Wind turbine grouts: foundations and tower flanges

Why a wind turbine needs grout

A wind turbine tower does not stand directly on the concrete. After alignment, a gap remains between the bottom flange of the steel tower (or the bearing ring of the anchor cage) and the foundation, and it is filled with grout. This layer, from a few centimetres to several tens of centimetres thick, transfers the weight of the tower, nacelle and rotor to the foundation and, most importantly, the alternating bending moment from the wind and the rotation of the blades. If the grout crumbles or settles, the anchor bolts lose tension, the joint starts to “breathe”, and repairing it on an operating turbine costs incomparably more than choosing the right material at the construction stage.

Onshore wind farm and grout under the flange of a wind turbine tower: anchor bolts with nuts, fresh grout covered with wet hessian

That is why manufacturers produce dedicated ranges for wind energy — ultra-high-strength cementitious grouts of classes C80/95–C100/115 and above, as well as special epoxy grouts. They differ from ordinary non-shrink grouts for pumps and columns in their strength and stiffness, their proven fatigue resistance and the fact that they are designed to be pumped in large volumes in a single continuous pour.

Where grout is needed in a wind turbine

  • Steel tower on an anchor cage. The most common onshore foundation arrangement: upper and lower steel rings, connected by long anchor bolts, are cast into the body of the foundation. The bottom tower section is set on shims and aligned, the gap under the flange is filled with grout, and only then are the bolts tensioned.
  • Baseplate or flange on cast-in bolts. The principle is the same: the grout is the only layer through which the load passes from the steel to the concrete.
  • Concrete and hybrid towers. The horizontal joints between precast concrete segments and the transition from the concrete part to the steel part are filled with high-strength mortar.
  • Offshore foundations. Monopile-to-transition-piece connections, piles in the sleeves of jacket structures, filling of voids under water — there are separate grouts for these, supplied in bulk and pumped continuously by the hundreds of tonnes.

Anchor cage: what matters for the grout

In an anchor cage foundation, the bolts pass through the concrete in protective sleeves and do not bond to it — so each bolt works in tension over its full length and can be tensioned to the design force. After the bottom tower section has been installed, the gap between the flange (or the top ring of the cage) and the concrete is filled with grout. The bolts may only be tensioned once the grout has reached the strength specified by the turbine manufacturer or the design: the entire compressive effect of the prestress is carried by the grout layer.

This leads to practical rules that recur in designs: grout must not get into the bolt sleeves (their tops are usually sealed before pouring); the annular gap is filled completely, without air pockets, by a continuous pour from one point or one sector; and the strength at the time of tensioning is confirmed by testing specimens cured under site conditions. Some grouts have been developed specifically for this arrangement — for example, the SikaGrout-9370 data sheet expressly states its use with anchor cages.

How tower grouting differs from equipment grouting

  • Fatigue. Over the service life of a turbine, the connection goes through hundreds of millions of load cycles. That is why not only compressive strength matters but also fatigue resistance proven by testing: manufacturers of wind turbine grouts refer in their data sheets to fatigue tests and certificates, and where no figures are given, the test reports should be requested before the material is chosen.
  • High sustained pressure. The tensioned bolts clamp the flange onto the grout with great force. A high strength class, a high modulus of elasticity and low creep are needed so that the layer does not “settle” and the bolt tension does not drop.
  • Thick layers and large volumes. The grout depth under a flange can be considerably greater than under a pump, and a ring several tens of metres long is poured in one go. The data sheets of wind turbine grouts give the minimum and maximum thickness, the diameter and length of the pumping hoses and the time the grout remains workable.
  • Early strength and the erection programme. The crane for erecting the tower is expensive, and bolt tensioning has to wait for the grout. Grouts differ in their strength after one day and in how quickly they gain strength in the cold — this has a direct effect on the programme.
  • Turbine manufacturer’s requirements. Wind turbine manufacturers set their own requirements for grout, and some grouts have been developed for a specific platform: Fosroc produces Conbextra VG for Vestas foundations and Conbextra SG for Siemens foundations, and the SikaGrout-9200 data sheet names onshore VESTAS foundations. The choice of material is agreed with the turbine supplier and the designer.

Cementitious or epoxy?

The bulk of the grouting under steel towers is done with ultra-high-strength cementitious grouts: they can be pumped, placed in thick layers and mixed on site in any quantity required. Epoxy grouts for wind energy are used where the connection design calls for them: they gain strength faster and have higher tensile strength, but they are more expensive, more sensitive to temperature and limited in the thickness of a single pour because of the heat of reaction. The two types are not interchangeable without the designer’s agreement — they differ in modulus of elasticity, creep and substrate requirements.

