Asphalt work

Introduction 

It is perhaps unfair to refer to any particular trade as special, for every operative in the industry demonstrates skills which are unique to the craft he follows. However, asphalt work is generally referred to as a specialist trade because it encompasses some unique features and is practiced by a relatively small number of craftsmen employed by a relatively small number of companies. To further emphasize this specialism, contracts for this type of work are normally let as a subcontract and most commonly to a nominated subcontractor. 

The most unique aspect of the trade is the material used. Unlike every other trade, the material, mastic asphalt, is handled by the operatives while it is hot, generally between 200 and 220 °C. Pitch mastic is laid at the lower temperature of 160 °C. 

This has two consequences. The first is that the work is dangerous; protective clothing is essential and safety procedures must be adhered to. 

The second is that a relatively rapid pace is needed in the application of the material, as it sets by cooling to ambient temperature. As soon as the material is drawn from the cauldron or mixer it starts to cool. When it is applied to a surface the rate of cooling accelerates for two reasons. First, the base surface is at ambient temperature and a base of masonry presents an enormous heat sink. Second, as the material is spread out,' more of it comes into contact with this cold base and with the cool air over it. 

Perhaps as a consequence of the specialist nature of the work there seems to be no well-defined procedure used by estimators for building up rates for measured items of work. Therefore the data and examples which follow are given only as a guide to one possible approach to achieve comprehensive financial cover for all expenditure. 

Before commencing examples it is proper to consider the logistics and practice in the trade. 

Underpinning

Introduction 

Underpinning is the transference of the load of a building to a lower stratum of the ground. This is done for several reasons: 

• To permit the construction of a new basement within or adjacent to the existing foundations which are above the level of the new basement.
• To provide a new foundation at a lower level than the existing, where the existing has failed due to, for example, shrinking clay or subsidence.
• To increase the load bearing capacity of the existing foundation.
• In extreme cases, to allow the whole building to be mechanically moved by installing a ground beam.

There are a number of ways to carry out the underpinning operation; the choice depends upon the stability of the existing structure. Except in the most simple cases and where the structure is in good condition, the work will be carried out by a specialist contractor. This contractor will employ stabilizing techniques such as the drilling of ground anchors and/or the construction of ground beams.

For a straightforward operation on a stable structure the contractor will price the work described in the bills of quantities. The constants used will be based upon those for the ground work, in situ concrete and masonry sections. 

Formwork of concrete

Formwork 
The materials employed to construct formwork for in situ concrete are generally softwood boards, plywood and sheet steel for working faces, supported on a softwood or steel framework, often in conjunction with proprietary props. Timber is still widely used in the making of formwork because of the variety of forms that it will allow. Plywood has generally superseded timber boards for forming the faces of members.

Sheets or boards must be of sufficient thickness to lake the weight of wet concrete. For plywood the thickness ranges from 25 to 50mm depending on the structures to be formed and the spans involved.

Timber forms for in situ work are usually unfit for further use after four to six uses. In the production of precast units in a factory, timber formwork will have up to twenty uses.

In accordance with the Working Rules, a carpenter required to reuse materials for concrete work is entitled to an extra payment.

Concrete work

In situ concrete 

Concrete is a mixture of cement, fine aggregate, coarse aggregate and water. The strength and durability of concrete are dependent on the proportions of materials in a particular mix. The methods for specifying prescribed and designed mixes of concrete, either site mixed or ready mixed, are covered by BS 5328. 

Designed mixes

Designed mixes are those for which the purchaser is responsible for specifying the performance, and the producer is responsible for selecting mix proportions that will conform with the particular performance requirements. 

Prescribed mixes 

Prescribed mixes are those for which the purchaser specifies the mix proportions that will produce a concrete with the required performance. There are two types of prescribed mix: 

Ordinary prescribed mixes 

These are specified in accordance with the requirements of BS 5328. The grade of concrete is prescribed in Table 1 of the standard, together with permitted types of cement and aggregate, aggregate size, etc. 

Special prescribed mixes

These generally meet the same requirements as ordinary prescribed mixes, but mix proportions in kilograms are given for each constituent material. 

Filling to excavations

The volume of fill given in the bill should be the volume of the void to be filled. Thus. whether the material used is to come from the excavation in the first instance or to be imported to the site. the estimator must be aware that the volume of material required is much greater, for it will require compaction after placing. For example, hard re fill decreases by 25 per cent of its original bulk after placing and compaction.

SMM7 gives two classifications of filling to excavations and filling to make up levels. Filling to excavations is generally more labour intensive or more difficult by machine since it involves back filling around the actual building works. Filling to make up levels is assumed to mean bulk full over more extensive areas either inside the building or in the open. where it would be possible to load in with a machine or by directly tipping a truck. The latter is obviously the cheapest solution. although not strictly speaking the best construction method since any fill should he laid down in layers and compacted before the next layer is superimposed.

