Western Cape Provincial Administration Chapter06 Revision 3.0 Latest

F

Materials Manual

Western Cape Provincial Administration

Department of Transport and Public Works Roads Infrastructure Branch

Materials Selection, Constraints and Design Procedures

Base

Subbase

Selected

Surfacing

Subgrade

Volume 2

Chapter 6 Revision 3.0

Materials Manual First Edition

Chapter 6 Materials Selection, Constraints

and Design Procedures

Western Cape Provincial Administration Department of Transport and Public Works Roads Infrastructure Branch

Western Cape Provincial Administration 2004 by the Western Cape Provincial Administration. All right reserved First Edition published 2004 2008 by the Western Cape Provincial Administration. All right reserved. First Edition revision 3.0 published 2008 Printed in the Republic of South Africa

SET: ISBN 0-620-29823-5 CHAPTER: ISBN 1-920158-07-3

WCPA Transport and Public Works Department Materials Manual Volume 2 Chapter 6

6 Selection, constraints & design procedures June 19, 2008 page 6-i

TABLE OF CONTENTS

MATERIALS SELECTION, CONSTRAINTS AND DESIGN PROCEDURES

Contents page 6-

INTRODUCTION..............................................................................................6-1

DESIGN CONCEPTS .......................................................................................6-2

SPECIAL PROBLEMS RELATING TO THE MINERAL CONTENT OF GRANULAR MATERIALS ..................................................................................................6-4

Introduction.......................................................................................................................................... 6-4 Micaceous Material.............................................................................................................................. 6-4

ROADBED.....................................................................................................6-5 Introduction.......................................................................................................................................... 6-5 Remedial Measures............................................................................................................................. 6-5

Expansive Materials..................................................................................................................................6-5 Collapsible Materials.................................................................................................................................6-6 Settlement Of Compressible Soils ............................................................................................................6-8 Flaws In The Structural Support ...............................................................................................................6-8 Non-Uniform Support................................................................................................................................6-8 Soluble Salts.............................................................................................................................................6-8 Highly Resilient Soils ................................................................................................................................6-9 Biological Activity ......................................................................................................................................6-9

Other Problems ................................................................................................................................. 6-10

CHEMICAL STABILIZATION............................................................................6-11 Introduction........................................................................................................................................ 6-11

Modification.............................................................................................................................................6-11 Cementation ...........................................................................................................................................6-12

Objectives Of Stabilization ................................................................................................................ 6-12 Improvement Of Sub-Standard Materials......................................................................................... 6-13 Cemented Materials .......................................................................................................................... 6-13

Strength ..................................................................................................................................................6-13 Cracking .................................................................................................................................................6-14 Erodibility ................................................................................................................................................6-16 Durability.................................................................................................................................................6-17

BITUMEN EMULSION TREATMENT..................................................................6-20 Introduction........................................................................................................................................ 6-20

Modification.............................................................................................................................................6-20 Stabilization ............................................................................................................................................6-21

Objectives Of Emulsion Treatment.................................................................................................... 6-21 Improvement Of Sub-Standard Materials By Modification ............................................................... 6-22 Improvement Of Materials By Stabilization ....................................................................................... 6-22

HOT-MIX ASPHALT BASE.............................................................................6-23 Introduction........................................................................................................................................ 6-23 Design ............................................................................................................................................... 6-23

SURFACING ................................................................................................6-25 General ..........................................................................................................................6-25

Introduction........................................................................................................................................ 6-25 Reseal Needs .................................................................................................................................... 6-25 Binder Selection ................................................................................................................................ 6-25


page 6-ii June 19, 2008 6 Selection, constraints & design procedures

TABLE OF CONTENTS

Compatibility Of Bituminous Binders With Various Aggregates.............................................................. 6-25 Bitumen-Rubber ..................................................................................................................................... 6-26

Aggregate Selection .......................................................................................................................... 6-29 Precoating ......................................................................................................................................... 6-29 Design ............................................................................................................................................... 6-29

Skid Resistance...................................................................................................................................... 6-29 Conversion Factors ................................................................................................................................ 6-31 Computation Of Average Least Dimension............................................................................................. 6-33

General Constraints .......................................................................................................................... 6-33 Geometry................................................................................................................................................ 6-33 Absorptive Behaviour Of Aggregates ..................................................................................................... 6-33 Limiting Hot Gross Spray Rates ............................................................................................................. 6-33 Application Of Second Spray.................................................................................................................. 6-33 Dilutions Of Emulsions............................................................................................................................ 6-33 Soluble Salts........................................................................................................................................... 6-34

Weather Limitations........................................................................................................................... 6-34 Crack Activity..................................................................................................................................... 6-35

Prime Coat .....................................................................................................................6-37 Introduction........................................................................................................................................ 6-37 Materials ............................................................................................................................................ 6-37 Design ............................................................................................................................................... 6-37

Spray Rate.............................................................................................................................................. 6-37 Tack Coat.......................................................................................................................6-38

Introduction........................................................................................................................................ 6-38 Design ............................................................................................................................................... 6-38

Spray Rate.............................................................................................................................................. 6-38 Otta Seals ......................................................................................................................6-39

Introduction........................................................................................................................................ 6-39 Materials ............................................................................................................................................ 6-39 Design ............................................................................................................................................... 6-39

Binder Spray Rates ................................................................................................................................ 6-39 Aggregate Grading Selection.................................................................................................................. 6-40 Aggregate Application Rates .................................................................................................................. 6-41 Special Considerations For Double Otta Seal And Combination Seals .................................................. 6-41

Sand Seal ......................................................................................................................6-42 Introduction........................................................................................................................................ 6-42 Materials ............................................................................................................................................ 6-42 Design ............................................................................................................................................... 6-42

Spray Rate.............................................................................................................................................. 6-42 Spread Rate ........................................................................................................................................... 6-42

Single Seal Of 6,7 mm Chips.........................................................................................6-43 Introduction........................................................................................................................................ 6-43 Materials ............................................................................................................................................ 6-43 Design ............................................................................................................................................... 6-43

Spray Rates............................................................................................................................................ 6-43 Spread Rates.......................................................................................................................................... 6-43

Single Seal Of 9,5 mm Chips Plus Sand Blind...............................................................6-44 Introduction........................................................................................................................................ 6-44 Materials ............................................................................................................................................ 6-44 Design ............................................................................................................................................... 6-44


Single Seal Of 13,2 mm Chips.......................................................................................6-46 Introduction........................................................................................................................................ 6-46 Materials ............................................................................................................................................ 6-46 Design ............................................................................................................................................... 6-46


Single Seal Of 13,2 mm Chips Plus Sand Blind............................................................6-47 Introduction........................................................................................................................................ 6-47 Materials ............................................................................................................................................ 6-47 Design ............................................................................................................................................... 6-47


Single Seal Of 13,2 mm Chips Plus Grit ........................................................................6-48


6 Selection, constraints & design procedures June 19, 2008 page 6-iii

TABLE OF CONTENTS

Introduction........................................................................................................................................ 6-48 Materials ............................................................................................................................................ 6-48 Design ............................................................................................................................................... 6-48

