WTA–YPA 2026 in the category "Practical"
The International Association for Science and Technology of Building Maintenance and Monuments Preservation e.V. annually awards the WTA-Prize for outstanding achievements in the fields of research and practice of building preservation and monument conservation.
This year's WTA Young Professionals Award in the cathegory 'Practical' for the first time was awarded to
Mr Aurel Froitzheim for his project work
'Restoration Concept for the Bonner Tor (Engl. Bonn Gate), Erftstadt-Lechenich, Germany'.
In a short presentation, Mr Froitzheim presented the main aspects of his work.
You can find the complete work here.
Introduction
The Bonner Tor (English Bonn Gate) is a historic city gate located in the center of Erftstadt-Lechenich near Cologne. It forms the northeastern entrance to the medieval old town and is an integral part of the historic city fortifications. The property is owned by the City of Erftstadt, while professional supervision lies with the Rhineland Regional Association (German: Landschaftsverband Rheinland; LVR).
The reason for closer examination arises from noticeable structural damage, particularly on the street-facing northern side of the gate.
As a central component of Lechenich’s medieval fortifications, the Bonner Tor reflects one of the most significant architectural monuments in the region. As a prominent city gate whose core structure dates back to the 13th–14th centuries and which has undergone multiple later alterations, it represents a highly complex object for architectural, material, and restoration research.
The objective of the master’s thesis/ project assignment within the Master Professional for Restoration of Historic Monuments program was to develop a well-founded monument restoration concept based on a comprehensive condition assessment, building-physics and material-science investigations, and a monument restoration evaluation. The study comprises:
- Historical contextualization of the gates structure
- Complete documentation of the existing building structures
- Technical and restoration-related investigations
- Damage analysis and identification of causes
- Monument restoration assessment
- Development of a restoration concept
- Manufacturing and examination of sample panels and test specimens
Historical Background – Key Events
ca. 1250-1350 First documented mention - Construction of the city fortifications
1642-1726 Destruction during the Thirty Years’ War - Replacement of the drawbridge with a stone bridge
1851-1897 Conversion into a prison - Vertical extension using brick masonry
1900-2023 Urban planning adaptations - 2005: most recent restoration measure
Localization of the Area of Investigation
The area of investigation comprises the street-facing northern side of the Bonner Tor, with particular focus on the damaged plinth zone. The examined building component consists of a roughly 1.5 m thick, double-leaf masonry structure with a loosely filled core.
The exterior surface shows an irregular mixture of clay brick and natural stone, arranged neither homogeneously nor in a clearly stratified manner. Varying stone formats and a heterogeneous combination of stone types indicate multiple construction phases and later additions.
Status assessment and mapping of damage
Observed damages include:
• Washed-out joints, loose mortar
• Weathering and loss of individual stones
• Plant growth in the joints
• Surface deposits
Exemplary Examinations
Joint Damage and Salt Contamination
Research question: Why are joint damages concentrated between heights of 0.50 and 1.30 m?
Method: Analysis of damaging salts
Result:
• Highest concentrations of damaging salts were detected between 0.5 and 1.3 m.
• Joint deterioration is likely associated with salt contamination,
presumably indicating rising damp and/or hygroscopic moisture.
Mirowski Pipe Test
Research question: Since deposits were detected: Were the masonry surfaces hydrophobized during a previous intervention?
Method: Investigation of capillary absorption behaviour using Mirowski pipes
Result:
• No hydrophobic agents were detected
• Significant differences in the absorption behaviour of the stones were observed
• Surface deposits influence absorption behaviour but are located outside the primary damage zone
Summary: Technological Assessment
| Damages | Causes | Findings |
| Washed-out joints, loose mortar | High salt load (chlorides and sulfates), e.g., due to road salt and emissions | Excessive load for the existing restoration mortar with white hydrated lime |
| Surface deposits | High moisture exposure (splash water) | New mortar approach, oriented towards the present structure: Option 1: hot-lime mortar Option 2: lime putty mortar |
| Weathering and loss of individual stones, respectively | Potentially incompatible mortars from previous measures (air lime mortar with brick dust) | Leave deposits unchanged, as they are not in the damaged area of the joints. |
| Plant growth in the joints | Remove damaged individual stones/bricks and replace them with suitable clay bricks |
Restoration Concept – Specific Measures
| Joint Repair | Stone Replacement | Maintenance |
Loose joint material is to be Sampling of mortars and | Only in cases of severe material loss: • Use of suitable reclaimed bricks • Preservation of existing natural stones wherever possible (consultation with a stonemason recommended) | Removal of vegetation Regular inspection of joint conditions and repair as required |
Execution
The practical implementation of the restoration concept was performed on sample panels, one for each mortar variant.
