INVESTIGATIONS ON EXPANSION ISSUES IN AGED CONCRETE ARCH DAM-A CASE STUDY

VV Arora1, V.P. Chatterjee1, B.R.K.Pillai2, Brijesh Singh1, A.K.Dhawan3, Arun Sood1

1National Council for Cement and Building Materials, Ballabgarh, Haryana 2Dam Safety (Rehabilitation) Directorate, Central Water Commission, New Delhi

3Egis India, DRIP Project, Central Water Commission, New Delhi

ABSTRACT

Several chemical reactions are able to produce swelling of concrete for decades after its initial curing, a problem that affects a considerable number of concrete dams around the world. Keeping in view that there are a large number of very important and valuable large concrete dams around the world and India that are entering “middle age”, the need to deal with “ageing” of the concrete in the structures is clearly increasing as many of them being near 50 years old. This study was done to review the expansion in concrete Arch dam which included (a) Evaluation of aggregates (taken out from concrete core) using Petrographic Analysis including morphological microstructural and Mineralogical analysis of the samples as per IS: 2386 Part VIII. (b) Detailed Mineralogical analysis covering the absence/presence of reactive aggregates prone to Alkali-Silica reaction. (c) Evaluation of concrete using Petrographic Analysis including study of pore structure and presence of micro cracks and abnormal reactive products. (d) Surface Morphology study of concrete samples by Scanning Electron Microscopic (SEM) method including detection of ettringite presence and fracture pattern. The petrographic analysis of coarse aggregate indicated aggregate type as Hypersthene-Granite. The Petrography analysis of concrete samples also indicated presence of onset of Alkali Silica reaction (preliminary stage). The sulphate in the form of pyrite was found from petrography and SEM studies and this was the reason for ettringite formation. The percentages of ettringite formation vary from less than 1 percent to 3 percent with respect to 5 percent to 8 percent of open air voids present in the concrete. The ettringite formation of order 1 percent to 3 percent was not likely to cause any expansion. The test results of petrography studies, Scanning Electron Microscopy, Colour test and Accelerated Mortar Bar test indicated signs of Alkali Silica Reaction in concrete, which would cause expansion in structure. Further investigations on engineering properties has also been completed and being published separately. 1.0 Introduction

Dams are built to serve for a long time of reliable and safe operation which is monitored for many dams since a long time period. The ‘age old’ can be characterized with an increase of structural deterioration, with an impaired structural safety and a remaining life expectancy for several years. Therefore, in order to execute an accurate and reliable analysis for concrete Arch dam, it is necessary to estimate the properties of concrete in dam appropriately. The two important chemical reactions which have a capability to cause swelling in concrete dams are Alkali Aggregate Reaction (AAR) and Ettringite Formation (EF). They lead to an expansion of the material and induce generally cracking and degradation of the mechanical properties. This implies problems in terms of serviceability, structural integrity [1,2,3] and durability since cracking favors the ingress of external species prone to initiate other degradations [4,5]. Because of their different chemical mechanisms, these two reactions are mostly studied separately in the literature. A large amount of research has been performed in the last decades for AAR [6,7] and EF [8,9,10]. However AAR and EF often act simultaneously in the field.

Due to similar macroscopic effects, some controversies remain in the identification of the deleterious causes [11,12]. To deal with the affected structures, it is thus necessary to precisely understand the chemo-mechanical effects of each reaction.

This paper presents a case study on the effect of Alkali Aggregate Reaction (AAR) and Ettringite Formation (EF) on behaviour of concrete arch dam including Petrographic and Mineralogical analysis of the samples covering the absence / presence of reactive aggregates prone to Alkali-Silica reaction and Ettringite Formation (EF). For this study, the concrete core samples were extracted randomly from all the accessible portions of the dam to represent the complete Arch dam. The concrete cores extracted for laboratory tests were visually inspected for voids, debris, joints, deterioration and other defects.