Spray Rates............................................................................................................................................6-48 Spread Rates..........................................................................................................................................6-48

Double Seal Of 13,2 mm & 6,7 mm Chips .....................................................................6-49 Introduction........................................................................................................................................ 6-49 Materials ............................................................................................................................................ 6-49 Design ............................................................................................................................................... 6-49

Spray Rates............................................................................................................................................6-49 Spread Rates..........................................................................................................................................6-50

Cape Seal ......................................................................................................................6-51 Introduction........................................................................................................................................ 6-51 Materials ............................................................................................................................................ 6-51 Design ............................................................................................................................................... 6-51

Spray Rates............................................................................................................................................6-51 Spread Rate............................................................................................................................................6-53

Design Sheet ..................................................................................................................................... 6-53 Slurry Mix Composition ..................................................................................................................... 6-53

Conventional Slow Setting Anionic Coarse Slurry .........................................................6-54 Introduction........................................................................................................................................ 6-54 Design ............................................................................................................................................... 6-54 Job Mix .............................................................................................................................................. 6-54 Weather Limitations........................................................................................................................... 6-55

Rapid Setting Rubber Modified Coarse Slurry ..............................................................6-56 Introduction........................................................................................................................................ 6-56 Design ............................................................................................................................................... 6-56 Job Mix .............................................................................................................................................. 6-56 Weather Limitations........................................................................................................................... 6-57

HOT-MIX ASPHALT SURFACING ....................................................................6-58 Introduction........................................................................................................................................ 6-58 General Framework For A Comprehensive System ......................................................................... 6-59 Aggregate Selection Subsystem ....................................................................................................... 6-60 Binder Selection Subsystem ............................................................................................................. 6-60

Physical Properties .................................................................................................................................6-60 Initial Mix Design ............................................................................................................................... 6-62

Asphalt With Conventional Binder .................................................................................6-64 Introduction........................................................................................................................................ 6-64 Materials ............................................................................................................................................ 6-64 Design ............................................................................................................................................... 6-64

Continuously Graded Asphalt .................................................................................................................6-65 Semi-Gap Graded Asphalt......................................................................................................................6-66

Bitumen-Rubber Asphalt................................................................................................6-68 Continuously Graded Asphalt............................................................................................................ 6-68

Introduction.............................................................................................................................................6-68 Design ....................................................................................................................................................6-69 Material Preparation ...............................................................................................................................6-69 Interpretation Of Test Data .....................................................................................................................6-71 Quality Control ........................................................................................................................................6-71

Porous Bitumen-Rubber Asphalt....................................................................................................... 6-72 Introduction.............................................................................................................................................6-72 Design ....................................................................................................................................................6-74 Minimum Binder......................................................................................................................................6-74 Maximum Binder.....................................................................................................................................6-75

FRICTION COURSES.....................................................................................6-76 Introduction........................................................................................................................................ 6-76

Design ....................................................................................................................................................6-77 Optimum Field Voids and Binder Contents.............................................................................................6-77 Minimum Aggregate Strength and Durability ..........................................................................................6-78 Minimum Binder......................................................................................................................................6-78 Maximum Binder and Inherent Mix Stability............................................................................................6-78 Minimum and Maximum Tack Application ..............................................................................................6-79


page 6-iv June 19, 2008 6 Selection, constraints & design procedures

TABLE OF CONTENTS

CONCRETE ................................................................................................ 6-80 Introduction........................................................................................................................................ 6-80 Materials ............................................................................................................................................ 6-80

Aggregate............................................................................................................................................... 6-80 Cement................................................................................................................................................... 6-80 Water...................................................................................................................................................... 6-81 Admixtures.............................................................................................................................................. 6-81

Design Criteria................................................................................................................................... 6-81 Mix Design Procedure ....................................................................................................................... 6-82

Introduction............................................................................................................................................. 6-82 Design Of A Trial Mix.............................................................................................................................. 6-82 Remarks On Design Procedure.............................................................................................................. 6-83

Durability............................................................................................................................................ 6-85 Aggressive Environments ....................................................................................................................... 6-85 Water In Contact With Concrete ............................................................................................................. 6-86 Analytical Tests Required....................................................................................................................... 6-87 Assessment Of Analytical Results And Recommended Countermeasures ............................................ 6-89 Soils In Contact With Concrete............................................................................................................... 6-90 Analytical Tests Required....................................................................................................................... 6-90 Recommended Countermeasures.......................................................................................................... 6-92

SEGMENTAL PAVING BLOCKS...................................................................... 6-93 Introduction........................................................................................................................................ 6-93 Design ............................................................................................................................................... 6-93 Constraints ........................................................................................................................................ 6-93

Junction Of Asphalt And Concrete Block Paving Surfacing.................................................................... 6-93

CONSTRUCTION WATER .............................................................................. 6-95 Determining Quality And Suitability ................................................................................................... 6-95

Analysis Of Water Results For Roadworks............................................................................................. 6-95 Analysis Of Water Results For Concrete ................................................................................................ 6-96

REFERENCES ............................................................................................. 6-97 Journals ............................................................................................................................................. 6-97 Manuals And Specifications .............................................................................................................. 6-99

FIGURES Figure 6-1: Layout of the Materials Manual ................................................................................................... 6-1 Figure 6-2: Proposed crushing curve .......................................................................................................... 6-16 Figure 6-3: Summary of relationships between erosion index and relative density of four types of

material ...................................................................................................................................... 6-18 Figure 6-4: Grading of coarse graded asphalt base.................................................................................... 6-24 Figure 6-5: Bitumen-rubber blend characteristics for 78% 80/100 pen bitumen with 20% rubber crumb

and 2% extender oil at different reaction temperatures of 180C, 200C and 220C ............... 6-28 Figure 6-6: Effect of surface properties on skid resistance ......................................................................... 6-30 Figure 6-7: Crack Movement for a typical cemented base.......................................................................... 6-36 Figure 6-8: Determination of M50 and CMSt from CMS data ....................................................................... 6-36 Figure 6-9: WCPA spread rate curve .......................................................................................................... 6-45 Figure 6-10: Radius of curvature versus asphalt strain............................................................................... 6-58 Figure 6-11: Surface Curvature Index (SCI) to maximum horizontal strain ................................................ 6-58 Figure 6-12: Granular base elastic modulus (EB) versus deflection basin parameters............................... 6-59 Figure 6-13: Comprehensive design system for asphalt with or without modified bitumen ........................ 6-61 Figure 6-14: Aggregate selection subsystem .............................................................................................. 6-63 Figure 6-15: Binder selection subsystem .................................................................................................... 6-63 Figure 6-16: Average dynamic creep results according to grading............................................................. 6-71 Figure 6-17: Example for road structure with porous asphalt ..................................................................... 6-72 Figure 6-18: Bitumen-rubber blend characteristics for different sources of bitumen, different percentages

rubber crumb at a reaction temperature of 180C .................................................................. 6-73 Figure 6-19: Example of a road structure with friction course (indicating water drainage mechanism)...... 6-76 Figure 6-20: Optimum coarse aggregate content........................................................................................ 6-84 Figure 6-21: Water map of South Africa giving an indication of potential aggressive ground waters ......... 6-87 Figure 6-22: Requirements for a corrosion survey ...................................................................................... 6-91


