TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport

RESEARCH REPORT 78

CORROSION OF REINFORCEMENT: AN ASSESSMENT OF

• TWELVE CONCRETE BRIDGES AFTER 50 YEARS SERVICE

By P R VASSIE

The views expressed in this report are not necessarily those of the Department of Transport.-

Bridges Division Highways and Structures Department Transport and Road Research Laboratory Crowthorne, Berkshire, RG11 6AU 1986

ISSN 0266-5247

CONTENTS

Page

Abstract 1

1 Introduction 1

2 The Investigations 1

2.1 Strategy 1

2.2 The tests 1

2.3 Visual observations 4

2.4 Presentation of results 4

3 Results and Discussion 6

3.1 Petrographic analysis and aggregate characteristics 6

3.2 Compressive strength, density and cement content 6

3.3 Depth of cover 6

3.4 Depth of carbonation and chloride content 6

3.5 Half cell potentials and corrosion 11

3.6 Resistivity 12

3.7 Sulphate content 12

4 Concluding remarks 12

5 Acknowledgements 13

6 References 13

7 Glossary of Gaelic bridge names 13

© CROWN COPYRIGHT 1986 Extracts from the text may be reproduced,

except for commercial purposes, provided the source is acknowledged

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced~ except for commercial purposes, provided the source is acknowledged.

CORROSION OF REINFORCEMENT: AN ASSESSMENT OF TWELVE CONCRETE BRIDGES AFTER 50 YEARS SERVICE

ABSTRACT This report evaluates the results from corrosion investigations on twelve 50 year old bridges on the A82 trunk road between Bridge of Orchy and Invergarry in Scotland. The bridges experience a harsh environment and have been treated with de- icing salts since the mid 1960's. Superstructural elements were surveyed by a contractor for reinforcement corrosion using half-cell potential, electrical resistivity, carbonation, chloride and cover measurements. These tests were supplemented by petrographic examinations of thin sections from cores and by measurement of compressive strength, density, cement and sulphate content to assess the condition of the concrete. The investigations showed that the beams and soffits of the decks have suffered the most corrosion as a result of the synergistic action of carbonation and chlorides. Data from the top steel of the decks were more difficult to interpret.

1 INTRODUCTION This overview of some aspects of the condition of twelve reinforced concrete bridges on the A82 trunk road between Bridge of Orchy and Invergarry is based on site measurements made between 1981-1983 on behalf of the Scottish Development Department (SDD).

The bridges were known to be suffering, in varying degrees, from cracking, spalling, leakage, frost damage and poorly compacted concrete. In addition there was evidence of corroding reinforcement. The site investigations used non-destructive survey techniques (Vassie, 1980) (Woodward, 1981) for the detection of corrosion which were developed at TRRL and have been successfully employed on Haddiscoe Bridge (Cavalier and Vassie, 1981). The bridges are of interest because they were constructed without waterproofing membranes at about the same time (1930-2) and have been exposed to broadly the same harsh environment at an altitude of approximately 250 m over Rannoch Moor and in Glencoe. De-icing salts have been used on the route for about 20 years.

The site surveys were designed to meet the needs of individual bridges rather than to provide comparative data for an overview. Nevertheless given the similarities of age and exposure a comparison is justified.

2 THE INVESTIGATIONS

2.1 S T R A T E G Y The bridges investigated are described in Table 1 and some are illustrated in Plates 1-4. The aim was to examine representative areas of each bridge; ease of access and other factors influenced the selection. Thus on some bridges most of the tests were carried out on the top surface of the deck while on others the emphasis was on the beams and soff i t . The structural elements examined on each bridge are shown in Table 2. On Etive and Tulla bridges, for example, the investigation was carried out on complete cross sections of the bridges over a length of six metres. In this report reference to work on the deck refers to the top surface unless specified otherwise.

2.2 THE TESTS The tests with their primary functions are listed in Table 3. Petrographic examination (ASTM 1977) can establish the presence of alkali silica reaction (ASR), carbonation, frost damage, microcracking, aggregate shrinkage and poor compaction in addition to characterising the cement and aggregate. Tests 3 - 5 (BSI 1971) give an indication of the concrete's durabil i ty while the extent or probabil i ty of reinforcement corrosion is indicated by tests 6-10. The risk of sulphate attack can be deduced from the results of sulphate analysis of the concrete as in test 11.

