Lightweight Concrete

process for manufacturing and placion high strength, low density concrete for offshore elements

The entire DemoSATH is made of concrete. It is a concrete with a density of 1.85 tonnes per m3 (concrete usually has a density of 2.45). When the project started, the working formula was not yet available. SAITEC, the client, was working on the formula together with the Santander Civil Engineering School. Ferrovial has successfully achieved this formula which, in addition to achieving the objective of having a density of 1.85, achieves a resistance of between 45 and 50 megapascals, which is maintained from the concrete plant to the construction site – since the density is altered and rises during transport and pumping. The problem with low density concrete is that as the density goes down, pumping becomes more complicated.

Ferrovial’s commitment was to help achieve the concrete formula. It was a challenge to obtain a concrete with such demanding specifications of density, strength and consistency and that was pumpable. After months of tests, we were able to successfully achieve this formula which, in addition to achieving the objective of having a concrete with a low density of 1.85Tn/m3 , reached resistances of over 50 Mpa, with a liquid consistency, high impermeability and pumpability.

Main objectives

Lightweight concrete is a conventional concrete modified by the introduction of air into the mass to reduce the density to below 2,000 kg/m3, whereas conventional concretes have densities in the range of 2,200-2,600 kg/m3. Density reduction by inclusion of air means that lightweight concretes have significantly lower compressive strength than conventional concretes. Thus, the ACI 213R guide establishes a classification according to the density of lightweight concrete, which considers structural lightweight concrete to be those with a density between 1,300 kg/m3 and 1,900 kg/m3, with a strength of more than 17 MPa.

However, within the scope of the DemoSATH prototype project, the design needs imposed by the floating platform for wind turbines required the use of a high-strength lightweight concrete, with a density of less than 2,000 kg/m3 , and with a characteristic compressive strength of over 50 MPa, self-compacting and pumpable, which represented a real technological leap with respect to known lightweight concrete technology.

In this context, the works team set out to develop a new procedure for the manufacture and installation of lightweight concrete with the required performance, with the support of the Machinery Service to develop the working formula.

Technical description

The main objective is the design of a low-density, high-strength, lightweight concrete for use in offshore elements such as wind foundations, capable of reducing the loads supported by the bracing, as well as withstanding the compression to which the float is exposed. The concrete defined for such use must be pumpable, highly impermeable and of a liquid consistency (almost compactable). In order to achieve this goal, the following tasks have been developed:

  • Definition of the theoretical reference dosage
  • Adjustment of the theoretical dosage, until the laboratory formulation is obtained.
  • Feasibility of plant manufacture, by means of density tests, percentage of occluded air, permeability and resistance at different ages and in different mixes and tests to determine the evolution of the density of the fresh concrete over time and its behaviour during transport and pumping.
  • Feasibility of the transport and on-site operations, by studying the influence of the duration of the transport of the concrete to the site and its pumping. To this end, tests were carried out on the same mix in the plant and on site to ensure the maintenance of the concrete properties over time.

How did we achieve this? With the use of low-density shale aggregate brought by ship from Norway to the Port of Bilbao, microsilica and high-end additives, including aerators that will produce air occluded in the concrete. In collaboration with Parque Maquinaria, the supplier of admixtures and the concrete plant, we started to test a working formula in the laboratory by adjusting and testing different raw materials, mainly looking for low densities. We developed this initial formula in the concrete plant, studying the behavior of the concrete in each mix and how the results of the tests varied (Abrans cone, occluded air, fresh density, resistance, penetration, permeability) with small variations in the percentages of aggregates, cement and additives used.
There were many variables and determining factors in achieving the ideal formula. We studied how and why there were variations in the specifications over time, with transport and pumping. One of the problems we observed on site was that when the density dropped below 1.82 Ton/m3, the concrete had difficulties to be pumped. This forced us to have an exhaustive control during the whole manufacturing process until it was placed on site, with a previous saturation of light aggregate according to established protocols and a control of the fresh density and the occluded air in each truck.
This low-density concrete is a very delicate concrete due to the amount of additives it contains and the water saturation process of the light aggregate, so we had to have exhaustive control at all stages of the process, from the saturation of the low-density aggregate until it was placed on site,” explains Enrique de las Cuevas, Site Manager.

Certified Innovative Technology

  • The innovative nature of this Project has been accredited by an External Entity, by obtaining certification as an R+D+i Project according to the UNE 166.001 standard.
  • The concrete developed within the scope of this project has been awarded First Prize in the Innovation category in the first edition of the concrete awards granted by ANEFHOP (National Association of Ready Mixed Concrete Manufacturers).