VN Paper on Minor Road Design

THE DESIGN OF PROVINCIAL MINOR RURAL ROADS IN VIET NAM Derek Fordyce, Director, Success Blossom Environmental (Viet Nam) JSC Engineer Pham Anh Quan, Director, Hung Yen Transportation Department, Hung Yen Province Ass. Prof. Dr. Hoang Ha, General Director, Ministry of Transport, Dept. of Science and Technology Engineer Nguyen Tu, Deputy Director, HRB Production, Phu Thien Phat Limited

Abstract Phu Thien Phat Limited is the sole producer and supplier of hydraulic road binder in Viet Nam. The binder represents the new generation of hydraulic binder to replace Portland cement. The binder is an alkali-activated pozzolan system based on coal fly ash; coal fly ash remains a waste product from power generation. Phu Thien Phat Limited have funded a three year programme to develop design guidance for road construction in Viet Nam; structural and foundation layers have been created from soil bound with hydraulic road binder. The design guidance has been developed based mainly on UK and EU experience. The mixtures produced are hydraulic bound mixtures (HBMs); road design relates principally to, but not limited to, the minor rural road network. Two types of soil have been defined in Viet Nam, Suitable Soils and Marginal Soils. Suitable soils are as dug coarse sands and gravels with specific limitations on grading and Liquid Limit and Plasticity Index; Marginal Soils are blends of alluvial soil and medium sands which are commonly found in the plains of Viet Nam. Single structural layer pavements have been developed that have a surface formed by stone dressing and bitumen seal or 3cm of cold proprietary asphalt. A simple linear elastic response model has been used to calculate structural layer thickness for different foundation bearing capacities and vehicle loading conditions. Vehicle loading has been divided into mixed traffic flow with light rigid chassis commercial vehicles and private cares, and private cars only. Traffic loading has been sub-divided into occasional vehicle movements through to high vehicle movements. Design Tables are provided from the output of the modelling. The output data in terms of structural layer thickness has been found to be wholly consistent with similar data produced in the UK based on observation of the performance of such roadways. The structural design life of roads is 20 years; the service life of a road surface will relate to the type of surface applied, a dressed and sealed surface or a cold asphalt surface. Guidance is also provided on the production and construction of pavement and foundation layers. It is believed that the work presented in this paper has the potential to form national guidelines using this particular form of sustainable low energy construction technology.

The authors wish to acknowledge the invaluable support provided by Nguyen Vu, Director, Phu Thien Phat Limited

Introduction Roads are layered systems formed by a pavement structure over a foundation. A pavement structure is formed by a base course and surface. The surface of a road requires remaining stable under all weather conditions and provide friction for the safe movement of vehicles. A road surface may be formed by the surface of the base course, or by one or two material layers as shown in Figure 1; a single layer is called the surface course, a two layer system is a binder course and surface course. The base course reduces the vertical contact stress from vehicle wheels to values within the capability of the soil foundation; the base course reduces vertical stress by shear strength, and flexural strength where the layer is a bound aggregate mixture. Surface

Base course

Surface course

Binder course

Base course

Base course Foundation

Foundation

Road surface is surface of base course

Foundation

Road surface is single layer

Road surface is two-layer system

Figure 1: Pavement construction

The foundation to a road is formed by a sub-base over a natural or reconstituted soil; the natural or reconstituted soil is called the subgrade. The surface of the subgrade is called the formation; a formation creates the line and profile of a road. The process of creating the formation is called earthworks; earthworks may involve cutting and trimming an existing soil, or it may involve fill operations and the creation of an embankment. The bearing capacity of a subgrade is measured empirically by the characteristic California Bearing Ratio (CBR); the ratio is expressed in percent. Where the bearing capacity of a subgrade soil is low, generally less than 5 percent CBR, an additional layer may be constructed called the capping layer. A capping layer is excavated soil or imported low-grade material that improves the bearing capacity of a weak subgrade. Where a capping layer is used the surface of the capping layer defines the formation, the surface of the existing soil then becomes the sub-formation. The sub-base creates a stiff platform for the movement of construction traffic and a surface on which to build the pavement; the sub-base also acts as a drainage layer ensuring a stable and low moisture condition within the base. Sub-base

Formation

Formation

Embankment

Embankment

Sub-formation

Cutting

Figure 2: Foundation construction

Capping layer

Capping layer

European approach to road design Europe has a unified series of Standards for materials used for road construction. Codes of Practice with road design vary between the countries forming the European Union (EU). The Codes of Practice referenced in this technical paper are based on UK research and development. UK Codes of Practice are not yet wholly consistent with EU Standards. The basis of current UK design is TRL Report TRL615. The work of TRL615 is embodied in the UK Design Manual for Roads and Bridges (DMRB) in two specific documents:  Volume 7, Pavement Design and Maintenance, Section 2, Pavement Design and Construction, Part 3, HD26/06, Pavement Design, and  Interim Advice Note (IAN) 73/06 Revision 1 (2009), Design Guidance for Road Pavement Foundations (Draft HD25). The design documents are complemented by two sets of specification documents in the Manual for Contract Documents for Highway Works (MCHW):  Volumes 1, Specification for Highway Works (SHW), Series 600, Earthworks, and Series 800, Road Pavements- Unbound, Cement and other Hydraulically Bound Mixtures, and  Volume 2, Notes for Guidance on the Specification for Highway Works, Series NG600 and Series NG800. UK design guidance enables the use of a wide range of construction materials in terms of aggregates and binders to form pavement and foundation layers; specifically, the approach enables pavement construction with low energy and secondary materials. In terms of hydraulic binders the new generation of alkali-activated binders is included; HRB 22.5E produced in Viet Nam by the Phu Thien Phat Company is a proprietary alkali-activated hydraulic binder. HRB 22.5E produces low energy road construction materials with soils that conform to EN 14227-1, Hydraulic Bound Mixtures- Specifications, Part 1, Cement Bound Granular Mixtures (CBGM), or conform to EN 14227-5, Hydraulic Bound MixturesSpecifications, Part 5, Hydraulic Road Binder Bound mixtures. HRB 22.5E produced by the Phu Thien Phat Company conforms to DD ENV 13282, Hydraulic Road BindersComposition, Specifications and Conformity Criteria. The basis of pavement design using recycled materials is TRL Report TRL 611. UK road design promotes a unified approach between fully flexible and flexible-composite pavement construction; flexible-composite pavements are an asphalt surface over a base course formed by a Hydraulic Bound Mixture. Flexible-composite pavements enable the use of a wide range of construction materials to form the base course. Road design is based on defining critical stress and strain values within the pavement using a linear elastic response model. The foundation is designed on the basis of equivalent half-space stiffness. Pavement design Fully flexible pavements are designed on the basis of horizontal tensile strain at the underside of the base course; flexible-composite pavements are designed on the basis of horizontal tensile stress at the underside of the hydraulic bound base course. Pavement design is divided by traffic loading into indeterminate life (long-life) and determinate life; the division is 80 million standard axles (msa). The fatigue relationship for hydraulically bound mixtures is highly sensitive to traffic induced stresses; two „K‟ factors are used to define a limiting tensile stress for pavement design. The two factors are, KHyd, which is the calibration factor for temperature, curing and fatigue effects, and KSafety, which is the calibration factor for risk with the design based on knowledge of material properties. The default value of KSafety is 1.0; the value of KHyd is dependent on the dynamic modulus and flexural strength of the hydraulic bound mixture. The equation using the K factors defining the limiting or critical tensile stress at the underside of a hydraulic bound mixture base course is, 𝝈r=(flexural strength.KHyd.KSafety)

(1)

