Ready mixed concrete for bored pile

[PAN]

The contractor advise me that ready mixed concrete must be used for bored pile work.

Are there any reference standards to reject concrete mixing at site for bored pile work? Please comment.

 

[dik]

With a properly written contract on supply of materials, CSA and ACI both have provisions for specifying concrete properties and CSA, at least, provides an outline of what is required for failure.

For cast in place piles (assuming they are friction piles), there is not much of a technical issue. They are generally lightly loaded for concrete. Shrinkage is not normally an issue. Some areas require suplhate resistance due to the soils. Is there a particular reason for wanting to reject. If a slump test is taken prior to depositing the load and it is outside spec then you can have the contractor reject the load, but slump (reasonable) has little impact on pile construction.

You should also spec that the top 10' are mechanically vibrated to help consolidate the concrete against the soil.

Dik

 

[BigHarvey]

Ready mixed concrete is a guarantee to have good quality concrete. In France you wouldn't be allowed to do anything else.

Concrete for piles are never vibrated and slump is a very important parameter: it should be around 18 cm and should keep this value as long as it needs for concreting the complete pile

 

[dik]

I'm sorry... but, I've had concrete for piles vibrated for over 30 years. If you try it, you'll notice that the level of concrete for a 40' long pile will drip about 4"... it consolidates the concrete against the soil.

Dik

 

[BigH]

Generally, BigHarvey is correct - with ready mix plants that have been properly vetted by DOTs or other agencies, you are more likely to have good quality concrete than not.
Now, say I am working in China - northwest. Nearest ready mix plant is 100km away or in Laos, which, to my knowledge has NO ready mix plants and doubtful of any vetting. Am I not able to pour concrete? Sure I can. I have used the portable 1/4 yd site mixers on many many bored piles, foundations, retaining walls, etc. They have been poured successfully. In fact, we watch the concrete being made on site in our supervising presence, we have control on the weights of the consitutents, we watch it being mixed and it is placed in the structure within minutes of mixing. Problems? No - other than for large pours, when you need several such mixers with no hiccups to prevent cold joints. We've had excellent concrete.
Not all jobs are in developed countries or inside large cities of the underdeveloped ones. Concrete can be made by portable mixers - but, you need to ensure good control on the consitutents and mixing. But wait, don't you need to do that too with ready mix plants? And, with ready mix plants and transit mixers, there is always the possibility of adding additional water 'on the way' or on arrival at the pour site. Doesn't happen with portable mixers - only problem with added water is when it is added during the mixing and you can observe that first hand - doubtful you do at a batch plant.

 

[cvg]

having ready mixed concrete is not a guarantee of high quality, if so then why do we still measure temperature, slump, check batching tickets for travel times and then take cylinders for compression tests? Because we are trying to assure the owner that the concrete has acceptable quality. With all that, I have rejected transit mixer loades that didn't meet the quality requirements. Same thing with onsite concrete production, except that you have a better opportunity to observe the production.

 

[BigHarvey]

Dik

In France, it is mandatory to put , depending on the codes you are submitted, around 400 kg of cement per m3. This gives a concrete which does not need to be vibrated to get a good density and watertightness without any vibration.
With slumps of 18 to 20 cm ( obtained with plasticizers )maintained throughout the concreting session

 

[BigH]

BigHarvey - you raise a point that has had me wondering for a while - why do codes (or specifications) mandate a minimum cement content (or is the cementitious material permitting fly ash to be substituted for cement)? It seems that if you specify the strength requirements, maximum w/c ratio - thus permitting plasticizers or superplasticizers for bumping up the workability at low w/c ratios, that the code/spec shouldn't care how much (or little) cement he uses so long as the project requirements are met.

I have seen minimum cement contents specified for 20 MPa concrete that forces the concrete (given the w/c ratio, etc) to end up with >30 MPa. Seems a waste of cement. Why don't they just do away with the 20 MPa and then specify 30 MPa?

Any take on it?

 

[dik]

Depending on the soil type, the cement content can have a marked effect on the durability of the concrete.

Dik

 

[BigH]

dik - okay but the codes and specs really don't address this "soil type" - I've never seen: "Well, if soil type
A, then use 400kg/m3; if type B, then use 375kg/m3." Of course, too, you will specify a different type (e.g., sulphate resistant) as needed.

