Post-Fire Bond Behavior of Cement-Based Post-Installed Reinforcing Bar Systems in Concrete
Presented By: Omar Al-Mansouri
Affiliation: CSTB
Description: When post-installed reinforcing bar systems (PIRs) are exposed to temperature increase, their bond is weakened. Within the evaluation of fire safety of PIRs, the bond-temperature relationship is determined by pull-out tests on PIRs bonded in concrete cylinders. This is done by applying a constant tensile load on the head of the rebar and a gradual temperature increase of the adhesive through the test using an electric oven, until pull-out failure at a certain failure temperature is reached. However, heat transfer continues during the post-fire phase due to thermal inertia of the heated materials. This paper presents an experimental study on postinstalled rebars bonded in concrete cylinders using a cement-based mortar. The aim is to evaluate the post-fire bond strength-temperature relationship by means of post-fire pull-out tests carried out according to the following steps: a constant tensile load on the steel bar and a progressive temperature increase applied up to several heating temperatures without reaching failure during the heating phase. Then, the system is cooled down to several temperature levels while keeping the load constant. Finally, the post-fire pull-out test is conducted at several cooling temperatures. The results are discussed and compared to the performance of a previously tested epoxy-based adhesive.
Post-Installed Rebar for Widening Concrete Bridge Slabs: Experimental and Numerical Study
Presented By: Dheeraj Waghmare
Affiliation:
Description: One of the methods to address the traffic congestion problems is to widen existing concrete bridge slabs. This is traditionally done by partly demolishing concrete and exposing the internal reinforcement to use it to form lap splices with the starter bars that can be used to form the connection with the new reinforcement in the extended parts of the slabs. In this work, an alternative, low-invasive solution is explored that consists of installing post-installed reinforcing bars to act as the starter bars for the extended portion of the slab. The method involves in making a diamond saw cut in the existing slab to visualize the exact location of the existing reinforcement, installing the post-installed reinforcement close to the existing reinforcement and using it as starter
bars, facilitating lap splices with reinforcement in new concrete. For safe designs, the embedment length of the bars should be enough to provide the required resistance.
Current design standards, such as ACI 318-19, allow the use of post-installed rebars if they meet AC308 qualifications. In this case, the post-installed rebar is designed as an equivalent castin straight rebar, which requires relatively large anchorage length. However, the high bond strength of post-installed rebar systems is often underutilized in these designs. Alternatively, rebars can be designed as adhesive anchors, considering concrete cone breakout and rebar bond pull-out failure modes. However, in reality, post-installed rebars are installed close to edges with minimum cover, and the failure is often governed by concrete splitting before any bond pull-out of the rebar or concrete cone breakout occurs. The formulations to design for concrete splitting are very different in the two approaches (design as reinforcing bar vs. design as adhesive anchor).
Assessment of Existing Anchorage to Concrete
Presented By: Lennart Elfgren
Affiliation: Lulea University of Technology
Description: There is a need to be able to evaluate the condition and capacity of existing anchors.
Several factors influence the remaining capacity as e.g.? What kind of environment surrounds the fastening? (humid/dry, high/low temperature, acid/non-acid)? Have any of the components of the fastening or its base material deteriorated since it was installed? What kind of loading history has the fastening been subjected to (static loading, impulse loads, accident loads, fatigue loads,seismic loads).? Many fastenings were designed when cracking of the concrete was not considered. If this is the case, a reduction in the capacity due to cracked concrete should be introduced where applicable. Factors as above may influence both the resistance R of a fastening and the load effect E on it. The influences may be both negative (e.g., deterioration or high loads) and positive (e g. increased concrete strength or loads that can be determined to be less variable than what was originally assumed). These new conditions may influence the partial coefficients in an enhanced evaluation or the variances in a reliability analysis. Two types of assessment will be presented here:
(a) For anchorages with sufficient background information available, theore cal assessment methods based on given values in approvals and equations.
(b) Assessments based on load testing on (some of) the anchors up to a specified load limit (or to failure).
Prediction of Long-Term Behavior of Bonded Fasteners Under Sustained Loads
Presented By: Doruk Gurkut
Affiliation: Department of Civil and Environmental Engineering
Description: After several tragic incidents, it became evident that long-term performance of bonded fasteners is vital; hence, must be assessed thoroughly. This paper reviews several existing methods that are proposed in widely accepted guidelines to evaluate the creep displacement of bonded fasteners under sustained loading conditions, which is the most important parameter to assess their long-term performance. A new method incorporating power law regression analysis is proposed to predict creep displacement of bonded fasteners. This method is based on the original stress- and time dependent equation of Findley, which enables the possibility of making arbitrary creep displacement predictions for various sustained load levels without making additional tests. The proposed method is applied to a data set obtained from a series of sustained load tests on a bonded anchor product and the obtained equation was found to accurately represent measured data. Creep displacement predictions were made and load ratio versus time-to-failure (LRvTTF) domain is obtained by considering different limit creep displacement values. The displacement predictions are meant to be evaluated by further testing at different sustained load levels.
Load Bearing Behaviour of Fastenings with Effective Embedment Depth Less than 30 mm (1.2 in.)
Presented By: Michael Eckstein
Affiliation: University of Stuttgart
Description: The Concrete Capacity method (CC) has been validated using experimental results in many studies. However, this approach is not without limitations. According to the current regulations, this method is only used for fasteners down to an anchorage depth of hef = 40 mm. The current developments in the field of structural engineering, which include the improvement of concrete compositions resulting in higher compressive strengths and better mechanical and physical properties, make it possible to build thinner structures made of reinforced concrete. Therefore, it is important to understand the behaviour of anchorages with smaller embedment depths and to test their applicability. This work focuses on the load bearing behaviour of fastenings with effective embedment depths less than 30 mm. Various factors influencing this behaviour and the durability of the fasteners on the concrete cover are investigated. For this purpose, over 284 tests were performed. Based on the results, a new design model is introduced for fasteners with small embedment depths. The results show that this model estimates the loads of the small anchorage depths (up to 30 mm) very well. Further research is necessary to extended and improve the introduced model.
A New Predictive Equation for the Factor asetup for the Design of Adhesive Anchors
Presented By: Akanshu Sharma
Affiliation: Purdue University
Description: One of the major failure modes specific to adhesive (bonded) anchors subjected to tension loads is the combined pullout and concrete failure mode, which is characterized by the partial bond pullout and partial concrete cone breakout. In the qualification of adhesive anchors, confined pullout tests are carried out to obtain the confined bond strength, which corresponds to the equivalent uniform bond stress along the depth of the anchor at peak load. To calculate the resistance against this combined failure mode, a so-called unconfined bond strength is used in design, which is obtained by multiplying the confined bond strength of the anchors with a factor called “asetup”. In the current codes, a constant value of 0.75 is recommended for this factor. Recent evidences have questioned this value since the tests have shown that the actual value of this factor may be much smaller than the recommended value. This paper investigates the influence of various parameters on the value of the factor asetup and proposes a new expression, which is more general and rational. Nonlinear 3D finite element simulations are used to quantify the influence of various parameters, numerical and test data from the literature is used to develop the equation, while additional testing is performed to further verify the validity of the equation.