The value of KHyd is given by the equation, KHyd=(0.368+5.27x10-5E-0.0351ff) (2) E is dynamic elastic modulus in GPa; ff is flexural strength in MPa Equation (2) is based on laboratory testing of materials that are in use; with new materials simulative testing is required to assess the value of KHyd and the use of a reduced value of KSafety may be appropriate with initial designs. With long-life pavements and traffic loading greater than 80msa, linear elastic theory is used to calculate the horizontal tensile stress at the underside of the hydraulic bound base, which is compared with the limiting value. For pavements with a traffic loading less than 80msa and where degradation of the hydraulically bound base is gradual, an empirical relationship has been defined for life to the point of structural maintenance. Log(N)=1.23x(SR.KHyd.KSafety+0.1626)2+0.2675 (3) N is msa; SR is strength ratio, the ratio of flexural strength to tensile stress at the underside of the hydraulic bound base determined using linear elastic theory With fully flexible and flexible composite pavements design charts based on equations (1) and (2) have been created in TRL615. With flexible-composite pavement design using linear elastic theory the key material characteristics of the based course are dynamic elastic modulus and flexural strength; both parameters are expressed as 360 day values. A linear relationship between dynamic elastic modulus and flexural strength has been defined for hydraulic bound mixtures (HBMs); the linear relationship is plotted to a log-log scale. To create design charts, nine Classes of HBM have been defined, H1 to H9, shown in Figure 11 of TRL Report TRL615; higher Classes (or higher qualities) of HBM have higher flexural strength for a given value of dynamic elastic modulus, as shown in Table 10 of TRL Report TRL615. The design charts in TRL615 have been recreated as a pavement thickness nomograph in Figure 2.1 of HD26/06. The design guidance in HD26/06 is based on mixtures bound with Portland cement, and mixtures have a minimum 360 day compression strength value of 15MPa. Experience has shown such mixtures develop linear cracks with curing; control of cracking is by pre-cracking the hydraulic bound base at 3m intervals along a road. Guidance on the process of pre-cracking is given in MCHW, Volume 1, SHW, Series 800, Clause 818. The surface of flexible composite pavements is asphalt; the thickness of asphalt controls the time for crack propagation from the surface of the base to the road surface. Crack propagation is traffic loading related and experience has shown equation (4) defines the asphalt thickness. HAsphalt=-16.05x(Log(N))2+101xLog(N)+45.8

(4)

Practical values of HAsphalt range from 180mm with long-life pavements down to 100mm with roads with a traffic loading of 3msa and less, as shown in Figure 2.1 of HD26/06. Foundation design The bearing capacity of a foundation has a significant impact on the performance of a road. The use of hydraulic bound mixtures for the sub-base layer of a foundation creates a condition that prohibits the vertical compressive strain at formation being used as a pavement design criterion. Vertical compressive strain was related to deformation that accumulated at formation level with traffic loading. Semi-rigid hydraulic bound sub-base, and bases, cannot deform in the manner of fully flexible pavement construction. As a consequence of the increased use of hydraulic bound sub-bases the design approach with the foundation is to control foundation stiffness. The design criterion is Foundation Surface Modulus. Foundation Surface Modulus is the ratio of applied surface stress to recovered strain. The design criterion is a composite value of foundation response from what is a multilayered system. The Foundation Surface Modulus is an equivalent stiffness value treating the foundation a half-space.

Four Foundation Classes have been defined by Foundation Surface Modulus:  Class 1, equal to and greater than 50MPa  Class 2, equal to and greater than 100MPa  Class 3, equal to and greater than 200MPa  Class 4, equal to and greater than 400MPa Draft HD25 defines two types of foundation design: Restricted Foundation Design and Performance Foundation Design. Performance Foundation Design Performance Foundation Design enables the use of a wide range of low energy materials to create the sub-base; the design is based on sub-base layer stiffness, which is defined by material design. Other than Class 1 foundations sub-base material is formed by hydraulic bound mixtures (HBMs). The stiffness of a HBM material sample (element) is defined by a compression test; the test defines the characteristic Modulus of Elasticity. The value of layer modulus used for design employs a factor with the element modulus for cracking that will occur through layer material curing and temperature gradients. The (calibration) factor applied to what is termed fast-setting HBMs, where they achieve more than 50 percent of their final specified (360 day) compressive strength at 28 days, is 0.2; with slow-setting materials the calibration factor is 0.1. Figures 4.2 to 4.5 in Draft HD25 show relationships between the thickness of sub-base as a single foundation layer and Subgrade Stiffness Modulus; Subgrade Stiffness Modulus is measured indirectly by dynamic cone penetrometer (DCP) that is converted to bearing capacity in units of percent CBR (California Bearing Ratio). There is an empirical relationship between bearing capacity in units of percent CBR and Subgrade Stiffness Modulus in units of MPa. The four Figures relate to the four Foundation Classes. For each Foundation Class there is a unique Layer Stiffness value range, this assures a limit on Layer Stiffness in relation to foundation stiffness preventing thin high stiffness sub-bases being specified for a Foundation Class. Figure 4.6 in Draft HD25 shows a capping layer and subbase specification for a Class 2 foundation as an alternative to a single HBM layer. Foundation Class 2 is taken as a standard foundation Unless there is assured experience of a foundation design a demonstration area requires being constructed; surface modulus is measured by direct testing using a Lightweight Deflectometer (LWD) or Falling Weight Deflectometer (FWD). With foundations LWD is the practical site tool, but it requires correlated in the demonstration area with a calibrated FWD. Restricted Foundation Design A Restricted Foundation Design is applied where material design and end performance testing are not appropriate, or not viable. As such materials with extended evidence of performance only are specified. Materials are capping material (MCHW, Series 600), and unbound mixtures and cement bound granular mixtures (MCHW, Series 800). Restricted Foundation Designs exclude Class 4 foundations. Single layer foundations thickness values for Class 1, 2 and 3 foundations are related to subgrade stiffness modulus in Figure 3.1 of Draft HD25. Figure 3.2 is the relationship between sub-base and capping layer thickness and subgrade stiffness modulus for a Class 2 (standard) foundation. End performance road design The analysis of a road construction uses a simple linear elastic model, a simple linear elastic model has been found to be adequate to predict stress and strain values within pavements. With the current UK approach layer elastic modulus values require definition, from experience, or from laboratory testing. Designs are simplified to three-layer structures: surface, base and foundation. With the foundation the modulus is the equivalent half-space stiffness value. The assumption with a pavement design is that the surface and base course layers are fully bonded, and there is full friction at the foundation interface. The loading is also simplified to the worst case of an axle end super-single wheel arrangement; the wheel load is taken as 40kN applied uniformly over a circular contact area or 151mm radius.

UK low volume road design HD26 pavement design guidance relates to roads that form the strategic network and principal roads with the non-strategic network; roads are designed to have structural capacity. For much of the non-strategic network more economic design is required. Two factors relate to the design of the minor, non-strategic, road network, which include roads carrying low volumes of traffic but which may be essential to local economies; the two factors are cost and sustainability. TRL Report TRL611 TRL Report TRL611 addresses the need for cost effective and sustainable road construction. TRL Report TRL611 is „A Guide to the Use and Specification of Cold Recycled Materials for the Maintenance of Road Pavements‟. The basis of pavement design is the use of recycled and secondary aggregates and low energy binders; low energy hydraulic binders may be naturally occurring hydraulic or pozzolanic materials, or hydraulic or pozzolanic materials that are by-products from industrial processes, such as blast-furnace slag and pulverised fuel ash. Pozzolans are the basis of alkali-activated binders; alkali-activated binders form the new generation of hydraulic binders to replace Portland cement. The grading limits for aggregate is wide, as with envelope A in EN 14227-1 relating to cement bound granular mixtures. Three overlapping grading envelopes are defined in TRL611, a fine grading envelope, a coarse grading envelope, and a grading envelope that is the overlap of the coarse and fine grading envelopes. The grading envelopes are shown in Table A1.1 of TRL611. Three grading Zones are defined by the overlapping envelopes. Zone A, the overlapping grading envelope, is suitable for all low energy mixtures; Zone A grading is similar to the 0/14 Hydraulic Road Binder Bound Mixture 2 of EN 14227-5. Zone B, the residual of the fine grading envelope, is suitable „in certain circumstances‟ to create low energy mixtures. Zone C, the residual of the coarse grading envelope, is prone to segregation and requires insitu mixing to form low energy mixtures. The grading Zones are shown in Figure 6.1 of TRL611. TRL611 follows the principles set out in TRL615, HD26 and Draft HD25, and MCHW, SHW, Series 800. As with TRL615, the principles in TRL611 relate to EU Standards but not in a wholly consistent manner. TRL611 covers the design of fully flexible and flexible-composite pavements. TRL611 follows TRL615 using the 9 strength Classes of HBM, H1 to H9, based on dynamic elastic modulus and flexural strength; values with the characteristics are 360 day values. The 9 Classes of HBM are shown in Figure 7.2 of TRL Report TRL611; values of dynamic elastic modulus and flexural strength can be predicted from compressive strength using the following general equations: E=(Log(ff)+a)/b