SMM7 also gives two thickness classifications. up to 250 mm and more than 250 mm, thus recognizing the additional labour in spreading thinner layers over wide areas.

Disposal of excavated material

The primary factors in disposal of excavated material arc as follows:

• What type and condition is the material?
• Is it suitable for back filling?
• Is it suitable for general filling?
• Has it to be disposed of in a particular way, in spoil heaps for later use, or to a coup? 
• Does the contractor have a choice?

Other factors arc the distance to spoil heaps or the proximity of a tip external to the site, the value of any surplus material, and whether or not the client wishes to retain a proprietary interest in it up to the point of disposal. For example, hand excavation followed by disposal into spoil heaps on site carried out using wheelbarrows might be reasonable if the wheeling distance was not too far. the ground was suitable, or barrow runs could be economically provided. A maximum distance might be in. the region of 150 m. Beyond this distance mechanical barrows or small dump trucks might be more suitable, bearing in mind that mechanical devices run on four wheels and need wider roadways, and that they bog down in poor soil conditions.

Wheeled or tracked plant engaged in bulk excavations such as reduced level or basement work could 'wheel' each bucket load to a disposal point. but this is only economic if the bucket is of reasonable capacity, the wheeling distance is less than 50 m and the machine can get out of the excavation to deposit the load, For distances beyond this it is considered more economical to utilize dump trucks or light tramway and hutches for on-site disposal and tipping lorries for off-site disposal,

When considering disposal_ the estimator must be aware that excavated material is measured in the bill as the volume of the void to be created. Le. the volume of compacted spoil removed. As soon as it is removed it will increase in volume, This phenomenon is called bulking and is ignored by the quantity surveyor when measuring disposal. However_ the estimator must allow in his rates for the fact that whenever he disposes of one meter cube of spoil as measured. the mm on site will he handling one and a bit meters cube of spoil.

Earthwork support

Earthwork support is measurable to all faces of excavation except the following.

• Faces not exceeding 250 mm high.
• Sloping faces where the angle of inclination is less than 45° to the horizontal.
• Faces which abut existing structures. 

In SMM7 measurement is classed in depth stages and in distances be opposing faces. It appears that the intention is to give gross areas of support where the total depth of excavation falls within any depth stage.

Up to 4 m between opposing faces it its possible to span across with struts of various kinds, e.g. timbers wedged in or proprietary telescopic struts of steel. Over 4 m it is usual to set up raking supports to the lower level of the excavation. although if this is below water level it may be better to use steel work for the shoring, and this will add considerably to the cost.

Other special conditions must be given in the bill. i.e. where the face is curved. below water level (there does not appear to be any provision for support partly below water level). next to roadways. next to existing buildings or in unstable ground. and finally where the support is left in place. The estimator is left to decide how to actually achieve the support and he will take into account the following additional considerations: 

• The most suitable material must be chosen. for example: solid timbers for waling, poling hoards or strutting; steel sheet. plywood, etc. for support; proprietor) struts spacing of all members considering the soil type:, proprietary close sheeting system.
• The number of uses must be estimated, i.e. the number of times each component can be reused before being discarded. Figures are quoted from 5 to 15 uses.
• Men working in the excavations require protective clothing.
• Outputs for excavation round timbering are generally halved.
• The proportion of various classes of excavation below the water table must be considered.
• Pumping. although given separately in the bill. must be realistically assessed. 

Working space at between footing and excavation edge.

The measurement for working space in SMM7 is now given in square meters, provided that the face of the work requiring space is less than 600 mm from the face of the excavation.. There is no minimum depth below which working space is not measurable; if it is required it must be measured, At first sight this change in SM M7 might appear to be quite radical, but to the estimator it only rationalizes what he has always had to do: assess for himself exactly how much working space would be required. whether or not any was given in. the bill!.

All previous methods of measurement have treated working space as a theoretical allowance., where the distance from the surface worked upon to the face of the excavation was set in accordance with the depth of the particular class of excavation, Invariably all other factors, such as the nature of the soil. were ignored. Some methods of measurement did try to relate some of the difficulty factors to the measurement, but the allowance was still theoretical.

