Research Project into Cast Stone

Executive Summary

The initial phase of the work included an extensive literature search of published work concerning Cast Stone. A comprehensive survey of production processes and products of the UK Cast Stone Association membership (23 companies) was carried out. This indicated that most companies used both wet and dry-cast Cast Stone production methods. Portland cement was the main binder used and all producers employed waterproofers. Parameters controlled or monitored in production included aggregate grading, moisture content of materials, ambient temperature and mixing time. Product performance was mainly assessed by cube strength and visual assessment and all companies carried out initial surface absorption tests (ISAT), with about half also using capillary absorption tests (CAT).

Based on the findings of the survey, it was agreed that only dry-cast, Cast Stone should be considered in the study. An initial set of tests on 28 dry-cast, Cast Stone samples obtained from 7 companies was carried out. Reference wet cast PC samples were manufactured at the University of Dundee and natural stone control samples also considered. It was found that there was a wide variation in mix compositions between companies with total binder and water contents ranging from 305 – 510 kg/m3 and 110 – 193 l/m3 respectively (water/cement ratio between 0.35 and 0.50).

Following this, a selected number of test samples were obtained which formed the main test series. This considered a range of additional permeation (water vapour transmission, air permeability, effective porosity and water accessible porosity), durability (carbonation and freeze/thaw) and weathering (efflorescence and particulate retention) tests. It was again found that virtually all test samples including natural stone failed to comply with the ISAT requirements, but all complied with those of the CAT. The ISAT measurements were also found to be unreliable, since in many cases drainage or ‘bounce-back’ under the cap occurred. The CAT was found to be suitable for identifying whether a waterproofer had been used. Effective and water accessible porosity, water vapour diffusivity and air permeability measurements indicated that none of these tests offered any significant improvement in the characterisation of the quality of dry-cast, Cast Stone compared to the existing methods.

All Cast Stone products, and particularly when waterproofed, were found to be highly resistant to carbonation. Efflorescence did not occur in any of the test samples and weathering in Cast Stone was similar to that in control natural stone. The data also suggests that dry-cast, Cast Stone is no less vulnerable to frost attack than natural stone.

The relationships between Cast Stone absorption properties to their durability and weathering properties were investigated. These call into question the widely held view that ISAT and CAT are directly linked to weathering. The results, however, suggest that the waterproofer used in all test samples is probably the most important characteristic controlling surface weathering performance. It was also found that none of the permeation tests considered were capable of differentiating between waterproofer effectiveness.

Additional tests were carried out to determine the effect of:

Binder types
Waterproofer dosage
Binder content
Although there was some variability in the samples produced by the manufacturers and, as a result, direct comparisons of the characteristics of the different test samples were not strictly possible, the results support the fact that the integral waterproofer strongly controls the performance of Cast Stone. It was also found that beyond 0.5% dosage by weight of binder, there was only a limited gain in performance and that there was potential to reduce typical waterproofer levels used. The third series of tests with various binder contents were inconclusive, except to further underline the importance of the integral waterproofer.

The study indicated that the performance and durability of Cast Stone was directly related to it’s underlying ‘quality’, as determined by its strength, and was not reliant wholly upon the use of waterproofer. To ensure that Cast Stone supplied from UKCSA manufacturers was of such a quality the minimum compressive strength was recommended to be increased to 35 MPa (35 N/mm2 ) for contracts commencing January 1st 2000 onwards. However, given it’s importance to the final performance of the material, it was also felt that specifications should consider making the use of an integral waterproofer mandatory.

The Full Report also concludes that: “In general, all dry Cast Stone Samples tested weathered in a similar way to the natural stone from the results of the accelerated weathering test.”

If you are considering cast stone, think UKCSA, think UKCSA Quality Mark and specify from the Full Manufacturing Members listing using the UKCSA Specification Document.