(5)

ff=c.fc (6) Where, E is dynamic elastic modulus in GPa, ff is flexural strength in MPa, fc is compressive strength in MPa and a, b and c are material constants. Table 2 in TRL615 shows values for the constants in equations (5) and (6) for HBMs including gravel and crushed rock aggregate. The values of constants used with low energy HBMs in TRL611 are those for gravel aggregate: a=0.773, b=0.0301 and c=0.11. TRL611 categorises roads on the basis of traffic loading in msa; the Road Type Categories are shown in Table 5.1 of TRL611. The five Road Type Categories are: 0 1 2 3 4

over 30 to 80msa over 10 to 30msa over 2.5 to 10msa over 0.5 to 2.5msa up to 0.5msa

Flexible-composite road design charts are provided for Road Type 2 and foundation Classes 1 to 4; the design charts are shown in Figures 7.3 to 7.6 of TRL611. The design approach follows that of TRL615 with a HBM base course and asphalt surface; however, for lower Road Type Categories an alternative (reduced) construction specification is promoted.

Low volume road design For low volume roads defined by Road Type Categories 3 and 4, and Road Type Category 2 up to 5msa, a combined base and sub-base HBM (or bitumen bound mixture) layer is promoted; the surface options are a single course of asphalt, 100mm or 40mm in thickness, or surface dressing (chip seal). The single structural layer is constructed directly on the formation. HBM single layer thickness values for subgrade CBR values are shown in Table 7.4 of TRL611; Table 7.4 is replicated here in Table 1. The values are based on observation of road performance and not on analytical design. The values of single layer thickness are based on Class 5 HBM; Class 1 HBM is the lowest (strength) Class and Class 9 HBM is the highest (strength) Class. The thickness values shown for the single structural layer require adjustment for HBMs of strength Class lower than H5, as shown in Table 7.5 of TRL611. Table 7.5 of TRL611 is replicated here in Table 2. Table 1: Low volume road specification (Replicated from Table 7.4 of TRL611) Subgrade CBR <2 2-4 5-7 8-14 >15

Road Type Category 3 Surface (mm)

2 Surface (mm)

4 Surface (mm)

Surface dressing

40

100

Surface dressing

40

100

Surface dressing

40

100

n/r n/r n/r n/r n/r

n/r 300 280 270 250

n/r 240 220 200 200

n/r 280 260 240 220

n/r 240 220 200 180

n/r 180 160 150 150

n/r 240 220 200 190

n/r 200 180 160 150

n/r 150 150 150 150

Note: n/r is not recommended

Table 2: Adjustment factors for HBM Class (Replicate of Table 7.5 TRL611) HBM strength Class H1 H2 H3 H4 H5 (or superior)

Thickness adjustment 1.66 1.45 1.28 1.13 1.00

Clause 2.14 of HD26 has added additional guidance to that in TRL611. The additional guidance suggests that adjustment factors may not be fully relevant; but, the additional guidance relates only to cement bound mixtures (CBMs). The interpretation of experience is that with HBMs using hydraulic road binders other than Portland cement the adjustment factors in Table 7.5 of TRL611 should be retained until more experience is gained.

Cracking of HBM mixtures The curing of hydraulic binders results in cracking of mixtures. Faster curing hydraulic binders resulting in mixtures with higher early strength have the potential to form discrete linear cracks across pavement base courses, or sub-bases. Slower curing binders resulting in mixtures with lower early strength create diffuse cracking within material layers. The event of cracking is lower layer strength compared with laboratory sample strength. This event is described explicitly in Section 2.1 of Draft HD25, and the event is a principle component of the KHyd factor in pavement design. The use of Portland cement as the hydraulic binder is most likely to result in discrete linear cracking of HBM layers. HD26 defines the compressive strength of 10MPa at 7 days as the criterion for the need to pre-crack pavement layers: Clause 14 of the notes on design chart Figure 2.1. MCHW, Series 800 Clause 818 describes the method of pre-cracking with pavement base courses. The equivalent tensile strength at 7 days for layer pre-cracking may be around 0.8MPa. Longitudinal cracking of HBM road layers may occur as the result of thermal gradients within the base course.

Specification of HBM mixtures EN 14227 is the European standard for specifying hydraulic bound mixtures. Mixtures are classified by compression strength (System I) or by modulus of elasticity and direct tensile strength (System II). Classification System II is inconsistent with UK design parameters.

Minor Provincial rural roads in Viet Nam Minor Provincial rural roads in Viet Nam are commonly 3m to 5m in width and carry a range of traffic in terms of traffic volume and vehicle weight. The most common vehicle using minor rural roads is the motorbike, but the roads are critical for the movement of commercial vehicles. Commercial vehicles on minor rural roads will generally be of the Light Rigid (LR) or Heavy Rigid (HR) chassis form. Commercial vehicles with a LR chassis have a Gross Vehicle Mass (GVM) between 4.5 tonnes and 8 tonnes; commercial vehicles with a HR chassis have a GVM exceeding 8 tonnes. Commercial vehicles with a GVM between 4.5 tonnes and 8 tonnes have two axles and include goods delivery vehicles and vehicles transporting construction materials including soils. Commercial vehicles with a GVM greater than 8 tonnes have three axles with a double rear axle configuration, and include vehicles transporting high volumes of construction materials including soils. Minor provincial rural roads will generally carry commercial vehicles with LR chassis, but these vehicles maybe overloaded. The rear axle-load of commercial vehicles with LR chassis may be in the range 4 tonnes to 8 tonnes; a 4 tonne rear axle load can be from overloaded small LR chassis vehicles, an 8 tonne rear axle load will result from overloaded LR chassis vehicles. An alternative design approach for Provincial roads in Viet Nam A design approach for Provincial roads in Viet Nam has been developed based on the guidance of TRL611. The design approach is to achieve low cost and sustainable construction with Provincial roads, and in particular with minor rural Provincial roads. The design approach is based on producing HBM using local soils, pulverised demolition and construction waste, natural and artificial aggregate, or a combination of those blended with Strength Class 22.5E hydraulic road binder conforming to DD ENV13282. The hydraulic binder is an alkali-activated pozzolan system produced in Viet Nam by the Phu Thien Phat Company. The HBM can form the sub-base with the foundation and the pavement base course with conventional road construction, or the single structural layer for low volume roads. The design approach has extended the work of TRL611. The design approach includes material design and structural design; the material design approach has extended the work reported in the United States Department of Defence document (United Facilities Criteria) UFC 3-250-11, Soil Stabilisation for Pavements in relation to hydraulic binders. The development of the alternative design approach has been in partnership with the Department of Transportation of Hung Yen Province, Viet Nam. The strength characteristics of HBM produced using a range of soils found in Viet Nam have been defined over a three year period. The strength characteristics are compressive strength, flexural strength and static modulus; the characteristics have been recorded as 28 day cured and soaked values. The characteristics have been defined as part of material designs with road construction. Road designs have all been single structural layer constructions with low volume roads; roads have had a surface formed by surface dressing or 3cm of proprietary cold asphalt developed by Colas in Viet Nam. Road designs have been carried out using linear elastic analysis of a two or three layer structure. The design approach is mechanistic-empirical, analytical design balanced by evidence of performance. The design of hydraulic bound mixtures Ahead of a mixture design soils are characterised in terms of their impact on mixture performance. The characteristics relating to impact on HBM performance are activity, durability and strength. The pH and organic content of a soil can impact on the hydration process (activity) of the hydraulic binder and thus impact on the strength of HBM. Sulphate content and swelling will impact on the durability of the HBM. Particle size distribution (grading), Plasticity Index (PI) and Liquid limit (LL), and strength (Los Angeles value and elastic modulus), will impact on the strength achieved by a HBM. Whereas the chemical characteristics of a soil can result in rejection of a source, the physical and mechanical characteristics will create specific impact on the mechanical properties of a HBM. The principal characteristics of a soil used to characterise a HBM in terms of key mechanical properties are: maximum particle size (85 percent of soil particles smaller than declared size), percentage of particles smaller than 0.063mm, PI and LL (of particles smaller than 0.5mm). Soils have been divided into two groups, Suitable Soils and Marginal Soils.

