Precast Mix Design Help. 1

[knightwadda]

I just start working for concrete company and i need a little help. i got a request for a tunnel mix design and i made one according to the specs. i received. but i would like to know what you guys think about my mix design.

1 YARD FT3
Cement 450 ASTM C-150 TYPE I / II 2.29
SLAG 450 ASTM C-989 FDOT APPROVED 2.47
57 ROCK 1500 ASTM C-33 9.48
PEAROCK 0 ASTM C-33 0.00
SILICA 1250 ASTM C-94 FDOT APPROVED 7.58
WATER LB. 265 ASTM C-260 4.25

ENTRAPED AIR 1.00% 0.27
Admix 1 49.00 OZ. WRDA 64 WATER REDUCER
Admix 2 0.00 OZ. DAREX AEA
Admix 3 24.00 OZ. ADVA 120 HRWR
Admix 4 5.00 GAL. DCI-S 0.67

Unit wt 145.0 YEILD 27.00
Water Cement 0.34
SLUMP 4 INCHES MAX

HERE'S THE SPEC.

Service life: 150 years
Environment class: External surfaces against ground: Extremely aggressive / Tidal zone no barrier

Internal surfaces: Slightly to moderately aggressive / not in contact with water/ Condition within 800 m from Ocean.
Nominal Cover: External surfaces: 2 inches
Internal surfaces: 1.6 inches
The covers must be assessed among calculation using life 365 model.
An over design might be applied, as set tolerances, for construction.
Density: ? 144 lb/ft3 [2300 kg/m3]
Class: V (Special) ? 6000 psi
apply an over design requirement of plus 1400 psi
Early strength (12 hours) shall be assessed on the base of compressive test results for the proposed concrete mixes.
Chloride content: ? 0.34 lb/yd3 [0.2 kg/m3]
Cementitious: ? 752 lb/yd3 [445 kg/m3]
Cement: OPC Type I/II – AASHTO M-85
Addition (Pozzolans and Slag-Std Specs - 346-2.3):
Cementitious replacement shall be assessed among calculation using life 365 model and mix trials adjustments to verify the properties of the fresh concrete.
o 20% Fly ash (FA- Std Specs -929-2) – 7 to 9% Silica fume (SF - Std Specs -929-3)
Or
o 60% GGBSlag (S - Std Specs -929-5)
Or
o 50% GGBSlag (S - Std Specs -929-5)- 10% Fly ash (FA - Std Specs -929-2)
W/C: ? 0.35
the ratios may be optimized among mix trials adjustments and calculation using life 365 model.
Corr. Inhibitor: ?5 gal/yd3 [25 l/m3] (Std Specs -924-2.7)
the quantities may vary among calculation using life 365 model
Superplasticizer: HRWR (Std Specs -924-2.6)- to be defined kg/m3
Slump of maximum 4 inches [100] mm. Tixotropic properties are required to achieved good mould filling under external vibration
Retarder: WRR (Std Specs -924-2.4) - to be defined kg/ m3
Slump retention ½ hour
Placement: N.A.
Laboratory trial mixes must be conducted on the basis of measurement of properties of the fresh concrete. This shall permit to optimize the use of superplasticizer and retarder.
Aggregates: Fine aggregate (Std Specs -902): DOT Natural silica sand
Max size coarse aggregates (Std Specs -901): 3/4” - Limestone size No. 67 or eventually 57

 

[Ron]

Well...a couple of quick points (keep in mind that asking such a broad question of a complex issue such as this will not get many answers. You are asking professionals to take their time to review a mix design in detail, which takes a fair amount of time to do properly)....

Your percentage of slag will cause a slow strength gain, so be prepared for low 7-day and possibly lower than anticipated 28 day strengths.

The specified cover is not consistent with their description of an aggressive environment...it should be more.

How will the concrete be placed? A 4-inch slump can be difficult to place.

If you do not have a record of service for this mix design, then you will have to go through a trial batch procedure and apply an overdesign factor (which is specified at 1400 psi, but you don't know if that's enough)

 

[cvg]

you really didn't follow the specs exactly, you omitted the fly ash...
this is for a pre-cast structure, so optimum curing conditions might speed up the strength gain. It might be good to ask your client how fast they want to strip the forms and how they plan to cure. that may affect your mix design.

 

[knightwadda]

Well am not worried about the early strength. i will be running a trial batch this week to check on the strengths, my main concerns are if i meet the specs. to the best of your knowledge.
as for cvg's comments about fly ash, it is optional as far as i can tell.

 

[knightwadda]

by the way Ron how do i increase the cover? with more sand and less rock?

 

[knightwadda]

CVG i see what u meant about the fly ash. thanks

 

[Ron]

knightwadda...cover is a section thickness issue, not a concrete mix design issue, except where tight cover will create problems with consolidation of concrete with large aggregate.

Density of the concrete can help the low cover issue, but not in a very aggressive environment.

 

[cvg]

cover for reinforcing steel in pre-cast elements is often thinner, anyway that is the engineer's responsibility, not the concrete suppliers. I don't see anything in the spec which indicates the type of reinforcing steel anyway.

 

[knightwadda]

that's what i was thinking too. as long as my concrete meets the slump requirement i am good.

 

[knightwadda]

i appreciate the help guys thanks a lot