2.0 Alkali Aggregate Reaction (AAR) and Ettringite Formation (EF) Studies including

Petrographic and Mineralogical Analysis

The Alkali Aggregate Reaction (AAR) and Ettringite Formation (EF) studies were conducted in all the three galleries (inside and outside) and accessible portions of downstream and upstream (inside portion) of dam. These tests mainly included (a) Evaluation of aggregates (taken out from concrete core) using Petrographic Analysis including morphological microstructural and Mineralogical analysis of the samples as per IS: 2386 Part VIII. (b) Detailed Mineralogical analysis covering the absence/presence of reactive aggregates prone to Alkali-Silica reaction. (c) Evaluation of concrete using Petrographic Analysis including study of pore structure and presence of micro cracks and abnormal reactive products. (d) Surface Morphology study of concrete samples by Scanning Electron Microscope including detection of ettringite presence and fracture pattern.

2.1. Petrographic and Mineralogical Analysis

The petrographic analysis was done for evaluation of aggregates (taken out from core). The study included morphological microstructural and Mineralogical analysis of the samples as per IS: 2386 Part VIII. Mineralogical details were analyzed covering the absence/presence of reactive aggregates prone to Alkali-Silica reaction. Thin sections of the selected samples were prepared. The samples were studied in NIKON POL-600E microscope under polarized light. The modal analysis, granulometry and microstructures were done using the Image Analysis System attached with the microscope. The analysis was also done for evaluation of concrete including study of pore structure and presence of micro cracks and abnormal reactive products. Representative in-situ samples were selected from the cores to carry out analyses of the concrete containing all the components. The study was done in Nikon Stereoscopic microscope. The analysis of various features was done using the software attached to the microscope. The test results are discussed in the following paragraphs.

2.2 Ettringite Formation (EF) Studies

Ettringite formation in concrete arch dam was checked by Scanning Electron Microscope (SEM) Study and optical microscopy study.

2.2.1 Scanning Electron Microscope (SEM) Study

The Surface Morphology study of concrete samples was done by Scanning Electron Microscopic (SEM) method including detection of ettringite and fracture pattern. This method is based on the interaction of electrons with matter. The resolution of Scanning Electron Microscope is high because of the low wavelength of electron beam. When electron beam falls on the sample, different signals will be emitted such as secondary electrons, back scattered Electrons, auger electrons, X-rays etc. Secondary electrons emission is the result of knocking out of orbital electrons of the atom by incoming electron beam. Secondary Electrons were used to study the morphology of

the sample. The emitted signals were detected using suitable detection systems. The test results are discussed in the following paragraphs.

2.2.2 Optical Microscopy Study

Polarising and stereoscope microscopes were used to study the concrete core samples extracted from Arch dam to detect ettringite formation. Selected in-situ samples of the concrete cores were studied first under stereoscopic microscope. On an average 150 runs were taken to establish the morphology and microstructure of ettringite grains. Thin sections of the same samples were prepared for polarising microscope studies. 150 to 200 sites were studied in each thin sections to study the granulometry and percent distribution of ettringite grains in the concrete sample.

2.3 Alkali-Aggregate Reactivity Studies

Alkali-aggregate reaction (AAR) is a reaction in concrete between the alkali hydroxides, which originate mainly from the Portland cement, and certain types of aggregate. Two types of AAR are currently recognized; these are alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR). As the names imply, these types of reaction differ in that they involve reactions with either siliceous or carbonate phases in the aggregates. To study Alkali Aggregate Reaction (AAR) following tests were conducted:

2.3.1 Optical Microscopy and Scanning Electron Microscopy

ASR rims were also checked for Alkali Aggregate Reaction products and results are discussed under heading of Petrography & Scanning Electron Microscopy on concrete core samples.

2.3.2 Colour Test- Method for Identifying Concrete Gels form by Alkali Silica Reaction

The detection of Alkali Silica Reaction swelling in concrete by staining was done by Regents of the University of California. The present investigation included the sequential application of solutions of each of two water soluble compounds to the concrete under investigation. Concentrated solutions of sodium cobaltinitrite and rhodamine B in water were prepared. The concrete surface to be examined was treated by pre-rinsing with water and subsequently applying each solution to the surface. After 30-60 seconds, the concrete was rinsed thoroughly with water. The treated surface would show yellow and pink regions where ASR gel was present, yellow regions indicated the presence of K-rich, Na-K- Ca-Si gels. While pink regions indicated alkali-poor gels. The final rinse step was required, since the yellow sodium cobaltinitrite solution would coat the entire concrete surface as would the pink rhodamine B solution, thereby obscuring the stained gel regions. The concrete core samples from all the randomly selected locations were rinsed with water and rinsed surfaces were searched for regions of yellow staining and regions of pink staining whereby K-rich i.e. Na-K-Ca-Si gels generated from ASR were identified by yellow staining and alkali poor, Ca-Si generated from ASR were identified by pink staining. The test results are discussed in the following paragraphs.

