflectometer and pavement distress surveys. The monitoring programme has indicated that the pavements constructed with fly-ash stabilised layers provide comparable stiffness to conventional pavements employing a cut-and-fill approach. No signs of distress have been observed in the pavements constructed with a stabilised layer. Thus, assigning layer coefficients for fly-ash stabilised soils based on correlations for granular sub-base materials appears reasonable until layer coefficients specific to fly-ash stabilised soils become available.

Finite element analysis of stress distribution on bituminous pavement and failure mechanism

This paper examines theoretically the possible mechanism of stress distribution on bituminous pavement layers using finite element method, the effects of a uniform vertical and horizontal pressure, hence stress spreading principles between tyre and pavement layers. The pavement modelling consists of a wearing, base course, granular sub-base and sub-grade. In the load application procedure, a dual wheel with the standard pressure was chosen. Results indicate that compressive and tensile stress is reaching a considerable value between wheels, where fatigue initiation may start from top and bottom of the layers accruing maximum stress. Although the vertical stress is effective within all the pavement layers, horizontal stress is only effective on wearing and base course, which is the main cause of fatigue related pavement failures.

Relationship Between Backcalculated and Laboratory-Measured Resilient Moduli of Unbound Materials

The resilient moduli of an unbound granular subbase (used at the Virginia Smart Road) obtained from laboratory testing were compared with those backcalculated from in situ falling weight deflectometer deflection measurements. Testing was performed on the surface of the finished subgrade and granular subbase layer shortly after construction. The structural capacity of the constructed subgrade and the depth to a stiff layer were computed for 12 experimental sections. The in situ resilient modulus of the granular subbase layer (21-B) was then back-calculated from the deflections measured on top of that layer. The back-calculated layer moduli were clearly stress-dependent, showing an exponential behavior with the bulk stress in the center of the layer. Resilient modulus test results of laboratory-compacted specimens confirmed the stress dependence of the subbase material modulus. Three resilient modulus models were fitted to the data. Although all three models showed good coefficients of determination ( R2 &gt; 90%), the K-θ model was selected because of its simplicity. The correlation between field-backcalculated and laboratory-measured resilient moduli was found to be strong. However, when the stress in the middle of the layer was used in the K-θ model, a shift in the resilient modulus, θ, was observed. This finding suggests that a simple shift factor could be used for the range of stress values considered.

Damages to residential buildings related to pyritic rockfills: field results of an investigation on the south shore of Montreal, Quebec, Canada

Numerous cases of garage and basement concrete slab distress at various levels of heaving and deterioration were noticed early after construction in the Montreal south-shore area (Canada). The use of clayey sedimentary rock containing pyrite as aggregates for granular subbase is responsible for most of the observed distress. The heaving of the slabs is related to the oxidation process of the pyrite, followed by sulphate crystallization causing the swelling of the compacted granular subbase. A first inspection survey was carried out in the summer of 1999 in an attempt to assess the extent and magnitude of the damage caused by pyritic rockfills. More than 200 houses showing distress associated with pyrite were investigated in three localities. Damage of the garage floor slab seems to occur earlier after construction and to be more severe likely due to the subbase thickness and to the lower quality of the rockfill. Five houses at various stages of reaction process were monitored. The results suggest that the rockfill swelling is quite fast and continuous. The heaving rate of some garage slabs might be as high as 1.2 mm/month.Key words: pyrite, shales, swelling rockfill, oxidation, sulphate attack, damage.

