Waste Minimisation

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Two reports have been published on this subject, partly because this is the most important issue and partly because this is where we probably had our largest failures.  Hopefully the lessons we have learned may be passed onto others so please read and comment on these reports and if you have personal experiences please tell us so we can pass them onto others

The Comely Green Sustainable Housing Project (formerly known as Lower London Road) has the stated aim of minimising the environmental impacts of the development over its design life.  The design life has been extended to 100 years, which means that the impact of operating the building will greatly outweigh those of building it (by at least 25:1 at current estimates).  The environmental impacts of building can be subdivided into the impacts of the materials and the impacts of the construcion process.The environmental impacts of the materials used have been discussed at length in other sections, such as the Materials Specification and Design Specification.  The environmental impacts of the construction process are covered here.

As environmental consultants for this project, Wren & Bell are collaborating with CIRIA, the Construction Industry Research and Information Association on their Research Project 582: Waste Reduction, Reuse and Recycling in Construction - Demonstration Projects, which is being undertaken by Scott Wilson.

The following is taken from some of the CIRIA/Scott Wilson material developed for the project.

Measurement of Waste on Site and the things which may affect it.

The inputs into a construction project can be spilt into resources and materials.

Materials are the items which will be left behind in the product, or will be utilised in the construction of temporary works.

Resources are things like time, energy, plant, workforce etc.

In order to produce evidence that waste minimisation works on site from a commercial point of view, both of these have to be considered.

In a road building contract, the materials may comprise a large portion of the total cost, in a more complex building operation or tunnel project, the resource costs associated with utilising these materials will be more important.

The first thing to be found out from the site is what records are already being kept and what information is already available.

Note: Any mention of "costs" should be taken to include "benefits" and vice versa.

Information which may already be available.

  1. The amount of material which the contractor’s or designer’s estimators thought would be required. This figure usually forms the basis of the "bill of quantities" in the contract and is the baseline for payments to the contractor.
  2. The actual amount of material which the contractor really thinks will be required.
    This is the level from which reductions have to be made.

    Note: These first two may well be different, and therefore there may be some barriers
    to its acquisition.
  3. The amount of material actually used to carry out the same or a similar operation previously.
    This is data which can be used as a benchmark to measure improvement against. This information can come from another site, contractor or even country.
  4. The amount of material which the site staff actually anticipate using for each operation or element. This data may not be known until shortly before the operation is carried out.
    It is particularly relevant where redesign of elements is carried out in order to reduce the costs. This process is often referred to as "value engineering" and on larger contracts may result in substantial savings in the use of both materials and resources.

 

There are points between the various stages in the supply chain where materials change ownership. (thick vertical lines in the table below.) These are normally marked in reality by the production of paperwork, copies of which find their way into the labyrinthine filing systems of the quantity surveyors and materials controllers of the site.

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The true cost to the environment of the production, supply and use of a material for the two parties is more than the portions of the chain represented by the top two arrows. The whole life cost of the material as represented by the bottom arrow does have an effect on both the supply cost and the market for particular products. The consideration of this whole life cost is one which is not fully developed yet in many industries, not just construction. How we consider this whole life cost by LCA or other means is unclear at present. Undertaking it is a complex task and would be very resource intensive for the CIRIA study.

Whatever factors are used for measurement, these transition / transaction points where a documentary record is created are the best location for gathering data. This can be done from vehicle movement checksheets, invoices, delivery tickets etc.

The best way in which to measure items for comparative purposes is by volume and / or mass or number. This allows comparison across variations in the same material type. i.e. a comparison of engineering bricks against ordinary facing bricks is straightforward if numbers alone are considered, if cost was used as the only measure, this would then produce non comparable data.

The data format which has the greatest effect is of course a monetary one, but only once it is in context. There should be no problems converting volumetric to monetary data when required as the accuracy of the converted data will be sufficient for carrying out any comparative assessments required to demonstrate that waste minimisation works.

Any other data on costs and benefits which is available and which can be linked in to materials can also be considered. This may include labour time, rehandling, additional storage, licensing and regulatory costs etc.

A complicating factor in the consideration of the rate of "use v waste" of materials on site is the affect that variations in quality and durability of different versions of the same item have on their wastage rates. For example one type of paving slab may be more prone to chipping through the use of a cheaper concrete mix in the precast factory. This cheaper variety of paving stone may look the same as a more durable and expensive stone. Addressing this durability issue may well be the key to reducing the wastage of this item, as well as helping with the provision of a finished surface with a longer lifespan.

When considering aggregates by weight or volume it should be remembered that variations in the moisture content of the material have an effect on not just the density of the material, but also the volume which can be compacted into place on site.

Therefore for consideration of the usage rates of aggregates and spoils on site some additional information will be useful.

This may include:

moisture content at the time of loading and delivery
dry density
optimum moisture content

Note that aggregates and fills are sold by weight but used by volume. Therefore it is in the interests of the supplier for a load to have a "reasonable moisture content", this reduces the volume of aggregate in the load and increases the amount of water.

In terms of making a comparative assessment across sites, the monetary value of the materials and waste is less important in terms of determining the environmental cost/benefits and process improvements than volumetric data.

Another complicating factor in the use of monetary data from the demonstrations for comparative purposes is that some materials vary widely in price and value (for reuse) according to geographical location. Although the primary reason for this is the transport cost involved, there is also the effect of market forces to deal with. This price difference may affect any cost benefit analysis of materials reuse, recycling disposal etc. This geographic variation in price is most important for aggregate based materials and the disposal / reuse of bulk materials such as spoil and demolition arisings.

Site auditing and observation will provide the one of the means for checking numerical data, but will also provide the opportunity for discovering what is being disposed of in skips.

It is proposed to audit skips as thoroughly as possible subject to safety constraints. It may be possible to set up some form of tally board or set of counters alongside skips to allow the workforce to assist in measuring how much is placed in the skips.

When analysing the flow of materials in and out of a site or operation there is a possible need to breakdowns the figures which may be reported in invoices etc.

Material coming onto site, and that which has been produced internally should be measured if possible to consider how much is derived from reused/ recycled sources, and how much is from virgin material sources.

If material is going off site it may useful to try and split the volume into materials which are being "exported" or sold on for reuse elsewhere and the volume which is going to disposal.

It is also important to try and differentiate between:

materials which can be reused easily or economically
materials which can be reused only with resource input ( may be uneconomic)
materials which are truly non recoverable and have to be sent for disposal.

As mentioned above the transport costs for materials, particularly bulky ones is an important component of not just the economic cost, but also the overall environmental cost of the material. Determining the sources of some of these high volume materials may also be useful in trying to include this cost into the overall cost : benefit analysis of the waste minimisation process.

Tracking in general detail the amount of time both in man-hours (i.e. effect of project on wages ) and variations to programme duration caused by the recycling project ( project cost effects) is also necessary to assist in proving the economic case.

 Examples of the elements which might be measured in different operations/

Simple concrete pour.

This is one of the closest operations on site to "just in time" ordering systems.

Engineers cube ups
Design volumes (with / without steel)
What volume is supposed to be on each load.

(concrete deliveries are sometimes only 5.5 m rather than 6m loads to avoid spillage from the rear of the wagon.)

What is the time delay between batching and placement.

(only 2 hours delay from one to the other is permitted for structural concrete.)

Weighbridge (plus wet density) to accurately find out the delivered volume.
Is there any over order
Is over-ordered concrete being efficiently used.
Is the concrete the correct mix for the job in hand, or is it just a "standard" 40/20 mix when 30/20 would do.

(There is a tendency to use a limited number of mix designs to cover all eventualities. Also there is often a desire for mixes to get to a minimum strength to allow formwork stripping as quickly as possible.)

How much excess cement is the Readymix supplier putting in to provide his error of margin on the 28 day strengths of the mix.

(Readymix suppliers sometimes give their mixes a cementitious content which is on the high side to ensure that their product meets the minimum strength requirements in the specification. This is more than is required to meet the real strength requirements if they are already utilising good QA procedures.)

Is there a need for an early strip for the falsework / formwork or can it be left.
Is the formwork correctly prepared with release agent to allow effective cleaning and easy reuse.

Laying concrete paving (slabs)

This is an operation where there are likely to be storage and handling problems on site.

How many did the designer / clients / architects think was required.
How many are marked on plan layout drawings.
What spacing are they being laid at.
Slabs ordered.
Slabs delivered.
Number used.
Number returned undamaged for reuse
Number damaged in storage.
Number damaged on site.
How many slabs could have been used, but were not due to cosmetic faults.

