Mandos para modelos con doble altura de trabajo (OPCIONAL)

Theoretically, in order to strengthen an existing slab-column interface, three basics can be addressed in order to achieve the required increase in capacity, i.e. (1) increasing the effective depth of the slab and adding flexural reinforcing, (2) increasing the area of load transfer, thus increasing the critical perimeter, (3) the addition of shear reinforcing.

Access to the affected slab-column connection will depend on the specific use of the structure. For instance strengthening of a bridge deck connection would require a method to be implemented from below the slab instead of from above in order to minimize interference with traffic.

5.1.1. Increasing the Effective Slab Depth

5.1.1.1. Slab strengthened with additional concrete and vertical bolts

By adding an additional concrete layer onto the existing slab, the effective depth can be increased, along with this the addition of longitudinal reinforcing is possible, both enhancing the punching shear capacity of the slab. In order to prevent delamination vertical reinforcing bars (shear reinforcing) need to be doweled into the existing concrete.

This solution seems fairly simple in principle, but poses numerous disadvantages and complications:

 Additional dead weight of concrete – consuming some of the added punching capacity

 Doweling vertical reinforcing into the existing slab is rather costly  Proper bonding of the concrete layover to the existing concrete

Fig. 5.1 Slab strengthened with additional concrete and vertical bolts

Recalculation of the punching shear capacity can now be done, taking the increased slab depth; new longitudinal reinforcing and the additional or new shear reinforcing into account.

5.1.1.2. Slab strengthened with additional concrete and bonded steel plate

Similar to the above solution, a concrete layover increases the effective depth of the slab. Additional longitudinal reinforcing is added by means of bonding steel plates on top of the existing concrete.

Possible problems with this strengthening method are:

 Additional dead weight of concrete – consuming some of the added punching capacity

 Bonding of the steel plate to the existing concrete substrate is often not done to specification. This can be due to the use of unskilled or inexperienced labour, improper use of epoxy bonding agents and/or incorrect preparation of the concrete substrate.

Fig. 5.2 Slab strengthened with additional concrete and bonded

Recalculation of the punching shear capacity can now be done, taking the increased slab depth and additional longitudinal reinforcing (provided by the steel plate) into account. The

contribution of the plate needs to be investigated further. Reasons for concern are the following:

 The plate may act as a bond breaker between the existing concrete and the new overlay, causing delamination of the slab  The cross sectional area provided by the plate is not necessarily

optimally utilized, mainly due to the fact that it is located quite far from the extreme tension fibre in the slab section

5.1.2. Increasing the area of load transfer

By increasing the critical shear perimeter of the slab-column connection, the punching shear capacity can be enhanced substantially. This can be achieved rather easily by increasing the column diameter with shotcrete, conventional concrete or self-compacting concrete – see Fig 5.3 and Fig

5.4.

The use of self-compacting concrete and shotcrete would be suited best for this application. Practically speaking one would not be able to cast

conventional concrete to the underside of the existing slab. The only options are one of the following: (1) cast the last portion of the column through the existing slab or (2) grout the last portion of the column with an expanding cementitious grout.

Shotcrete and self-compacting concrete can easily be cast to fit snugly to the existing slab.

If not properly addressed load transfer to the column can be a problem. Due to differential creep the new concrete column will shorten and will consequently not be loaded as envisaged. This can be problematic, especially when casting the column head through the existing slab. Differential shrinkage of the old- and new concrete will cause the new

This remedial measure is cost effective, durable and reliable. However, it can be time consuming and rather expensive to install.

Fig. 5.3 Punching capacity increased with added column head

Fig. 5.4 Punching capacity increased with increased column cross section

The punching shear capacity of the slab-column connection can now be calculated using the enlarged column cross section, thus resulting in a bigger critical perimeter being considered, rendering a higher shear capacity.

5.1.3. Installation of additional shear reinforcing

The slab-column connection can be strengthened by means of installing additional shear reinforcing. The new shear reinforcing would typically consist of short reinforcing bars grouted into holes drilled in the slab.

5.1.3.1. Doweling additional bars into the existing slab

This remedial method will be very easy and fast to install, as well as relatively inexpensive.

Fig. 5.5 Punching capacity increased with additional shear

reinforcing

The increased punching shear capacity can now be calculated taking the additional shear reinforcing into account.

If a proper adhesive grout – e.g. HILTI HIT-RE 500® _{or similar – is}

used, the bonded bars should be at least as effective as bars cast into the slab during construction. For 450MPa reinforcing bars pullout tests on bars grouted with these grouts mostly result in the bars yielding before concrete pullout failure.

5.1.3.2. Slab strengthened with vertical bolts

In the case where an insufficient quantity of shear reinforcing is provided, additional shear reinforcing can be added by merely perforating the slab around the column to install steel bolts into the slab – Fig 5.6.

Fig. 5.6 Slab strengthened with vertical bolts

Due to the fact that micro cracking of the slab takes place during failure, the strength of the bolt is utilised along its length only if it is properly bonded to the concrete – simply assuming that the restraint provided by the bolt heads alone, is not adequate.

5.2. Repairing slab-column connections showing distress