The use of permeable materials for backfilling trenches

The use of cementitious permeable backfill materials containing pulverized fuel ash based aggregates for trenches was investigated by means of laboratory tests and a field trial. Single-sized Lytag and Aardelite aggregates were used in this investigation. Laboratory measurements included unconfined compressive strength, 10% values, flowability, permeability and freeze–thaw resistance. These results were compared with those of foamed concrete. It was found that the flowability of Lytag and Aardelite depended in part on the spherical shape of the particles and therefore low water–cement ratio was required to achieve a workable mix which does not need compaction. Aardelite mix showed high potential for backfilling trenches because during pilot testing it performed as well as Lytag and it was much cheaper than the latter aggregate. However, more work is needed to develop a mix design. A field trial was conducted on a Lytag mix and its performance was compared with that of granular type 1 backfill. The trench backfill materials complied with the Highways Authorities and Utilities Committee specification and required no compaction. Field trial showed that Lytag backfill performed better than type 1 granular sub-base in terms of having lesser long-term settlement. A cost estimate of the backfill materials suggested that the Aardelite backfill is similar to granular based sub-base. In the long term it may be cheaper, as it may require little or no maintenance.

Hot-Mix Asphalt Overlay Design Concepts for Rubblized Portland Cement Concrete Pavements

The Illinois Department of Transportation’s (IDOT’s) first interstate rubblization project was constructed in 1990 on I-57 near Pesotum as part of a rehabilitation study. Hot-mix asphalt overlays (HMA OL) were constructed over a rubblized jointed reinforced concrete pavement (JRCP) with a granular subbase. Excellent performance was achieved on the project, which had accommodated 7.5 million equivalent single-axle loads through 1998. Periodically collected falling weight deflectometer (FWD) data indicate the sections have retained their structural capacity and integrity. HMA fatigue distress has not developed. The success of the project prompted IDOT to consider portland cement concrete pavement (PCCP) rubblization and HMA OL a viable and cost-effective rehabilitation option. Rubblization is particularly appropriate for eliminating reflective cracking and for use when PCCP patching quantities are high or concrete deterioration is in an advanced stage. HMA OL fatigue considerations control the HMA OL thickness requirement for rubblized PCCPs. HMA OLs for rubblized PCCPs, generally in the 150-mm to 250-mm (6-in. to 10-in.) range, are thicker than those used in the traditional PCCP rehabilitation and HMA OL projects. IDOT uses mechanisticempirical (M-E) flexible pavement design procedures for full-depth asphalt and conventional flexible pavements. M-E–based HMA OL design procedures for rubblized PCCPs are being developed. In the M-E design procedures, the tensile strain at the bottom of the HMA layer is used to consider HMA fatigue. The structural behavior and HMA OL fatigue performance of the Pesotum sections are used in the initial development of the M-E–based HMA OL design procedures for rubblized PCCPs. Results from other rubblized PCCP projects are being used to refine and validate the M-E design concepts.

Plasticity Modeling Applied to the Permanent Deformation Response of Granular Materials in Flexible Pavement Systems

Mechanistic design methods for flexible pavements are usually based on limiting only two critical pavement responses: tensile strain at the bottom of the asphalt layer for fatigue damage and vertical compressive strain at the top of the subgrade for overall pavement rutting. Rutting in granular base and subbase layers is assumed to be small for all cases. Research was conducted to develop a fundamental method for incorporating rutting of granular base and subbase layers in flexible pavement thickness design. The research concentrated on investigating the potential for using constitutive relationships based on the flow theory of plasticity to limit permanent deformations in granular layers of pavement systems. The flow theory of plasticity is the most popular of the three plasticity theories that have been applied to soils and granular materials. The total strain is viewed as the sum of the reversible elastic component and the irreversible plastic component. A yield function is introduced to differentiate between elastic and plastic behavior. This yield function is a function of the stress state and may change on loading and unloading to describe cyclic hardening behavior. To use a flow theory model to predict the response of a granular pavement layer requires relatively complex finite-element analysis and sophisticated laboratory testing that may not be warranted for normal pavement design. A method is presented for using the yield surfaces from a flow theory model as a design criterion for limiting permanent deformations in granular layers. Only the results of triaxial strength tests are needed to implement the method.

