Admixtures

Putting a heavy track vehicle on concrete is probable expecting more from the concrete than it is capable of delivering, even a high quality 4000 PSI concrete.

Diamonds are the hardest substance in the world but if you hit a diamond with a hammer it will shatter. The same thing is happening with the concrete under the point load of a track vehicle. The aggregate as well as the cement paste is being shattered.

Concrete is hard but it is also brittle. What is needed is something that can absorb the impact and transfer the load to the concrete. A ductile and malleable surface treatment such as a metallic aggregate floor hardener would help. A surface treatment that can be compressed instead of being brittle would help protect the base concrete.

ACI defines HRWR as a water-reducing admixture capable of producing large water reduction or great flowability without causing undue set retardation or entrainment of air in mortar or concrete. ASTM set the limit for water reduction of HRWR at a min of 12%. Neither ASTM nor ACI define MRWR. These products have been developed out of an industry need. Most all of them pass ASTM C494 Type A – Water Reducing Admixture Standards. Some of the even pass the requirements for HRWR. The main difference is the overall water reduction that can be reached at the higher end of the dosage range. Most MRWR would also start to have some excessive retardation once dosages go higher. Another key point is that some formulations of MRWR have a workability enhancer added, which can results in better finishability. All-in-all however, the application and the water reduction determines whether the product is considered a MRWR. In many applications that are typically more suited for MRWR, HRWRs have been used very successfully.

Traditionally HRWR were either based on Naphthalene or Melamine. Today most of the new products are based on polycarboxylate polymers. These new products are still formulated to meet ASTM C-494 Type F or G. The biggest difference is the versatility in these new products. Since the polymers are synthesized, they can be specially designed for specific applications. No additional admixtures would be needed for higher earlier strength or longer slump retention. These PC-based products are also much more efficient in water reduction than traditional HRWR. Dosages for regular concrete applications are therefore lower and at higher dosages

Accelerating admixtures are very commonly used in the winter months in all concrete applications. In cold weather the set times of concrete are delayed, which results in additional placing, finishing and curing costs and can delay the construction schedule. Using accelerating admixtures can offset the effect of cold weather and generate higher earlier strengths. Accelerating admixtures can be used in any mix designs. The main aspect to consider when using in combination with High-Range Water Reducers (HRWR) is the effect it can have on the air content and the dosage of air entraining admixture. Furthermore, as Accelerators are typically used at high dosage rates, the water content of the mix should be adjusted accordingly. Another point to consider when using Calcium Chloride based accelerators is corrosion. Typically mixes containing HRWR are used in structural applications, the use of calcium chloride at high dosages may initiate corrosion and lead to structural damage. In these cases, the use of a non-chloride accelerator is recommended. In rare occasions depending on the chemical composition of the accelerator, and often in combination with the use of fly ash, the use of accelerator can lead to discoloration of concrete.