Strain Rate Induced Strength Enhancement in Concrete: Much ado about Nothing?

When concrete impact and penetration simulations are discussed, the question of increased strength due to high strain rates arises. Many concrete material modelers cite and use the seminal work of Bischoff and Perry (1991), or the widely accepted standard reference for concrete Comite Euro-International du Beton (1993) or CEB for short. Bischoff and Perry amassed a large amount of concrete laboratory data addressing strain-rate induced Dynamic Increase Factors (DIF) or the ratio of the measured dynamic to quasi-static strength. Figure 1 is taken from Bischoff and Perry (1991) and shows the large amount of data they collected, along with the strain-rate equations recommended in the CEB for two concrete strengths. The data shows a large amount of scatter in reported strength increases. The depicted CEB equations approximately bound the data. The CEB recommended strain-rate induced strength increase equations are: [ ... ] "It should be noted that the sharp increase predicted at rates greater than 30/s is only tentative, and other recent recommendations [113]1 have also been made which disregard this effect for concrete strength in compression." The data collected by Bischoff and Perry clearly indicates there is some measurable increase in unconfined compressive strength of concrete with increasing strain rate, and we can accept the CEB formulass as being representative of the data. However, the unanswered question is "Does this unconfined compression data translate into the simulations of interest, e.g. blast and penetration of concrete targets, and in particular, do the strain-rate forms used in constitute models?" The above question is addressed in two parts: 1. What do simulations of dynamic unconfined compressive strength tests predict? 2. What do the corresponding simulations of dynamic confined compressive strength tests predict? And, what data, if any, can be used to verify these models.