Water Softeners: a problem for concrete?

You have just had your septic tank pumped and the pumper informs you that the concrete is corroding.  He then tells you that your water softener may be causing the problem because salt is detrimental to concrete.  The only thing he may be correct of is stating that salt (chlorides) can cause concrete degradation.

Concrete spalling due to chlorides is more complicated than I wish to share in this blog.  In my archives you will find an explanation of the effect they have and some products which will reduce or eliminate this effect.  What I do want to share is a response for 2009 regarding the affect water softeners have on concrete septic tanks.  Here is the link to that article: http://www.onsiteinstaller.com/editorial/2009/12/softeners-and-concrete

For information on concrete corrosion due to bacteria, visit my other blog: concretecorrosion.blogspot.com.  I explain this in detail and provide links for further study.

Concrete is the most widely used building material in the world.  It has existed for thousands of years, and it will continue to provide centuries of service.  Don't let hype and misinformation cause you to question such a fantastic, sustainable, and economical building material.  Go concrete!

ASTM C31 Making and Curing Concrete Cylinders in the Field

I am continuing to post summaries of ASTM standards related to precast concrete quality control testing.  This blog post will focus on making and curing concrete test cylinder in the field.  I will contrast this with some of the requirements in ASTM C192, the practice for making and curing cylinders in a laboratory.  ASTM C31 is typically required by specifying agencies to prove product strength conformance, whereas ASTM C192 is typically used to validate a concrete mix design.  NPCA Plant Certification, often a requirement for precasters, specifies ASTM C31 in section 5.3.5.1.

Practice C31 is to be used for concrete having a measurable slump.  It is not intended for "zero slump" concrete, also called dry cast.  ASTM C1176 has a comparable procedure for these samples.  Also, refer to an article written by Carl Buchman, P.E. on ASTM C1176 by going to this link http://precast.org/tag/astm-c1176/.  This practice is used for Self-Consolidating Concrete with the exception that the concrete is placed in one lift and rodding is omitted.

The practice of making and curing concrete cylinders in the field is important for the following reasons:

• (SC) strength acceptance as compared to the specified minimum;
• (SC) mixture proportioning and consistency;
• (SC) quality control;
• (FC) determination of an in-service date;
• (FC) as a method of comparison to other concrete made with the same design;
• (FC) to verify curing methods or methods of protecting the fresh concrete;
• (FC) for form stripping and removal.
  (SC) = Standard Curing; (FC) = Field Curing

For the purpose of this post, I will only mention cylinders.  Cylinders can vary in size, having two minimum requirements.  The length must be two times the diameter, and the minimum diameter is to be three times the nominal maximum size of the coarse aggregate.  Cylinders made with a coarse aggregate exceeding 2" must be sieved.  The most common cylinder sizes are 6"x12" and 4"x8", with the 4"x8" cylinders becoming the most predominate size.

Concrete used to make compressive test cylinders must be representative of the placed concrete.  samples are to be taken from concrete after any water or admixtures are added.  Concrete used in other QC testing (i.e.; ASTM C231, air content) is not to be used to make compressive cylinders.  

Molds are to be rigid enough to hold their shape.  They are to be be capable of holding water.  A mallet made of rubber or rawhide weighing 0.75-1.75 pounds is to be used for conventional slump concrete.  A rod measuring at least 4" more than the length of the mold having a hemispherical tip on at least one end shall be used.  The diameter of the rod shall be 5/8" on molds greater than or equal to 6" in diameter, and a 3/8" diameter rod shall be used for smaller molds.

Concrete compression tests are required to be performed by those individuals holding a certification as an ACI Grade I Field Testing Technician.  Conventional concrete cylinders are made by filling the mold in three equal lifts, rodding each layer 25 times.  The mallet is to be used to tap the sides of rigid molds 10-15 times after each lift is rodded.  An open hand is to be used on light gauge single use molds.  Strike a filled mold using the tamping rod.  A float or trowel may be used if permitted.  Final leveling should be done after moving the filled mold to its final curing location.

Section 10 of ASTM C31 references two methods of curing.  Section 10.1 is Standard Curing; Section 10.2 is Field Curing.  The method of curing will be specified by the agency with jurisdiction, by the customer, or as stated in the company quality control manual.  The NPCA Plant Certification Manual (v10.0) specifies that the "specimens shall be cured in a manner similar to the curing of the concrete products represented by the specimens."  This is usually defined as field curing as described in section 10.2.

Standard Curing - The molds will be stored on a surface level within 1/4" per foot.  Store cylinders for a period of up to 48 hours in an ambient temperature ranging from 60-80 degrees Fahrenheit.  The environment shall also prevent moisture loss.  For high strength concretes (6,000 psi or greater), the temperature range shall be 68-78 degrees Fahrenheit.  The storage temperature shall be controlled, and a data logger shall document the minimum and maximum temperature throughout this period.  This information shall be recorded.  There are multiple methods to achieve these requirements.  See note 6 of ASTM C31 for a list.

