High-Density Super Workable Concrete for an MRI Facility
How a 4,000 kg/m³ concrete was designed, trialled, pumped and placed when conventional concrete was not an option.
When Conventional Concrete Wasn't an Option
A new MRI facility at a medical centre presented an unusual materials engineering challenge.
The proposed MRI room was located within a constrained, irregular triangular area adjacent to an existing building. Available room space and boundary limitations meant that the conventional approach of using very thick walls constructed from normal-density concrete could not be accommodated within the available footprint.
The project therefore required another approach: significantly increase the density of the concrete so the required shielding performance could be achieved within the restricted wall dimensions.
The target was a concrete density of approximately 4,000 kg/m³ – around 4 tonnes per cubic metre.
The client had been considering having specialised shielding bricks manufactured from imported iron ore aggregate. While potentially providing the required density, this was a costly solution, would be labour intensive to construct and the project was operating within a limited timeframe.
The question put to us was simple:
Could we develop a concrete solution instead?
The Challenge
The client had sourced specialised high-density iron ore aggregate from the United States.
The materials available for developing the concrete included:
GP cement;
slag;
20 mm high-density iron ore aggregate;
manufactured sand produced from the iron ore;
a limited quantity of locally sourced fine sand; and
locally available concrete admixtures.
The concrete needed to achieve three critical properties simultaneously:
Density. Strength. Workability.
Achieving density alone was not enough.
The concrete also had to be capable of being conventionally batched, transported in agitators, pumped and successfully placed through heavily reinforced wall sections.
This created what initially appeared to be two conflicting requirements:
Heavy-density aggregate and super workable concrete are not two things you would normally expect to put in the same sentence.
This was the first time I had been faced with developing a concrete of this type.
Designing the Concrete
The first stage was understanding the supplied materials.
Using the individual material gradings, the aggregate structure and proportions were developed around both the required density and the workability necessary for construction.
One of the major limitations was the amount of conventional fine sand that could be incorporated into the mix.
Normally, increasing fine material can assist in developing the paste characteristics needed for a highly workable concrete. In this case, however, increasing the locally sourced fine sand would simultaneously reduce the overall density.
The mix therefore needed to rely heavily on the manufactured iron ore sand, including its material passing the 75 µm sieve, together with the GP cement, slag and a minimum quantity of conventional fine sand to develop the required paste matrix.
The challenge was effectively to create enough paste to allow the concrete to flow without diluting the very property we were trying to maximise – density.
Developing the Rheology
The target was a spread of approximately 650–700 mm.
Simply producing a concrete that spread to 700 mm was not the objective.
The mix needed sufficient rheological stability to allow the paste to move through and around the extremely heavy aggregate while also carrying that aggregate through the concrete matrix without excessive segregation.
A high-range superplasticiser was used in conjunction with a workability-modifying admixture.
Numerous admixture combinations and dosage rates were evaluated.
The balance involved:
water demand;
superplasticiser dosage;
workability-modifying admixture dosage;
cementitious content;
paste volume;
manufactured iron ore fines;
conventional fine sand content; and
coarse-to-fine high-density aggregate proportions.
After numerous adjustments, we achieved the behaviour we were looking for.
The concrete flowed.
More importantly, the heavy aggregate moved with it.
Laboratory testing demonstrated that the developed mix could achieve the required:
✓ Density✓ 650–700 mm spread✓ Stability✓ Strength
But making it work in a laboratory was only the beginning.
The next question was much bigger:
Could concrete approaching 4 tonnes per cubic metre actually be batched, transported, pumped and placed under real construction conditions?
Building a Full-Scale Trial
Before approval for construction, a full-scale field trial was undertaken close to the concrete plant.
Rather than simply pumping the concrete onto the ground, a representative structure was constructed to reproduce the actual placement conditions as closely as practical.
An L-shaped wall section was constructed to represent the height and width of the proposed walls and incorporated reinforcement representative of the actual structure.
The trial was designed to evaluate the entire process:
Batching → Loading → Transport → Pumping → Pump interruption → Restart → Flow through reinforcement → Flow around a corner → Filling → Formwork pressure → Hardened concrete.
The trial was observed by the design engineers and medical physicists involved with the project.
Trial Day – Would It Actually Pump?
The concrete was batched, loaded into an agitator and transported to the nearby trial structure.
On discharge, it looked unusual.
The concrete appeared relatively coarse and harsh compared with what would normally be expected from a super workable concrete.
This was a consequence of the design.
We could not simply add additional conventional fine sand to improve its appearance because doing so would reduce the density. The mix instead relied on the manufactured iron ore fines, GP cement, slag and carefully controlled fine sand content to generate the paste required for flow.
