Results of an experimental investigation on the properties of rapid hardening concrete are reported. Six potential cements that can develop 2000 l bf/in² in 3 hours and workable duration of about 20 minutes were selected based on an extensive literature search. Preliminary strength and workability tests were conducted for these six selected cements. Based on the results of the preliminary investigation, three cements were selected for further investigation. The variables evaluated were: (i) cement type, (ii) cement content, (iii) water – cement ratio, (iv) use of latex, (v) influence of retarding admixture, and (vi) ambient temperature. The response variables were: (i) strength gain with time, (ii) slump loss with time, (iii) workability under vibration, (iv) plastic and drying shrinkage, and (v) relationship between compressive strength and modulus of rupture. The following are the major findings.

• It is possible to formulate a workable concrete that can provide 2000 lbf/in² compressive strength in 3 hours.
• A modulus of rupture of 350 lbf/in² can be easily achieved at 3 hours.
• The strength gain under flexure mode is more rapid that the strength gain under compression loading.
• Retarding admixtures can be added to increase the workable duration to 25 minutes. The concrete flows well under vibration.
• The concrete retains its flowable characteristics better if the mix was kept under constant movement.
• Trial mixes should be made if the coarse aggregate is different from the 0.375 in maximum size trap rock used for the investigation.
• If the ambient temperature is between 65 and 80° F, mix proportions presented in the conclusion section will provide a workable mix for 25 minutes.
• If the ambient temperature is between 80 and 90° F, the retarder dose can be increased by 20 percent. If there is no need for extended workable time, the admixture dosage can be maintained at the same level.
• If the ambient temperature is between 50 and 65 ° F, the admixture dosage should be reduced by 50 percent.
• If the ambient temperature is less than 50° F placement is not recommended, unless heated water is used for the mix and heating blankets are used for curing for at least 3 hours. The concrete should be maintained at about 72° F for a minimum of three hours.
• Rapid Set Concrete is more susceptible to plastic shrinkage cracking as compared to ASTM Type I cement concrete. Therefore, the exposed surface should be protected with curing membrane or wet blanket to avoid any water loss. The surface protection can be applied as soon as the surface becomes hard.
• Rapid Set Concrete shrinks less and therefore cracks less under restrained conditions. It might be possible to formulate a mix that will not crack due to drying or autogenous shrinkage.