In a previous contribution (Van Balen 2003) we have advocated to integrate the chain of preservation into the authenticity debate of conservation. A particular role was identified for "workmanship".
A study on repair of ancient masonry structures of a fortress (Fort IV), part of the great defence works in Belgium in the middle of the nineteenth century in Mortsel, close to Antwerp in Belgium and the execution of the repair work, have shown that not only analysis is required for identifying mortar to be used for repairing historic masonry, but that also the exchange with craftsmen is essential in the final decision on the mortar composition to be used. The fortress corridor around Antwerp was one of the largest construction sites in Europe in the middle ofthe nineteenth century. Documentation on the construction technologies used those days is available and was used in the technical analysis preceding the conservation works.
In the second part the remarkable consensus on workmanship of masonry mortar between masons from various origins is explained. Ongoing laboratory study on workmanship of mortars revealed that when masons working in different geographic locations (three from Flanders, one from Wallonia, one from the Netherlands and one from France) are requested to identify workability of mortar, they seem to agree quite well. Their agreement seems to be more coherent than standardized tests do. This finding together with the previously explained experience indicates that workmanship is to be considered into the preservation process, as many other interdisciplinary issues.
Evaluating the heritage values of an architectural fabric, craftsmanship embedded in it should be considered, but also the continuation of practice has heritage values and is part of the preservation process. Not all available workmanship is compatible with this process but it can be made compatible if the context is appropriate.
2 REPOINTING WALLS IN FORT IV 2.1 Construction of Fort IV
Fort IV was part of the defence system the Belgian Government built in the middle of the XIX°C to make a stronghold (National "Reduit" or bomb-free shelter) in Antwerp for the ultimate retreat of its government in case of Belgian's occupation. Antwerp was chosen as it was situated on the Scheldt River that could provide an escape route to the open sea and to England (Gils 1997). Antwerp was also one marching day further from the French and German border than Brussels. A series of eight fortresses at the south and east of Antwerp was planned in 1859, later in the nineteenth century extensions of the system were planned at the west side of the river and to the North East.
Construction works started in 1860 at Fort III, shortly followed by the other forts based on the job description made by the Ministry ofWar in 1859. This document is a valuable tool of information to understand the construction technology and the materials used at that time. It should be understood that at that time this was one of the largest construction sites in
Europe. It required a lot of building material, the organization of many brick factories in the clay ground in the valley of the Scheldt River.
Fort IV was one of the last fortresses to be vacated by the Belgian Army at the end of the twentieth century. It is now property of the local town of Mortsel and managed as a recreation area with social and cultural functions. Also conservation of nature is considered of great importance, sometime counteracting the need for preservation of the built fabric. One example of the latter is the conservation of the trees on the vaulted underground construction while their roots damage the original water evacuation system.
Within a EU Interreg IIIb project "Crossing the Lines" a methodological approach of repointing has been experimented within the project activities dealing with restoration techniques. The findings hereafter are results from that project in which the Department of Civil Engineering and the Raymond Lemaire International Centre for Conservation at the K.U. Leuven were involved as requested by the Town of Mortsel.
The research presented here is about the re-pointing of the facades of the fortress. Beside the study of the re-pointing investigation was carried out on the production of replacement bricks as well as on climate control in these constructions with particular boundary physical conditions: massive walls covered by earth and with limited possibilities for aeration.
For the repair of parts of the masonry repointing was necessary. The study and the application of the mortar that would closely match with the existing mortar and that could replace the original mortar, involved the following steps:
1. Historical research on the used construction materials and on lime mortar technology.
2. Determination of the composition of the original mortar through analysis.
3. Determination of the properties of the original and repair mortar.
4. Formulation of the repair mortar based on above conclusions and interpretations.
5. Hiring skilled craftsmen and suitable technology, in order to assist developing and
6. Applying repair mortar.
The 1859job description and some comparative material allowed understanding the types of materials used and the way they were applied in the construction of Fort IV This information was however not univo-cal as some of the descriptions could be interpreted in different ways. It is also known from construction practice, that what is executed does not always correspond with what was prescribed. Therefore analysis on the materials themselves could complete the picture.
The work prescription included the following articles that are relevant for understanding the mortar used:
Art 2: Mode of execution.
§9 Masonry: foundation walls and underground masonry are made from stone (Fr.: moellons), from concrete or bricks; all other masonry will be made from bricks. Shall be used: ordinary mortar, cinder track (Fr.: cendrée)-mortar or hydraulic mortar according to the indications given by the commander of the engineer corps. The vault covering (Fr.: chape) are made from hydraulic mortar.
Art. 3: Materials to be delivered by the contractor:
§11 Lime - lime for masonry will be hydraulic with similar quality as lime from quarries from the neighbourhood of Tournai; reduced (Fr.: réduite) into putty, plunged immediately in water, and subjected to the needle test (Vicat Needle?) it will have to set after 24 hours under water.
