Materials and Structures (2017) 50:249 https://doi.org/10.1617/s11527-017-1115-6 ORIGINAL ARTICLE Bond of steel reinforcement with microwave cured concrete repair mortars P. S. Mangat . Shahriar Abubakri . Konstantinos Grigoriadis Received: 30 June 2017 / Accepted: 17 November 2017 / Published online: 28 November 2017 Ó The Author(s) 2017. This article is an open access publication Abstract This paper investigates the effect of A unique relationship exists between bond strength microwave curing on the bond strength of steel and both compressive strength and porosity of all reinforcement in concrete repair. Pull-out tests on matrix materials. Microwave curing reduced shrink- plain mild steel reinforcement bars embedded in four age but despite the wide variation in the shrinkage of repair materials in 100 mm cube specimens were the repair mortars, its effect on the bond strength was performed to determine the interfacial bond strength. small. The paper provides clear correlations between The porosity and pore structure of the matrix at the the three parameters (compressive strength, bond steel interface, which influence the bond strength, strength and porosity), which are common to both were also determined. Test results show that micro- the microwave and conventionally cured mortars. wave curing significantly reduces the bond strength of Therefore, bond-compressive strength relationships plain steel reinforcement. The reduction relative to used in the design of reinforced concrete structures normally cured (20 °C, 60% RH) specimens is will be also valid for microwave cured elements. between 21 and 40% with low density repair materials and about 10% for normal density cementitious Keywords Microwave curing Á Bond strength Á mortars. The corresponding compressive strength of Shrinkage Á Porosity Á Compressive strength Á Concrete the matrix also recorded similar reduction and repair microwave curing resulted in increased porosity at the interface transition zone of the steel reinforcement. Abbreviations d Diameter of mild steel bar (mm) l Length of the steel bar in contact with the P. S. Mangat (&) Á S. Abubakri Centre for Infrastructure Management, Materials and repair (mm) Engineering Research Institute, Sheffield Hallam F Applied force (KN) University, Howard Street, Sheffield S1 1WB, UK fb Bond strength (MPa) e-mail: [email protected] fc Compressive Strength (MPa) S. Abubakri SD Standard deviation e-mail: [email protected] MIP Mercury Intrusion Porosimetry K. Grigoriadis vp Loading rate of pull-out test (N/s) School of Natural and Built Environment, Queens s Bond stress (MPa) University, David Keir Building, Stranmillis Road, r Radius of pore (nm) Belfast, Northern Ireland BT9 6AX, UK P Applied pressure (Pa) e-mail: [email protected] 249 Page 2 of 16 Materials and Structures (2017) 50:249 p Porosity patch to provide uniform heating across its depth. Less c Surface tension of mercury ((N/m) than 1 h of microwave exposure applied in the first h Contact angle between mercury and concrete stage of hydration is sufficient to achieve the desired (°) curing temperature [5–7]. There is already consider- able research [8–12] on the use of early age microwave curing to accelerate strength development of concrete. The expected advantages of microwave curing have 1 Introduction led to research around the world on developing microwave prototype systems [13–15] for commercial Reinforced and pre-stressed concrete infrastructure in use. the world has approached a stage where repair is often Most of the research on microwave curing of necessary due to various concrete degradation mech- concrete has focused on normal concrete materials anisms, abnormal loading or poor workmanship. with limited attention to steel reinforced concrete and Although most concrete construction is highly durable to in situ concrete repair which incorporates both in all kinds of environments, the concrete repair and proprietary repair mortars and steel reinforcement. maintenance market has grown exponentially in recent Microwave curing of concrete with steel reinforce- decades due to significant durability failures. It is ment raises two basic concerns. The first is safety essential to regularly monitor the performance of relating to the electrically charged or reflected reinforced concrete infrastructure and carry out timely microwave energy causing sparks and overheating at repair to extend the service life of the concrete the steel surface. The second concern is the effect of structures and to prevent failure. However, factors microwave curing on the bond between steel rein- affecting the durability of repair itself have become a forcement and concrete, which is fundamental to the concern in recent years as most repairs prove unsat- composite action of reinforced concrete. The first isfactory within a few years [1, 2]. concern has been addressed by exposing parts of steel The rapid development of strength at early age reinforcement protruding from concrete specimens to during the application of repair is highly desirable as it microwave powers of up to 420 W without recording can provide an efficient and