The Restoration of Ancient Brickwork Chimneys

The restoration of ancient brickwork chimneys

D. ~osia',G. pistone', G. ~iva~& A. violante' ' Department of Structural Engineering, Politecnico di Torino, Caste110 del Valentino, Turin, Italy

' Departnietit of Construction Processes in Architecture, Venice University Institute of Architecture, Tolentirzi, Verzice, Italy

Abstract

The debate concerning themes of the conservation of the obsolete building heritage is particularly lively today, ranging from interests more strictly associated with museums (the safeguarding of machines, devices and tools; the documentation of productive cycles; period photography exhibitions) to the problem of the reuse of large empty containers - originally factories - in the urban fabric of large towns. Chimneys, much more than large building volumes? are fascinating "signs" of human activity in a past that is still recent, yet concluded. Unlike the large factory containers, these "points of reference" can no longer be recovered for functional use, at the most they can only act as a reminder of the past.

But the conservation comes up against quite difficult problems: stood abandoned for decades, they are nearly always in a state of advanced decay, against which the poor attention that has been devoted to them has prevented the development of an effective consolidation technique that would respect their original conception. They are continuously being demolished or cropped; only in rare cases are steps taken to reinforce them, applying means of consolidation that are frequently invasive and eye-catching. The research presented here aims to document what is left in two areas that have a large number of these structures (Piedmont and Veneto), and tries to develop a method of investigating their state of conservation which would allow prompt intervention to deal with their pathological condition, respecting the original structures as far as possible.

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1. Introduction

In a previous research presented at the STREMAH Conference in 1997, three chimneys were examined out of the many encountered in the study carried out on this aspect of the building heritage in Northern Italy [l] [2], as they were considered to be significant examples of a wide panorama as regards their structural characteristics. The chimneys were selected for their representative nature with reference both to the period of their construction and to the particular constructive features, to be subjected to F.E.M. analysis. The oldest of the structures analysed, the one in Venice, dates back to 1879, but was probably rebuilt in 1903 when the premises were enlarged, and was originally 35 metres tall, while the second chimney examined, of 1907, is one of a pair of twin chimneys located at Codigoro (Ferrara), each 55.0 metres tall; the third, at Valle Mosso (Biella), built in the period between 1920 and 1935, is 47.5 metres tall.

Our interest was particularly focussed on certain typical sources of stress, such as wind and extremes of temperature, which could have influenced the durability of the structures: an adequate understanding of their structural behaviour enables us to identify characteristic aspects of weakness and consequently to take action with maintenance work and reinforcement aimed at limiting the typical damage observed in chimneys, such as vertical cracks and the loss of straightness of profile. With ths aim another chimney of medium height was recently studied, which can be compared with the others. Before examining in details the results obtained in this last case, we shall briefly recall the conclusions reached with the structures previously examined.

2. Chimneys in Venice, Codigoro and Valle Mosso

Although all three chimneys examined are made with a double stack, as may be seen from the project drawings available for all the structures, and built with the shafts solidly set into the massive base ('justifying the assumption of the constraint condition of a truss embedded in the ground), their structural characteristics present significant differences which are also expressed in different structural behaviour. The chimneys in the nord-est of Italy region, of an earlier date, have external stacks characterised by a greater thickness (the chimney in Venice is the most squat). This circumstance allows the structures examined to be assimilated rather to the simple flue type, considering the reduced section of the inside flue and the fact that it does not present significant connections with the outside stack. The Piedmontese chimney at Valle Mosso, which is the most recent, has instead a structure composed of two very thin concentric rings, connected by vertical ribs and horizontal circular crowns of masonry so as to form a very light honeycomb structure. This results in very different forms of structural behaviour: a considerable rigidity and stability against overturning for the chimneys in Venice and Codigoro and, on the other hand, high flexibility with reduced overturning margins for the chimney at Valle Mosso. The vibration periods, in seconds with

respect to the first vibrating mode, were respectively 1.26 for the chimney in Venice, 1.59 for the one at Codigoro and 2.34 for the one at Valle Mosso. From the point of view of their state of preservation, none of the three chimneys presented significant lesions or signs of failure of the foundation at the time of investigation. The chimney at Codigoro was in an excellent state of repair, due to the maintenance work carried out and to the attention that it receives regularly, with the precaution of putting the two flues into operation from time to time, thus keeping at bay the well-known processes of decay caused by the frost and by the differences in temperature between parts that are differently exposed. Of the other two chimneys, the one in Venice especially presented accentuated phenomena of decay of the bricks and mortar on the northern side and at the base; the top of both chimneys had also been demolished for safety reasons (the one in Venice has been lowered to about 27.5 metres).