6 Selection, constraints & design procedures June 19, 2008 page 6-v

TABLE OF CONTENTS

TABLES Table 6-1: Summary of Deflection Basin Parameters ................................................................................... 6-2 Table 6-2: Behaviour States Defined by Deflection Basin Parameters......................................................... 6-3 Table 6-3: Kantey-Brink criteria for expansive potential................................................................................ 6-5 Table 6-4: Van der Merwe criteria for expansive potential ............................................................................ 6-6 Table 6-5: Minimum compaction requirements for collapsible soils (after Weston)...................................... 6-7 Table 6-6: Tentative allowable roadbed collapse values............................................................................... 6-8 Table 6-7: Proposed erodibility criteria for lightly cemented materials, C3 and C4..................................... 6-17 Table 6-8: Erosion test results on the material from Eastern Cape ............................................................ 6-17 Table 6-9: Residual UCS strength............................................................................................................... 6-19 Table 6-10: Selection of the appropriate design approach for bitumen emulsion treated layers ................ 6-20 Table 6-11: Recommended quality limits for substandard materials .......................................................... 6-21 Table 6-12: Recommended PIs for modification with bitumen emulsion.................................................... 6-22 Table 6-13: Target grading for continuously graded asphalt base -37,5mm............................................... 6-23 Table 6-14: Test requirements for continuously graded asphalt base ........................................................ 6-24 Table 6-15: Suitability of binder-aggregate combination ............................................................................. 6-26 Table 6-16: French texture depth guidelines ............................................................................................... 6-30 Table 6-17: Minimum values of skidding resistance for different sites........................................................ 6-31 Table 6-18: Characteristic SFC values in the Western Cape Province....................................................... 6-32 Table 6-19: Suggested conversion factors for converting net cold residual binder to spray rate at average

spray temperature ................................................................................................................... 6-32 Table 6-20: Gradient limits for surfacing...................................................................................................... 6-33 Table 6-21: Minimum hot gross spray rates ................................................................................................ 6-33 Table 6-22: Road surface temperature for application of binders ............................................................... 6-34 Table 6-23: Interim crack movement classification ..................................................................................... 6-35 Table 6-24: Recommended tack coat spray rates....................................................................................... 6-38 Table 6-25: Choice of Bitumen binder ......................................................................................................... 6-40 Table 6-26: Hot bitumen spray rates for primed rates (l/m2)....................................................................... 6-40 Table 6-27: Preferred aggregate gradings .................................................................................................. 6-40 Table 6-28: Appropriate grading for Otta Seals........................................................................................... 6-41 Table 6-29: Aggregate application rates for Otta Seals .............................................................................. 6-41 Table 6-30: Emulsion spray rates for a 6,7 mm seal................................................................................... 6-43 Table 6-31: Spray rates for 6,7 mm seal for lightly trafficked by-passes .................................................... 6-43 Table 6-32: Values of p for determining spray rate for double seal ............................................................ 6-49 Table 6-33: Determination of E80s heavy vehicle....................................................................................... 6-52 Table 6-34: Values of p for determining spray rate for Cape Seal .............................................................. 6-52 Table 6-35: Special circumstances warranting a reduction of spray rates.................................................. 6-52 Table 6-36: Tolerances for grading of aggregate for conventional slow setting anionic coarse slurry ....... 6-54 Table 6-37: Tolerance for grading of aggregate for rapid setting rubber modified coarse slurry ................ 6-57 Table 6-38: Mix Properties........................................................................................................................... 6-62 Table 6-39: Test requirements of semi-gap and continuously-graded mixes ............................................. 6-66 Table 6-40: Performance indices for binders and asphalt mixes ................................................................ 6-69 Table 6-41: Porous asphalt requirements ................................................................................................... 6-75 Table 6-42: Friction Course mix requirements ............................................................................................ 6-78 Table 6-43: Friction Course tack application rates ...................................................................................... 6-79 Table 6-44: Workability related to slump and vibro-consistometer.............................................................. 6-82 Table 6-45: Maximum water-cement ratio for different cement strength..................................................... 6-82 Table 6-46: Water content ........................................................................................................................... 6-83 Table 6-47: Approximate relative density for components of concrete ....................................................... 6-85 Table 6-48: Tests required for testing aggressiveness for water ................................................................ 6-88 Table 6-49: Recommended limits for assessing aggressiveness of water ................................................. 6-89 Table 6-50: Precautionary measures to protect concrete piles ................................................................... 6-92 Table 6-51: Water sample test results......................................................................................................... 6-95


6 Selection, constraints & design procedures June 19, 2008 page 6-1

INTRODUCTION

MATERIALS SELECTION, CONSTRAINTS AND DESIGN PROCEDURES

INTRODUCTION This chapter gives the background needed to adequately select and improve materials to be used for road construction, as well as to design composite materials, such as asphalt surfacing. Topics covered include problems and constraints with:

roadbed stabilization seal application rates asphalt mix design wearing course selection concrete mix design, and usage and application of materials.

This chapters part in the overall layout of the Materials Manual is shown in Figure 6-1 on page 6-1 .

The chapter is laid out in accordance with the sequence of pavement layers starting at the roadbed and progressing to the surfacing layers. Concrete mixes are also covered at the end.

1 Management proceduresfor monitoring and control

2 Materials standards

3 Commentary on testmethods

4 Sampling methods

7 Construction equipment control

Chapter 5MATERIALS

INVESTIGATION AND REPORTING

Chapter 6MATERIALS SELECTION,

CONSTRAINTS AND DESIGN PROCEDURES

Chapter 8QUALITY ASSURANCE

Chapters 9-20ACCEPTANCE CONTROL

Chapter no.

INPUTS ACTIVITIES

This chapter

Figure 6-1: Layout of the Materials Manual


page 6-2 June 19, 2008 6 Selection, constraints & design procedures

INTRODUCTION

DESIGN CONCEPTSIn order to produce an appropriate rehabilita-tion design the behavioural characteristics of the pavement need to be understood particu-larly with respect to flexibility. It would be inappropriate to apply a stiff or rigid overlay on a highly flexible pavement. Deflections and curvature created by wheel loads can be measured and are used in the de-signs. The typically used parameters are, for

convenience, summarised in this section. Different deflection basin parameters and ap-propriate measuring devices are summarized in Table 6-1 on page 6-2. Horak also introduced the concept of the dif-ferent behaviour states and guideline criteria using the deflection bowl parameters, as out-lined in Table 6-2 on page 6-3.

Table 6-1: Summary of Deflection Basin Parameters

PARAMETER FORMULA MEASURING DEVICE

REFERENCE

Maximum deflec-tion

maxY

or 0

Benkelman beam Lacroix deflectograph

Kennedy et al Asphalt Institute (1978)

Radius of curva-ture

mm 127 =r

)1(2r

=R0r0

2

Curvature Dehlen (1962a)

Spread ability ( )[ ]

mm 305 spaced ...

