Idea Transcript
Instituto Superior Técnico Universidade Técnica de Lisboa
Connections Between Concrete Layers with Different Ages Bruno Baleia Casal Civil Engineering Department, Instituto Superior Técnico, Universidade Técnica de Lisboa, Portugal
Abstract
Regarding the design of structures, the connections between different elements play a main role in the efficiency of the structural system. These connections stand out, for example, in casting joints or in precast element joints, being an inherent feature in any building where structural concrete is used. The interface between concrete layers with different ages in the given examples must assure enough shear strength as a main requirement to resist the applied actions on the structure. Under this scope, the present dissertation aims to study the behavior of connections between concrete layers with different ages. Initially, the most relevant research projects, which represent significant advances at the time of their disclosure, are presented and assessed. Furthermore, this dissertation includes a synthesis of the most used behavior models that describe the shear mechanism: the adhesive bond, the dowel action and the friction. Later, an assessment and comparison of the most relevant normative expressions in Europe is performed, aiming to related these expressions to the mechanism models of the presented studies. Finally, the effects of shear are evaluated from an analytical point of view, with the presentation of two case studies regarding the shear requirements for safety of composite elements. Keywords: Concrete Connections, Adhesion, Bond, Friction, Dowel Action, Shear Resistance
1. Introduction The connections between concrete layers with different ages may occur in a wide range of situations, from structures rehabilitation and repairing to the construction of new buildings both with precast or cast-in-situ elements. The use of these solutions originates the so-called composite elements and, as a main requirement, these elements must assure enough shear strength in their interfaces. However, the shear stress transfer mechanism between two concrete layers is a complex phenomenon that involves the combination of different interactions and depends on several parameters that influence the transmission process, such as the amount of reinforcement crossing the interface, the compression resistance of the weaker concrete, the roughness of the interface, the presence of cracking or the stress caused by normal forces across the interface. This subject has been continuously studied over time, a justifiable attention that has provided several research projects since the 60´s. As a consequence, there is a large number of design expressions nowadays, which are included in several documents, model codes and normatives. These equations, which assess the behavior and/or resistance characteristics through the use of variable parameters, were calibrated in order to obtain design values similar to those from empiric tests.
It is within this scope that this document is based, which was prepared on the assumption of providing, at first, a better understanding of the physical model of the shear mechanism in a composite element. This assessment is therefore the basis to apply the design expressions in a judicious and assertive way and to reach the most efficient construction process of each solution.
2. Shear Transfer Mechanism This work starts with a research over the projects already performed under the basis of the presented subject. Regarding the most relevant studies, it is possible to summarize the ones which were fundamental to develop designing models and some important research works. 2.1. Hanson, 1960 In 1960, Hanson [12] presented the second part of a study in which the connections in bridges, both with precast or castin-situ elements were analyzed. In those tests, the following variables were considered: adhesive bond, roughness, keys and the shear reinforcement. Initially, the author performed 62 push-off tests in concrete elements. To establish a common basis of comparison between various contact surfaces in the push-off tests, the effect of stirrup reinforcement was isolated by subtracting the loaddeformation curve for stirrup-only (specimens with contact
2
B. Casal
surface smooth and unbounded) from the curves of all individual test specimens. The nature of failure in these tests is illustrated by the shear-slip curves in Figure 1. Stirrup Effect Subtracted For All Curves
4,14
Initial Peak
Average Shearing Stress (MPa)
3,45 2,76
Bond and Roughness
Bond and Roughness with Key Rough Unbonded with Key
2,07
Rough Unbonded
1,38 Smooth Bonded 0,69
test curves are conservative for a smooth bonded connection, representative for a rough bonded connection and inconclusive for a rough unbounded connection. 2.2. Birkeland and Birkeland, 1966 In 1966, Birkeland and Birkeland [8] published an article presenting connections in precast concrete construction. For the first time it was explained the shear friction analogy. When a joint between two rough surfaces is required to transfer shear forces, the shear will be resisted by friction, resulting from an external force (Figure 2 a) or from reinforcement crossing the joint (Figure 2 b). If the crack m-m is rough, sliding motion along it will cause a separation δ of the two halves. If the reinforcement is placed across the interface, the separation will develop tension T in the reinforcement, resulting in compression across the interface. The roughness may be visualized as a frictionless saw tooth ramp, where the angle of the teeth is the friction angle Ø and thus the friction coefficient is tanØ. P
0
0,127
0,254 Slip (mm)
0,381
In the second part of this study ten T-shaped girders were tested. This test allowed to compare the results of shearing stress developed in girders and push-off specimens. In order to establish this comparison, the variables of bond, roughness and stirrups were combined in the girder-slab contact surface. According to Hanson, it may be stated that the push-off tests give a good representation of the character of the stressslip curves for the girders testes. Quantitatively, the push-off
m
m
Figure 1 – Typical Shear-Slip Curves [12]
With these first results, the author reached some interesting conclusions, as follows: The effect of bond might be called a rigid type of connection. It is noted in Figure 1 that specimens for which bond was utilized as part as connection, developed a high shearing stress at low slip. When roughness and bond are utilized together, an early peak is followed by an immediately decreasing and posterior increasing of shearing capacity. The slope of the increase is dependent on the amount of stirrup reinforcement. In this case, Hanson suggests that the early peak stress is probably a suitable basis for practical design. The shearing stress of keys cannot be added to the contribution of bond and roughness. The slip movements required to develop the keys are greater than the movements for a bonded surface and it appears that bond must be destroyed in order for a key to act. For specimens with roughness unbounded some benefit from the keys was indicated by a slight increase in the average stress at the maximum points on the shear-slip curves. The key added also an initial peak stress at low slip which made the connection more rigid. Although the concrete strength was not investigated as a variable, some tests provide some indication of the effect of variation in strength of concrete. Shearing stress appears to be approximately proportional to the concrete strength.
P
V
0,508 V
P
μP N (=P) a)
Reforço m
V
T
V
T
V
m δ V
T b)
T Ø T.tanØ
Figure 2 – Shear friction hypothesis [8]
The ultimate shear capacity will be reached at yield of the reinforcing. If this taken as the yield stress of the reinforcement, then: (2.1) tan ∅ The value of tanØ depends on the type of concrete and the interface roughness. For artificially roughened joints it is recommended 1.4 and for ordinary joints 0.8 to 1.0. 2.3. Mattock and Hawkins, 1972 Mattock and Hawkins [16] developed a study in witch it was investigated the role of concrete strength, shear plane characteristics, reinforcement, and direct stress on the shear transfer strength of reinforced concrete. This work was based on 3 previous studies where monolithically cast push-off, pull-off and modified push-off specimens were tested. This last one was performed with an obliqued shear plane that originated the development of a compressive normal force across the plane. The following highlight the main conclusions: A pre-existing crack along the shear plane will reduce the ultimate shear strength and increase the slip in all levels of load.
B. Casal
For the same parameter ρfy, the changes of spacing, the strength and the size of reinforcement do not influence the shear strength. For values of ρfy