NGA2 Key to Materials

NGA2 Key to Materials

NGA2.0 Introduction

For guidance on the terminology for asphalt mixtures refer to PD6691

NGA2.1 Hot Rolled Asphalt (HRA) Mixtures

1) HRA design mixtures give better resistance to deformation where queuing of heavy traffic is likely to occur and may also be more economical to lay.

2) Type C mixtures use fine aggregate of a coarser grading than Type F mixtures - usually associated with the use of crushed rock fines. Such mixtures tend to be stiffer and are less well suited to the reinstatement of small excavations.

NGA2.2 Stone Mastic Asphalt (SMA) Mixtures

NGA2.2.1 General

1) SMA is a high stone content, gap graded material which has the voids between aggregate particles essentially filled with a bitumen-rich mortar.

2) SMAs are very rich in bitumen to the extent that measures have to be taken to prevent bitumen from 'draining' from the mix during transport. Most commonly bitumen drainage is prevented by addition of either polymer modifiers or cellulose fibre.

3) True SMA is intended to be virtually impermeable and has very good resistance to deformation by virtue of its 'stone to stone' skeleton. The level of texture depth achieved is largely a function of the material design, texture depth achieved is therefore likely to be more consistent than with HRA, where the application rate and the embedment of the surface applied coated chippings is critical.

NGA2.2.2 Specification

1) There is a European Standard for SMAs, BS EN 13108-5 and, in the UK, Published Document PD6691 "Guidance on the use of BS EN 13108 Bituminous mixtures - Material specifications".

2) There are also a number of proprietary mixes that fit the generic description of an SMA.

3) HAPAS approved thin surface course systems may also comply with the HAUC requirements but there is no guarantee. Some HAPAS materials can have void contents in excess of that permitted in this specification. Purchasers should make it clear to suppliers that the work has to comply with the air void specification.

NGA2.2.3 Transport

1) As with all asphalt materials it is important that temperature loss is minimised during transport, handling and storage to allow effective placement and compaction. The high bitumen content of mixtures means that provided material temperatures remain elevated then compaction is relatively easy.

2) Transporting should comply with the requirements of this specification and BS 594987.

3) To minimise the risk of segregation and temperature loss it is preferable to use the material direct from the delivery vehicle or hot box. Material should never be tipped on to adjacent surfaces for use sometime later.

NGA2.2.4 Preparation

1) Preparation should comply with this specification and BS 594987. A polymer-modified bond coat is preferable when using SMA.

NGA2.2.5 Laying

1) Placement of the SMA should be done in such a way as to avoid segregation of the mix. Where possible this is to be accomplished by careful use of a shovel. The use of a rake is to be avoided. Care must be taken with the use of some "hot boxes" as these can also cause segregation of mixes through the discharge augers.

2) Areas showing segregation should be removed as these are not acceptable at any time.

3) Where initial skid resistance of >40 SRV is required (all carriageway applications) or in areas where equestrian usage is likely the material should be gritted using a clean, dry, crushed quartzite or similar very hard angular aggregate to a grading similar to that shown in Table NGA2.1 or 3mm steel slag shall be applied evenly to the surface during the initial rolling, i.e., whilst the material is still warm. The rate of application shall be to provide about 80% surface coverage (approximately 1000 grammes per square metre). Where the authority uses a lightly coated grit for the treatment of SMA this may also be suitable.

4) After the final rolling all surplus aggregate should be removed before the reinstatement is opened to traffic.

5) It is important to take care that the application of grit should not reduce compliance with any texture depth requirement.

Table NGA2.1 - Recommended Aggregate Grading for Gritting SMA Surfaces
BS test sieve: % passing
6.3 mm 100
5.0 mm 95-100
3.35 mm 66-90
1.18 mm 0-20
600 μm 0-8
75 μm 0-1.5

NGA2.2.6 Compaction

1) Compaction of the material is best carried out using a smooth wheeled roller. Vibration may be applied provided that this does not bring about excessive movement of the bitumen to the surface of the layer, i.e. "bleeding". On narrow reinstatements compaction equipment in accordance with Table A8.3 may be used but care must be taken to ensure that there is no excessive loss of surface texture or bleeding.

NGA2.3 There are no Notes for Guidance and NGA2.4

NGA2.5 Structural Layer Thickness Tolerances

Excessive layer thicknesses of HRA surface course can lead to localised rutting likely to exceed the surface profile performance requirements set out in Section S2.2.

NGA2.6 - NGA2.10 There are no Notes for Guidance