For all the examples considered, numerical simulation was carried out, checking - both individually and by overlapping the effects - the consequences of the following possible different actions: dead load, wind according to Italian rules, second category seism (corresponding to a conventional acceleration of a/g =

0.07) and temperature range in limit conditions when not in operation (winter shade -20°C and sun +20°C). For the materials, the following hypotheses were made, based on the experimental results obtained from similar masonry of the same period: Young modulus E = 3000 MPa, Poisson coefficient l/m = 0.2 and specific weight 18

KNI~~. All three chimneys are stable at their dead weight, confirming the good workmanship of their masonry. The thermal stress to which they are subjected after having been abandoned, above all in winter, produces quite high states of tensile stress which could have caused localised cracks, particularly on the north wall of the stack. This type of phenomenon has frequently been encountered in abandoned chimneys and suggests a possible explanation for the appearance of the characteristic vertical lesions found on the north wall of many chimneys. The analysis of the horizontal loads due to wind and above all to seism indicates perturbing states of stress which, while they could probably be absorbed by masonry in a good state of preservation during operation, allowing the redistribution of peaks of tension, lead us to be very prudent in considering the structural reliability of stacks that have remained out of use for a long time if they are subject to dynamic actions of this kind.

3. The chimney at Ceva

The brick chimney at Ceva (Piedmont - Italy) (fig. 1) was built in 1908 to serve the new mill of the "Anonima Cotonificio di Ceva" for the mechanical weaving of cotton, which initially provided employment for 400 male and female workers. The company had to close in 1931, after which the premises were used for other production purposes until 1979. Since then it has had no precise function but, in memory of its productive past, there remains on the territory the sign of this elegant brick chimney, 42.35 metres tall.

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It is a double stack chimney with a circular section and truncated conical vertical

development, made of shaped solid bricks and, with all probability, using external scaffolding, as is shown by the clear traces of "scaffold holes" on the whole height of the shaft, sealed with mortar. The chimney is made up of a base (h = 2.05 m), only slightly pronounced,

containing the access door for maintenance work, the shaft itself (h = 34.32 m)

and the top (h = 5.98 m), complete with coping. The base, which is cylindrical, has a constant diameter of 3.82 metres and is a few centimetres wider than the shaft. The internal flue remains cylindrical for the whole height and has a diameter of 1.34 metres. A service ladder made of metal rungs embedded in the internal masonry is still present, though unsafe. Today the truncated conical shaft is tilted towards the south, with a shift of the axis, from the base to the join with the top, assessed at 40 cm: the inclination causes a rigid rotation and is due to settlement of the foundations. The survey was very accurate as regards the external geometry, in particular the present deformation and the verticality; on the other hand it was not possible to inspect the inside of the flue, but only to view it very approximately through the bottom opening, as the access ladder is now unsafe. It would also have been interesting to investigate the cavity wall between the two stacks, especially to check the type of reciprocal connection. It is documented that the cavity wall was filled with concrete about forty years ago, at least in the bottom part, to improve the stability of the structure; unfortunately there are no precise data on the level reached by the filling, which was presumably kept fairly low, considering the difficulty of reaching high levels with the casting.