Cabling

This is a real can of worms. The estimated lengths of cable required may vary substantially from the actual amount used. Therefore measurement of this type of operation may be tricky. (plumbing and ductwork may also be difficult to reconcile against tender figures.)

What is the ordering route on site, do individual gangs of electricians order their cable, or does it get ordered through a central store.
Is new cable held in a central store or distributed to a number of locations immediately upon delivery.
Is the supply chain management and ordering system linked to the labour and work schedule management. If the two are separate, do they talk to each other?

Bricklaying and blockwork.

How many did the designer / clients / architects think was required.
Are they being laid at the correct spacing.?
Number ordered.
Number delivered.
Number used.
Number returned undamaged for reuse
Number damaged in storage.
Number damaged on site.
How many bricks could have been used, but were not due to cosmetic faults.

Some of these numerical items would have to be approximated and derived from sampling undertaken on site.

Despite the variety of figures to measure given above, the best way to determine how effective waste minimisation is to block and brickwork is probably by auditing good practice on site.

Formwork

(Falsework is the scaffold which holds formwork up.)

For some construction elements such as columns and wall facings it is already common to find reusable steel forms being used. Similarly flat panels mounted on "strongbacks" or "soldiers" are often reused many times. These are often made from expensive high quality resin injected ply which is resistant to mechanical damage.

Problems in the effective use of materials and the avoidance of unnecessary wastage increase inversely with the size of the formwork. Smaller elements such as box outs and forms for kickers are nor reused as often as they might.

One of the problems with formwork which is forming an acute angle, i.e. is "in" the concrete rather than around it, is that it has to be broken down to get it out. Often this has to be done in a confined space and may lead to breakage and subsequent disposal of the material.

Pressured by time and labour cost, it is often not economic to de-nail such small pieces if they are no longer to be used. This restricts their reuse in manufacture of other wood derived products and can hamper chipping prior to burning in waste to fuel processes.

Measuring the use of wood products in formwork would be difficult in comparison to the purchased volume. Hope fully none of the formwork is going to stay in the works, and so it should all leave the site, the question is how much utility has been derived from it in the meantime. This is probably best measured by observation and audit.

 

A selection of some of the individual items which might be measured on the demonstration projects.

NB: Cost of disposal is not included in the "value" of items.

 

Item High Volume / numbers High Unit value High Volume + Value
240v electric cable

X

3 phase cable

X

admixtures

X

aggregate

X

banding
blocks

X

cable tray

X

X

X

cement

X

cladding

X

cutter discs

X

X

X

diesel

X

dry lining

X

ducting

X

engineering bricks

X

X

X

facing bricks

X

fibre optic

X

X

X

filler

X

fixtures and fittings

X

X

X

floor coverings

X

X

X

formwork

X

glass

X

glazing units

X

hilti nails

X

insulation - noise

X

insulation - thermal
joists

X

light bulbs

X

mains electricity

X

manholes

X

mastic

X

mortar

X

mouldings

X

X

X

nails

X

paint – exterior

X

paint – interior

X

pallets

X

paper

X

paving stones

X

pavoirs

X

petrol

X

pipe – foul

X

pipe – gas

X

pipe – steel

X

X

X

pipe – water

X

plants (green)

X

plaster

X

 

Item High Volume / numbers High Unit value High Volume + Value
plastic sheet

X

plasticisers

X

plywood

X

PPE

X

X

X

propane/butane
rope

X

sand

X

scaffold – aluminium

X

scaffold – kwik stage

X

X

X

scaffold – steel

X

X

X

screws

X

sealer

X

switches

X

telecom cabling

X

X

X

time !

X

X

X

topsoil

X

X

X

turf

X

water

X

waterproofing
white goods

X

wire

 

Some of the trades which might be measured on a demonstration projects.

(This is a selection)

Admin
Bricklaying
Carpentry
Concrete placement
Electricians
Flooring
Glaziers
Groundworks
Landscaping
Materials delivery
Plumbing
Waste removal
Welding
Welfare provision.

Waste minimisation audits are undertaken at the Comely Green site on a frequent (but unannounced) basis.  As the results of these audits are appraised, the results will be posted here.