Tests 1-5 and 11 were based on cores. These were taken only from the bridges surveyed more extensively and then only a l imited number were cut from some of the structural members investigated. Cover and half cell potentials (ASTM, 1980), tests 6 and 8, were usually measured at the nodes of a grid, typical ly 500 x 500 mm and these represent the most comprehensive set of measurements. Resistivity measurements were restricted to decks and were relatively few, typical ly f ifteen per deck-span. The analysis of the concrete for chloride content was based on dust dril l ings sampled over a 1 m x 1 m grid. The powdered concrete samples from the drillings were collected from the outer 25 mm although a l imited number of profiles were determined by collecting dril l ings or slicing cores at successive depths of 25-50 ram, 50-75 mm and 75-100 mm. The non-destructive tests were supplemented by visual surveys.

. . " • , - .

Neg. no. R 8 1 / 8 5 / 1 A

Plate 1 Tulla Bridge (Tied arch

|

Neg. no. R81/{]5/gA

Plate2 GiubhasBridge (Typical of the small, single span bridges investigated)

p f . .

Plate 3 Lairig Eilde Bridge

Neg. no. R81/85/10A

4 q .~ ' ,

1

Neg. no. R81/85/6A

Plate 4 Invergarry Bridge

3

TABLE 1

Description of bridges investigated

Bridge

Lairig Eilde

Cabar

Etive

Tulla

AIIt Na Feadha

AIIt Ruigh

Meannachlack

AIIt Na Muidhe

Giubhas

Molach

AIIt Na H'achlaise

Invergarry

Type

Deck supported on continuous beams spanning between cross heads. 4 spans. Masonary piers.

Single span simply supported skew slab

Single span tied arch

Single span tied arch

Single span simply supported skew slab

Twin span simply supported skew slab

Date of Construction

1932

1932

1932

1932

1932

1932

Single span simply supported skew slab

Single span simply supported skew slab

Single span simply supported

Single span simply supported Early

4 Hunched drop beams supporting a skew, integrally cast deck

3 span arch

1932

1932

Early 1930's

1930's

Early 1930's

Early 1930's

Map Ref.

NN 183 563

NN 193 563

NN 252 547

NN 312 443

NN ~ 563

N N 176 567

N N 189 562

NN 119 566

NN 265 539

NN 272 534

NN 316 486

NH 308 011

2.3 V I S U A L O B S E R V A T I O N S Detailed observations were made on the beams and soffits surveyed, but only a cursory inspection was made of the decks once the surfacing had been removed. All the decks showed some evidence of frost damage that is frequently encountered in the wet salty concrete found on unwaterproofed decks. The damage usually extended to a depth of about 15 mm from the surface. At Tulla where the cover was noticeably low there were signs of Iocalised corrosion in areas of frost damage.

The beams and soffits tested exhibited a wide range of defects: cracking, spalling, rust staining, leakage, lime deposits, delamination, efflorescence, poor compaction and map cracking. These defects varied in severity and extensiveness, but most soffits and beams showed more than one defect.

To summarise the visual observations the following grading is used in Table 4:

Grade 1--Good condition Grade 2--Minor and isolated defects Grade 3--Moderate occurence of defects Grade 4--Major or widespread occurrence of

defects Grade 5--Major and widespread occurrence of

defects.

2.4 P R E S E N T A T I O N OF R E S U L T S The results are simplified and summarised in Table 4 so that overall comparisons between the different bridges and their particular structural elements are not obscured by the large quantity of basic data obtained from the surveys.

Measurements of depth of cover are classified in terms of the proportion with depth less than 50 mm and with depth less than 30 mm. Half cell potentials are classified in terms of the Van Deveer criteria (Van Deveer, 1975) which have been shown to give the following correlation on bridges:

Potentials less negative than -200 mV against saturated Cu / CuSO4-- no corrosion Potentials more negative than -350 mV against saturated Cu/CuSO4--corrosion

Intermediate values give no clear indication of the risk of corrosion occurring on the reinforcement. Measurement of the electrical resistivity of the concrete is classified in three bands (Vassie, 1980) that give some guidance to the rate of reinforcement corrosion that could occur if the passivity of the steel was to be broken down by de-icing salts or carbonation. Measurements of chloride concentration made at different positions in the surface 25 mm of the concrete are reported as a range and a mean value in terms of percentage total chloride ion by weight of cement. Variation of chloride concentration with depth is reported as the mean value at each depth. Compressive strengths, densities, cement contents and sulphate content of the concrete are reported as mean values. Carbonation depths were determined by using phenolphthalein spray on cores and also from thin sections. Because comparatively few carbonation determinations were made the results are reported as a range.

