Thursday, March 28, 2013

Concrete slump

Wet concrete is rather mushy, and it flows a little.  If you try to make a peak out of it, it slumps.
Slump is an important characteristic of freshly mixed concrete, and it is used as a specification for concrete.  It is a measurement of workability, or flowability, or viscosity.

Slump is measured using a specific standardized test.  The freshly mixed concrete is formed into a conical cylinder and consolidated, or tamped on.  The cone form is then removed, and the amount of slump is measured.

Here are the specifications for a slump test (taken from Wikipedia):

Principle   The slump test result is a measure of the behavior of a compacted inverted cone of concrete under the action of gravity. It measures the consistency or the wetness of concrete.[
Metal mold, in the shape of the frustum of a cone, open at both ends, and provided with the handle, top internal diameter 102 mm, and bottom internal diameter 203 mm with a height of 305 mm. A 610 mm long bullet nosed metal rod, 16 mm in diameter

Procedure  The test is carried out using a mould known as a slump cone or Abrams cone. The cone is placed on a hard non-absorbent surface. This cone is filled with fresh concrete in three stages, each time it is tamped using a rod of standard dimensions. At the end of the third stage, concrete is struck off flush to the top of the mould. The mould is carefully lifted vertically upwards, so as not to disturb the concrete cone. Concrete subsides. This subsidence is termed as slump, and is measured in to the nearest 5 mm.

Interpretation of results    The slumped concrete takes various shapes, and according to the profile of slumped concrete, the slump is termed as true slump, shear slump or collapse slump. If a shear or collapse slump is achieved, a fresh sample should be taken and the test repeated. A collapse slump is an indication of too wet a mix. Only a true slump is of any use in the test. A collapse slump will generally mean that the mix is too wet or that it is a high workability mix, for which slump test is not appropriate.   Very dry mixes; having slump 0 – 25 mm are used in road making, low workability mixes; having slump 10 – 40 mm are used for foundations with light reinforcement, medium workability mixes; 50 - 90 for normal reinforced concrete placed with vibration, high workability concrete; > 100 mm.

Limitations of the slump test   The slump test is suitable for slumps of medium to high workability, slump in the range of 25 – 125 mm, the test fails to determine the difference in workability in stiff mixes which have zero slump, or for wet mixes that give a collapse slump. It is limited to concrete formed of aggregates of less than 38 mm (1 inch).


Differences in standards

The slump test is referred to in several testing and building codes, with minor differences in the details of performing the test.

United States

In the United States, engineers use the ASTM standards and AASHTO specifications when referring to the concrete slump test. The American standards explicitly state that the slump cone should have a height of 12-in, a bottom diameter of 8-in and an upper diameter of 4-in. The ASTM standards also state in the procedure that when the cone is removed, it should be lifted up vertically, without any rotational movement at all.  The concrete slump test is known as "Standard Test Method for Slump of Hydraulic-Cement Concrete" and carries the code (ASTM C 143) or (AASHTO T 119).


Typically, when you order ready mix concrete (delivered in a concrete truck), you’ll specify a slump.  It is usually anywhere from around 3 to 6 inches.  3 inch slump is pretty thick concrete, 6 inch slump is pretty smooth.  If you use superplasticizer, as discussed here, then the concrete is more workable, and will have a higher slump while still having a low water-to-cement ratio (indicated: w/c).  A low w/c is desirable, and makes for better concrete.

Sometimes a zero slump concrete is desirable.  For example, concrete block machines use zero-slump concrete.  This mix has low water content (usually 5-7%) and a correspondingly low w/c.  Because block are made so fast, they must be removed from their forming mold immediately after they are made.  These blocks should not slump at all, or they will be deformed.  Zero slump.

Tuesday, March 19, 2013

On Fukushima: I told you so!

