Bio-based mortar for concrete repair

BIO-BASED MORTAR FOR CONCRETE REPAIR

Delft University of Technology

Faculty of Civil Engineering and Geosciences Microlab, M & E

P.O. Box 5048 2600 GA Delft The Netherlands m.g.sierrabeltran@tudelft.nl

KEYWORDS: Concrete repair, Bio-based agent, Fibre-reinforced mortar, Shrinkage

ABSTRACT

Compatibility between the repair material and the concrete substrate ensures that the repair system will stand the stresses induced by restrained shrinkage as well as chemical changes. In this research, a cement-based mortar reinforced with a low percentage of polymer fibres is studied. Under tension stress this material behaves ductile and develops multiple micro-cracks before failure. A bio-based agent, present in the mortar, will improve the durability of the repair system by promoting crack healing and improving the bonding with the concrete substrate. The bio-based agent consists of bacteria that are compatible with concrete and organic compounds that serve as food for the bacteria. Previous research has shown the capacity of this bio-based agent to produce calcite-bio-based minerals, filling up cracks and reducing the concrete permeability.

INTRODUCTION

Currently available concrete repair systems are often not compatible with the original concrete material, which translate into early detachment and poor durability. Additionally, these systems are largely based on environmental unfriendly materials such as epoxy systems, acrylic resins or silicone-based polymers. This paper focus on the development of a concrete compatible and fully sustainable bio-based repair system, that features better bonding and improved durability and sustainability characteristics when compared to existing repair systems.

A special type of high performance fibre reinforced cementitious composite (HFRCC) called Engineered Cementitious Composite (ECC) has been studied as repair material for concrete structures (Li 2009, Zhou 2011) because of its capability to deform to high tensile strains under load. As a repair material, ECC cracks when subjected to differential shrinkage but it is capable of carrying more tensile load and to accommodate larger tensile strain than other repair systems (Zhou 2011).

ECC was micromechanically designed to have large values of strain capacity with a low percentage of randomly distributed polymer fibres (Li 1993). Because of the presence of fibres ECC develops multiple micro-cracking prior to failure. The crack width remains below 1 millimetre (Li 2002). Conventional ECC is designed without coarse aggregates and with only a small amount of fine sand, in order to control the fracture toughness of the matrix (Li 1993). This characteristic leads to a higher cement and binder ratio and eventually to a high value of shrinkage (Buffenbarger et al. 1998). Li (2009), Yang et al. (2007) reported drying shrinkage values of 1200x106 to 1800x106 for conventional ECC. In similar drying conditions of 20°C and 60%

relative humidity, normal concrete has a drying shrinkage strain of 400x106 to 600x106 (Neville 1995).

To improve the durability of the concrete repair system as well as to improve the bonding with the concrete substrate, this paper proposes a bio-based agent to be included in the mortar mix. The bio-based agent consists of alkali-resistant bacteria and a food source for the bacteria. When applied in concrete, this bio-based agent has the capacity to produce calcite-based minerals inside cracks reducing the permeability of the concrete (Jonkers 2011, Wiktor and Jonkers 2011). The first steps into the development of the bio-based mortar repair systems are presented in this paper.

EXPERIMENTAL PROGRAM

In this paper the experimental program covers two parts of the research investigation. In the first part, four different ECC-type mixtures are characterized by their mechanical properties and drying shrinkage. Based on these results, two mixtures are chosen for further investigation as bio-based mortars. In the second part, the influence of the healing-agent in the mechanical properties of the mortars was investigated.

Materials

The materials used are cement type CEM I 42.5N, blast furnace slag (BFS), fly ash (FA), limestone powder (LP), sand, poly-vinyl-alcohol (PVA) fibres, and superplasticizer (SP). BFS and fly ash have a potential of pozzolanic reaction and these reactions need to be activated by the hydration products of Portland cement. In the mix design, BFS, FA and Portland cement were considered as cementitious materials, and the limestone powder was considered as inert filler material. Table 1 gives the mix proportion of the ECCs.

Table 1 Mixture compositions of ECCs

Mix ID Mix 1 Mix 2 Mix 3 Mix 4

Cement, kg/m3 233 440 526 516 BFS, kg/m3 543 132 - - FA, kg/m3 - 560 631 620 LP, kg/m3 775 - - 413 Sand, kg/m3 - 440 405 - Water, kg/m3 416 374 365 384 SP, kg/m3 8 16 16 10 PVA fibres, kg/m3 26 26 26 26  0.27 0.33 0.33 0.25

The sand has an average and maximum grain size of 250 m and 500 m respectively. The maximum particle size in Mix 2 and 3 correspond to the maximum particle size of the sand. In Mix 1, the maximum particle size is 150 m, corresponding with the maximum particle size of the cement and LP. The maximum particle size in Mix 4 is 200 m corresponding to the FA.