Suitable Soils Suitable Soils are coarse sand and gravel soils. Data generated in Viet Nam has related to coarse sand and fine gravel, which is a particle size range 0.6mm to 6mm. The mechanical properties of HBM measured are compressive strength, static modulus and flexural strength. Cured and soaked values of the three mechanical properties have been recorded for a range of hydraulic binder content between 5 percent to 12 percent by mass of a mixture and for a mass content of soil of particles less than 0.063mm within the soil mass between 20 percent and 50 percent. Optimum mechanical values with the three characteristics are in the range 30 percent to 35 percent of particles less than 0.063mm in the soil mass. The strength characteristics relate to a Class 22.5E hydraulic binder only, at this stage. For each soil source a material design has included a range of hydraulic binder content and strength measurements have been made with time up to 28 days, generally; some data has extended time measurements. With each strength characteristic, two relationships have been plotted relating strength with time and hydraulic binder content. Strength characteristics have also been correlated: flexural strength with compressive strength, flexural strength with static modulus, and compressive strength with static modulus. Curing of samples was sealed curing at 20C; soaked data was defined by additional samples being seal-cured for 21 days then immersed in water maintained at 20C for 7 days. Raw laboratory data was adjusted to achieve consistent relationships for strength with time and hydraulic binder content, and between strength characteristics, thus enabling best estimates of 28 day strength data to be recorded and best estimates of strength characteristic relationships. Strength gain profiles show soil HRB including a strength Class 22.5E hydraulic binder to be fast-setting as defined in Draft HD25. Ratio of 7 day to 28 day compressive strength values range from 0.5 to 0.8; the ratio is dependent on compressive strength of the mixture. The ratio of 7 day to 28 day compressive strength for cement bound mixtures (CBMs) quoted in TRL615 as 0.84, which is consistent with Viet Nam data in terms of strength of mixture. Strength characteristics of Suitable Soil HBM in Viet Nam The relationship between strength characteristics of soil from Viet Nam has been plotted using adjusted 28 day soaked data; this approximately translates to 360 day cured data as used in TRL615 and HD26. Generally the relationship between flexural strength and compressive strength is linear, as is the relationship between flexural strength and elastic modulus. Non-linearity has been recorded with angular weak soils. The relationship between static modulus and flexural strength has been created for 28 day data with soils having a maximum particle size between 0.6mm and 6mm, with a range of content of particles less than 0.063mm from 20 percent to 50 percent by mass of the soil, and for a hydraulic binder content of between 5 percent and 12 percent by mass of the mixture. The static modulus has been converted to dynamic modulus using the equation quoted in TRL615, Es=1.08Ed-9.07

(7)

The equation was derived for cement bound mixtures and can only be taken as an approximation for soil HBMs. The static modulus range was found to be 0.1GPa to 0.3GPa, which converts to a dynamic modulus range of 8.50GPa to 8.75GPa, the modulus range is small given the range of hydraulic binder content and fines content (particles less than 0.063mm). The measured range of flexural strength was found to be from 0.20MPa to 0.75MPa, which is more significant. The data shows, 1. The quality of the mixtures, and Class of HBM, is affected more by hydraulic binder content than fines content 2. As expected, the sensitivity of the relationship alters as the fines content increases above 35 percent and the mixture moves from coarse particle control to fines control 3. The data suggests than the limit of fine content for suitable soils should be 20 percent to 40 percent by mass of a soil 4. Mixtures with a hydraulic binder content in the range 8 percent to 10 percent by mass of the mixture have a dynamic modulus of around 8.60GPa and flexural strength around 0.55GPa

5. Mixtures with a hydraulic binder content in the range 10 percent to 12 percent by mass of the mixture have a dynamic modulus of around 8.65GPa and flexural strength around 0.65GPa 6. Mixtures with a hydraulic binder content in the range 8 percent to 10 percent by mass of the mixture are in the transition between Class H1 and Class H2 HBM as defined in TRL615, and 7. Mixtures with a hydraulic binder content in the range 10 percent to 12 percent by mass of the mixture are in the transition between Class H2 and Class H3 HBM Figure 3 shows the relationships defined using Suitable Soil HBM laboratory data. The data relates to 28 day soaked sample values, the Viet Nam material condition for design. The soaked strength values were generally in the range 70 percent to 80 percent of cured values. Dynamic modulus (GPa) 8.50

8.70

8.60

0.80 20 percent percent 0.70

Flexural strength (MPa)

0.60

30 percent percent 35 percent percent 40 percent percent

0.50 0.40

Fines content Binder content

0.30 0.20

50 percent percent

0.10 0 0.10 0.20 Static modulus (GPa)

0.30

Figure 3: Relationship between flexural strength and modulus for suitable soil HBMs

For the construction of rural Provincial roads the hydraulic binder content range of 8 percent to 10 percent by mass of a mixture is taken as the most economic; for the hydraulic binder content range structural design using soil HBM mixtures over the fines content range of 20 percent to 40 percent should be taken as Class H1. For structural analysis using a simple linear elastic model the element (laboratory) dynamic modulus can be taken as 8.60GPa; this has been translated to a layer modulus for design of 1.7GPa (0.2x8.6GPa); flexural strength for design should be taken as 0.19MPa (0.55x0.35, where 0.35 is taken as KHyd). With 8-10 percent strength Class 22.5E hydraulic binder by mass of a mixture Suitable Soil HBM in Viet Nam is Class 1 HBM. Suitable Soil HBM layer characteristics for structural design are dynamic modulus of 1.7GPa and flexural strength of 0.19MPa. Suitable Soil HBMs may be used to create pavement base layers or single structural layer, or they may be used to create the sub-base with foundations. For soils with a maximum particle size greater, or smaller than the range 0.6mm to 6mm the effect, from data analysis, will be equivalent, very generally, to a shift in binder content. An increase in maximum particle size will broadly be equivalent to an increase in binder content;

a reduction in maximum particle size will broadly be equivalent to a decrease in binder content. The latter event is shown with what is described as Marginal Soil. Marginal Soils Marginal Soils have been defined by a maximum particle size within the range 0.2mm and 0.6mm with fines content in the range 20 percent to 50 percent of the soil mass. Marginal Soils will generally have a (target) fines content in the range 30 percent to 40 percent of the soil mass. Whereas Suitable Soils are generally „as dug‟ material, Marginal Soils are generally created by blending medium sand with alluvial soil. The addition of medium sand to alluvial soil is to control the percentage of fines in a blend. Marginal Soils are created by design; Marginal Soils are blends of alluvial soil and medium sand to achieve a maximum particle size between 0.2mm and 0.6mm and fines content of between 30 percent and 40 percent of the soil mass. The source of sand to blend with an alluvial soil is dredged river sand, or a river sand deposit. The grading of river sand varies and care is needed to source medium sand. Likewise the grading of alluvial soils varies, and care again is needed in selecting a suitable source of alluvial soil. Marginal Soil design requires laboratory testing where there is no experience of the grading of sand or alluvial soil sources. Strength characteristics of Marginal Soil HBM in Viet Nam The strength characteristics of HBM created using a designed Marginal Soil has been found to be relatively insensitive to fines content and binder content in the range 7 percent to 12 percent by mass of a mixture. The optimum strength characteristics of marginal soil HBM is with a fines content of between 30 percent and 40 percent of the blended soil mass; for a hydraulic binder content in the range 8 percent to 10 percent of the mixture optimum strength characteristics are a dynamic modulus 8.5GPa and flexural strength around 0.15MPa. With 8-10 percent strength Class 22.5E hydraulic binder by mass of a mixture Marginal Soil HBM in Viet Nam is below Class 1 HBM. Marginal Soil HBM layer characteristics for structural design are dynamic modulus of 1.7GPa and flexural strength of 0.05MPa. Marginal Soil HBMs may be used to create low volume single structural layer pavements, or they may be used to create the sub-base with low volume roads, or capping layer with foundations for all road types. Unsuitable soils for HBM base layers Soils or soil blends represented by fine sands, with a maximum particle size in the range 0.06mm and 0.2mm, are unsuitable for producing HBM and for road base construction. Such soils can be stabilised by hydraulic binders and used to provide subgrades with enhanced bearing capacity. Limiting characteristics of suitable and marginal soils The laboratory data with soils from Viet Nam have been at, or beyond, the fine material boundary for Zone B as defined in TRL 611. The grading of Suitable Soils used in Viet Nam to date has a maximum particle range lower than Zone B but a fines content in the range 20 percent to 40 percent; this is consistent data. As the maximum particle size increases the lower limit of fines content can reduce to 5 percent, as shown with Zone B soils in TRL 611. With Marginal Soils the fines content has higher limits of 30 percent to 40 percent, again this is consistent. The lower limit of fines content avoids the use of poorly graded coarse or medium sand. This is one aspect of defining useable soils and expanding on the statement in TRL 611 relating to Zone B soils being “only suitable in certain circumstances”. Limits have also been defined for Liquid Limit and Plasticity Index: for Suitable Soils LL is limited to 50 percent and PI to 20, for Marginal Soils LL is limited to 30 percent and PI to 7. This is consistent with data in UFC 3-250-11.

