A Lean Journey: Twas a 5S Christmas

A Lean Journey: Twas a 5S Christmas: The folks over at Graphic Products received this great holiday poem which they shared in their newsletter. I thought it would be great to ...

Making the grade - The required compressive strength in concrete design

A few years back, I discussed the merits of the NPCA Plant Certification program with the owner of a small precast plant.  He did not feel the need to become certified.  His exact statement was something like: “I’ve made 5,000 psi concrete for 30 years.”  That is a strong statement, and one that hopefully was supported by fact.  In a non-threatening manner, I responded with a question: “do you have one piece of paper that documents the evidence of this claim?”  He was silent.

A quality system will not magically transform the average precaster into super precaster.  And, the strength of concrete is only one part of making a quality product.  As I think about the comments made by this precaster, I wonder just how many folks understand the compressive strength requirements.  Jay Shilstone (2012), in his blog “Missed it by that much” – Concrete tests and f’c, got me to thinking about the importance of this quality principle. 

The product design calculations, or possibly the customer, will state a minimum compressive strength requirement.  The desire is that no concrete compressive test result will be lower than the specified strength.  All processes have variation, and concrete is no different.  A population set of data will be distributed in such a manner that when plotted by a curved line, the data points will form a bell shaped curve.  In a normal distribution curve, the apex of the curve will represent the mean or average.  Theoretically, 50% of the data will be to the left, and 50% will be to the right of the mean.

ACI 318 defines the standard method for determining the target compressive strength required to assure that 99% of the time the compressive strength will be greater than the specified strength.  According to ACI 318 section 5.1.1, the average strength is called the required strength or f’_cr.  The specified strength is noted as f’_c.  Section 5.3.2.1 provides the formulas for establishing the required strength when the specified strength and sample standard deviation are known.  In the formula below, s_s is the sample standard deviation.

Specified compressive strength, psi	Required average compressive strength, psi
f’c ≤ 5000	Use the larger value computed from Eq. (5-1) and (5-2) f’cr = f’c + 1.34ss                  (5-1) f’cr = f’c+2.33ss – 500             (5-2)
f’c ≥ 5000	Use the larger value computed from Eq. (5-1) and (5-2) f’cr = f’c + 1.34ss                  (5-1) f’cr = 0.90f’c+2.33ss              (5-2)

                              Table 5.3.2.1 from ACI 318

The standard sample deviation is calculated from “30 consecutive tests or two groups of consecutive tests totaling 30 tests” (ACI, 2005).  A modification factor is allowed when the number of tests is less than 30 but greater than or equal to 15.  When the number of consecutive tests is less than 15, the required average strength will be [f’_cr = f’_c + 1200] when the specified strength is ≥3,000 psi and ≤5,000psi.  For a specified concrete compressive strength of >5,000 psi, the required average strength will be [f’_cr = 1.10f’_c + 700].

For the precast producer, making consistent concrete with less variability will reduce the sample standard deviation.  This will allow the precaster to produce a concrete design with a required average strength that is lower, while still meeting the over design requirements of ACI 318.  This might result in a more economical concrete mix, and it will also provide a better batch-to-batch consistency for concrete products.  So, the next time you tell someone that “you make 5,000 psi concrete”, consider the statistical variation.  Maybe you do, and maybe you don’t.

References:

ACI. 2005. Building Code Requirements for Structural Concrete (ACI 318-05). American Concrete Institute: Farmington Hills, MI.

Shilstone, Jay. 2012. “Missed it by that much – Concrete Tests and f’c. Accessed on September 27, 2012 from http://www.commandalkonconnect.com/2012/09/26/missed-it-by-that-much-concrete-tests-and-fc/.

Concrete Cracking

I heard the statement one time that "all concrete cracks."  It is true that concrete is a composite that is very strong in compression, but weak in tension.  This makes it susceptible to cracking.  And, reinforced concrete is actually designed for cracking.  The reinforcement is engaged as the concrete section bends and yes, cracks.  But the statement above is not all its cracked up to be.

There are several different types of cracks, and most of them can be avoided with a proper mix design and best production practices.  The following is a list of types of cracks that are typical in concrete:

• Plastic Shrinkage
• Dry Shrinkage
• Alkali-Aggregate Reaction
• Alkali-Silica Reaction
• Corrosion
• Structural Overload

For this blog, I want to focus on Dry Shrinkage cracking.

Concrete is typically made with more water than is necessary for hydration.  The excess water evaporates or "bleeds" out.  As this occurs, there is noting to fill the volume left behind.  As the concrete drys it will naturally shrink in volume.  But, if the concrete is constrained by the formwork or reinforcement, then the concrete cannot shrink which causes internal stressing.  