On March 30, 2012 I wrote a blog entry entitled “Review of Fukushima Daiichi Nuclear Disaster”.  In this entry, I briefly discussed the necessity of providing adequate venting to prevent the escape of radioactive waste to the outside in the event of loss of power and a resulting explosion.  Here is what I wrote:

Venting channels to help relieve the high pressures resulting from any explosion inside the reactor are readily provided in a domed containment structure. These vents are typically filled with boron-based sand or aggregate, and activated carbon. This sand, aggregate and activated carbon remove radioactive material from explosive gasses as they pass through the venting channel, before they vent to the outside atmosphere.”

In response to this entry, some anonymous source posted the following remark on May 2, 2012:

Containment is supposed to be virtually leak-tight under all Design Basis conditions. Venting Channels defeat this purpose.”

I replied on May 2, 2012:

“It is impossible to contain a supersonic explosion without venting, or else the containment structure becomes a "bomb". See this discussion by Arnie Gunderson of Fairewinds. Evidence seems to indicate "inadvertent criticality" as cause of explosion. In order to begin to control the effects of such an explosion venting is required.

The same anonymous source then replied once more, also on May 2, 2012:

“An explosion is beyond Design Basis except for Steam Generator Faults and LOCAs. These are supposed to be complely contained within containment. Any explosion such as "inadvertent re-criticality" is not containable without loss of containment integrity.”

At this point, I realized that due to the language used, the defensive posturing, and the inability to admit a design flaw in an inherently flawed design; that I was most likely communicating with an engineer or Public Relations employee from General Electric (GE).  The nuclear reactors involved in the Fukushima Daiich disaster are GE Mark I reactors.  There are currently 31 reactors very similar to those at Fukushima Daiichi in the US. 

In an effort to be clear, to emphasize a solution, and, furthermore, in an attempt not to antagonize or further alienate this anonymous commenter, I offered the following, final remark in our exchange (also on May 2, 2012):

“The containment structure is not adequate to contain an explosion without venting. (I think we are saying the same thing?) As proven by experience, explosions happen at nuclear reactors. The containment structure should be designed to withstand an explosion, this necessitates inclusion of proper venting. The existing structures at Fukushima were improperly designed and built. They need to be fixed, they need better containment (with venting) since explosions can and do happen.”

My hapless anonymous commenter did not respond.

Today, March 19, 2013, The US Nuclear Regulatory Commission (USNRC) posted an entry on their blog, entitled “NRC Commission Approves More Post-Fukushima Upgrades to Nuclear Plants.”  In this blog entry, the NRC demands strengthened venting at the 31 nuclear reactors in the US which are similar to the GE reactors involved in the disaster at Fukushima Daiichi.

Quoting from today’s USNRC blog entry (3/18/13):

“The venting systems at Fukushima played a role in their nuclear crisis, and the NRC issued an Order to the 31 plants with similar designs to take action. The plants either had to install vents or improve their existing venting system. The goal was to make sure the vents can operate during the early phases of an accident, even if the plant lost all power for an extended time.

In their latest decision, the NRC Commission votes to further strengthen these vents. The NRC staff has 60 days to finalize an Order for these enhancements. Generally speaking, these additional requirements mean the vents could handle the pressures, temperatures and radiation levels from a damaged reactor, and that plant personnel could operate the vents under these conditions.

As part of the same decision, the Commissioners directed the staff to begin a formal rulemaking on filtering methods that would prevent radioactive material from escaping containment in an accident, either through new filter systems or a combination of existing systems. The staff will develop the technical analysis, a proposed rule and then a final rule. Throughout this process, the public and various stakeholders will have opportunities to submit comments and attend meetings to ask questions. And there will be many future posts about the progress!”

To my anonymous commenter:  I told you so!

Luckily, there has not been a nuclear disaster in the US since the March 11, 2011 tsunami disaster in Japan.  Hopefully there will not be a disaster before proper venting is provided to the 31 nuclear reactors in the US which are inherently flawed and pose a significant risk in the meantime.  If any nuclear reactor operators are seeking an effective, affordable solution to this engineering challenge, I urge you to contact me.  I was right when I posted on March 30, 2012; I am right now.