Rotary Bored Piling Specialists

Concrete for Piling

There is no prescriptive “Right” or “Wrong” concrete mix for piling, as the best mix may be determined by differing circumstances. This is intended to be helpful by demonstrating examples of a thought process to examine those characteristics which may be critical to its operational use.

For example,

A successful pump mix may not be a suitable piling mix:

Reason:
 

Steel pipes and concrete delivery hose used for pumping concrete are impervious and prevent free water in the mix from leaving the concrete, thereby maintaining its workability.

That same concrete may behave differently when in contact with (ground) which is able to absorb the free water!

Consistent good flow is a vital requirement for:
 

Tremie operations: under water / with polymer fluid / with bentonite

Potential Problem - a concrete mix which has a higher bleed potential could cause... read more

Potential Problem - a concrete mix which has a higher bleed potential could cause problems with tremie operations, particularly under polymer fluid. The polymer fluid inside the pile bore is likely to have a head pressure well above the external ground water pressure, in order to facilitate the excavation process, and prior to the concrete being placed in the bore, the polymer acts to create a semi / impervious membrane, supporting the bore by the internal excess pressure. When the concrete is placed however, and comes into contact with the surrounding ground, the high alkaline environment of the concrete breaks down the polymer membrane at the ground / concrete interface, and if the ground is able to absorb water then the difference in head pressure may cause the concrete to bleed free water laterally into the ground, and stiffen the concrete even under fluid. This stiffening can be enough to block the tremies at the base of the pile.

Secant wall male piles

Potential Problem - Secant walls are most likely to encounter water leaks at the... read more

Potential Problem - Secant walls are most likely to encounter water leaks at the inter-section of where the male pile cuts into the female, as this is the point of contact between the piles and is also the point where the concrete section is at its most narrow. In order to gain a good bond between the piles, the concrete from the male pile must fully flow into the space occupied by the casing, before ground water or debris from the ground can fill that space. Secant wall male piles act in bending and potentially may be heavily reinforced. This reinforcement may inhibit the flow of the concrete (outward through the reinforcing cage, and upward from below), allowing ground water and debris to occupy the space intended for the bond between male and female piles, before the concrete can fill it.

Secant wall female piles

Potential Problem - A secant wall female pile mix ideally should still comprise good... read more

Potential Problem - A secant wall female pile mix ideally should still comprise good characteristics of being well graded and have a good workability and low bleed, but the concrete for secant wall female piles need to have a controlled rate-of-gain of strength, so that, when forming the male piles, the females are not so hard that they cannot be cut at a production rate which is economically viable. The female piles however still have to reach a sufficient long-term strength for durability. These subtly competing requirements can lead to problems. A concrete which has a high bleed potential and formed in ground which is dry is likely to bleed excess water into the surrounding ground, thereby reducing the water cement ratio of the female pile, and accordingly increasing its strength, making the wall harder to cut than anticipated.

Problem-free concrete pumping

Potential Problem - A poorly graded mix, has greater potential to segregate the coarse... read more

Potential Problem - A poorly graded mix, has greater potential to segregate the coarse aggregate from the cement and fine-sand paste, particularly when pumping upward. If this occurs the coarse aggregate may interlock and block.

CFA pile formation and reinforcement installation

Potential Problem - Concrete for CFA piles require all of the properties of a good mix... read more

Potential Problem - Concrete for CFA piles require all of the properties of a good mix for “Problem-free concrete pumping” but may also require the addition of retarders in hot weather conditions, as the large surface area of the concrete delivery hose may raise the concrete temperature potentially leading to early stiffening. In order to effectively install a reinforcement cage into a column of concrete in a CFA pile, the concrete must be able to displace and flow around the reinforcement, as the cage is inserted.

Suggested Solutions
Tremie operations: under water / with polymer fluid / with bentonite

Potential Problem - a concrete mix which has a higher bleed potential could cause... read more

Potential Problem - a concrete mix which has a higher bleed potential could cause problems with tremie operations, particularly under polymer fluid. The polymer fluid inside the pile bore is likely to have a head pressure well above the external ground water pressure, in order to facilitate the excavation process, and prior to the concrete being placed in the bore, the polymer acts to create a semi / impervious membrane, supporting the bore by the internal excess pressure. When the concrete is placed however, and comes into contact with the surrounding ground, the high alkaline environment of the concrete breaks down the polymer membrane at the ground / concrete interface, and if the ground is able to absorb water then the difference in head pressure may cause the concrete to bleed free water laterally into the ground, and stiffen the concrete even under fluid. This stiffening can be enough to block the tremies at the base of the pile.

