Forensic Engineers and Consultants

Archive: Fires & Explosions

What You May Not Know About Using a Concrete Test Hammer

When assessing potential problems in concrete structures, consider a non-destructive test using the concrete test hammer, AKA “rebound hammer,” before investing a lot of time and money needlessly replacing or destructively testing the concrete structure.  The use of rebound hammer tests should be considered before you or your client decide to drill multiple core samples. Large areas of the concrete structure suspected of having potential strength problems can be tested quickly with a rebound hammer.  Analysis of those results can narrow down specific areas for more rigorous testing.

A rebound hammer consists of a spring-loaded steel hammer which, when released, strikes a steel plunger in contact with the concrete surface. The spring-loaded hammer must travel with a consistent and reproducible velocity. The rebound distance of the steel hammer from the steel plunger is measured on a scale attached to the frame of the instrument, giving you the Hr number.

Compressive strength versus rebound number

The rebound hammer test is used to assess the in-place uniformity of concrete, to delineate regions in a structure of poor quality or deteriorated concrete, and to estimate in-place strength development. The non-uniform area can become a target area for further testing. The further testing is used to establish the relationship between the rebound number and the compressive strength.

Rebound Hammer

For a given concrete mixture, the rebound number is affected by factors such as moisture content of the test surface, the method used to obtain the test surface (type of form material or type of finishing), and the depth of carbonation. These factors need to be considered in preparing the strength relationship and interpreting test results. To estimate strength in an existing structure, establish the relationship by correlating rebound numbers measured on the structure with the strengths of cores taken from specified locations.

Uniformity is not the same as strength. It should be noted that according to the American Concrete Institute (ACI) Provision of ACI 301-16, a rebound hammer cannot be directly used for in-place strength testing. However, this same provision allows use of a rebound hammer in accordance with ASTM C805/C805M, if specified by the Architect/Engineer, only “to evaluate uniformity of in-place concrete or to select areas to be cored.” At the same time, Provision states: “Results of in-place strength tests will be evaluated by Architect/Engineer and are valid only if tests are conducted using properly calibrated equipment in accordance with recognized standard procedures and an acceptable correlation between test results and concrete compressive strength is established and submitted.” However, no specific non-destructive testing methods or equipment for in-place strength evaluation are listed. For a review of in-place methods for estimating concrete strength, including the rebound hammer, refer to ACI 228.1R.

Drilling for core sample

Concrete specimen

This testing procedure might help avoid having to use ground penetrating radar and then drilling core samples for compressive strength tests. It is designed to provide data in the field on concrete structures without causing damage and gives an immediate indication of uniformity.  It is economical, quick, and simple to use.  Its results can be used to estimate compressive strength for the structure.  If you have concrete in question, consider the use of a rebound hammer test along with a thorough visual inspection as part of the first steps in the investigation of the structure.

Carlos Zarraga has more than 9 years of engineering experience in the structural field specializing in building design, building components and foundation design.  Carlos has designed and analyzed structures, supervised designers and drafters, prepared construction documents and provided on-site duties for field supervision and inspection of construction projects. Certified in RISA 3D, RISA Foundation and RISA Connection, he is well-versed in the analysis of foundation failures.   He often determines the root cause of failure and the resulting scope of damage.  He has designed retrofits to existing structures in addition to repairing construction defects.  He also has experience in the industrial and petrochemical industry designing structures for materials handling facilities and industrial buildings.  Carlos holds a Bachelor of Science in Civil Engineering from the University of New Orleans.

The Demise of Insulation on Electrical Wiring


Unlike fine wines and some types of cheeses, not everything ages well.  Such is the case with the materials used as insulation of electrical wiring.  While the copper metal used as the conductor in many wire types will last virtually forever, the cladding used to protect and insulate the wire allowing electrons to flow to their final destination does not. Read More

Ammonia – The Good, The Bad, The Smelly… Part Two

Now that you know what ammonia is (see Part One here), how it behaves, and how to safely store it and work with it, let’s look at some areas in industry where it is used.

Anhydrous ammonia has a use in pollution control.  Industrial boilers and power plants burn coal or natural gas to make steam and/or electricity. When the fuel is burned using air as the oxygen source nitrogen gets exposed to the heat as well because air is 79% nitrogen.  The nitrogen gets oxidized and forms several compounds referred to as NOx (NO, NO2, NO3).  NOx compounds are harmful to Read More

Construction Techniques to Prevent Water Penetration at Windows

Windows, and their interface with the exterior walls, are an important part of a building’s envelope that resists the intrusion of water. Most builders take many precautions to protect a house from water damage. One of the most important factors in keeping the water out is the installation of window flashing, a thin material that prevents water from seeping in around a window. Read More


Ammonia – The Good, The Bad, The Smelly… Part One

Ammonia is a compound consisting of one nitrogen atom and three hydrogen atoms and is denoted by the formula NH3. Its boiling point is -28°F at atmospheric pressure, so unless it is under pressure, it is gaseous at room temperatures. Therefore, pure ammonia is typically stored under pressure in a liquid form. Household ammonia is only 5-10% NH3, the remaining 90-95% is water. Ammonia is extremely soluble in water. It is often depicted  like this: Read More

The CE Mark and What Should It Mean to You? Part One


Two little letters, CE.  Perhaps you have seen those two letters on a machine nameplate or some other equipment.  What is the meaning behind those two stylized letters and how does it drive the design of safer machinery?  Let’s take a closer look. Read More

Shedding Some Light on Fluorescent Light Fixture Fires


Lighting systems in buildings and other structures have undergone changes over the years.  Many of these changes have occurred as manufacturers have developed more efficient lighting methods.  Lighting loads can represent the largest category of electrical load in many buildings, thus improved lighting efficiency may significantly lower your power bill and can lengthen time between lamp changes. Read More

Structure Fires in Eating and Drinking Establishments


Eating and drinking establishments see an average of 7,410 structure fires per year based on a 2017 report published by the National Fire Protection Association (NFPA). The report analyzed available data from the U.S. Fire Administration’s National Fire Incident Reporting System (NFIRS) and the NFPA’s annual fire department survey for the years 2010-2014.

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Hidden Heat: The Unseen Hazard of a High Resistance Connection


A typical residence can have upwards of 10,000 feet of electrical conductors installed, most of which are buried in the walls, attics and crawlspaces.  A commercial building can have 100,000 to upwards of 1 million feet of electrical conductors.  At each device such as a switch or a receptacle are at least three, and typically six or more connections of these conductors within a junction box.  The connections can be in the form of twisted connectors, screw terminals, push in terminals and crimped connectors.

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Distributed Control Systems…Data is the Key

Wouldn’t it be great to have a built-in camera to let you see exactly what went wrong before an incident?  In many manufacturing instances there is, chemical plants especially.  The computer system that operates the plant is called a Distributed Control System (DCS) and it has the capacity to monitor thousands of process variables (flow rates, temperatures, pressures, levels, valve positions, pumps on/off) simultaneously.

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