After initial curing of standard cured cylinders, and within 30 minutes of de-molding, place cylinders in water on all surfaces having a temperature of 73.5 +/- 3.5 degrees Fahrenheit.  The water tank of storage room must comply with ASTM C511, unless sulfur mortar caps are used.

Field Curing - Store the cylinders of concrete as near the concrete represented as is possible.  For precast concrete operations, this can be the plant environment if indoors, or outside if product is cured outside.  Protect the cylinders for wind or direct sunlight.  The same moisture and temperature of the product should be used for the test specimens.  If the product is covered during curing, cover or cap the cylinder molds.  If the concrete is not covered, then the concrete in the mold should not be covered.  It is important that samples cast for field representation be de-molded when the representative product is stripped.  If the concrete is moved outdoors, the concrete samples might also move outdoors at this time.  This should be defined in the quality control manual, and may be specified by the customer.

The report shall contain an identification number, a location, the date, time, and technicians name.  The slump, air, and fresh concrete temperature shall also be recorded.The curing method (Standard or Field) should be noted of the final record.  This record may also be used for recording the strength of the cylinders as specified by ASTM C31.  I will review this test method in my next post.

ASTM C231 Air Content Test Explained

In this next series of blogs, I am going to examine the various ASTM Standard Test Methods used in testing concrete in a precast operation.  The first blog will be on ASTM C231, the Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method.

ASTM C231 is the test method most commonly used by precast quality control personnel.  The NPCA Plant Certification program requires that this test be performed daily to verify the total air content present in the concrete sample.  Note that the air measurement attained in this sample may differ from the actual air content of the cured concrete it represents.

The scope of ASTM C231 specifically addresses concretes (or mortars) with dense aggregates.  It is not suitable to use this method when testing for air content of lightweight aggregate concrete or concrete made with air cooled blast furnace slag.  For these materials, ASTM C173 is to be used.  Also, ASTM C231 is not applicable for concretes often referred to as dry cast.  These mixes are not plastic, and they are often used for the manufacture of concrete pipe or masonry units.

While this ASTM standard shows two different meters, a Type A and a Type B, this blog will only refer to the Type B meter.  It is the experience of the author that this is the most widely used air meter by precast concrete manufacturers.  The Type B meter has an air pump, two petcocks, an air release valve, and a bleeder valve.  The bowl is typically ¼ cubic foot in volume, although if this bowl is used for computing density, the volume must be verified.  The bowl should have a diameter that is equal to the depth within a range of ±25%.

There is a strike-off bar and a strike-off plate for leveling the concrete sample.  The strike-off bar can be used when only using the sample for an air test using this standard.  If a density test (ASTM C138) will be performed with the same sample of concrete being prepared for the air test, then a strike-of plate must be used.

The air meter needs to be calibrated at least every three months.  ASTM C231 provides instructions for the calibration process in the appendix of the standard.  The calibration can be done by the plant personnel with knowledge of the procedure.  Documentation should be kept of the calibration, especially if the producer must provide evidence to a DOT or for Plant Certification.  The calibration date and initial pressure (if applicable) are to be present on the meter.

The test method is straight forward; the bowel is filled as noted in the standard.  Using Self Consolidating Concrete does not require three lifts or rodding as required in conventional mixes.  The bowel is filled to approximately 1/8” over the top of the bowl.  The concrete is struck off, and the sealing surface is cleaned.  Refer to the test standard for the exact procedures.  When filling with water through the petcock, the second petcock is to remain open.  Once water is seen exiting the second petcock, the meter is to be jarred gently to expel any trapped air.

With the aid bleeder valve closed, the meter air chamber is pumped to the initial pressure mark for the meter.  It is very important to leave the petcocks open at this point.  Why?  This allowed the technician to verify that the seals between the air chamber and the bowel are not leaking.  If there is an air leak, bubbles will form in the petcocks.  Once the initial pressure is reached, wait a few seconds to allow the compressed air to cool.  Gently tap on the gauge to assure an accurate reading.  Add or bleed air as necessary.  Release air into the bowel.  Tap the gauge to make sure there is an accurate reading.  This reading is the apparent air reading.  The sample air content is the computed by subtracting the aggregate correction factor from the apparent air reading.

What is an aggregate correction factor?  ASTM C231 is very clear that even when using dense aggregates, it is possible to have air within the pores of the aggregate.  Since this may affect the air content within the matrix of the concrete paste, this must be calculated and subtracted from the apparent air reading.  See ASTM C231, section 6, for the exact procedure for computing the aggregate correction factor.