2.3.3 Accelerated Mortar Bar Test

Accelerated Mortar Bar Test (AMBT) as per ASTM C-1260 was carried out on coarse aggregate samples removed from concrete cores. The AMBT consists of preparing mortar-bar in the same way as for conventional tests as per IS: 2386 (Part VII) i.e., by proportioning one part of cement to 2.25 parts of graded aggregates by mass, a fixed water to cement ratio i.e., 0.47. The sample after 24-hours was de-moulded and then cured in hot water at 80ºC for 24-hours. Finally, the specimen is stored in 1N NaOH solutions at 80ºC for 14 days. The length change observations are to be taken in hot condition i.e., within 20 seconds after taking out from the solution. The samples are stored in plastic containers & the use of glass or metal container for this purpose is not recommended as the same get corroded by NaOH solution. As per ASTM criteria, the aggregate showing 14 days

expansion less than 0.10 percent were classified as innocuous, whereas the aggregates showing more than 0.20 percent expansion were classified as potentially reactive. For aggregates showing expansion between 0.10 percent and 0.20 percent, the results were to be supported by other test. In this study one additional sample was also kept in water for 14 days at 80ºC.

3.0 Test Results and Discussions

3.1 Petrography Analysis

The optical microphotographs of petrography study done on coarse aggregate sample are given in Figure 1. The findings of petrography study on coarse aggregate are given here under: 3.1.1 Coarse Aggregate: The coarse aggregate sample had been taken from the concrete core. This is a medium grained textured partially weathered Hypersthene-Granite. The major mineral constituents were orthoclase-feldspar, quartz, hypersthene and plagioclase-feldspar. Accessory minerals were pyrite, microcline-feldspar and iron oxide. Subhedral orthoclase grains with rounded grain margins were uniformly distributed in the rock. Orthoclase grains were highly fractured and partially shattered. Orthoclase grains present at the contact of mortar were highly corroded and brittle in nature. Grain size of quartz varied from 18µm to 478µm with an average of 264µm. Majority of quartz grains are in the size range of 200 µm to 260 µm. The strained quartz percentage is about 16% and their undulatory extinction angle (UEA) varied from 190 to 210. Lath shaped hypersthene grains were partially altered. Prismatic plagioclase grains with sharp grain margins were partially fractured and shattered. Euhedral pyrite grains with sharp grain margins were scattered throughout the sample. Reaction rims were developed mostly on the margins of pyrite grains. In few instances pyrite grains were completely consumed during hydration reaction. Subhedral microcline were partially fractured, shattered and alerted. Subhedral to anhedral iron oxide grains with corroded margins were randomly distributed in the rock. Iron oxide grains were brittle and fragile in nature.

Figure-1 Optical microscopic images of Aggregates

The modal composition is: (a) Trade Group: Granite (Igneous Rock), (b) Petrological name: Hypersthene-Granite, (c) Particle shape: Irregular and (d) Surface texture: Crystalline.

Modal Composition of the Coarse Aggregate (Results in %)

Sl. No.

Rock Type Minerals

Ort

hocl

ase

Fel

dspa

r

Qua

rtz

hype

rsth

ene

Pla

gioc

lase

F

elds

par

Pyr

ite

Mic

rocl

ine

feld

spar

Iron

Oxi

de

1 Hypersthene Granite 35 27 18 14 3 2 1

Findings: - The sample studied was medium grained textured partially weathered random sample of Hypersthene-Granite. The strained quartz percentage and their UEA are within permissible limits. Feldspar grains are partially fractured, shattered and altered. Reaction rims are developed mostly on the margins of the euhedral pyrite grains. In few instances pyrite grains are completely consumed during hydration reaction. The quality of the coarse aggregate is fair.

