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Precast Deck Connection Slim Fastener

Development of the precast deck connection Slim Fastener
Shortens the deck connection time in slab replacement work by up to 50% and realizes high durability in the joints

Obayashi Corporation has developed Slim Fastener, which greatly reduces the time required for slab replacement work and realizes high durability in the joints, by using Slim-Crete, which we developed as room temperature-hardening, ultra-high-strength fiber-reinforced concrete, in the connection of precast decks.

Overview of Slim Fastener

In recent years, renewal work on road bridges has been progressing nationwide due to the increasingly decrepit expressways. When decks are replaced as part of the renewal work, it is necessary to restrict the flow of vehicle traffic, and generally precast decks produced in a factory in advance are used in order to complete the work in as short a construction period as possible.
The adjacent precast decks installed on the steel main girders delivered to the sites lap for a prescribed length the rebars which protrude out from both sides of the decks, the rebars for reinforcement are laid out, and the decks are connected to that part by implanting high-strength concrete. On the sites, the bar arrangement work for reinforcing the joints and the formwork and falsework assembly and dismantling work for implanting the concrete took a lot of time and effort; furthermore, it was necessary to apply an epoxy coating with an antirust effect to the rebars inside the joints in order to ensure durability.

The procedure for erecting precast decks in slab replacement work

The Slim Fastener developed by Obayashi Corporation on this occasion can shorten the length of the rebars protruding from the deck and reduce the width of the joints to half of the conventional width, thereby reducing the amount of concrete placed, by using Slim-Crete, an ultra-high-strength fiber-reinforced concrete, for the connection of the precast decks instead of high-strength concrete.
Moreover, rebars for reinforcing the joints are no longer necessary, and also the width of the joints becomes narrower, so the falsework that was necessary on the undersurface of the joints is no longer necessary. Furthermore, Slim-Crete is also a dense material, so corrosion factors have difficulty penetrating inside the concrete, meaning that the antirust processing of the rebars is also no longer necessary.

The features of Slim Fastener are as follows.

  1. Great shortening of construction time due to the reduction of site work

    With this construction method, Slim-Crete, an ultra-high-strength fiber-reinforced concrete, is used for the connection of the adjacent precast decks. For the joints using the conventional construction method, rebars were lapped at the prescribed length, rebars for reinforcement were added at the site, formwork and falsework were installed on the undersurfaces of the decks, and high-strength concrete was implanted.
    The Slim-Crete used for the connections in this construction method is ultra-high-strength fiber-reinforced concrete, so the adhesion strength with the rebars is high and it is not necessary to lap the rebars protruding from the adjacent decks. The rebars can be shortened and due to the ultra-high-strength and effect of the steel fiber the rebars for reinforcement are no longer necessary. The width of the joints is half of the conventional width, so the amount of concrete placed can also be reduced by half, and in addition the falsework for the formwork is no longer necessary due to the decrease in weight. With this construction method, light buried formwork is installed on the joints of the precast decks at a factory in advance, so all of the formwork and falsework assembly and dismantling work on the undersurfaces of deck joints, which were previously implemented on sites, has become unnecessary.
    Due to these factors, for the same costs as with the conventional construction method, the work hours required for the connections of the decks are shortened by up to 50% of the conventional work hours, realizing a great shortening of the construction period for slab replacement work.

    Precast decks: comparison of connection methods
  2. Long-term use in harsh environmental conditions is possible

    The design service life of general concrete is about 50 years, but the design service life of Slim-Crete has been certified as 100 years in the technology evaluation by the Japan Society of Civil Engineers, and it is a material that is resistant to cracking and has a high bonding strength with precast decks. Furthermore, in this construction method, the connection boundary surfaces of precast decks are made into a special uneven shape called multiple shear keys, thereby enhancing the shear capacity and bonding effect of the boundary surfaces.
    In the test to confirm the long-term fatigue durability, no cracking, etc. was recognized in the joints even with respect to repeated loading equivalent to 100 years of use, confirming that the bonding of the precast decks is maintained. The material itself is dense and offers high resistance to neutralization and salt water damage, so antirust processing such as epoxy coating is not necessary for the internal rebars, and long-term use is possible even in harsh environmental conditions.

It is also possible to apply Slim Fastener to the decks of newly constructed bridges and roads and railroads in shield tunnels for which rapid construction is necessary; Slim Fastener is not limited to slab replacement work on expressways, etc. only.
Note that this construction method is a technology which was developed through technology exchanges with Kraemer North America, LLC, an American subsidiary of Obayashi Corporation. Furthermore, it won the FY2019 Japan Society of Civil Engineers Technology Development Award.

A room temperature-hardening, ultra-high-strength fiber-reinforced concrete material which is a mixture of ultra-high-strength mortar with ultra-high-strength steel fiber. In addition to being ultra-high-strength, it also has high durability. General ultra-high-strength fiber-reinforced concrete is manufactured in factories equipped with heat curing facilities in order to deliver the prescribed performance, but Slim-Crete delivers the prescribed performance with curing at normal temperature, so implantation of Slim-Crete at construction sites is possible. Slim-Crete won the Grand Prize at the 18th Infrastructure Technology Development Awards and the FY2016 Japan Society of Civil Engineers Technology Development Award.

The formwork which is not dismantled after the concrete has been implanted. A part of the structure.

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