1005/=

31

0

3210

=

+++

S

Dynaflect Vaswani (1971)

Area ( )( )

+

++

0302

01

//2

2+16=A

Falling weight deflec-tometer (FWD)

Hoffman and Thompson (1981)

Shape factors ( )( ) 2310

1201

FF

=

=

FWD Hoffman and Thompson (1981)

Base Layer Index (previously SCI)

BLI = 0 - 300 Benkelman beam Road rater FWD

Anderson (1977) Kilreski et al (1982) Molenaar (1982)

Middle Layer In-dex (previously BDI)

MLI = 300 - 600 Road rater Kilareski et al (1982)

Lower Layer In-dex

LLI = 600 - 900 FWD

Deflection ratio Qr = r /0 r =0/2

FWD Claessen and Ditmarsch (1977)

Bending Index BI = 0/a a = deflection basin

Benkelman beam Hveem

Slope of deflec-tion

SD = tan -1(0 - r)/r r = 610 mm

Benkelman beam Kung (1987)

Tangent slope ST = (0 - r)/r where r is determined by a polynomial function

Benkelman beam FWD

University of Dundee (1980)

Radius of influ-ence

RI = R/0 R is the distance from 0 to where the basin is tangent to the horizontal

Ford and Bisset (1962)



INTRODUCTION

Table 6-2: Behaviour States Defined by Deflection Basin Parameters

GRANULAR BASE PAVEMENT APPROXIMATE

TRAFFIC CLASS BEHAVIOUR

STATE Ymax mm

BLI mm

MLI mm

LLI mm

ES100 Very stiff 0,10

ASPHALT BASE PAVEMENT APPROXIMATE


STATE Ymax mm

BLI mm

MLI mm

LLI mm

ES100 Very stiff 0,08

CEMENTED BASE PAVEMENT APPROXIMATE


STATE Ymax mm

BLI mm

MLI mm

LLI mm

ES100 Very stiff 0,10 >0,08

Note IDM Deflection and Bowl Parameters PRESS = the pressure of the load plate (diameter 300 mm) on the pavement in kPa (standard = 550 kPa). LOAD = the load transferred by the plate to the pavement in kN (standard = 40 kN). D1 = deflection of the pavement surface at the centre of the load plate, m D2 = deflection of the pavement surface at 200 mm from the centre of the load plate, m D3 = deflection of the pavement surface at 300 mm from the centre of the load plate, m D4 = deflection of the pavement surface at 600 mm from the centre of the load plate, m D5 = deflection of the pavement surface at 900 mm from the centre of the load plate, m D6 = deflection of the pavement surface at 1200 mm from the centre of the load plate, m D7 = deflection of the pavement surface at 1800 mm from the centre of the load plate, m Ymax = D1 = maximum surface deflection which is an indication of bearing capacity of the total pavement, m BLI = (previously SCI) Base layer index = D1 - D3: indication of stiffness of the surfacing, the base and sometimes the subbase (depending on the thickness of these layers) MLI = (previously BDI) Middle layer index = D3 - D4: indication of stiffness of the subbase and upper selected material LLI = (previously BCI) Lower layer index = D4 - D5: indication of stiffness of the selected material and roadbed SPD = Spreadability = 100([D1 + D2 + D3 + D4 + D5 + D6 + D7]/7D1): is an indication for the ability of the pavement to transfer loads. E-AASHTO = Modulus of elasticity of the roadbed, MPa, calculated with the following equation1: E-AASHTO = (20p)/(Dr.r) p = plate pressure, kPa Dr = deflection at distance r, mm r = distance from the centre of the load plate, mm The deflection D6 or D7 is normally used.

1. AASHTO guide for Design of Pavement Structure. AASHTO, Washington D.C., 1986.



MINERAL CONTENT OF GRANULAR MATERIAL

SPECIAL PROBLEMS RELATING TO THE MINERAL CONTENT OF GRANULAR MATERIALS

INTRODUCTION A granular pavement layer consists of the natural materials that must meet the standards as given in Chapter 2, Materials Standards. Special attention should be paid to the impact of the material characteristics affecting performance in order to obtain a uniform, high resilient modulus to support the following layer. As the resilient modulus increases with increasing density, decreasing saturation, and increasing angularity, it is important to identify and eliminate materials that will reduce compactability and which are prone to saturation and water sensitivity. The purpose of this section is to give guidelines for the identification of some problem materials that will lead to the construction of substandard layers.

MICACEOUS MATERIAL Weathered rock or soil that contains more than 10 percent of mica2, especially coarse-grained muscovite, should be avoided for use in any layer of a pavement and particularly the subbase and base layers.

2. Tubey, L.W. A laboratory investigation to determine the effect of mica on the properties of soils and sta-bilized soils. Ministry of Transport, Roads Re-search Laboratory, Research Note No. RN/4077/LWT (unpublished), 1961.

Muscovite, the light or white mica3, also occurs in igneous rocks, particularly in the acid types like granite and pegmatite, but is also an important and widely occurring component of certain sedimentary and metamorphic rocks. Muscovite is an extremely elastic mineral whose platy crystals, if bent, will always tend to recover their original shape. The spring action affects the compactability of muscovite-containing soil or weathered rock. Densities obtained are usually less than 1770 kg per m for subbase layers.

Obtaining the specified density can be problematic when using weathered rock or soil of high micaceous content. With increasing contents of mica, the liquid limit and plastic limits of soil increase while the plasticity decreases. This is caused by the numerous voids contained in a micaceous soil. The high void ratio results in the soil retaining an abnormally high content of free water resulting in a high Liquid Limit. Also, the large internal surface area of micaceous soils, caused by the platy mica minerals, allows a larger quantity of water than usual to be retained in the soil when the cohesion between the soil particles breaks down and this results in a higher Plastic Limit.

3. Weinert, H.H. The natural road construction materials of Southern Africa, NITRR, CSIR, Pretoria, 1980.



ROADBED

ROADBEDINTRODUCTION

The roadbed consists of the natural in situ material on which the pavement layers are constructed. The centreline materials survey should give a good picture of the in situ materials along the route. The purpose of this section is to give guidelines on remedial measures for in situ materials within the material depth that have unusual characteristics, e.g., expansive and collapsible materials.

REMEDIAL MEASURES

EXPANSIVE MATERIALS

INTRODUCTION For soils to behave expansively, three concur-rent conditions are necessary. They are:

A clay profile (this includes constituents of the smectite group of clay minerals, i.e., saponite, nontronite or montmorillonite): the greater the amount of clay in the soil the worse the potential for heaving.

A deep water table level: the greater the depth of the water table, below the surface, the greater the potential heave, since the swelling will be accumulated over much a greater depth.

A desiccated soil, i.e., the moisture con-tent is very low: heaving conditions may be an-ticipated in climates where evaporation from a free water surface is much greater than rainfall. This particularly occurs when there is a large seasonal variation in rainfall.

Weston4 has found that expansive clays will detrimentally affect the road if the following circumstances exist:

4. Weston, D.J. Expansive roadbed treatment for southern Africa. Proceedings of the International Conference on Expansive Soils, Denver, 1980, pp 339-360.