Suggested Solutions

The solution here is to have a well graded mix, increase the super-fine particles (cement / GGBFS) and incorporate anti bleed agents into the concrete. The potential problem of a concrete with a high bleed potential when using a tremie operation with bentonite, is that the membrane effect of the bentonite potentially remains in place during and after the tremie operation effectively preventing the lateral release of fluid from the concrete. If the concrete has too much free water in the mix, this could initiate a vertical bleed, potentially causing washout of the cementitious particles. All tremie operations have the same “general requirements” in that, the concrete must be able to flow outward between the bars of a reinforcing cage, at a rate sufficient to maintain a near identical rise in the level of concrete in the area outside of the cage, to the level of concrete contained within the inner core of the pile. If the concrete in the inner core becomes significantly higher, then there is a risk that concrete may flow outward at a higher level, trapping material or fluid, and causing integrity problems to the outer pile concrete. This risk is exacerbated by closely spaced reinforcement, potentially reducing the flow rate or causing aggregate interlock. The solution here is to increase the workability, ensure that the mix is well graded to prevent interlock and to reduce the coarse aggregate size.

Secant wall male piles

Potential Problem - Secant walls are most likely to encounter water leaks at the... read more

Potential Problem - Secant walls are most likely to encounter water leaks at the inter-section of where the male pile cuts into the female, as this is the point of contact between the piles and is also the point where the concrete section is at its most narrow. In order to gain a good bond between the piles, the concrete from the male pile must fully flow into the space occupied by the casing, before ground water or debris from the ground can fill that space. Secant wall male piles act in bending and potentially may be heavily reinforced. This reinforcement may inhibit the flow of the concrete (outward through the reinforcing cage, and upward from below), allowing ground water and debris to occupy the space intended for the bond between male and female piles, before the concrete can fill it.

Suggested Solutions

The solution here is to have very good workability, a smaller aggregate size and a high flow for the concrete. Note: (The higher the rate of extraction of the pile casing, the greater the flow of concrete required to fill the interface between male and female piles.)

Secant wall female piles

Potential Problem - A secant wall female pile mix ideally should still comprise good... read more

Potential Problem - A secant wall female pile mix ideally should still comprise good characteristics of being well graded and have a good workability and low bleed, but the concrete for secant wall female piles need to have a controlled rate-of-gain of strength, so that, when forming the male piles, the females are not so hard that they cannot be cut at a production rate which is economically viable. The female piles however still have to reach a sufficient long-term strength for durability. These subtly competing requirements can lead to problems. A concrete which has a high bleed potential and formed in ground which is dry is likely to bleed excess water into the surrounding ground, thereby reducing the water cement ratio of the female pile, and accordingly increasing its strength, making the wall harder to cut than anticipated.

Suggested Solutions

The solution here is to create a mix which has good workability and low bleed properties, but low rate of gain of strength. Limiting the overall cementitious content of cement and GGBFS will restrict the rate of strength gain, but lead to an un-cohesive mix of low workability, but this can be rectified by the introduction of further super-fine particles of a not-cementitious nature (e.g. very fine limestone dust) by the addition of anti-bleed agents to the concrete a low rate-of-strength-gain female mix can be produced.

Problem-free concrete pumping

Potential Problem - A poorly graded mix, has greater potential to segregate the coarse... read more

Potential Problem - A poorly graded mix, has greater potential to segregate the coarse aggregate from the cement and fine-sand paste, particularly when pumping upward. If this occurs the coarse aggregate may interlock and block.

Suggested Solutions

The solution would be to improve the grading, increase the quantity of super-fine particles (cement / GGBFS) and reduce the free water by the use of plasticisers, but be careful of superplasticisers, as (although they are very effective water reducing / plasticising agent), as the concrete starts to stiffen, they can exhibit a more sudden reduction in performance than a normal plasticiser

CFA pile formation and reinforcement installation

Potential Problem - Concrete for CFA piles require all of the properties of a good mix... read more

Potential Problem - Concrete for CFA piles require all of the properties of a good mix for “Problem-free concrete pumping” but may also require the addition of retarders in hot weather conditions, as the large surface area of the concrete delivery hose may raise the concrete temperature potentially leading to early stiffening. In order to effectively install a reinforcement cage into a column of concrete in a CFA pile, the concrete must be able to displace and flow around the reinforcement, as the cage is inserted.

Suggested Solutions

The key requirement here is to have a low bleed property for the concrete so that the concrete cannot loose water from the mix into the surrounding ground and prematurely stiffen. This should maintain good flow characteristics, even after the concrete has been in contact with the ground during the pile formation. If problems occur, then try increasing the quantity of super-fine particles such as cement / PFA / GGBFS / micro-silica, and also the addition of anti-bleed agents.

Get In Touch


For more information about our services and how we can help you with your projects, please call us on 01476 570781 or leave us a message below.