Bugholes in Precast

Are you bugged with bugholes?  Bugholes are air bubbles that show up on the surface of the concrete after it is stripped.  They are a problem for both precast and pour-in-place concrete.  In this blog, I will provide some tips for evaluating the root cause of the bugholes and ultimately preventing their reappearance.

Often, these tiny, visible air bubbles are only a cosmetic issue, and they are not detrimental to the performance of the concrete.  When they become larger and cover more area, it is something that should be evaluated.  Occasionally these voids are large enough, or so numerous that it renders the product defective, requiring immediate corrective action.

Bugholes are the result of a void in the concrete paste.  A very common cause of bugholes is poor consolidation practices.  Either the concrete is insufficiently vibrated, or it is actually over vibrated, and the result is a void, or series of voids in the concrete.  The general rule for vibration is to vibrate until the breaking bubbles stop.  These are the smaller bubbles that pop up to the top.  Over vibration will cause bubbling of the concrete at the exposed surface, and this may be creating entrapped air.

Stinger vibrators must be used properly.  When I visit a precast plant, it is rare that I see the best practices being used.  These consolidation tools are inserted vertically into the concrete at a rate of 1’ per second.  The withdraw should be half that speed at 1’ every two seconds.  The vibration produces a radius of action.  This is the area of consolidation at a certain distance from the head of the vibrator.  Each insertion of the vibrator should be separated enough that the radius of action will overlap by about 2”.  A stinger vibrator must never be used to move concrete laterally.  This will cause segregation.

Another cause of bugholes is form release agent; either the lack of it, the wrong type, or over application.  There are two basic types of release agents: barrier and reactive.  A barrier release agent is typically oil based and it creates a physical barrier with between the paste and the form surface.  A reactive type will chemically react with the concrete paste to form a soap that creates a slippery surface, preventing the concrete from sticking to the forming surface.  The manufacturers of these products can provide information on the best product for your application.

Something else that can cause bugholes is a poorly designed concrete mix.  The best concrete will have well graded aggregates with various sizes from very large to very small that naturally consolidate and fill the voids.  The paste will be sufficient to fill the void space and glue every aggregate particle together.  The amount of water is kept at a minimum to reduce bleeding.  Bleeding is the transport of the excess water from the concrete to the surface.  Contact a mix design specialist for advice on your mix design.  Often, your cement or admixture supplier will have a specialist who can provide the advice you need.

Self Consolidating Concrete (SCC) has improved the aesthetic qualities of concrete since its introduction in the 1980’s.  Today, many precast concrete plants are able to place a very strong, high quality concrete with very little or no vibration.  The flowability of SCC produces a surface that is virtually bughole free.  The added cost of the mix is offset by the saving realized in labor and the improved customer satisfaction.  For more information on SCC, contact the National Precast Concrete Association (www.precast.org), and ask for the SCC White Paper.

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.

The Effect of Bleed Water in Concrete Products

I just had a conversation with a producer who is experiencing grout leakage, sand streaking, and the appearance of crack like fissures, often called “worm trails” in the finished product.   This producer uses ready-mixed concrete, and they recently reduced the size of their coarse aggregate from 3/4” rock to 3/8” pea gravel.  The problems that this producer is seeing are mostly the result of excessive bleeding in the concrete mix.

What is meant by the term “bleeding” in concrete?  Bleeding is the term used from the result of gravity that occurs when heavier materials in the placed concrete settle causing the lighter materials (mostly water) to rise toward the surface.  Bleed water is often important to finishers when placing floors, driveways, patios, etc., and it allows the concrete to be properly finished.  In precast, the concrete pieces are so large that the bleed water cannot easily escape.  This leaves the “worm trails” that form along the casting walls that nearly always are vertical and they have the appearance of a crack.  Bleeding also causes scalling and crazing at the horizontal surface due to the higher water percentage in the paste as the water migrates to the surface.

There are three things that a producer can do to improve this condition and reduce the amount of “bleeding” that is occurring.  First, the water to cement ratio (w/c) needs to be as low as practical.  Water is used to cause Portland cement to react and become a paste that hardens within a few hours.  Cement requires about one pound of water for every four pounds of cement to completely hydrate.  That is a w/c of 0.25.  Any additional water is called water of convenience.  Also, a w/c of 0.40 is required to even begin to develop a slump for placement.  Most concrete mixes have about one pound of water for every two pounds of cement, which is twice the amount of water that is necessary for cement hydration.  Chemical admixtures called water reducers allow less water to be necessary to provide flowability, and this will also help reduce the amount of bleed water.

The second modification that a producer can do to improve/reduce the bleed water is to increase the amount of fines in the mix.  A volumetric concrete mix design begins with a determination of the coarse aggregate ratio.  For conventional concretes, this is typically 60-65% of the absolute volume of all materials.  If the producer wishes to reduce bleeding in the concrete, and they have reduced the water to cement ratio to a point where further reductions begin to have an adverse effect on economy, then the next step is to lower the coarse aggregate ratio gradually.  A mix design formula following ACI 211 for the volumetric proportioning of concrete is the method used.  If the producer already has a properly proportioned mix, then begin by reducing the coarse aggregate by 25 pounds and increasing the amount of fine aggregate by the same amount.  This is not an exact relationship, but it will help the producer to achieve a mixture with a lower potential for bleeding.

