This commentary is intended to highlight the codes and standard practices of the American Concrete Institute to bring awareness to the solution and address the common problems during concrete placement.
By: DNV North American Civil Engineering Team
Onshore wind turbine generators (WTG) are typically supported by reinforced concrete foundations which are cast in place. As wind turbines are getting larger and larger, so do wind turbine foundations. A 5 MW turbine may need an 80-foot diameter reinforced concrete foundation to provide support. Such a foundation will require a volume of concrete in the range of 850 to 900 cubic yards (mass concrete) and take seven to nine hours to complete the on-site concrete placement. As the size of the foundations and placement durations have increased, so do the placement challenges associated with mass concrete. It is important for concrete placement to meet American Concrete Institute (ACI) guidance as well as industry standards of practice—which is a main challenge across the wind industry. This commentary is intended to highlight the codes and standard practices to bring awareness to the solution and address the common problems during concrete placement.
Industry codes and standards
The following industry codes and standards are applicable for cast-in-place WTG foundation concrete:
- ACI 301, Specifications for structural concrete
- ACI 318, Building code requirements for structural concrete
- ACI 305.1, Specification for hot weather concreting
- ACI 306.1, Specification for cold weather concreting
- ACI 308.1, Specification for concrete curing
- ACI 309R, Guide for consolidation of concrete
- ASTM C172, Standard practice for sampling freshly mixed concrete.
Concrete placement challenges
- WTG foundation mass concrete placement faces the following main challenges:
- Hot and cold weather
- Interruptions (rain, lightning, equipment breakdown, force majeure events)
- Consistent concrete delivery
- Mass concrete consolidation
- Long duration of concrete placement for laborers
- Monolithic pours where the base and pedestal concrete are placed during the same pour. This limits the possibility of adequately vibrating the last top layer of base concrete near the bottom of the pedestal formwork due to possible concrete runout from the completed pedestal.
Hot and/or windy weather can create curing problems related to the increased rate of cement hydration at high temperature and increased evaporation rate of moisture from the freshly placed concrete, that can cause extensive shrinkage cracking of the concrete surface that requires repair. Cold weather, on the other hand, can cause damage to concrete due to early age freezing. If established procedures are followed, newly placed concrete can avoid high or low temperatures during curing that can adversely affect foundation performance.
Interruptions to the concrete placement can cause cold joints, i.e., the interface between two concrete layers cast at different times, to effectively form a plane of weakness between layers within the WTG foundation. A cold joint affects the structural integrity of the turbine foundation, is expensive to repair and causes delays to project schedule.
Mass concrete consolidation
Of the list, mass concrete vibration is often overlooked and based on DNV experience has caused more problems to a wind project than all other placement challenges combined. Some of the main issues are summarized below:
- Vibration is not applied
- around embedment ring and vertical reinforcement, e.g., highly congested area (Figure 1)
- during the final lift of concrete
- during the knitting together of concrete layers within the structure
- Little to no vibration on the sloped part of the foundation
- Use of vibrators to move concrete laterally
- Inadequate equipment used
- Challenges of proper training and limited availability of experienced labor for large foundations
- Challenges of labor providing consistent vibration for the duration of long concrete pours
- Non-compliance with ACI standards and industry best practices (Figure 2)
These issues may lead to different concrete quality concerns such as the cold joint and honeycombing as shown in Figures 4 and 5, respectively. In minor or some moderate cases these concerns may be able to be repaired or patched. In a worst case scenario, the only solution available would require the removal of the defective foundation and replacing the foundation with a new one, see Figure 6.
|Figure 1 - Top mat reinforcement congestion close to the embedment ring||Figure 2 - Spacing of immersion vibrator insertions shall not exceed 1-1/2 times the vibrator’s radius of action per ACI 301|
According to ACI 309 “Guide for consolidation of concrete”, concrete consolidation is the process of inducing a closer arrangement of the aggregates in freshly mixed concrete during placement by the reduction of voids by vibration. Concrete vibration is required to remove excessive entrapped air in freshly placed concrete; if allowed to harden with the entrapped air, the concrete will contain voids and be poorly bonded to the foundation reinforcement. The resulting concrete foundation will have low strength, high permeability, and poor resistance to deterioration.
Given the large volume and concrete delivery methods (typically by 10 cubic yards per concrete trucks), WTG foundation concrete is placed in layers (lifts). It is important to consolidate concrete by vibration to knit each layer of freshly placed concrete into the in-place concrete, i.e., ensure that the vibrators penetrate through the current layer to the previous layer.
Industry standard, for WTG foundation mass concrete consolidation, is performed per the ACI 309 standard and industry best practices. Some of the key vibration best practices are summarized below:
- The lifts should be built up with multiple layers 12 to 20 inches thick; depending on the aggregate size, or by the foundation designer’s specifications
- For effective consolidation of mass concrete, the vibrator crew should follow a systematic procedure. The vibrators should be inserted nearly vertically into the tops of the deposited concrete at uniform spacings; and penetrate a minimum of 6 inches into the previously placed layer; the distance between insertions should be approximately 1-1/2 times the radius of vibrator influence, which depending on the stiffness of the mix, and the type and size of the vibrator means insertions would be approximately every 15-25 inches apart over the whole area surface of the base and pedestal
- Vibration at each point should continue until entrapped air ceases to escape. Depending on mixture and slump, this time will usually range from 10 to 15 seconds, or per the designer’s specifications.
Figure 3 – Mass concreting of a wind turbine foundation
Figure 4 – A cold joint shown at the edge of a wind turbine foundation
Figure 5 – Lack of concrete vibration shown at the edge of a wind turbine foundation
Figure 6 – Wind turbine foundation demolition due to interrupted pour
Preventative measures to mass concrete consolidation issues
WTG foundation mass concreting is challenging; however, the following measures can be taken to ensure better quality of the foundations:
- Regular training for the concrete placement crew on compliance with ACI standards and industry best practices
- Involvement of the foundation design engineer, e.g., providing guidance or support to the development of an industry standard mass concreting plan and providing on-site supervision
- Use of the right equipment and manpower
One of the key take-aways is that, as wind turbine foundations are getting larger in size, it is important to provide adequate manpower and equipment, i.e., increasing the number of laborers and vibrators; whereas one to two vibrators may have been sufficient for smaller foundations.
DNV is available to discuss the details of the concrete consolidation issues or any aspects of WTG foundation construction, as we have a detailed familiarity with the industry standards worked with every day. Please reach out to Claire Haack of DNV Civil Engineering or any of the DNV independent engineering project managers.