Site procedure

  • Substrate. Laitance and weak concrete are removed down to sound aggregate and the surface is cleaned. For cementitious grouts, the substrate is pre-soaked for the time stated in the data sheet and standing water is removed before pouring; for epoxy grouts, it is kept dry.
  • Formwork. Strong and grout-tight all round the ring, with extra height to provide a head and with air vents. Cement paste leaking through gaps leaves honeycombing at the edge.
  • Mixing. Only forced-action, high-shear mixers, with the amount of water strictly within the data sheet limits. Before starting, a trial batch is checked, and a reserve of material and a standby mixer or pump are prepared: stopping halfway round the ring is not acceptable.
  • Placing. Continuously, from one side or one point, by pump or through a head hopper, without vibration unless the data sheet permits otherwise.
  • Testing. Specimens are taken from the batches (cubes or prisms, by the method given in the data sheet and the design); their results are the basis for tensioning the bolts. The temperatures of the air, the substrate, the dry mix and the water are recorded.
  • Curing. Exposed surfaces are covered immediately with wet hessian, polythene sheeting or a curing compound. If the edge dries out in the wind and sun, cracks form at the edge of the flange.

Heat, frost and wind: Central Asian conditions

Wind farms in Uzbekistan and neighbouring countries are built on open steppe and desert sites. In summer, the flange steel and the concrete heat up in the sun far more than the air, while the upper application limit of wind turbine grouts is usually in the range of +30…+40 °C. In hot weather, pours are scheduled for the night or early morning, bags and water are kept in the shade, chilled water is used and the work area is shaded. In winter, the lower limit — from 0…+2 to +5 °C depending on the grade — calls for a heated enclosure, heating of the substrate and mixing with warm water within the limits permitted by the technical data sheet; the grout is protected from freezing until it has gained strength. Strong, dusty wind speeds up the evaporation of water from fresh grout, so covering it immediately after pouring is not a formality but a condition for achieving the design strength.

What to look for in the technical data sheet

  • the strength class (for example, C80/95, C90/105, C100/115) and the compressive strength at 1, 7 and 28 days, stating the specimen shape and temperature;
  • the strength after one day and at low temperature — this determines when the bolts can be tensioned;
  • the modulus of elasticity and the flexural tensile strength;
  • confirmation of fatigue resistance — which tests and certificates the data sheet refers to;
  • the minimum and maximum pour thickness, and whether and how far the grout can be pumped;
  • the application temperature range and the time the grout remains workable;
  • expansion and shrinkage;
  • which wind turbine platform the grout was developed for, if the data sheet says so.

See also: grouts — overview, cementitious grouts, epoxy grouts, anchoring and anchor grouting, equipment grouting.

Materials for wind turbine foundations

Below are the reference guide’s cards with properties taken from the manufacturers’ technical data sheets. The figures are for guidance: for the design, use the values from the current revision of the data sheet for the version that will actually be supplied.

Cementitious grouts for onshore wind turbines

  • SikaGrout-9370 — Sika — formerly MasterFlow 9370; for anchor cage foundations, class C80/95, layer 25–250 mm, application from +2 to +40 °C.
  • SikaGrout-9300 — Sika — formerly MasterFlow 9300; with metallic aggregate for increased ductility, at least 120 MPa at 28 days, layer 30–200 mm.
  • SikaGrout-3320 — Sika — fatigue-certified, with a reduced carbon footprint, 115 MPa at 28 days, layer 20–500 mm.
  • SikaGrout-3350 — Sika — rapid-hardening, fatigue-certified, approx. 120 MPa at 28 days, layer 20–500 mm.
  • SikaGrout-3200 — Sika — rapid-hardening grout for tower joints and baseplates, approx. 90 MPa at 28 days, layer 10–200 mm.
  • SikaGrout-9200 — Sika — formerly MasterFlow 9200; for onshore VESTAS turbine foundations, at least 135 MPa at 28 days, layer 25–600 mm.
  • SikaGrout-9400 — Sika — formerly MasterFlow 9400; for tower flanges and anchor bolts, from 130 MPa at 28 days, layer 25–600 mm.
  • SikaGrout-4316 — Sika — precision grout with a hydraulic binder; according to the manufacturer, fatigue-tested to DNV ST C502 for wind turbine grouts.
  • Fosroc Conbextra BB92 — Fosroc — for structural connections in onshore foundations and concrete segment towers, at least 90 MPa at 28 days, gap 10–150 mm.
  • Fosroc Conbextra VG — Fosroc — for Vestas wind turbine foundations, class C100/115, gap 25–400 mm.
  • Fosroc Conbextra SG — Fosroc — for Siemens wind turbine foundations, class C80/95, gap 25–150 mm.
  • Fosroc Conbextra TA — Fosroc — for tower foundations onshore and in the tidal zone, class C100/115, layer 25–400 mm.
  • PAGEL V1/30HF, V1/60HF, V1/0HF — Pagel — ultra-high-strength grouts of class C100/115 for steel tower segments, at least 60 MPa after one day, application from +0.5 °C.
  • PAGEL HF50 — Pagel — class C100/115, at least 115 MPa at 28 days; its listed uses include wind turbine grouting.
  • PAGEL V1/50HF — Pagel — class C90/105 for wind energy installations and precast elements.
  • Ducorit A2G — ITW (Densit) — class C70/85, fine-grained, layer 10–150 mm, for steel tower flanges and concrete tower joints.
  • Ducorit R5G — ITW (Densit) — class C100/115, 120 MPa at 28 days, layer 50–350 mm.
  • Ducorit S5R — ITW (Densit) — class C110/125 for structural connections in onshore and offshore foundations.
  • Mapegrout WT-S — Mapei — ultra-high-strength cementitious grout for onshore wind turbine flanges and anchor bolts.
  • Mapefill 130 WT — Mapei — high-flow non-shrink mortar for anchoring onshore wind turbine baseplates, 130 MPa.
  • Fosroc Conbextra BB72-IN — Fosroc — free-flow non-shrink grout for joints of precast wind turbine towers.