Low volume road construction specification Two forms of low volume Provincial roads have been constructed in Viet Nam, which relate directly to the specification for such roads in TRL611: a single structural layer over a subgrade with a surface formed by 30mm of proprietary cold asphalt, and, a single structural layer over a subgrade with a surface formed by surface dressing. The roads have been designed using a simple linear elastic model and based on the structural characteristics defined from laboratory material design and testing; to date, material design has always been carried out with Marginal Soils. With the surface formed by proprietary cold asphalt the stiffness modulus of the layer used in design has been taken as Class B1 cold recycled material as defined in TRL611 Table 7.6; the modulus of Class 1 B1 cold asphalt is 1.9GPa. This event makes the cold asphalt similar in modulus to soil HBM, whether the soil is suitable or marginal. Low volume road Design Tables for Viet Nam Eight Design Tables have been produced in a manner similar to Table 7.4 of TRL611. The Design Tables are based on HBMs produced using suitable and marginal soils and for the two construction specifications of base plus 3cm of cold asphalt and base plus surface dressing. The Design Tables have inputs of subgrade bearing capacity in units of percent CBR and traffic loading in terms of volume of traffic. Subgrade bearing capacity is divided into three categories: existing soil foundation, which translates to new (green field) construction on soil, or improved soil, with CBR value in the range 5 percent to 10 percent; new soil foundation, which translates to ground that has been designed as a road foundation with CBR values in the range 15 percent to 30 percent; and, existing road foundation, which translates to traffic loaded and consolidated subgrades with CBR values in the range 45 percent to 60 percent. Traffic loading has been divided into Occasional Flow, which translates to up to 10 vehicles per day, Low Flow, which translates to 11-50 vehicles per day, Medium Flow, which translates to 51-250 vehicles per day, and, High Flow, which translates to 251-1000 vehicles per day. Given the width of minor roads and the event of vehicles moving along the middle of the road traffic flow values are two-way. Designs are based on a single loaded wheel configuration with a circular contact area having a radius of 0.151m for LR chassis commercial vehicles and axle weights equal to and greater than 2 tonnes, and a contact area radius of 0.1m for private vehicles with an axle weight of equal to and less than 2 tonnes; for LR chassis commercial vehicles single wheel load designs are more onerous than dual wheel load designs. Whereas with commercial vehicle loading TRL615 shows a design wheel load of 40kN (80kN axle weight), or 4 tonnes, designs with low volume roads in Viet Nam have been carried out for a range of axle weight, and thus range of wheel loads. Design Tables 3, 4, 5 and 6 relate to commercial vehicle loading; Design Tables 9, 10, 11 and 12 relate to private vehicle loading. Roads in Viet Nam may be unrestricted in vehicle use, other than through road width, or roads may have restricted use with the provision of concrete pillars or blocks limiting the width of vehicle to private cars. Private cars in rural Viet Nam are commonly 4x4‟s. Unrestricted roads will carry mixed commercial and private vehicle traffic; restricted roads will carry private vehicles only. The two situations create two sets of Design Tables. With private car loading the default design loading is an axle weight of 1 tonne; a 2 tonne axle weight loading is provided as the vehicle over-load design scenario. With mixed traffic loading of commercial vehicles and private cars the default design loading is commercial vehicles with a 6 tonne axle weight, an 8 tonne axle weight is provided as the vehicle over-load design scenario; a 4 tonne axle weight is also provided where there may be restricted vehicle use. The damage effect of private vehicles is low compared with commercial vehicles; using the fourth power damage law, over 1300 private cars with 1 tonne axle weight are equivalent to one 6 tonne commercial vehicle axle weight. The use of the Design Tables is shown in the Flow Chart of Figure 4. For design the traffic loading categories have limiting formation vertical compressive formation strain values to achieve similar design lives and enable the specification of an economical construction: Occasional Flow 600µε, Low Flow 400µε, Medium Flow 200µε, and

High Flow 100µε. The design life of a road before structural intervention is required is 20 years.

Table 3: Composite pavement construction using Suitable Soils for base course HBM base course thickness (mm) (Content of Class 22.5E hydraulic binder 8-10 percent by mass of mixture)

Low Flow (400µε)

Medium Flow (200µε)

High Flow (100µε)

(Up to 10 vehicles per day) Vehicle axle weight (tonnes)

(11-50 vehicles per day) Vehicle axle weight (tonnes)



61

82

43

61

82

43

5

195

245

1504

255

305

195

370

435

305

10

1704

220

1504

230

275

1704

340

405

275

15

1604

2104

1504

2154

255

1554

320

385

255

30

1904

2004

1704

1904

2254

1704

280

355

225

45

2104

2204

1904

2104

2204

1904

265

335

205

60

2204

2304

2004

2204

2304

2004

255

325

195

61

82

43 435

Requires full pavement construction

43

Requires increased surface thickness

82

Existing Soil foundation

61

New Soil foundation

Occasional Flow (600µε)

Existing road foundation

Foundation

CBR (%)

405

385 355

335

325

Notes 1. Design thickness for standard commercial vehicles with LH chassis 2. Design thickness for overloaded commercial vehicles with LH chassis 3. Design thickness for commercial vehicles of limited gross weight 4. Base in compression 5. Grey filled boxes are the condition of base stress control on base thickness

Table 4: HBM pavement construction using Suitable Soils for base course HBM base course thickness (mm) (Content of Class 22.5E hydraulic binder 8-10 percent by mass of mixture)

Low Flow (400µε)


High Flow (100µε)





61

82

43

61

82

43

5

225

275

1804

285

335

225

400

465

335

10

2004

250

1704

260

305

2004

370

435

305

15

1904

2404

1704

2354

285

1854

350

415

285

30

2204

2304

2004

2104

2554

2004

310

385

2554

45

2404

2504

2204

2404

2504

2204

295

365

2354

60

2504

2604

2304

2504

2704

2304

285

355

2254


61

82 Requires full pavement construction

43

Requires increased surface thickness

82


61

New Soil foundation



Foundation

CBR (%)

43 465 435

415 385

365

355

Table 5: Composite pavement construction using Marginal Soils for base course HBM base course thickness (mm) (Content of Class 22.5E hydraulic binder 8-10 percent by mass of mixture)

Foundation

CBR (%)


Low Flow (400µε)


High Flow (100µε)





82

43

61

82

43

61

82

43

61

82

43

1604

10

1904

New Soil foundation

5

15

1904

2104

1804

2204

30

2304

2354

2154

2304

2504

2154

2304



61

1604

1804

45

2304

2504

2304

2304

2504

2304

2304

60

2704

2704

2504

2704

2704

2504

1804

2704

2504

325


Table 6: HBM pavement construction using Marginal Soils for base course HBM base course thickness (mm) (Content of Class 22.5E hydraulic binder 8-10 percent by mass of mixture)

Foundation

CBR (%)


Low Flow (400µε)


High Flow (100µε)





82

43

61

82

43

61

82

43

61

82

43

1904

10

2204

New Soil foundation

5

15

2204

2404

2104

2504

30

2604

2654

2454

2604

2804

2454

2604



61

1904

2104

45

2604

2804

2604

2604

2804

2604

2604

60

3004

3004

2804

3004

3004

2804

2104

3004

2804


Elastic analysis

Comparison with TRL 611 design data can be made with Medium and Heavy commercial vehicle flow, which translates to Type 4 and Type 3 UK roads. The comparison of data is shown in Tables 7 and 8; in the comparison the interpretation of TRL 611 thickness values has accounted for the thinner 3cm cold asphalt surface. The comparison of base thickness values is reasonable suggesting the use of the simplified elastic model is valid and the proposed Design Tables are valid as a starting point ahead of full performance experience.