4

TABLE 2

Tests and their locations on each bridge

Tests Performed Bridge Test Locations (For code see Table 3)

Lairig Elide , 2 , 3 , 4 , 6 , 7 , 8 , 1 0 , 2 , 3 , 4 , 5 , 6 , 7 , 1 0

7 ,8 ,10 4 , 6 , 7 , 8 , 9 , 1 0

Cabar 2 , 3 , 4 , 5 , 7 , 9 , 1 0 , 1 1 6 , 7 , 8 , 9 , 1 0

Etive

Part of four Longitudinal Beams 1 Part of two Transverse Beams 1 One Soffit span 6, Four Deck spans 3,

Deck 1, Soffit 3,

Soffit Transverse Beams Tie Beams Hangers Arches The above test locations covered

a section of length 6 m Deck

Soffit Transverse beams Longitudinal tie beams Hangers Arches A section of length 6 m was sampled

for each location

80 per cent of the Deck

Tulla

Deck

Deck

Deck

Soffit

Soffit

Soffit Four Drop Beams

Deck (7 panels)

6 , 7 , 8 , 1 0 1 , 2 , 3 , 4 , 5 , 6 , 7 , 9 , 10, 11 3 , 4 , 5 , 6 , 7 , 9 , 1 0 10 10

6 , 7 , 8 , 9 , 1 0

6 , 8 , 1 0 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 10, 11 3 , 4 , 6 , 7 , 8 , 10 8, 10 10

AIIt Na Feadha 6, 10

AIIt Ruigh 6, 9, 10

Meannachlack 6, 9, 10

AIIt Na Muidhe 6, 9, 10

Giubhas 1, 2, 3, 4, 5, 6, 7, 8, 10

Molach 1, 2, 3, 4, 5, 6, 7, 8, 10

AIIt Na H'achlaise 1, 2, 3, 4, 5, 6, 7, 8, 10 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 10

Invergarry 6, 8, 10

TABLE 3

List of non destructive tests used

Code Name Information produced

1

2

3

4

5

6

7

8

9

10

11

Petrographic examination from thin sections of cores

Aggregate characteristics from core examination

Compressive strength of concrete from cores

Density of concrete from cores

Cement content of concrete from cores

Depth of concrete cover to the reinforcement

Depth of carbonated concrete from the surface

Half-cell potential

Concrete resistivity (Electrical)

Chloride ion content of the concrete

Sulphate content of concrete

Occurrence of ASR, frost damage, micro- cracking, aggregate shrinkage, poor compaction and sulphate attack

Aggregate type, shape and grading. Shell content

General quality of the concrete

General quality of the concrete

General quality of the concrete

Regions at risk from deicing salts and carbonation

Regions of depassivated reinforcement

Regions of depassivated reinforcement

Areas capable of supporting high corrosion rates

Regions of depassivated reinforcement

General quality of the concrete

3 RESULTS A N D D I S C U S S I O N

3.1 PETROGRAPHIC ANALYSIS AND AGGREGATE CHARACTERISTICS

Cores from eight elements were subjected to this test. The fol lowing were features of common occurrence: microcracking, carbonation, leaching, voids, high water-cement ratio, poor compaction, non-uniformity, elongated and shrinkable aggregates. Microcracking usually resulted from frost action or aggregate shrinkage. The depth of carbonation determined by microscopic analysis of thin sections was often high and tended towards the upper end of the range of values produced by phenolphthalein spray on the cores. The evidence of leaching from the thin sections corresponds with visual evidence of extensive leaking and lime staining associated with joints and cracks in the slabs. The voids were often associated with elongated coarse aggregate particles and the frequent occurrence of this type of aggregate also resulted in a marked tendency towards orientation of the coarse aggregate. The high water- cement ratios, poor compaction and non-uniformity of the concrete can be explained by l imitations of site construction techniques in the 1930's with small batch production of concrete and ineff icient methods of mixing and compaction.

3.2 COMPRESSIVE STRENGTH, DENSITY A N D CEMENT CONTENT

The compressive strength and density of the concrete were in the range 33-67 N.mm-2 and 2270-2380 kg.m-3 respectively on all the bridges with

the exception of AIIt Na H'achlaise and Tulla, which had somewhat lower strengths. There appears to be greater variability between the strengths of individual cores in any one bridge than would be thought desirable for modern concrete. Cement contents were variable with some high values at Etive and low values at Tulla.

