Andrew Pott and Jamal Elkaissi, Colorado Department of Transportation
Photo: Steve Yip, CDOT
The definition of high performance concrete continues to evolve in Colorado. Technological advances, as well as performance requirements, continue to raise the bar for what we consider “high performance.”
High Performance Concrete
Back in the 1930s, high performance was sought by using a higher concrete strength, 3000 psi (21 MPa) Class A concrete for bridge structures as opposed to the 2000 psi (14 MPa) Class B concrete. As the effects of freezing and thawing cycles became better known, air entrainment was required for the portions of bridges and structures exposed to the environment. This “high performance concrete” is now considered standard practice. As tight urban environments forced us to increase our span lengths while decreasing our structure depths, increased concrete strength again was the goal of our higher performance concrete. An article in HPC Bridge Views Issue No. 3 on the I-25 over Yale Avenue Bridge is a good example of this.
Some higher performance concrete results simply from technology changes in the industry. As cement particles were ground finer and finer, we found ourselves with higher earlier strength concretes. Along with these concretes came the potential for increased thermal cracking and higher setting temperatures.
The Colorado Department of Transportation’s (CDOT) latest foray into higher performance concrete was an attempt to decrease permeability and achieve less cracking in our bridges decks that use Class H and HT concretes. These concretes are used for bridge decks that do not receive a waterproofing membrane. Although having the same minimum strength of 4500 psi (31 MPa) as our Class D and DT mixes, the timing for strength requirements was changed from 28 to 56 days and cementitious requirements were reduced to a range of 580 to 640 lb/yd3 (344 to 380 kg/m3) in order to achieve the desired characteristics. Lower permeability was obtained with the addition of fly ash and silica fume and lower cement content. The lower permeability helps slow the ingress of moisture and chlorides thus protecting the reinforcing steel. The lower cement content helps reduce shrinkage cracking. The addition of fly ash and silica fume also provided higher strength to offset the lower cement content.
Class H mixes have achieved the objective of less cracking, but at a cost. The specification for Class H concrete requires testing for cracking tendency per AASHTO provisional standard PP34 Standard Practice for Estimating the Cracking Tendency of Concrete. (See HPC Bridge Views, Issue No. 45) With test capabilities available at only two facilities in the state, this presented a challenge for the first projects. This challenge has now been reduced with the addition of new capabilities at other testing facilities.
Another unforeseen challenge of the Class H mixes is the silica fume content. The availability of silica fume in 25 lb (11 kg) bags only, does not lend itself readily to large batch mixes. In addition, because of the small particle size, the silica fume can be a hazard to the workers exposed to it. A study is currently underway to develop a new mix design to remove or at least reduce the silica fume content. An alternate testing method to the AASHTO PP34 test is also being sought to ease the testing requirements for this mix class.
High Strength Concrete
CDOT’s requirements for higher strength, cast-in-place concretes are found in the Class S mixes including S35 at 5000 psi (35 MPa), S40 at 5800 psi (40 MPa), and S50 at 7250 psi (50 MPa). In order to gain the extra strength, the cement content range was extended. Unfortunately with increased cement content comes the risk of increased cracking tendency. A task force is looking at how to optimize the mix designs for best overall performance, e.g. increase the strength without increasing the cracking tendency. Class PS concrete is used for precast items such as girders and deck panels and is typically high strength as well, with strengths reaching up to 14,000 psi (97 MPa). This is attained by controlling aggregate size, water-cementitious materials ratio, and admixtures. The challenges for the mixes with the higher strengths are in achieving consistent properties. Fortunately, CDOT enjoys a great working rapport with local precasters in overcoming these problems.
Lessons Learned
One important lesson learned from research and experience is to use high performance concrete teamed with proper design and construction practices. A “high performance concrete” installed with poor construction practices will generally only result in a poorly performing concrete. The Class H concrete in bridge decks cracks less than the Class D concrete mix when installed properly. If installed improperly, it will crack just as much as a Class D mix, if not worse.
As part of improving construction practices, pre-placement conferences and test placements are critical, especially when using new concrete mix designs. Mixes with silica fume or other admixtures may finish differently and test placements provide the contractor with experience. At a minimum, a pre-placement conference can make the contractor aware of potential differences. Curing methods are crucial to reducing cracking.
Completely crack-free bridge decks, curbs, and sidewalks are difficult to obtain due to the restrained drying shrinkage. Even with the elimination of negative moment cracking at the piers through alternative structural designs, shrinkage cracking will still create challenges. High performance concrete can mitigate this cracking, but is only one component to making deck systems last 75 to 100 years. Secondary protection systems such as reinforcement corrosion protection and bridge deck waterproofing systems are important components as well.
Continuing research is also crucial in this holistic approach to longer lasting deck systems. This research needs to consider not only concrete mix design, but design details, construction details and practices, and supplementary protection systems as well.
The Challenge
Like other states, Colorado is challenged to develop high performance concretes to achieve the reliability and dependability necessary to meet the desired 75- to 100-year service life for our bridges. Combined with proper design and construction techniques, these materials will help achieve this goal. CDOT looks forward to the process of properly implementing the high performance concretes of today, while developing the high performance concretes of tomorrow.
Additional Information
Specifications for the classes of concrete and their intended use are available in Section 601 of the CDOT Standard Specifications for Roads and Bridge Construction.