shrinkage of slabs 1

[oneintheeye]

does anyone know of typical values for shrinkage coefficeient? My computer model is asking me for it. It doesn't seem to tie in with BS8110 which i would use values of shrinkage strain to determine deflections from if doing by hand.

 

[corus]

Presumably you mean the coefficient of thermal expansion for shrinkage. Try googling it for the slab material you have.

corus

 

[Teguci]

Shrinkage of concrete is related to:
Average Humidity
Longterm Compressive Strain (prestress)
Volume to Surface Ratio
Temperature During Construction
Type of Curing
A good resource for volume change is the PCI Design Handbook. For a typical interior 4" floor slab, a volume change of 420 millionths can be expected (about 1/4" over 50'). The bigger issue is usually slab curling due to uneven shrinkage of the top surface vs bottom.

 

[apsix]

As Teguci said, and also:
Aggregate type
Sand grading
Cement type / amount
Water/cement ratio
Additives and cement replacements

AS3600 (Austr.) recommends using 850 x 10-6 for normal class cement, unless shown otherwise by testing.

 

[rapt]

herewegothen,

BS8110 Part 2 defines a method of calculatiung the final in-service shrinkage you can expect for a member of a specific shape and humidity conditions. It is based on an assumed Basic Shrinkage value which it does not define because the method it was copied from (CEB-FIP 1970) did not define this as a variable. It is simply dependent on the concrete strength.

However, concrete shrinkage varies a lot depending on the components used to make it as Apsix listed above. The main one is aggregate and sand type and this varies significantly around the world and even in local areas depending on the source of the materials.

So the BS8110 model is a bad model as it does not allow for these variations.

The Basic Shrinkage can vary from about 400 * 10-6 (with lots of additives) to 2000 * 10-6 (Hong Kong standard design figure). In Australia it varies from about 600 to 1200 * 10-6 without special additives.

Some computer programs (like RAPT) ask for the Basic Shrinkage to be defined and calculate the final shrinkage from that based on the different methodologies defined in different countries. This allows the design to allow for the concrete available in his area for a design rather than using a random gues as the code has used.

 

[druminman]

Teguci,

Linear length change due to sustained load (compressive or tensile) is creep, not shrinkage. The phenomena for creep and shrinkage are similar, but not inclusive, and both are dependent significantly on time and environmental conditions. ACI209 provides guidelines for estimating creep and shrinkage for simple applications.

Patrick

 

[ciarabelle]

Ciria Guidance document C660 gives guidance in addition to providing Spreadsheets for calculating expected shrinkage and early age thermal movement.

 

[LittleWheels]

CIRIA is the way to go.

 

[Lion06]

I got this graph from someone on here a while back. I'm not sure where it's from, but check it out.

Additionally, ACI has something on this as well that accounts for aggregate type, etc, but I can't remember where it is. I'll look for it again. I'm remembering that it was a bit of a mess to wade through.