Protection of aged concrete structures: application of bio-based
impregnation system
Virginie Wiktor1*, Henk M. Jonkers1
(1) Delft University of Technology, Delft, The Netherlands
Abstract: This paper focuses particularly on the ageing of concrete due to micro-crack formation or freeze/thaw which results in an increased permeability of the concrete. The bacteria-based repair system presented in this paper aims at recovering the concrete permeability thanks to bacteria-induced calcium carbonate precipitation inside cracks/porosity.
The performance of the bacteria-based repair system in laboratory and field application were very promising. The laboratory results showed that the crack sealing capacity of the repair system is very good as the cracks were completely sealed after impregnation. Results from field application are also very good as treated concrete had a significant higher resistance to freeze/thaw and only cracks that were not impregnated with the bacteria-based repair system were still heavily leaking.
Keywords: Concrete, crack-repair, bio-based system, biodeposition
1
Introduction
Concrete structures are, in the course of their lifetime, constantly ageing. They are exposed to a number of degradation processes such as carbonation, chloride ingress or freeze/thaw, which directly impact and degrade the properties of the material. Nevertheless, concrete structures can reach a service life of 50 years or longer as these ageing mechanisms are taken into account in the codes and predictive models used in the design phase of the structure.
However, early-age deterioration is not easy to predict and the formation of micro-cracks for instance lead to a significant increase in the permeability of the concrete. The subsequent ingress of aggressive corroding substances can lead to the premature corrosion of the reinforcement and early failure of the structure.
To maintain the integrity of the concrete structure measures for maintenance and repair have to be undertaken. Nowadays a wide range of repair products, such as for instance epoxybased fillers or silane-based water repellent, is available for concrete. However, besides their high cost, their short term efficiency and negative impact on the environment are an issue for the repair industry. Biodeposition, a method by which calcium carbonate (CaCO3) precipitation is induced
by bacteria, has been proposed as an interesting alternative approach to protect building materials against ageing.
Among the various pathways involved in Microbial Induced Precipitation (MIP), enzymatic hydrolysis of urea in calcium rich environment is the most commonly used system. While it has been successfully applied as surface treatment in practice to limestone monuments, it has been considered only on a laboratory scale for cementitious material and crack repair. Also, besides cost issues, MIP using ureolytic bacteria might generate other problems, such as environmental nitrogen loading due to the production of ammonia during the hydrolysis of urea or negative effect to the material itself due chemical reactions with ammonium salt [1]. In addition, the time
*
required for substantial amount of bacterially induced calcium carbonate may hold back the acceptance of MIP as efficient repair technique by the building industry.
Another interesting pathway for MIP is the metabolic conversion of organic salts through bacterial respiration. Successfully applied in self-healing concrete [2], this concept has been implemented by the authors for the development of bacteria-based repair system for concrete structures. In this way, a liquid ingress system for concrete transports a bio-based agent into aged porous/cracked concrete which results in reduced permeability and increased service life of constructions. The performance of the bio-based repair system in laboratory and in practice on 2 aged concrete parking garages are presented and discussed in this paper.
2
Materials and methods
2.1 The bacteria-based repair system
The bacteria-based repair system combines advantages of both, traditional repair system for concrete (fast reacting, and short term efficiency), and bio-based methods (more sustainable, slow process, and long-term efficiency).
The repair system consists in concrete compatible bacteria [3] and feed which produce calcite-based minerals decreasing concrete porosity. This system is composed of two solutions:
(i) Solution A – sodium silicate (alkaline buffer, 4.8 g/L), sodium-gluconate (carbon source for bacteria growth, 125 g/L), yeast extract (vitamins for bacteria, 1g/L), alkaliphilic bacteria (1.6x108 spores/L).
(ii) Solution B – calcium lactate (calcium for CaCO3 precipitation) or calcium nitrate (nitrate
source for denitrification when O2 is depleted and calcium for CaCO3 precipitation,
500g/L), alkaliphilic bacteria (1.6x108 spores/L).
The denitrification is the biological reduction of nitrogenous oxides to gaseous products during an aerobic (no oxygen) bacterial growth. This means that under the metabolic conversion of calcium nitrate, N2 and CaCO3 are produced.
The sodium silicate in solution A ensures alkaline pH in the system and formation of a gel inside the crack. Although not very strong, this gel allows a rapid sealing of the crack (within few hours) and optimum environment for bacteria to precipitate calcium carbonate. By the time the gel becomes too weak, substantial amount of CaCO3 has been precipitated to seal the crack.