355

Table 7: Comparison of UK-Viet Nam HBM base course thickness values 1 Thickness values for composite pavement construction using Suitable Soils for the base course

Medium Flow

High Flow

Axle weight (tonnes)

CBR 5 10 15 30 45 60

6 TRL611 370 365 340 335 320 300 280 280 265 255

1


8 TRL611 435 430 405 400 385 365 355 350 335 325

1

4 TRL611 435 430 405 400 385 365 355 350 335 325

1

Notes 1. Values based on Table 7.4 TRL611 using adjustment factor of 1.66 for Class H1 HBM

Table 8: Comparison of UK-Viet Nam HBM base course thickness values 2 Thickness values for HBM pavement construction using Suitable Soils for the base course

Medium Flow

High Flow


CBR 5 10 15 30 45 60

6 TRL611 400 400 370 365 350 330 310 310 295 285

1


8 TRL611 465 465 435 430 415 400 385 360 365 355

1

4 TRL611 465 465 435 430 415 400 385 360 365 355

1

Notes 1. Values based on Table 7.4 TRL611 using adjustment factor of 1.66 for Class H1 HBM

Construction

Base soil HBM

Base with 3cm of cold asphalt surface Base with stone chippings and sealed

Suitable Soil HBM Marginal Soil HBM

Design Chart

Formation bearing capacity

Pavement loading

CBR (percent)

Vehicle axle weight (tonnes) Volume of vehicles (per day)

Measured using DCP

Thickness of base Figure 4: Flow chart for use of Design Charts

Table 9: Composite pavement construction using Suitable Soils for base course HBM base course thickness (mm) (Content of Class 22.5E hydraulic binder 8-10 percent by mass of mixture)

Medium Flow

(400µε)

(200µε)

(100µε)

(Up to 10 veh per day) Vehicle axle weight

(11-50 veh per day) Vehicle axle weight



(tonnes)

(tonnes)

High Flow

(tonnes)

(tonnes)

22

11

22

11

22

11

22

5

1003

1053

1003

140

140

215

215

310

10

1003

1003

1003

1003

1003

1153

1153

180

15

1003

1003

1003

1003

1003

1003

1003

170

30

1003

1053

1003

1053

1003

1053

1003

1503

45

1003

1203

1003

1203

1003

1203

1003

1403

60

1053

1253

1053

1253

1053

1253

1053

1303


11

New Soil foundation

CBR (%)

Low Flow

(600µε)


Foundation

Occasional Flow

Notes 1. Design thickness for standard private vehicles 2. Design thickness for overloaded private vehicles 3. Base in compression 4. Grey filled boxes are the condition of base stress control on base thickness 5. Red coloured figures are less than the minimum of 150mm base thickness

Table 10: HBM pavement construction using Suitable Soils for base course HBM base course thickness (mm) (Content of Class 22.5E hydraulic binder 8-10 percent by mass of mixture)

Occasional Flow

High Flow

(200µε)

(100µε)



(tonnes)

(tonnes)

(tonnes)

(tonnes)

22

11

22

11

22

11

22

5

1153

1353

1153

170

170

245

245

340

10

1003

1003

1003

1003

1003

1453

1453

210

15

1003

1003

1003

1003

1003

1253

1253

200

30

1203

1203

1203

1203

1203

1203

1203

180

45

1303

1503

1303

1503

1303

1503

1303

1803

60

1353

1553

1353

1553

1353

1553

1353

1603


11

New Soil foundation

CBR (%)

Medium Flow

(400µε)



Foundation

Low Flow

(600µε)



Table 11: Composite pavement construction using Marginal Soils for base course HBM base course thickness (mm) (Content of Class 22.5E hydraulic binder 8-10 percent by mass of mixture)

Medium Flow

(400µε)

(200µε)

(100µε)





(tonnes)

(tonnes)

High Flow

(tonnes)

22

11

5

1003

1053

1003

10

1003

1003

1003

1003

1003

1153

1153

15

1003

1003

1003

1003

1003

1003

1003

30

1203

1353

1203

1353

1203

1353

1203

45

1353

1453

1353

1453

1353

1453

1353

1453

60

1453

1553

1453

1553

1453

1553

1453

1553


22

11

(tonnes)

11

New Soil foundation

CBR (%)

Low Flow

(600µε)


Foundation

Occasional Flow

22

140

11

22

215


Table 12: HBM pavement construction using Marginal Soils for base course HBM base course thickness (mm) (Content of Class 22.5E hydraulic binder 8-10 percent by mass of mixture)

Low Flow

Medium Flow

(600µε)

(400µε)

(200µε)

(100µε)





(tonnes)

(tonnes)

22

11

5

1003

1353

1303

10

1103

1303

1103

1303

1103

1453

1453

15

1203

1303

1203

1303

1203

1303

1253

30

1503

1653

1503

1653

1503

1653

1503

45

1653

1753

1653

1753

1653

1753

1653

1753

60

1753

1853

1753

1853

1753

1853

1753

1853


22

11

(tonnes)

11

New Soil foundation

(tonnes)

High Flow


Foundation

CBR (%)

Occasional Flow

22

170

Notes 1. Design thickness for standard private vehicles Elastic analysis 2. Design thickness for overloaded private vehicles 3. Base in compression 4. Grey filled boxes are the condition of base stress control on base thickness 5. Red coloured figures are less than the minimum of 150mm base thickness

11

22

245

Critical aspects with minor road design The critical aspects with minor road design relate to the generated road stress and strain conditions, the durability of the surface, and the control of the quality of the soil. Generated road stress and strain conditions Design Tables 3 and 4 show with High Flow that either a thicker surface course will be required, or full pavement construction may be required. TRL 611 suggests that the limit of the single structural layer specification is 5msa; with High Flow over-loaded commercial vehicles the traffic loading exceeds 5msa. With the default loading of 6 tonnes and High Flow the need is for a surface course exceeding 3cm, or the use of a hot asphalt surface. There may also be a limitation to surface life with Medium Flow conditions in mixed traffic. Three conditions of base stress and subgrade strain exist within a road: the underside of the base is in tension and the structural life of the road is controlled by subgrade strain; the base is fully in compression and the structural life of the road is controlled by the subgrade strain; or, the base compression stress controls the structural life of the road. The three conditions are noted with all the Tables. The conditions where the structural life of the road is controlled by base compression stress are high subgrade bearing capacity and high generated vertical subgrade strain; the conditions where the underside of the base is in tension and the structural life is controlled by vertical subgrade strain are lower subgrade bearing capacity and lower generated vertical subgrade strain. With Marginal Soils and the data in Design Tables 5 and 6, and for private vehicle loading, the base is generally in compression, and with higher subgrade bearing capacity the base compression stress controls the structural life of the road. Increasing base thickness over the values shown in the Design Tables will result in reduced vertical subgrade strain, but will create and, or increase the value of horizontal tensile stress at the underside of the base that can quickly exceed the working tensile strength of the HBM. The tensile strength of soil HBMs is low; the tensile strength of Marginal Soil HBM is very low. As stated in TRL Report TRL 615 HBMs are stress sensitive. Degradation of soil HBMs will be from repeated stressing as a result of the movement of vehicles over the surface of a road; repeated compression stress will degrade a soil HBM as well as repeated tensile stress. Surface durability Care is required with a surface formed by dressing the surface of the base course with stone chippings and sealing the surface with emulsified bitumen. The durability of the surface will be limited, and additional layers of surface dressing will be required over the structural life of a road, or a double-raked layer, or a $multiple surface dressed layer, should be considered at the stage of construction. With lightly loaded roads there may be situations where no surface treatment is applied; in such circumstances it is critical that the surface of the base achieves full strength through keeping the surface moisture during the critical early curing period of the HBM. If this is not carried out then the surface will degrade with vehicle movement and the movement of surface water. Soil grading The control of soil grading and blend proportions is critical with any bound aggregate system. The selection of Suitable Soil deposits requires an assessment of the potential variation in soil grading; Marginal Soils can have more grading control as they will generally be blended soil systems. However, with Marginal Soil the criticality is the maximum particle size. The use of fine alluvial soils and critically fine sand sources will create Unsuitable Soils. The consequence of the use of fine soil and sand sources for the production of soil HBM will be discrete layer cracking. The standard hydraulic binder content using a strength Class 22.5E binder is currently 8-10 in Viet Nam. Including this quantity of Fast Setting hydraulic binder within fine soils creates similar conditions to those defining the need for the pre-cracking of HBM layers; the binder creates shrinkage stresses within a layer that result in discrete layer cracking. Discrete layer cracking will be seen as transverse road cracking at a regular interval along the length of the road; the time taken to achieve critical strength may result in the full crack pattern taking many months to be evident. Ambient air temperature will also impact of the rate of crack