The execution process can additionally be viewed via the accompanying QR codes:
| Preparations | Variant 1: Hot-Lime Mortar | Variant 2: Lime Putty Mortar |
| https://youtu.be/FG0xMIPvn0c | https://youtu.be/i2C3iJGshis | https://youtu.be/AI5P-8BkW7I |
Process Steps
1. Production of Sample Panels
a) Documentation of the initial condition and existing damage
b) Sample panel using lime putty mortar
c) Sample panel using hot-lime mortar
2. Raking out damaged joints and removing loose mortar
3. Removal of weathered stones
4. Cleaning and pre-wetting of the masonry surface
5. Installation of individual stones using hot-lime mortar
o Use of suitable reclaimed bricks from a comparable period of
manufacture
| Variant 1 – Hot-Lime Mortar | Variant 2 – Lime Putty Mortar | |
| 6. | Grouting joints Mixed in small batches and applied immediately at temperatures above 40 °C: • 4.5 parts sand (0–8 mm) • 0.5 parts brick dust • 1.0 part quicklime CL90 • 1.75 parts water Applied in layers by hand pointing. | Dry-slaked in small portions in advance in the mixing vessel and applied the following day: |
| 7. | Post-processing Joints are lightly re-wetted after application and finished using a wooden jointing tool | After several hours of drying time, joints are roughened and reworked with a jointing tool |
| 8. | Follow-up procedures The sample panels are covered with jute fabric and kept moist at regular intervals | |
Comparison of Condition
Before and After Treatment
Laboratory Investigation
Preparation of Test Specimens
Strength Development of the Mortar Variants
Research Question: Does the hot-lime mortar develop excessive strength? (Target compressive strength approximately 2–8 N/mm²)
Method: Compressive strength testing of mortar prisms
Result: Neither mortar variant exhibits excessive strength. The target strength range is likely achieved by both systems.
Advantages and disadvantages of the mortar variants
| Variant 1: Hot-Lime Mortar |
| Advantages |
| Application: • Higher early strength enables efficient pointing of deep joints • Rapid setting allows early finishing and secure placement of individual stones • Low shrinkage tendency |
| Laboratory Results: • Water absorption comparable to brick masonry, reducing stress between stone and mortar • Mechanical strength compatible with historic brick masonry • High elastic modulus, potentially allowing load-bearing functions • High capillary-active porosity enabling damage-free salt crystallization and accelerated drying |
| Disadvantages |
| Application: • Increased time pressure due to rapid setting • Enhanced occupational safety requirements when handling quicklime |
| Laboratory Results: • Very high capillary-active porosity may lead to increased risk of frost damage • Slightly improved, but still low, bond strength compared to lime putty mortar |
| Variant 2: Lime Putty Mortar |
| Advantages |
| Application: • Longer open time allows controlled and large-scale application • Reduced occupational hazards during handling |
| Laboratory Results: • Mechanical strength compatible with historic brick masonry |
| Disadvantages |
| Application: • No possibility of spontaneous re-mixing • Difficult adjustment of consistency; risk of soiling stone edges • Risk of shrinkage cracking in deep joints, requiring layered application |
| Laboratory Results: • Low bond strength; test specimens disintegrated upon removal |
Conclusion
Air-lime mortars are suitable for use in historic masonry only when understood as sacrificial materials.
Their high porosity, moisture absorption capacity, limited bond strength, and moderate mechanical strength serve to protect historic masonry from moisture and salt damage, allowing replacement when necessary.
Recommended Approach
A combined application of hot-lime mortar and lime putty mortar on the Bonner Tor as a suitability test:
• Hot-lime mortar for deep preparatory pointing and stone replacement due to its higher early strength and
potential load-bearing capacity
• Lime putty mortar for surface-level final pointing due to its longer open time and safer handling
Potential for Optimization
Improved plasticity, the use of less reactive limes, and the addition of pozzolanic materials could enhance durability. Further suitability testing is required.
Outlook
Sustainable restoration requires regular maintenance, continuous monitoring, and, where necessary, the ongoing development of appropriate mortar systems.