The PVA fibres have a diameter of 39 m and a length of 8 mm. The fibres have a tensile strength of 1620 MPa, elastic modulus of 42.8 GPa and a maximum elongation of 6.0%. The fibres have been coated with a proprietary oiling agent 1.2% by mass. The superplasticizer Cretoplast SL-01 was used in this research. The amount of superplasticizer was adjusted for each mixture to achieve consistent rheology properties for proper fibre distribution and workability. The use of limestone powder

with Portland cement has many advantages, including durability and workability (Tsivilis et al. 2002). Thus, in Mixes 1 and 4 prepared with LP, a lower water-to-binder ratio () and smaller amount of superplasticizer were necessary.

The healing agent considered for this research is an alkali-resistant spore-forming bacteria and calcium lactate as a nutrient source for the bacteria. Lightweight aggregates (LW) were impregnated with the bacteria and food. The LW particles have sizes ranging between 0.25 and 2 mm.

Test methods

For each mixture, fresh state ECC was cast into prisms with dimensions 40x40x160 mm3 and thin beams with dimensions 240x60x10 mm3. The specimens were demolded after 24 hours and moist cured in plastic bags at 95% relative humidity (RH), 25°C for 7, 28 and 60 days. Some of the prisms were cut into 40-mm cubic specimens for compression tests and the thin specimens were cut into thin beams of 120x30x10 mm3 for four-point bending tests.

The four-point bending tests were performed under displacement control at a loading rate of 0.01 mm/s. The span length of the flexural loading was 110 mm with a 30 mm centre span length. During flexural test, the load and mid-span deflection were recorded.

Drying shrinkage measurements were made on all mixtures. The drying shrinkage of three prism specimens was measured up to 120 days after an initial one day curing in the mould. The drying shrinkage specimens were stored in a drying room at 19±1°C and 52±2% RH.

RESULTS AND DISCUSSION Compressive strength

Figure 1 shows the average of the compressive strength as determined from four cubic specimens at the age of 28 days. The compressive strength of Mix 1, the mix with considerably lower cement content, is as expected lower than for the other mixes. This material fulfils the compressive strength requirements for repair material of concrete structures Class R3 according to the standards NEN 1504-3. The other three mixes have compressive strengths higher than 45 MPa, fulfilling the requirement for Class R4 in the same standard.

0 10 20 30 40 50 60

Mix 1 Mix 2 Mix 3 Mix 4

Com p re s s iv e s tr engt h, M P

Flexural performance

The flexural test results at different ages are summarized in Table 2. The table presents the average flexural strength (modulus of rupture) and the ultimate mid-span deflections at the peak stress for each mixture. Each result in this table is the average of four tests.

Table 2 Flexural strength and ultimate deflection of ECCs Flexural strength, MPa Ultimate deflection, mm Mix ID

7 days 28 days 60 days 7 days 28 days 60 days

Mix 1 9.6 10.6 13.8 6.9 4.6 4.1

Mix 2 7.8 9.5 10.1 6.0 5.6 3.7

Mix 3 8.5 11.5 10.5 6.7 4.8 4.8

Mix 4 8.4 10.2 10.4 5.3 6.6 5.0

The flexural strength increases with increasing age, apart from Mix 3, as shown in Table 2. For Mix 3, the test results after 28 days show a decrease of flexural strength. Specimens of this mix were tested at 90 days to confirm this tendency. At this age, the average flexural strength was 9.9 MPa, a lower value than the average strength at age 60 days, 10.5 MPa.

The ultimate deflection capacities of Mixes 1, 2 and 3, decrease with increasing age. Similar behaviour has been observed in other ECC type materials (Sierra-Beltran 2011, Yang et al. 2007, Lepech and Li 2006). Mix 4 exhibited a maximum deflection capacity at 28 days.

Figure 2 shows the typical flexural strength-deflection curves at 28 days for each of the mixtures. As can be seen in these figure, all ECC thin beams have similar behaviour under blending load, they deform similarly to a ductile metal plate through plastic deformation. In all the specimens, the first crack started within the mid-span at the tensile face. As presented in Figure 2, the slope of the load-deflection curves is similar for all mixes. This slope represents the stiffness of the thin beams.

0 2 4 6 8 10 12 14 0 1 2 3 4 5 6 7 Mid-Span Deflection (mm) F lex ur al S tr engt h ( M P a Mix 1 Mix 2 Mix 3 Mix 4

Fig. 2 Typical flexural strength-deflection curves of ECCs at 28 days.

Drying shrinkage

The drying shrinkage measurements for all mixes are shown in figure 3. Each value in this figure represents the average measurements of three specimens.

Mix 1 has the highest shrinkage, while mixes 3 and 4 have the lowest. The presence of sand particles in mixes 2 and 3 contributes to reduce the drying shrinkage. In Mix 4, it is possible that unhydrated fly ash particles serve as fine aggregates that restrain the shrinkage, thus contributing to a lower shrinkage compare to the other mixes (Sahmaran et al. 2007). -3500 -3000 -2500 -2000 -1500 -1000 -500 0 0 20 40 60 80 100 120 Age (days) Dr y ing s h ri nk a ge ( x 10-6 Mix 1 Mix 2 Mix 3 Mix 4

Fig. 3 Drying shrinkage strain versus drying shrinkage time for ECCs.

