Shear strength of epoxy anchors embedded into low strength concrete

https://doi.org/10.1016/j.conbuildmat.2012.09.020Get rights and content

Abstract

Chemical anchors are getting more frequently used to connect structural elements. The studies regarding the chemical anchors embedded in low strength concrete are very limited in the literature. However, the compressive strength of the concrete may be 10 MPa or lower in many strengthening applications. Steel bars having 12, 16 and 20 mm diameters have been selected as the anchor rod in this study. They have been embedded in to concrete blocks with 5.9 and 10.9 MPa compressive strength. Solvent-free epoxy based three component chemical adhesive has been used for the connection between concrete and anchor bar. The depth of holes is 10, 15 and 20 times that of the anchor diameter. The anchors have been embedded such that they are sufficiently away from the free edge so as not to cause any concrete failure. The load–displacement cycles of all anchors have been obtained by reversed cyclic tests with incremental displacement. The obtained results indicate that increasing the anchor diameter have decreased the shear strength. Even though the anchor damage has been caused by steel failure, a decrease in shear capacity was observed with the lower strength concrete.

Highlights

► Cyclic shear tests were conducted on anchors embedded to low strength concrete. ► The obtained results indicate that increasing the anchor diameter have decreased the shear strength. ► A decrease in shear capacity was observed for lower concrete strengths. ► A reduction factor is introduced depending on the bar diameter and concrete strength. ► Establishing an upper limit for the anchor bar diameter in the related standards is proposed.

Introduction

Anchors that are used to provide the connection between two different elements can be categorized under two categories as cast-in-place and post-installed anchorages. Post-installed anchorages could be manufactured using different methods such as mechanical, grout or chemical. The behavior of cast-in-place anchors [1] and post-installed mechanical anchors [2] have been studied considerably well in previous studies conducted in the past years. The design of cast-in-place anchorages [3] and mechanical anchorages [4] has been consigned to a reliable procedure as a result of these studies. Although the use of chemical adhesives in the construction sector goes back to late 1960s [5], the studies on the use of chemical anchors especially used for strengthening applications are relatively recent and a standard for specifying the design principles of such anchors has not yet been established, also under the influence of wide variety of materials [6].

Chemical anchors are embedded in the holes set up in the hardened concrete. The diameter of the drilled hole is at most 50% larger than that of the bar diameter [2]. Chemical adhesives are among the best solutions providing the bonding forces between the concrete and the steel [7]. Chemical anchors have begun to be widely used starting in the 1990s with the development of high resistance adhesives of polyester, vinylester and epoxy type [8], [9]. Nowadays, many products are available in the market in terms of chemical adhesives. However, the bonding resistance of epoxy type products is usually higher than that of esther based products [10]. Many parameters such as the cleanness of the drilled hole, the method of drilling, the humidity level of the concrete and temperature may affect the bond strength in addition to the type of adhesive [2].

The first studies on chemical anchors go back to the early 1980s [11]. Most parts of those studies are based on experimental studies for determining the tensile strength of the anchors. The effect of different factors on the anchor tensile strength has been investigated in those studies. Factors such as the adhesive thickness, the type of filler material added to the adhesive [12], the embedment depth [8], the anchorage diameter [13], [14], the steel resistance [15], the edge distance [16] and the distance between the anchorages [17] have been investigated in parts of these studies. In some studies, it has been observed that the concrete strength and the type of aggregate were also taken into consideration [10], [14]. A study investigating the behavior of both the single and the group anchors has been recently completed [2]. Studies are also present regarding the anchor tensile strength under dynamic loads in response to increases in the loading speed [18], [19]. Some studies that have focused on the tensile behavior of chemical anchors are concerned with partially bonded anchors [1], [20].

In addition to the studies on the subject of the anchorage tensile behavior, the studies on the topic of its shear behavior have been very limited. Only one study could be accessed on the shear behavior of chemical anchors [17]. Studies on this topic are mostly carried on mechanical and cast-in-place anchors [21], [22]. In other conducted studies, the shear resistance of single anchors and double bolted group anchors [23] the shear-tension interaction [24] has been investigated. In another study on anchors near the free edge and the dynamic behavior of anchor groups [25] with and without hairpins have been used. The hairpinned samples were observed to have more resistance at the end of these experiments.

Basically concrete anchors attain load capacity by two failure modes as steel and concrete failure [26]. The steel failure determines the shear capacity of anchors that were embedded usually further away from the free edge. When shear stress is applied on the anchor, the steel rod embedded in the concrete is damaged through the bending behavior. At that moment, local crushing is observed in the concrete around the steel [27]. The concrete based failures take place as a breakout failure when the anchor is closer to the free edge and as a pryout failure when it is further away [3]. The 45° Cone [21] and concrete capacity design methods [28] to determine the capacities regarding these failure types were developed based on these available test data. The method presented in the PCI Design Handbook [29] is another method that may be used as an alternative [6]. In addition to these, several studies regarding the anchor strength determined from neural networks are also present [26], [30].

In most of the previous studies, the anchors were embedded to normal or high strength concrete. However, in many buildings, concrete possessing lower strength values than these are encountered [31], [32]. Therefore, the shear strength of chemical anchors on low strength concrete is still an important subject area. The shear behavior of chemical anchors has not been studied thoroughly. In this study, the behavior of epoxy bonded anchors embedded in low strength concrete under reversed cyclic shear loads is investigated.

Section snippets

Test specimens

As it is known, many strengthening applications are performed on buildings that are made of low strength concrete. Because of that, in order to better represent the practice, the anchors have been embedded in concrete elements with low compressive strength [31], [32]. Anchors were embedded in two concrete grades with different compressive strength, which are named as C5 and C10, in scope of the study. Other parameters that have been used in this study are the anchor bar diameter and the anchor

Test results

The hysteresis curves for anchors under shear loading that have been embedded into C5 and C10 grade concrete are given in Fig. 4, Fig. 5. The shear load capacity has been observed to increase with increasing diameters in the obtained curves, as it was expected. However, with increasing embedment depth, a significant change could not be observed in capacity except for the C5D12L24 sample. The hysteresis curves indicate that the anchorages possess different capacities for push and pull

ACI318 anchor design method for shear loads

In ACI318 Appendix D three different failure modes were defined upon reaching the anchor shear capacity: steel failure, concrete breakout failure of near edge anchors and concrete pryout failure. These modes of collapse are shown in Fig. 7. In situations where the anchor bars is closer to the edge, the ultimate concrete cone capacity generally governs the anchor capacity and where it is further away from the edge, the shear capacity of the anchor bar is the main determining factor.

Taking steel

Conclusions

Considering the concrete classes and the materials that have been used, the following implications for anchors presenting steel failure under shear could be obtained from this study:

  • 1.

    Although the load capacity of the adhesive anchors that have been embedded away from the edges increases with increasing diameter, the ultimate shear stress levels of the anchor bars decreased.

  • 2.

    In order to provide the required anchor area, through the use of many small diameter anchors rather than fewer large

Acknowledgements

This study has been supported by the Turkish Scientific Research Council (TUBİTAK) through Project No. 107M572 and by Eskişehir Osmangazi University (ESOGÜ) through the Scientific Research Project No. 200815012. The authors greatly acknowledge the support by TÜBİTAK and ESOGÜ.

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