generation. Fine graded (unsuitable) soils require a reduced hydraulic binder content, around 2-3 percent by mass of a mixture, to create diffuse cracking within a layer; the soil will be stabilised and have enhanced bearing capacity but little structural strength.

Foundation design using soil HBM With foundation design based on Draft HD25 layer modulus requires being defined. With soil HBM, material modulus has not been measured in the same manner as required in Draft HD25. However, a Restricted Foundation Design approach can be taken where sub-base material is specified by strength Class, strength Classes being HBM C3/4 and C5/6. These materials will create a Class 2 foundation. Class H1 soil HBM will achieve strength Class C3/4 with coarse sand and fine gravel mixtures and with 10-12 percent binder by mass of a mixture. With medium and coarse gravel mixtures, or using crushed building or demolition waste the hydraulic binder content will reduce to 8-12 percent by mass of a mixture, or less.

Conclusions on minor road design using soil HBM The data from the design and use of soil HBM in Viet Nam has extended that reported in TRL 611. The data has shown that fine grained aggregate and soils can create practical road construction materials. The materials created are low energy materials that maximise the use of local soils avoiding the importing of high energy quarried aggregate, and including an alkali-activated hydraulic binder based on an industrial by-product in coal fly ash. Structural design using a simplified linear elastic model and laboratory defined mixture strength characteristics has shown a good correlation with Suitable Soils with the output from TRL Report TRL 611. Suitable Soils in Viet Nam can create Class H1 HBM with a 22.5E hydraulic binder conforming to EN13282. Marginal Soils are a class of soil defined in Viet Nam for low-lying areas that have sand and alluvial soil deposits. These areas are extensive in Viet Nam. Marginal Soil HBMs require care in production as they are blended soil systems; sources of sand and alluvial soil require careful, but standard, analysis ahead of use. Fine sand deposits and fine alluvial soil will easily reduce a Marginal Soil to an Unsuitable Soil; the result will be cracking of a road. Unsuitable Soils can only be stabilised and can be used to provide the foundation to a pavement. However, stabilised Unsuitable Soils will enable the use of Marginal Soil HBMs to be used with low bearing capacity subgrades. Marginal Soils can readily be translated to Suitable Soils when secondary aggregate and construction and demolition waste is used to create the aggregate structure with a HBM. Secondary aggregate can be Furnace Bottom Ash; a common demolition waste, and production waste in Viet Nam is fired clay brick. Pulverising fired clay brick and blending with a fine soil can create a Suitable Soil for the production of HBM. The maximum particle size with pulverised fire clay brick will be between 20mm and 40mm. Such materials can create high performance HBM suitable for the construction of the base and sub-base layers with more than minor traffic routes. For optimum control on material quality with demolition and construction waste pulverisation is best carried out using mobile crushing plant with pulverised waste fractions being blended with soil and hydraulic binder in a high shear mixer. This process is best suited where roadways in an area can utilise the output from such a facility. For individual projects demolition and construction waste can be processed insitu. The insitu processing of brick and other demolition waste requires care in achieving a suitable soil grading. Experience allows this to be carried out by observation; otherwise, a short series of sieves is required to assess a grading during the pulverisation and mixing process. The use of alkali-activated hydraulic binders with local soils enables the creation of stable surfaces for the movement of people, animals and vehicles. Whereas Suitable Soils can provide the structural layers to vehicle loaded roads, Marginal Soils are well suited to creating the structure with lightly loaded roads used by private cars, motorbikes, people and animals. For walkways and the limited movement of motorbikes stabilising Marginal Soils, or Unsuitable Soils, may be adequate.

Pavement construction Pavement construction with minor roads involves two elements, earthworks to create the formation, and pavement works that include the single structural layer base and sub-base, and the surface. Earthworks are specified in MCHW Volume 1, SHW, Series 600, Earthworks, and Volume 2, Notes for Guidance on the SHW, Series NG600; pavement works relating to the single structural layer are specified in MCHW Volume 1, SHW, Series 800, Road Pavements- Unbound, Cement and other Hydraulically Bound Mixtures, and Volume 2, Notes for Guidance on the SHW, Series NG800. Where a single layer cold asphalt surface course is applied some guidance is provided in TRL Report TRL611, but the system is proprietary with specific guidance on placement required. Earthworks Generally, with minor roads in Viet Nam soil pathways and roadways exist that are upgraded. As such the creation of a new road formation involves profiling the existing pavement surface, or cutting into the existing soil. The profile of the formation should create the profile of the surface to the (new) pavement; the profile of the pavement surface should allow surface water drainage through longitudinal profile and transverse profile, as a camber or cross fall. MCHW, Volume 1, SHW, Series 600 Clause 616 defines the tolerance of the formation profile, which is +20mm and -30mm, and the compaction of the trimmed formation, which is by using a towed vibrating roller of at least 1800kg/m width of roll following Method 6 in Table 6/4 for a 250mm thick layer. Where the construction of the single structural layer is not immediate then a 300mm protection layer is required where the process of cutting the formation has started, described in Clause 603, or the process of cutting the formation is postponed. An assessment of the bearing capacity of the subgrade is required to enable the use of the Design Tables. Subgrade soils in the plains of Viet Nam are silty soils or possibly sandy silts with alluvial soils; where the existing subgrade drainage is poor the subgrade may only achieve a 5 percent CBR bearing capacity. Where the bearing capacity of the subgrade is measured the dynamic cone penetrometer (DCP) should be used, as described in Draft HD25. The DCP is a very simple and low cost tool that can be transported easily. Where the subgrade bearing capacity is in the range 5 percent and 10 percent CBR and Marginal Soil forms the single structural layer and the roadway requires being capable of supporting the movement of commercial vehicles, the upper level of the subgrade requires stabilisation, by insitu or juxta-situ method (dig out, mix and replace) to form a soil improvement layer, or capping layer. Interestingly the characteristics specified for a cohesive silty material able to be stabilised by hydraulic binder (Class 7E material) follow very closely those defined for HBM in Viet Nam. The subgrade soil may comply with a Class 7E Earthwork material, which is a standard material for stabilisation, or it may require a specific material design approach and the use of a demonstration area. Clause 613 of MCHW, Volume 1, SHW, Series 600 covers the requirements of a capping layer, and Clause 614 covers the stabilisation process of soils. Clause 614 allows the use of a towed vibratory roller or pneumatic-tyred-roller (PTR); target compaction moisture content is defined by Moisture Condition Value (MCV). The target MCV value with silty cohesive soils should be less than 12. Along with the DCP the Moisture Condition Machine is a low cost and simple site tool that avoids complex site testing. Pavement works The single structural layer is a HBM. The nature of the hydraulic binder in Viet Nam enables a mixture to be classified as a Cement Bound Granular Mixture (CBGM), EN14227-1, or Hydraulic Road Binder Bound Mixture (HRBBM), EN 14227-5. The soil grading in Viet Nam is unlikely to comply with EN14227-1, rather the mixture created will comply with Clause 6.8 of EN 14227-5 HRRBM 4. The processes involved in creating a HBM road layer in Viet Nam are unlikely to use a mixin-plant method of material production, rather a mix-in-place method, or variant of that process, is used. The mix-in-place method is described in Clause 816 MCHW, Volume 1, SHW, Series 800. Clause NG 813 MCHW, Volume 2, SHW, Series NG800 is the most important in describing how best to create a HBM structural layer.

