Mark Gaines, PE, Washington State Department of Transportation, Mohammad Sheikhizadeh, PE, David Evans and Associates
The aging and deficient bridge infrastructure of the country is facing a replacement crisis. As funding for replacement of deteriorated bridges is becoming scarcer, there is a renewed interest by many states in constructing bridges that can provide long service life with minimal maintenance costs. Early age concrete bridge deck cracking is one of the major contributory factors affecting durability and loss of service life in bridges. Cracks in bridge decks can provide a direct pathway for water and salts to penetrate deep into the slab, potentially initiating slab reinforcement corrosion, and a potential process of rapid deck degradation.
Over the past 15 years, the Washington State Department of Transportation (WSDOT) has become increasingly concerned with bridge deck cracking. In 1996, WSDOT made significant changes to the requirements for bridge deck concrete mixtures. Some of the key characteristics of this revised mix include:
- 660 lb/cubic yard cement (minimum)
- 75 lb/cubic yard fly ash (minimum)
- ¾” nominal maximum aggregate size
With a high cement content, these modifications resulted in concrete with excellent compressive strength (typically 6500+ psi at 28-days) and very low permeability. Unfortunately, this mix is prone to early-age shrinkage cracking. On recent projects constructed since 2005, both the size and the frequency of the shrinkage cracks have increased. It’s difficult to determine exactly why deck cracking has become worse in recent years, but possible causes include changes in the chemistry or fineness of cement, variation in fly ash properties, and modifications in the chemical formulation of admixtures.
WSDOT has led research in recent years to determine the root cause of this problem and has used the results of research at Washington State University in field experimentation to control bridge deck cracking. Based on these efforts, WSDOT developed a new performance-based specification that has been used on approximately ten projects since 2011. Instead of placing requirements on minimum cementitious content, the new mix uses performance requirements, including:
- 28-day compressive strength of 4000 psi (minimum)
- 28-day drying shrinkage of 320 microstrains (maximum) (AASHTO T160)
- 56-day rapid chloride permeability of 2000 coulombs (maximum) (AASHTO T277)
- Scaling resistance, visual rating less than or equal to 2 after 50 cycles (ASTM C672)
WSDOT also made changes to the concrete placement and curing requirements. New requirements eliminate curing compound, instead requiring fogging to prevent moisture loss from fresh concrete. Once the finishing of the concrete is complete, pre-soaked burlap or other finishing blankets are immediately laid out on the wet deck. Soaker hoses are placed and deck is covered with reflective sheeting as soon as the concrete achieves initial set. After the 14-day wet cure is completed, diamond grooving is used to provide longitudinal tining of the deck necessary for skid resistance. These changes to the concrete mix and finishing/curing practices have significantly reduced deck cracking as illustrated in Figures 2 and 3.
Fig. 3. (right) – View of the underside of a bridge deck constructed in 2013 using WSDOT performance-based bridge deck specifications
Although implementation of the new specifications has reduced deck shrinkage cracking, minor cracking is still evident. One factor that can significantly contribute to early transverse deck cracking is the differential between deck peak hydration temperature and ambient temperature. The strain due to this temperature differential can be as much as 300 micro-strains (Figure 4).
Performance-based deck concrete mixes with differential temperatures limited to 24°F have displayed significant reduction in transverse cracking. However, the Washington Association of General Contractors has expressed concern about including this limit in construction contracts due to unpredictability and the contractual risk it would expose them to.
WSDOT has been monitoring the hydration temperature signatures of performance-based concrete mixes and has found a remarkably high coefficient of correlation of R2 = 0.98 among concrete mixes in terms of initial hydration temperatures at different ambient temperatures and varying amounts of cementitious content (Figures 5 & 6).
The regression analysis holds true for mixes with up to 20% substitution with Supplementary Cementitious Materials (SCM), primarily Class F fly ash and slag cement. With the aid of the above graphs simplifying predictability of concrete mix hydration temperatures, it may become more acceptable to the contracting community to include limiting differential temperatures to 24°F in the specifications.
In summary, allowing performance-based concrete mixes with specific outcome requirements, best practices with curing and texturing, and considering the curing environmental factors in construction specifications can result in crack-free bridge decks.
For more information, please contact Mark Gaines of WSDOT at [email protected], and Mohammad Sheikhizadeh of David Evans and Associates at [email protected].