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Curing Concrete in the HEAT!
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?
Hide and Seek with Concrete
Watch as ETC demonstrates a Ground Penetrating Radar (GPR) Test.
What’s So Special About 28 Days?
When placing new concrete, the contractor or supplier may instruct you to wait an odd number of days before testing it to see if it is strong enough. That number will probably be 28 days. Why? Is it because the supplier’s favorite month is February or because 30 sounds too casual?
As the days add up after the concrete has been placed, compressive strength increases. The question is how did the concrete industry pick 28 days and not 27 0r 29? It’s because at day 28, the strength of the concrete is about 99% of its maximum value and after this day increases in strength are expected to be miniscule.
After one day of setting, the concrete generally reaches 16% of its maximum strength, at day three it’s usually about 40%. By day seven the concrete’s strength is usually around 65% of its design value, and at day 14 it would be expected to be around 90%. After 28 days, the concrete is typically at 99% of its intended strength. It’s clear that the concrete gains strength rapidly in the initial days after casting, but it may take over a year to reach its full compressive strength.
Stop! Take 5
What a fun treat to spend time with several of ETC’s first-rate clients outside of the office. Nice to put business aside for an evening and enjoy the stories of family, old times and dare I say politics ?! No! I promise, no politics were discussed…..however I do believe we solved a few of the world’s problems in a mere couple of hours!
The best part of my job is getting to enjoy the company of others. ETC has the best group of “others” I know!
New Technology with Paint
In the future we might be able to “see” the health of a building by scanning the paint with a handheld device. We would be able to tell how a building is deflecting and understand better why a defect is occurring. It looks like it might be a while before this technology is commonplace, but with the speed at which technology comes to market, it could be sooner than we think.
Peeling Back A Mystery
Recently, we were asked to investigate a building where mold was growing in the walls. During the investigation, we came across a product that we had not encountered before. It was a sheetrock panel with a shiny aluminum backing.
These panels are made by laminating special kraft-backed aluminum foil to the back surface of regular drywall panels. After reviewing the product literature, the panels offer some desirable qualities as they are intended to create a vapor retarder that helps prevent interior moisture from entering via exterior walls, it helps maintain comfortable room humidity all year-round, and creates a comfortable temperature during the appropriate seasons.
After reviewing the product limitations, we found that this drywall was not ideal for our situation. This was primarily because the interior walls were decorated with vinyl wallpaper, which created a double vapor barrier on the interior wall. This condition trapped moisture and created a place for mold to grow. Unfortunately, because the foil backing was not visible, no one knew that a problem was created by adding the wallpaper. Sometimes even simple cosmetic changes can lead to unexpected consequences.
How To Add Tile To Your Balcony
Are you looking for a way to update your balcony or outdoor walkway? Does a nice decorative tile cover sound nice?
As an alternate to an existing concrete balcony or walkway, there is an option to cover it with ceramic tile. Although the industry does not recommend adhering tile directly to a concrete slab, this can be achieved with the right knowledge and a proper installation. The reason the industry does not recommend this decorative option is due to the high risk of hidden structural damage occurring under the tile because, contrary to popular belief, tile is not waterproof.
The Handbook for Ceramic Tile Installation by the Tile Council of North America, Inc. (TCNA) recommends installing a waterproof membrane under the tile for outdoor walkways and balcony applications to protect the concrete from moisture. In these situations, proper water drainage is critical with the concrete slab sloped at a minimum of ¼” per foot to promote drainage away from the building.
If possible, the installation of a drainage course under the tile would be preferable as this material does just what its name implies and allows water to flow out from under the tile.
Of course, you will need to be sure that the tile, grout, adhesive, and other materials to be used are suitable for exterior use, as the hot sun and freeze/thaw conditions can wreak havoc on improperly selected materials.
A few companies are producing prefabricated balcony coverings systems using tile, wood, and carpet, which are specifically designed for exterior installation, and can be easily installed. These newly available products incorporate a drainage course so that they can be laid over a membrane to prevent concrete deterioration. Proper slope (drainage) is also needed to resist mold and mildew growth.
In certain buildings like apartments, condominiums, etc. tile cannot be installed without advance approval from a controlling authority (such as the condominium’s Board of Directors). So check this carefully before you get started.
The final check required is to make sure that the weight of the new tile will not overload or dramatically reduce the available load carrying capacity of the balcony. You will need a structural engineer to figure this out before buying the tile.
Proper maintenance and material selection are critical for ensuring the longevity of outdoor structures. Learn about flashing your wood balcony to prevent water damage and extend its lifespan. For landscaping and structural needs, our retaining wall material guide offers insights into choosing the best options for durability and aesthetics. If you’re considering carpet on balconies, understand the potential risks and benefits before installation. Additionally, stay informed about the signs and solutions for concrete deterioration to maintain safety and structural integrity in your spaces. Explore these essential guides for expert advice and recommendations!
A MUST READ!
Decoding Wall Cracks: Lessons learned from last summer’s earthquake.
IT”S A MUST READ!!
http://etc-web.com/ArticleDecodingWallCracks.pdf
Who Needs a Window Office?
In the world of engineering, corner and window offices are overrated. On the other hand, an experienced lift operator and something strong to tie off to definitely is a nice business perk! Reserved parking is not too bad either, just drive through the orange cones and stay close to the heavy equipment. For some reason, no one wants that spot. Hmmm…..
Yes, engineers are a breed of their own. I’m just so pleased he have an office full of very talented ones that don’t mind heights!!
Proud Moments for the Carlsons
It has been a busy time in the Carlson family this spring, as two kids graduated within two weeks of each other. Christopher completed a double major in Criminology and Government (yes, these are two different majors) at the University of Maryland – College Park and will be entering the University of Richmond School of Law in the fall. Molly graduated from Chapelgate Christian Academy as one of the top students in her class and will study Kinesiology at the University of Maryland – College Park this Fall.
Way to go proud parents, Chris & Pam! You sure did something right, but I’ll have to say…we were crossing our fingers for another engineer! Hmmm, there’s still hope for Tommy : )
Special Inspections
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.