Influence of fibres and hardening accelerator on concrete for rigid pavements

Rigid highway and airfield pavements have been widely used in many countries for several decades. Accordingly, the task of repairing such pavements is always relevant. Many materials have been developed for the repair of concrete pavements, where high-early strength concretes are used most often. The setting rate is important for repair materials from the standpoint of being able to re-open the highways or airfield runways to traffic in the shortest possible time. Repair concretes must also provide the required design compressive strength, flexural strength and sufficient durability.

It is important to note that for repair materials, adhesion to the material of the structure under repair is one of the most important quality indicators. In most cases, when performing full depth repair of rigid highway and airfield pavements, the repair material is placed in the appropriate recess, which is located at the site of the defect. If necessary, the edges of the recess are further processed to obtain an edge with an angle close to a straight line – that is, the mass of repair material can partially work as a stand-alone slab. However, due to dynamic loads, in particular when braking a vehicle, in the case of adhesion loss to the concrete base, the repaired area can collapse faster due to the smaller thickness of the repair material layer compared to the main pavement. In addition, in most cases, defects in the pavements are formed in the joint areas and the recess will not be surrounded by ‘old’ concrete on all sides. Thus, the task of ensuring the adhesive strength of concrete and fibre-concrete for rigid highways and airfield pavements is important. This task should be solved while simultaneously ensuring the necessary quality indicators of concrete, in particular high-early strength.

Road surfaces are constantly exposed to high traffic loads, sharp temperature drops and aggressive exposure to thawing agents in winter. This negatively affects the structure and properties of concrete (Fan and Zhang, 2021). To repair cement-concrete pavements, the full depth repair technology is widely used (Guyer, 2021; Zhao et al., 2019). Restoration of the weakened properties of the ‘old’ concrete and its additional protection requires an effective multifunctional material. To repair concrete with high strength at early age and at the design age, a complex modification with a superplasticiser and an accelerator is most often used (Boykova et al., 2015; Van Dam et al., 2005).

However, for repair mixtures, an important characteristic that affects the functionality and durability of the repaired structure is the adhesive strength (Fan and Zhang, 2021). The adhesion boundary is often the weakest part of the repaired structure. The contact layer is also most often damaged due to stress when braking vehicles (Zhao and Wang, 2011).

Boykova et al. (2015) obtained a repair concrete with a strength of at least 55 MPa and high frost and wear resistance by using a complex admixture. The developed modified concrete had an adhesive strength of 3.2 MPa to the concrete pavement of grade C32/40, which ensured solidity and integrity of the repair area and high load-bearing capacity of the structure. Peng et al. (2011) obtained geopolymeric concrete for the repair of the rigid pavements by using hardening activator, which gained flexural strength of 3.3 MPa and compressive strength of 44 MPa in 8 h. Also, geopolymeric concrete provided the necessary adhesion and shrinkage of the material during hardening.

Sangkyu et al. (2020) showed that the introduction of epoxy resin into the cement mortar increases its adhesive strength to the concrete base and also increases the flexural strength by 40% and the compressive strength by 35%. Shrinkage deformations of the material decreased by almost twice, compared with control samples.

Kabiri and Zanotti (2019) noted that the performance of the repaired areas depends on the repaired surface, namely the contact area ‘old coating–repair material’. The positive effect of dispersed reinforcement on adhesive strength was confirmed, depending on the type, size and amount of fibre, as well as the stress–strain state of the repair area. It was found that steel fibres can be more effective at lower dosages compared to polyvinyl alcohol fibres.

Szymanowski (2019) described the main methods for determining the adhesive strength, namely the pull-off method and the bending tension method. Special attention was paid to the importance of the base preparation for the adhesive strength of the applied material. The positive effect of increasing the adhesive strength when applied to a textured surface of a concrete base with a removed cement wash and treated with a primer was shown. The importance of using dispersed reinforcement and modern modifiers to improve the physical and mechanical properties of the applied material on a concrete base was noted. Alireza et al. (2020) and Changqing et al. (2021) studied the adhesive strength of ultra-high-performance fibre-reinforced concrete (UHPRC) with steel fibre to a concrete base using the shear method. The compressive strength of the concrete base was 43 MPa and the compressive strength of UHPRC was 126 MPa. The adhesive strength to the raw and rough concrete base was 2.8 and 6.3 MPa, respectively. With the help of computer simulation and finite element modelling (FEM), a comparison was made with the obtained experimental data. The results were very close to the full-scale experiment.