Mechanical properties of bio-based mortar

Based on the laboratory tests results discussed above, mixes 2 and 4 were chosen to further develop the bio-based repair mortar systems. These mixes developed low drying shrinkage while exhibiting the higher compressive strength. The flexural behaviour of these mixes is comparable in strength and deflection capacity to the other mixes.

In Mix 2 the sand was partially replaced by the lightweight (LW) particles containing the healing agent. In Mix 4, the limestone powder was partially replaced by the particles with healing agent. In both mixes a higher amount of water and superplasticizer was necessary to achieve consistent rheology properties. The mixture compositions with and without healing agent are presented in Table 3.

Table 3 Mixture compositions of selected ECCs with and without healing agent

Mix ID Mix 2 Mix 2 H Mix 4 Mix 4 H

Cement, kg/m3 440 420 516 479

BFS, kg/m3 132 126 - -

FA, kg/m3 560 532 620 575

LP, kg/m3 - - 413 323

Sand, kg/m3 440 361 - -

LW with healing agent,

kg/m3 - 59 - 60

Water, kg/m3 374 374 384 389

SP, kg/m3 16 16 10 14

PVA fibres, kg/m3 26 26 26 26

 0.33 0.35 0.25 0.28

Specimens of Mixes 2 H and 4 H were prepared, casted and tested in the same way as the samples without healing agent, as described above under the Test method section. The LW particles are evenly distributed within the samples, as was observed with the

stereomicroscope and the Environmental scanning electron microscope (ESEM). Figure 4 presents a microscopic image of a Mix 4H sample where the LW particles can be seen.

Fig. 4 Lightmicroscopic image of LW particles with healing agent distributed through a Mix 4H sample.

The average compressive strength at 28 days of the mixes with healing agent is higher than the average compressive strength of the mixes without healing agent, as shown in figure 5. This may be an effect of the presence of calcium lactate in the healing agent. Jonkers and colleagues (2009) reported the same effect.

0 10 20 30 40 50 60 Mix 2 Mix 4 Com p re s s iv e s tr engt h (M P Without LW With LW

Fig. 5 Average compressive strength (MPa) of selected ECCs with and without bacteria, at 28 days.

The presence of LW particles with healing agent has an effect on the flexural behaviour of ECC. Figures 6 and 7 present typical flexural strength-deflection curves, at 28 days, of Mixes 2 and 2 H and Mixes 4 and 4 H, each. The flexural strength of Mix 2 H, 9.3 MPa, is slightly lower than that for Mix 2, 9.5 MPa. In a similar way, the flexural strength of Mix 4 H, 9.6 MPa, is lower than that of Mix 4, 10.2 MPa. These reductions are attributed to several factors. In the first place, the mixes with healing agents have higher water-to-binder ratio and higher amount of superplasticizer. In the second place, the small particle size of limestone powder (mean particle size 7.78 m)

and sand (mean particle size 250 m) compared to the size of the LW particles (mean particle size 900 m). The smaller particle sizes contribute to a better packing of the cement-based matrix around the fibre, and a better packing leads to an improvement of the fibre-matrix interface properties and the ductility of the composites without healing agent. Nevertheless, the flexural behaviour of both Mix 2 H and mix 4 H containing healing agent, remain acceptable for an ECC type material. Both mixes developed multiple cracking prior to failure and exhibited a mid-span deflection of more than 5 mm which is considerably greater than the ductility of conventional concrete. 0 2 4 6 8 10 12 0 1 2 3 4 5 6 7 Mid-Span Deflection (mm) F lex u ral S tr e n g th ( M P a Mix 4 Mix 4 H

Fig. 6 Typical flexural strength-deflection curves of Mix 4 and Mix 4H.

0 2 4 6 8 10 12 0 1 2 3 4 5 6 7 Mid-Span Deflection (mm) F lex ur al S tr engt h ( M P a Mix 2 Mix 2 H

Fig. 7 Typical flexural strength-defection curves of Mix 2 and Mix 2H.

CONCLUSIONS

This paper presents the first results in the development of a bio-based mortar for concrete repair. The proposed material is cement-based; it is reinforced with polymer fibres and includes lightweight aggregates impregnated with a healing agent.

Initially, four different ECC-type materials were studied and based on their mechanical properties and drying shrinkage capacity, two of them were chosen for further studies. Both materials fulfil the compressive strength requirements of the standards NEN 1504-3 for repair materials for concrete structures Class R4.

In the chosen materials the filler, either limestone powder or sand, was partially replaced with lightweight aggregates impregnated with a healing agent consisting of alkali-resistant bacteria and a food source. The presence of the healing agent resulted in an increase of compressive strength of the mixtures. The flexural strength and deflection capacity, on the other hand, decreased slightly when the healing agent was included. Nevertheless, the composites with healing agent exhibited acceptable strength and ductility for an ECC type material.

Currently, the drying shrinkage capacities of the two bio-based mortar repair mixtures are being measured. Further research in this ongoing development also includes the assessment of healing capacity of the bio-based mortar and the bonding properties with aged concrete.

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