Filtration Rolls and Wattles

Tensar Sedi-maxFiltration rolls and wattles offer a good alternative to straw bales for silt and erosion control on construction projects by intercepting and retaining suspended solids, and slowing water flow across a slope. Generally speaking the rolls are laid in shallow trenches, and are staked in place. As most products appear to be fully biodegradable, they can be incorporated into permanent works, and I noticed an interesting variation on this where the stakes were “living wood” (I assume willow) which subsequently sprouted as the rolls degraded.

Filtration rolls are more intended for permanent works, whilst straw wattles are more cost effective options for temporary works. Both of the Tensar products (SediMax-FRTM Filtration Rolls and SediMax-SWTM Straw Wattles) are fully biodegradable – the rolls filled with a straw/coconut fibre mix with a biodegradable netting cover, and the wattles with compressed straw cyclinders wrapped in a photo-degradable netting. Wattles are light and manhandle-able and easily staked along contours of disturbed slopes, wrapped around inlets and drains, along the banks of watercourses – in fact, across anywhere water is likely to flow on a construction site – a practical, controlled, and more long-lasting alternative to straw bales

(Click on the image above to go to the SediMax page on the Tensar site)


Volumetric concrete mixers

Utranazz mixerJust spotted this in The Construction Index, and thought it was worth a quick note. Not something completely new, but it looks like Utranazz and Elkins may have solved one of the biggest problems in this kind of concrete mixer – conveyor belt replacement – by developing a slide-out belt cassette.

Hopefully, if this proves to be practical, we may see more of this sort of unit on the road rather than traditional drum mixer deliveries, allowing exact quantities to be delivered, and part-load charges to be eliminated.

Silt Fencing

Silt Fence - HytexSilt management on UK construction sites is frequently limited to providing a wheel-wash at the site exit, but controlling silt movement on site can keep haul roads and access areas cleaner, minimising the need for such facilities. A useful tool in a silt management strategy for sites is the installation of silt fences in vulnerable areas to prevent silt running off sloping land onto areas you want to keep clean.

Silt fences are generally constructed of woven geotextiles, which are “heeled” into the ground to seal the bottom, and supported on timber stakes to hold them upright. Under heavy rainfall, silt-laden water is retained and filtered by the fence, keeping the silt in place whilst allowing substantially cleaner water to drain away. Once the rainy period has ended, the retained silt can be returned to the land from which it originated, and the silt fence is once again ready for the next period of wet weather.

Silt Fence DWInstallation can be by hand for shorter lengths, but on larger sites, mechanical installation is more practical, with attachments available for commonly used machinery.

Whilst silt fences are frequently employed to control silt run-off from open land in other sectors (such as forestry) as well as in construction, they are also useful for controlling dust-laden run-off from demolition operations, and around stockpiles of crushed materials.

(Click on any image to go to the relevant pages of the manufacturers’ website)

CMB Rubble Crusher

CMB CrusherI came across this new, useful piece of plant at this year’s Recycling and Waste Management show at the NEC in September. It caught my eye as it offers the opportunity to crush and reuse hard wastes on smaller sites instead of paying for their removal and the import of pre-crushed secondary aggregates. As you can see, in contrast to larger crush & screen units that require a low-loader for movement between sites, the unit is readily towable behind a Landrover or larger van, and can be easily moved from site to site. Looking at the company’s information, it can handle between 10 and 50 tonnes per hour depending upon the jaw setting – from smaller 15mm aggregates at the lower rate, to 100mm at the upper. It’s jaw crusher can also handle some pretty large lumps of concrete and if you watch the video you will see it rapidly reduce scrap concrete kerbs and blockwork into useful secondary aggregates.

Note that if you want to operate this under an EPR2010 T7 exemption, there is a limit of a maximum of 20 tonnes produced in any hour. You will also need to register a U1 exemption to reuse any crushed waste on site. Need help with waste permits & exemptions? See my Services – Waste Management page

(To go to the company’s website to see more information on this equipment, just click on the image)

Puralytics LilyPads

Puralytics LilypadRecently, I came across this novel product that looks like it may be really useful for dealing with low-grade persistent pollution, such as oil films on water. It’s a floating plastic mesh impregnated with photo-reactive nanoparticles that destroy pollution simply by the action of sunlight. The inventor also claims that it can eliminate E.coli and Salmonella, but as the pads are a long-term control measure rather than a short-term fix it may not be to practical to deal with high-impact pollution such as raw sewage or milk. However, compared to ordinary hydrophyllic absorbent mats, the new LilyPads destroy the pollution rather than simply absorbing it, which means that at the end of it’s working life, it’s not soaked with hydrocarbons, so no hazardous waste to dispose of. Worth keeping an eye on.

(If you want more information on LilyPads, just click the image to go to the Puralytics site)

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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Timber detailing

The importance of keeping trapped water away from timber!

Whilst visiting a venue for a sustainability event, I noticed dark staining around the bottom of each of the glulam columns – obviously due to water problems – surprising in a building less than a year old. Closer inspection revealed how the detailing of the support foot  has created water-traps leading to the staining, and worse – delamination of the ends of the columns in one case. (4)

The building is a gently curving linear structure, with externally exposed twinned columns supporting an overhanging green roof, forming a colonnade to the building. As the front is open over its length, lateral stability is provided by a couple of cross-braced bays using steel ties towards each end. (1) It’s also in a hilly countryside location, and so quite exposed to the weather.

The ends of the twin glulam columns sit hard against a horizontal galvanized steel plate which is presumably load-bearing, and there appear to be three routes that permit water to get into the junction between these two. The most significant is the connection between the twin glulam columns and the support foot which is made using a fabricated steel “H” section, through which the columns bolt. (2&3). The section is open at the top and forms a ready channel for driven rain to fall to the plate at the bottom. There are no obvious signs of drain holes in the plate (although some may be hidden), and so it is probable any water getting in can only get out by weeping across the end-grain timber at the support foot junction

Additionally, between the two columns at the base a small section of the base plate is exposed (2), again permitting standing water to collect and wick into the junction between timber and the plate. Finally, there are no drips at the base of the glulam, so any water running down the face of the columns can wick into the junction with the support foot.

So how could it have been improved?

The simplest solution would probably have been to effect full load transfer between the columns and support through the bolted connection, avoiding the need for the timber to bear on a base plate – no wet horizontal surface = no problem.

However, working within the current detail – cap and seal the H-section to avoid water ingess down the centre, cut away the small exposed section of base plate either side between the columns, and recess the base plate into the bottom of the columns to form a drip. Not an ideal solution, but one that would have discouraged water ingress. There’s also potentially bedding the detail on silicone seals and sealing around the sections, but relying on this to keep the end-grain dry is a long-term maintenance issue.

The staining above the connection to the bracing is also indicative of another potential long-term problem that could lead to deterioration of the glulam. (5) For this staining to occur, water must clearly stand on the horizontal top of the bracket and soak the timber, and probably wicks down between the bracket and timber too. Ideally, the top surface of the bracket would have sloped away from the timber, with the bracket bedded on a coat of wet stain or varnish as the bolts were tightened to seal the joint properly.