2.2 Laboratory testing of the bio-based repair system
Mortar discs (∅ 17cm and h=2cm) were cast with ordinary Portland cement and aggregates (0-4mm) in plastic buckets as described by Wiktor & Jonkers[3]. The specimens were kept 28 days in sealed conditions at room temperature and then tested for 3 points bending test until failure. The bottom of the buckets is removed and the cracked mortar discs were glued in the buckets (Fig. 1).
Figure 1 Preparation of mortar specimens for water permeability test
~ 2cm
a. Casting of
a. Casting of mortarmortardiscsdiscs
bucket
∅= 17 cm - h= 14 cm
b.
b. BreakingBreakingof of specimenspecimen
F
d.
d. SpecimensSpecimensare are gluedgluedinsideinsidethe the bucketsbuckets c. The
c. The bottombottomof the of the bucketbucketisisremovedremoved
bucket
bottom removed ~ 2cm
a. Casting of
a. Casting of mortarmortardiscsdiscs
bucket
∅= 17 cm - h= 14 cm ~ 2cm
a. Casting of
a. Casting of mortarmortardiscsdiscs
bucket
∅= 17 cm - h= 14 cm
b.
b. BreakingBreakingof of specimenspecimen
F
b.
b. BreakingBreakingof of specimenspecimen
F F
d.
d. SpecimensSpecimensare are gluedgluedinsideinsidethe the bucketsbuckets c. The
c. The bottombottomof the of the bucketbucketisisremovedremoved
bucket
bottom removed
d.
d. SpecimensSpecimensare are gluedgluedinsideinsidethe the bucketsbuckets c. The
c. The bottombottomof the of the bucketbucketisisremovedremoved
bucket
bottom removed
c. The
c. The bottombottomof the of the bucketbucketisisremovedremoved
bucket
bottom removed
Permeability test is performed before and 28 days after impregnation with the bacteria-based repair solution: the amount of water permeating through the crack in 1 hour is recorded. The permeability value before repair is noted P1 and the one after P2.
The difference between these 2 values gives an estimation of the efficiency of the crack repair. To evaluate the resistance of the repair to ageing, the specimens are then subjected to five freeze/thaw cycles, and a third permeability test is performed afterwards (Fig. 2). Control specimens are impregnated with tap water. Calcium lactate is used as calcium source in solution B.
Figure 2 Impregnation of the mortar specimen with the bacteria-based repair system and evaluation of the crack-sealing efficiency by means of water permeability test
The decrease in permeability after impregnation and after 5 freeze/thaw cycles is given by equation 1 and 2 respectively:
Pdecrease after impregnation=
(
)
2 1 1 100 P P x P − (eq 1)
Pdecrease after freeze/thaw=
(
)
3 1 1 100 P P x P − (eq 2)
2.3 Field testing of the bio-based repair system
In order to validate laboratory results, the bacteria-based repair system has also been applied in practice, on 2 parking garages named PG1 and PG2. In both cases, the concrete deck was suffering from cracking which resulted in a significant leaking of the structure. Also, in PG2, the concrete pavement on each side of the access ramp was aged and damage due to freeze/thaw.
Figure 4 Parking garage 1 (PG1) – (a) detail of a leaking crack before application of the bacteria-based repair system, (b) spraying of the bacteria-based repair system
In PG1, an area of 2x4m was sprayed with the bio-based repair system on both side of the concrete deck (Fig. 4). In PG2, part of the concrete pavement (area of 2x0.5m) and three cracks (1-3mm wide) of the concrete deck were impregnated with the bacteria-based repair system (Fig. 5).
Figure 5 Parking garage 2 (PG2) – (a) spraying of the bacteria-based repair system on concrete pavement, (b) crack impregnated with the bacteria-based repair system
In both cases, solution A and solution B were each poured in a sprayer, and manually applied at the surface of the concrete in layers until saturation of the concrete treated area. Solution B contains calcium nitrate instead of calcium lactate as calcium source. In this way more calcium can be added and the system also has a nitrate source for bacterial growth in case the concentration in O2 is limited.
Two months after the application of the bacteria-based repair system the crack sealing performance was visually assessed in PG1.
In PG2, the crack sealing efficiency was assessed by means of water permeability test performed on site. Rectangular wooden frames (1x0.5m) were placed on top of 3 treated- and 3 untreated cracks on the concrete deck. The wooden frames were sealed with silicon glue prior pouring 5L tap water. As the crack goes through the whole thickness of the deck, the sealing efficiency was assessed by monitoring visually, from the other side of the deck, how much water was dripping through the crack. Also, 6 cores were drilled from two different locations on the concrete pavement: 3 from the treated area and 3 from an untreated part on the same side of the access ramp as control specimens. The resistance to freeze/thaw and deicing salt was then evaluated in laboratory. The cores were tested according the NPR/TS 1239-9 and NEN-EN 13877-2. The test was performed by Cugla B.V. (Breda, the Netherlands).