The minimum compacted HBM layer thickness should be 150mm, MCHW, Volume 2, SHW, Series NG800 Clause NG813-7. Tables 9, 10, 11 and 12 show in red compacted layer thickness values less than 150mm. For light loading defined by motorbikes and small private cars the reduced single layer thickness value may be adequate; no single layer has been laid less than 150mm in compacted layer thickness in Viet Nam to date. Insitu layer production is by mobile pulverising equipment; in Viet Nam and to date the addition of hydraulic binder is by 50kg bag of material. Two aspects are important. Calculations require knowing the bulk moist mass density of the soil to get an accurate estimate of bags per square metre of surface to achieve a binder content of 8-10 percent by mass of a mixture. Secondly, the pulverising mixers in Viet Nam can reduce cohesive materials to agglomerates but they have difficulty reducing the cohesive materials to a maximum particle size less than 28mm as defined in MCHW, Volume 1, SHW, Series 600, Clause 614-7. MCHW, Volume 2, SHW, Series NG800, Clause NG813-1 states that a secondary process can be used to create an insitu mixture layer; this approach has commonly been used in Viet Nam. Material is excavated from source and hydraulic binder added to a stockpile of source material. Calculations are required to define the number of whole 50kg bags of hydraulic binder to be added to a partially compacted and moist volume of stockpile soil. The hydraulic binder is blended with the soil stockpile using a hydraulic bucket. The process does cause degradation of the larger cohesive agglomerates; residual oversize cohesive agglomerates are removed manually, or are broken down manually. Water can be added to the stockpile until the MCV of the mixture achieves the target value defined by laboratory design. The mixing process is continued until the stockpile has uniform colour and there are no free volumes of hydraulic binder. The pre-blended mixture is then transported to location and spread by dozer or grader. The mixture is then treated as an insitu mixing process and a pulverising rotavator is used to ensure agglomerates are less than 28mm, either by mechanical action, or again, by residual agglomerates being removed manually. MCHW, Volume 2, SHW, Series NG800, Clause NG813-7 states that pre-blended stockpiles cannot be formed ahead of material placement unless a slow-setting hydraulic binder is used, and then only in certain circumstances. A Class 22.5E hydraulic binder is a fast setting hydraulic binder. Layer compaction must be by vibrating roller and PTR, or by PTR using a 30kN wheel load. Fine mixture will exhibit surface shear under the action of a vibrating roller. This may be reduced with the use of a PTR, or the layer is laid +30/40mm thicker than required and the surface trimmed to level; after trimming no further compaction is required with the layer. This condition is described in MCHW, Volume 2, SHW, Series NG800, Clause NG813-18. The trimming method does not eliminate the need for the use of a PTR. A critical situation that exists in Viet Nam is the drying out of the surface of a layer of HBM during compaction. This creates conditions where reduced strength will exist in the surface of the layer and cause performance issues for a road. Where layer surface drying is possible because of heat or wind then the surface should be wetted up during compaction, as described in MCHW, Volume 2, SHW, Series NG800, Clause NG813-12. With multiple layer construction full bond is required between layers. This requires the surface of the lower layer to be moist, and there may be a need to scarify the surface of the lower layer ahead of placing the upper layer, as described in MCHW, Volume 2, SHW, Series NG800, Clause NG813-13. Early trafficking of the base course depends on the surface construction. For an exposed HBM base there should be no trafficking for 7 days; for immediate trafficking the Immediate Bearing Index should be at last IPI40 for fine mixtures, or IPI50 for mixtures with at least 50 percent crushed hard aggregate, as defined in MCHW, Volume 2, SHW, Series NG800, Clause NG813-16. Immediate Bearing Index is the cured (unsoaked) CBR value of the compacted HBM.

Pavement surface The surface to pavements of minor roads in Viet Nam have been a single 3cm layer of proprietary cold asphalt, or surface dressing, or, in some cases, no surface treatment. With a 3cm surface course the material is mixed on site using a rotating drum concrete mixer. A blend of medium sand and fine gravel is mixed with a proprietary emulsified bitumen produced by Colas. The mixture is spread by hand as with mastic asphalt; compaction is by light weight PTR. With a surface dressing, 10-20mm stone chipping are spread on the surface of a single structural layer that has been partially compacted. The spread rate is around 60 percent shoulder to shoulder. After final compaction and the embedment of the surface chippings the surface is sprayed with the same emulsified bitumen used for the production of the 3cm cold asphalt layer. The rate of spread of the emulsified bitumen requires being greater that 0.3kg/m2 of surface; in compliance with MCHW, Volume 1, SHW, Series 800, Clause 813-15. Where no layer curing treatment is applied with roads carrying only motorbikes, people and animals the surface the single structural layer requires being kept moisture until the full strength of the mixture is achieved, by mist, fog, or light spray of water as described in MCHW, Volume 1, SHW, Series 800, Clause 813-15. If this is not carried out there will be a lack of surface durability.

Sustainability of low volume road construction The structural single HBM layer will degrade with trafficking and time. Repeated compression stress will degrade an HBM layer as much as repeated tensile stress. Degradation of the structural HBM layer will define the life of the roadway to the point at which it requires reconstruction. Evidence of the need to consider structural maintenance will be wheel track deformation that should be limited to 20mm as the value for action. Soil HBM can be recycled with the use of a pulverising rotavator to breakdown the HBM layer in the manner of fired clay brick waste. Adding a hydraulic binder and mixing the system insitu will reconstruct the structural layer in the manner of the original construction; additional waste products may be added to increase the maximum particle size of the mixture, again as with the original layer.

Conclusions The development of soil HBM for the construction of pavement and foundation layers in Viet Nam has been shown to be viable. The work of the last three years with the construction of sections of the minor rural road network has enabled the production of material and structural design guidance that is consistent with and extends both UK and US current information. Two classes of soil have been defined for the construction of rural minor roadways in Viet Nam, Suitable Soils and Marginal Soils. Limits for the chemical, physical and mechanical characteristics of the two Classes of soil have been defined, along with the mechanical characteristics of soil-based hydraulic bound mixtures including strength Class 22.5E hydraulic road binder. The hydraulic road binder is an alkali-activated pozzolan system based on pulverised fuel ash. The soil HBMs are sustainable low energy materials with the potential to reduce the cost and emissions associated with road provision and maintenance in Viet Nam. Structural Design Tables have been produced for single structural layer pavements in the manner promoted in the UK for low volume roads. The tables provide structural layer thickness values for roads carrying mixed vehicle traffic and vehicle limited traffic. Mixed vehicle traffic includes private cars and commercial vehicles with Light Rigid chassis. Design vehicle loading for mixed traffic flows is a 6 tonne axle weight; structural layer thickness values for an 8 tonne over-load and 4 tonne limited load condition have also been provided. Where traffic loading is private car the design axle weight is 1 tonne, with information on a 2 tonne over-load condition. Traffic volume is an input to the Design Tables along with foundation bearing capacity. The structural design life of roads is 20 years; the service life of a road surface will relate to the type of surface applied, a dressed and sealed surface or a cold asphalt surface. A dressed and sealed surface will have limited service life with medium and high mixed traffic flows, possible no more than 2 or 3 years. The work presented in this paper has the potential to form national guidelines using this particular form of sustainable low energy construction technology.

VN Paper on Minor Road Design

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