Penggang et al. (2020) used modified steel fibre-reinforced concrete with a design strength of 49 MPa as a repair material. The repair material was applied to smooth and rough surfaces of ordinary concrete with a strength of 41 MPa. The influence of shrinkage cracks on the adhesive strength of the two-layer coating ‘old concrete–repair material’ was studied. It was established that an increase in the roughness of ‘old concrete’ reduces the shrinkage deformation of repair steel fibre-reinforced concrete and increases its adhesive strength.

Tian-Feng et al. (2020) and Boyu and Rishi (2021) studied the adhesive shear strength at different inclination angles of experimental samples with different roughness of the concrete adhesion surface and adhesion tensile splitting strength. The obtained results showed a positive effect in increasing roughness on the adhesive strength, regardless of the inclination angle of the adhesion surface.

Javidmehr and Empelman (2021) carried out experiments to determine the effect of surface treatment on the adhesive strength of concrete. A positive effect of surface treatment on increasing adhesive strength was found. However, the results were non-linear, with an increase in surface roughness both in a full-scale experiment and using computer simulation, which requires additional research. In general, the positive impact of modified concretes and fibre-reinforced concretes in layer-by-layer reinforcement or repair is emphasised.

Hae-Won et al. (2021) studied the adhesive strength of reinforced rigid pavement layers using FEM. Based on the data obtained for the Republic of Korea, a catalogue was created taking into account the existing pavement parameters and the recommended parameters for their reinforcement layers.

Thus, the above data confirm the effectiveness of modifying concrete mixtures with modern chemical admixtures and dispersed reinforcement. However, in the studies described above, the effects of hardening accelerator on the adhesive strength of composites was not studied. This is an important factor for repair concretes and fibre-reinforced concretes, because in most of them early strength is achieved by the introduction of hardening accelerators (Guyer, 2021; Kroviakov et al., 2021; Van Dam et al., 2005; Zhao et al., 2019). Also, in the studies described above, there is a positive effect of roughening the repair surface; however, it should be noted that such preparation leads to additional labour costs. Therefore, the present study is focused on investigating the adhesive strength of the untreated surface of old concrete.

In recent years, the repair of rigid highway and airfield pavements has increasingly used fibre-concrete with different fibre types (Eisa et al., 2021; Kabashi et al., 2018; Tian and Trevor, 2018). Fibre reinforcement increases the compressive and flexural strength of concrete, reduces shrinkage deformation and increases crack resistance (Farid et al., 2018; Kroviakov and Kryzhanovskyi, 2021; Pang et al., 2018). Also, fibre-reinforced concrete resists dynamic loads well, which is especially important for rigid highway and airfield pavements. Additionally, an important technological technique for improving the adhesive strength of repair concrete for rigid pavements is the treatment of the contact surface with a primer (Zhao and Wang, 2011).

The objective of this research is to determine the influence of steel fibre and accelerator on the adhesive strength of concrete for the repair of rigid highway and airfield pavements, taking into account the treatment of the contact surface with a primer.

The research results are important for improving existing methods of repairing rigid road and airfield pavements. The use of fast-hardening fibre-concrete allows a quick re-opening of the repaired area to traffic. At the same time, the problem of adhesive strength of the repair concrete to the old concrete remains important. To ensure the quality and durability of the repair area, high-strength repair concretes must have sufficient adhesive strength to the concrete of the old pavement.

To study the adhesive strength of road repair fibre-reinforced concrete to the existing ‘old concrete’ grade C32/40, compositions with Portland cement CEM II/A-S 42.5 ‘Dyckerhoff Cement Ukraine’ in the amount of 400 kg/m³ were used. Granite crushed stone with a fraction of 5–20 mm and quartz sand with a fineness modulus of 2.45 were used as aggregates. The grading of aggregates is shown in Figure 1. The bulk density of aggregates was determined according to BS EN 1097-3 : 1998 (BSI, 1998) and ASTM C 29 (ASTM, 2017) standards and amounted to 1370 kg/m³ for crushed stone and 1440 kg/m³ for sand. The grain size of aggregates satisfies the requirements of ASTM (2018) and DSTU (1995, 2013).

