By Gert Schmidt
Hindle Mason Projects (HMP) has mastered the art of structural strengthening over more than 30 years. This is a field that complements our comprehensive range of concrete repair and protection services.
At some point in their service lives, concrete structures need to be strengthened. This may be due to design or construction errors, functional changes, design code updates or lack of maintenance. Structural strengthening also becomes necessary when the structural system has changed, or the asset has incurred serious damage over time.
Irrespective of the system type deployed, structural strengthening remains substantially more cost-effective than replacing a building in its entirety.
This again emphasises the importance of proactive maintenance and timely repairs.
The best structural strengthening method
As an expert, we first evaluate all failure modes to determine the best structural strengthening method to deploy.
Incorrect methods used may, for example, achieve flexural strength but result in shear failure. This is opposed to increasing load bearing capacity. Refer to Flexural Strength of Concrete – EngineeringCivil.org. Shear Cracks Explained (How Shear In Reinforced Concrete Works) – Structural Basics provides more information on shear. Also note Load Bearing Capacity – an overview | ScienceDirect Topics.
Other factors also need to be considered. For instance, when a critical element is strengthened, another one can become critical.
Our teams investigate the condition of the entire structure to avoid implementing undetermined changes in the structural system.
Crucial information that we need includes details of past modifications to the as-built drawings. This is in addition to actual sizes of the concrete elements and their concrete mix composition. We will also note the location and size and cause of cracks and spalling. This is in addition to the location and extent of reinforcing corrosion. This, while also accurately noting the quantity of reinforcing steel used and where it has been placed. We will also evaluate applied loads.
However, one of the most critical factors to consider is the concrete surface. Concrete must be able to transfer loads from existing structures to applied strengthening mechanisms. Therefore, we may have to remove and replace existing surface concrete when strengthening elements.
We also select systems that reduce maintenance and repair needs.
Of course, there are a host of other factors that also need to be considered. These include site access limitations, operating time schedules, as well as budget and financial limitations. Also of significance is the required useful life of the structure, environmental aspects and weather.
Structural strengthening via retrofitting
One of the ways that we achieve structural strengthening is via retrofitting concrete elements.
This technique is also referred to as section enlargement, involving placing a reinforced concrete jacket around existing elements. This increases load-carrying capacity or stiffness.
However, a major drawback of this system is that it increases the size of concrete elements.
We first remove the deteriorated concrete and corrosion from the exposed reinforcing bar. The surfaces are then cleaned and prepared to ensure a strong bond with the repair material.
The other traditional method that we use entails bonding steel plates or flat bars to structural elements using epoxy adhesives. Dowels or bolts are glued to holes drilled in concrete elements to strengthen the bond.
A disadvantage of this method is that it cannot be used to strengthen severely damaged concrete elements.
Refer to Untitled-4.
Structural strengthening via post-tensioning
We also achieve structural strengthening via post-tensioning, a quick and cost-effective technique.
This method has been very effective in increasing the flexural and shear capacity of reinforced and prestressed concrete elements. This is by correcting excessive deflections and cracking after the elements have undergone structural repairs. Refer to. Flexural Capacity – an overview | ScienceDirect Topics.
Post-tensioning forces are provided by conventional prestressing tendons or high-strength steel rods installed outside the elements. Tendons are connected to the structure at anchor points usually at the ends of the concrete elements. Cracks are repaired using methods, such as epoxy injection [epidermix Crack Injection System | a.b.e.®] Spalling patching must also be undertaken to ensure that prestressing forces are distributed uniformly across sections of elements. Refer to Spalling concrete: the causes, and control, repair & protection methods | Mapei.
Bolted to structural elements, end anchors consist of steel fixtures or cast-in-situ reinforced concrete blocks. Uplift forces are provided by deviation blocks fastened at high or low points of structural elements.
Refer to Post-Tensioning- Methods for Reinforcing Concrete – Concrete Network.
Structural strengthening with ferrocement
HMP is also equipped to undertake structural cement with ferrocement [What is Ferrocement? | Concrete Society].
Ferrocement is a thin composite material that is reinforced with wire meshes. They are uniformly distributed in continuous layers with relatively small diameters.
This method improves cracking resistance, flexural stiffness and the ultimate loads. However, this is dependent on the complete action between the ferrocement layers.
Concrete beams need structural strengthening
Reinforced concrete beams need structural strengthening when existing steel reinforcement or cross-sections are insufficient. These concrete elements will also have to be reinforced when their loading has been increased.
The system that we deploy depends on whether we want to enhance beam section flexural or shear capacity. This can be achieved by installing steel plates in flexural zones or new steel rods/stirrups without enlarging sections. Alternatively, we can add new steel and enlarge sections. Ferrocement technology or even post tensioning can also be deployed for this application.
Refer to 2016-3-4-1-Ozbek.pdf.