Figure 1: Chimney at Ceva

4. Numerical tests carried out on the chimney at Ceva

The tests were carried out with the F.E.M. Algor programme. Since there were no data on the internal parts of the cavity wall, we opted for a type similar to that documented for the chmney at Valle Mosso, which is geographically not far from the one at Ceva [3]. As we said in point 2, the structure was found to be very light: in the present case the horizontal connecting crowns were not reproduced. The two tubular elements that make up the structure were simulated with brick elements: the outer one has a truncated conical section with a constant thickness, while the inner one has a cylindrical section. In the absence of a direct inspection of the cavity wall between the two flues, 4 connecting crowns were hypothesised for a first series of models and 8 for a second series. The documentation available on similar cases often indicates an arrangement with 4 connecting septurns, but there is no lack of cases with double that number of elements and even more: the two hypotheses highlight specific features of behaviour that are quite marked in some aspects, to which we shall return shortly.

Tensor

Figure 2: State of tension at the base of the chimney

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1 Tensor

Figure 3: Cross section show vertical tensile

The models were composed respectively of 7452 nodes and 4440 elements and of 7628 nodes and 4784 elements. The assumptions concerning the simulated isotropic stress-resistant material with linear behaviour were as follows: specific weight 18 KN/~~,Young modulus E = 2500 MPa and Poisson coefficient llm = 0,25. The actions considered, besides the dead weight, were, firstly, wind, seism and temperature. We also wanted to simulate the deterioration of the material, especially on the north side, gradually varying the modulus of elasticity in a radial direction, so as to reproduce the deterioration of the mechanical properties of mortar and bricks as a result of exposure to severe atmospheric conditions.

5. Results of the research

The action of the dead load shows stress gradients that increase regularly downwards, reaching a value of 0.6 MPa, perfectly compatible with the material, for which a breaking stress of about 10 MPa may be estimated. This value is based on tests on masonry of the same period in the geographic area in which the chimney is located, performed with "strong" bricks and common lime mortar, well organised from the point of view of texture [4]. Wind action was presumed in agreement with Italian regulations: having defined the geographic area and the characteristics of the territory in the vicinity of the construction, the wind may be assumed as a static action which presses on the leeward side with 0.7 KN/~~and decompresses the windward side with half the value. The analysis shows a vast surface at the base of the chimney (fig. 2) which is certainly subject to a state of tension on the material, with stress values

varying between 0.15 and 0.45 MPa; the situation concerns both the base and, above all, the external stack. The concern produced by this static situation may be mitigated with the following considerations: the cross section of the plan (fig. 3) shows how the vertical tensile stress is limited to the external circular crown; consequently the centre of gravity of the resistant section still remains sufficiently within the perimeter of the chimney, though at the cost of maximum stress on the compressed edge, reaching values of 2 MPa;

the circular profile certainly attenuates the state of decompression on the windward edge, so the value prescribed by the regulations may be considered slightly overestimated. The result of the analysis is consistent with previous findings on the other chimneys and clearly stresses how the stability of these structures with respect to the action of the wind is guarantees in unsatisfactory conditions, due to the possible restriction of the section and the risk of the formation of horizontal lesions at the joins in the mortar. Since the latter are not generally visible in normal conditions, they certainly become a vehicle of aggression for mortar in conditions of damp and frost, favouring the gradual decay of the material, with loss of its mechanical characteristics. From the seismic point of view the chimney, characterised by periods of 2.44 s

(fxst mode)and 0.5 S (second mode) has a similar behaviour to the one at Valle

Mosso, with aggravation of the stability problem already mentioned in the event of wind.

Figure 4: Base of the chimney

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The effect of the progressive deterioration of the material (fig. 4) was considered, bearing in mind that the consequent worsening of the mechanical characteristics appears, as regards deformation, as a decrease of the modulus of Young: it is therefore reproduced by gradually lowering the modulus of elasticity along the perimeter on the northern exposed half of the chimney. The deformation gives a good visual reproduction of the curved vertical profiles to be seen on some abandoned chimneys (fig. 5). As regards resistance, clearly the vertical tension values, slightly varied with respect to the entire modulus, must however by compared with breaking tensions that are at times quite reduced with respect to the value of 10 MPa taken as reference.