The road is sealed with bituminous or concrete surfacing. The finished road is within 0,5 m and 5,0 m above the natural ground. There is an expansive clay roadbed which extends from the natural ground surface (i.e., not buried under other material) to a depth of less than 3,0 m. An average annual rainfall of 500 mm to 800 mm that is concentrated in the summer months. Material is potentially expansive when the Atterberg Limits of the material exceed the Kantey-Brink5 criteria given in Table 6-3 on page 6-5.

Table 6-3: Kantey-Brink criteria for expansive potential

ATTERBERG LIMITS CRITERIA Linear Shrinkage, % critical >8 marginal 5 8 Liquid Limit, % > 30 Plasticity Index > 12

Alternatively, the modified Van Der Merwe criteria6 for weighted PI may be used to iden-tify expansive potential of material, as given in Table 6-4 on page 6-6.

Refer also to the paper by Hanafy7 where the characteristic swelling-shrinkage curve of des-iccated expansive clay is developed. Very sig-nificant volume change can occur in some clays, in excess of 30 percent in certain in-stances.

5. Kantey, B.A. and Brink, A.B.A. Laboratory criteria for recognition of expansive soils. NBRI Bulletin no. 9, 1952, pp 25 - 28.

6. Wilson, L.C. Discussion, Speciality Session B. Pro-ceedings of the Sixth Regional Conference for Af-rica on Soil Mechanics and Foundation Engineering, Durban, 1976, pp 167-168.

7. Hanafy, E. Swelling/Shrinkage Characteristic Curve of Desiccated Expansive Clays. Geotechnical Test-ing Journal, GTJODJ, Vol. 14, No. 2, June 1991, pp 206-211.

WCPA Department of Transport and Public Works Materials Manual Volume 2 Chapter 6


ROADBED

Table 6-4: Van der Merwe criteria for expan-sive potential

WEIGHTED PI1 EXPANSIVE POTENTIAL 32 Very high expansion Note

1. The weighted PI = PI.(% passing 425 m)

TREATMENT Possible methods available to reduce or elimi-nate the effects of heaving of materials identi-fied as expansive in road construction are8:

Control of free water by effective drain-age.

Control of density and moisture content during compaction: it has been found that compaction above OMC increases cohesion and internal friction.

Pre-wetting of expansive materials has been tried with varying success. The main dif-ficulty is in obtaining thorough and uniform wetting.

Lime stabilization or treatment: lime has been found beneficial in modifying the characteristics of expansive clays. It is important, however, to ensure thorough mixing across the full width of the road and to sufficient depth. Lime treatment is generally best used in conjunction with other methods of reducing swell.

The use of pressure to reduce swelling: this is achieved by using surcharge loads of at least 5 m. This is clearly not often an option.

Raising gradelines sufficiently to prevent damage from free water.

The use of impermeable membranes to prevent moisture variation.

8. Concrete pavement construction over expansive soils. Portland Cement Institute, 1970.

The removal of the potentially expansive material and replacement with suitable mate-rial.

The use of ionic stabilizers (sulphonated oils): these need to be tested with the expan-sive soil (for example cation exchange) to de-termine whether they are effective. The fol-lowing can be achieved:

Increased density (with sufficient effort) Reduced permeability and hence less

susceptibility to moisture variations With some ionic stabilizers, chemical al-

teration of the clay particles and a conse-quent reduced susceptibility to moisture.

Weston5 gives two methods of estimating the amount of swell (in millimetres). This is an important step in assessing suitable counter-measures. The estimates can be used to rank the swell potential of the expansive clay at one of four levels. Appropriate treatment measures can be selected.

COLLAPSIBLE MATERIALS INTRODUCTION This is really a special case of consolidation settlement where inundation, under load, causes the collapse of the unstable soil fabric. This is due to the softening of the bridging material, which holds the coarser grains apart. The requirement for both loading and water distinguishes it from pure settlement where only loading is required. These cementing bridges are usually one of or a combination of the following: clay, iron oxide, carbonate, gyp-sum, mica and possibly even salt.

Collapsible soils are generally of aeolian or hillwash origins. However, highly weathered granites and felspathic sandstones are also known to cause this problem. According to Weston9, collapsible soils show all of the fol-lowing characteristics:

9. Weston, D.J. Compaction for collapsing sand road-beds. Proceedings of the Seventh Regional Con-ference for Africa on Soil Mechanics and Founda-tion Engineering, Accra, 1980.



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They are predominantly sandy, i.e., more than 60 percent of the material between 2,0 mm and 0,6 mm and less than 20 percent less than 2 m in particle size.

They have a relative compaction (in situ) of less than 85 percent of Mod. AASHTO.

Oedometer tests on the material have a collapse, measured under future service stresses, of greater than one percent.

Potentially collapsible material in the roadbed should be assessed as follows:

soil profiling to provide a descriptor DCP testing to indicate relative

strength or homogeneity, but not density in situ density to measure relative

compaction laboratory grading to indicate sand

content oedometer test to indicate collapse

potential laboratory compaction to establish

Mod.AASHTO and hence relative com-paction

Atterberg Limits (on both -0,425 mm and -0,075 mm) to determine plasticity

A quick on site check for collapse potential can be done as follows:

dig a hole at least 300 mm x 300 mm x 300 mm;

stockpile the excavated material care-fully; and

backfill the hole with the stockpiled ma-terial. Do not compact the backfill.

If after the hole is filled no surplus stockpiled material remains the material is probably col-lapsible.

TREATMENT Weston10 has made a detailed study of col-lapsible sands. He recommends the following minimum compaction for areas with collapsi-ble soils - see Table 6-5 on page 6-7.

10. BS 1377 : Part 5 : Compressibility, permeability and durability tests. British Standard Methods of Tests for Civil Engineering Purposes, 1990.

Table 6-5: Minimum compaction require-ments for collapsible soils (after Weston)

DEPTH m

MINIMUM MOD. AASHTO COMPACTION, %

0,0 - 0,5 90 0,5 - 1,0 85

Compaction at depth can be achieved by nor-mal compaction methods when the sand is saturated. This may not be an option in the dryer areas where these sands are often found. It may thus be necessary to use pneumatic, im-pact or vibratory rollers depending on the site.

Roller types, number of passes and moisture contents should be decided by the Engineer after site trials. Monitoring of the compaction should be carried out preferably by in situ density measurements, or by DCP tests. The DCP should be calibrated to the in situ density, at different levels, for the given material.

If the compactions, at depth, given in Table 6-5 on page 6-7 cannot be achieved, then further oedometer collapse testing of the compacted material should be carried out. If the measured values exceed those given in Table 6-6 on page 6-8, then additional densification is required.

Refer also to TRH 911, TRH 1012 and TRH 1513 for further design guidelines.

