Compressive strength of micropile-to-grout connections J. Veludo a , E.N.B.S. Júlio b,⇑ , D. Dias-da-Costa c a ICIST, Department of Civil Engineering, Polytechnic Institute of Leiria, Campus 2, Morro do Lena, Alto do Vieiro, 2411-901 Leiria, Portugal b ICIST, Department of Civil Engineering, Instituto Superior Técnico, Technical University of Lisbon, Lisbon, Portugal c INESCC, Department of Civil Engineering, University of Coimbra, Coimbra, Portugal article info Article history: Received 28 October 2010 Received in revised form 10 May 2011 Accepted 13 June 2011 Available online 12 July 2011 Keywords: Bond Strength Confinement Retrofitting Interface Micropile Grout Push-off tests abstract Strengthening foundations with micropiles is progressively being used, due to the major advantages that this technique presents. Nevertheless, the influence of some relevant parameters in the overall behavior of the retrofitted foundations has not yet been studied. Generally, micropiles are installed in holes drilled through the existing RC footing, which are then filled with grout. The efficiency of the load transfer mech- anism depends on the bond strength of both the micropile–grout and the concrete–grout interfaces. This paper describes an experimental study performed to specifically study the influence of the follow- ing parameters on the bond strength between micropile–grout interface: hole diameter; embedment length of the micropile; and level of confinement of the grout mass. Thirty micropile–grout specimens were submitted to monotonic push-off tests, until failure. Bond strength was found to increase with a decrease of the hole diameter and with an increase of the confinement level. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction RC footing strengthened with steel micropiles is today a wide- spread method. However, in practice, it is empirically applied. In reality, although the parameters that influence the connection capacity are identified, these have not been quantified in a compre- hensive way. In general, micropiles are applied to the existing RC footings through predrilled holes. After the micropile is installed, the hole is filled with non-shrink grout. It is known that the effi- ciency of the connection depends on the bond strength of both the micropile–grout and the grout–concrete interfaces but this has not been characterized so far. To increase the bond strength of the micropile–grout interface, it is common practice to weld steel rings or a steel spiral around the perimeter of the casing. To improve the bond strength of the grout–concrete interface, sometimes grooves are chipped into the wall of the hole. Detailing depends mainly on the required capacity [1–3]. Attention must also be paid to the reinforcement of the existing concrete footing since this can be inadequate for the strengthened situation. In this case, the lateral and the axial con- finement of the existing footing should be increased to improve the connection capacity. As mentioned, few experimental studies have been conducted on this subject. Gómez et al. [4] performed push-off tests to study the connection capacity of smooth micropiles, grouted in rein- forced concrete footings through holes drilled using a jack ham- mer. The authors concluded that the connection capacity is controlled by adhesion and friction at the micropile–grout inter- face and that the residual capacity of the connection is entirely fric- tional and dependent on the confinement provided by the footing reinforcement. It is also concluded that the connection capacity in- creases with the decrease of the hole diameter and that the embed- ment length has little influence on the bond strength. Contrarily to micropile–footing connections, the bond mecha- nism between reinforcing bars and concrete is well studied and it is generally accepted that it is controlled mainly by three differ- ent parameters: (1) chemical adhesion between concrete and steel; (2) friction between concrete and steel; and (3) bearing of the rebar ribs on the surrounding concrete [5–8]. Two different bond failure mechanisms can be observed: splitting failure or pull-out failure. Various studies have also been performed in grouted ribbed re- bar or cable bolts. Moosavi et al. [9] studied the bond of cement grouted reinforcing bars under constant radial pressure. These authors state that the properties of both grout and confinement play an important role in developing the bond capacity. Hyett et al. [10] performed several pull-out tests of grouted cable bolts using a modified Hoek cell and concluded that the bond strength at the cable bolt–grout interface is more related to friction rather than to chemical adhesion. The authors also state that the 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.06.007 ⇑ Corresponding author. Tel.: +351 218 418 258; fax: +351 218 497 650. E-mail addresses: joao.veludo@ipleiria.pt (J. Veludo), ejulio@civil.ist.utl.pt (E.N.B.S. Júlio), dcosta@dec.uc.pt (D. Dias-da-Costa). Construction and Building Materials 26 (2012) 172–179 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat