Managing Concrete Test Data

Thanks to Kenneth for this great post!

Get better results by taking care of technicians and cylinders.

By

Kenneth C. Hover

 

On some jobs, concrete test cylinders “don’t get no respect.”

Sometimes we forget to let the testing company know we are pouring. Sometimes between the truck chute, the pump, and stacks of materials there isn’t much space for the testing technician to work. Sometimes the technician’s work area isn’t level, and sometimes it is just plain unsafe. Sometimes the technician has to carry the freshly cast cylinders a good distance to store them for the first 24 hours onsite, and we rarely use a curing box to physically protect the cylinders and maintain the specified air temperature. We argue that the test cylinders are not a fair representation of the actual concrete in the structure: Differences can include water and air adjustments, consolidation, concrete and air temperature, and moisture control. And we like to complain that the cylinders are not handled gently enough between the site and the lab.

Although many of these concerns are justifiable, and while properly making and handling cylinders is not easy, we have to face the facts: By the time the cylinders are broken, the dust settles, and the results printed and distributed to the owner, designers, contractor, and concrete producer, those test results generally are considered to legally indicate the actual strength of the concrete. Test results are the stand-ins for actual concrete performance, especially in the early stages of the project before the concrete in place has had an opportunity to speak for itself. It is during these early days in the life of the concrete test results are accepted as the primary indicators of concrete quality, and in these same early days, the owner is making the decision to pay or not pay, or to release or withhold the retainer. When the cash flow stops, the test cylinders suddenly get a lot of respect and attention!

Credit: Portland Cement Association

The safer and more comfortable the technician, the more he or she can concentrate on your tests.

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Credit: Portland Cement Association

The safer and more comfortable the technician, the more he or she can concentrate on your tests.

The importance of tests

As an analogy, consider the task of hiring someone fresh out of school or a training program. If that job candidate has no actual experience to date, you might place a lot of importance on things like grades or standardized test scores (even though we might question the relationship between test scores and real-world, productive capability). When evaluating a job applicant with several years of relevant professional experience, however, their actual performance counts far more than standardized test scores.

Similarly with concrete, until the structure has been in service for awhile and experienced a few seasons of freezing or thawing, or been loaded to a significant fraction of its design load, the actual long-term performance of the concrete is unknown—we need test scores such as air content, unit weight, and compressive strength to give us confidence. Once that same structure has performed its desired function, carried its intended load, and survived its expected environment for awhile, those test scores are no longer looked upon as the primary evidence of acceptability.

10 Things on Testing to Cover in the Prepour Conference

– Why certified technicians are necessary

– The routine for communicating placing schedules, start times, frequency, and numbers of tests required, as well as who gets the notifications

– The desired ages of concrete when tested

– The need for testing to meet contractor needs: Do you need early-age tests to provide information for removing forms or shores, or for stressing PT tendons, or for applying construction loads?

– The locations for sampling concrete and conducting tests (Note: Samples taken from other than the chute of the ready-mix truck are nonstandard, and a method and protocol for obtaining such nonstandard samples needs to be defined in detail to limit variability.)

– The requirements for the level and safe testing locations

– Cleanup requirements

– The location of the curing boxes and who is providing the curing boxes

– How to get access for transporting cylinders back to the lab

– The procedures and contact info for disseminating test results

Of course, we can always fall back on the ACI 318 Building Code provision that core tests can be authorized “if the likelihood of low-strength concrete is confirmed,” and this often gets us out of trouble, especially given the acceptance criteria that “the average of three cores is equal to at least 85% of f´c and if no single core is less than 75% of f´c.” But even if the cores turn out OK, how much time and money was lost in the process? It costs a lot more to extract a core than to make a cylinder—wet-coring is messy, and all of this takes time to organize, drill, cure, test, report, and then wait for the green light to get back on schedule. Wouldn’t it pay to encourage reliable compression tests in the first place?

Accounting for the technician

Let’s start by planning a convenient work zone for the test technician. Everybody wins when it’s easier for that person to properly sample concrete, perform air and slump tests, make cylinders, and store them in a safe, temperature-controlled environment. Uneven, unlevel surfaces are miserable for slump and air tests, and bad conditions usually increase the apparent slump. The same bad ground makes it harder to consolidate a cylinder. If certification programs wanted to test applicants under real-life conditions, they would place the candidate between the truck and pump, on wet rocky ground, balancing on a nasty piece of plywood on the edge of the excavation.

Of course the actual conduct of the tests is in the hands of the testing technician, which is why most of our standard specifications require certification. Top-grade testing companies do a great job of making sure their people are well trained and certified, but it does not hurt the contractor to reinforce this need and even to verify certification.

Credit: Portland Cement Association

According to the commonly required ASTM C172, Standard Practice for Sampling Fresh Concrete, testing starts by collecting the sample from the truck chute.

