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.