Concrete wash-out

The washing out of ready-mix concrete lorries on construction sites after delivery of each load is a common occurrence, normally being carried out in a designated “wash-out” area, or, on more crowded sites, into a purpose-built wash-out unit. This unit separates the solid materials from the washout water, and treats the separated water to reduce its alkalinity before discharge to a foul sewer under a trade effluent discharge consent. Whilst many sites accept this as the norm, it is not the only solution, nor in many cases, the most environmentally-friendly one.

The Environment Agency offers guidance how washwaters should be managed on site in their Regulatory Position Statement (“Managing concrete wash waters on construction sites: good practice and temporary discharges to ground or to surface waters“). In Appendix 1 (good practice guidance) the Agency clearly advise that “As far as possible concrete mixing or delivery lorries should return for washout to the batching plant with only chutes being washed out on site.” This is repeated in the EA’s “PPG6: Working at construction and demolition sites” which again states that ready-mix lorries should return to the batching plant for washing out. (Section 7, p.41: Essential pollution prevention).

Clearly, there are benefits to this approach for the contractor, who doesn’t have to allocate space and manage a washout point on site or the waste arising from its use, nor is there standing time for vehicles using (or waiting to use) the wash-out point on site. And, as the majority of mix design codes around the world permit a percentage (typically 5%) of suitable recovered materials to be used in subsequent concrete mixes, returning the 1% – 4% of concrete that remains in the drum after discharge to the plant appears to make economic sense for the batching plant too.

Steelfields washout reclaimerA recent press release by Hanson UK (11 September 2012) refers to two new concrete production facilities in Glasgow noting: “In addition, a water reclaimer allows returned materials and wash-out from trucks to be separated. The solids – mainly sand and aggregate – go back into stock for reuse and the water is filtered and pumped into the supply tanks.” Clearly, batching plants are increasingly prepared for this approach, and are making use of the returned materials, with batching and mixing plant manufacturers such as Steelfields offering wash-out reclaimers as standard equipment.

However, to make batching plant wash-out a workable solution, two conditions have to be met:

  1. The site has to be close enough to the batching plant to ensure that the lorry can return and wash-out (or reload with an identical mix) before the residual concrete starts to set. The rule of thumb to meet this condition is normally maximum 20 minutes return travel time between the plant and site. (But see also 7 March 2014 addendum at bottom!)

  2. The lorry has to be able to return to the batching plant without losing any of the concrete remaining in the drum or chute onto the public highway – hence the EA recommending that the chute is washed out on site.

The first of these is clearly dependent upon the relative location of the plant and site, and the traffic conditions between the two whilst deliveries are taking place. If the travel distance between the two is too long, there is nothing to be done – wagons must be washed out on site or the residue will begin to harden inside the drum.

Until recently, meeting the second condition has been more problematic, with returning lorries losing small but troublesome quantities of concrete onto the road, not only creating uneven road surfaces once set, but also the risk of cracked windscreens and damaged paintwork for other road users whilst still fresh – and consequent insurance claims for the ready-mix suppliers. To minimise this risk, the EA recommend washing out the chutes only before returning to the batching plant, but even this requires a wash-out point on site.

Concretesock montageToday, however, not even this is necessary, thanks to the development of  Concretesocks – simple inexpensive closures that fit over the end of the delivery chute before returning to the batching plant – sealing the end of the chute and completely eliminating the risk of loss of material on the roads. And, on fast turn-around sites, wash-out becomes unnecessary except at breaks as the vehicle can return and refill before the previous mix has begun to set – reducing waste and making more productive use of the delivery vehicle – and in doing so, reducing costs.

So, given that batching plants are increasingly able to reuse materials from washing out delivery mixers, and loss of materials on the roads no longer need be a concern, why are contractors still using (and paying for) concrete wash-out points, or even expensive washout plant, on construction sites close to batching plants?

7 March 2014: At the invitation of Karl Goff, the inventor of the Concretesock, I subsequently had an entertaining and enlightening chat with “Brian the Driver” who has been using this product every day for over 2 years now, and who happily “rolls for an hour” without washing out when returning the the batching plant, “feels naked” if he doesn’t have a sock on the chute when travelling on the roads – and typically gets an extra load a day in as he just reloads with the same mix without washing out at all … just once at the end of the day!