11. TRH 9: Construction of road embankments. CSIR, NITRR, Pretoria, 1982.

12. TRH 10: The design of road embankments. CSRA, Department of Transport, Pretoria, 1994.

13. TRH 15: Subsurface drainage for roads. CSRA, Department of Transport, Pretoria, 1994.



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SETTLEMENT OF COMPRESSIBLE SOILS

INTRODUCTION These generally involve soft alluvial, estuarine and swamp soils, clays and mine slimes. These conditions require deep site investigations consisting of deep trial pits, boreholes and/or probing such as quasistatic (Dutch) probes or continuous SPTs. Undisturbed samples need to be taken so that in situ density, grading, Atterberg Limits, organic content and oedometer (consolidation) tests can be carried out in the laboratory.

TREATMENT Adequate drainage, preloading and/or reinforc-ing is usually required. Refer to TRH 912, TRH 1013 and TRH 1514 for further design guidelines.

FLAWS IN THE STRUCTURAL SUPPORT

INTRODUCTION These flaws are secondary sinkholes, subsi-dence in dolomitic and limestone terrain, min-ing subsidence and slope in stability.

TREATMENT Special measures such as filling, reinforcing, drainage detailing, bridging and retaining structures may be needed. For further guide-lines see Reference 14.

NON-UNIFORM SUPPORT INTRODUCTION This results from wide variations in soil types or varying conditions in a single soil type.

TREATMENT Varying and/or deepening of the pavement structure may be required.

SOLUBLE SALTS INTRODUCTION Under certain circumstances, salt may migrate upwards and cause cracking, blistering or loss of bond of the surfacing, disintegration of ce-mented layers and loss of density of untreated layers. The upper limit of soluble salt content of pavement material has been recommended

14. Van Wyk and Louw. Various reports on project C404, Danilskuil to Kuruman.

Table 6-6: Tentative allowable roadbed collapse values

ALLOWABLE COLLAPSE1,2,3 DEPTH BELOW ORIGINAL GROUND

LEVEL (D) Saturation possible

Saturation unlikely

Saturation very unlikely

D 0,5 m 1% 1% 1%

0,5 < D 1,0 m 1% 2% 3%

1,0 < D 1,5 m 1% 3% 4%

Road surface total settlement4, assuming saturation to a depth of 1,5 m

15 mm 30 mm 40 mm

Notes 1. The collapse should be determined in a standard oedometer using soil specimens 75 mm diameter by 19 mm

high. (Different dimensions may give different collapse values.) (British Standard Methods of tests for civil engi-neering purposes. BS 1377, Part 5: 199011 - Compressibility, permeability and durability tests, 4. Determination of swelling and collapse characteristics).

2. The collapse should be measured under a vertical pressure equivalent to the overburden, construction and traffic stresses.

3. The future roadbed moisture conditions should be assessed and the appropriate collapse values selected. 4. Differential settlements will be less (25 - 75 percent) than this.



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as 0,2 percent15. This level is only valid if there is no source of additional salt below the pave-ment. In such instances special measures to minimize upward migration of salts may be needed.

Possible measures to minimize upward migra-tion of the salts are16,17 as follows:

The use of a barrier to prevent salt migra-tion. This can be in the form of a plastic water-proof membrane, asphalt priming or coating or uniform graded macadam or gravel without fines and sand.

Ensuring that the existing salt content of material used does not exceed 0,2 percent by mass.

The use of only fresh water for construc-tion.

Minimize salt migration by programming the pavement construction to avoid wet/dry cy-cles during the construction of each of the sub-sequent layers.

Ensuring that each compacted layer is covered with the material of the next layer within 14 days (the uncompacted cover mate-rial may be left indefinitely, but once com-pacted it must also be covered within 14 days).

The use of an inactive crushed stone base, placed and then compacted within 30 days.

Sealing the completed base within 21 days.

The use of a more impermeable bitumi-nous surfacing (e.g., Cape Seal). This should include the shoulders, to prevent the wet/dry cycles, which promote migration of salt. Care should be taken when using this precautionary measure to ensure that the bituminous surfac-ing remains impermeable. In this regard it may

15. Netterberg F. Salt damage to roads- an interim guide to its diagnosis, prevention and repair. IMIESA 4, 1979

16. Horta, J.C. Salt Heaving in the Sahara. Geotech-nique, Vol. XXXV, No. 3, Sept. 1983.

17. Obika, B. and Freer-Hewish, R.J. Soluble salt damage to thin bituminous surfacings of roads and runways. Australian Road Research, Vol. 20(4), December 1990, pp 24-41.

well be advisable to consider the use of modi-fied binders, such as bitumen-rubber or SBR latex, which offer additional flexibility and du-rability.

HIGHLY RESILIENT SOILS INTRODUCTION Excessive deflection and rebound of highly resilient soils can occur during and after the passage of a load. This can occur in organic material, ash, micaceous and diatomaceous soils.

TREATMENT Special compaction techniques or the removal of the material may be required.

BIOLOGICAL ACTIVITY

INTRODUCTION

Moles Road damage due to the activities of moles appears to be caused mainly by two members of the Bathyergidae Family, the Cape dune molerat (Bathyergus suillus) and the Namaqua dune molerat (B. janetta). These are neither moles or rats, but are more closely related to porcupines. The molerats potential for causing damage to the road structure is illustrated by the following facts18:

The Cape dune molerat has been known to reach a body length of up to 360 mm (ex-cluding the tail), and a mass of almost 900 g. It is the largest known completely subterranean rodent in the world. The Cape dune molerats extensive burrowing under highways and rail-way tracks causes sagging of the roads and lines. They can be found in both sand dunes and sandy loam areas. Their tunnels range be-tween 150 and 220 mm in diameter and are typically 400 to 650 mm below the surface. Total burrow lengths measure up to 420 m.

18 Skinner, J.D. and R.H.N. Smithers. The Mammals of

the Southern African Subregion. University of Preto-ria Press, 1990.



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A single individual occupies a burrow. Nesting chambers have been measured between 400 and 900 mm below the ground level. Average monthly extensions of the burrows range between 15 and 100 m. They are most active when the moisture content of the substrate is highest, in winter and during the breeding season. It is thus apparent that the molerat is most active in the wet season, when the road structure is most vulnerable to damage. The molerats operate at the level of the lower selected level where sands layers are frequently used.

Termites Termites can create voids under a road, which leads to differential settlement.

TREATMENT

Moles There are two primary courses of action to pre-vent damage to the road structure caused by molerat activity in the typically sandy envi-ronment where they are generally found. These are either to use mole barriers, or to stabilize the lower pavement layers.

Mole barriers: mole barriers typically consist of a continuous wall of 6 mm thick unpressed fibre-reinforced-cement sheets placed vertically in the road verge. The sheets are placed from the bottom of subbase level down some 0,9 to 1,5 m, depending on the in situ ground conditions and the normal water table level. Bear in mind that molerats have been known to dig deeper than 1,5 m. Careful backfilling of the trenches is important and stabilization of the backfill above the sheets, at the subbase base levels, is advisable.

The advantage of mole barriers is that they can be added after the road has been constructed. They are, however, expensive and make no contribution to the strength of the road structure.