Some times drying shrinkage occurs where the concrete is exposed to the air while still in the form.  The water evaporates more rapidly on exposed surfaces, and therefore the precast element contains small micro-cracks.  In reinforced concrete, due to the constraint of the steel, the cracks will appear somewhat equally spaced along an edge.  If the concrete is fiber reinforced, it may still crack, but instead of a 0.002" crack every 6", the cracks are not visible, and there may be dozens within a 6" span.

If you are a precaster who manufacturers septic tanks, you may notice drying shrinkage more in a certain season.  This is easily explained by the theory of drying shrinkage.  When the dew points are low, and the day time high is 30+ degrees above it, then the relative humidity is low.  New concrete contains moisture that is necessary for hydration, and as long as the water is present, the concrete hydration cycle will continue for weeks.  New concrete placed on the yard in storage will dryout where the surface is exposed to air.  A small septic tank is usually stored as a complete, sealed unit, ready for installation.  The relative humidity inside is high, but the outside surface is losing moisture.  This internal stress will lead to hairline cracking on the outside.

Sometime a crack is misdiagnosed as a dry shrinkage crack.  Here is a scenario from my past that will illustrate this.  A 1,250 gallon mid-seam septic tank was developing a "smiley" crack after 3-7 days in the yard.  We had two sets of forms to make this same size tank, and the crack only appeared on the castings from one of these forms.  We thoroughly inspected the castings after they were stripped.  No cracks.  Since we did not see a crack until the casting was about a week old, we were certain that it was caused by drying shrinkage.  But why did it only affect the castings off one of the two forms?

I realized that any cracks in the castings were most apparent after a rain.  As the concrete dried, the moisture retained in the crack longer, making it visible.  With this knowledge, I established a new QC procedure: using a wet rag, the QC inspector would wipe the casting walls prior to removal from the plant.  Sure enough, we found the "smiley" crack that had eluded us for several weeks.  It was there all along, and was not caused by drying shrinkage.  The actual cause was discovered to be a faulty air regulator on the air cylinder for the collapsing core of the form.

If you are seeing cracks your concrete products, perform a root cause analysis to identify the primary and contributing causes.  One or more of these will be the root cause of the cracking.  Don't just accept the statement that "all concrete cracks."  Many of the causes can be eliminated with minor, inexpensive changes.  The customer perception of the quality of your precast products will improve as a result.

The Importance of Compression

Earlier this year I wrote a blog titled "why is my concrete tank leaking?"  In that blog I mentioned that volume was one of the important characteristics of a quality seal with a compression sealant.  The volume of sealant needs to be the minimum amount of sealant needed to fill the joint and create a watertight seal.  The volume of sealant does not change as the sealant is compressed.  Actually, the sealant only changes shape.

As the photo to the left shows, the sealant begins as a shape that typically represents a square.  The force of the lid will be applied to the top surface of the sealant, causing the sealant to flow laterally.  As the sealant begins to compress, the surface area making contact with the lid increases.

The photo to the right shows the sealant compressed 25%; the remaining gap is 75% of the original height of the sealant.  The width of the sealant has increased 28%.  The for applied by the lid is now distributed over a larger area.

Concrete Sealants, Inc. advises 50% as the minimum amount of compression necessary to produce a watertight seal.  More compression is optimal, but less than this will result in a reduction in the effectiveness of the seal.  Also at this point, the sealant has now increased in width 100%, and the surface area has also doubled.

Somewhere around 75% - 85% compression, the sealant will have reached the maximum amount of compression that will typically occur.  And, while it is possible to exert enough force to compress the sealant, the resistance per square inch continues to increase, as does the amount of surface area where the force is applied.  At 75% compression, the width has increased nearly 400%.
This is a very basic concept of the effect of compression.  Sometimes a precaster will want a wide sealant that is short.  This is not the best option to obtain a watertight seal.  If the sealant is already wide, then obtaining 50% compression will be very difficult since the force will be distributed over a larger area to begin with.  We call this the "snow shoe effect", referring to the ability for someone to walk on snow with special shoes that distribute the weight over a larger area.  Taller sealants usually allow for more compression, and the result is a better seal.
For more information on sealants, sealant sizes, and the effect of compression in obtaining a watertight seal, contact Concrete Sealants, Inc. at 800-332-7325.  A sales representative will be happy to assist you in finding the right sealant size for your application.

Concrete Dusting

Is your concrete "dusty"?  Sometimes I see concrete that is so dusty that it appears a bottle of baby powder just exploded.  You might think that this dust is coming from the environment: the dust from your plant, the gravel roadway, or other source.  More often this is a natural effect of the concrete hydration process, and there are several methods that can be used to minimize the dust.