The concrete entered the pump.
It pumped straight through.
Despite the extreme aggregate density and unconventional appearance, there were no significant pumping issues.
Then We Stopped the Pump
Continuous pumping was only part of the test.
On the actual project, there would inevitably be interruptions while agitators were changed over.
We therefore deliberately stopped the pump.
The concern was that the extremely heavy aggregate could settle while stationary, potentially making it difficult or impossible to restart the line.
Following the planned interruption, the pump was restarted.
The concrete pumped again without issue.
That was an important result.
The mix demonstrated stability not only while moving but during the type of interruption that could reasonably occur during construction.
The Placement Test
The next test was perhaps the most important.
The pump hose was positioned at one end of the L-shaped trial wall.
Rather than continually moving the hose, we wanted to see how far the concrete itself could travel.
Pumping commenced.
The high-density concrete moved through the reinforcement, progressively filled the wall, travelled around the corner of the L-shaped structure and continued filling the adjoining section.
We watched as concrete approaching 4,000 kg/m³ flowed through the reinforced structure and continually filled the form.
No mechanical vibration was used.
This demonstrated that the concrete had achieved what initially seemed contradictory – very high density combined with the flow characteristics required for a super workable concrete.
Testing the Formwork
The trial also addressed another important engineering consideration: formwork pressure.
Highly workable concrete can impose significant lateral pressure on formwork.
When that concrete has a density approaching 4,000 kg/m³ rather than approximately 2,400–2,500 kg/m³, the implications become even more significant.
The full-scale trial allowed the design and construction teams to observe both the concrete and the behaviour of the formwork under realistic placement conditions.
The placement trial was successful.
But we wanted to know what was happening inside the wall.
Cutting the Trial Wall Open
A concern remained that because the concrete had not been mechanically vibrated, apparent success from the outside did not necessarily prove complete internal consolidation.
After the trial concrete hardened, the wall was cut vertically from top to bottom and completely through its thickness.
A section was then pushed over to expose the internal concrete.
This allowed the project team and medical physicists to physically examine the internal structure and look for:
segregation;
settlement of the heavy aggregate;
voids;
honeycombing;
incomplete filling around reinforcement; and
inconsistencies through the height and thickness of the wall.
Cores were also extracted to provide further verification of compaction and density.
The destructive examination and testing confirmed that the concrete had successfully filled and consolidated throughout the structure.
The mix was approved for use on the project.
From Trial to Construction
The next challenge was delivering the solution under actual site conditions.
Site logistics were difficult.
Access was through a shopping centre car park and the concrete plant was approximately 20 minutes from site, with the delivery route involving a significant climb to the project location.
The density of the concrete also placed a major restriction on transport.
At approximately 4,000 kg/m³, a 3 m³ load contained around 12 tonnes of concrete.
Agitator loads therefore had to be limited to a maximum of approximately 3 m³ to manage vehicle weight limitations.
The project was divided into two Saturday pours:
Pour 1 – Walls
Pour 2 – Roof
6:00 am – The Wall Pour Begins
The first wall pour commenced at approximately 6:00 am.
The original placement methodology was based on approximately 500 mm lifts around the structure.
Understandably, there was considerable concern around the potential formwork pressures generated by a highly workable concrete approaching 4 tonnes per cubic metre.
On pour morning, the site engineer made the conservative decision to reduce the initial lift height to approximately 200 mm per pass.
The pump progressively moved from corner to corner, placing controlled lifts around the structure.
The approach reduced risk but also significantly slowed the rate of placement.
As the pour progressed and the behaviour of the concrete and formwork was observed, confidence increased.
Once the walls reached approximately halfway, the lift height was increased to approximately 500 mm.
Without this change, the pour risked extending well into the night.
One Lesson at the Concrete Plant
Considering the unusual nature of the concrete, production proceeded remarkably well.
There was, however, one incident that demonstrated how differently a concrete of this density needed to be handled.
One agitator driver stopped the mixer bowl while travelling from the batch sock to the slump stand.
With conventional concrete this may not have created a major problem.
With this concrete, it did.
The extremely heavy aggregate settled and concentrated a significant mass of material in one position inside the agitator.
When the driver attempted to restart the bowl, the hydraulic system struggled to overcome the load.
The bowl had to be carefully rocked backwards and forwards until the material redistributed sufficiently for normal rotation to recommence.
The truck successfully travelled to site and discharged its concrete, but due to concerns with the hydraulic system it was removed from the remainder of the pour.
It produced a simple but important operational lesson:
With concrete approaching 4 tonnes per cubic metre, keep the agitator bowl turning.