Structural strengthening of columns
Structural strengthening of reinforced concrete columns is undertaken using various traditional methods.
One such technique is reinforced concrete jacketing. The size of the jacket and the number and diameter of the steel bars used is based on our structural analysis. Before deploying the technique, we need to temporarily reduce loading with hydraulic jacks.
Confinement can also be achieved by adding steel plates to the surface of columns.
Steel jacketing is used when we cannot increase the cross-sectional area. It is manufactured in two shell pieces that are welded together around columns.
Another method entails installing horizontal angles to slabs and to the top and bottom end of columns. Thereafter, four vertical steel angles are applied at the corners of the columns and welded to the angles. Steel strips are then welded to the vertical angles at specified distances around the column perimeter.
Confinement with external pressure is another way that we strengthen beams, columns and beam-column connections. It entails applying external confining pressure along the element length through a set of elongated elements. They are then fixed between with strips.
This technique increases the strength and ductility of reinforced concrete elements without raising their weight. Furthermore, the technique reduces lateral strains, internal cracking and volume increases when adding more loads to the elements.
Refer to Steel Jacket – an overview | ScienceDirect Topics.
Advances in structural strengthening
HMP has kept pace with advances in structural strengthening systems, including fibre-reinforced polymer (FRP) materials.
It is extremely lightweight with 6,2kg of FRP materials able to substitute 175kg of steel plates. This provides ease and speed of installation. Combined with excellent corrosion resistance properties, FRP materials reduce installation and maintenance costs.
FRP is the only method for strengthening elements in areas that cannot be accessed with machinery.
We have installed FRP to the tension side of concrete beams, girders and slabs to provide additional flexural strength. HMP has also enhanced the flexural strength of beams and girders by applying FRP materials on their sides.
HMP also wraps columns to enhance ductility due to induced concrete confinement.
Refer to Fiber Reinforced Polymer (FRP) Fabrics and fiber-reinforced-polymer–frp–composite-systems-brochure.pdf.
Effective beam structural strengthening technique
FRP is a very effective beam structural strengthening technique.
Composites are bonded to the tension and/or side faces of concrete beams. This provides additional tensile reinforcement and increases element strength in bending by orienting the fibres along beams’ longitudinal axes.
Its efficacy depends mainly on the integrity of the bond achieved between the composite and surface. Therefore, anchor bolts will sometimes be added to the ends of FRP plates to prevent debonding and peeling.
We have used various bonding techniques to improve the shear capacity of reinforced concrete beams. The shear effect of FRP external reinforcement is maximised when fibre direction coincides with maximum principal tensile stress.
Maximum principal stress trajectories in shear-critical zones of structural elements subjected to transverse loads form an angle with member axes. However, sometimes it is more practical to install external FRP perpendicular to the axis direction with the principal fibre direction.
We structurally strengthen concrete slabs
We have also deployed this system to structurally strengthen simply supported reinforced concrete slabs. This has been done by bonding FRP strips to soffits along the required direction.
FRP strips are applied in two directions to structurally strengthen two-way slabs. Strengthening is concentrated in the centre, considering possible collapse mechanisms, with strips terminating far away from the edges.
Structurally strengthening existing reinforced columns
Structurally strengthening existing reinforced concrete columns with FRP is based on a well-established fact. This is that lateral confinement of concrete can substantially enhance axial compressive strength and ductility. Refer to Concrete Compressive Strength – an overview | ScienceDirect Topics. The Role of Ductility in Construction for Flexible and Resilient Designs is also another important reference.
FRP provides a means for confinement without increasing stiffness, while also facilitating rapid fabrication of cost-effective and durable jackets.
The jackets are loaded mainly in hoop tension to confine concrete subjected to axial compression. This ensures that both materials are used to their best advantages. Both the strength and ultimate strain of the concrete is enhanced, while the tensile strength of FRP is used effectively.
FRP wraps can also carry tensile forces around the perimeter of columns. This is as a result of lateral expansion of the column when loaded axially in compression. Constraining lateral expansion confines concrete, increasing its axial compressive capacity. Refer to Bending and axial force.
Rectangular columns or those square in cross section can be confined at corners only with negligible resistance to lateral expansion. This is provided along the flat column.
Another HMP structural strengthening technique
Another HMP structural strengthening technique is near-surface mounted reinforcement.
The technique entails bonding a composite rod in a pre-sewn grooves along the concrete cover on the tension side. This is achieved with an epoxy paste or cement grout.
It increases load-carrying capacity of existing reinforced concrete elements. The method is particularly suited to flexural strengthening in the negative moment regions of slabs and decks. This is where external reinforcement is subjected to mechanical and environmental damage and, therefore, requires protective cover.
Learn more about HMP. www.hindlemason.co.za
Schmidt is a Director of HMP