3.3 DEPTH OF C O V E R This was predominantly less than 30 mm for all beams and soffits investigated. The three decks on which depth of cover was measured had significantly higher values, with typically less than a quarter of the deck area with cover less than 30 mm and more than a quarter with cover greater than 50 mm.

3.4 DEPTH OF CARBONATION AND CHLORIDE CONTENT

The depths of carbonation encountered in beams and soffits were particularly variable with values generally in the range 5-30 ram. Cover to the reinforcement was typically less than 30 mm so that it is evident that the carbonation front had reached the reinforcement over significant areas of these elements while remaining areas were likely to be suffering from local or partial carbonation, at cracks for example.

On the three decks tested for carbonation the measured depths were both smaller and more uniform than on the beams and soffits. The highest value obtained was 10 mm and on two decks at Lairig Eilde and Etive no carbonation was detected. This limited carbonation found on decks is not unusual and is

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thought to be due either to the road surfacing or the wet condition of the concrete.

Sixty three percent of chloride measurements on the beams and soffits have values above 0.2 per cent total CI by weight of cement and thirty-seven percent above 0.35 per cent total CI by weight of cement whereas, in decks, chloride levels were considerably higher, with the mean value greater than 0.5 per cent on all decks except AIIt Na Muidhe.

The reduction in chloride concentration with depth in decks indicates that the main source of the chloride in the concrete is de-icing salts. However, it is possible that some of the aggregate could have been of marine origin with associated salt, because shells were observed in some of the thin sections.

The concentration of chloride in concrete above which there is a chance of reinforcement corrosion occurring is not clearly established and various contributors to the literature have suggested values ranging from 0.2 to 0.4 per cent total chloride ion by weight of cement (Reading, 1982) (Building Research Establishment, 1982). The current British Standards (BSI, 1972) (BSI, 1978) advocate less than 0.35 per cent for 95 per cent of samples with the remaining 5 per cent of samples having chloride concentrations elss than 0.5 per cent. These standards are not entirely appropriate as they refer to chlorides that were present in the mix. Research relating to de-icing salts entering hardened concrete (Vassie, 1984) indicates that threshold values near the lower end of the range are more appropraite for the bridges under discussion.

The values of chloride concentration and their variability indicate that some parts of these bridges are likely to have corroding reinforcement although the higher depths of cover found on decks would tend to reduce its incidence. On beams and soffits the chloride level was often close to the threshold value and in these instances the probability of corrosion is likely to be strongly influenced by other factors, notably the depth of carbonation. The reduction in pH which results from even partial carbonation of the concrete lowers the value of the threshold chloride concentration necessary to depassivate the reinforcement. The extensive carbonation encountered on beams and soffits implies that this phenomenon could increase the probability of corrosion of reinforcement in these elements.

In general, depths of cover of less than 30 mm on beams and soffits and of about 50 mm on decks has proved inadequate to prevent carbonation (fifty years exposure) and de-icing salts (twenty years exposure) reaching and depassivating the reinforcement with the consequential occurrence of corrosion and concrete disruption in some places.

3.5 HALF CELL P O T E N T I A L S A N D C O R R O S I O N

There was only l imited opportunity to make direct visual examination of corroded reinforcement on these bridges, particularly the decks. Corrosion of the deck reinforcement at Tulla was the Iocalised type whereas corrosion of beam and soff i t reinforcement on several bridges was a combination of Iocalised and general types. General corrosion produces cracking and spalling of the concrete but comparatively small reductions in bar cross-section; it is usually associated with carbonation. Conversely Iocalised corrosion does not disrupt the concrete but often results in major loss of section and is associated with wet, chloride contaminated concrete. These observations are typical of decks, beams, soff i ts and piers examined in other bridge investigations (Cavalier and Vassie, 1981; Vassie, 1984).