Grouts for offshore wind turbines

  • SikaGrout-9500 — Sika — formerly MasterFlow 9500; DNV-certified, at least 135 MPa at 28 days, layer 25–600 mm, placing from 0 °C.
  • SikaGrout-9650 — Sika — formerly MasterFlow 9650; supplied in bulk, class C60/75, layer 50–1000 mm.
  • SikaGrout-9800 — Sika — formerly MasterFlow 9800; for pile-to-sleeve connections, class C90/105, layer 30–600 mm.
  • Ducorit S8 — ITW (Densit) — supplied in bulk with pneumatic transfer, at least 90 MPa, layer 30–700 mm.
  • Ducorit D4 — ITW (Densit) — ultra-high-strength grout for wind turbine foundation and oil and gas structural connections.
  • Ducorit S5 — ITW (Densit) — ultra-high-strength grout for wind turbine foundation and offshore structural connections.
  • Ducorit S2 — ITW (Densit) — very-high-strength grout for wind turbine and oil and gas foundation connections.
  • Ducorit S1 — ITW (Densit) — high-strength grout for wind turbine foundation and offshore structural connections.

Epoxy grouts for wind turbines

  • Five Star XP 250 Epoxy Grout (Wind) — Five Star Products — 172.4 MPa (25 000 psi) at 28 days, pour depth 50.8–457.2 mm (2–18 in) in a single lift.
  • Five Star XP 230 Epoxy Grout (Wind) — Five Star Products — ultra-high-strength grout for wind turbine towers, 158.6 MPa (23 000 psi) at 28 days.
  • Five Star SP Epoxy Grout PG — Five Star Products — pumpable, 113.8 MPa (16 500 psi) after one day; its listed uses include wind turbine bases.
  • Five Star DP Epoxy Grout PG — Five Star Products — pumpable, low-exotherm, pour depth 12.7–152.4 mm (0.5–6 in); the manufacturer includes it in its range of solutions for wind energy.
  • SikaFlow-648 — Sika — formerly MasterFlow 648; a three-component epoxy grout whose listed uses also include wind turbines.
  • SikaFlow-690 — Sika — very-high-strength flowable epoxy grout for wind turbines, pumps and compressors.

Frequently asked questions

Can a tower flange be grouted with an ordinary non-shrink grout?

Only if the foundation design and the turbine manufacturer’s requirements expressly allow it. As a rule, they specify a strength class, a modulus of elasticity and proven fatigue resistance that general-purpose grouts do not achieve or have not been tested for.

When can the anchor bolts be tensioned?

Once the grout has reached the strength specified by the turbine manufacturer or the design, and this has been confirmed by testing specimens cured under site conditions. The time depends on the temperature: the data sheets of wind turbine grouts give the strength after one day and, for some grades, also at low temperature, but the decision is based on the actual test results.

Can grouting be carried out in winter?

Within the temperature limits stated in the technical data sheet, and with protection against freezing until the grout has gained strength. The lower limit differs from grade to grade: for example, +2 °C for SikaGrout-9370 and SikaGrout-9300, +0.5 °C for PAGEL V1/30HF, V1/60HF, V1/0HF, and +5 °C for Fosroc Conbextra BB92, SikaGrout-3320 and SikaGrout-3350. At lower temperatures, a heated enclosure and heating of the substrate and the material are needed, as recommended by the manufacturer.

Why are wind turbine grouts so strong?

The layer under the flange is compressed by the tension in the anchor bolts and at the same time takes the alternating load from the rotor. High strength and stiffness reduce deformation and creep, maintain the bolt tension and provide a margin against fatigue.

How do offshore wind turbine grouts differ from onshore ones?

Offshore grouts are designed for pile and transition piece connections, which are grouted in large volumes, often under water or in the tidal zone, with the grout fed from hoppers on a vessel. Onshore wind farms in Central Asia use grouts for onshore foundations; offshore grades are generally not needed here.

What should be agreed before ordering?

The grout grade — with the designer and the turbine supplier; the specimen test method and schedule — with technical supervision; the availability of the documents required to use the material in Uzbekistan — with the supplier. Foreign certificates do not replace national requirements.