INTERNATIONAL SCIENTIFIC CONFERENCE
CIBv 2010
12 – 13 November 2010, Braşov
EXISTING CONCRETE SILOS – LESSONS FROM INSPECTION,
DIAGNOSIS, REHABILITATION AND STRENGTHENING
Augustin POPAESCU*, Ovidiu DEACONU**,
*Faculty of Civil Engineering, Universities Ovidius Constanta and Transilvania Brasov, Romania
**Faculty of Civil Engineering, University of Transilvania Brasov, Romania
Corresponding author: Augustin Popăescu, E-mail: popaescu@gmail.com
Abstract: The paper presents the behavior of some prestressed soya bean silos and
of two other reinforced concrete grain silos, in service for more than 25-30 and 40
years respectively. All of these silos have suffered multiple durability
deteriorations for which an in-depth inspection was performed. Studies focused on
diagnosis and level of deteriorations, on rehabilitation and strengthening and even
replacement of prestressing system. A decision as to level of intervention took into
account the estimated remaining service life of each silo. The paper deals with the
practical application of repair and strengthening methods and it also recommends
inspection intervals.
Keywords: concrete silos, inspection, diagnosis, rehabilitation, strengthening.
1. INTRODUCTION
The first part of the paper presents two cases of soya beans coupled silos of seven hexagonal
cells each made of prestressed concrete and the second part presents two cases of grain silos made
of reinforced concrete and having cylindrical cells.
An in-depth inspection was performed for each silo, followed by a diagnosis of deteriorations
identified and recommendations for rehabilitation and strengthening.
The practical application is presented with the focus on repair and strengthening methods and
recommendations are made as to inspection intervals.
2. FIRST PART - COUPLED SILOS OF HEXAGONAL CELLS
The two coupled soya beans silos from the oil factories in Buzau and Urziceni consist of seven
hexagonal cells each (figure 1). The side of each hexagonal cell is 3.75 m and about 32 m in height.
Figure 2 presents a horizontal cross section.
256 Augustin POPAESCU, Ovidiu DEACONU
Fig.1 View of coupled silos Fig.2 Cross section of hexagonal silo
The structure was achieved in 70s, by slip form method of concrete walls with the planned
C16/20 class concrete. The prestressing operation provided strands of 12 mm dia., of two half ring
anchored alternatively on concrete blocs. Their protection was provided by coatings for strands
(multiple layers) and by concreting for anchorages.
2.1 Inspection
After 25-30 years of service some broken strands were noticed and an in-depth inspection was
necessary using climbing platforms for access to the wall at different heights.
The in-depth inspection covered the concrete itself and the prestressing system, including
vertical bearing concrete ribs and blocs, strands and anchorages.
The results of inspection on the silos made after about 25-30 years of service showed that the
prestressing system presented significant deteriorations due to the corrosion of strands and
anchorages, which were similar in all four hexagonal cells batteries.
The next figures show various zones of intensive corrosion on the strands bearing support –
fig. 5, at the strands exit from blocs – fig.4, the anchorages in blocs after removing the concrete
protection – fig. 3, 6, 7, and 8.
Fig. 3 Blocs examination Fig. 4 Strands exit from blocs Fig.5 Strands on the bearing zone
Existing concrete silos – lessons from inspection, diagnosis, rehabilitation and strengthening 257
Fig.6 No sensible anchors Fig.7, 8 Anchorages with sensibility to corrosion (hardened only)
Behavior differences can be observed between the anchorages no sensible to corrosion – fig.6
and the anchorages more sensible to corrosion like in hardened steel only – fig. 7, 8.
It was also found that it was impossible to provide a trust protection by coating on more than 1
km of strands for one hexagonal silo.
2.2 Diagnosis
The hexagonal silos showed a significant deterioration of the prestressing system due to the
corrosion of strands in local zones and of anchorages, main causes[3], [4] of which are:
- defective protection of strand at the exit from blocs and on the bearing zone; impossible
application of coating behind the strands which practically remain uncoating;
- low quality, noncompact concrete for anchorages protection;
- use of hardened steel anchorages with a high corrosion sensitivity.
Many of strands and / or anchorages shoued an in-depth deterioration, that can strongly affect
the safety of the prestressing system and of silo itself, as well as the safety in service. This led to the
decision to replace the prestressing system so as to conserve the initial design service life.
2.3 Substitute prestressing system
The substitute prestressing system consisted in strands provided in polypropylene tubes filled
with protection stable grease. The anchors leaned on additional bearing steel blocs – fig.9 and were
protected by sleeve nut filled with grease – fig.10.
Fig. 9 Additional bearing steel blocs Fig.10 Anchorage protection in sleeve nut
258 Augustin POPAESCU, Ovidiu DEACONU
The in-depth inspection was performed in two cases after 3 or 5 years intervals from the
replacement of prestressing, providing access along height of the wall. The inspection results
showed a good behavior of the system – see fig. 11, 12, including anchorages leaning on bearing
steel blocs and protected by sleeve nuts filled with grease.