Since this study is a continuation of the experimental plan described by Kos et al. (2022), the variable factors of the fibre-reinforced concrete repair mixture (X₁ – amount of hardening accelerator; X₂ – amount of steel anchor fibre) and the amount of superplasticiser were not changed, as presented in Table 1. The workability of fibre-reinforced concrete repair mixtures was 5–8 cm. The change in the water/concrete ratio depended on the amount of fibre and hardening accelerator in the repair composition. Figure 2 shows a photograph of the steel hooked-end fibre with ultimate tensile strength of 1150–2300 MPa. Table 2 shows the specifications for SikaRapid 3 non-chloride hardening accelerator and MasterGlenium SKY 608 superplasticiser.

The upper and lower limits of the variable factors (−1, +1) were chosen using the methods of experimental–statistical modelling (Lyashenko and Voznesenskiy, 2017) and methods of multi-parameter design of concrete and fibre-reinforced concrete (Dvorkin et al., 2012). This technique can significantly save labour costs for the experiment and reduce the amount of batches necessary.

Figures 3 and 4 show diagrams of the compressive and flexural strength of the studied fibre-reinforced concrete at the age of 2 and 28 days.

The value of adhesive strength of concrete and fibre-reinforced concrete to the concrete base was determined from BS EN 1542 (BSI, 1999) using the Proceq Dyna-Z16 adhesimeter. A layer of repair concrete composition 30 mm thick was applied to concrete samples of grade C32/40 with a size of 30 × 30 × 10 cm, the age of which was 1.5 months. The surface of the sample met the requirements of BS EN 1542 (BSI, 1999). After 3 days of hardening, a metal plate measuring 10 × 10 cm was glued to the layer of repair concrete with Sika AnchorFix-1 quick-setting adhesive. After 28 days of the repair material curing, the concrete plate was detached from the substrate and the adhesive strength was determined; see Figure 5.

Also, the adhesive strength of concrete and fibre-concrete was determined by the method of flexural strength of beams with a size of 10 × 10 × 40 cm. Half of the beams were made of ‘old’ concrete grade C32/40, which was made in advance and at the time of application of repair concrete was about 2 months old. The second half were made of concrete or fibre-concrete repair composition, which were placed into the form with the half of ‘old concrete’. The new concrete was in contact with the old concrete through the end surface of the ‘old’ half. Using this technique, the adhesive strength was determined by flexural strength at 3-point load scheme (Figure 6).

Photographs of the process of determining the adhesive strength by the flexural strength method and an example of the broken sample are shown in Figure 7.

The adhesive strength of all nine investigated repair concrete and fibre-concrete compositions was determined from two types of pre-treatment of the sample surface, which simulated the surface of the repair area: wetting the surface of the base, designated as R (MPa); treating the surface of the base with primer ISOGRUNT in the amount of about 0.25 kg/m², designated as R_P (MPa).

Kos et al. (2022) found that with hardening accelerator SikaRapid 3 set at 1.2% of the cement content and 70–90 kg/m³ steel fibre, the flexural concrete strength at the age of 2 days is 8.5–9.3 MPa. At the design age of 28 days, the flexural strength of concrete without fibre ranges from 7.0 to 8.5 MPa and of concrete with 70–90 kg/m³ fibre ranges from 15.5 to 17.5 MPa. However, the use of a hardening accelerator reduces the flexural strength of concrete at the design age by 10–12%.

At the same time, the hardening accelerator significantly increases the concrete compressive strength at the age of 2 days from 46.5–52.6 MPa to 56.4–61.5 MPa. Fibres also have a small positive effect on early compressive strength. Fibre-concrete with 70–90 kg/m³ fibre and 1.4–2.4% accelerator already had a compressive strength of at least 60 MPa at the age of 2 days. At the design age, due to the use of dispersed reinforcement with steel fibre, the concrete compressive strength for rigid highway and airfield pavement repair increases by 8–10 MPa. However, the use of a hardening accelerator reduces the compressive strength at the age of 28 days by 6–9 MPa.

Thus, taking into account the specifics of the requirements for repair concrete, the required strength is ensured by the use of 70–90 kg/m³ steel anchor fibre and 1.4–2.0% hardening accelerator SikaRapid 3.

The nine-point experimental plan and the data of the adhesive strength of concretes and fibre-reinforced concretes are shown in Table 3.