Approximately half the structural elements where half- cell potentials were measured had substantial areas where, on the basis of the Van Deveer criteria, the reinforcement was probably corroding. In about half these cases chloride appeared to be the primary cause of depassivation while in the remaining cases the cause was probably a combination of chloride contamination and carbonation of the concrete. Structural elements from beams and soffits of four of the eight bridges tested had a high proportion of measurements (40 per cent more negative than - 3 5 0 mV) indicating corrosion on the Van Deveer criteria whereas a high proportion of readings from all three decks tested suggested corrosion. It was not possible to assess how successful the half-cell potential method was in locating corroding reinforcement because controlled destructive examination was not part of the investigative programme. Nevertheless a crude assessment may be made by comparing the potential measurements with visual observations of rust staining, cracking and spalling in Table 4. In eight out of nineteen elements where visual evidence was available the potential values indicated extensive corrosion and in the remaining eleven cases the potentials indicated that corrosion would be limited. There was satisfactory agreement between half cell potential values and the visual evidence in 7 of the 11 cases where only limited corrosion was predicted and in 6 out of the 8 cases where the Van Deveer criteria predicted corrosion. These results show that the Van Deveer criteria should not be used in isolation.

Potentials are best interpreted using contour maps (Vassie 1980, 1984). Where corrosion is active the configuration of the iso-potential contour lines obtained by mapping half cell potential data provides a useful pointer to whether the type of corrosion taking place is Iocalised or general. If the contour lines are closely spaced, indicative of relatively high potential gradients, then the corrosion is probably Iocalised. General corrosion is associated with lower potential gradients and results in considerably

11

shallower contours. At Etive and Lairig Elide the half- cell potentials measured on the decks were predominantly in the corrosion range, typically - 6 0 0 mV against saturated CuSO4 but the iso- potential contour maps showed low potential gradients indicati_ng that the corrosion was not Iocalised. This result was unexpected because experience at Tulla and from unwaterproofed decks elsewhere predicted the occurrence of Iocalised corrosion. The half-cell potential data from the decks at Etive and Lairig Elide suggests that either there is general corrosion resulting from extensive carbonation wi thout chloride contamination or that the steel is passive with potentials lowered because the transport of oxygen through the concrete is unusually low. The first explanation is improbable since there is clear evidence that carbonation is not significant and chloride contamination is extensive in these decks. The second explanation is preferred because the deck concrete was very wet, a circumstance known to limit the transport of oxygen through the concrete. The plausibil ity of passivity at such negative potentials was supported by subsequent examination of small areas of reinforcement which showed the steel to be uncorroded. This phenomenon is found in reinforced concrete in deep water (Wilkins, 1980) but it is important to note that it has not previously been found on bridges and that this interpretation should therefore be treated with caution until confirmatory evidence becomes available. Nevertheless this emphasises the danger of using the Van Deveer criteria wi thout also considering potential gradients and the importance of specialist involvement in half- cell potential surveys, particularly for data interpretation.

3.6 RESISTIVITY This test was only used on seven elements of which six were decks. The values obtained were generally less than 12 Kohm. cm indicating that the concrete was quite porous with a high moisture content. This is normal for concrete made in the 1930's and used wi thout waterproofing in bridge decks in an area of high rainfall. These low resistivities suggest that corrosion could proceed at a significant rate if the reinforcement became depassivated. The deck at AIIt Ruigh had by far the highest resistivities indicating that either the concrete had a lower moisture content or a lower porosity than the other decks tested. Lower porosity is the more likely explanation because the level of chlorides found in the deck suggests that the moisture content was probably high.

3.7 SULPHATE CONTENT Where measured the sulphate content was not high enough to cause sulphate attack on the concrete. The presence of ettr ingite in the microcracks of some of the thin sections, although a characteristic of sulphate attack, is not unusual in concrete more than a few years old and this observation by itself is not suff icient to indicate sulphate attack.;

4 CONCLUDING REMARKS

The quality of the concrete in these bridges was reasonably good considering the batch-mixing and compaction procedures in use during the 1930"s. The compressive strength was generally satisfactory but there was a tendency towards high water-cement ratios and orientation of the coarse aggregate particles because of their elongated shape. The properties of the concrete were noticeably non-uniform.

On the basis of depth of cover, carbonation and chloride measurements all the bridges would be expected to suffer corrosion or further corrosion of their reinforcement during the next ten years. This is supported by half-cell potential measurements and visual evidence that beams and soffits are already corroding in four out of eight bridges. There is insufficient visual evidence to make definite deductions about the corrosion of deck reinforcement. It appears that while there is sufficient chloride and moisture to produce corrosion, the supply of oxygen is critical, being sufficient at Tulla to produce Iocalised corrosion whereas at Etive and Lairig Elide, where the cover was greater, it was not sufficient to cause corrosion. This is the first indication that there might be situations where oxygen supply is a critical factor limiting corrosion in bridge decks. Reducing moisture levels in such a situation could increase the oxygen supply and initiate corrosion in chloride contaminated concrete. The decks of the smaller bridges were found to have no top steel, but the chloride and moisture levels were sufficient to support the view that reinforcement, had it existed, would have corroded in normal circumstances.