3.1.2 Concrete core sample: The petrographic analysis was also done for evaluation of concrete including study of pore structure and presence of micro cracks and abnormal reactive products. The hard and well compacted concrete core samples composed of homogenously distributed cement, coarse aggregate and fine aggregate from all the randomly selected locations studied are shown in Figure-2. The selected samples were studied in polarizing and stereoscopic microscopes. Based on petrographic studies it was also observed that, coarse aggregate samples were rich in three types of feldspars (orthoclase, plagioclase and microcline). Plagioclase grains of both the components (coarse and fine aggregate) were less affected by alteration and other type of hydration reactions. Microcline grains were partially affected by hydration reaction. Orthoclase grains present in coarse aggregate were affected more than other feldspar. In few orthoclase grains in both of components migration of reacted products were deposited along the weak planes of the grains. Few plagioclase grains are also show reaction products along the weak planes of the grains. However, alterations of minerals were not very common hence petrographically it is concluded that both the coarse and fine aggregates were partially affected by hydration reactions and their hydration products. The petrography analysis of concrete samples indicated presence of onset of Alkali Silica reaction (preliminary stage) and examination of ASR rims indicated that the infection were due to presence of orthoclase.

Figure-2 Optical microscopic images of Concrete Samples

3.2 Ettringite Formation (EF) Studies

3.2.1 Scanning Electron Microscopic (SEM) Study

The concrete core samples were tested under SEM study and their test results are given in Figure-3 and 4. Based on study carried out it was seen that numerous microcracks were observed at the interfacial zone and also in the paste. Ettringite formation of size ranging less than 2 microns to 60 microns was found in most of the samples. Formation of microcracks was identified around interfacial zone in most of the samples. Pyrite (FeS2) crystals were present as minor constituents in Hypersthene granite (upto 1 to 4 percent).Microscopic studies reveal that pyrite mainly outsourcing Sulphur for formation of ettringite. Pyrite was normally randomly distributed in the Hypersthene-granite (coarse aggregate component). Initially the pyrites present on the aggregate-mortar boundaries had released Sulphur for formation of ettringite. Ettringite formation may aggravate if the pyrite present inside the aggregate starts participating in the reaction. Spread of this type of ettringite formation was very slow and it

might continue further in future. ASR and Thaumasite were also identified in the few samples under SEM study.

Figure-3 SEM Images indicating Ettringite in Concrete samples

Figure-4 SEM Images indicating Ettringite in Concrete samples

3.2.2 Optical Microscopy Study

Sulphide minerals were found in all the coarse aggregate fractions. Quartz grains were present in both coarse and fine aggregates used in the concrete. Feldspar grains were partially fractured with numerous micro structural features. Majority of the sulphide minerals were partially reacted during hydration reaction. Scattering of sulphide minerals was also observed in the matrix of the concrete samples. These grains were highly reacted on the margins. Very fine hydrated products were developed in the concrete. These hydrated products were rod shaped and in cluster form. These grains were most likely ettringite and CSH. Radiating needles very fine and sharp in nature were developed as flowery structure in the samples. These formations were mostly observed in the open voids of the concrete. Rod shaped hydrated products were mostly translucent with curvilinear structure. Very fine crystalline grains were also developed on the surfaces of these rods. The reacted portions of the sulphide minerals indicated that they might be the main source of ettringite formation. This was also confirmed by the detailed SEM studies. Ca (OH)2 formation was also observed in the samples, which were mostly present as globular clusters in the concrete. To ascertain the ettringite formation in terms of percentage, ten samples from each concrete core were taken under observation. The studies were carried out with 1000 counts from each core. The percentages of ettringite formation vary from less than 1 percent to 3 percent with respect to 5 percent to 8 percent of open air voids present in the concrete. The ettringite formation of order 1 percent to 3 percent is not likely to cause any expansion. Ettringite crystals in air voids and cracks typically up to 2 to 4 micrometers in cross-section and 20 to 30 micrometers long in the concrete have been reported in literature. Under conditions of extreme deterioration, and repeated wetting and drying, ettringite crystals can appear to completely fill voids or cracks. However, ettringite, found in

this benign state as large needle-like crystals, should not be interpreted as causing the expansion of deteriorating concrete (reference “Portland Cement Association, PCA R & D, Serial No.2166”.

