We took this photo recently while on a project. It shows a reinforcement bar that was partially cut by a core drill when a plaza drain was being installed and highlights the reason that all slabs should be scanned with a Ground Penetrating Radar (GPR) before drilling.
A GPR can locate embedded steel reinforcement and allow you to miss the bars. Cutting rebar or tendons can compromise the load carrying capacity of the structure. Cutting into other embedded items (such as conduits for electrical service or communication lines)could disrupt building operations or even be hazardous.
Call ETC for all of your GPR needs and we will bring our 3D machine out to make sure you do not damage your building.
The Baltimore-Washington area is rich with historic structures. These buildings are usually beautiful (or at least visually interesting) but, more importantly they’re reminiscent of our nation’s heritage and essential to understanding America’s culture. Realizing this, owners and managers of historic buildings, or any older structure really, must carefully consider all treatment options before deciding to restore, rehabilitate, or reconstruct.
Restoration stresses the building’s historic character. Whenever possible, materials from the most significant time in the building’s history are conserved, while removal of materials from other periods is generally permissible.
Rehabilitation typically entails retention of as much of the historic elements of a structure as possible; however, it’s accepted that more materials have deteriorated beyond salvation and there is more latitude for replacements.
Reconstruction is the last resort and involves the demolition and replacement of non-surviving or non-functioning building components or even entire structures. This option offers limited opportunities to save the authenticity of the building, but may be necessary if it’s no longer structurally sound.
Myriad factors would dictate the most appropriate strategy, including the physical condition of building components, historical importance, planned usage of space, and building codes. Historically significant structures in this region will usually fall under the purview of historical preservation organizations and their mandates could trump other considerations.
Of course, the best strategy is preservation. Conscientious maintenance and timely, proper repairs can extend the useful lives of building elements hundreds of years, obviating, or at least minimizing, the need for more drastic measures
Cantilever balconies are commonly seen protruding from the façade of condominiums and apartments. However, this arrangement can lead to problems arising from heat transfer and condensation, resulting in mold growth.
Typically, steel framed cantilever balconies have beams that extend into the building and connect to the structure. Concrete balconies are usually cast integrally with the rest of the floor. During winter months, the exposed balcony structure becomes cold and when it meets the warm building interior, typically near a sliding glass door, thermal transfer increases, as does the possibility for condensation and mold growth. Building owners should be aware of this possibility and watch for condensation forming in the area under the carpet or wood flooring, near the balcony.
Fortunately, for new buildings with cantilever balconies, products are now being produced to prevent condensation and mold by inserting a thermal break (insulation) between the exterior and interior portions of these structures. These new balcony structural inserts can carry significant weight, while preventing interior heat transfer. The most common materials used for these products are stainless steel and fiberglass reinforced laminate composites. Although these are new products, they appear promising.
“A man does not plant a tree for himself, he plants it for posterity.”
– Alexander Smith
When planting trees around your property and near your buildings, think in the long term. That small tree will eventually become big, maybe even really big. We get calls related to foundation problems and wall cracks caused by the roots of big trees which were placed too close to the building many years before. Tree roots can exert excessive pressure on the building foundation and walls and lead to structural damage, foundation movement, and water infiltration. Unfortunately, to prevent additional damage to the building the big beautiful tree that has been around for so long typically has to be removed.
Keep on the lookout for Japanese Knotweed – it is on the list of the top 100 worst weeds, worldwide. Locally, it has spread from Nova Scotia to North Carolina, so those in the Mid-Atlantic region should be alert and remove this pervasive weed. The plant is so tenacious that it is known to cause damage to concrete and grow through asphalt pavement. This weed is so nasty it even caused delays during some of the construction for the London Olympic venues.
Read more about this damage causing plant and how to get rid of it on these two links
Some people may say that concrete is a paradoxical material: it is strong and yet fragile; it is mundane and yet remarkably versatile. But more often than not, this material is taken for granted as the surface of everyday elements such as streets and sidewalks.
As discussed in our September 26th, 2011 post “Curing Concrete in the Cold,” temperature plays a crucial role in the outcome of newly placed concrete. Curing concrete in temperatures above 80° Fahrenheit can be as challenging as doing so in temperatures below 35° Fahrenheit (see “Curing Concrete in the Cold ”)
Concrete cured at high temperatures will have a high rate of evaporation, causing uncontrolled thermal cracking, which in turn compromises the concrete strength and durability. Laboratory testing has proven that concrete improperly cured under high temperature conditions can lose as much as fifty percent (50%) of its service life. Concrete naturally produces heat as it is mixed and cured. So placing a material that has internal heat on a hot day is quite challenging.
The first step in ensuring adequate concrete curing and reducing the temperature of concrete are taken at the batch plant by adding ice as part of the batch water, using chilled batch water, or cooling the concrete with liquid nitrogen.
Then, it is up to the Contractor to ensure additional adequate conditions. The concrete placement should be scheduled as early in the day as possible to avoid the hottest part of the day. Advanced planning and timing should also be performed to avoid delays in delivery, placement, and finishing. If long haul times cannot be avoided, it is possible to include a retarder as part of the mix design to prevent fast setting. However, the amount of retarder is limited by the work intended, as elevated amounts of retarder will crust the top surface of a slab while the underlying concrete remains soft.
