Site-batched or ready-mixed?

Site-batched or ready-mixed?

Owen, Glyn

During 2000, Miller Civil Engineering placed more than 25,000m^sup 3^ of concrete in locations stretching from Aberdeen to Torquay. Ready-mixed companies supplied most of the concrete for these, but at three locations concrete was hatched on site – Roundhills Sewage Treatment Works, Channel Tunnel Rail Link contracts 350 and 410 and the example which is covered later in this article, Torbay Sewage Treatment Works.

In the past, Miller used site batching mainly for remote contracts or large-scale projects where significant quantities of concrete had to be produced, such as the Glendevon (24,000m^sup 3^) and Megget (36,000m^sup 3^) dams. On the Skye Bridge, 15,000m^sup 3^ of site-hatched concrete were used.

The main factors to be considered when comparing and evaluating site hatching against ready-mixed supply are:

* Location of contract and accessibility

* Classes of concrete and purpose of the structures, e.g. water-retaining

* Technical requirements

* Quality issues

* Total quantity to be produced

* Type and size of each structural element and likely pour sizes

* Availability of local ready-mixed concrete and back-up supply

* Availability of suitable aggregate sources

* Programme

* Type of contract – design and build, traditional client design, or partnering.

At tender stage, a decision between the two methods must be made after careful evaluation of the commercial and technical options, along with the most efficient and practical work programme.

If the ready-mixed option is considered, the current prices per cubic metre are investigated. Depending on the contract timescale, various increases are added to accommodate inflation, annual cost price rises (whether justified or not) and diesel price fluctuations. If concrete is required outside the supplier’s normal working hours, additional costs must be included, together with part load and waiting time charges. This final element can account for 5-10% of the total order. At this stage, the concrete costs can be finalised, with the assurance of the QSRMC scheme for quality (or equivalent such as BSI) and a concrete supply that meets all relevant standards.

For site batching, the tender preparation is more onerous and, as with an enormous crossword puzzle, takes considerable thought to complete. Compared with the ready– mixed option, a similar amount of material still needs to be conveyed by road. However, the number of road journeys can be substantially reduced if suitable material conforming to the relevant standards can be excavated locally or derived from the recycling of concrete. Aggregate lorries may deliver during off-peak hours, avoiding traffic and delays. Concrete trucks may also be able to use site roads, avoiding traffic and possibly deriving tax benefits, unlike readymix trucks. All the relevant operations must be costed before a final price per cubic metre can be calculated. This calculation is undertaken by Miller on a comprehensive spreadsheet containing the major plant and material elements.

Example

The Torbay Sewage Treatment Scheme is a strategic partnering project with South West Water as the client, Hyder Consulting as design managers, Degremont UK as process managers and Miller Water as civils managers. The environmental and political sensitivities of the scheme necessitated an in-depth analysis of the effects of site-batching on the area, with contributions by all the project partners.

In addition to the normal considerations made when evaluating the feasibility of a site batching plant, other factors addressed were:

* Programme options

* Operational opportunities

* Perceptions of the local residents

* Full risk analysis.

A significant factor in assessing the impact of site hatching at Torbay was road congestion during the peak holiday period between June and September, when the local population can increase by more than half. Ready-mixed concrete delivery cycles were calculated at a turnaround of one and a half hours, increasing substantially during the summer and introducing a significant element of operational uncertainty, with possible effects on quality. Delivering an estimated 15,000m^sup 3^ of ready-mixed concrete by road to the works would have been problematic, as the programme included pours up to 360m^sup 3^ per day, requiring a constant supply to complete scheduled pours in the working day.

After discussions with local company E. & J.W. Glendinning, a scheme was developed for supplying concrete to the project. The intended purchase of a new Steelfields batching plant was brought forward 12 months by Glendinning for temporary installation at the Torbay site. Concrete aggregates were supplied from local quarries and concrete plant backup and technical facilities from an existing Glendinning ready– mixed plant at Ashburton, around 45 minutes from the site. This arrangement brought further efficiencies though opportunistic programme management, such as amalgamating the testing regimes between Glendinning and the project.

The main advantages of an on-site batching plant were found to be:

* Control of quality and specification requirements

* Tight controls on concrete ordering and production, almost eliminating wastage

* Instant availability, with concrete produced at short notice

* During the winter, the on-site flexibility permitted installation of a steam boiler to keep concrete temperatures around 20 deg C, increasing pour rates by 50%

* Time saving through opportunistic programme management

* Elimination of charges for part loads, and for opening the batcher outside normal hours

* Concrete testing practices of supplier and purchaser were combined and site supervision was undertaken by representatives from both companies

* Flexibility to change mix design.

Inevitably, the on-site method has some disadvantages:

* The effects of plant breakdown: a backup supply may be hours away and can limit production

* The site-batching company is liable for quality control.

On-site batching as a partnership between Miller and Glendinning proved to be the best method in Torbay but, for the Greater Glasgow Sludge Solution Scheme, there were benefits to using ready-mixed concrete. To comply with the Urban Waste Water Treatment Directive, which prevents dumping of sludge at sea, extensive treatment works are under construction at Daldowie and Shieldhall. The quantity of concrete required is in the region of 13,000m3 over a year. The supply contract was awarded to RMC Scotland as they have six concrete plants using the same raw materials within a ten-mile radius. There was no advantage in considering an on-site set-up as supply and technical backup were guaranteed.

The specification required partial replacement of the Portland cement content by either ggbs or pfa, which proved problematic in controlling 28-day compliance strength. Since it is a designand-build contract, this was solved by the facility to test at 56 days, with concrete conforming to the designer’s specification.

Some suppliers can produce pfa-blended cements at various replacement levels in easily handled containers – one-tonne bags, for example – and this is a significant factor in small-scale or site batching operations.

The concrete industry faces some uncertain times in respect of recent and imminent takeovers, impending aggregate taxation and unpredictable fluctuations in fuel costs. On the positive side are the considerable advances in technology and design of batching plants, providing accurate and consistent weighing of constituent materials. At the same time, mobile batching units, built around articulated trailers, are becoming increasingly popular. These can be running within 24 hours and supply up to 24m3/hour.

In short, ready-mixed concrete supply versus on-site hatching is a question of `horses for courses’, the choice being determined by the circumstances, technical aspects and cost benefits.

Glyn Owen, James Mawson and Alan Johnson, Miller Water

Copyright The Concrete Society Feb 2001

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