CONTROL OF THAUMASITE FORMATION IN CONCRETE: CURRENT APPROACH AND RESEARCH NEEDS

Transcription

1 INTERNTIONL SEINR: The Thaumasite Form of Sulfate ttack of oncrete ONTROL OF THUSITE FORTION IN ONRETE: URRENT PPROH ND RESERH NEEDS Dr Ewan Byars entre for ement and oncrete University of Sheffield This paper discusses the current approach taken in BRE Special Digest 1 to the specification of concrete for thaumasite attack resistance, with respect to the lessons learned from the research at Sheffield University raises questions on the issues of: i) The effects of source of aggressive chemicals (clay or pure solution) on rate of attack of concrete ii) iii) iv) The potential to afford an additional protective measure by better backfill compaction (reduction of surrounding clay permeability) The need for a specification on minimum cement contents The current specification for aggregate carbonate contents and the need for an additional clause that considers alternative carbonate sources and v) The potential need to reclassify SRP with respect to Thaumasite resistance

2 ontrol of Thaumasite entre for ement and oncrete Formation in oncrete urrent pproach and Research Needs Dr Ewan Byars entre for ement and oncrete University of Sheffield

3 Occurrence of Thaumasite entre for ement and oncrete In arch 2002, the TEG reported a significant number of new cases of thaumasite formation in cementitious construction, including: 16 Highways gency cases (Gloucester/Wiltshire) 6 cases associated with sulfate bearing brickwork 1 Highways gency case in o. Durham 2 internal sand/cement render cases contaminated with gypsum 2 cases of slab heave on sulfate-bearing fill 1 kerb failure in a drainage adit 1 set of harbour steps exposed to seawater Notably, 2 cases were observed where the concrete was made with siliceous aggregates

4 urrent Specifications Overview of SD1, Pt. 2 entre for ement and oncrete 1. ssess ggressive hemical Environment for oncrete (Table 2) 2. Define cement type to be used (Table 3) 3. Determine ggregate arbonate Range (Table 4) 4. State Structural Performance Level (Table 5) and member size 5. Use 1, 2 and 4 to determine Design hemical lass (Table 7) 6. Use output from 5 and 3 in Table 6 to obtain max W/ and min cement content

5 ggressive hemical Environment entre for ement and oncrete Important Factors Presence of sulfates - generally salts of calcium, magnesium and sodium Presence of sulfides particularly pyrite which may oxidise after excavation and prior to backfill to produce sulfuric acid then sulfate salts on neutralisation with clay minerals or concrete surface obility of groundwater for transportation and refreshment of aggressive species at concrete surface cidity of groundwater

6 obility of Groundwater entre for ement and oncrete urrent Definitions Static permanently dry or clay permeability < 10-6 m/s obile water can flow through soil, permeability > 10-6 m/s Highly obile water is flowing through soil e.g. under hydraulic head Research Questions i) What is the effect of different degrees of clay density after ii) iii) compaction on backfill impermeability? How does this relate to rate of thaumasite formation? an we provide contractors with with backfill compaction guidelines as an dditional Protective easure?

7 lay Density X Example ttack Rate Y entre for ement and oncrete How does attack rate vary with degree of compaction?

8 ggressive hemical Environment for oncrete (E) entre for ement and oncrete The sulfate, sulfide, acid and mobile groundwater values in the current SD1 lead to: 7 Design Sulfate lasses with 16 sub-e classes (Table 2) These, along with structural and member size details, feed in to Table 7 in SD1 Part 2 to give concrete Design hemical lasses (D) then using Table 6, with aggregate carbonate content details, gives target values of W/ and minimum cement content

9 ggressive hemical Environment for oncrete (E) entre for ement and oncrete Insert table 2

10 entre for ement and oncrete

11 Design hemical lass (based on Tables 3,4 SD1 Pt 2) entre for ement and oncrete

12 oncrete Quality entre for ement and oncrete Research Question Do we need to specify minimum cement content? Low-water concrete (made with maximum aggregate content and appropriate use of superplasticizers and mix proportioning) can be made at the relevant w/c ratios and suitable workability with most gravel aggregates and some crushed rock aggregates.

13 ggregate arbonate Ranges entre for ement and oncrete Research Question Effect of carbonate sources other than from aggregate?

14 Digest 363 lass 2 Solution Siliceous gg. OP arbonaceous gg. entre for ement and oncrete Results from Sheffield Study

15 Digest 363 lass 2 Solution Siliceous gg. PL arbonaceous gg. entre for ement and oncrete Results from Sheffield Study

16 Digest 363 lass 3 Solution Siliceous gg. OP arbonaceous gg. entre for ement and oncrete Results from Sheffield Study

17 Digest 363 lass 3 Solution Siliceous gg. PL arbonaceous gg. entre for ement and oncrete Results from Sheffield Study

18 Digest 363 lass 3 Solution Siliceous gg. PF arbonaceous gg. entre for ement and oncrete Results from Sheffield Study

19 Digest 363 lass 3 Solution Siliceous gg. SRP arbonaceous gg. entre for ement and oncrete Results from Sheffield Study

20 Digest 363 lass 3 Solution Siliceous gg. GGBS arbonaceous gg. entre for ement and oncrete Results from Sheffield Study

21 Digest 363 lass 4B Solution PF Siliceous gg. arbonaceous gg. entre for ement and oncrete Results from Sheffield Study

22 Digest 363 lass 4B Solution Siliceous gg. SRP arbonaceous gg. entre for ement and oncrete Results from Sheffield Study

23 Digest 363 lass 4B Solution Siliceous gg. GGBS arbonaceous gg. entre for ement and oncrete Results from Sheffield Study

24 ementitious Groups (from Table 3, SD1 Part 2) entre for ement and oncrete

25 ementitious Groups entre for ement and oncrete SRP GGBS Research Question Should Sulfate Resisting Portland ement be reclassified in terms of its resistance to thaumasite formation?

26 Special Digest 1 entre for ement and oncrete Features : 16 E lasses 3 structural performance levels 3 concrete member thicknesses 128 Design hemical lasses corresponding to the above Practical Questions Is this an overly complex approach to a final product whose cement content varies by only 100kg/m 3 and W/ by 0.2? ould the methodology be simplified by having less categories? Should we specify concrete W/ ratio in increments of less than 0.5 for finer tuning?

27 ontrol of Thaumasite Formation in oncrete entre for ement and oncrete Recommendations relating to SD1 from the Sheffield Study larify the relative aggressivity of clay conditions versus solution chemistry and adjust Table 2 (needs further research) Provide details of the effects of backfilled clay permeability in terms of compacted densities for control of water mobility (needs further research) larify the effects of carbonate from aggregates versus other sources and adjust Table 4 (needs further research) Revise Table 3 cementitious grouping to take into account the susceptibility of SRP and possibly other cementitious combinations to thaumasite attack (from current research)

28 ontrol of Thaumasite Formation in oncrete Final onclusions entre for ement and oncrete 1. To date, concrete containing 45% GGBS has provided much better resistance to both thaumasite and acid attack than other uncoated cementitious combinations 2. ll concrete coated with bitumen has remained unaffected by thaumasite attack