Prior to the placement, the forms, subgrade, and reinforcement need to be soaked to ensure that unsaturated materials do not absorb the moisture from the concrete mixture. Once the concrete is at the job site, water may be added to the mixture to adjust the slump only at the time of the truck arrival, and only if the mix design allows it. In the event that water is added, it shall not exceed the volume listed on the batch ticket provided with each truck load. Once the concrete is in the process of being placed, water must not be added to the mixture. Doing so will greatly compromise the concrete. To make things even more interesting, concrete must be placed within 90 minutes from the time it was mixed in the truck.
After the top surface of the concrete has been given a finish, moisture should be prevented from evaporating by covering the elements with soaked burlap or cotton rugs. It must be ensured that the coverings remain continuously wet so that they do not absorb water from the concrete during the first seven days after placement.
Who would have thought that such a dull material would require such a meticulous procedure to ensure it reaches its true potential?
Over the last two years, we have found that local jurisdictions are more frequently requiring that concrete repairs, as well as other building restoration or retaining wall projects, are being classified as critical structures and/or needing special inspections. Special inspections have been included in the Building Code for a while, so this is not something new. A special inspections program involves additional paperwork (typically referred to as a Statement of Special Inspection) to obtain a building permit and requires that a Special Inspection Engineer of Record be used (in addition to the Structural Engineer of Record) on the project. All engineering consultants involved in the critical work, as well as the building owner, and contractor must sign the Statement of Special Inspection.
The Special Inspector can also be the one of the Engineers of Record, such as the Structural Engineer that designed the repairs. As you might imagine, the local jurisdictions have requirements over which portions of the critical work must be specially inspected and how often the inspections will occur. Additionally, the jurisdiction must approve who the special inspector will be. Sometimes the inspector (and not just someone in the company) must be a licensed professional, such as a Professional Engineer, or someone who is a certified inspector with credentials obtained from WACEL, AWS, or another recognized inspector certification entity. The contractor cannot retain the special inspector, as this could resent a conflict of interest.
During the project, inspection reports, signed and sealed by a Professional Engineer that performed or supervised the special inspections, are often submitted to the County at intervals during the project. Additionally, a project competition form must be submitted to the county and stamped by the Special Inspector.
The special inspection requirements can increase the cost of the field inspections on a project, but it does help ensure that quality work that complies with the Building Code and the project specifications is provided.
An effective corrosion protection plan is an essential aspect of the design, repair or maintenance of reinforced concrete structures such as parking garages and balconies or steel building frames wrapped in masonry, which are exposed to the elements. Severe corrosion of the embedded steel must be avoided so that the structure maintains its full strength.
Corrosion of steel embedded in concrete or masonry is an electrochemical process that occurs in the presence of moisture and oxygen, which can accelerate if the structure is exposed to deicing salts or a salt-water environment. Once started, corrosion will continue until it is controlled.
To control corrosion, the steel can be coated with a protective material such as zinc, paint, or epoxy. Alternately, the electrochemical process can be mitigated with cathodic protection. Coatings generally only protect the steel until the barrier is breached, while cathodic protection provides a more active means to address corrosion.
In cathodic protection, zinc anodes may be attached directly to the steel to alter the electrochemistry and force the steel to become the cathode and be protected while the zinc anode “sacrifices” itself. Anodes can be used in new construction as well as in the repair of existing structures.
Another form of cathodic protection, applies a small electrical current directly to the steel, which prevents the electrical process of corrosion from occurring. This system requires a power supply, a sacrificial anode, and instrumentation for monitoring.
Selection of the appropriate level of corrosion protection for an existing structure is based on many factors. Among them are the level of corrosion damage, environment around the structure, potential for corrosion activity, the cost and design life of the corrosion protection system, and the expected service life of the structure.
Concrete can degrade and fail due many causes and sometimes the reason is not quite evident. We often use sounding techniques such as tapping with a hammer or chain dragging as well as chemical analysis or electrical measurements to diagnose concrete problems. When these simple tests prove inconclusive, we can send a concrete sample from the structure to a laboratory for petrographic analysis.
Concrete petrography is an effective technique for investigating the quality, workmanship, durability and defects of concrete. It involves the examination of the concrete with an electron microscope that can magnify the sample by up to 10,000,00 times and allows for the description of hardened concrete in the lab, using specialized techniques borrowed from those used in the study of rocks.
The analysis can provide a wide array of information that cannot be determined in the field such as:
Information on the composition and construction of the concrete sample,
Identification of the aggregates, the quality of the cement paste, and the bond between them,
Determination of the water to cement ratio, which is a major factor in concrete strength,
Assessment of the concrete’s resistance to damage by freeze and thaw cycles, sulfate attack, alkali-aggregate reactions, aggregate durability, and carbonation, and,
Evaluation as to whether a failure was caused by a faulty mix design or poor workmanship during placement and curing.
We do not use this service too often, but when we need to know more about the concrete, petrography often gives us the answers that we are seeking.
