We are asked this question a lot, the answer is no, if the grout is solid and healthy there will be no loss of grout. However, if the grout is compromised from age and weathering showing cracks and pulling away from the edge of the slabs then it is likely to come out. The other factor to keep in mind is the material type of the grout being used especially in newer patios, it is very common these days for landscapers to use cheaper quality brush in products, they will use these for the ease and speed of use. These products often claim to set as hard as cement when in reality they are often fragile and weak and even whilst using appropriate reduced levels of pressure these weak products are susceptible to coming loose. This also goes for if a weak sand and cement mix or a very dry mix is used. If the grout is a good quality epoxy bound resin or a strong sand and cement mix, then even with higher levels of pressure the grout will remain intact.