Cement stabilization: it is recommended that the uppermost sand layer of the pavement structure be stabilized with 1,5 to 2 percent cement to a depth of 300 mm to minimize the impact of mole excavations under or through the road prism. In addition, cemented subbase layers provide a bridging structure in the event of undermining at lower levels.

The advantages of stabilization is that it both decreases the water sensitivity of the road prism, reduces pavement deflections and adds to the overall strength. The disadvantages are the cost and the fact that stabilization must be carried out during construction.

Termites Cement stabilization or insect poisons are the only preventative options. Stabilization is envi-ronmentally preferable.

OTHER PROBLEMS Other rarer problems to be aware of include polluted soils, combustible soils (peat, colliery spoil), sulphate attack, acid attack, corrosion of metal culverts, wind erosion and deposition. The treatment of these special problems will be dealt with on a needs basis.



CHEMICAL STABILIZATION


INTRODUCTION Chemical stabilization is achieved by mixing an active substance with a road building mate-rial. The material may have had a mechanical stabilizer (binder) added. The resulting chemi-cal reaction causes a cementing action, which makes the material stronger and more durable. As a secondary result the chemical reaction may also reduce or eliminate certain unsatis-factory properties such as plasticity. Grading is not a critical factor, but a lack of fines can result in poor strength gain.

Chemical stabilization is used for a wide range of purposes such as to:

Increase bearing strength, increase the tensile strength, especially in the early stages, by cementing the granular material, decrease the moisture content of wet ma-terials in clay cuttings during rainy seasons in order to expedite construction, reduce or eliminate the formation of harmful salts, i.e., soluble sulphate is changed to non-active sulphate compounds, reduce the plasticity (active clay con-

tent), improve the compactibility of clayey ma-terials due to alteration of the small clay particles to bigger particles, making the ma-terial more friable and workable, and make a material less susceptible to mois-ture ingress.

Granular layers can be chemically treated to achieve some or all of the above by

modification, modification and cementation, or cementation.

However, it must be remembered that in the case of lime, these two processes, modification and cementation, overlap.

MODIFICATION The main objective of modification is to alter certain undesirable characteristics of a material and make it suitable for use in the pavement. Modification applies only to those materials, which have clay, or other lime reactive mate-rial in them. The purpose of modification is to neutralise the negative effects of clay minerals.

Lime is usually the most effective and cheapest modifier. The addition of lime leads to a change in the state of aggregation of the clay particles as a result of a cation exchange. The physical properties of the material itself are thus changed. This process is generally termed modification and takes place fairly rapidly. In addition, normally over long periods of time and under favourable conditions soil-lime pozzolanic reactions may take place. As a re-sult hydrates similar to those encountered in cement are formed, leading to a cementing ac-tion. The processes of cementation and modifi-cation are not necessarily distinct and may oc-cur simultaneously.

Lime has the ability to reduce plasticity and for acidic, sulphide and/or sulphate contaminated materials, to prevent salt damage. For success with modification, it must be remembered that road limes react differently with different types of clay minerals. Therefore, the most appropriate type of lime must be determined. All types of road lime from feasible lime sources near the project should be used in the Design Verification (Laboratory) stage, to determine the most cost effective type of lime stabilization.

The three types of lime are as follows:

Calcitic road lime: This has a high concentration of calcium hydroxide (available free lime) with no or very little magnesium hydroxide, e.g., lime from Titan Lime Works, Moorreesburg, Ulco Lime Works and P & B Lime Works, Bredasdorp.

Magnesium road lime: This has a lower concentration of calcium hydroxide with a




higher level of magnesium hydroxide. Sources in this province are unknown.

Dolomitic lime: This has more or less equal calcium and magnesium hydroxides, e.g., Cape Lime Works, Langvlei, Robertson.

However, road lime either from Moorreesburg or Ulco is superior in comparison to lime from Bredasdorp. Lime from Robertson needs a higher content to be added to achieve the same effect as the others. The transport cost is a factor to take into consideration when selecting a source of lime.

The sampling and testing procedures are car-ried out after the stabilising agent has been mixed in, as described in Chapter 4, Sampling Methods, Sampling of Pavement Layers.

CEMENTATION This includes cement, cement-slagment and lime-slagment blends. Cementation is due to the formation of strongly cementitious hydrates which bond soil particles together. The hydration proceeds largely independently of the aggregate and does not rely on chemical interaction between cement and aggregate for development of strength. Some lime is produced during the hydration which may react with any clay particles present in the manner described below.

Cementation of subbase and selected layers is sometimes required to satisfy the pavement de-sign. Limited road funds may also prescribe stabilization of an existing base as a first stage rehabilitation. In a following stage this layer would become a cemented subbase.

When a cemented layer is required, the un-treated material shall comply with the specifi-cation for the granular material for that layer.

Portland cement is normally the cementing agent used to achieve high tensile strength in the early stages. If early strength is not required, a mixture of cement and slagment, or lime and slagment mixture may be used. Some lime, as in the case of cement, may be produced during hydration, and this may react with any clay minerals present in the mixture.

In the case of road lime, cementation may also be achieved. Sufficient lime is added to satisfy the initial consumption of lime (ICL) for modi-fication of excess active clay minerals present. Thereafter, excess lime is used for the forma-tion of cementing compounds by soil-lime poz-zolonic reactions. Normally this reaction takes place over long periods of time.

Where the early gain of tensile strength is needed preference shall be given to cement, cement-slagment, or lime-slagment blends, in this order. The selection of the stabilizing agent will depending on the quality of material to be used and whether it is available.

Increased density improves the strength gained. Therefore, the minimum compaction for the upper cemented subbase layer is in-creased to 96 percent of Modified AASHTO density.

The trial section offers an opportunity to do the following:

a compaction study; assess the effectiveness of various compaction equipment; establish if the higher specified compaction can be achieved, and calibrate a second nuclear gauge and/or the sand replacement method to be used as a back-up.

In the case of a second gauge or sand replacement method giving a higher mean percentage compaction of more than one percent point, this value should be added to the minimum specified compaction limit for the specific layer, rounded off to the nearest 0,5 percent.

OBJECTIVES OF STABILIZATION

The objectives of stabilization of pavement layers are:

Reduction of construction costs by improving the properties of substandard, readily available materials where stabilization is a cheaper alternative than the procurement of materials complying with the relevant




specification. The material may remain essentially a granular type, i.e., its strength is derived largely from inter-granular friction, although some increase in cohesion will result should cementation take place.

The achievement of tensile strength due to an increase in cohesion as a result of cementation. A cemented layer is required to satisfy criteria set for the following interrelated characteristics:

Strength Cracking Erodibility Durability.

The construction of a platform by stabi-lizing a wet and/or soft roadbed. This expe-dites the construction process.

IMPROVEMENT OF SUB-STANDARD MATERIALS

DESIGN CRITERIA Since the main requirement is modification, tests shall be performed on uncured material, irrespective of the type of stabilizer, although it will most likely be lime. The design criteria are dependent on the properties that the treatment is intended to improve. Generally, the only material properties under consideration in this respect are Plasticity Index and California Bearing Ratio. In view of the inherent variability of materials and non-uniformity of mixed-in stabilizer, the target values for materials design should be chosen such that this variability is accounted for. The guidelines are given here:

Target CBR = CBRmin + 25 Target PI = PImax - 2

DESIGN PROCEDURE The objective is to determine the optimum content of the appropriate stabilizer for construction. Since it is well known that materials react differently to the addition of various types of lime (calcium, magnesium or dolomitic type), all the feasible options should be investigated in order to minimize costs.

