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.

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/.

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.