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Carbonated aggregates

Carbonated aggregates have recently featured in the news as a result of the negative carbon footprint of Lignacite’s “Carbon Buster” concrete block – the first concrete block to achieve this. Needless to say, the block uses “manufactured” aggregates – artificial aggregates who’s manufacture converts carbon dioxide gas into solid carbonate products, giving the material its carbon negative credential – sufficient to more than offset the carbon emissions of the cement used in block’s manufacture, with a net capture of 14kg of CO2 per tonne of manufactured block.

(Carbonation is a naturally occurring chemical process where atmospheric carbon dioxide slowly reacts with the hydrated Portland cement matrix of concrete, altering its chemical composition. In doing so, it reduces the alkalinity of the concrete from a pH of 12.5 to 8 or less and exposes any underlying steel reinforcement to the risk of corrosion.)

Patented by Carbon8 Aggregates, Accelerated Carbonation Technology speeds up this process and is capable to turning waste into a material the Environment Agency have confirmed is a “product” (i.e. the carbonation process is an agreed waste recovery process) suitable for virgin aggregate replacement saying “Concrete blocks made from Carbon8 Aggregate … show no worse detriment to the environment or human health than blocks made with virgin aggregate”. The process mixes APCr (Air Pollution Control residue) with carbon dioxide, sand, cement and water to make C8Agg®, an inert carbon-negative aggregate suitable for many applications.

Carbon emissions of cement and concrete

Whilst the production of Portland cement is very energy-intensive and gives off virtually as much CO2e as the weight of product it produces (approximately 910 kg CO2e per tonne of cement), it is only one component of concretes and mortars. In addition, OPC is normally blended with cement replacement materials such as ground granulated blastfurnace slag (ggbs – 67 kg CO2e per tonne) and fly ash (pfa – 4 kg CO2e per tonne). As a result, the weighted average for all cements and blends sold in the UK is approximately 850 kg CO2e per tonne. (Cradle to factory gate)

Reference to the Sustainable Concrete website indicates that in 2012, the average carbon intensity of UK concrete was 79.4 kg of CO2 per tonne of concrete produced, a 23% reduction from the 1990 baseline, with a target reduction to 72.2 kg CO2 per tonne by 2020. Reinforcing bar typically adds a further 10 kg of CO2 per tonne of concrete used.

Carbonation & secondary concrete aggregates

Production of the cement used in concrete is recognised as one of the largest single contributors to greenhouse gas emissions, making up 5% of global CO2 emissions. However, once cast, concrete begins to carbonate, reabsorbing some of the carbon dioxide given off in its manufacture, the rate of carbonation being dependent primarily on the porosity of the concrete matrix, and to a lesser extent, its aggregates

Recent research in Japan has shown that CO2 uptake by the cement hydrate increases significantly when particle sizes are small and are subjected to alternate wetting and drying. Examination of concrete from crushing plants has indicated a typical carbonation uptake of 11kg of CO2 per tonne of crushed concrete aggregate – about 14% of the average UK production carbon dioxide emissions per tonne.

Belin has taken this further and looked at the benefits that enhancing carbonation of recycled concrete aggregates may have on its performance as a material. Recognising that porous weak cement matrices in old concrete leads to poor durability, the researchers found that by accelerating carbonation in crushed concrete aggregates, porosity was dramatically reduced, making the treated aggregates comparable to natural sands and stones commonly used in concrete manufacture.

Which seems like a win-win opportunity for the construction industry. By accelerating the carbonation of selected post-demolition crushed concrete, not only is the end product more durable and suitable for use in concrete rather than being relegated to fill, but in doing so it also absorbs a proportion of the carbon dioxide given out during its initial manufacture. It will be interesting to see if carbon sequestration through accelerated carbonation treatments of crushed concrete becomes more common over the next few years in projects looking to reduce their carbon footprint.

(This is an extract from a longer fully referenced article on this topic available from BPS Eco Ltd)