Should there be any doubt with regard to the uniformity of the material from a particular source, then the design shall be carried out on at least three representative samples from the source and the mean of the data obtained from these tests shall be used for the design.

Soil-stabilizer mixtures shall be prepared as described in paragraph 3.1.1 of Method A9 of TMH 1. The mixtures shall be tested in the normal manner as for uncured material. The stabilizer content, expressed as a mass proportion at the specified density, and yielding satisfactory improvement of the material, should be the content specified, provided it is not less than a practical minimum. As a guide for minimum stabilizer content (in situ mixing) the following figures are given:

STABILIZER LIME CEMENT

Minimum content normal control% by mass

1,5 -

Minimum content tight control & pockets % by mass *

2,0 1,75

*Pockets of cement shall be used.

CEMENTED MATERIALS STRENGTH

DESIGN CRITERIA As stated above, the objective of achieving tensile strength largely applies to cemented subbase. Consequently, for reasons of economy, raw materials shall comply with the specifications for subbase (CBR >45 percent), unless otherwise authorized.

Since cementation is required to meet the required strength, tests shall be performed on cured material. The stabilizer will generally be cement, or a mixture of cement and slagment, or lime and slagment. In certain cases cementation may, as described above, be achieved by the addition of lime only. However, since the development of tensile strength with respect to time for lime-soil mixtures remains somewhat indeterminate,




preference shall be given to cement, cement-slagment or lime-slagment blends.

DESIGN PROCEDURE The development of tensile (cohesive) strength is gauged by means of the cured, unsoaked Unconfined Compressive Strength (UCS) as described in TMH 1, Method A14. Moisture-density tests should be carried out at varying stabilizer contents, e.g., 1, 2 and 3 percent. The laboratory design UCS (unsoaked) at 7 days and 100 percent Mod. AASHTO density shall be a minimum of 0,75 MPa and maximum of 3,0 MPa (refer TRH 1419, C3-C4).

Tests shall be performed at 24 hours (cured at 70C) and at 7, 14, 28 days, and if possible, at 90 days (especially for ferruginous materials) to assess the rate of strength gain. This could be useful information when adjudicating bor-derline results during construction. To provide a rapid answer for construction control the 24-hour rapid cure method can be calibrated against the standard 7 day test.

The maximum UCS value is only a guide and may be exceeded where it would result in an impractically low stabilizer content. However, very high strengths could result in reflection cracking. In such a case another stabilizer agent could be selected which would give a lower (while yet acceptable) strength.

The stabilizer content shall be that which yields satisfactory mixture strength. Recom-mended minimum proportions given above shall also apply.

CRACKING CRACKING IN CEMENT-TREATED LAYERS Cracks in cement-treated layers cannot be avoided and must be accepted a feature of cement treatment. However, cracking may cause structural and maintenance problems. A description is given of the mechanism of crack

19 TRH 14: Guidelines for Road Construction Materials,

C3-C4. CSRA, Department of Transport, Pretoria, 1985.

formation and the means of controlling and accommodating cracks so that they do not have an adverse effect on the performance of the pavement.

There are essentially two types of cracks in cement-treated layers; they are: traffic-associated cracks, and cracks that are not caused by traffic and that are usually referred to as initial cracks. Non-traffic-associated cracks, such as cracking caused by expansive clays and unstable em-bankments, are not discussed here.

Initial cracking Initial cracking is caused by drying shrinkage, or thermal effects, or both, and is independent of traffic. In the majority of cases drying shrinkage is probably the main cause of initial cracking and volume changes due to tempera-ture variations may be regarded as a contribu-tory cause. The soil type, the compaction mois-ture, the rate of drying and the cement content are factors that influence the degree and nature of the cracking.

CRACKING IN LIME-TREATED MATERIALS Initial cracking develops in lime-treated pave-ment layers and the cracks form rectangles, like the cracks in cement-treated layers. The shrinkage characteristics and rate of strength development of lime-treated materials are gen-erally different from those of cement-treated materials. Usually the cracks in lime-treated materials are narrower, less extensive and therefore less significant than those in cement-treated materials. However, some lime-treated materials, such as some calcretes and sand-stones, may crack as badly as cement-treated materials.

CONTROL OF SHRINKAGE CRACKING DURING THE DESIGN STAGE Design strength Materials with a high stabilizer content are more susceptible to cracking than materials with a low stabilizer content. The design strength should, therefore, be as low as possi-ble, but still be consistent with the structural and durability requirements of the pavement. It




is often possible to achieve adequate strength with a low stabilizer content.

Position of treated layer in pavement Cracks that reflect through the bituminous surfacing can usually be prevented if the cemented material is confined to the subbase layers and the base consists of untreated material. The initial cracks in the subbase do not usually reflect through an untreated base (G1, G2 or G3) of 150 mm or more. This is probably the most effective way of preventing shrinkage cracks from reflecting through the surfacing. However, instances have occurred where cracking in the subbase has reflected through 200 mm of an untreated base and bituminous surfacing. Where the base of an existing road requires cementation as part of a rehabilitation strategy, an appropriate bitumen-rubber seal or equivalent should be considered in the overall design strategy.

Material properties Some materials shrink more than others. Chert gravel is an example of a material that shrinks considerably when treated with cement. Many lime-stabilized calcretes and sandstones exhibit shrinkage cracking.

CONTROL OF SHRINKAGE CRACKING DURING THE CONSTRUCTION STAGE - DESIGN INPUT IN PROJECT SPECIFICATIONS Stabilizer Content The stabilizer content should not be increased excessively during construction simply to en-sure that strength requirements are met. Spreading, mixing, compaction and curing, should be of a high standard so that a uniform mix is obtained both vertically and horizon-tally. This is so that the specified strength can be achieved with the lowest possible stabilizer content.

Compaction Moisture Content The degree of cracking is proportional to the amount of moisture lost on drying and thus the wetter the material on compaction, the greater the degree of cracking. The compaction moisture content should not exceed OMC.

Construction practice to ensure effective curing should be carefully evaluated. The most practical solution for curing of a cemented subbase is to cover it by spreading the next layer within 48 hours.

Delay Between Mixing And Compaction Cracking in plastic materials stabilized with lime can sometimes be reduced by mixing the lime with moist material and then delaying the compaction until tests show that the plasticity has been reduced to acceptable limits. This de-lay should be incorporated in test procedures for determining density and strength.

Delay Of Surfacing If it is convenient, delay the construction of the bituminous surfacing (where applicable) until some or most of the shrinkage cracking has taken place and the tendency of cracks to re-flect through the surfacing may be reduced. The layer must, however, be cured. This delay is not recommended for lime-stabilized mate-rials owing to the likely increased risk of car-bonation.

Settlement If excessive settlement take

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