3
Results and Discussion
3.1 Laboratory testing of the bacteria-based repair system
The results of the water permeability test are presented in figure 6. The cracks impregnated with the bacteria-based repair system are completely sealed as no water was leaking from the crack. For the control specimens on the other hand, the water permeability is even higher 28 days after the impregnation. It is also interesting to note that after 5 freeze/thaw cycles the permeability of control specimens tremendously increased while the specimens treated with the bacteria-based repair system still exhibit a lower permeability than the initial value.
These results are very encouraging as only the specimens treated with the bacteria-based repair system showed improved properties. However, in order to validate these good results, the
bacteria-based repair system should be tested on real concrete structure, outside in a non-controlled environment.
Figure 6 Water permeability results of control and specimen treated with the bacteria-based repair system
3.2 Field performance of the bacteria-based repair system
-Parking garage 1 – PG1
The performance of the bacteria-based repair system for the first field application in PG1 was assessed qualitatively by visual observations. The pictures taken on PG1 two months after the impregnation with the bacteria-based repair system are shown on figure 7. It is worth noting that the pictures were taken only a few days after a raining episode. It can clearly be seen that the cracks that have not been treated with the bacteria-based repair system (Fig. 7a) seem wet and are lined with white precipitate formed due to ingress water through the cracks. On the other hand, all the treated area is dry meaning (Fig. 7b) that cracks treated with the bacteria-based repair system are not leaking anymore.
Figure 7 Visual inspection of cracks 2 months after application of the bacteria-based repair system in PG1 – (a) non treated area, the arrow points white precipitate still leaking from the crack, (b) treated area, no
-Parking garage 2 – PG2
For the second field application in PG2, the performance of the repair system is assessed more quantitatively. The water permeability test to assess the crack sealing efficiency was performed on site 2 months after the application of the repair system. The results were very encouraging as only cracks that were not treated with the bacteria-based repair system were still heavily leaking along their full length (Fig. 8a). Two cracks impregnated with the repair system had only few localized dripping spots (Fig. 8b), and the third one didn’t leak at all.
Figure 8 Observation of water leaking through the cracks during water permeability test – (a) control crack, (b) crack treated with the bacteria-based system
Relating to the freeze/thaw resistance of the concrete, the specimens treated with the bacteria-based repair system had a significant better resistance compared to the control as they exhibited 48% less mass loss (scaling) than the control.
These results from field experiment confirm the good performance of the bacteria-based obtained in laboratory.
4
Conclusion
This paper presented the application of bacteria-based repair system as preventive solution against ageing of concrete. It focuses particularly on the ageing of concrete due to micro-crack formation or freeze/thaw which results in an increased permeability of the concrete. The bacteria-based repair system presented in this paper aims at recovering the concrete permeability thanks to bacteria-induced calcium carbonate precipitation inside cracks/porosity.
The performance of the bacteria-based repair system in laboratory and field application were very promising. The laboratory results showed that the crack sealing capacity of the repair system is very good as the cracks were completely sealed after impregnation. Results from field application are also very good as treated concrete had a significant higher resistance to freeze/thaw and only cracks that were not impregnated with the bacteria-based repair system were still heavily leaking.
This results are very encouraging for the application in practice of the bacteria-based repair as preventive measure against ageing of concrete. The bacteria-based repair is now being optimized in order to improve the freeze/thaw resistance and to raise the crack-sealing efficiency on site to 100%. The next step is to evaluate the long-term durability of the repair.
5
Acknowledgment
The authors would like to thank Cugla B.V. (Breda, The Netherlands) for testing the resistance of the concrete cores to freeze/thaw. Also, the financial support from Agentschap NL (IOP grant SHM12020) for this work is gratefully acknowledged.
6
References
[1] Dhami NK, Reddy SM, Mukherjee A (2012) Biofilm and microbial applications in biomineralized concrete. In: Seto J (eds) Advanced topics in biomineralization. In Tech, Rijeka, pp 137-164.
[2] Wiktor V and Jonkers HM (2010) Quantification of crack-healing in novel bacteria-based self-healing concrete, Cement Concrete Comp. 33:763-770.
[3] Wiktor V and Jonkers HM (2012) Application of bacteria-based repair system to damaged concrete structures. Proceedings of the 2nd International Workshop on Structural Life Management of Underground Structures, Daejeon 19-20 October 2012 pp.31-34.