First of all, the dust is not typically a problem for most concrete products.   Over time, as the concrete is exposed to the environment, the mild acids from rain and carbon monoxide will reduce or eliminate this dust.  The dust, or powder, is calcium oxide (lime).  The primary problem with the dust is that paints and coating will not stick to the dusty surface.  Also, since the lime reacts quickly with acids to neutralize it, the acidic degradation will etch the concrete and lead to more problems later in the life cycle.  In some cases, as with Microbial Induced Corrosion, the reaction will form calcium sulfate (gypsum) which further reacts to form an expansive gel within the concrete.    


In the hydration process, water is used to hydrate cement to form a gel called C-S-H, or Calcium Silicate.  For every unit of gel created, about two units of a by-product called calcium hydroxide are released.  Some, but not all of the calcium hydroxide is used to form other compounds.  Any calcium hydroxide that is left will eventually dehydrate, leaving free lime on the surface.


One of the most frequent causes of surface dusting, especially in precast concrete, is poor curing.  Concrete products are often stripped from forms while the hydration process is still occurring.  The form provides an impermeable skin that hold the moisture in the concrete.  When stripped, the moisture evaporates quickly unless the concrete is placed in a moist curing environment.  


Another cause of dusting is a mix design with too much water.  This is especially true in flat work such as floors, sidewalks, and parking lots.  The excessive water evaporates from the surface too quickly leaving behind a weak and porous surface.  This is made worse as foot and vehicle traffic "grind" the surface into a fine powder.


There are several methods which can be used to reduce or eliminate the dusting effect in your concrete products.  In this blog, I will explain three methods that I advise precasters to follow.  The first method involves mix design, the second method requires better curing practices, and the third method is a product that can be applied after stripping the casting.


The best way to produce high quality products with durable, dust free surfaces, is to use an appropriate mix design.  Have an engineer or mix design specialist create a volumetric mix that follows the requirements outlined in ACI 211.1, the publication for proportioning normal weight concrete.  A low water to cement ratio is important for durability and low porosity.  In addition, select a material to use as a supplement to the ordinary Portland cement.  Some examples are: slag, fly ash, silica fume, colloidal silica, etc.  The first three are readily available in the United States. Colloidal silica is the latest technology, and is gaining support in the concrete community.  Each of these are secondary cementitious materials (SCM's), and they react with the free calcium hydroxide by product in the hydration process to form calcium silicate.


In addition to a good, high quality mix design, proper curing is essential.  Concrete hydration occurs in stages.  The first three phases occur within the initial 10 to 20 hours, and involve a releasing large amount of heat during the hydration.  Stage four will primarily occur over the first few days, but continues for weeks.  Actually, as long as concrete has access to moisture, the curing process of stage four will continue.  Concrete that is exposed to air at an earlier stage will have a lower strength and an increased porosity.  The chart below shows how significant this effect is.

The last method I will mention is the application of a penetrating reactive sealer.  There are several types of concrete sealers on the market.  For the purpose of this blog, I am going to focus on the reactive silicate type.  A reactive sealer is one with very small molecules which penetrate deeply into concrete.  Some claim to penetrate several inches, although this is more of a property of the capillaries in the concrete than the physical properties of the chemical.  Most of these are now available in water based formulas which do not have serious environmental concerns.  One brand in particular is made by Concrete Sealants, Inc. in New Carlisle, OH.  It is called ConBlock SH.


Products like ConBlock SH penetrate the concrete and react immediately with the lime (calcium oxide) to form calcium silicate (C-S-H gel).  The reaction begins initially, making the concrete dust free within a few minutes.  The complete reaction takes a few weeks.  Some of the benefits to the concrete include a more durable, hard, abrasion resistant surface.  It also reduces the porosity of the concrete.  The concrete has better resistance to many acidic products like acid rain, carbonation, vinegar, and pickle juice.  The surface is also more resistant to freeze thaw damage when exposed to a chloride solution.  

The photo above shows two blocks made from the same, non air-entrained concrete.  Each were cycled 100 times through freezing and thawing with a chloride solution on the surface.  The block treated with ConBlock SH outperformed the untreated block.  Another benefit of ConBlock SH is that is is a great primer for other paint, flooring, or sealant application.  To learn more about ConBlock SH, click on the hyperlink, or call 800-332-7325.  If you are not a precast concrete company, then contact your local precaster and ask them how you can purchase ConBlock SH.


Concrete dusting is a naturally occurring process.  It can be reduced or eliminated using good production practices.  Commercially available products can be applied to concrete surfaces soon after they are removed from the mold, or many years later, to improve the surface qualities of most concrete surfaces.  

Leading with Vision

Recently, I watched a movie titled 7 Days in Utopia.  The story is about an amateur golfer, Luke Chisholm, who unexpectedly finds himself stranded in Utopia, Texas.  It just so happens that the eccentric rancher, Johnny Crawford, who provides him with a place to stay for the week, is a former PGA tour pro.  Johnny convinces Luke to accept an offer to mentor the talented young golfer in the fine points of the game.