An exception was found in a few local zones of tube hose connectors where a short movement
of tube appeared due to temperature variation. The tube hose connectors were repaired.
It was recommended to carry on surveys, such as in-depth inspection, at 3 to 5 years intervals
after the application of a substitute prestressing system.
Fig. 11 Actual view of strands in tube and Fig.12 Grease state of anchorage after
anchorage sleeve nuts removing sleeve nut
3. SECOND PART - REINFORCED CONCRETE CYLINDRICAL CELLS SILOS
The first grain silo from Urziceni storage base built 25 years before the inspection is composed
of three batteries of 12 cylindrical cells, one cell of 7 m in diameter and 36 m in high, with the wall
thickness of 18 cm. The slip form method of construction was used .
Multiple deteriorations, especially noted in one battery, mainly affected the concrete cells
wall, having an advanced state of cracking and numerous deficiencies, such as lack of compactness,
due to the slip form method of concrete walls construction.
Fig.13 Urziceni silo Fig. 14 External cell surface before inspection
Existing concrete silos – lessons from inspection, diagnosis, rehabilitation and strengthening 259
The second silo from Dragos Voda storage base, built 40 years before the inspection is
composed of 6 cylindrical cells, one cell having a diameter of 7.30 m and 20 m high, with a wall
thickness of 15 cm. Two stellate cells are formed between the cylindrical cell walls.
Fig.15 Dragos Voda silo Fig.16 Deteriorations on external cell surface
The main deteriorations of this silo were observed to concrete and reinforcement on external
cells surface only.
3.1 Urziceni cylindrical cells silo
Inspection: An in-depth inspection was performed using climbing platforms for access to both
sides of wall, including internal one and was made including hammering the concrete and removing
the detachable or broken layer of cement mortar and detecting the entire zone of damaged concrete.
Fig.17 Deep deteriorations in concrete Fig.18 Detached bars cover, cavities
The examination started from the signs of visible deteriorations, which consisted in developed
cracks of more than 0.3-0.5 mm, detached or broken cement mortar and even deep deteriorations of
concrete.
The inspection showed deep deteriorations in concrete, multiple local small deteriorations,
such as cavities, holes (some of them pierced), detached bars cover, inconsistent concrete, unfilled
holes provided for slip form supporting bars – Fig.17, 18, 19, 20, 21.
260 Augustin POPAESCU, Ovidiu DEACONU
Fig.19 Hole, view from outside Fig.20 Hole, view from inside Fig.21 Deep damages in
concrete
Diagnosis of deteriorations: The slip form casting of the planned C16/20 class concrete in
cells wall was unfortunately defective and the main concreting deficiencies were:
- portions of concrete cast-in place after setting strength started;
- local cavities or casting interruptions;
- segregation areas (lack of compactness), impurities.
These deficiencies were the main cause of the further deteriorations, such as:
friable / expelled concrete, concrete settling; cracks, deformations;
buckling of reinforcing vertical bars (sometimes placed outside of horizontal ones);
reinforcement corrosion (not active).
Alternative repair / strengthening solutions: The defects and deteriorations may further
adversely affect the structure function and its strength and stability [7] .
The proposed strengthening method in similar cases was to cast an interior reinforced concrete
layer, i.e. a supplementary reinforced concrete cell, but in this case the application of an interior
concrete layer to each cell implies a lot of concrete and steel, with high costs.
For the actual state of deteriorations and having in mind the repair / strengthening cost
reduction, a restoration of the structural concrete continuity was considered alternatively as a
limited repair / strengthening, adequate for an additional shorter service period and lower expenses.
For the structural concrete repair and strengthening, we recommended different procedures,
using special reliable chemical products which become available in the country at the time.
So, for filling of all the cavities or holes, the concrete mix of concrete class C20/25 included
aggregates of to 3mm or 7mm, an acrylic bonding agent to be applied on old concrete surface and a
super plasticizer, so as the concrete vibrating became not needful. Other small deficiencies of the
concrete, as well as of the reinforcement concrete cover, were repaired using epoxy bonder or
epoxy concrete. The cracks exceeding 0.3 - 0.5 mm were injected using low viscosity epoxy resin.
3.2 Dragos Voda cylindrical cells silo
Inspection: An in-depth inspection was performed using a kind of nacelle crane used for
access on electrical poles, removing the detachable cement mortar.
Fig.22 View of outside affected zones Fig.23 In-deep reinforcement corrosion
Existing concrete silos – lessons from inspection, diagnosis, rehabilitation and strengthening 261
Fig. 24, 25 Expelled concrete in zone with reduced concrete cover
The main concrete and reinforcement deteriorations were observed only on the outer side of
the cylindrical walls, especially at form panel joints and they consisted in:
- detachments of the reinforcement concrete cover ,where the thickness measured 5-15 mm;
- segregation areas;
- adherent or in-deep corrosion of the reinforcement in this zone;
- concrete deterioration due to freeze-thaw, see Fig.22, 23, 24 and 25.