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Credit: Portland Cement Association

According to the commonly required ASTM C172, Standard Practice for Sampling Fresh Concrete, testing starts by collecting the sample from the truck chute.

With so much going on during a major concrete placing operation, it is easy to overlook the post-pour fate of concrete test cylinders, which are to remain onsite for up to 48 hours after casting, and then transported to the testing lab. If those cylinders were made near the point of concrete discharge, it is likely they will be in the way after the trucks and pump depart. So they often are moved to temporary storage, and if this happens a few hours after casting, the concrete might be at its most fragile. It is difficult to lift and transport a concrete cylinder to minimize damage. Plastic caps are not only handy for reducing drying of the top surface, but they also reinforce the mold so it is stays circular during handling.

But even when the cylinders are gently moved to a safer place, ASTM C31-09, “Standard Test Method for Making and Curing Concrete Test Specimens in the Field,” requires “Immediately after molding and finishing, the specimens shall be stored for a period up to 48 hours in a temperature range from 60° F and 80° F [16° C and 27° C] and in an environment preventing moisture loss from the specimens. For concrete mixes with a specified strength of 6000 psi [40 MPa] or greater, the initial curing temperature shall be between 68° F and 78° F [20° C and 26° C].” Although there are a few jobsites in which the air temperature will not dip below 60° F nor rise above 80° F for a couple of days after casting (Waikiki in January comes to mind, but a field trip is required for verification), such limited temperature swings cannot generally be relied upon. Curing boxes therefore are required most of the time, winter and summer.

Credit: Portland Cement Association

A curing box equipped with a heater and thermostat is useful whenever ambient air temperature will drop below the minimum temperature specified in ASTM C31, (68° F for fc ¥ 6000 psi, 60° F otherwise).

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Credit: Portland Cement Association

A curing box equipped with a heater and thermostat is useful whenever ambient air temperature will drop below the minimum temperature specified in ASTM C31, (68° F for fc ¥ 6000 psi, 60° F otherwise).

Using curing boxes

For some reason, few in the design-build-supply-test chain like to provide or take responsibility for curing boxes. Part of the problem might be confusion between standard-cured and field-cured cylinders. We have been talking about initial standard curing conditions in the field which are followed by standard lab-curing conditions until the specimen is tested. The purpose of standard curing is to control thermal and moisture conditions and to isolate the inherent properties of the concrete as-delivered from the variable conditions of the jobsite. Sometimes we want to get a handle on the effects of the actual site environment on concrete properties, so we intentionally expose test cylinders to field conditions. However, this doesn’t always work as well as it seems like it ought to, owing to the fact that test cylinders heat up and cool down toward ambient air temperature far faster than the actual structure they are supposed to represent. Cylinder specimens are far more likely to cook in the summer and freeze in the winter than the structure being tested. So unless field-cured cylinders are specified or required by the contractor, the initial curing temperature limits pertain, and a cure box is the best way to stay in compliance.

Credit: Kenneth C. Hover

With this highly effective curing box used by the Washington DOT, the cooler is partially filled with water to increase thermal mass and slow temperature changes.

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Credit: Kenneth C. Hover

With this highly effective curing box used by the Washington DOT, the cooler is partially filled with water to increase thermal mass and slow temperature changes.

Another point of confusion is the weird effect of temperature on concrete strength gain. Although it is true that higher concrete temperature accelerates hydration of cement, faster hydration can lead to poorer quality of hydration products and a reduction in 28-day strength. High temperature curing generally increases strength at an age of up to about three days, but decreases 28-day strength. It is not unusual for reported concrete strengths to drop in the summer, due largely to hot concrete cylinders baking in the sun. I have observed a surface temperature of 124° F for 6×12 concrete cylinders in black plastic molds after a few hours in the afternoon sun in June in upstate New York. The concrete was an ordinary sidewalk mix, and 28 days later no one would have thought to lay the blame for low strength on whoever chose to place the specimens at the base of a west-facing stone wall (an unintentional solar oven). On an earlier project in the same location, I monitored a 4×8 cylinder that froze solid overnight while the well-protected structure stayed toasty warm. Given a curing box helps achieve specified 28-day compressive strength in both summer and winter, it is just silly to not make one a standing part of concrete placing operations.

It makes great sense, therefore, to include testing procedures and logistics as part of your prepour conferences (see 10 Things on Testing to Cover in the Prepour Conference). By increasing the chances of getting acceptable concrete strengths, the retainer you save may be your own.

Concrete Curling

 

Curling is the phenomenon when the surface of the concrete shrinks greater than the middle and bottom of the slab. Mud drying does the same thing, see picture.

curling

The bowl like affect it causes within the jointed areas creates raised areas along those joints. In warehouse applications it creates speedbumps and wear for forklifts and wheeled vehicles.  It makes flooring a real challenge.