In one of the scenes, Johnny tells Luke to meet him on the golf course at 8am.  When Luke shows up, he does not find Johnny ready for a round of golf, but instead he is sitting at an easel, painting a oak tree.  He asks Luke to tell him how he would play a difficult lie behind the big tree.  Luke provides a quick response which was not acceptable to his mentor.  Instead, Johnny explains that Luke must "see" this shot in his mind before he ever swings a club.  Then he provides Luke with an assignment: paint the shot on canvas.


Being a leader requires the same skill that was being taught to the young golfer.  Leaders need to "see" the terrain that lies ahead, and then plan their shot before swinging the club.  Since we do not always know what is going to happen in the future, sometimes this requires multiple scenarios.  Dr. John Maxwell states that "leaders see before others, and leaders see more than others."  We are not referring to the sense of sight either.  Successful leaders have a vision of where they want to go today, tomorrow, next month, and five years from now.  And the choices they make today are based upon the future state they desire.


The first skill that you need to succeed as a leader is a clear vision.  Create a picture of the future state so clearly in your mind that you could paint it on canvas.  Then make your decisions based upon making the shot that gets you closer this goal.  Talent is necessary, and must not be ignored.  But talent is not enough.  And vision without talent will only lead to disappointment.  But when your natural, God given talent makes contact with your clear and well defined vision, you are sure to reach the green.


Okay, so what does this have to do with precast concrete?  Everything.  This is an industry that was created by people with a vision.  Concrete is one of the oldest known building materials.  For centuries forms were created on site and the concrete was poured in place.  The precast industry envisioned a process that that allows these products to be produced in a factory under controlled conditions.  This new process adds value to the customer by eliminating the hassles at the job site and the delay in waiting for the concrete to reach strength.


Success in our industry came from the creativity of seeing (vision) what can be accomplished before anyone else knew it was possible.  The entrepreneurs who were skilled in concrete and and construction (talent) began forming precast concrete companies.  The result is an industry of innovative people who create endless possibilities.  Stop swinging the club long enough to see the shot in your mind, and you will create a new future for this industry.

Reinforcement in Precast - The Basics

My blog this week will focus on the reinforcement used in precast concrete units.  Before I begin, let me make it clear that I am not a licensed professional engineer, and my comments come from a combination of experience, education, and 15 years of practical experience.  (Also, I recently slept at a Holiday Inn Express!)  My past experience includes working as a designer where we were required to make some simple calculations in house as part of the production drawing process.

Let’s start out with the basic question of why we reinforce concrete.  Concrete is a very strong composite material with compressive strengths of usually 5,000 psi or more.  And while concrete can handle such great forces in compression, it is weak in tension, roughly 1/10 of the compression.  Tension is the force of being pulled apart.  A simple concrete beam supported on each end with a force applied downward in the middle will create a natural tendency to “bend” the concrete.  The middle of concrete is the neutral zone.  Above the neutral zone the concrete is in compression, and below the neutral zone the concrete is in tension.

If the concrete was not reinforced, then the concrete would crack in the middle and collapse.  

If the reinforcing steel is placed in the concrete, but it is located in the upper half called the compression zone, it will crack severely, but the reinforcing steel will likely prevent a collapse.  

On the other hand, if the reinforcement is located in the bottom half of the concrete, also called the tension zone, the concrete will bend and until the steel is engaged by the tension forces.  Minor cracking may occur, but nothing serious.  And most importantly, the reinforced concrete section will be ready to continue to perform its designed function.

The location of the steel reinforcement is critical to its function.  Also, the concrete must bond with the steel to transfer the forces.  There are a couple of important facts to consider in placing the steel.  The steel can be affected by the environment and the alkalinity of the concrete can reduce this, but only if there is a sufficient amount of concrete covering the steel.  There is also a minimum amount of concrete covering required for the steel to effectively receive the tension forces.  The second important consideration is the distance that the center of the steel is from the force or compressive load.  This distance is used in calculations to determine the amount of steel required, or the amount of load permissible.

 

Do you see the steel bars?

I know some manufacturers who still “hand place” steel bars in the concrete product after the concrete is poured.  This practice is concerning for several reasons.  Can you be sure of the location and exact placement of the steel?  Does the steel have enough concrete cover to prevent corrosion leading to spalling of the concrete and further degradation?  And did the act of placing the steel create voids within the concrete that can weaken the structure?  In the case of a parking block, just drive around and look at these products in use.  I see a lot of them that were made with “hand placed” steel that look terrible in just a few years.

This is just a quick blog on the importance of the location of reinforcing steel in precast concrete units.  Proper placement will affect the overall quality and longevity of your products.  Your reputation and the reputation of the precast industry will be judged by the perception of quality seen.