The following experimental and statistical models were designed from the data given in Table 3 (Dvorkin et al., 2012; Lyashenko and Voznesenskiy, 2017), which show the influence of various factors on the adhesive strength determined by the pull-off method with the wetting of the sample surface (R) (Equation 1) and treating the surface with primer (R_P) (Equation 2)

and the adhesive strength determined by the flexural strength method (R_ctk – wetting the surface, R_ctk.P – primer treatment)

In Equations 1–4 the levels of variable factors were coded in the range from −1 to +1 (Dvorkin et al., 2012; Lyashenko and Voznesenskiy, 2017). According to the experimental–statistical models, the diagrams in Figure 8 were drawn. Analysis of these diagrams and data from Table 3 show that all the concretes and fibre-concretes studied are characterised by a sufficiently high adhesive strength to old concrete, from 2.3 MPa using the pull-off method and from 2.05 MPa using the flexural strength method. By treating the surface of ‘old’ concrete with Isogrund primer, the adhesive strength of the repair composition increases by 0.10–0.24 MPa, which is 6–8% when determining the adhesive strength by the pull-off method and 8–10% when determining the adhesive strength by the flexural strength method – that is, the technological method of applying the primer can be recommended for the more loaded areas of repaired pavements, especially at the joint areas and at the slab edges.

Steel fibre-reinforcement significantly increases the adhesive strength of repair compositions. The adhesive strength determined by various methods increases by 0.16–0.19 MPa (7–9%) when using 50 kg/m³ fibre. When reinforced with 90–100 kg/m³ steel fibre, the adhesive strength increases by 0.24–0.36 MPa (11–15%) in comparison with non-reinforced concrete. This effect is explained by the increase in the strength of the repair composition due to fibre-reinforcement, and the concrete shrinkage reduction, which was established in previous studies of the properties of repair concrete (Kos et al., 2022). Using dispersed reinforcement with steel fibres, the adhesive strength increases due to less shrinkage during hardening. Thus, the shear stresses are reduced at the interface area between the repair concrete and the old concrete. Using the SikaRapid 3 hardening accelerator reduces the adhesive strength of concrete and fibre-concrete, which is explained by the decrease in the strength of the repair material at the design age. However, as can be seen from the diagrams in Figure 8, this decrease is fully compensated by fibre reinforcement.

The maximum adhesive strength of fibre-concrete determined by the pull-off method is 2.8 MPa without primer treatment and 3 MPa with Isogrund primer treatment, while the maximum adhesive strength of fibre-concrete determined by the flexural strength test (3-point load application) is 2.3 MPa without primer treatment and 2.5 MPa with treatment, which in turn reflects a more realistic behaviour of the ‘old concrete’–repair material system. The road pavement works as a slab on an elastic base; therefore, flexural strength is a more significant characteristic.

Fibre-concretes developed for rigid highway and airfield pavement repair through the application of effective superplasticiser with hardening accelerator and anchor steel fibre have high compressive and flexural strength at early and in design age. In order to ensure the necessary strength properties of concrete, the optimal amount of anchor steel fibre is 70–90 kg/m³ and that of hardening accelerator SikaRapid 3 is 1.4–2.0%.

Also, modified repair concretes have a sufficiently high adhesive strength to old concrete, from 2.30 MPa tested by the pull-off method and from 2.05 MPa tested by the flexural strength method. Due to dispersed reinforcement with steel fibre, the adhesion strength of repair concrete increases by 7–15%. The addition of a hardening accelerator insignificantly reduces the adhesive strength of concrete. By treating the contact surface of the ‘old’ concrete with a primer, the adhesive strength is further increased by 6–10%.

Thus, modified fibre-reinforced concrete meets all the basic requirements for rigid highway and airfield pavement repair materials. Use of such concretes allows the quick resumption of traffic movement and ensures high-quality combined work of the repair material with the base.

Further research should be aimed at determining the effect of hardening accelerator and steel anchor fibres on early adhesive strength (1–3 days), which is especially important for ensuring the durability of highway and airfield pavement repair areas. Also, taking into account the development of computer modelling, it is necessary to pay attention to the development of software for determining the adhesive strength of repair concrete because, according to Aman et al. (2022), there is no single methodology and there are significant differences in the results obtained. Therefore, it is necessary to accumulate, first of all, experimental data on the determination of adhesive strength, taking into account different stress–strain states.

R

adhesion strength with water treating (pull-off method)

R_ctk

adhesion strength with water treating (flexural strength method)

R_ctk.P

adhesion strength with primer treating (flexural strength method)

R_P

adhesion strength with primer treating (pull-off method)

X₁

hardening accelerator, % cement content (coded value)

X₂

steel fibre, kg/m³ (coded value)