Are your compressive test cylinders representative of your product?
Article by Carl S. Buchman, P.E.
Carl Buchman is NPCA’s Senior Technical Consultant.
Every quality control technician knows how to make concrete test cylinders for “wet” or slump mixes. ASTM C31 and ASTM C192 detail exactly how this is to be done, including molding requirements either by rodding or vibration. But if your mix is so dry that you actually leave a hole when you withdraw the rod or vibrator, what do you do?
For “dry” or zero slump concrete mixes, ASTM C1176 provides similarly detailed instructions for making cylinders, using a vibrating table and a metal compacting weight.
That’s it, you’re in good shape? I don’t think so – you are done only with the easy part. If the density of the cylinder you made on a vibrating table isn’t the same as that of the product, you are not getting realistic results. It doesn’t matter if your cylinders are to be used to evaluate stripping strength, shipping strength or 28-day strength; if the compaction or density of the product is not replicated in the test cylinders, the cylinders will not be truly representative of your product. Machine-made concrete isn’t automatically the same; variations arise from certain machine adjustments, variations in the concrete mix and even timing changes.
Drilling test cores is another way to determine the actual compressive strength of the product itself. ASTM C42 covers this topic (including sawed beam samples). But this is not a convenient method for establishing release or demolding strengths. This article will outline a very detailed method to achieve the same density within the cylinder as your manufacturing equipment achieved in the product.
The procedure will appear at first to be lengthy and cumbersome. But with the proper, simple lab equipment setup and a few tries, you will find it goes very quickly; and even the seemingly complicated calculations are very straightforward (and easily programmed for computer input and solution).
The method for making these cylinders is essentially the same as that explained in ASTM C192 (the vibrating table and the top metal weight). The difference is this: With ASTM C192, the cylinder is compacted by the process until the specified end point is achieved; with this procedure, the cylinder is compacted until the same compacted density is achieved in the cylinder as has been produced by the machine-made product.
This procedure was first developed by Don Logan at Stresscon Corp. for use in making concrete test cylinders that would be representative of the concrete placed and compacted by Stresscon’s hollow core placing machine. It is just as applicable to any machine-made product as long as you can obtain a hardened piece or sample representative of the product being made.
The first step is to determine the specific gravity of the product actually being made. Saw off or break a piece of concrete from the finished product, perhaps from a broken or rejected piece; a cored sample from a good piece also works. Each sample should be thoroughly brushed clean to remove residual matter produced from any cutting operations. Weigh the piece and record it as W 1 (this procedure works best with a gram scale). Next you must coat the piece with wax. Melt some clear sealing wax (as used in canning) in a small pan on a hot plate in the lab. Using a small brush, coat the sample with enough wax to seal it everywhere so it will not be able to absorb any water. When the wax has dried, weigh the sample again and record it as W 2.
This next step requires some unique equipment setup to complete. As shown in the sketch on page 10, arrange a large gram scale to weigh the suspended sample. Be sure to zero out the scales after attaching the hanging apparatus. Suspend the waxed sample in water. Make sure the immersed sample is not touching the sides or bottom of the container and that the line suspending the sample is free and clear of any interference back to the scale. Record this weight for each sample as W 3.
The next step is easier than it looks: Calculate the unit weight of the sample.
SG sample = W 1 ÷ [ W 2 – W 3 – (W 2 – W 1 ÷SG wax) ] and assume the specific gravity of wax = 0.9. Then the unit weight (density) of the concrete sample = SG sample x 62.4 lbs./cu. ft.
OK, now make a test cylinder of the same density. The procedure is to determine the exact amount of concrete to be placed into the test mold, and then follow standard and applicable procedures for filling the mold.
Start by determining the volume of the cylinder mold you will be using (this needs to be done only once). The cylinder volume = cylinder height x ( ð x d 2 ÷ 4). This can be done for any size mold, but here are the calculations for a 4-inch by 8-inch cylinder mold where a unit weight of 145 pounds per cubic foot is to be replicated.
Cylinder volume = 8” x ( ð x (4”) 2 ÷ 4) = 100.53 in 3, and 100.53 in 3 ÷ 1728 in 3 / ft 3 = .05818 ft 3.
Now use this number to establish the weight of fresh concrete to put into the cylinder:
.05818 ft 3 x 145 lbs./cu. ft. = 8.44 pounds of fresh concrete mix.
Weigh out half the desired amount of concrete first and place it in the 4-inch by 8-inch cylinder mold. Using a vibrating table and floating weight, or simply using the weight as a tamper, tamp by hand until the concrete is at an even level measuring 4 inches down from the top of the cylinder mold. Weigh out the second half of the total fresh concrete needed and place it in the cylinder mold. Compact this layer even with the top of the mold. Be careful that no material is lost due to spillage. The use of a small collar temporarily placed around the top of the cylinder mold to hold all the mix without spillage may be of some help. Finally, the same steps should be taken with this mold as any other: Cover the mold in some way to protect it from moisture loss, and cure the mold either with the product or using a method to match that of the product.
Some final thoughts: This method cannot give you cylinders of the same product density as that of product made the same day. But it will give you samples representative of the product as recently made as you wish, like one day earlier. It allows you to continuously monitor the actual product density, and produce test cylinders replicating that density, and thus truly reflective of that product. Also, the accuracy of this procedure is no better than the care of the technicians involved or the accuracy of the measuring equipment. Remember to have all scales used in your lab calibrated at least annually by an outside agency. Lastly, there are simplifications of this procedure you can use, such as skipping the wax and pre-soaking the sample piece, but they are not as accurate; you can work these out yourself if you wish.