3.3 Alkali-Aggregate Reactivity test

The studies on the Alkali Aggregate Reaction (AAR) included (a) Optical Microscopy and Scanning Electron Microscopy for AAR detection, (b) Colour test method i.e. method for identifying concrete gels form by Alkali Silica Reaction and (c) AAR study on coarse aggregate removed from the tested concrete cores using accelerated mortar bar test as per ASTM C-1260. The test results are discussed under following paragraphs.

3.3.1 Optical Microscopy and Scanning Electron Microscopy

ASR rims were also checked for Alkali Silica Reaction products (Figure -5). The results obtained revealed that initial stage of ASR was observed on the boundaries between coarse aggregate and cement mortar. Aggravated ASR reactions were more on the boundaries between partially altered coarse aggregate used and mortar part. The thickness of ASR rims varies from few microns to 50 microns in size. In few instances, the pores containing crystalline mass were observed on the boundaries of ASR rims. Pore distribution in different cores was varied too much. When pores were studied under microscope, it was observed that ettringite formation had taken place with three types i.e. Crystalline, semi-crystalline and gel. Grain size variation was large. Upstream Core samples showed more ettringite than downstream samples.

Figure-5 Optical Microscopy study on ASR Rims

Microscopic studies suggested that K-bearing feldspars (Orthoclase) were more prone to alterations which lead to Alkali-Silica Reaction (ASR). In numerous grains of orthoclase, microcracks were developed due to ASR reactions. By products generated due to theses reactions were deposited in-situ. However, this development in orthoclase grains had disturbed the morphology and binding characteristics of concrete. Impact of this was not very rigorous but it might show more aggressive reaction with ageing of concrete. Due to this ageing effect on concrete, the three types of feldspars and pyrite present in the coarse aggregate component may also disintegrate and transform into either in other mineral or by product. However, this disintegration would most likely get deposited in-situ. Microcline grains were also partially affected by ASR but effect of ASR on Microcline was less aggressive than orthoclase. In few instances, some plagioclase grains had also shown effect of ASR on the grain boundaries.

3.3.2 Colour Test

The concrete core samples extracted randomly covering entire structure was tested for presence of ASR using colour method. The treated samples were searched for regions of yellow staining (K-rich i.e. Na-K-Ca-Si gels generated from ASR) or regions of pink staining (alkali poor

i.e. Ca-Si generated from ASR). The test results indicated light to dark pinkish colour in all the samples indicating presence of ASR.

3.3.3 Accelerated Mortar Bar Test

The test results of Accelerated Mortar Bar Test indicated that the net expansion in coarse aggregate and fine aggregate (mortar part) sample is 0.04 percent and 0.03 percent respectively. The Petrography analysis and Scanning Electron Microscopy studies of concrete samples also indicated presence of onset of Alkali Silica reaction (preliminary stage). Though this expansion indicated that the aggregate was not under potentially reactive category and the typical pattern cracking due to ASR would not occur but length change could be caused even by small amount of ASR expansion.

4.0 Conclusions

Based on the investigation done and literature studied, it is concluded that: i. The petrographic analysis of coarse aggregate indicates aggregate type as Hypersthene-Granite.

The Petrography analysis of concrete samples indicates presence of onset of Alkali Silica reaction (preliminary stage).

ii. The sulphate in the form of pyrite is found from petrographic, SEM and XRD studies and this is the reason for ettringite formation. The percentages of ettringite formation vary from less than 1 percent to 3 percent with respect to 5 percent to 8 percent of open air voids present in the concrete. The ettringite formation of order 1 percent to 3 percent is not likely to cause any expansion.

iii. The study on expansion potential of aggregates expansion indicates that the aggregate is not under potentially reactive category and the typical pattern cracking due to ASR will not occur but length change can be caused even by small amount of ASR expansion. With the end restraints in case of arch dam, this small expansion may also add to the movement of mid point towards upstream side.

Acknowledgements

The authors would like to acknowledge the laboratory technical officers and technical assistants concerned for their assistance in carrying out this sponsored projects.

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