JAGUARI HYDROPOWERPLANT - EVALUATION, DIAGNOSIS AND CONTROL OF A STRUCTURE AFFECTED BY ALKALI-AGGREGATE REACTION

JAGUARI HYDROPOWERPLANT – EVALUATION, DIAGNOSIS AND CONTROL OF A STRUCTURE AFFECTED BY ALKALI-AGGREGATE REACTION

Flavio M. Salles 1*, Julio C. Pínfari 1, Selmo C. Kuperman 2, Camilo Mizumoto 1, Haroldo de Mayo Bernardes 3

1 Companhia Energetica de São Paulo-CESP, Sao Paulo, SP, Brazil

2 DESEK Ltda, Sao Paulo, SP, Brazil

3 Sao Paulo State University “Julio de Mesquita Filho”-UNESP, Ilha Solteira, SP, Brazil

Abstract

Comprehensive inspections performed at the water intake of CESP’s Jaguari hydropowerplant, raised the possibility that cracks in concrete could have been caused by alkali-aggregate reaction. Triortogonal joint meters were installed and cores extracted for petrographic analysis, proving that cracks were caused by AAR and that actions were needed to evaluate the level of expansion and the structural behavior of the intake. A monitoring system comprising rod extensometers, crack meters, triortogonal meters, LVDT was provided and a new optical instrument was developed to check movements. Drilled cores were sent to the laboratory to be subjected to petrographic analysis, expansion tests, compressive strength and modulus of elasticity. AMBT, CPT and ACPT tests were also performed. In order to check the occurrence of gel a staining technique was applied in the concrete. This paper summarizes the research and development project that is allowing CESP to better monitor and study the intake.

Keywords: Hydropowerplant, water intake, alkali-aggregate reaction, instrumentation, concrete tests

1 INTRODUCTION

The Jaguari Hydropowerplant (HPP) belongs to CESP - Companhia Energetica de Sao Paulo and is located in the Paraiba do Sul River, near Jacareí city in São Paulo State, Brazil. The plant started operating in 1973 and has two generating units totaling an installed capacity of 27.6 MW. The water intake is a tower type structure 63m high. Water reaches the powerhouse through a penstock with 572.5 m in length and diameter of 1.55 m Figure 1depicts an aerial view of the site and Figure 2 presents the front view and sections. In 2000 CESP detected a pattern cracking in the concrete of the water intake, powerhouse and spillway that raised concerns. An aggregate considered as reactive with the alkalies of cement, the analysis of cracking pattern and the existence of exudation of white material in some cracks led to the diagnosis of possible alkali-aggregate reaction (AAR).

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Triortogonal joint meters were installed to follow-up movements and cores extracted for petrographic analysis. Further inspections, tests and studies performed in 2003 proved that the cracks were caused by AAR and that actions were needed in order to evaluate the level of expansion and the structural behavior of the intake. The discovery of AAR requires detailed knowledge of the movements of the affected structure, with additional instrumentation to measure movements of cracks, joints and the structure as a whole, as well as the rate of concrete expansion. In 2007 CESP started a thorough monitoring system of the intake comprising the installation of rod extensometers, crack meters, triortogonal meters, the development of a new optical instrument to check relative movements between the stoplog steel slots, and also a LVDT for the same purpose. These efforts were sponsored by a Research and Development Project from ANEEL, the Brazilian National Agency for Electrical Energy. These instruments aim to monitor all movements of the water intake, since the displacement of this structure may cause the locking of the gate, jeopardizing the region's water supply and generation of electricity. Cores were drilled and sent to the laboratory to be subjected to petrographic analysis, expansion tests and to determine compressive strength as well as modulus of elasticity. A staining technique for the concrete was also introduced. Actions taken and results are described [1, 2, 3].

2 IDENTIFICATION OF AAR

2.1 Investigations

A detailed visual inspection was performed on the water intake to evaluate its structural aspects and concrete cores were extracted and subjected to mineralogical and petrographic analysis. The most important aspects noticed during the inspections were: occurrence of severe map cracking; presence of leaching products from the hydrated cement paste and probably AAR gel on the concrete surface, with the formation of white precipitate that filled various cracks in the water intake slab, as shown in Figure 3; cracks in the water intake slab, bypassing the columns of the travelling crane, with openings of up to 10 mm. The emerged part of the water intake shows severe random cracking with most of them presenting openings in the order of millimeters. The columns, with rectangular section, that support the crane also show vertical cracks on all sides. Data collected from the triortogonal meters showed increase in the openings of 0,5mm/year. Petrographic and mineralogical analysis confirmed that AAR led to the cracking of the structure.

2.2 Visual inspection of the extracted cores

Concrete cores were visually inspected in the laboratory with naked eye and with magnifying glass in order to verify the possible presence of typical AAR elements such as pores filled with reaction products, cracks (both in the aggregate and the mortar), reaction rims, dark spots on the mortar or around the aggregates, detachment between the aggregate and the paste. Such elements are shown in Figure 4.

2.3 Petrographic Analysis

The petrographic analysis were based on ASTM C856/95 [4] and were performed on cores taken from a slab of the water intake, from one column that supports the crane and also in some aggregates to identify potentially reactive minerals. Techniques applied were: macroscopic and microscopic observations through stereoscopic microscopy (reflected light), optical microscopy (transmitted light) and SEM. Aggregates were classified according to their petrographic analysis and mineralogical description, identifying deleterious minerals according to their textural and structural shape. The coarse aggregate has as main minerals quartz and feldspar, as secondary minerals mica, anfibolite and opaque, showing as reactive components quartz and feldspar with undulatory