Eight Hours Later
Despite the restricted 3 m³ loads, controlled lift heights, challenging access and unusual density of the material, the wall pour was successfully completed in approximately eight hours.
The concrete remained pumpable and workable throughout the operation.
The laboratory concept had now become a full-scale concrete structure.
Pour Two – The Roof
The second pour was undertaken the following Saturday and involved the roof of the MRI enclosure.
This time the team had the benefit of everything learned during the previous week's wall pour.
Batching, agitator management, truck movements, delivery sequencing, pumping and placement were all better understood.
The result was a considerably smoother operation.
The roof pour progressed successfully and was completed by approximately midday.
The high-density concrete had now been successfully used to construct both the walls and roof of the MRI enclosure.
Hardened Concrete Performance
Testing undertaken following construction confirmed that the concrete achieved the required engineering properties.
Compressive Strength
The concrete achieved the specified 40 MPa compressive strength at 28 days.
Density
Test results achieved densities in excess of 4,000 kg/m³, confirming that the required high-density characteristics had been maintained through production and placement.
Drying Shrinkage
Shrinkage performance was also very good for such an unusual concrete system, with samples recording approximately 550 microstrain at 56 days.
Final Verification – Radiographic Testing
The final test was the completed structure itself.
Following construction, non-destructive Radiographic Testing (RT) was undertaken to evaluate the completed high-density concrete enclosure and identify any localised discontinuities or variations that could affect the required shielding performance.
The results confirmed successful performance throughout the vast majority of the structure.
There was, however, one localised area above the doorway that did not achieve the required result.
An investigation was undertaken to understand why.
During placement of the walls, as the pump hose was moved around the structure for each successive lift, small quantities of concrete had dribbled across the formwork above the door opening.
Rather than becoming incorporated into the continuously flowing mass of concrete, this material progressively accumulated in the area above the doorway.
This created a localised area that did not achieve the same uniform consolidation as the surrounding concrete.
Importantly, the investigation indicated a localised placement issue rather than a failure of the high-density concrete mix.
The affected area was subsequently rectified using lead shielding, allowing the enclosure to achieve the required shielding performance.
The Outcome
The project began with a problem that could not readily be solved using conventional concrete.
The geometry of the site, available room dimensions and boundary constraints meant that simply increasing wall thickness was not an option.
A specialised iron ore brick solution was being considered, but it came with significant cost, construction and programme implications.
Instead, the concrete itself was engineered around the constraints of the project.
The result was a purpose-designed super workable high-density concrete exceeding 4,000 kg/m³, incorporating imported iron ore coarse aggregate and manufactured sand together with locally sourced cementitious materials, fine aggregate and admixture technology.
It achieved:
Density exceeding 4,000 kg/m³
40 MPa compressive strength at 28 days
650–700 mm target spread
Approximately 550 microstrain shrinkage at 56 days
Successful conventional batching
Successful transportation in controlled 3 m³ loads
Successful pumping
Successful pump stopping and restarting
Flow through heavily reinforced sections
Flow around changes in wall direction
Successful placement without conventional vibration
Successful construction of both walls and roof
Full-scale trial verification through destructive examination and coring
Final verification of the completed structure using non-destructive Radiographic Testing
More Than a Mix Design
Perhaps the most important lesson from the project was that developing a specialised concrete cannot stop at achieving a result in the laboratory.
A laboratory mix achieving 4,000 kg/m³ would have been of little value if it could not be batched, transported, pumped and successfully placed on site.
The solution required consideration of the complete concrete process:
Material characterisation → grading → particle packing → paste design → admixture rheology → density → laboratory development → batching → transport → pumping → formwork pressure → placement → consolidation → hardened performance → final verification.
There were also lessons along the way.
Keep the agitator turning.
Understand the effect of concrete density on formwork pressure.
Trial the actual placement conditions, not simply the concrete.
And with super workable concrete, pay attention not only to the main body of the pour but also to small amounts of material accumulating in areas such as openings and penetrations.
Ultimately, the project demonstrated what can be achieved when concrete is designed around the engineering problem rather than attempting to make a conventional concrete solution fit an unconventional project.
Heavy-density aggregate and super workable concrete might not sound like they belong in the same sentence. On this project, they had to.
The result was a practical, pumpable and constructible high-density concrete solution that successfully turned an unusual materials challenge into a completed MRI facility.
Have a concrete project that doesn’t fit a standard mix design?
CGE works with producers, contractors and project teams to develop practical concrete solutions for unusual materials, performance requirements and construction constraints.
Talk to us about your project.
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