Corrosion in decks was caused by chloride de-icing salts. Their mean chloride content typically exceeded 0.5 per cent chloride ion by weight of the cement whereas carbonation depths were less than 10 mm. On beams and soffits the cause of corrosion was the synergistic effect of carbonation, typically of depth 5-30 mm, and chlorides from de-icing salts with mean concentrations about 0.2 per cent chloride ion by weight of cement.

The data from these bridges emphasises the dangers in making diagnoses on the basis of one or two tests in isolation. For example chloride, resistivity and numerical half-cell potential data on two decks predicted extensive Iocalised corrosion but this was contradicted by the shape of the iso-potential contour map. The combination of all these data together with the depth of cover lead to a more consistent interpretation. Similarly on some beams and soffits the results from chloride and carbonation measurements separately did not predict much corrosion, but the combined data were consistent with the level of corrosion observed.

12

5 ACKNOWLEDGEMENTS The work described in this report was carried out in the Bridges Divison of the Highways and Structures Department of TRRL. Thanks are due to the Chief Bridges Engineer of the Scottish Development Department for permission to use the data collected on his behalf by St Albans Testing Services.

6 REFERENCES

American Society for Testing Materials. 1977. The standard recommended practice for the petrographic analysis of concrete. ASTM C 85677.

American Society for Testing Materials. 1980. Standard test method for half cell potentials of reinforcing steel in concrete. ASTM C 87680.

British Standards Institution. 1971. Methods of testing concrete. BS 1881.

British Standards Institution. 1972. The structural use of concrete. CP 110, Part 1.

British Standards Institution. 1978. Steel, Concrete and Composite Bridges. BS 5400, Part 8;

Building Research Establishment. 1982. The durability of steel in concrete, Part 2, Department of the Environment. BRE Digest 264, Garston.

Cavalier, P G and Vassie, P R. 1981. Investigation and repair of reinforcement corrosion in a bridge deck. Proc. Instn. Civ. Engrs. Part 1, 70, Aug., 461-480.

Reading, T J. 1982. Chloride Content Limits recommended by ACI Committee 201. Concrete Construction, 777-779.

Van Deveer, J R. 1975. Techniques for evaluating reinforced concrete bridge decks. J. Am. Concr. Inst. Dec. 697-704.

Vassie, P R. 1980. A survey of site tests for the assessment of corrosion in reinforced concrete. Department of the Environment. Department of Transport TRRL Laboratory Report No. 953, Crowthorne, Transport and Road Research Laboratory.

Vassie, P R. 1984. Reinforcement corrosion and the durability of concrete bridges. Proc. Instn. Civ. Engrs. Part 1, 76, Aug. 713-23.

Woodward, R J. 1981. Case studies of the corrosion of reinforcement in concrete structures. Department of the Environment. Department of Transport TRRL Laboratory Report No. 981, Crowthorne, Transport and Road Research Laboratory.

Wilkins, N J M and P F Lawrence. 1980. Fundamental mechanisms of corrosion of steel reinforcement in concrete immersed in sea water. Concrete in the Oceans. CIRIA/UEG.

7 GLOSSARY OF GAELIC BRIDGE NAMES ALLT LAIRIG EILDE (lairig elde) CABAR ETIVE TULLA ALLT NA FEADHA

(allt na faidh) (allt na fei(e))

ALLT RUIGH (allt rui) MEANNACHLAH (mennaxlax) or

(mjaunaxlax) obscure. ALLT NA MUIDH (allt na muie) GIUBHAS MOLACH (adjective)

(AIIt Moch) ALLT NA H'ACHLAISE INVERGARRY

'stream' 'Pass or gulley of the hind' 'a pole' Name of a district and the river running through it. 'a hillock' Probably 'stream of the deer' In both cases the pronunciation is virtually the same

'stream of the foot of the mountain The last element is from 'clach', a rock. The first element may be a corruption of 'meanbh', small; if so, then 'little rock'. 'stream of the churn' 'Scots fir' 'rough'. May refer to a stoney bed

'stream of the hollow' Inbhir (mouth of a river) + Garry. The mouth of the Garry

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