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Maximum water cement ratios 3
- Thread starter jike
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3
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Should you be specifying the water cement ratio at all?
I ask the question as this is a subject undergoing much review in the UK, and the 'latest' thinking is that the designer should specify the requirements for the performance of the concrete, i.e. target/min strength, intended lifespan, ground conditions, fire resistance, chemical resistance etc... and leave the actual design of the mix to the concrete producer. The idea is that the designer is responsible for specifying what the concrete needs to meet, the producer is responsible for prodiving a mix that will meet the specification.
A lot of problems with concrete mixes have been blamed on the specification of max water/cement ratios, giving a mix that when it turns up on site is too stiff to place, and needs additional water to increase workability. For the designer to specify the required workability along with the other requirements for the mix, the producer has to provide a mix that not only is workable but also durable.
This is quite a departure from traditional concrete specifications, and only time will tell how succesful this approach will be, but the idea is there that the designer is responsible over the issues he has control over, and the concrete producer is responsible for providing a mix that will perform.
I ask the question as this is a subject undergoing much review in the UK, and the 'latest' thinking is that the designer should specify the requirements for the performance of the concrete, i.e. target/min strength, intended lifespan, ground conditions, fire resistance, chemical resistance etc... and leave the actual design of the mix to the concrete producer. The idea is that the designer is responsible for specifying what the concrete needs to meet, the producer is responsible for prodiving a mix that will meet the specification.
A lot of problems with concrete mixes have been blamed on the specification of max water/cement ratios, giving a mix that when it turns up on site is too stiff to place, and needs additional water to increase workability. For the designer to specify the required workability along with the other requirements for the mix, the producer has to provide a mix that not only is workable but also durable.
This is quite a departure from traditional concrete specifications, and only time will tell how succesful this approach will be, but the idea is there that the designer is responsible over the issues he has control over, and the concrete producer is responsible for providing a mix that will perform.
I agree with Iandig. Far to often design mixes are far too stiff to place. Without sufficent placticity it is hard to get the concrete placed well around the bars resulting in poor bond. In hot weather stiff mixes willl begin to set before being properly consolidated. Add to this that designs today in general seem to have a lot more bar than they used to, forms are more congested than ever.
Super P has somewhat reduced the need for water, but it does not solve all problems. If actual strength is no where near allowable, a little water can go along way to a better product.
On the other hand some of these residential guys pour walls with what looks like a milkshake. That is a little too wet.
Super P has somewhat reduced the need for water, but it does not solve all problems. If actual strength is no where near allowable, a little water can go along way to a better product.
On the other hand some of these residential guys pour walls with what looks like a milkshake. That is a little too wet.
If all you're concerned about is meeting a compressive strength criterion, the water-cement ratios can vary all over the board. If you have a concern about durability, reduced shrinkage (and cracking), and integrity of the concrete, the water-cement ratios should be closely watched and matched with the rest of the mix constituents to produce quality concrete.
The ranges I gave for different locations will produce good durability. If you design for durability, the compressive strength will follow...always. But concrete has to be designed for the application. For instance, the 4000 psi concrete used for columns and beams shouldn't be the same concrete as the 4000 psi floor slab. Different applications, different design parameters. To further this concept, the "flatwork" should be designed with the lowest amount of cement that will achieve the desired strength and durability, coupled with the largest practical coarse aggregate size so that the water demand is reduced, the shrinkage is reduced, and the overall performance durability of the floor is enhanced. Conversely, the concrete in a tightly reinforced column needs more cement to affect good bond to the rebar and smaller coarse aggregate to prevent segregation and voids. Due to the higher cement content and the smaller aggregate, the water demand is increased and the combination of those produces concrete that shrinks to a degree that would be unacceptable in a floor slab but works fine in a column.
The ranges I gave for different locations will produce good durability. If you design for durability, the compressive strength will follow...always. But concrete has to be designed for the application. For instance, the 4000 psi concrete used for columns and beams shouldn't be the same concrete as the 4000 psi floor slab. Different applications, different design parameters. To further this concept, the "flatwork" should be designed with the lowest amount of cement that will achieve the desired strength and durability, coupled with the largest practical coarse aggregate size so that the water demand is reduced, the shrinkage is reduced, and the overall performance durability of the floor is enhanced. Conversely, the concrete in a tightly reinforced column needs more cement to affect good bond to the rebar and smaller coarse aggregate to prevent segregation and voids. Due to the higher cement content and the smaller aggregate, the water demand is increased and the combination of those produces concrete that shrinks to a degree that would be unacceptable in a floor slab but works fine in a column.
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