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, New Delhi

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. This study was conducted on concrete core samples extracted randomly from various locations of dam covering entire structure. The petrographic analysis of coarse aggregate indicated aggregate type as Hypersthene-Granite. The optical microphotographs of petrography study done on coarse aggregate sample are given in Figure 1. 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.

Figure-1 Optical microscopic images of Aggregates

Orthoclase grains were highly fractured and partially shattered and orthoclase grains present at the contact of mortar were highly corroded and brittle in nature. Subhedral quartz grains with

rounded grain margins were well graded but randomly distributed. 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. The modal composition obtained is: (a) Trade Group: Granite (Igneous Rock), (b) Petrological name: Hypersthene-Granite, (c) Particle shape: Irregular and (d) Surface texture: Crystalline. 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 were also showing 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

The concrete core samples were also examined under Scanning Electron Microscope (SEM) and their test results are given in Figure-3.

Figure-3 SEM Images indicating Ettringite in Concrete samples 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 1-4%). Microscopic studies revealed that pyrite was mainly outsourcing Sulphur for formation of ettringite, which was randomly distributed in the Hypersthene-granite (coarse aggregate component). 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 & 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”. ASR and Thaumasite were also identified in the few samples under SEM study.

ASR rims were also checked for Alkali Silica Reaction products. 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. Very minute crystals also formed on the surface of concrete i.e. on mortar part. Due to the 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. 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 movement of mid point towards upstream side. Further investigations on engineering properties has also been completed and being published separately.

Conclusion: 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). 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. 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.