economic repair. This any adverse effect [5]. The second aspect of rein- enables minimum disruption to traffic, for example, forcement bond strength is addressed in this paper. It during localised repair in concrete pavements. Precast deals with the bond of plain steel reinforcement industry can also benefit from rapid strength develop- embedded in different concrete repair materials. ment by increasing their turnover. Another important Another aspect of bond strength which is of critical economic advantage provided by high early age importance for the efficient performance of concrete strength development is to enable concrete construc- repair is the interfacial bond between the substrate tion in cold weather [3]. This will allow continuous concrete and patch repair, which has been reported construction activity in winter, thereby improving the elsewhere [16]. economics of the construction industry in cold regions The composite performance of reinforced concrete of the world. depends on the efficient interaction between the steel Conventional heating methods are currently used in reinforcement and the concrete matrix, which relies on the concrete industry to achieve high early-age their bond strength. Plain steel bars provide a true compressive strength of concrete. However, there are bond failure mechanism with concrete, which includes limitations with these methods [4] and they cannot be chemical adhesion and frictional resistance. The pull used efficiently, safely and economically especially out strength of deformed reinforcement bars overes- for in situ repair or for new construction on site. timates the basic bond strength by including the Microwave technology, on the other hand, is poten- mechanical interlock strength of the concrete [17]. tially a very effective method to cure cementitious The simple pull-out mechanism of the plain rebar repair materials at early age and develop strength embedded in concrete is made more complex by the rapidly. Unlike conventional heating, microwave bursting forces generated by the wedge action of ribs curing generates heat from within concrete or a repair on the rebar which is confined to different degrees by the concrete cover and confining reinforcement Materials and Structures (2017) 50:249 Page 3 of 16 249 [18, 19]. The bond behaviour is influenced by factors Repair Material 1 A proprietary shrinkage-com- such as the rib geometry, their relative surface area pensated, rapid hardening cement mortar with pul- [20] and interlock with concrete lugs [21]. The fib verised fuel ash. Density of the fresh mix was 2260 kg/ model design code [22] provides a basic expression for m3. rebar bond strength, relating it to the square root of Repair Material 2 A proprietary polymer-modified concrete strength while also incorporating other cement mortar, fibre-reinforced and shrinkage-com- empirical coefficients which depend on many param- pensated. Density of the fresh mix was 1730 kg/m3. eters such as geometry of the section, strength, Repair Material 3 A proprietary lightweight, low diameter and surface geometry of reinforcement, permeability, polymer-modified cement mortar. Den- concrete cover, bar spacing and confinement of sity of the fresh mix was 1540 kg/m3. reinforcement [22]. However, the main aim of the Repair Material 4 A mortar with CEM II/A-L 32.5 research reported in this paper is to isolate and R Portland-limestone cement [24], coarse sharp sand determine the effect of microwave curing on rein- (50% passing a 600 lm sieve) and w/c ratio of 0.45. forcement bond strength without introducing other Density of fresh mix was 2200 kg/m3. factors which may interfere with the clarity of results. Therefore, plain reinforcement bars have been used in 2.2 Microwave curing regime the experimental programme to provide a basic bond mechanism. In practice also, many reinforced concrete A Sharp Model R-2370 commercial microwave oven structures which have been constructed with plain with a maximum actual output power of 1320 W was steel bars in the past have reached their repair stage used for curing the specimens. It was calibrated and the bond of this reinforcement in repair patches according to ASTM F1317 [25] and BS EN 60705 has become important. [26]. The microwave oven could be set to generate The main parameters of microwave curing which power at incremental levels of 10% up to 100% of its are likely to affect the interfacial bond strength are the maximum output. The microwave frequency of the porosity and pore structure of the matrix at the steel oven was 2.45 GHz. A 10% power level was used to interface, the shrinkage of the matrix and its com- generate actual output power of 132 W to microwave pressive strength. Therefore, in addition to pull out cure specimens. Lower powers of 92 and 60 W were tests of reinforcement, the test programme of this also used for some tests. investigation also determines the effect of microwave curing on the porosity and pore structure of the matrix 2.3 Details of specimens, casting, curing by mercury intrusion porosimetry, shrinkage and and testing compressive strength by standard test methods [23].