Yesterday's initial slump test result was a little awkward. I was betting with people around since the batching/mixing sound is so harsh, i would say it is around 70mm slump based on my experience. A marginal fail if based on their design mix, their designed slump should be around 100±25mm. The variance have increased to ±30mm based on the latest specification BS EN 206:2013 Table 23 under my discretion. For that reason I anticipated slump 70mm for a marginal pass when i heard the harsh sound during batching.

35mm slump which actually falls under Category 1 based on BS EN 206 Part 1:2000 when the design supposed to be under Category 3. The second batch achieved 100mm flat and right. By changing water content, they have breached the design free cement/water ratio from the intended 0.43 to perhaps 0.48 which is close to the marginal value of 0.50 set for robustness. This is essential when you are constructing bridges. So what should they do? Well it is easy or convenient to change water ratio but they actually should revise their design for their superplasticizer for better workability at low free cement/water ratio as long it complies with BS EN 934.

The intention was to set the free cement/water ratio as low as possible is to ensure denser concrete which as you all know are the prime mover for change of BS8110 Part 1 1985 to 1997(Rev Nov.2005) for durability. There is no longer a practice to ensure foundation which is exposed to extreme condition to have additional 5N/mm² concrete grade to counter the permeability/durability issue. The change of grade ensure lower permeability due to low air pockets from batching with less water and low free cement/water ratio. So the key is always about making changes on your additive for workability, not the free cement/water ratio.

Slump was not discussed in detail in BS EN 206 Part 1:2000 specification. It mentioned slump classes and no more than that. The latest BS EN 206:2013 however changed the classification into three class which is easier to understand. D.2.3 Fresh Concrete clause specified slump where;
A. 150mm is for concrete placed in dry condition;
B. 180mm for concrete placed by pumping or by tremie pipe in submerge conditions under water; and
C. 200mm for concrete placed by tremie pipe in submerged conditions under a support fluid.

The additional change from the shift of the older specification to the in-place specification would be the introduction of slump flow test test based on BS EN 12350 Part 8 which compliment this slump test which formed Part 2 of the same specification. This was use previously when i need to approved the pre-bagged grout back in 2009 in a marine structural repair works. It was known as flow cone test.

So a simple slump test is not as easy as most think. There are a lot of parameters involved to translate your design mix to workable concrete that complies with concrete utilization and the holy mother book; BS EN 2006:2013. Lastly, they should furnish us with standard deviation from past 15 batches for statistical analysis in order to complete the derivation for standard deviation shall the value for mean target strength is not extended to 6 to 12kN.