The problem then that the estimator faces is twofold. It is necessary to decide first how much space the workmen will need, and secondly how many external and internal corners there are on the plan! The first problem reviles answers to the following:

Breaking Out / remove excavated material from site

This is the term applied to the removal of material from. the excavations which requires the use of equipment other than pick and shovel. These materials are defined in SMM7 as:

1. Rock
2. Concrete
3. Reinforced concrete
4. Brickwork, block work or stonework
5. Coated macadam or asphalt

It is important to note that it is relatively easier to break out any of these materials if they have been laid down in a thin layer say up to 300 mm thick. It is possible to come across rock in such layers, although there are usually several such layers interleaved with softer material; nevertheless, these are easier to remove. Hand tools such as pick and shovel are of little use against concrete, high quality brickwork or bituminous macadam. A compressor and pneumatic drills are required to break up these materials, whether in hulk or in layers. The hire cost of a compressor and drills or even of electrically powered hammer drills is so low that wedges and hammers are falling out of use.

Once broken up, the material may be loaded for disposal with either a hand shovel or a machine. Outputs and multipliers for excavation in earlier tables reflect the use of appropriate mechanical means of breaking out difficult materials. Generally this means using a compressor with two drills equipped with suitable points. It should be noted that the outputs for hand or machine loading for disposal, while being very different from each other, will not vary much from material to material. In addition, these materials will all bulk up by 33 to 50 per cent. Breaking out with explosives is such a rare occurrence in building works, and so specialized, that it has not been included.

Machine excavation

The ubiquitous tractor with backhoe is not the only machine available. Giant earth moving and excavating Ling equipment is capable of moving mountains. Miniature machines can be driven through a I m gap to give machine capabilities in back. gardens or even inside buildings themselves.

Here we must repeat the earlier caveat regarding the suitability of a particular machine for the type of excavation being tackled. Texts on contractors' plant treat the subject of choice of appropriate equipment in some depth.

With regard to soil conditions, machine excavation is not subject to the same variation in output as hand work, providing the correct type of plant can be procured. The output multipliers in Table give some guide as to variations in performance in different soil conditions.

Hand excavation

Not all excavation is possible using machinery. Nor is there one type of machine which is suitable for all classes of machine excavation.

For example, excavation of foundation trenches using a backhoe on a wheeled tractor unit will not give accurate alignment of the trench, and the teeth on the bucket will disturb the bearing surface of the trench. Further, it is not possible to dig to a precise level and depth with sufficient accuracy. The backhoe will therefore take out the bulk of the excavated material - say 90 per cent and the trench will be trimmed to level for the foundation by hand. Hand trimming to width may also be required where there is extensive timbering.

Hand excavation may also be necessary for small isolated pits for pads and manholes, excavations for foundations to entrance steps, and so on, either because it is uneconomical to bring a machine to the work or because access is restricted. In the latter instance one could also consider work inside an existing building or in a back garden or enclosed yard or area.

On the subject of trimming excavations by hand, it should be noted that no matter how the excavation is done, the smaller and more confined types of excavation have a higher proportion of trimming to volume of excavated material. Generally in the past this has been taken into account by using lower outputs per hour when calculating the overall rate, which is based purely on volume. However, SMM7 and previous editions have included provision for the preparation of excavations to receive concrete foundations.

Excavation: general remarks - Construction Technology

The rules of measurement for SMM7 Section D2 have been drafted with machine excavation in mind as the principal method. Any excavation which the estimator judges can be carried out only or better by hand must be priced within the constraints of these rules. The implications of this for both the estimator and the person preparing the bill of quantities are discussed later in the chapter.

Excavation is measured in eight classes or types of excavation:

1. Top soil for preservation;
2. To reduce levels;
3. Basements and the like;
4. Pits (give total number);
5. Trenches up to 300 mm wide;
6. Trenches over 300 mm wide;
7. For pile caps and ground beams between caps;
8. Benching sloping ground to receive filling.

For the first class, state the average depth. For the remaining classes, give the maximum depth in stages: up to 250 mm deep; up to 1 m deep; up to 2 m deep; and so on in stages of 2 m. In addition, a starting level for the excavation should be provided where the particular class of excavation commences beyond a depth of 250 mm below existing ground level.

The sub classification into depth stages with a given starting level is a blanket requirement of SMM7.

Excavation and filling - Construction Technology

Site preparation


This section is concerned with the removal of trees, tree stumps, bushes, scrub, undergrowth and hedges. A tree or tree stump is defined in SMM7 as having a trunk of at least 600 mm girth at 1 m above ground level or at the top of the stump. The assumption can only be made that anything smaller is a bush.


Guidelines for estimating for the removal of trees and stumps might include the following:

• Trees: Time for squad to fell; hire of chain saws; fuel and oil; sharpening; axes, wedges, pince bars and mauls; protective clothing; time to cut up and burn the slash; disposal of the main limbs and trunks to a tip or to a firewood merchant.

• Stumps: Time to excavate around larger stumps by hand, or by machine if numbers or size warrant; men excavating or hire of machine and operative; disposal of stumps to tip (firewood merchants seldom want to know about stumps).