Dave Parker reports from Wales on how waterproof self-compacting concrete has come to the aid of the archaeologists.
Right up to the moment that the diggers excavating the new Newport Theatre & Arts Centre orchestra pit struck ancient timber, main contractor E Turner & Sons operations director Don Bowles was expecting his main challenges to involve the complex geometry of the £11.5M project.
Made up essentially of three connected buildings, a 500 seater main theatre with associated facilities, 150 seater studio theatre, and cafe bar/kitchen complex, the angular centre sprawls along the right bank of the tidal River Usk immediately opposite the town's bus station.
Simply setting out the groundworks promised to be a major challenge.
'There's not a right angle in the job, nor a gridline, ' Bowles reports. 'But until 20 June everything was going far better than we had expected.'
One of the anticipated problems with the site was its proximity to the river whose tidal range of up to 12m is one of the highest in the world. But 'in fact this is one of the driest sites ever, ' says groundworks subcontractor Churngold Construction site manager Ian Manning, 'which just shows how good this clay here is.'
Such welcome impermeability came at a high price. As the well-preserved lower hull of a massive 14th century merchant vessel began to emerge from the clay, everyone involved realised that here was a major archaeological treasure (see box). It soon became obvious that the seven weeks allowed in the 85 week contract programme to accommodate archaeology would stretch to months.
Some 95% of the intact hull lay neatly inside the cofferdam, with the keel virtually on the same level as the planned underside of the base slab. However some 10 predriven precast concrete piles had been punched through the clinker built hull prior to excavation.
Leaving the timbers in place was not an option. The theatre had to have an orchestra pit, and a large section of the basement walls were already poured. After vigorous local campaigning, the £3.5M needed to remove the timbers and reassemble them in a new basement under the theatre foyer was raised.
Most of this sum is allocated for the rescue operation and treatment of the timbers. The 7m deep basement will add an extra £1.5M to the original contract value.
Like the 4.8m deep basement under the main theatre, its walls will be poured with high technology waterproof self compacting concrete (SCC).
Bowles says the decision to go down this route rather than adopt the more traditional external tanking was influenced by a number of crucial factors. 'Sika, the system supplier was prepared to give a very good guarantee, and this confidence was impressive.
'In addition, two Sika technicians would be at the readymix depot and then on site for every pour.'
Few on site had seen modern SCC in action until the pour of the first trial panel. 'The way it flowed was a revelation, ' Manning reports. 'And the finish on the trial panel was outstanding.'
The promise was not fulfilled when the shutters were stripped after the full height pours on the 2.5m high by 250mm thick basement walls, however. 'In fact the finish on the outside, where the back shutter received less preparation was better than on the inside, ' Manning says.
'Sika's response was that traditional precautions aimed at obtaining a high quality finish, such as oiling the shutter, could be counterproductive. A more absorbent surface would give a better result.'
Apart from the extra effort needed to ensure an effective seal at stop ends, Churngold found the SCC easy to handle, Manning adds. The Injectoflex system was, he reports, little more complicated than a conventional waterbar system.
'We could easily have used conventional vibration in the walls as rebar density wasn't excessive, ' Bowles points out.
'But a key factor was the boxouts for the services. The SCC ensured perfect compaction right up to the underside.'
Walls on the new basement will be 400mm thick. To avoid a major redesign of the structural steelwork, the basement will be spanned by massive concrete transfer beams up to 1m deep and 2.5m wide.
Turner took no chances with the second cofferdam, now nearing completion. Trial bores went down 17m, and large trial pits were taken out across the site.
Bowles admits he had nightmares about the discovery of yet more ship remains. 'I got a nasty shock when old timber turned up in one of the trial pits recently, ' he says.
'Luckily, these were judged to be unconnected fragments from an old jetty, so it looks as though we are all right.'
Concrete technologies separated by more than eight decades were combined to produce the self-compacting concrete (SCC) in the centre's basement walls.
Watertighteness was ensured by Sika 1, a poreblocking admixture first used back in 1910 to waterproof rail tunnels through the Swiss Alps, and the low water/cement ratio possible with the advanced Viscocrete 2015 superplasticiser. Adding the viscosity modifier Viscocrete Stabiliser allowed it to be poured without bleeding or segregation.
Structural engineer the Clarke Bond Partnership chose the non-SCC version of the mix for the base slab. For extra security and compliance with BS8102 Grade 4, Sika's Injectoflex system was specified for all construction joints.
Basically a neoprene hose fitted with three hydrophilic swelling strips, glued into position on the base slab with a hydrophilic sealant, the system offers the possibility of injecting a waterbased resin should this be required.
Concrete production fell to Tarmac Western. The basic waterproof concrete mix for the base slab featured a maximum water/cement ratio of 0.45 and minimum cement content of 350kg/m 3. Sika 1 was partnered by the less expensive Sikament 10 high range water reducing admixture. A more radical design was needed for the SCC, however.
Sika's experience suggested that a true SCC would need a fine aggregate content of at least 50% of the total plus a high cement content in the range 420kg/m 3-480kg/m 3. After trials and the production of a sample panel on site, the final mix design use a 48:52 ratio of 20mm to 5mm blended limestone and blended marine sand and limestone dust.
Cement content was 480kg/m 3, water/cement ratio 0.40 and target slumpflow was 675mm.
Described as '50 years older than the Mary Rose, and in much better condition', the vessel discovered beneath the site of the Newport Theatre & Arts Centre has got marine archaeologists really excited.
Tree-ring dating puts its construction at around 1466, and artefacts already recovered suggest trading with the Iberian peninsula, a routine voyage for a ship whose estimated dimensions are a length of 25m and a berth of 8m.
Despite the loss of the upper sections of the hull, archaeologists are convinced the ship is an early form of the cog or caravel used by later transatlantic explorers, the most advanced marine technology of the time.
The site is on the edge of former monastery land, leading to speculation that the ship had been docked for repairs and was abandoned during the Wars of the Roses or after Henry VIII's seizure of monastic wealth during the Reformation.