Reliability Assessment of Existing Structures

Inspection can be carried out on existing structures, according to EU standards and National Requirements.

In this way, the static mapping and the creation of the static load carrier of the building are accurately implemented, which is then resolved with appropriate software, to find the static adequacy of the building.

The static mapping of the building in a non-destructive way using scanning equipment, creates with precision the static identity of the building, which is the unique tool, for any future intervention.

 Stages of inspection:

  • Level I: Visual Inspection, General Checks and Self-frequency measurement.
  • Level II: Data Acquisition through use of NDT Equipment and other testing methods.
  • Level III: Structural Analysis / design of the retrofitting (upon request)

 Inspection Level I / Visual Inspection and Documentation of findings

Visual inspection is the most basic NDT method and typically the first step in the evaluation of concrete structures. It can provide a huge amount of information, crucial to form a strategy for the inspections to follow. Broad knowledge of structural engineering, concrete materials and construction methods are needed to extract the most information possible. It is generally used for the identification of cracks, peeling and other imperfections on the surface and interior of the concrete. Simple tools for concrete crack size measurements might be needed.

The visual inspection method, although very simple and cheap, is extremely efficient, especially when combined with a different method.

It has the obvious limitation that only visible surface can be inspected. Internal defects cannot be noticed and no quantitative information is obtained about the properties of the concrete.

During a visual inspection, structural damage is detected, and recorded through drawings, pictures and videos, such as:

  • Cracks (position and size)
  • Corroded reinforcement bars
  • Moisture problems
  • Anomalies on concrete and steel (concrete peeling, material continuity and uniformity, signs of steel corrosion and concrete carbonation, exposed/broken rebars)
  • Measurement of the building’s frequency is performed, in order to create an evaluation tool, following any future structure alterations.

All the outputs of level I inspection, are provided in a detailed “Level I Technical Assessment Report”.

Inspection Level II / Data Acquisition

This procedure is more involved and time-consuming. It includes visual checks, measurements of building and soil properties and simplified analysis.

This inspection makes use of non-destructive testing (NDT) equipment, such as concrete magnetic/ultrasonic scanners and rebound hammers to provide additional and more reliable information of the concrete’s and rebars’ condition.

The Inspection Level 2 for existing buildings, can also include Destructive Tests, depending of the NDT results and the results of Level 1 Inspection.

All the outputs of level II inspection, are provided in a detailed “Level II Technical Assessment Report”.

NON-DESTRUCTIVE TESTING

1. Non-destructive tests on concrete

  • Concrete rebound hammer: This method is applied with the impact of a metallic mass on the surface of the examined material. Its rebound is measured on a calibrated gauge which translates it to compressive strength. It is applied to:
  • Estimate the material’s strength.
  • Estimate the material’s homogeneity.
  • Compare the quality of a structural element’s to another.
  • Nail extraction test: A nail launcher penetrates the concrete with a nail of specified dimensions. After 10 min, the force required for its extraction is measured with a dynamometer and translated to compressive strength. The method is applied to:
  • Estimate the concrete’s strength.
  • Estimate the concrete’s homogeneity.
  • Ultrasonic Pulse Velocity test (UPV): This method is based on the propagation of stress waves through concrete. A transmitter produces ultrasonic waves which are received by a receiver. The wave propagation velocity is estimated by the wave’s travel time.

The wave propagation velocity is directly correlated to the modulus of elasticity, Poisson ratio, density and geometry of the material. The method is applied to:

  • Estimate the concrete’s/masonry’s
  • Detect concrete internal incontinuities.
  • Detect pipes.
  • Estimate cracks’ size.
  • Estimate the depth of damaged layers.
  • Phenolphthalein carbonation test: The rebars are protected by corrosion through a very thin surface layer of iron oxide, which is created by the high alkalinity of its surrounding concrete. Concrete carbonation is the chemical reaction between the calcium hydroxide that exists in the concrete’s water and the atmosphere’s carbon dioxide, which leads to the loss of this layer. It is a sign that the rebars’ corrosion takes place and an indicator of the concrete’s durability. Carbonation can be detected with the use of phenolphthalein spray.
  • Infrared Thermography: This method is based on the principle that heat flows at a different rate on different materials. This leads to differences on the surface temperatures. An infrared thermography camera is able to ‘see’ and measure the infrared energy radiated by these surfaces, which is invisible to the human eye. It has proven to be an accurate and efficient method with many applications in civil engineering, as to:
  • Detect delaminations on the material’s surface.
  • Predict buckling, caused by thermal expanding, of exterior tiles on tall buildings.
  • Detect voids in the concrete’s core.
  • Detect concrete bands in brick walls.
  • Detect underfloor heating and thermal bridges, to assess the energy performance of buildings.
  • Detect moisture and heat leak caused by insulation problems of buildings.
  • Measure the liquid level of tanks.
  • Detect slab leaks.

2. Non-destructive tests on reinforcement steel:

  • Magnet scanners: Magnet scanners’ function is based on the interaction of the reinforcement bars and a low frequency electromagnetic field. The method is applied to:
  1. Detect reinforcement bars.
  2. Estimation of the bars’ diameters.
  3. Estimation of the concrete’s cover.
  • Eddy Current Testing: The steel is exposed to electric current. This causes changes to the current. The interpretation of these changes gives information about:
  1. The imperfections and surface cracks on the steel.
  2. The hardness of the steel.
  3. The paint thickness.
  • Leeb rebound hardness test: A defined mass impacts with a known energy on steel. The steel’s hardness is estimated by the energy loss after the impact.
  • Method UCI (Ultrasonic Contact Impendance): This method is performed by a device containing an oscillating mass that comes in contact with the steel. Following this, the surface hardness is estimated as a function of the change in natural frequency of the oscillator, before and after the impact.
  • Wenner method: Electric current is applied to two positions on the concrete’s surface. The voltage response of a corroded bar is different when compared to a non-corroded one. The method is applied to:
  • Detect rebar’s corrosion.
  • Measure rebar’s corrosion potential.
  • Measure rebar’s corrosion rate.
  • Measure the concrete’s resistivity.