yes / ACI Committee 544’s repeated drop-weight impact test for concrete is often criticized for large variations within the results. This paper identifies the sources of these large variations and accordingly suggests modifications to the ACI test. The proposed modifications were evaluated and compared to the current ACI test by conducting impact resistance tests on 40 specimens from two batches of polypropylene fiber-reinforced concrete (PPFRC). The results obtained from both methods were statistically analyzed and compared. The variations in the results were investigated within the same batch and between different batches of concrete. The impact resistance of PPFRC specimens tested with the current ACI test exhibited large coefficients of variation (COV) of 58.6% and 50.2% for the first-crack and the ultimate impact resistance, respectively. The corresponding COV for PPFRC specimens tested according to the modified technique were 39.4% and 35.2%, indicating that the reliability of the results was significantly improved. It has been shown that, using the current ACI test, the minimum number of replications needed per each concrete mixture to obtain an error below 10% was 41 compared to 20 specimens for the modified test. Although such a large number of specimens is not good enough for practical and economical reasons, the reduction presents a good step on the development of a standard impact test.

This study describes the strength, toughness and strain-rate sensitivity of fibre-reinforced cement-based foams subjected to variable loading rates. Drop-weight impact tests were conducted on beams with cast density between 475 - 1200 kg/cu.m. The study shows that under quasi-static loading, the compressive strength, elastic modulus and the modulus of rupture of plain mixes scale with the square of the relative density. On the other hand, the flexural toughness factor scaled linearly with it. Fibres were seen to increase the flexural strength at all rates of loading, regardless of cast density. Further, cement based foams were seen to be strain-rate sensitive. The resistance of cement-based foams to sulphate exposure was also investigated. Heavier cement-based foams are more susceptible to sulphate attack and perform poorly with an increase in the duration of exposure when compared to the lightest mix which showed improved responses up to 30 days of exposure due to self-healing. / Structural Engineering

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