A few things to drastically decrease curling:

  • Use a well graded mix. All Duke City mixes use combined gradation technology.
    • So called GAP grading requires more water to fill the voids left by large aggregate next to small aggregate. Intermediate aggregates established by combined gradations fill that void.
  • DO NOT ADD ACCESS WATER TO THE MIX- the more water in the mix the more it shrinks.
    • Use water reducers to increase slump for workability
  • Since the cause is uneven evaporation, top drying out faster than the bottom; create similar conditions for the surface as well as the subgrade.
    • Don’t over wet the subgrade unless you can drown the surface
    • VERY IMPORTANT—Keep the surface wet with curing methods
      • Wet burlap and keep wet
      • Dike and flood surface area
      • Cover in plastic to seal in moisture
    • Hot-Windy days are worse conditions than rainy days

Curling can be reduced using the right tools and techniques.

 

Flyash and Concrete

There has been a lot of talk about flyash in New Mexico lately.  If you listen to the talk you have probably heard our market has been put on a Flyash allocation by the sole supplier, Salt River Materials Group. So what does that mean to you the concrete customer short term and long term?

 

If you have been around this market for a while you know the evolution of flyash here in Albuquerque. If not, a quick story.  Flyash, a byproduct of coal burning power plants, was introduced to us in the early 80’s.  At that time we replaced up to 20 percent with flyash. At that time it was called “Hamburger Helper” among other things since it allowed the supplier to use less cement but still achieve strengths, although it took a little longer. It is much finer than cement but it is spherical rather than crystal shaped.  This acts as ball bearings and helps workability and pump ability.  The complaints at the time were that it was sticky and hard to finish with the increase of super fines.

About this time, middle to late 80’s, the interstate concrete in the area was starting to fall apart after only 15 or so years. Upon further investigation ASR, Alkali Silica Reactivity, was found to be the culprit.  The Alkali in the cements were interacting with the Silica in the aggregates of this area causing a gel in the concrete attaching itself to particles in the mix.  When water migrated into the concrete after final set it would cause this gel to expand and break up the concrete from the inside.

Testing with another Class of Flyash , F, with lower Alkali, proved to mitigate the ASR. The Class F flyash would soak up great amounts of the Alkali in the cement thus reducing half of the equation.  Today the City of Albuquerque and NMDOT both require 25% Class F flyash in their approved mixes just for this reason.  We here at Duke City have been a big proponent of using Class F flyash in all exterior applications to mitigate ASR influencing architects and engineers not familiar with the ASR issue in our area to consider specifying Class F flyash.

Fast forward to today. Because of maintenance issues, lower natural gas prices, lower usage of electric power among other factors, the supplier of area has allocated flyash to its customers based on prior usage. Four Corners PNM coal burning power plant is the main provider for Class F flyash for Colorado, New Mexico, Arizona and California.  We credit Salt River Materials Group for classifying a consistent flyash out of this plant.  The other sources, not so much.

This allocation actually has an end date, April 10, 2016, but the writing maybe on the wall for the future supply of Class F flyash. You have all heard the attack on coal from the environmental sector and if clean natural gas pricing remains low flyash maybe be a thing of the past.

What then do we do to mitigate ASR?

Since the ASR “Gel” requires water, interior mixes wouldn’t have the same risks as exterior mixes exposed to moisture, therefore those mixes could go to straight cement. The risks would be greater for subgrade applications, but not as severe as exposed concrete to the elements.

As you may or may not know, Duke City Redi-Mix coarse aggregate is Basalt. The other coarse aggregate used in our market is the natural river rock in the area which is considered very reactive, with a  high silica content. Basalt is considered non-reactive.  We do use natural sand, as do all producers in the area, which is also very reactive. Duke City is half way to resolving the issue due to the fact our coarse aggregate is not reactive.

The current alternatives to mitigate ASR are through chemical admixes, namely Lithium, using alternate sources of aggregates or other pozzolans. Duke City is currently testing a lot of the available alternatives.  The availability, workability of the mixes and cost of these alternatives will make this transition a challenge for all of us.

 

  • Lithium cost is in the $25 per gallon range. The dosage averages one gallon per yard depending on the severity of the reaction. AND Lithium is known to accelerate set times.
  • Since all natural sands in this state are silica based we must crush a non-reactive aggregate down to use as a fine aggregate. Natural sand is spherical, great for pumping and finishing. Crushed fines are angular. Not really pump able and very tough to finish. AND, get this, it will cost $10 to $12 more per yard.
  • Other pozzolans are being considered including pumice. Testing is underway.

 

In light of all of this I believe Duke City will be able to get through this initial shortage of flyash without resubmitting mixes or losing any integrity in any of our mixes. Looking to the future we will be able to keep the increase in costs to a minimum using the alternatives mentioned earlier due to the fact that we use a non-reactive coarse aggregate and must just deal with the natural fine aggregate.  Please be assured Duke City Redi-Mix will address these issues with the highest integrity and commitment to quality you have grown to expect.