Fresh concrete testing takes time, and there are a number of standards to adhere to.
The information collected during this process and the way concrete is handled at this step greatly informs the rest of the quality control process and project as a whole.
This goes double when testing self-consolidating concrete (SCC).
So do yourself a favor and make sure to do it right.
According to the NPCA Quality Control Manual for Precast Concrete Plants, there are three ways to achieve a certification for applicable concrete testing:
- Become an American Concrete Institute (ACI) Concrete Field Testing Technician – Grade 1.
- Become an industry-recognized ACI equivalent
- Have a professional engineer observe and sign off on their ability to perform these tests.
Though not all precasters use SCC, many rely on SCC’s flexibility. SCC is slightly more expensive than traditional concrete, but the time saved through its ability to flow into the voids of complex molds – as opposed to tapping or vibrating the concrete – offsets the additional cost.
However, testing SCC looks a bit different compared to testing traditional concrete.
Standards provide all of the information a precaster needs, but the language is not intended to be instructional. This is where supplementary manuals such as ASTM R0030-19 provide commentary and insights to help interpret the standards.
Learn the standards. Follow the manuals. Then use these tips inspired by the standards, the manuals and experience to maintain proper practices.
Slump testing (ASTM C143) assesses the consistency and workability of fresh concrete by measuring the degree of deformation that occurs when a sample of freshly mixed concrete is placed and compacted. At the center of these tests is the slump cone.
Fill the cone with concrete, strike-off the top surface, then remove the cone in a steady, vertical motion.
One of the keys to completing this test successfully is to control how quickly to remove the cone. ASTM C143 says it should take five seconds, plus or minus two seconds, to lift the cone 12 inches.
The difference in height between the top of the cone and the settled concrete is then measured and recorded as the slump.
Traditional concrete doesn’t really spread. It slumps. Which is why this test works.
SCC, on the other hand, can spread out anywhere between 20 inches and 30 inches. So with SCC, look at slump flow instead.
Slump flow testing measures a few different things:
- Filling ability. How well it gets into every void of the mold.
- Static segregation. How well the coarse aggregate uniformly disperses.
- Passing ability. How well it goes around imbedded objects such as rebar.
Filling ability follows ASTM C1611. It requires a bigger base. Most testers use a large plastic platform but otherwise the same primary tool.
That is, the slump cone – only upside down.
R0030-19 advises that the test is easier with the smaller end of the cone facing down, though ASTM C1611 gives the option between upright or inverted.
Place the slump cone in the center of the base and pour the concrete sample into the cone without rodding or other consolidation. Vertically lift the cone and let the concrete spread.
It should flow “like hot lava, not like water,” as R0030-19 puts it. Measure the widest and narrowest diameter, then calculate the average between the two.
At this point, visually inspect the “SCC patty” for static segregation. A visual inspection helps gauge how well the SCC does what it is supposed to do (self-consolidate).
This is ranked on a scale of 0 to 3 called the Visual Stability Index (VSI), with 0 as the most stable mix and 3 as the least. A VSI of 0 or 1 is ideal, but 2 may be acceptable. Some mixes have a lot of liquid on the outside and aggregate clumped in the middle, producing what looks like a halo around edge of the patty. A large halo is a 3.
After slump flow, it is time to test the SCC’s passing ability, according to ASTM C1621. Use the same orientation as with the slump flow filling ability test. This time, use a J-Ring, which is a tool that restricts SCC’s flow in order to measure how well it passes through obstructions.
This test works the same as before. Results should be a little less than 90% of the slump flow measure.
Measuring temperature is fairly straight-forward but important. Temperature also affects workability, setting time and long-term durability.
As the NPCA QC Manual notes, warm concrete gains strength faster, but its strength at later ages is lower than cool concrete. ASTM C1064 requires the temperature test to be completed within five minutes after placing a thermometer in concrete.
Unit weight (density)
Density is another straightforward test – just a field scale and a bucket.
Here’s a tip: Save a step by using an air meter bucket. That way, once done measuring unit weight, immediately move on to air entrainment.
Regardless of the container, know its volume and weight. Ideally, set the scale to zero with the empty container on it. Then, simply add concrete to the container and weigh it.
Based on the concrete weight and the bucket’s dimensions, calculate the concrete’s density.
This is important because it provides information about the proportion of materials in the mix in order to verify compliance with specifications. If adjustments need to be made to the mix because of unit weight falling outside of the specified limits, take another measurement after the mix is adjusted.
Density is another indicator of workability and flowability, and it is closely related to the consolidation and air content of fresh concrete.
When performed in accordance with ASTM C138, density may be substituted for ASTM C231 or ASTM C173 after a correlation between air content and density has been established.
Another exception from the NPCA QC Manual says that, unless otherwise specified, dry-cast concrete density does not need to be tested.
Following the density test, concrete is already in the air meter bucket. Use this setup for the next test: air entrainment.
There are two options for normal-weight concrete:
- The pressure method (ASTM C231).
- The volumetric method (ASTM C173).
The pressure method is more commonly used, because the test is easier and faster. The volumetric method is preferred – and often required – for lightweight concrete or concrete using porous aggregate. The volumetric method is more accurate than the pressure method in that context.
In accordance with ASTM C231, the pressure method measures the air content as a percentage of the total volume of the concrete.
First, clean the rim of the bucket to get a tight seal.
Then, put the cover assembly on the bucket and seal the cover with clamps.
Close the main air valve, open the petcocks and add water into one petcock until water emerges from the other petcock.
The key is to make sure to expel any air trapped inside the air meter.
Pump the canister to the specified pressure, then release air into the bucket after closing both petcocks.
Based on how much the pressure drops, measure how much air was in the concrete as it compresses the “old air” with “new air.”
Good testing goes a long way
When it comes to testing SCC, the NPCA QC Manual notes that the air content of an SCC mix can affect the desired mix properties, recommending that air content be tested regularly with slump flow testing and VSI observations.
At the center of fresh concrete testing is the data a precaster needs. Many field technicians have turned to mobile field apps for data collection, enhancing efficiency and transparency in the QC process.
Ultimately, proficiency comes down to understanding the best practices of performing these tests. Do these tests by the book, with the help of some tips and tricks along the way, and getting certified should not be a roadblock for your plant.
Scott Grumski is the vice president of platform development for Forney LP and a member of the NPCA QA/QC Committee.