The third component that a producer can do to reduce the effects of excess bleed water is to add an air entraining admixture (AEA).  The AEA does two things: it takes up a small amount of volume formerly occupied by other materials, and it creates some fluidity to the mix that allows for less water needed for the convenience of placement.  In effect, and AEA can be described as tiny ball bearings in the mix allowing it to be more flowable.

Don’t let excessive bleed water continue to affect your concrete’s appearance and quality.  It is possible to take a few simple steps to solve the root cause of the underlying problem.  The result will wow your customers with a better looking precast concrete product that is more durable and higher quality.

Why is my concrete tank leaking?

Watertight concrete tanks are important to specifiers and regulators. The customer's level of expectation is that the product will perform as expected. But occasionally, a leak is noticed at the seam or joint of the product. Why is this occurring, and how can it be fixed?

When a water retaining structure is leaking, the first response is to find fault with the sealing material. While this cannot be ruled out, there are several other potential causes that are contributing to the leak rather than a failure of the sealant. Sealants manufactured to recognized standards such as ASTM C990, have been designed and performance tested to withstand hydrostatic forces present in over 99% of precast concrete products.

PLACEMENT

The first issue that should be mentioned is placement. There is not an exact rule of thumb that works for every type of casting. The placement is dependant on the type of joint, the size of the joint, the weight of the casting, and the volume or size of sealant. Based upon testing, I have found that there are two rules of thumb that I follow. The first is to place the most volume of sealant within the annular space of the joint. The annular space is the intentional void between the angled surfaces of the opposing sections of the joint. Filling the annular space provides the best seal and allows the highest amount of compression. The second rule of thumb is to place the sealant in the joint nearest the hydrostatic force. If this is a water containment structure, then sealant should be placed nearest the inside of the joint.

Placing the sealant always means that at least two ends of the preformed sealant must be joined together. This should be done in a manner that best forms a continuous bead. The sealant should never be stretched. It is best to avoid overlapping and side-by-side placement. The ends should be cut at 45 degree angles, and then worked together by hand to form a continuous piece.

In a square or rectangular structure, the sealant should never be started on a corner. Because three point are all that are truely ever in one plane, leaks are often found in corners. It is best to begin and end the sealant a minimum of 12" from a corner.

The final point to consider is that squeeze out is not always good. Squeeze out is where sealant material is extruded out of the joint. Material that squeezes out of the joint is material that is not being used to fill the joint. With squeeze out, the purchaser does not know if there is a 1" band of sealant in the joint, or a 4" band. A wider band of sealant in the joint will always produce the best seal.

VOLUME

Creating a watertight joint is as much about using the correct volume of sealant material as it is putting it in the right place. Volume is defined as the area in the crossection of the sealant material. Volume is determined by multiplying the height by the width. For instance, a sealant with the crossectional sizes of 0.60H x 0.75W has a volume of 0.45 in. sq. To find the area needed, there are two methods. One is to lay several strips of sealing material across the joint surface, using baby powder to prevent adhesion, and then seting the next section onto the first section. by cutting off the ends of the sealant outside of the joint, and then rolling the compressed material into a roll, the required crossectional volume can be calculated. Use the highest volume of the pieces to determine the proper size.

It should be noted that wider is not better. A common misconception held by many is that a wider pice of sealant is better than a narrow piece. This is a false assumption. Actually, it is best to have a taller piece of sealant and allow the sealant to compress as much as possible. Compression of at least 50% is preferred by most sealant manufacturers. As the sealant is compressed, it gets wider and thinner. At some point, the sealant will not compress any further. This is a point where the resistance force is eaqual to the force applied (weight) over the area of sealant. Time and temperature can affect this.

ADHESION

The final point I will touch on in this blog is adhesion. Sealant has to stic to the concrete to be most affective. Most sealant materials will naturally stick to clean, dry concrete. But if the joint contains dirt, rocks, concrete debris, and dust, the sealant will not stick. The joint surface should be clean and dry, and any dust that is present should be revoved. Primers can improve adhesion in one of two ways: either create a good mating surface imbedded into the concrete surface, or by creating a sticky surface for the sealant to be applied to. Also, most concrete has some level of porosity.

Tunnel Tanks

Here is a good article on the installation of tunnel tanks.

http://www.jepcosales.com/Fall2010.pdf

I have created a blog called Concrete Corrosion where I am going to explain the process of Microbial Induced Corrosion. Please follow me at concretecorrosion.blogspot.com

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