The behaviour of the chimney exposed to heat effects appear to be very interesting. In the investigations carried out in the past, the heat effects were considered with reference to the differences in value that occur between the sector exposed to the sun and the sector exposed to the north, for example on particularly cold winter mornings when the weather is fine; states of horizontal tensile stress had been observed, which were certainly not very high, but potentially responsible for the onset of vertical cracks. Even during operation of the chimney the conditions may be no less severe; the temperature of the furnaces normally exceeds 200°C, quickly bringing the masonry of the thin internal masonry cylinder to the same value, while the air chamber between the two flues, even though interrupted by the presence of septums, remains at a temperature very close to that of the environment. The result of thls is the formation of a layer of joint action between the internal element which tends to expand the external one and the external one which opposes the movement.

Displacement

Figure 5: Curved vertical profile on abandoned chimneys

The comparison between the two deformations on the plan (fig. 6) shows how the arrangement with 8 septums guarantees a far more regular behaviour than the one with 4 septums. However, it is above all with regard to tensions that the greatest differences are seen. The joining of the septum on the internal flue produces, in the horizontal section of the flue, a state of biaxial compression in both arrangements, but causes tensile stress on the external flue in a direction tangential to the perimeter where the septums rest on the wall: in the case of 4 septums the tensile stress reaches values of 0.18 MPa against 0.05 MPa in the case of 8 septurns. In the intermediate sections between the septums the situation is inverted: on the external surface of the flue the values reach 2.2 MPa (4 septums) and 1.9 MPa (8 septums), while the internal surface remains compressed (0.35 MPa i 0.3 MPa). On the other hand: in the same sections the external surface of the bearing cone is compressed and the internal surface under tensile stress, with maximum stress values of 0.4 MPa in the arrangement with 4 septums. In brief we may observe how in both arrangements the flue tends to maintain a continuous inner ring that is all compressed, due to the state of joint action produced by the expansion prevented by the radial septums. The two principal tensions are both negative on the whole ring, which indicates a regular and fundamentally stable pressure curve. Thls favourable condition in obviously more accentuated in the case with 8 septums. Though the external flue is much more rigid, it is in a critical situation, especially where the septums join the flue, where there is high tensile stress, especially in the case with 4 septums: the pressure curve, which is very irregular, is not as stable as that of the internal flue, due to the free expansion that the external flue may perform. The condition observed may explain the appearance of vertical lesions, widely reported in the past in masonry chimneys and repaired by fitting metal rings encircling the outside.

Figure 6: Comparison between two deformations on the plan

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6. Conclusions

The studies carried out on chimneys in Piedmont and nord-est of Italy suggest the following hypotheses for interpreting the phenomena of obsolescence repeatedly observed in these structures, in particular the appearance of the typical curved profile, especially when they are no longer in use, and long vertical cracks on the external walls. The first phenomenon, which may be more generally associated with the physical and mechanical decay of the masonry, would seem to originate in the cyclical stress caused by the wind, which is able to cause horizontal micro-cracks in the joins in the mortar and thus favour aggression by atmospheric agents (particularly frost), especially in structures that are no longer in use. The second aspect would appear to originate in the strain generated in the two flues due to the connecting septums, when the difference in temperature between the two surfaces is relatively high (really high in normal working conditions). Widely spaced arrangements of ribs with a function of connecting rather thin stacks produce high horizontal strain on the external flue, with possible breakage of the material. Subordinately, the same effect could be produced, as documented by the research in [3],by the difference in temperature between the south and the north face, when subjected to relatively high At values.

References

[l] Pistone G., Riva G., Problemi di recupero delle vecchie ciminiere in opera laterizia: esperienze in Piemonte e nel Veneto, Costruire in laterizio no 79 gen- feb 200 1.

[2] Bosia D.,Pistone G., Riva G., Le ciminiere in opera muraria, Ambiente Costruito, no 1 2001. [3] Pistone G., Riva G., Zorgno A., Structural behaviour of ancient chimneys, 5' Conference on Structural Studies, STREMAH '97, Comp. Mech. Publications, Southampton and Boston, vol. 3, Advances in Architecture Series.

[4] Mattone R., Pasero G., Pavano M,, Pistone G., Roccati R., Prove sperimentali su campioni di varie dimensioni volte alla determinazione delle caratteristiche meccaniche delle vecchie murature, Proceedings at 6' International Brick Masonry Conference, Roma? 1982.