92 Damage to concrete structures
Wind conditions can be very influential on the problem of early-age thermal cracking, especially immediately after demoulding. Also solar radiation can have a significant influence. Typically, a higher wind speed can
lead to larger thermal gradients within the element. A more pronounced
solar effect in combination with day and night cycles can also increase the
early-age thermal crack risk.
4.4.2.4 Technological parameters during execution
During execution, the early-age thermal crack risk can be significantly
influenced by the formwork conditions and by cooling measures. An insulated formwork will increase the maximum temperature in the hardening
element, increasing the risk of early-age thermal cracking due to external
restraint. However, it will decrease the thermal gradient inside the hardening element, thus reducing the risk of early-age thermal cracking due to
internal restraint. By all means, the timing of demoulding should be carefully checked in case of massive hardening concrete elements. Inappropriate
demoulding when still having elevated temperatures in the core of the
element can lead to a substantial increase in thermal gradient and, thus,
to an increased crack risk.
4.4.3 Mitigation
Considering the influencing parameters as mentioned in the previous section, some measures can be listed in order to avoid the risk of early-age
thermal cracking in hardening concrete elements:
• A cement type with low heat of hydration can be considered. This can
be a Portland cement with reduced C 3 S content, designated as a low
heat (LH) cement. As an alternative (and a more common solution
in Europe), blended cements can be used, such as blast furnace slag
cements.
• A partial cement replacement by puzzolan materials can also help
reduce the heat of hydration. Puzzolan materials react more slowly
than Portland cement and thus reduce the heat production rate.
Puzzolans can help to reduce the cement content, while still maintaining long-term strength of the concrete. However, their influence
on the long-term durability behaviour of the concrete should also be
investigated, duly considering the type and nature of the puzzolan.
• A coarser aggregate type could be applied, considering the massivity
of the element. This will lead to a lower required paste volume, which
is helpful in further reducing the cement content and, thus, the heat
of hydration.
Wind conditions can be very influential on the problem of early-age thermal cracking, especially immediately after demoulding. Also solar radiation can have a significant influence. Typically, a higher wind speed can
lead to larger thermal gradients within the element. A more pronounced
solar effect in combination with day and night cycles can also increase the
early-age thermal crack risk.
4.4.2.4 Technological parameters during execution
During execution, the early-age thermal crack risk can be significantly
influenced by the formwork conditions and by cooling measures. An insulated formwork will increase the maximum temperature in the hardening
element, increasing the risk of early-age thermal cracking due to external
restraint. However, it will decrease the thermal gradient inside the hardening element, thus reducing the risk of early-age thermal cracking due to
internal restraint. By all means, the timing of demoulding should be carefully checked in case of massive hardening concrete elements. Inappropriate
demoulding when still having elevated temperatures in the core of the
element can lead to a substantial increase in thermal gradient and, thus,
to an increased crack risk.
4.4.3 Mitigation
Considering the influencing parameters as mentioned in the previous section, some measures can be listed in order to avoid the risk of early-age
thermal cracking in hardening concrete elements:
• A cement type with low heat of hydration can be considered. This can
be a Portland cement with reduced C 3 S content, designated as a low
heat (LH) cement. As an alternative (and a more common solution
in Europe), blended cements can be used, such as blast furnace slag
cements.
• A partial cement replacement by puzzolan materials can also help
reduce the heat of hydration. Puzzolan materials react more slowly
than Portland cement and thus reduce the heat production rate.
Puzzolans can help to reduce the cement content, while still maintaining long-term strength of the concrete. However, their influence
on the long-term durability behaviour of the concrete should also be
investigated, duly considering the type and nature of the puzzolan.
• A coarser aggregate type could be applied, considering the massivity
of the element. This will lead to a lower required paste volume, which
is helpful in further reducing the cement content and, thus, the heat
of hydration.