Low-grade quarry products, reclaimed aggregates and inert wastes — Their use in unbound mixtures for road pavements

The UK Specification for Highway Works appears to encourage the use of low-grade quarry products and a range of reclaimed aggregates and inert wastes in earthworks. However, in practice, materials used in earthworks are judged against requirements designed to assess the compactibility of soils. Compliance with “compactibility” criteria rather than “fit for purpose” criteria can lead to the use of higher quality aggregates than is strictly necessary. For many years U.K. clients have had supplies of good quality granular subbase material available at low cost. The use of such aggregates in non-structural fill layers presents few risks, but diverts the aggregate from more appropriate end uses, such as asphalts. Proponents of the greater use of wastes and reclaimed aggregates face the same dilemma. The development of European (CEN) Standards may influence the situation. These Standards are designed to ensure that barriers to trade are removed for “products placed on the market”. The new Standards aim to be “blind” to the source of the material and to reflect the wide range of national practices across Europe. The CEN work may give a status to a number of tests that are more appropriate to the control of unbound mixtures than those currently used in the U.K.

Characterization of Base/Subbase Materials under Repetitive Loading

Abstract Testing of granular materials for evaluation of Resilient Modulus (MR) suffers from the lack of a standardized test method. Although AASHTO over the past six years (1986 to 1992) has proposed three different testing procedures (T274-86, T292-91I, and T294-92I), some state transportation departments have adopted their own MR testing procedures. In this study, the factors affecting the measurement of MR such as the testing method used and stress applied and its sequence are investigated. In addition, the updated methodologies proposed by various agencies for MR of granular materials and the MR values obtained are reviewed and evaluated. It is found that the applied stress sequence has some effects on MR of granular materials. For the two aggregate types investigated in this study, with the same location of LVDT and dynamic waveform, the T294-92I testing procedure yields higher MR than that obtained by using the T292-91I testing procedure. The stress sequence in T294-92I infers a stiffening and strengthening effect on the specimen structure as the stress level increases from low to high. The variation of MR values due to testing procedure is found to be higher than those due to the aggregate source. The MR values obtained in this study are found to be lower than those reported in the literature. Furthermore, with the updated testing procedure for MR, the correlation between the structural layer coefficients and MR of granular base and subbase proposed by the AASHTO needs to be modified accordingly.

Design approaches for geotextile separators beneath the granular sub-bases of unpaved roads

The purpose of a geotextile separator beneath a granular sub-baisse to prevent the road aggregate and the underlying subgrade soil from intermixing. In the absence of a geotextile, contamination of the sub-base aggregate by soft subgrade soil could reduce its load bearing capacity and thereby limit the life of the pavement A number of different approaches have been proposed for the design of geotextile separators beneath unpaved roads. The most widely accepted of the methods based on simple hand calculations are those of Giroud and Noiray, and Sellmeijer et al. This Technical Note presents a simplified summary of these two design approaches and suggests some modifications.

PAVEMENT STRUCTURAL ANALYSIS USING FALLING WEIGHT DEFLECTOMETER

This paper describes a method for evaluating a structural capacity of an asphalt pavement with falling weight deflectometer deflection data. A back-analysis was used to estimate in situ elastic moduli of a granular subbase and a subgrade in the pavement. A tensile strain at a bottom of the asphalt layer and a compressive strain in the subgrade surface under the standard wheel load were calculated with the BISAR. A number of allowable loading repetition corresponding to the tensile strain and the compressive strain were estimated with an asphalt strain criteria and a subgrade strain criteria obtained by many researchers.The asphalt strain criteria was very useful imformation for a theoretical design method on asphalt pavements and overlay design, because there was a correlation between the cracking data and asphalt strain criteria in the national highway 337.

Stability of gravel sub-bases : Earland, M G; Pike, D C Transport and Road Research Laboratory Research reportN64, 1986, 17P

As part of an investigation into the stability of unbound granular subbases a series of trafficking trials has been carried out with loaded lorries on several types of crushed rock and gravel subbases. The results of these trafficking trials are correlated with shear strengths measured in a 300 mm shear box machine modified to test wet aggregates with plastic fines. Precision estimates of this test method are given. It was found that many gravel subbases can perform as well as crushed rock type 1 subbases. The shear box test, however, was only able to distinguish between the least and most stable; for most subbase materials a trafficking trial would be needed to verify their stability. (a)