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Time between concrete cast in two lifts (layers)

Time between concrete cast in two lifts (layers)

Time between concrete cast in two lifts (layers)

Hello All,
A large raft (7,000 m3)is to be cast in two lifts with each layer thickness of 1.5 to 2.0 meters. The mix contains Flyash and Microsilica with strength of 40 Mpa (ASTM). We would like to know what shall be the minimum time gap between casting next layer (second and final layer). The contractor wants to cast after 24 hours of casting the first layer which will be disaster because the preceding layer hasn't achieved the required strength to receive the second layer plus the heat of hydration differences between first and second layer may create a thermal shock and cracks may occur.

Another problem is cold joint which will occur if two layers are cast at different periods.

Please send us your valuable suggestions.

Thanks in advance.

RE: Time between concrete cast in two lifts (layers)

When I looked at my past work the biggest pour i got involved was about 4000m3 but I have dealt with a lot of steam turbine foundation all the way to 800MW.

I am retired now but in the past I did manage to persuade the contractor to pour all the raft foundations in one operation lasting days.

To me if the contractor can re-start the pour 24 hours later he might as well to arrange the shifts to do one big pour. There will be a heat problem and many thermal couples have to be installed, extensive insulation to be applied and the concrete would have to mix with ice to lower the pouring temperature.

I am lucky in a way that turbine foundation is a dynamic foundation and risk of the machine dynamically coupled only to the upper lift (if poured in two halves) in the vibration is something no turbine manufacturer could take. If the foundation is subjected to vibratory load for life any crack and separation in concrete will grow with time and never stop. I have seen machine foundations with the upper layer vibrating without the lower layer participating.

Therefore if the foundation resists only the static load there would not be a compelling case to insist on a single pour because the two layers should have no reason to move relative to each other.

For the two-pour operation I would prefer to see the first layer hardened first, its surface specially prepared for the key-in integration (with the surface scabbled if hardened or jet washed when it is still green). I normally insist on the surface to have physical keys by dropping large wooded section on top of the fresh concrete and removed them later to leave depressions behind. Fresh cement adhering to the exposed rebar above the first pour will also need to be removed and cleaned.

I would go for at least a 3-day gap mainly because the first layer should be prepared properly. Concrete should be hard enough after 24 hours to avoid being damaged by the vibrators' action from the second pour. However both cement replacement materials are known to slow down the concrete setting time and you are likely to need high performance super-plasticizer in the mix too. The longer the gap the better heat dissipation for the first pour so that the differential temperature at the interface is reduced.

RE: Time between concrete cast in two lifts (layers)

Is there any standard/spec specifying minimum time gap between two layers/pours? ASTM/ACI/BS ?


RE: Time between concrete cast in two lifts (layers)

Quote (concretebaby (Civil/Environmental) (OP))

Is there any standard/spec specifying minimum time gap between two layers/pours? ASTM/ACI/BS ?

I am not aware of one.

Concrete has initial and final sets. Once the final set has bee reached the green concrete should be allowed to harden and not disturbed. Therefore if you pour the next lift the basic requirement is to have the concrete hardened enough to support the workmen and not to be re-mixed again.

However I would venture to say the minimum time gap between the two lifts is dictated by the reasonable time required to prepare the construction joint so the two can be key in properly. Every code has specification governing the preparation of the construction joint. In the standard practice the Contractor will try his best to get the first lift surface prepared, the supervising engineer will have to inspect. The green light is only given to carry out the next pour when everything meets the required standard.

It would not be sensible to attach a time scale between two lifts as a lot of factors can change it. Even in raft adequate time must be allowed to erect the additional reinforcement, formwork, fixings, embedments, inserts.... for the next lift. This is to assume one has at any time access to infinite concrete supply as it is no small matter to guarantee 3500m3 concrete without interruption.

The structure also can have different requirements for its construction joint preparation. For the best result the common reinforced concrete codes in USA mandate the surface roughened with an amplitude of 1/4" in a manner that will not leave loosened particles, aggregate, or damaged concrete at the surface. Thoroughly clean the surface of foreign matter and laitance, and saturate it with water.

For the above reasons I doubt if the minimum time gap, if exists, can be contractually enforced as it make no technical sense. I did have engineers who make contractors spent week to prepare the receiving surface of the next lift. A construction joint is a plane of weakness to be avoided if possible. If it is unavoidable then due care must be exercised to ensure the interface to be prepared as per design code.

RE: Time between concrete cast in two lifts (layers)

Take a look at ACI 207.1R, Guide to Mass Concrete. It addresses these issues.

RE: Time between concrete cast in two lifts (layers)

Why not allow a construction (cold) joint?....especially in mass concrete.

RE: Time between concrete cast in two lifts (layers)

Cold joints are done all the time in Dam construction.

Mike McCann, PE, SE (WA)

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