Diagnosis of deteriorations: After about 40 years in service the Dragos Voda silo showed a
good behavior in terms of strength and stability, less in terms of durability.
These deteriorations were caused by the external whether action combined with an insufficient
concrete cover of outside layer of the reinforcement and of concrete segregation.
Repair solutions: The repair solutions for durability deteriorations were aimed at ensuring the
intended remaining service life of the silo.
An evaluation of the maximum load capacity showed insufficient design reinforcement of the
two stellate cells and the need for these cells to be filled up to 50% of capacity.
4. CONCLUSIONS
All silo structures examined have suffered in terms of durability, but no other damages or
other deterioration signs from seismic action or differentiated settlements were found.
The hexagonal prestressed concrete silos
The hexagonal prestressed concrete silos suffered, after about 25-30 years of service,
from multiple durability deteriorations of prestressing strands and anchorages.
The causes for the corrosion of strands in local zones and of anchorages were the defective
strands protection by coating, low quality, non-compact concrete for anchorages protection, use of
anchorages with a high corrosion sensitivity.
The decision to replace the prestressing system was taken for the purpose of conserving the
intended design service life of the concrete silo.
The chosen substitute prestressing system consisted in strands in polypropylene tube filled
with protection stable grease. The anchors leaned on additional bearing steel blocs and were
protected by sleeve nuts filled with grease.
The inspection made in two cases after 3-or 5-year intervals as from the replacement of
prestressing system showed a good behavior of the system .
262 Augustin POPAESCU, Ovidiu DEACONU
It was recommended that surveys, such as in-depth inspections, should further be made at 3-5
year intervals after the substitute prestressing system application.
The cylindrical reinforced concrete silos
In the first case of Urziceni grain silo, the casting in slip form of concrete in a cells wall was
unfortunately defective, followed by portions of concrete cast in place after the setting strength
started, local cavities or casting interruptions, segregation areas (lack of compactness), impurities,
friable / expelled concrete, concrete settling.
The limited strengthening to reestablish the structural concrete continuity appeared to be a
more favorable solution and was recommended for a shorter additional service period and lower
expenses.
The use of modern chemical products to provide high bond and strength levels between old
and new materials and of completion materials offers a reliable rehabilitation solution for a limited
strengthening of grain silo cell.
In the second case of the Dragos Voda storage base silo the main concrete and reinforcement
deteriorations were located only on the outer side of the cylindrical walls, especially at form panel
joints, the main causes being the external whether action along with the insufficient concrete cover
of the outside layer of reinforcement and concrete segregation.
For the repair of structural concrete we recommended different procedures using special,
reliable chemical products, such as epoxy bonder or epoxy concrete and an outside concrete surface
protection by impregnating it with chemical products supplied by SYMONS - DAYTON Corporation
USA [6].
REFERENCES
1. CEB Bulletin 243 (1998), Strategies for Testing and Assessment of Concrete Structures, Lausanne,
May 1998.
2. fib CEB-FIP, Structural Concrete, bulletin 3, Lausanne, December 1999.
3. POPAESCU A. Extended inspection of durability of existing concrete structures, FIP Symposium,
London, The Concrete Society, Slough, U.K., 1996.
4. POPAESCU, A., Relation between quality control and durability of prestressed concrete units,
Simpozion International Quality Control of concrete structures, Rijks Universitait Gent,12-14 June,
1991.
5. POPAESCU A., DUMITRACHE I., MATEESCU I., Rehabilitation of a concrete chimney by
prestressing rings, The 1st fib Congress, Osaka, Session 8 Durability, 2002.
6. SYMONS Chemical Products Handbook, 1993, Symons Corporation, Des Plaines, IL 60018, USA
7. POPAESCU A., DEACONU O., Rehabilitation of a grain concrete silo constructed by slip form
method, The International Conference on Concrete Repair, Rehabilitation and Retrofitting, Cape
Town, South Africa, 21-23 Nov. 2005, Taylor & Francis/Balkema, Leiden, The Netherland, ISBN
0415396549.
8. POPAESCU A., GEORGESCU D., SCHIAU AM., SVEICA V., Inspection and diagnosis for
durability of existing reinforced and prestressed concrete structures în Romania, Civil
Infrastructures Systems for the Next Century, US National Science Foundation & Polish Academy
of Science, Cracow, Poland, October, 1996.
9. DEACONU O., Study concerning the behavior to durability of the structures from reinforced
concrete subdued of natural conditions of exploitation, PhD thesis, University of Transilvania
Brasov, Romania, 2009.
Received September 20, 2010