§12 Sand - the necessary sand for the preparation of the mortar will be siliceous, free from every mixing, without clay, it will feel coarse and will be sieved (Fr.: passé à la claie); sand for foundations will be without mixing from earth or clay.
§13 Cinder track (Fr.: cendrée) - ashes will come from factories and mainly from forges, it may not be mixed with other substances; it will be sieved.
§14 Hydraulic basis - the substances that will serve as a hydraulic basis will be subjected to the following tests:... mixed evenly with ordinary mortar and subjected to the needle test, they will have to set after 48 hours of submersion
§16 Bricks - bricks for a certain construction will derive from the same producer (Fr.: fabrication), they should have similar dimensions and appearance;
§18 Mortars - ordinary mortars will be composed of one part of slaked lime (Fr.: chaux éteinte) and one part of sand.
Information on mortar composition could be summarised as follows:
- Cinder track mortar will be made from two (2) parts of extinguished lime, one (1) part of sand and one (1) part of cinder track;
- Hydraulic mortar 1 (for drowned works) will be composed of one (1) part of trass of Andernach (or another hydraulic basis approved by the War Department); (this trass is a pozzolanic material exracted in Andernach, Germany)
- Hydraulic mortar 2 (for vault capping) will be made of six (6) parts of slaked lime, one (1) part of sand and four (4) parts of Andernach trass (or another hydraulic basis approved by the War department);
Ambiguity and freedom of action is also included in the descriptions as:
- In case exceptional circumstances require, the use of other type of mortar or the dosage will be determined by the commander of the engineer corps.
- If the mortar is prepared by labourers, it will have to be mixed and will be beaten during two days, it will be used the third day after it has been re-beaten (with as less as possible water) as to gain a degree of liquidity (workability?) for its use.
- Mortar preparation can be accelerated using machines; in that case the commander of the engineer corps willjudge on the modifications that have to be carried out on the previous stipulation.
Those historical descriptions elucidate which materials were used, how they were mixed and in which proportions. Aspects of workability could also be found in conjunction with the preparation of the mortar. As well workability as other circumstances allowed for adjustments on the prescribed methods. It is clear that other information available today is necessary to decide on the composition of the repointing mortar.
2.2.2 Chemical analysis of mortar
Samples for further analysis were identified and were extracted from the building.
- M1 Bedding mortar from sampled core 1.
- M1p Pointing mortar from sampled core 1.
- M2 Bedding mortar from sampled core 2.
- M4 Bedding mortar from sampled core 4.
- M5 Bedding mortar from sampled core 5.
- M7 Bedding mortar from sampled core 7.
- M8 Bedding mortar from sampled core 8.
- M8p Pointing mortar from sampled core 8.
Results of the chemical analysis are given in Table 1. Proper interpretation of those results also requires the evaluation of the thin sections which were made from some samples.
From the analysis possible theoretical composition of the original mortar could be estimated. Today's reference materials were used for the calculation taking profit of a program developed for that purpose. Based on the possible mixtures that fit with the chemical analysis the following conclusions were drawn:
1. There is a difference in mortar composition between the bedding and the pointing mortar;
2. The bedding mortar has approx. 50 weight% of sand and 50 weight% of binder and most mortars have a moderately hydraulic binder (except M7).
3. The pointing mortar is richer in binder and as will be explained later different sand with less glau-conite was used. The pointing in an average has 67 weight% of binder and 33% of sand and the binder is moderately hydraulic. The difference in sand may also be an indication that pointing was carried out in another construction phase.
In the next table a re-composition is given based on the combination of lime hydrate and trass. In the last column an alternative binder composition is given for which a higher fraction of the binder can be recomposed. The compositions are theoretical as depending on the circumstances of hydration versus carbonation but also depending on the composition of the raw materials that have been used; the final chemical composition may alter for a same mortar composition.
From written historical sources we should conclude that normal mortar (including pointing?) should be made from 1 volume of lime and 1 volume of sand which in terms of weight proportions mean 30 weight% of dry lime hydrate and 70 weight% of sand. This is different from the results found in the chemical analysis that would correspond to 2.5 volumes dry lime hydrate to 1 volume of sand. However the description makes reference to lime putty, made from hydraulic lime. This means that the density of the lime will be higher and that the water fraction of the putty is considered part of the binder.
The preparation: slaking, beating of lime for 2 days includes the risk that if the lime contains a considerable amount of hydraulic and reactive particles, they might hydrate and turn into granulates (they will not be able to contribute to the binding anymore) which changes the apparent proportions as identified from the analysis.
2.2.3 Thin sections of mortar including mortar-brick interface The thin sections have been prepared from the samples taken from the masonry. As a general outcome of the
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