Ian Longworth of BRE Construction Division reviews some of the ground engineering aspects of the Thaumasite Expert Group's recent report on the thaumasite form of sulfate* attack.
In the past decade a new form of sulfate attack has been found in buried concrete in the UK. Two cases of the thaumasite form of sulfate attack (TSA)1, 2 have been investigated by BRE in the foundations of domestic properties on Lower Lias Clay in the Cotswolds area.
In March 1998, TSA was also found to have caused deterioration of concrete in the foundations of some M5 motorway bridges in Gloucestershire, built 30 years ago on Lower Lias Clay. The concrete was affected to a depth of up to 50mm, the surface being transformed into a soft white mass (Figure 1).
These foundations were constructed with good quality concrete which had been specified, in accordance with contemporary guidance, to cater for the perceived ground sulfate conditions. Investigations of the TSA cases found that the ground conditions had been significantly changed by construction processes. The concentrations of sulfate in the Lower Lias Clay had been enhanced to detrimental levels by oxidation of sulfides, mostly in the form of pyrite, due to the clay being excavated and used as backfill.
Concern about the unexpected occurrence of TSA in buried concrete prompted construction minister Nick Raynsford to set up an Expert Group3 to review its occurrence and to produce interim adviceand guidance on the implications for both existing structures and the construction of new ones. BRE staff participated in the Expert Group, and part of their role was to provide technical advice on sulfate occurrence and assessment.
This article describes the ground conditions that contributed to the occurrence of the thaumasite form of sulfate attack in the M5 bridges' foundations and reviews the procedures recommended by the Expert Group to cater for such site conditions in new construction.
TSA in buried concrete
Thaumasite's chemical composition is CaSiO3.CaSO4.CaCO3.15H2O. Field and laboratory studies by BRE1,2 and others have shown that TSA may occur in buried concrete if the following conditions are simultaneously present:
the concrete has a hydrated calcium silicate phase, typically owing to use of Portland cements
there is a substantial concentration of sulfate in ground or groundwater
the groundwater is copious and mobile
there is abundant carbonate - generally in coarse and/or fine concrete aggregates
temperatures are relatively low (generally below 15degreesC).
Occurrence of TSA in M5 bridge foundations
The bridge foundations affected by TSA were on the Gloucestershire section of the M5 motorway and were built in about 1970. They were either constructed in excavations in the Lower Lias Clay which were then backfilled with the same clay, or passed through embankments constructed of Lower Lias Clay.
A study of the affected foundations identified the following causes contributing to TSA occurrence:
all of the TSA-affected bridge foundations were constructed of concrete containing carbonate-rich aggregates
all sites of affected elements were very wet, due in part to inadequate drainage and the original excavations acting as sumps
all of the affected foundation elements were in contact with large volumes of reworked, initially unweathered, Lower Lias Clay which contained pyrite. Oxidation of this pyrite led to enhanced sulfate concentrations in soil and groundwater. Sulfide-consuming bacteria may have assisted in this process.
The oxidation process produced distinctive changes in the appearance of the Lower Lias Clay. Initially stiff, well structured and dark blue- grey material was found 30 years later to be mostly soft, structureless and olive brown.
Table 1 shows the sulfate content measured in the original site investigation and also the sulfate contents found in the recent investigations of the most severely affected bridge foundation. The determinations of total sulfate by acid extraction show a substantial increase in sulfate concentration. At the original investigation the site would have been assessed as Class 1 for sulfate in respect of concrete design (BRE Digest 90). It would now be assessed, overall, as Class 3.
Assessment of sulfate class for new construction
Historically, the potential for sulfate concentration to be increased as a result of oxidation due to disturbance has not been taken into account in guidance documents for concrete design, such as Digest 363 and BS5328. Research is needed to establish a new standard procedure for this. In the interim, the Report of the Thaumasite Expert Group extends current guidance by recommending (Figure 2) that if buried concrete is likely to be exposed to disturbed sulfide-bearing ground, then the 'potential' sulfate concentration in the disturbed ground should be calculated from the measured total sulfur content, as follows:
potential sulfate (SO4)% = sulfur (S)% x 3.0
where the sulfur content can be measured by techniques given in BR 2798.
The new procedure gives a conservative estimate of the potential sulfate content since any sulfur within organic matter and minerals such as barite, both of which are more inert than pyritic sulfur, are included.
When assessing sulfate class for buried concrete design, the report recommends the following options, which take into account the potential sulfate content:
(i) Classify the ground assuming the worst case scenario in which all the sulfide in the disturbed ground is converted to sulfate, and use a specified correlation for sulfate class against potential sulfate content (see Table 3 inset in Figure 2). It should be noted that this correlation does not take account of the fact that the commonly occurring calcium sulfate (gypsum) is far less soluble than magnesium and sodium sulfates. In most cases, therefore, the resultant sulfate class will be very conservative in respect of concrete mix design.
(ii) In the case of new construction in sulfide bearing ground, consider changing the design or method of construction so that concrete will not be exposed to sulfate which might result from oxidation of sulfides (see the options below). This will allow the derived potential sulfate value to be set aside and the traditional procedure of Digest 363 and BS 5328 to be followed in its entirety.
Ground engineering measures to avoid exposure of concrete to enhanced sulfate levels
As an alternative to specifying concretes which will withstand high sulfate levels potentially resulting from oxidation of sulfide-bearing ground, the following design measures are recommended in the Report for buried concrete construction:
Design construction works so that disturbance (eg cut and fill) of the ground is minimised. As well as providing an opportunity for sulfides in unweathered clays to oxidise, leading to formation of sulfates, disturbance can also increase ground permeability and increase hydraulic gradients, making groundwater more mobile and more likely to carry sulfates to concrete. The use of piled or trenchfill foundations which avoid excavation and backfilling may be appropriate.
Design excavations within a clay stratum so that they do not become sumps with high concentrations of sulfate-rich groundwater. Specifically avoid backfilling with sulfide-bearing clay. Other backfills should be well- compacted to minimise voids and drainage paths.
Design ground drainage so that any flows of mobile groundwater which might be coming from a sulfate source are intercepted. At the same time, avoid construction features that can channel water to buried concrete structures or adjacent backfill. At the M5 bridges, water leaking from French drains which passed through the backfilled excavations contributed to the severity of the TSA.
Apply a protective coating of a material such as bitumen or resin. Note, however, that the coating needs to be of proven durability.
1 Crammond, NJ and Halliwell, MA (1995). The thaumasite form of sulfate attack in concretes containing a source of carbonate ions - a microstructural overview. Second Symposium on Advances in Concrete Technology. ACI SP154, pp357-380.
2 Crammond, NJ and Nixon, PJ (1993). Deterioration of concrete foundation piles as a result of thaumasite formation. Sixth Conference on the Durability of Building Materials. Japan. 1993, Vol 1, pp295-305.
3 Department of Environment, Transport and the Regions (1999). The thaumasite form of sulfate attack: risks, diagnosis, remedial works and guidance on new construction. Report of the Thaumasite Expert Group. DETR, January 1999.
4 Building Research Station (1968). Digest 90: Concrete in sulfate-bearing soils and groundwater.
5 Building Research Establishment (1996). Sulfate and acid resistance of concrete in the ground. BRE Digest 363.
6 British Standards Institution (1997). BS 5328. Methods of specifying concrete, including ready-mixed concrete.
7 British Standards Institution (1990). BS 1377: Methods of test for soils for civil engineering purposes. Part 3: Chemical and electrochemical tests.
8 Bowley, MJ (1995). Sulfate and acid attack on concrete in the ground: recommended procedures for soil analysis, BRE Report BR 279.