 

[cvg]

i think the reason for minimum cement content requirements is that it is much easier to add a little extra cement to the mix and ensure that the concrete has adequate strength, than it is to discover afterwards that the concrete doesn't meet the desired strength and then be faced with the difficult task of chipping it out, hauling it away and doing it all over. In addition to the quaility control issues, adding more cement (or fly ash) gives a product which is more dense and impermeable. This usually results in a more durable concrete. This is especially useful where it is an exterior structure subjected to freeze thaw or if it is a structure expected to hold or convey water.

 

[BigHarvey]

In France the subject is clear : the 400 kg are for density of concrete becaus we can't vibrate it. If you construct a diaphragm wall you will see that with 400 kg , your concrete is watertight without vibration and without waterproofing additive. Given the allowable stress in concrete for deep foundations , it is obvious that it is not for strength reasons.
It is also true that in our codes the type and quantity of cement can be influenced by the soil from an environmental point ( sulphates and so on )

 

[Zambo]

To go back a couple of steps - I agree with BigHarvey regarding concrete slumps/vibrating of piles. The comment from dik about 40' piles is superfluous as a 40' bored pile is not the norm, 40 - 100m would be typical.

 

[dik]

Doesn't matter... vibrating helps consolidate concrete against the soil... if you have a long shaft, then use the vibrator for as deep as it will reach... I've never worked in an area where 120-300' was the norm... areas I've encountered are generally less than 50'... also, the 4" was a minimum (sorta), I've often encountered the concrete to drop a foot... You can suit yourself, but, I'll continue to vibrate them...

Dik

 

[BigH]

Zambo - I agree with dik on the length of the bored piles. There really is no "typical" length - it really depends on the locale of work, local practice and the like. In the Toronto area of southern Ontario (Queenston Shale), bored piles are often in the order of 20ft (6m) deep. My first job in southern Ontario was inspection of large belled battered bored piles (drilled shafts, among several other 'names') that were only about 25 ft deep and were, in fact placed in tension due to large power line transmission tower footing. In India on my job there - there were 40m deep - In Bangkok, of course, they can go to 75 m or more to get down into the second sand stratum.

We vibrated the concrete - but in shallow bored piles with sufficient shaft diameter, it was not uncommon to have workers down inside the cage in order to handle the elephant trunk. With respect to BigHarvey's experience, they are taking advantange of, I presume, plasticizers to increase the slump (workability) so that the final product is not much unlike self-consolidating concrete.

I don't think that vibrating the concrete really 'presses in' the concrete to the side wall - there just isn't enough energy being imparted to 'force the issue'. Still, to ensure that you do not have honeycombing with lower slump concretes, vibration is not a bad idea.

 

[Zambo]

BigH,

yes it is clear we are talking about different forms of construction. I am talking about bored piles for civil engineering structure in the order or 1.5m diameter and lengths 50-100m. In this case bentonite or polymer is used as excavation support, and I really do not see that it is practical to use vibrators at this depth and through the bentonite.

The concrete has a high slump so that the concrete is self compacting. The piles are of course tested and there is not a problem with uncompacted concrete.

 

[PAN]

Thanks for all comments. I work in Thailand. In this country, we can use whether ready mixed concrete or site mixer.

 

[BigH]

Great country - I will be there on a bit of R&R come early June. Was involved with bored piles there as Rama IV elevated highway was being constructed. We were involved with the foundations for the higher elevated skytrain - that didn't go later on.

 

[Zambo]

BigH,

you were maybe working with Thai Bauer - good company. I see that they are picking up work again in Thailand and we are also talking to them about work in Vietnam. The new airport link had a lot of bored piling rigs last time I had a drive past, Bauer, Pylon (previously Power P I think) Ital Thai Trevi.

 

[BigH]

It was quite a while ago, but I had contact with them and they were very helpful - got a lot of load test data off of them - not sure they actually did the production or not; likely. I noted on many that when the adhesion was reached, the piles 'plunged'. Up to that point, using Tomlinson's adheasion factors yielded excellent comparisons (no end-bearing). They have good reputation.