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Bowers basebed technical data

This technical data sheet was compiled by the Building Research Establishment (BRE) at the request of Albion Stone and is updated by Albion Stone to incorporate current test results. The 866 tests have been carried out in accordance with current European standards by the BRE on Albion Stone's behalf, or by other accredited testing houses. The early test data that pre-dates the introduction of Euro-codes has been included providing the test methods were very similar. The work carried out by the BRE on this technical data sheet has been undertaken as a paid commission and does not represent an endorsement of the stone by the BRE.

This data includes the Lowest and Highest Expected Values (LEV & HEV) using the statistical calculations from the Euro-codes. We are confident that these results give a good indication of the stones value, but as it is a natural material we, like other stone producers, are unable to guarantee individual results for specific stones. Instead, we recommend that an appropriate factor of safety is used to ensure satisfactory performance, Annex 1 of the Technical Manual provides further information, but we suggest that a suitably qualified stone consultant with geological and testing experience is employed to provide further information.

Petrography

Material Description
Cream fine to medium oolitic and bioclastic limestone containing bioclasts (5mm in width by 20mm in length)

Petrographic details
The rock was a grain supported oobiomicrite made up of predominantly rounded micritic ooliths showing concentric structure and ranging from 50 mm to 300 mm in diameter, numerous bioclasts up to 5 mm by 20 mm in size and sporadic irregular quartz grains 100 mm nominal size. There was an apparent pressure adhesion of the grains and some matrix between the ooliths and this results in occasional intergranular voids. Sporadically sparry calcite infilled or partially infilled intergranular spaces or replaced central parts of bioclastic debris.

The ooliths consisted of micritic calcite and the bioclasts generally exhibited original aragonitic texture.

The rock contained occasional voids 100 mm nominal size. The micrite was also microporus with pores beyond the resolution of a transmitted light petrological microscope. No deleterious constituents or features were observed.

Strength

Compression - BS EN 1926
Lowest Expected Value 27.41 Mpa
Highest Expected Value 100.22 Mpa
Average: 55.39 Mpa from 54 tests

Flexural Strength - BS EN 13161
Lowest Expected Value 4.06 Mpa
Highest Expected Value 11.50 Mpa
Average: 7.16 Mpa from 68 tests

Breaking Load at Dowel Hole (at 75mm thick) - BS EN 13364
Lowest Expected Value 3,459 N
Highest Expected Value 5,950 N
Average: 4,601 N from 20 tests

Durability

Water Absorption - BS EN 13755
Lowest Expected Value 3.33%
Highest Expected Value 8.42%
Average: 5.21% from 119 tests

Density - BS EN 1936
Lowest Expected Value 2,124 kg/m³
Highest Expected Value 2,331 kg/m³
Average: 2,226 kg/m³ from 287 tests

Porosity - BS EN 1936
Lowest Expected Value 12.69%
Highest Expected Value 18.37%
Average: 15.36% from 448 tests

Saturation Coefficient - BS EN 1936
Lowest Expected Value 0.67
Highest Expected Value 0.85
Average: 0.76 from 251 tests

Salt Crystallisation - BS EN 12370
Lowest Expected Value 25.29%
Highest Expected Value 100%
Average: 62.18% from 81 tests

Technical summary

Prepared by: Dr T Yates, BRE (Building Research Establishment)

Durability and Weathering
It is important that the results from the sodium sulphate crystallisation tests are not viewed in isolation. They should be considered with the results from the porosity and water absorption tests and the performance of the stone in existing buildings. Stone from the Portland Basebed is traditionally acknowledged as being less durable than Whitbed but it has been used extensively where a faster rate of weathering is acceptable or where its working qualities were required. It is possible to compare the results for the Basebed Stone from Bowers Quarry to those collected from buildings, exposure trials and tests on quarry samples collected by BRE during the last 70 years. This shows that the stone compares well with the traditional view of Portland Basebed. Previous research at BRE has shown that Portland limestone which has a low saturation coefficient (>0.72), a high microporosity (>11.0 of the stone by volume) and an increased amount of micritic matrix will weather more rapidly than Whitbed when used on buildings. The results summarised on these sheets show that most of the samples tested are of this type. The crystallisation test results show the stone to be Class D -E which BRE Report 141 suggests that it is suitable for plain walling and cladding. The results from the other tests suggest that soundest stone may well perform better than this class in the current environment. Where more severe exposure conditions are expected, for example high concentrations of sulphur dioxide or severe frosts, or where a long life is required (for example >50 years) then it may be desirable to use a more durable stone (e.g. Portland Whitbed). When using Bowers Basebed it is especially important that the detailing of the stonework is designed to offer the maximum protection to rainwater and rainwater runoff.

Based on current research it seems likely that the stone would weather at a rate of between 3 and 4 mm per 100 years but it could be greater in severe exposures or on the edges of stonework.

(Weathering rates are based on the BRE interpretation of historical data dating from 1932)

 

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