Controlling Cracks in Bridge Decks

Abstract

Laboratory studies incorporating crack-reducing technologies with those included in specifications for low-cracking high-performance concrete (LC-HPC) bridge decks and analyses of data from crack surveys and construction observations on more than 50 concrete decks are described.

The laboratory investigations include the combination of supplementary cementitious materials (SCMs), slag cement and silica fume, as partial replacements for portland cement, internal curing through use of pre-wetted lightweight aggregates (IC), and calcium oxide-based (CaO) or magnesium oxide-based (MgO) shrinkage compensating admixtures or expansive additives (SCA) with LC-HPC. A modified version of ASTM C157 in which length-change measurements begin 5½ ± ½ hr after casting concrete is developed and used to evaluate swelling and shrinkage including the combined effects of supplementary cementitious materials (SCMs), internal curing (IC), and MgO- and CaO-shrinkage compensating admixture (SCAs) on shrinkage of concrete specimens designed according to LC-HPC specifications. The results show that the modified version of ASTM C157 helps to capture the early-age behavior of concrete mixtures. IC is effective in reducing 0 to 20-day drying shrinkage in concrete, but the opposite is observed regarding 20 to 180-day drying shrinkage. SCMs induce increased first-day expansion and reduce shrinkage. A further increase in first-day expansion and a reduction in shrinkage is obtained when IC is used in conjunction with SCMs. The SCAs evaluated in this study reduce the tendency to develop shrinkage strain. The CaO-based SCA induces the more rapid expansion of greater magnitude, while the MgO-based SCA expands more gradually. When the CaO-based SCA is incorporated in a mixture containing SCMs or SCMs and IC, expansion is further increased, an observation that cannot be made for mixtures incorporating SCMs with the MgO-based SCA.

Analyses of crack survey results and construction observations for more than 50 bridge decks are used to better understand the principal factors affecting cracking, evaluate the effects of construction practices on cracking, and assess the effectiveness of crack-reducing technologies, such as synthetic fibers and IC. The results indicate that bridge deck cracking increases with age. Paste content (volume of cementitious materials plus water) is the most dominant factor affecting cracking, and parameters such as slump, compressive strength, and air content have much less effect. Decks cast with concrete with paste contents exceeding 27.2% exhibit substantially greater cracking than those with lower paste contents, regardless of other factors. The incorporation of a crack-reducing technology such as IC in the decks cast with concrete having paste contents exceeding 27.2% cannot overcome the negative effect of the greater paste content on cracking. Individual contractors and poor construction practices, particularly poor consolidation and overfinishing the concrete, can significantly affect cracking even when the decks are cast with a low-shrinkage concrete (paste content limited to 27.2%) even where a crack-reducing technology is used. Bridge decks with precast partial-depth concrete deck panels with cast-in-place concrete toppings show excellent cracking performance if the topping has a low paste content and good construction procedures are used. Greater average crack widths correspond with greater crack densities.