These relatively dense deposits of hydration products surrounding (and sometimes encapsulating) the cement grain serve as diffusion obstacles to water and hydration products. Therefore, further hydration is hindered, producing a much more open pore structure than that of comparable materials with a normal rate of hydration. Hence, strength gain acceleration in cementitious materials generally has a negative effect on their transport properties.
Based on this analysis, it can be concluded that for concrete and other cementitious materials—especially those exposed to severe environments—the rate of strength gain is critical to durability. Materials with slow strength gain (e.g. those containing fly ash or slag) might perform more satisfactorily under these conditions.
Repair materials with acceptable minimum early-strength properties should be used. If practical, their compressive strength should be specified at a stage later than the traditional 28 days. It should not be in excess of what is necessary for load-carrying purposes. The ‘specified’ strength values should be kept at levels similar to the actual ‘in-place’ compressive strengths.
Cement and aggregates
When ready-mixed concrete is specified as a repair material, the ‘more-cement-is-better’ rule tends to wrongly prevail. Any attempt to produce durable cement-based material comes up against a dilemma. If a small amount of cement is added, the material is relatively crack-resistant, but permeable. If a large amount of cement is mixed into the concrete, the material becomes stronger and more impermeable, but less crack-resistant.
In fact, if cement is added until extremely low permeability is achieved, the material becomes more brittle and has much less creep relaxation to sustain high tensile stresses induced by drying and autogenous shrinkage. In other words, it is impermeable between the cracks, but in the end, its true permeability can become substantially higher than the lower-strength material.
Therefore, durability cannot practically be achieved between the extremes of either too little or too much cement. One of the main reasons for more extensive cracking and the reduced durability of ‘high-performance’ concrete and other cementitious materials is these materials have higher cement contents, higher paste volumes, higher moduli of elasticity, and lower creep.
The specifications reviewed by the authors followed standard material manufacturers’ recommendations, such as the need for incorporating aggregates in thicker repairs:
When thickness of the repair exceeds 50 mm (2 in.), the repair mortar should be extended with 10-mm (3∕8-in.) coarse aggregate.
The specifications also require the same aggregate quantity be used, regardless of the material composition and repair specifics (e.g. thickness, spacing of reinforcing steel, and clearance from reinforcement to the bottom of repair cavity).
A crack-resistant, ‘durable’ repair material should not have a deficiency of any aggregate particle size. The adequate aggregate size distribution minimizes void content, as the incrementally small particles fill these spaces. The goal is to pack as much aggregate into the material mixture as practically possible, thereby reducing the amount of paste needed to fill the voids between particles.
Drying shrinkage of a concrete repair material is one of the major factors influencing the overall repair durability. However, not a single limitation for shrinkage was found in the specifications of concrete as a repair material in the cases studied. Pre-packaged repair materials may be limited to certain shrinkage values, but without any indication to what age of the material and test conditions this is to be applied, such requirements are useless.
Of equal concern is the current myth that specifying low water-to-cementitious materials (w/cm) ratios reduces shrinkage. A low w/cm ratio may increase strength and density, but it is unlikely to reduce ultimate shrinkage (i.e. self-desiccation shrinkage and drying shrinkage). For given constituents, it is not the w/c ratio, but the total water and paste content of the mixture that has the greatest influence on the material’s shrinkage and cracking potential.
Cement paste acts as a binder, filler, and finishing aid. However, it is also the phase undergoing shrinkage in concrete. Unrestrained neat cement paste can shrink four to five times more than concrete prepared with the same paste. Therefore, any reduction in paste quantity will make the greatest contribution to reducing shrinkage and cracking, along with improving durability (as far as adequate consolidation can be achieved).
Often, ‘high-performance’ concrete possessing a w/cm ratio of about 0.25 is unnecessarily specified. In so doing, designers unintentionally create an epidemic outbreak of self-desiccation and cracking. Water-reducing admixtures are quite effective in modifying some concrete properties, but they may not necessarily reduce the amount of shrinkage. Sometimes, the opposite is true.